Figure 2 illustrates forerunning activity from 1976 until the great Tokachi-oki earthquake of 2003, which occurred offshore of the northern Japanese island of Hokkaido. Its mechanism involved thrust faulting along the downgoing plate boundary. Lay and Rhode conclude [19] that little slip occurred along the shallowest part of the plate boundary to the southeast. The GCMT hypocenter of the mainshock, a measure of its centroid location, is shown as a large blue circle. It is situated within the areas of large horizontal slip in 2003 (solid black and dashed lines) as determined from tide gauge data and seismic-wave modeling [17] [18].

Forerunning activity of Mw ≥ 5.0 occurred almost exclusively outside either of the two areas of computed maximum slip. Those forerunners were distributed along the peripheries of three sides of the areas of high slip in the coming mainshock. Almost all aftershocks of Mw ≥ 5.0 (Figure 3) also occurred outside of those area of high slip. More aftershocks than forerunning activity took place closer to the trench.

The area ruptured in 2003 was broken previously in 1952 during a somewhat larger event whose areas of maximum slip extended farther southeast toward the deepest part of the trench [18]. Nearly the same great asperity broke in 1952 as in 2003 [17]. Hence, those two great events cover a complete seismic cycle of stress buildup and release for which the distribution of high slip has been computed for each mainshock. The zone ruptured previously in 1853 [18].

In summary, the 2003 shock, like other subduction-zone events described later, was characterized by a central large region that was nearly devoid of forerunning events in the preceding decades as well as devoid of aftershock activity. Forerunning activity occurred on the sides of what is interpreted as a great asperity that ruptured in 1952 and 2003.

Figure 4 shows forerunning activity of shocks Mw > 5.0 until the mainshock as well as the adjacent Mw 7.7 Tocopilla event two days later. Regions of high slip associated with the 2014 mainshock were determined from local and distant seismic data, GPS networks, other geodetic data and offshore tsunami gauges—one of the best earthquakes with multiple, different sets of forerunning data.

The centroid location (blue square) of the mainshock lies within the computed zone of highest slip, 3 m, (solid black ellipse). Nearly all-forerunning activity of Mw ≥ 5.0 was located west and up dip of the two GCMT locations, within the slip contours of 0.5 and 3 m but not as far as the deepest part of the trench. The mainshock generated only a relatively small tsunami [20], which is consistent with small slip close to the trench. Aftershocks (not shown) were in nearly the same areas as the forerunning events [21].

Smaller earthquakes located using data recorded by a local network occurred to the east and downdip of the centroid shown in Figure 4 [22]. Many small events occurred in the two weeks before the mainshock updip of what [22] call the main asperity. Some of those shocks occurred within the upper plate, something that could not be ascertained solely from the data in Figure 4. The two sets of small earthquakes form what has been called a Mogi donut [23], i.e., seismic activity surrounding the quiet coming rupture zone of the mainshock. Thus, the “donut” consisted of a hole in seismicity surrounded by many forerunning earthquakes.

The distribution of data from small events confirms the use in this paper of moderate-to-large forerunning earthquakes in mapping great asperities decades to years ahead of their breaking in great shocks.

A group of coastal GPS stations accelerated toward the trench in the eight months prior to the 2014 earthquake [24]. Small repeating earthquakes likely indicated that aseismic slip took place along the plate boundary during the foreshock sequence [25]. A slow slip event using GPS observations was detected preceded the 2014 earthquake [26]. Significant changes in the rates of microseismicity extending back as far as July 2013 are interpret as large-scale unlocking of the plate interface [27].

The Iquique and Tocopilla earthquakes of 2014 were unusual in that neither rupture zone was the site of a very large shock since the giant earthquake of 1877 [5]. Thus, the 2014 rupture areas had been long-standing seismic gaps. The two largest events of 2014 were situated more than 200 km along strike from the great southern Peruvian earthquake of 2001 (described later) and well north of a 1977 shock of Mw 7.7 [21]. Hence, forerunning activity in Figure 4 is unlikely to have been affected by nearby great shocks along the plate boundary to either the northwest or the south. The plate boundaries adjacent to the two 2014 events remain seismic gaps that could be sites of future earthquakes of about Mw 7.8 to 8.3.

Cumulative seismic moment for shocks of M ≥ 5.5 (Figure 5) for the area of Figure 4 from 1976 until the mainshocks of 2014 indicates a nearly exponential increase starting about 2007, seven years before the mainshock. Following the great 2001 Peruvian shock farther to the northwest, the rate of events of magnitude ≥ 4.5 increased in the vicinity of the 2014 sequence [28]. The b-values, a measure of the number of large to small events, were anomalously low for 5.5 years in the coming rupture zone of the 2014 earthquake [29]. Values of b were normal (larger) outside that zone. Low b values suggest that the coming rupture zone in Chile was the site of high stress ([16], p. 186).

In summary, forerunning activity in Figure 4 was limited to the length along strike of the plate boundary that subsequently broke in 2014 during the two shocks of Mw 8.1 and 7.7. That observation as well as data from the local seismic network might have been used to make a long-term estimate of the length of the plate boundary along strike that would later rupture in those two earthquakes. Little forerunning activity occurred within the central zone where slip exceeded 3 m. Precursory forerunning changes in b-value, the formation of a Mogi donut, precursory slow slip events and accelerating seismic moment release occurred prior to the 2014 mainshock. This is one of the best indications of a variety of changes prior to a great earthquake. Together, they might have been the basis for issuing either a hazard watch or a prediction for time scales of about 5 to 10 years.

GPS observations were made during the decade that preceded the giant 2010 Maule earthquake, during the mainshock itself and afterwards—the first for any giant shock. The two regions of slip larger than 8 m (solid curve, Figure 6) in the mainshock as deduced from geodetic and seismic observations are located to the north and south of the seismic moment centroid (red diamond) and the epicenter of initial fault rupture in the same area. None of the forerunning activity of M ≥ 5.0 in Figure 6 occurred in either of those two zones of slip greater than 8 m. Forerunning activity was located well to the north and south of the centroid and outside of the two areas that slipped more than 8 m.

Two forerunning events also occurred east of the centroid of the mainshock. Nearly all the aftershocks of the 2010 event also occurred outside of the two inferred regions where slip exceeded 8 m [30]. A similar distribution of earthquakes took place before the 2010 mainshock [31]. Those authors calculated somewhat different amounts of slip in the mainshock than that by [32] that is shown in Figure 6. Nonetheless, the three sets of authors show maxima in slip

both to the north and south of the 2010 centroid. It ruptured bilaterally to the south and then to the north [32]. Substantial slip occurred westward to the trench as determined from tsunami observations [31]. The rupture zone of the 2010 mainshock was locked beforehand, i.e., well coupled [33].

