Iceland displays a complex tectonic spanning Upper Tertiary to present. Above a hotspot [67] [68], the American and Eurasian Plates separate at a rate of 2 cm/yr in the direction of N105˚E [69] along several active plate boundaries, which are loci of intense earthquakes, magmatism, and geothermal activity (Figure 1(a)). The active rifts are the Kolbeinsey Ridge, the Northern and the Eastern Rift Zones (NRZ and ERZ), and the Western Rift Zone (WRZ). The transform segments are the Tjörnes Fracture Zone (TFZ) and the South Iceland Seismic Zones (SISZ), and the microplates are in Grímsey-Tjörnes-Dalvík and Hreppar [70]. Oblique rifting occurs in Reykjanes Peninsula (RP) [39] [71], Reykjanes Ridge [38] [72] [73], and around Grímsey [74]. The oceanic crust consists of basaltic and acidic series, with intercalated hyaloclastites and sediments. The oldest series date from 16 - 15 Ma, emitted from the now extinct Snæfellsnes Rift Zone [75] [76] [77], and possibly 24 Ma, emitted from the Northwest Rift Zone [78]. At least one Tertiary transform segment is considered in central Iceland [76] - [81]. In West Iceland, the Tertiary structures are cut by the Quaternary intraplate Snæfellsnes Volcanic Zone (SVZ) [79]. Icelandic rift segments and the microplates that formed within rifts are characterised by normal faults, eruptive fissures, and central volcanoes arranged in swarms that strike N-S in NRZ, but NNE/NE in WRZ, ERZ, West Iceland and Hreppar Microplate. The transform segments primarily identified by earthquakes [82] [83] [84] are with dextral motion along the TFZ, and sinistral motion along the SISZ, both having internal Riedel shears, e.g. [29] [85]. As a result of a continuous tectonic since Upper Tertiary, the crust is structured with flexures, erosional unconformities, and an intense fracture network [70] [86] [87] [88] that reflect severe stress field fluctuations over time [34] [89] [90] [91].

The 7 analogues are in the intraplate outcrops of West Iceland, but mostly within the active SISZ, at its intersection with the WRZ, and within the RP oblique rift (Figure 1(b)).

• In South Iceland, the 3.4 - 0.7 Ma bedrock consists of basalts, hyaloclastites, local acidic rocks and intercalated sediments in Hreppar [92]. Based on mineral assemblages [93], the bedrock in Hreppar is eroded down to 0.7 km. The interglacial lavas (<0.7 Ma) cover mostly the series of Hreppar while the late glacial formation and the 8600 yrs Þjórsá lava [94] are more widespread in the SISZ. Reykjanes is covered by the post glacial lavas 14,500 yrs - 13^th Century, with isolated hyaloclastite ridges < 0.115 Ma [95], which are found down to 400 m in wells above older bedrock, e.g. [96]. The transform zone of SISZ is considered to have an overall E-W strike, with a width of ~25 km, a length of 80 km or more [97], a sinistral motion and a constant slip-rate of 19 ± 1 mm/yr [29] [98]. A recent study, however, demonstrates that the SISZ likely strikes ENE and could be bounded by ENE oblique-slip sinistral faults [88]. The literature agrees that from WRZ to the RP, the transform zone strikes ENE, is up to 20 km wide, and is highly oblique [71] [99]. Oblique

rifting and magmatic phases seem alternating on the RP, and the plate boundary undergoes presently transtension, with both sinistral motion and extension [100]. Earthquakes are frequent in the SISZ and on the RP, reaching M_L 7. The last major events in the SISZ are the 2000 and 2008 earthquakes (SIL network), and those on the RP are the earthquake swarms of 1972 [101], continued to 1973 [102], as well as those in 2013-2015 [103] [104]. The surface ruptures of earthquakes in both the SISZ and on the RP are primarily the conjugate N-S dextral and ENE sinistral strike and oblique slip faults [29], and secondarily the WNW, NW and E-W Riedel shears [88] [105] [106]. The surface ruptures display left- and right-stepping en échelon arrangement, indicative of dextral and sinistral motions. It is noteworthy that in Reykjanes, the WNW and NW structures are prominent both at the surface [107] and in resistivity anomalies at depth [108].