The aftershock zones of the Mw 7.9 1985 Valparaíso and 2010 Maule overlapped about 160 km at the northern end of the 2010 rupture zone (Figure 7). Computed slip in the 2010 mainshock was relatively small there. Slip at the southern end of the 2010 zone between 37˚ and 38˚S was small, the same area that broke in the foreshock of about Mw 8.1 the day before the giant Chilean shock of May 22, 1960 [34]. The rupture zone of the giant 1960 Chilean shock itself broke from 38˚ to 46˚S, mostly to the south of Figure 6.

Aftershock zones, and by inference the main rupture zones, of several adjacent great and giant earthquakes touched one another without significant over or underlap according to [2] and [4]. Better mapping of the 1985 and 2010 zones shows, however, significant overlap of their aftershocks. That overlap occurred in the region where computed slip in 2010 was relatively small. Many of the great and giant mainshocks reported later in this paper also exhibit relatively small slip at their ends along strike where their aftershock zones overlap. It is understandable that displacements in large shocks do not decease instantaneously to zero at the ends of their rupture zones since doing so would result in a stress singularity.

The 1985 shock broke part of the seismic gap remaining from the great 1906 Chilean earthquake of about Mw 8.2 to 8.5. The 1906 rupture length from 32˚ to 35˚S was well determined from observations of coastal uplift [7]. Its northern part broke again from north to south in 1971 (Mw 7.8) and 1985 (Mw 7.9). Nishenko [7] stated, “The southern segment of the 1906 zone (from 33.5˚ to 35˚S) has been quiet for large shocks since 1916, and may be the next segment to rupture.” The 2010 Maule shock did rupture that remaining part of the 1906 rupture zone as well as the gap to the south that last broke in the great earthquake of 1835 [7]. It is not surprising that two great asperities, not one, ruptured in 2010 since the whole zone broke in separate great shocks in 1906 and 1835. The northern extent of rupture in 1835 is poorly known, however [7].

An earthquake in 1928 near 35˚S of revised magnitude Mw 7.7 [11] probably was not nearly as large as the 1835 and 1906 earthquakes. The 1928 felt area was aligned east-west and may not have been a thrust event along the plate boundary [7]. An earthquake of revised magnitude 7.6 in 1939 occurred farther east within the upper plate with a strike-slip mechanism. Since it did not break the plate boundary, it did not rupture the seismic gaps remaining from the 1835 and 1906 earthquakes. A poorly located event in 1914 of revised Mw 7.4 occurred in the same region. In any case, however, the plate boundary between 34.5˚ and 38˚S was a seismic gap for many decades before 2010.

Many GPS observations were used to compute plate coupling along the Chilean subduction zone between 18˚ and 38˚S [33]. They find that the region is subdivided into seven well-coupled (locked) segments interspersed with at least six low-coupling zones (LCZs). For the three most recent earthquakes of Mw greater than 8—Maule, Iquique and Illapel—Métois et al. [33] report that co-seismic asperities correlate well with highly coupled segments while LCZs to their north and south behaved as barriers that stopped rupture. They argue that the first-order mechanical interpretation of plate coupling in the rate-and-state formalism implies that the LCZs largely creep while the coupled segments remain locked during inter-seismic loading.

Métois et al. [33] call the region to the south between 37˚ and 38˚ the Araco LCZ. It was the site of earthquakes smaller than the giant shocks of 1960 and 2010 including foreshocks the day before the giant earthquake of 1960 [34]. Many authors unfortunately plot a single rupture zone for the earthquake of May 1960. The region between 37˚ and 38.4˚S has behaved differently from the zones of giant shocks to its north and south. The Araco LCZ is discussed more later in conjunction with slip in the giant 1960 event.

Olsen et al. [35] conclude that much of the south-central Chilean margin from 32˚ to 46˚S contains unusually strong, well-drained sediments. Their study included the rupture zones of the 2010 and 1960 giant earthquakes. They found an exception where the Mocha fracture zone is being subducted between 37˚ and 38˚S, i.e., at the Araco LCZ. They infer that it experiences localized overpressure, delaying compaction of incoming sediments and weakening the plate interface. High Poisson’s ratios and inferred very high fluid pressures in the region were inferred by [30]. These factors are associated with poor coupling along the plate interface and an abundance of forerunning activity to the 2010 mainshock, as well as to the 1960 mainshock (described later).

Figure 8 shows that cumulative seismic moment increased from 1995 until the 2010 Maule mainshock in the region between the 1985 and 2010 rupture zones shown in Figure 7. Accelerating cumulative moment in the southern part of the 2010 zone is dominated by a single large shock. Whether either acceleration of seismic moment was, in fact, casually related to the coming 2010 earthquake is debateable. Deducing a strong connection will require knowledge from future great events.

In summary, most of the plate boundary along the 2010 rupture zone is well coupled and is inferred to consist of two great asperities. Two regions of high slip were mapped, which extend westward as far as the outcrop of the plate boundary at the trench. Little forerunning or aftershock activity occurred in those zones of highest slip. Most forerunning activity occurred near the northern and southern boundaries of slip in 2010 in what has been called low-coupling zones (LCZ). They are inferred to consist of smaller asperities. The northern LCZ was the site of an increase in seismic moment release in the several years before the 2010 shock.

More than 18,000 lives were lost in the giant Japanese earthquake and tsunami of 2011. The World Bank estimated the economic costs as $ 235 billion, the costliest natural disaster in world history. More than 127,000 buildings totally collapsed and an additional one million were partially damaged. Additional longer-term financial losses have yet to be tallied from radioactive leakage, evacuation of hundreds of thousands of people, and ongoing attempts at cleanup from major explosions and meltdowns at four of the nuclear reactors at nearby Fukushima Daiichi.

Earthquakes of M ≥ 5.5 in the decade before the giant 2011 shock occurred either between the 4 and 24 m slip contours in Figure 9, on the periphery of the 24 m contour or outside them to the southwest near the shock of magnitude 7.9. It occurred about 30 minutes after the mainshock. All but one of the forerunning thrust earthquakes within the 24 m contour occurred along its periphery. As in several other subduction zones, none extended as far east as the deepest part of the trench.

The 2011 source area was unusual since between 1978 and two days before the mainshock several events of Mw greater than 7.0 took place (Figure 10). One of them, known as a Miyagi-oki earthquake, occurred within the 24 m contour and

four outside it but within the 4 m contour. The region ruptured previously in additional Miyagi-oki events of M ≥ 7.0 in 1915, 1933, 1936 and 1939. Most of the M ≥ 7 Miyagi-oki shocks broke separate asperities [17].

The locations of the five largest forerunning earthquakes since 1976 in Figure 10 surround the short-period hypocenter, the point of initial rupture in the 2011 mainshock (blue triangle). Rupture in 2011 expanded well beyond those five. The large magnitudes and numbers of those forerunning earthquakes are unique among the events studied in this paper. The dimensions of the main asperity broken in the largest, Mw 7.6 in 1978, was about 45 by 25 km [17]. The asperities of those five events were large enough for stresses to have interacted significantly with one another.

Ikuta et al. [37] conclude, … “the plate boundary was accumulating the full slip deficit (i.e., it was totally locked) during at least the last 15 years [before 2011] in a very limited area with a diameter of a few 10s of kilometers, which overlaps with the maximum co-seismic slip area.” They suggest this relatively small area, or asperity, was the last locked section of the plate boundary to fail in 2011. They show that M 7 class Miyagi-oki earthquakes of the previous 80 years only released part of the slip deficit that was being accumulated. They conclude that those M 7 rupture zones moved again in the mainshock. Apparently, the giant earthquake could not occur until several of the relatively large asperities associated with Mw ≥ 7 events ruptured and increased stresses on the last-to-break asperity [37].

The large foreshock of March 9 was followed two days later by additional moderate-size earthquakes whose locations approached the short-period epicenter of the coming giant shock (blue triangle in Figure 10). Many geophysicists now regard that forerunning sequence as a precursor to the giant earthquake. Nevertheless, until the mainshock occurred, those events do not seem to have been identified as precursors.

The 2011 mainshock was recorded by an exceptionally great number of seismograph stations and geodetic observation points both on and offshore. This permitted slip to be well calculated as far to the southeast as the deepest part of the Japan Trench. The average fault displacement over the entire area of rupture in 2011 was about 10 m. Horizontal slip close to the trench was an extraordinary 50 to 80 m. Loveless and Meade [38] used GPS data to calculate the percentage of locking (coupling) along the main plate boundaries of Japan. They found strong coupling in the zone that broke in 2011.

Forerunning thrust activity of Mw > 5.5 in Figure 10, however, did not extend to the deepest part of the trench. Very few shocks as small as Mw 5.0 (not shown) occurred nearer the trench than the centroid location. The rocks closest to the deepest part of the trench are inferred to be mostly in the velocity-strengthening slip regime. Displacements close to the trench in many other subduction zones have been poorly mapped. Some geophysicists concluded previously that those zones either creeped continuously with time or moved episodically in slow-slip events. Instead, that region ruptured more than 24 m in 2011 (Figure 9 and Figure 10).

Ikuta et al. state [37], “… suggest deeper fault sections played an important role in the accumulation of a large slip deficit near the trench, which is where the large co-seismic slip was reported in studies using seismic waves.” They claim the M7-class asperities prevented large slip from occurring closer to the trench until the mainshock. In that sense, the latter region appears to be very unusual among subduction zones.

Several authors [39] [40] [41] [42] reported slow slip in the years to about a decade before the mainshock. Two of them placed it at depth along the plate boundary to the west of the centroid. The time interval between small repeating earthquakes decreased with time, another indication that slow slip was increasing downdip before 2011 [43].

Nanjo et al. [44] showed that b-values decreased from a normal value of about 1.1 in the 5 to 20 years before the giant 2011 earthquake. They found it then decreased further in the five years before 2011 to a very low value of 0.45 within the coming rupture zone. Such low b-values are often taken as an indicator of relatively high stress [16]. These are among several examples of intermediate-term (1 to 10 years) and long-term (10 to 30 years) precursors.

Figure 11 shows cumulative seismic moment released between 1976 and the 2011 mainshock. If moment release had been occurring uniformly with time, the cumulative release would form a straight line of positive slope. Instead, it increased more rapidly starting between 2003 and 2005. It also increased at the time of the Mw 7.6 shock in 1978 but then decreased until 2003.

As stated earlier for Chile, the Japan subduction zone is divided into segments like the 2011 rupture zone that are characterized by strong coupling. Nishikawa et al. [45] and references therein] report that the regions to the north of 39˚N and south of 37˚N were the sites of numerous slow events on a variety of time scales. The region in between them was the main slip zone of the 2011 earthquake.

In summary, the 2011 mainshock was unusual in that it was preceded by abundant activity of magnitude 7 and greater, which did not extend as far toward the trench as the centroid of the mainshock. A great asperity that finally broke in the mainshock is inferred to have extended from the three large easternmost Miyagi-oki earthquakes to the outcrop of the plate boundary at the trench. Cumulative seismic moment accelerated between about 2004 and 2011. Others reported various precursory activity from several years to one or two decades before the mainshock.

The great Kuril Islands thrust earthquake of 2006 filled part of a major seismic gap between the 1952 Kamchatka shock of Mw 8.9 to the northeast and the 1963 Kuril earthquake of Mw 8.5 to the southwest. A smaller event of Mw 7.8 in 1915 likely broke part of the 2006 zone. Another shock of Mw 7.5 in 1918 probably ruptured to its southwest [2] [5]. A major seismic gap still exists between the rupture zones of the 2006 and 1952 earthquakes.

The depth of the moment tensor solution for the 2006 rupture was computed to be only 14 km, an indication it occurred along the shallow part of the plate boundary. A similar conclusion was reached by [46] from their analysis of the codas of P waves. The 2006 event produced a tsunami of only moderate size [46]. The 2006 shock was followed two months later by a great normal-faulting event of Mw 8.1 just to its east within the down-bent Pacific plate. The 2006 earthquake triggered normal-faulting events before the great 2007 normal-faulting shock.

Maximum slip [47] was centered in the middle of that deduced by [46] in Figure 12. I do not think the result in [47] is as accurate since it was based on GPS observations to the southwest, (many in Hokkaido and one site in a southern

Kuril Island). Steblov et al. [48] inferred that slip was concentrated at its southwestern end. They, however, used data from a few widely spaced new GPS stations in the Kuril Islands. I regard the maximum slip distributions computed by [46] as the most reliable and show them in Figure 12.