• The crust in West Iceland formed at the SRZ between 16 - 15 and 5/7 Ma, then in the WRZ from 6 to ~3 Ma [75] [77]. The series consist of basalt and intercalated sediments, which, based on zeolite assemblages [93] are eroded down to 1.5 km depth. Erosional unconformities separate the stratigraphic units of the two Tertiary rifts (Figure 2(a)). Rift-parallel structures strike NNE to NE, but the same Riedel shear pattern as in the SISZ and RP is also present. Dyke injection into Tertiary N-S dextral oblique fault segments similar to those in the SISZ indicates that a sinistral Tertiary transform zone may have connected the two Tertiary rift segments [81] (Figure 1(a)). The Quaternary intraplate volcanism (SVZ) consists of WNW segments with dextral motion [76] [86], and the 1974 intraplate earthquakes at its eastern tip show primarily extension along all fracture sets [109]. The SVZ appears to fall within the Tertiary transform zone, but superimposed on all sets of underlying Tertiary faults and dykes [81].

• The highest geothermal gradients are considered to be in rift segments [110] (Figure 1(b)) with reservoirs reaching up to 380˚C at 2 km depth [111]. In most cases, however, such reservoirs are located at the intersection of rift and transform zones. Low and medium temperature gradients are generally on rift shoulders and within the SISZ where temperatures reach 175˚C at 2 km depth [111]. Structural permeability that facilitates the leakages of cold or hot water is traditionally attributed to rift-parallel extensional fractures in Iceland. However, structural analyses in many areas of Iceland show that rift-parallel fractures represent up to 1/3 of any fracture population [70] [88] [107].

Area 1 is covered by the 6 to ~3 Ma basaltic lavas on the hills and by the 10,000 yrs old silt and clay in the valleys. Geothermal manifestations were mapped in three clusters in the younger sediments (Figure 2(a)), located at some 10 km to the southwest of the 1974 earthquakes (Figure 2(b)). From north to the southwest, the geothermal clusters and the number of mapped features within them are Hurðarbak (60); Deildartunga (56), Kársnes (36), Kleppjárnsreykir (30); and Klettur, Runnar (207), summing up to a total of 389 geothermal features.

The older fractures in the bedrock present several sets, but two peaks at NNE to NE (parallel to rift) and ENE to WNW stand out. All sets have dip- and strike-slips, but the shear motions are complex as the same set can display opposite sense of motions indicating a polyphase tectonic (Figure 2(c)). The aftershocks of 1974 earthquakes spread in an area of ~25 km long and 9 km wide, with seismic lineations oriented ENE, NNE, and then N-S and WNW (Figures 2(a)-(c)). The 389 mapped geothermal features align dominantly N-S and ENE,

secondarily NNW, and lastly WNW, but not NNE parallel to the rift. These regional alignments fit relatively well the seismic lineations rather than the dominant strikes of the underlying older bedrock fractures (Figure 2(d)).

The geothermal manifestations of Klettur-Runnar are selected for the analysis here as they are the longest cluster (Figure 2(e)). The features mapped in detail are few cold springs (1˚C - 9.9˚C), a majority of warm (10˚C - 49.9˚C), a few hot springs (50˚C - 83.1˚C), and one borehole (64.3˚C). Warm soil with occasional steam, and a few metre-scale zones free of snow and ice during winter were also mapped indicating possible nearby geothermal source. The host fractures to the leakages are covered by superficial formation, and are deduced from the alignments of the leakages, falling in the continuation of the fracture sets of the same strikes in the immediate surrounding bedrock. These permeable fractures are not surface ruptures of 1974 earthquakes, but reactivated fractures of the bedrock reflecting the present-day tectonic activity that keeps the fractures open for fluid flow. As such, the most dominant permeable set appears to be NNW, which is also the orientation of the bigger springs (Figure 2(f)), then NE, N-S, and WNW. They are, however, not aligned ENE similar to the main seismic lineations of 1974 earthquakes. Despite the leakages align on regional fractures, the flow to the surface is equally facilitated by fracture intersections (Figure 2(d) and Figure 2(e) and Figure 2(g)).

Area 2 is covered by Þjórsá lava and fini-glacial loose overburden, but the lava and tillite belonging to Hreppar formation (3.1 - 0.7 Ma) crop locally at Heiðartangi where the leakages of cold water were mapped (Figures 3(a)-(g)). Although Hreppar formation was formed at the rift and should be dominantly with NNE structures, the most frequent older bedrock fractures here are the ENE sinistral and N-S dextral oblique-slip faults, and several ENE and one N-S dyke segments (Figure 3(a) and Figure 3(b)). Area 2 is the site of the 1896 earthquakes, but also adjacent to the site of the 21 June 2000 earthquake that occurred to the west of Urriðafoss. The main surface ruptures of these earthquakes are the N-S dextral and ENE sinistral conjugate sets cutting the Hreppar formation and the Þjórsá lava [29] (Figure 3(a) and Figure 3(b)). Detailed mapping of the historic surface ruptures by this study, however, shows that regionally, along with the N-S and ENE structures, other sinistral Riedel shears striking WNW and ENE also ruptured the surface (Figure 3(b)). Albeit minor, all sets also display dip-slip. The mapped leakages are cold springs, filled and dried-up ponds, and rare warm springs up to 24˚C. They align clearly on the N-S dextral, then equally on the ENE and NW sinistral fractures, but less on the E-W sinistral and the NNE extensional fractures. At the tip of Heiðartangi near the shore of Þjórsá, the leakages appear through the tholeiite lava, which is cut heavily by N-S, ENE, NW and WNW joints (Figure 3(c)). Such joints are widespread between Heiðartangi and Urriðafoss (Figure 3(d) and Figure 3(e)), sometimes forming

narrow brecciated fault zones critical for fracture permeability. Clear examples of fracture permeability along NW and ENE sinistral, and N-S dextral oblique-slip faults are the cold leakages in Heiðartangi (Figure 3(c) and Figure 3(f)).

Although Heiðartangi is caught between two major N-S dextral and two ENE sinistral oblique-slip regional fractures, the leakages align on regional N-S and ENE structures but more so on local NW fractures (Figures 3(f)-(h)). Clearly, the permeable fractures are the bedrock structures reactivated during earthquakes, with leakages mostly along the main fracture segments rather than at the fracture intersections.

Area 3 in Akbraut is at some 17 km farther to the northeast of Heiðartangi and displays leakages of cold water (Figures 4(a)-(g)). The outcrop is within the SISZ (Figure 1(b)), very near the northern boundary of the transform zone and adjacent to the southern limit of the extinct Stóra-Laxá central volcano within the Hreppar Microplate. The central volcano is covered by the interglacial lava < 0.7 Ma, while Þjórsá lava and superficial formations cover the lowland around Akbraut. The outcrop analogue is on a small hill made of the same interglacial lava as in the Stóra-Laxá central volcano (Figure 4(a) and Figure 4(c)). Although the NNE rift-parallel fractures are widespread in the interglacial lava of the central volcano, overall, the older bedrock regional fractures in Hreppar and SISZ are dominantly the N-S dextral and ENE sinistral, then the WNW/NW sinistral oblique-slip faults (Figure 4(b)). The surface ruptures of the historic and the 2000 earthquakes are located to the north, east, and south of Akbraut, and they are dominantly N-S dextral.

The leakages of cold water in Akbraut are springs, filled or dried-up elongated ponds that form clusters, as well as three abandoned shallow wells (Figure 4(c)). The leakages form distinct northerly, NNE, ENE and WNW/NW clusters, with left- or right-stepping arrangements similar to surface fractures (Figure 4(d) and Figure 4(e)). They are indeed directly located on the traces of the underlying N-S dextral, and the NNE rift-parallel structures, then on the traces of ENE sinistral, and finally the NW and WNW oblique-slip sinistral shears fractures (Figure 4(f) and Figure 4(g)). Given these frequencies, permeability is dominantly controlled by the Riedel shears of the transform zone. Although the total length of each individual permeable fracture is unknown, the leakages seem to be in the middle sections of the fractures rather than by their tips. This does not exclude that fracture intersection also plays a role in increased permeability.

Area 4 in Laugar (Figure 5(a)) is to the northeast of the island of Árnes, and at some 5 km to the east of Akbraut. The outcrop is within the SISZ and includes both low temperature geothermal activity and a few small leakages of cold water.