Forerunning activity (Figure 12) occurred southwest of the 7 m slip area but inside the one meter contour. Many of those events occurred as foreshocks in the few months before the 2006 main event. The southern half of the zone of 7 m slip in 2006 included few forerunning thrust shocks between 1996 and the mainshock. Many forerunning thrust events of 5 < Mw < 6 occurred in the northeastern half of the zone of 7 m slip along with four earthquakes with reverse-faulting mechanisms.

Rupture in 2006 propagated mainly to the northeast within the zone of high slip in Figure 12. Ref. [46] also states, “large secondary bursts are associated with two northeastern patches of enhanced slip.” Fujii and Satake [49] deduced that the largest tsunami source occurred at its northeastern end. The Bulletin of the International Seismological Centre computed a short-period hypocenter at 46.68˚N, 153.21˚E and a shallow depth of 12 km, which places it near the southern part of the zone of high slip but to its west. One possible explanation for forerunning activity in the northeastern half of the zone of maximum slip is that the southwestern half ruptured first and triggered somewhat slower slip and a tsunami in the northeastern half. If so, they appear to define two large asperities that broke in 2006.

In summary, forerunning thrust earthquakes occurred in the northeastern half of the zone of computed highest slip but not in its southwestern half. The southwestern half may be interpreted as a great asperity with slip initiating nearby. Estimates of magnitude and rupture length of the coming 2006 mainshock based solely on forerunning shocks of Mw > 5.0 would have been too small. The mainshock occurred in a long-standing seismic gap. Cumulative seismic moment release from 1976 until the mainshock was nearly constant except where it increased abruptly during the one-year period of precursory activity.

Plate boundaries that are usually regarded as poorly coupled off Java, the Lesser Sunda Islands of Indonesia, the Marianas, Tonga and the Kermadec islands were examined to see if any of their few great earthquakes were preceded by significant forerunning activity. The answer was no. The data for Java are presented here; the other areas are described in the Supplement.

The plate boundary south of Java is very different from that to its west off Sumatra in that many known earthquakes of magnitude 8 and greater have occurred south of Sumatra but none off Java and the Lesser Sunda islands to its east. Only one known event greater than Mw 7.0 (it of 7.3) took place off Java between 1916 and 1977. Only a single moderate-size earthquake occurred from 1976 until 1994 within 150 km of the 1994 mainshock in Figure 13. Very low activity took place between 1994 and the 2006 mainshock.

The thrust events of 1994 and 2006 of Mw 7.8 and 7.7 occurred very close to the Java trench. Their computed slip was small. Each was followed by aftershocks involving normal faulting, not thrust events. Their zones of maximum (one meter) slip were not sites of forerunning thrust earthquakes from 1977 through early 2019 but few other places in Figure 13 were.

Abercrombie et al. [50] report that the 1994 earthquake occurred near a seamount that is currently in the initial stage of being subducted. They interpret the 1994 event as occurring at a locked asperity in an otherwise largely decoupled subduction zone. They show that aftershocks surround the seamount, an indication that it was, in fact, a major asperity. The 2006 mainshock appears to be very similar to that of 1994. Each generated a large tsunami. A seamount along an otherwise decoupled plate boundary is well coupled [53].

In summary, nearly the entire plate boundary south of Java is poorly coupled. Forerunning thrust events prior to the 1994 and 2006 were nearly non-existent. Hence, asperities of future large events like them are unlikely to be identified ahead of time by long-term forerunning activity of moderate magnitude. Work is needed to identify them ahead of time using other techniques since their occurrence can generate damaging tsunamis.

The Costa Rican mainshock of Mw 7.6 was smaller than the other events studied. Nevertheless, patterns of forerunning earthquakes and high slip [54] [55] are like those for the Iquique, Chilean shock in Figure 4. Most forerunning activity in Figure 14 occurred outside and updip of the slip zones of 0.5 and 2 m but not as far as the deepest part of the trench. Other forerunners took place along strike to the northwest and southeast. Larger aftershocks in 2014 (not shown) also occurred mainly updip of those high slip zones [55] [56].

The Costa Rican thrust earthquake of 2012 is nearly unique in that major slip (Figure 14) occurred beneath land, i.e., the Nicoya Peninsula, a thick ophiolitic accumulation of oceanic crust that was previously emplaced along that coast of Costa Rica. The Peninsula is only 60 km from the deepest part of the Central American trench. Thus, the two regions of high slip in Figure 14 were well-determined using GPS data from stations on land. Since large slip in 2012 did not extend as far as the trench, it is understandable that a significant tsunami was not generated [56]. The plate boundary beneath the central part of the Nicoya Peninsula was largely locked, i.e., tightly coupled as determined from GPS data, which commenced in 2000 [55] [56].

The 2012 zone ruptured previously in large earthquakes in 1950 (Mw 7.75, 1992 [11]) and 1853 [56]. The 2012 rupture zone had been identified as a major seismic gap for many years.

Seismic tremor and low-frequency earthquakes were observed at depths below the M discontinuity of the upper plate and most tremor occurred in adjacent areas of the megathrust that experienced little seismic slip in the 2012 mainshock [54].

In summary, forerunning thrust earthquakes nearly surround the rupture zone of the 2012 mainshock. The interior quiet zone is identified as a great asperity that broke in 2012. The zone was identified ahead of time as a major seismic gap.

Forerunning activity of Mw ≥ 5.0 to the 2016 Ecuadorian mainshock occurred outside the zones of computed slip of 1 and 3 m (Figure 15). The main centroid was situated inside one of the two zones of 3 m slip, like the patterns for many mainshocks in this paper. Most forerunning events occurred on the northeastern and southwestern sides of the regions of maximum slip and mainly in two bands extending from the coast outward to the trench.

That zone is identified as a great asperity.

The 2016 rupture zone broke previously in the Ecuador-Colombia earthquake of 1906 of about Mw 8.5. The 1906 zone subsequently ruptured from south to north in the sequence 1942, 1958 and 1979 (Mw 7.7, 7.3 and 8.2 [11]). The 1958

and 1979 shocks were located along strike to the northeast of the 2016 rupture area. The 1942 epicenter fell within the 2016 zone of high slip in Figure 15.

Mothes et al. [58] conclude that the 2016 shock broke two main asperities on either side of the 1942 rupture area. I do not think the 1942 earthquake is well enough located to claim it ruptured a separate, relatively small asperity. Its Mw is like that of 2016, indicating its rupture zone and that of 2016 probably were of similar size. The region to the northeast last broke in 1979. It is a likely place for a future great shock. The 2016 rupture zone was a seismic gap for many decades.

The 1958 and 1979 zones were well coupled out to the deepest part of the trench but the 1942 and 2016 segments were well coupled only about halfway to the trench [58]. The segments of the plate boundary south of the 2016 rupture, where the Carnegie Ridge is being subducted, are poorly coupled except between about 1.2˚ and 1.4˚S where significant relief occurs on the plate interface [59]. The latter region had significant seismic activity prior to the 2016 earthquake, as did the zone just to its north.

In summary, the 2016 Ecuadorian earthquake ruptured a quiet zone that was surrounded by forerunning and aftershock activity. That region is regarded to be a great asperity.

Most forerunning activity to the Mw 8.0 earthquake of 2007 offshore of Pisco in central Peru was situated either on the periphery of the slip contour of 2 m or to its southeast. Aftershocks also occurred either to its southeast or within its northwestern part. A zone about 75 km along strike was the site of no thrust forerunning or aftershock activity (Figure 16).

In summary, most of the thrust foreshocks were located outside the zone of slip larger than 2 m. A zone about 75 km along strike can be regarded as a great asperity that broke in the mainshock.

This was one of the smallest events studied, Mw 7.7. Forerunning and aftershock activity in Figure 17 were located entirely outside, but adjacent to, the computed 1 m slip contour. Most of that activity occurred updip and closer to the trench than the centroid of the mainshock. The main event generated a large tsunami.

In summary, the few forerunning events and aftershocks surround a quiet zone that can be identified as a large asperity.

The Peruvian shock of 2001 was one of the largest earthquakes studied. It broke a significant part of the seismic gap remaining from the giant shock of 1868 but well short of the rupture zone of the 2014 Iquique event to the southeast (Figure 4).

Rupture started at the short-period hypocenter (blue-green square in Figure 18) at the far northern side of the 1 m slip contour and progressed southeasterly where slip greater than 4 m took place in a second sub-event (Bulletin International Seismological Centre). Two forerunning shocks occurred within the northern 1 m sub-region zone and another on its northwestern-most side (Figure 18). Other forerunners were scattered throughout the rest of the rupture zone as well as in the area just to the southeast of the 1 m contour.

In summary, the patterns of forerunning activity to the Arequipa mainshock are like those of the great Kuril earthquake of 2006 described earlier (Figure 12) in that rupture started first in a sub-region with few forerunning events and progressed along strike where a second sub-event was part of the mainshock. The 2001 mainshock likely ruptured two great asperities. The 2001 region had been a significant seismic gap since it last ruptured in a giant shock in 1868. The remainder of the 1868 zone to the southeast remains a seismic gap.

Most forerunning and aftershock activity associated with the Antofagasta earthquake of 1995 was located either on the periphery or outside the 1 m slip contour in Figure 19. The centroid of the mainshock is located just inside the 4 m slip contour. A shock of Mw 7.5 occurred eight years earlier on the periphery inside the 1 m contour. Chlieh et al. [61] conclude that slip occurred “fairly uniformly” during the mainshock and that all slip inversions give a consistent picture of the distribution of displacements during the earthquake with a single asperity slipping 5 to 10 m.

Rupture started at the short-period location just to its north at the blue square near the Mejillones Peninsula (MP). The area offshore of the peninsula, between 23˚ and 23.5˚S in Figure 19, was identified [33] as a low-coupling zone (LCZ) much like that at the southern end of the Maule rupture zone (Figure 6). While the Mejillones LCZ was the site of little co-seismic slip in the mainshock, long-term aseismic slip within it is not known [33]. No known large earthquakes, however, have ruptured directly beneath the Peninsula [33].

In summary, forerunning activity to the 1995 mainshock as well as aftershocks occurred on the periphery of the 1m slip contour. Two forerunners also took place within the central 4 m slip contour.

The 2007 mainshock, was one of the smallest examined. Forerunning thrust activity in Figure 20 was concentrated to the south of the centroid of the mainshock and southwest of the two zones where slip was calculated as one meter or greater. That activity was concentrated offshore beneath the Mejillones LCZ, which was described in the previous section. The small cumulative seismic moment of forerunning thrust events (not shown) exhibits upward curvature but is dominated by a single larger earthquake.

In summary, forerunning activity was concentrated south of the regions of high slip beneath the Mejillones low-coupling zone. The rupture zone was quiet ahead of time and may be defined as a major asperity. The GCMT location farther west than the high slip contours indicates that slip likely extended farther west closer to the deepest part of the trench.

Many forerunning shocks of M ≥ 5.0 from 1976 until the 2015 mainshock were located inside the 2 and 5 m slip contours of Figure 21. Slip was modeled using a combination of seismic and offshore tide gauge data to better constrain displacements near the trench [64]. Those authors concluded that slip between the 2 and 5 m maximum in Figure 21 extended out to nearly the outcrop of the plate boundary at the trench. The region inside the 5 m contour between the centroid and about 50 km to the north was the site of only one forerunning shock and no aftershocks. It might be picked from forerunning activity as the great asperity that broke in the mainshock.

The 2015 region previously broke in 1943, both in events of Mw 8.2 [11]. The Illapel rupture zone and the Atacama seismic gap to the north are separated by the Challenger fracture zone (not shown), which is being subducted at the trench. The two ends of the 2015 rupture were identified as low-coupling zones [33].

The region to the north of the Illapel shock last ruptured in the giant Atacama earthquake of about Mw 8.7 in 1922. Very little activity of even moderate size has occurred in that seismic gap to the north of 30.5˚S from the start of the GCMT catalog in 1976 through 2019.

In summary, the distribution of forerunning activity to the 2015 earthquake is not clear with respect to the two regions of high computed slip. It is possible to pick a large region just to the north of the centroid of the mainshock that contained only a single forerunning shock within the 5 m contour as a great asperity. No aftershocks occurred in that region.

The Mw 9.6 shock in south-central Chile was the largest earthquake between the start of instrumental seismology in the 1890s and August 2020. I use a combination of different types of locations to examine it: centroids since 1976 and short-period epicenters from 1920 to 1976.