The Þjórsá lava and the superficial formations cover the lowland, but the hills immediately surrounding Laugar are made of the tholeiite lavas of Hreppar formation, and by the interglacial lavas (Figure 5(a)). Laugar is adjacent to the site of the 1630 earthquake. Additionally, the N-S and ENE oblique-slip surface ruptures of the 1896 earthquakes cut directly the bedrock, the Þjórsá lava and the superficial formation in this site. Although the NNE rift-parallel structures should be dominant in the bedrock that was formed in the rift, the older regional fractures here are dominantly the northerly dextral and the ENE sinistral oblique-slip Riedel shears, and secondarily the NNE and WNW structures (Figure 5(b)).

The mapped features in this site are the surface ruptures, the altered geothermal zones, a few warm and cold springs, and 9 wells (Figure 5(c)). The surface ruptures are the right-stepping ENE en échelon sinistral segments, a few WNW and NW undetermined fractures mainly to the northeast, then a few left-stepping N-S dextral arrays. Geothermal manifestations are warm swampy soil (Figure 5(d)) and a steamy elongated pond (Figure 5(e)) with 16.4˚C to 22.3˚C temperature that occupies a narrow ENE zone in the western part of the area, in continuation of the ENE surface ruptures. A few warm springs outside of the main ENE altered zone are 26.4˚C to 47˚C, with evidence of current alteration mineral deposition (Figure 5(f)). The hottest features related to the ENE altered zones are two warm springs (23.8˚C and 39.8˚C) and two short linear features aligned E-W and ENE (32.5˚C and 28.1˚C). The cold springs farther to the northeast are with low discharge, and they are near most of the drilled wells that reach up to 42˚C.

The structural data demonstrate that geothermal activity is mainly along the ENE sinistral surface ruptures and secondarily along the N-S dextral source fault, with the hottest producing well (63˚C) at the intersection of these two sets. The cold springs, on the other hand, are on the traces of NW fractures. This analogue reflects the strong control of fracture permeability by the Riedel shears of transform zone where the leakages are along the fracture traces rather than by their tips (Figure 5(g) and Figure 5(h)).

Located in the SISZ, Area 5 in the island of Árnes is midway between Akbraut and Laugar, and displays prominent leakages of cold and warm water (Figures 6(a)-(j)). Its geological context is similar to Laugar, in that it is at the sites of two earthquakes, i.e., the 1896 and the 17 June 2000, and at a short distance from the Stóra-Laxá central volcano. The interglacial lava of this volcano crops out in the site of the analogue, which is surrounded by Þjórsá lava and the late glacial sediments (Figure 6(a)). The older bedrock structures are identical to Laugar, with dominantly N-S dextral, ENE sinistral, and NNE rift-parallel structures, and to a lesser degree the WNW fractures. Surface ruptures of the same strike cut the younger formations of Árnes where, however, the NNE fractures are the least frequent (Figure 6(b)).

Area 5 is at the bend of the Árneskvísl river and the mapped features consist of surface ruptures, low temperature geothermal alteration, leakages of cold water, two ENE well-developed fractures with up to 4 m dip-slip, and one borehole (Figure 6(c)). The surface ruptures of the 2000 earthquakes are in two locations [29]. To the northwest, they strike N-S, have a left-stepping arrangement typical of dextral motion, and align with the river, while to the southeast they strike NW (Figure 6(c)). In this study, we mapped additional surface ruptures in the surroundings of the N-S segments, and they strike WNW, NW and E-W. Geothermal manifestations in the central part of the site are a clay zone without steam and water, that strikes NW and bends to E-W (Figure 6(c)). In the NW striking section of the alteration zone, N-S, WNW and NW secondary fractures host the alteration mineral deposition (Figures 6(d)-(g)), while in the E-W section numerous “clay pots” are similar to sinkholes of surface ruptures, and they align E-W and N-S.

The cold leakages are in two areas. One is a cluster of N-S springs in the continuation of the N-S surface ruptures, as well as WNW perpendicular to them. The other is a narrow E-W graben in the middle of the site, bounded by small E-W parallel normal faults with up to 0.5 m dip-slip (Figure 6(c) and Figure 6(h)). Within the E-W graben, there are several sinkholes filled with water and arranged in a left-stepping array typical of sinistral motion (Figure 6(h)).