Epicenters of forerunning activity of mb ≥ 6.0 from 1920 to the 1960 mainshock were all located outside of the three computed high-slip zones where displacements exceeded 20 m (Figure 22). The epicenter of the mainshock, which is not a centroid but a short-period location, marks the initial start of rupture. Rupture in the mainshock propagated to the south for nearly 900 km to the plate triple junction where the South Chile rise intersects of Chilean subduction zone. The 1960 sequence was initiated the day before the mainshock by an event of about Mw 8.1 [34] within the Araco low-coupling zone (LCZ). The Araco LCZ was the site of abundant forerunning activity as well as subsequent shocks between 1960 and 2020 (Figure 23 and Figure 24). Figure 24 shows abundant forerunners to the Maule shock of 2010 in the Araco LCZ between 37˚ and 38.4˚S.

The 1960 event triggered many normal-faulting earthquakes (Figure 23 and Figure 24) just to the west of the plate boundary. Very little thrust activity occurred between 1999 and 2020. The earthquake of Mw 7.6 in 2016 broke outside the 1960 20 m slip contours but within the 5 m contour (Figure 23). While large, it was much smaller than the giant 1960 shock.

A considerably smaller moment release from displacements along the coast in the 1960 mainshock was calculated by Barrientos and Ward [65] than by Cifuentes and Silver [66] using teleseismic long-period data. The former speculated that greater slip occurred closer to the trench and accounted for the difference. The large number of normal-faulting events west of the plate boundary in Figure 23 and Figure 24 is in accord with that hypothesis. The 1960 event generated a huge tsunami.

In summary, no forerunning thrust mechanisms of Ms ≥ 6.0 occurred from 1920 until the May 1960 mainshock within the three zones where computed slip exceeded 20 m. Those three zones are identified as huge asperities. Forerunning activity was concentrated just to the north within the Aruco low-coupling region where several large shocks commenced the day before the giant earthquake. Forerunning shocks to the Maule earthquake of 2010 to the north also were concentrated in the Aruco LCZ. Precursory activity to future great events should be sought within the Aruco region and perhaps at the southern end of the 1960 rupture zones.

The giant Sumatra-Nicobar-Andaman earthquake of 2004 ruptured from its southern end off the coast of Sumatra near 3˚N to its northern end near 14˚N, one of the longest ruptures known (Figure 25 and Figure 26). Computed slip was greater than 10 m along several segments of the subduction zone, a common feature for giant earthquakes. I use the slip contours of [67] since they incorporated local and distant seismic and geodetic information in their computed displacements.

Several forerunning earthquakes with thrust mechanisms occurred near the southern end of the rupture zone in Figure 25. Many strike-slip mechanisms occurred within the 10 m slip zones off Sumatra and the Nicobar Islands. A few thrust events occurred farther north (Figure 26) near the Andaman Islands. The 10 m slip zones off the Nicobar and Andaman Islands were the sites of only a single forerunning thrust earthquake. Several thrust mechanisms, however, can be seen in Figure 25 and Figure 26 with unusual dips to the southeast to west.

In summary, several large forerunning thrust shocks of 6.0 < Mw < 7.2 occurred near the southeastern end of the coming 2004 rupture zone. Otherwise, forerunning thrust shocks were rare. Reverse and strike-slip mechanisms and thrust solutions with unusual dips took place in and near the 10 m slip zone off Sumatra prior to the giant earthquake. The 10 m slip zones off the Nicobar and Andaman Islands were very quiet prior to the great shock. The entire 2004 rupture zone had been a seismic gap for many decades before 2004. The 2004 mainshock likely broke at least two great asperities.

The giant Nias-Simeulue shock of 2005 occurred three months after the 2004 Sumatra-Andaman event. A number of forerunning thrust earthquakes of Mw > 6.9 occurred between them (Figure 27). The computed 6 m slip contour for the Nias-Simeulue shock, the site of its centroid of the mainshock, was nearly devoid of forerunning activity. Considerable forerunning thrust mechanisms can be seen in Figure 27 to the southeast of the 2.4 m slip contour. Figure 28 shows an abrupt termination of aftershocks of the 2004 giant shock about 150 km to the northwest of the 2005 centroid.

In summary, the region that slipped six or more meters in 2005 was nearly devoid of forerunning thrust activity. It can be regarded as a great asperity. Many thrust shocks occurred to the northwest and southeast of both the 2.4 slip zones.

The northern Sumatran earthquake of 2010 ruptured a 100-km length of the plate boundary between the giant shocks of 2004 and 2005 [69]. Using GPS data [69], it broke between the two northwestern regions of 2.4 m slip in the 2005 earthquake (Figure 27), i.e., a zone that remained unbroken in 2004 and 2005. The mainshock consisted of two large events about 12 s apart. Most forerunning thrust activity occurred seaward of the computed 1.75 m rupture contours. Its centroid and many aftershocks were located similarly (Figure 29). None took place within the 2.5 m contour.

In summary, all but one of the forerunning shocks to the north Sumatra earthquake of 2006 took place outside of its two zones of major displacement. Its slip contours define two major asperities that broke in 2010.

The earthquakes of Mw 8.5 and 7.9 ruptured offshore of central Sumatra 12 hours apart in 2007 (Figure 30). They were preceded by a strike-slip shock of Mw 7.8 in 2000 (large black X, bottom right of Figure 30). Rupture started near the southeastern end and propagated to the northwest in the first and larger event. Feng et al. [71] deduce that the second shock of Mw 7.9 broke two zones: 1) within the northern end of the 5 m contour surrounding the Mw 8.5 event and 2) another near 2˚S. The centroid of the Mw 7.8 earthquake being between the two is consistent with their interpretation of two separate patches of 5 m displacement.

The Mw 8.5 shock was nearly devoid of forerunning activity within either the 5 m slip contours and none within that of 15 m. Abundant thrust forerunning activity occurred farther to the southeast along strike of the plate boundary. Two forerunning thrust and several high-angle reverse earthquakes occurred within the 5 m slip zone near 2˚S. The source time function consisted of two pulses [73]. The 2007 rupture zones broke previously in a giant mainshock in 1833. It had been a major seismic gap for many decades prior to 2007.

In summary, forerunning thrust activity did not occur within the 15 m slip contour, consistent with its being a giant asperity associated with the Mw 8.5 earthquake.

The 2010 shock ruptured seaward of the two events of 2007 along the shallowest part of the plate boundary seaward of the Mentawai Islands. It generated a large and destructive tsunami, like that off Java in 2006 [74]. Unlike the Java shock (Figure 13), several thrust foreshocks occurred on the periphery of the higher slip zone of Figure 31.

In summary, the 2010 earthquake, which occurred close to the trench, ruptured a quiet 5-m slip zone that was surrounded by forerunning thrust shocks. That zone is identified as a major asperity. The 2010 event generated a large tsunami.