Data from this site show that regardless of temperature, the permeable fracture sets are the Riedel shears of the transform zone, striking N-S dextral, E-W to WNW sinistral, and NW (Figure 6(i) and Figure 6(j)). The NW segment, however, shows evidence of dextral motion, which is opposite to the typical Riedel shears of the same strike. As this segment ruptured with the main dextral N-S fault during the 2000 earthquakes, it is likely a splay of the main N-S source fault. The only borehole, with ~50˚C, falls on this NW segment. The leakages in Area 5 occur mostly along the fracture traces although fracture intersection has contributed to local opening for the flow of cold and hot water.

Area 6 in Reykjafjall (Figure 7 and Figure 8) is located at the intersection of the SISZ and the Hengill fissure swarm of the WRZ (Figure 1(b)). It is one of the sites of the 2008 earthquake doublet, and displays high temperature geothermal activity. The surface ruptures and changes in geothermal activity during this earthquake were described by [105], and some of the initial data are used here to discuss fracture permeability in the context of rift and transform zones.

Regionally, the interglacial to fini-glacial lavas, the hyaloclastites (0.78 Ma - 11,000 yrs), and the >0.78 Ma lavas cover Reykjafjall, while postglacial lavas (<11,000 yrs) are in the lowland (Figure 7(a)). Bedrock faults and dykes strike NNE parallel to the rift. But they also strike N-S due to continuous seismic activity related to transform faulting, with the latest evidence during the 2008 earthquake doublet that had a mainshock on N-S structures and aftershocks on N-S and NW fractures [113] (Figure 7(b) and Figure 7(c)).

The mapped features in Area 6 are the surface ruptures of the 2008 earthquake located on top of Reykjafjall, a few boreholes and geothermal manifestations on the foot of Reykjafjall. The geothermal resource has been long exploited there but increased activity in this earthquake (Figure 7(d) and Figure 7(e)). The surface ruptures are in a narrow area in Reykjafjall (Figure 7(d)), and appear as linear breaks and sinkholes, arranged in left-stepping N-S and right-stepping ENE arrays typical of dextral and sinistral motions. These young structures also display dip-slips (<1 m) and opening Figures 7(g)-(i)), indicating that they are oblique-slip fractures, but push-ups were rarely observed. A cluster of segments that ruptured during the earthquake blends with some of the N-S surface ruptures without displaying a coherent en échelon arrangement, and could represent the tearing apart of the edge of Reykjafjall. To the south, several NW-striking surface ruptures are organised in right-stepping arrays typical of sinistral motion (Figure 7(d)). It should be noted that the bedrock is also cut by older N-S and ENE faults that have up to 10 m dip-slips (Figure 7(e)), as well as by NW (Figure 7(g)), and WNW faults (Figures 8(i)-(k)), some of which have associated intense fault gouge/breccia (Figure 7(f)). The 2008 surface ruptures are often above the older fractures and reflect reactivations of the underlying fracture sets.

In temperature, the geothermal manifestations range from hot (75˚C - 99.5˚C) to warm (13˚C - 31˚C). The metre-scale manifestations consist of fumaroles that broke the surface during the earthquake (Figures 8(b)-(e)), reactivated bursting fumaroles and mud pots (Figure 8(f)), boiling hot spring (Figure 8(g)), and soil altered by steam and fumarole (Figure 8(h) and Figure 8(i)). These manifestations form distinct clusters, which are wider to the south than to the north. The clusters align dominantly northerly, ENE and NW, with en échelon arrangements similar to the dextral and sinistral young surface ruptures. Additionally, a few E-W and WNW older segments filled with alteration in wide fault gouges (Figure 8(j) and Figure 8(k)) are also present.

Although Area 6 is partially at the intersection of rift and transform zone, the data demonstrates that permeability is controlled by the N-S, ENE, NW, WNW and NW Riedel shears of the transform zone (Figure 8(l) and Figure 8(m)) that are reactivated fractures from the basement. Permeability is primarily along the traces of the younger Riedel shears, although fracture intersection enhances the flow.