None of the few thrust foreshocks occurred within the solid line that indicates major computed slip in 1996 (Figure 32). Das and Henry [75], however, found that their well-located pre-events took place very close to their zone of maximum slip, which they did not find for the other great shocks they examined. The tsunami reached heights of 7 m in many areas (Bulletin International Seismological Centre). The mainshock consisted of a relatively simple pulse [73].

In summary, a major asperity may be present near the centroid. The number of forerunning events was not sufficient for them to be used to map the coming rupture zone.

Many thrust earthquakes of Mw > 6.5 occurred along a narrow band of the subduction zone in the Solomon Islands from 1977 through 2018. Events of Mw > 7.0 from 1915 to 1977, while likely not as well located, also were situated near that same zone.

Figure 33 is centered on the largest known shock in the Solomon Islands, that of 2007 of Mw 8.1. Little forerunning activity of Mw ≥ 5.5 occurred from 1977 until the main event within either the western or eastern zones of large slip or between them. The moment tensor solution for the mainshock was situated just inside the western, i.e., the larger, zone of forerunning activity. Abundant forerunners occurred just to its northwest as well as trenchward.

A very young plate boundary formed to the south of the Solomon Islands along the north-south zone of rough topography [76] near 156.5˚E (Figure 33). It now divides the Australian plate to the east from the Woodlark plate to its west. High slip in 2007 was concentrated on either side of where that zone intersects the subduction plate boundary. That triple junction, however, experienced significant uplift during the 2007 mainshock [76]. The region between the western and eastern zones of maximum slip in 2007 has not experienced a larger known earthquake than Mw 6.4 from 1977 to 2018 or Mw > 7.0 since 1915. That very young subduction region near the triple junction may not be capable of rupturing in a very large earthquake. Instead, it may move mainly aseismically.

The rupture propagated mostly unilaterally to the northwest from the short-period epicenter, which was located just to the southwest of the eastern region of high slip in Figure 33 [76]. The greatest slip occurred in the western zone.

In summary, little forerunning activity occurred within either the western and eastern zones of high slip or between them. The two major asperities that broke in the mainshock could have been defined ahead of time using the forerunning shocks. The zone between them, the site of a young triple junction, may move aseismically.

The shock of 2016 off the west coast of the island of Guadalcanal at a centroid depth of about 46 km had a reverse-faulting mechanism. Thrust and reverse forerunning earthquakes in Figure 34 surround a quiet zone near the centroid of the mainshock. Rupture started at the southeastern end of that quiet zone near 10.79˚S, 161.28˚E. To my knowledge no one has published a detailed slip distribution.

In summary, the forerunning activity surrounding the centroid of the mainshock appears to define a great asperity extending about 50 to 80 km along strike. That asperity itself was quiet for decades ahead of the 2016 mainshock. A detailed computation of slip is needed for it.

The great Santa Cruz earthquake of 2014 ruptured two separate patches of high slip shown by the solid and dashed black lines in Figure 35 and Figure 36. Most foreshocks (Figure 35) in the year before the mainshock occurred between the two areas of higher computed slip not inside them. It is understandable that the centroid location of the entire mainshock is located between the two high-slip patches.

Figure 36 shows forerunning activity from 2003 to 2013 omitting the foreshocks in the previous figure. The regions of highest computed slip were devoid of thrust forerunning activity. Several thrust forerunning events occurred in the northwestern 2 to 7 m slip area. Activity in the southeast corner of Figure 36 may represent continuing aftershocks of the Mw 7.8 shock of 2009.

In summary, the two zones of highest slip in the 2014 mainshock were devoid of forerunning activity. They are taken as defining two great asperities that ruptured in 2014. Foreshocks occurred between the two high slip zones.

The 2009 event took place along a subduction zone that extends from the southern South Island of New Zealand southwestward to Macquarie Island (Figure 37) and dips to the southeast. Much forerunning thrust activity in the decades before the 2009 mainshock took place to its northeast and southwest beyond the computed slip contours [78]. Aftershocks took place closer to those contours and to the centroid of the mainshock.

In summary, a quiet zone of forerunning activity was centered near the high-slip area and the centroid of the mainshock. The forerunning activity in the figure occurred beyond them. The rupture zone itself likely was the site of a great asperity.

Forerunning activity of Mw ≥ 6.0 from 1976 until the great Kuril Islands earthquake of 1994 surrounds a very quiet inner zone in Figure 38 (i.e., it defines a very elongated shape or “Mogi donut” [23]. The location of the centroid of the mainshock was situated within that elongate zone of forerunners. The short-period locations (not shown) for the same events also outline a similar donut shape but about 25 km to the northwest, probably an artifact of not considering properly the velocity structure of the downgoing plate.

Modelling of seismological and geodetic data for the 1994 mainshock indicates it consisted of two main sub-events [79]. Since those geodetic observations were all to the southwest in Hokkaido except for a single observation on the nearby Kuril Island of Shikotan, they were not certain a shallow-dipping plane parallel to the strike of the subduction zone could be excluded. Nevertheless, they favored a steeply dipping fault (solid black line in Figure 38) extending from a depth of 40 to 60 km within the downgoing lithospheric plate. They called it a lithospheric earthquake. Their model of the rupture zone lies in the southwestern half of the quiet part of the donut.

The centroid depths of the two reverse faulting aftershocks in Figure 39 were 54 and 60 km. The depths of thrust aftershocks within 100 km of the 1994 mainshock were 33 to 60 km, in general agreement with the depth estimates of [79].

Thus, rupture in the 1994 mainshock appears to have been either within the downgoing Pacific plate, a complex event with varying dips and depths or both.

Most aftershocks of Mw ≥ 5.5 from the main event of 1994 until the 1995 adjacent shock occurred just to the northwest of the seismic centroid (Figure 39) and outside the quiet zone (i.e., the hole of the donut) in Figure 38. Few aftershocks occurred along the southeast side of the donut. The distribution of aftershocks favors rupture along a fault parallel to the plate boundary, albeit one of steep dip.

The 1994 earthquake did not occur on the plate boundary but instead on a northwesterly striking fault according to [80] [81]. I think they placed too much emphasis on a possible connection between the 1994 shock and an event at a depth of about 100 km farther to the northwest in 1978 of Mw 7.8. I conclude they each picked the wrong nodal plane as the causative fault.

In summary, forerunning thrust events outline an elongate donut that was free of such shocks in its interior. The centroid of the mainshock was inside, indicating it likely defines a great asperity that broke in 1994. More work is needed to resolve whether the mainshock, in fact, ruptured within the downgoing Pacific plate, including double-difference computations for the centroid data of the events in Figure 38. A rupture within the lithosphere, as advocated by [79], is like that in the great Tongan earthquake of 2006 as described in the Supplement. Great earthquakes within the downgoing lithosphere, however, are uncommon compared to great plate boundary events. The 1994 event probably did not re-ruptured part of the plate boundary that broke in the Mw 8.2 event of 1969 just 25 years earlier and hence did not violate the seismic gap hypothesis. That part of the plate boundary itself likely remains a seismic gap.

Forerunning activity to the great Kuril earthquake of 1995 surrounds a quiet zone that includes the centroid of the mainshock (Figure 40). The 1995 mainshock took place to the northeast of the 1994 event (Figure 38). Rupture in 1995 consisted of a simple pulse according to [73] but the Bulletin of the International Seismological Centre states it broke in two main pulses 22 s apart. Schwartz [82] computed the distribution of slip for the mainshock along strike but not with depth. Schwartz found that computed high slip in 1995 overlapped that of the giant shock of 1963 but with rather large uncertainties.

In summary, forerunning thrust activity defines a quiet zone that is interpreted to be a great asperity that broke in 1995. The amount of overlapping slip in the 1963 giant and 1995 great earthquakes is uncertain.

Abundant forerunning activity with thrust mechanisms occurred on all sides of the centroid of the Aleutian mainshock of 2003 (Figure 41). The mainshock was one of the smallest events studied. Foreshocks were prominent (Figure 42) from January 2003 until the main event in November. Aftershocks occurred solely to

the east of the mainshock. Its short-period epicenter as determined by the Bulletin of the International Seismological Centre was located about 50 km east of the centroid, indicating the mainshock ruptured from east to west. Amchitka Island, the site of three U.S. underground nuclear tests is located near 51.5˚N, 179˚E.

Rupture in 2003 consisted of a large simple pulse followed by later smaller slip pulses [73]. Most forerunning thrust earthquakes occurred outside the areas of high computed slip but three forerunning event took place within the larger slip contour.

In summary, much forerunning thrust activity to the 2003 mainshock occurred closer to the Aleutian Islands than the centroid and zones of computed high slip. Three thrust events occurred within the main high-slip contour. All but one foreshock took place outside.

Almost all forerunning thrust events (Figure 43) of the Aleutian earthquake of 1996 occurred on the island side of its centroid. Abundant forerunning activity took place to the west of the centroid and of Adak Canyon (near 177.3˚W).

Tanioka and Gonzalez [84] modelled tsunami data for the 1996 and 1986 earthquakes. They concluded that the downdip width of the mainshock of 1996 was 30 km or less. The 1986 and 1996 rupture areas were well coupled seismically but the region to the east of 174˚W was not [85]. Displacements were modelled for the 1996 mainshock [83]. The zone containing 50% of moment release

is devoid of forerunning activity in Figure 43. All but one of the forerunning events occurred trenchward of that contour. This is consistent with the finding that the 1996 shock generated a tsunami that propagated across the Pacific [85].

Modelling the distribution of slip along strike for the 1986 shock, Boyd and Nábêlek [86] found it consisted of two zones of high moment release to the east of the 1986 centroid (Figure 43) and a zone of smaller release about 23 km to its west. Thus, high slip in the two mainshocks does not appear to overlap. The average slip from the seismic moment over the entire rupture 1986 zone was close to that calculated assuming full plate coupling since the 1957 giant earthquake [86]. Their estimates for the two zones of highest slip in 1986, however, were greater than those calculated for the potential slip built up since 1957 assuming full coupling.

The occurrence of the 1986 and 1996 earthquakes so soon after 1957 led several earth scientists to claim the seismic gap hypothesis was false. Since most great earthquakes have occurred in long-standing seismic gaps where sufficient potential displacement could have built up by plate motion, I sought other explanations for the occurrence of the 1986 and 1996 Aleutian events only 29 and 39 years after the giant 1957 mainshock. The centroids of the 1986 and 1996 shocks were located farther south and more trenchward than nearly all of the 1996 aftershocks and possibly those of 1986 (Figure 44). Thus, slip in 1996 and possibly 1986 may have ruptured more trenchward than the 1957 earthquake. Boyd and Nábêlek [86] did not estimate how the release of seismic moments varied in the dip direction; nor did Johnson et al. [87] for the 1957 earthquake.

Moment release was small in 1957 to the east of the 1986 rupture zone [87]. Thus, we do not know how much areas of significant moment release in 1957 overlapped or not with those in 1986 and 1996. The short-period epicenter of the 1957 shock near 51.63˚N, 175.41˚W was close to that for 1986 and about 45 km north of the 1986 centroid.

Another explanation is that the 1957 shock did not release the stress that had been built up earlier. By that hypothesis, the 1986 and 1996 earthquakes released stresses that had been stored up for longer than 29 and 39 years.

In summary, forerunning and aftershock activity to the 1996 earthquake occurred mainly north of its centroid and the zone of highest slip, indicating the asperity that broke in it may have been located farther south. Detailed information on slip in the 1957 mainshock is needed to ascertain if the occurrence of the 1986 and 1996 events violated the seismic gap hypothesis by rupturing so soon.

The great Kamchatka mainshock of 1997 was a thrust event along this well-studied subduction zone. The event was unusual in that the highest slip region was situated beneath land of the Kronotsky peninsula. Neither the forerunning thrust nor aftershock activity in Figure 45 and Figure 46 occurred within the two computed regions of high slip. The forerunners surround the quiet zone on three sides.

The plate boundary to the north of 53˚N is only moderately well coupled according to [88] [89] compared to that to the south, which includes a great shock in 1923 and the giant earthquake of 1952 [90]. The 1997 mainshock took place in a long-standing seismic gap between the 1923 event and that of 1971 to the

northeast. The two papers find that small foreshocks and postseismic slip in the two months after the 1997 mainshock occurred outside of the main asperity that ruptured co-seismically. They report that aseismic afterslip during the next two months released as much seismic moment as the mainshock. The rupture in 1997 consisted of a simple pulse [73].

In summary, forerunning activity did not occur within the computed high slip region but was concentrated along its periphery on three sides. It defines a great asperity that likely broke in the mainshock. The mainshock took place in a well-defined seismic gap.