The last outcrop analogue is Area 7 (Figures 9(a)-(j)) in the Reykjanes oblique rift, and displays high temperature geothermal activity. The area is covered by the basaltic and picrite lavas (14,400 yrs to 13^th Century), and by small hyaloclastite ridges (<0.115 Ma) (Figure 9(a)), with minimal erosion. Seismic activity related to transform faulting, without magmatism, is continuous in the area. The last two major earthquake swarms date from 1972 and 2013-2015 and both swarms show seismic lineations aligned ENE, then WNW, N-S, and E-W (Figure 9(a)). However, since late 2020, a new swarm of earthquakes with transform character became active in Fagradalsfjall to the east of the study area. The earthquake swarm culminated in an eruption that started on the 19 March 2021 and lasted for several months.

The features mapped in Area 7 are the bedrock fractures, the surface ruptures of earthquakes, and geothermal activity with distinct structural organisations (Figure 9(b) and Figure 9(c)). The basement faults and eruptive fissures strike dominantly NE, near the NNE strike of the rift. However, a majority of the older faults, along with a few local dykes and crater rows, strike N-S, ENE, WNW and NW (Figure 9(d) and Figure 9(e)). The younger surface ruptures are metre-scale push-ups, sinkholes and open fractures. Although their common strikes are N-S and ENE [39], they equally strike WNW and NW in Reykjanes [107]. The dip-slip of the older faults is <0.5 m to ≥20 m, with the highest values along ENE and N-S oblique-slip structures (Figure 9(d)), then along the NNE extensional faults. The dip-slip of the younger surface ruptures reaches up to 1 m, and their aperture is generally 0.5 m (Figure 9(f)). Both the bedrock and the younger fractures have left- and right-stepping en échelon arrangements typical of dextral motion along the N-S, WNW and NW sets, as well as sinistral motion along the ENE set (Figure 9(d)), and are thus oblique-slip.

Surface geothermal manifestations crop out in an area of ~1 × 2 km (Figure 9(b) and Figure 9(c)), considered to be above the centre of the geothermal reservoir into which 37 wells < 3 km were drilled, and the IDDP-2 well was deepened to 4.5 km depth reaching > 400˚C [114]. The mapped manifestations range from 99˚C to 11˚C, and they consist of intense to minimally altered soil, fumaroles, steam vents, bursting mud pots, and green moss with steam escaping from fractures (Figure 9(b) and Figures 9(f)-(i)). Fumaroles and mud pots are generally < 1 m in diameter, but the biggest ones at Gunnuhver are 2.5 m diameter, strongly venting water and steam (Figure 9(i)).

Several features emerge from Area 7 (Figure 9(j)). First, two ENE sinistral oblique-slip fractures (L and G on Figure 9(j)) act as the boundary of the geothermal reservoir. Regional craters arranged in an ENE en échelon array appear farther to the east-northeast and fall on the continuation of the southern reservoir boundary fault, indicating that magma injects also into oblique-slip faults [107]. Second, the shear fractures and the few NNE extensional structures compartmentalise together the reservoir into smaller blocks. Within the reservoir, geothermal manifestations align dominantly on the main ENE, N-S and WNW/NW oblique-slip segments (Figure 9(j)). Furthermore, the analysis of tracer recovery in wells has shown that one ENE sinistral and one N-S dextral oblique-slip fractures in the middle of the reservoir act alternatively as carrier and barrier structures to the flow, depending on the direction of incoming injected water [107]. Rift-parallel structures have been favoured as flow paths for geothermal fluid and magma. However, the bulk of data shows that fracture permeability is controlled by the oblique-slip Riedel sets of the transform zone where leakages are mostly along the fractures although opening at fracture intersections increases the fluid flow.

Figures 10(a)-(g) compile the strikes of older regional fractures, the young surface ruptures, and the permeable fractures of each study area, along with a quick correlation with literature data regarding earthquakes [101] [103] [104] [109] [113] [115] and stress fields [91]. Having in mind that the structures in the 7 studied areas are at different stages of tectonic evolution, they can be interpreted as follows.

The structures in Area 1 (Figure 10(a)) are in an older crust, which formed over millions of years at the plate boundaries and shifted away. These bedrock fractures are reactivated since the Quaternary volcanism was emplaced, and presently undergo occasional intraplate earthquakes. The density and strike-range of basement fractures are much wider, so are the peaks of the most frequent sets (NNE to NE and ENE to WNW). The reactivated fractures hosting all the regional geothermal manifestations strike dominantly northerly, NW, ENE, and WNW. The same permeable fractures are found in the area to the south of the geothermal system but with different frequencies. It is noted that the permeable fracture sets do not reflect the dominant ENE seismic lineations of the 1974 earthquake swarms that occurred at some 10 km distance, but they fit with the fault plane solutions (FPS) of those events. The stress fields proposed by [91] show a dominant normal fault regime compared to strike-slip regime, with severe fluctuations in the direction of Shmax (Figure 10(a)). Similar fluctuations in the paleostresses were observed in various parts of West Iceland, e.g. [89] [116], as much as in the FPS of the single 1974 earthquake event.

The structures in Areas 2 to 7 are at a younger stage of tectonic evolution as they are still being formed and reactivated within the active plate boundaries under frequent earthquakes (Figures 10(b)-(g)), thus not fully developed and with a narrower strike-range than bedrock structures. Both among the bedrock and the surface ruptures, the dominant structures are the N-S and ENE, then WNW and NW oblique-slip fractures, and secondarily the NNE rift-parallel structures. Albeit with slight differences in frequency, the same sets control the permeability of cold and hot water, be it within the SISZ transform fault (Areas 2 to 5), at the intersection of rift and transform zones (Area 6), or at the oblique rift (Area 7). The mapped bedrock, younger, and permeable fractures also correlate well with earthquake data, namely the seismic lineations and the FPS of the 2000 earthquakes in Areas 2 to 5 (Figures 10(b)-(e)), the 2008 earthquake in Area 6 (Figure 10(f)), and the earthquake swarms in Area 7 (Figure 10(g)). In terms of stress fields, strike-slip regime is dominant in Areas 2 to 5 and 7, with slight fluctuations in the direction of Shmax, but no defined stress regime and Shmax are proposed covering Area 6 [91].

All mapped cold and hot water leakages, and their host fractures, are reported

on Figures 11(a)-(g)) for an overview of their geometries. Using the simple shear model, attempts were also made to deduce the directions of maximum (Ϭ₁) and minimum (Ϭ₃) principal stresses compatible with the permeable fractures of each area, unrelated to the regional stress fields and regimes proposed by [91].

In the intraplate Area 1 (Figure 11(a)), permeable fractures fall in the continuation of older structures of the same strikes in the immediate bedrock, which have developed over millions of years, coalesced into longer structures, and lost their initial en échelon geometries. Regardless of temperatures, the leakages align along the main NW, N-S, NE/ENE and WNW oblique-slip fractures rather than by their tips. They also appear as much along the main fractures as at the fracture intersections, with no major difference in the volume of the flow. As a result of their long-term activity and reactivations, older fractures of the same set sometimes show opposite sense of motions, thus incompatible with a single stress field. Therefore, at least two sets of stress fields are required to explain all the mapped structures in this area. One set would be with a NNE Ϭ₁ and WNW Ϭ₃, compatible with N-S to NW dextral and NE to ENE sinistral oblique-slip faults. Another set would be with a NNW Ϭ₁ and ENE Ϭ₃, compatible with the WNW dextral oblique-slip fractures that are parallel to the Quaternary WNW-striking dextral SVZ.

The fracture geometries and the stress fields differ in Areas 2 to 7 within the active plate boundaries (Figures 11(b)-(g)). The majority of the dominant oblique-slips, and the less frequent NNE rift-parallel permeable fractures still present en échelon arrangements, indicating that they are surface expression of deeper structures. In these areas, the leakages are primarily along and in the middle section of the main fractures or on their splays (e.g., Areas 5 and 7), rather than by the fracture tips or in the stepovers between segments. Occasionally, leakages are at the fracture intersections.

In Areas 2 to 7, where the transform segment is a regional shear zone, the permeable fractures act as its internal Riedel shears, accommodating the overall sinistral motion of the transform zone. They are the main N-S to NNW dextral (R’) and ENE to E-W sinistral (R) conjugate sets, the second order shear fractures striking WNW to NW sinistral (P), and occasionally the NNW (X) also possibly sinistral. The NNE/NE extensional fractures represent (T) within the transform zone and are the main rift-parallel fractures outside of the shear zone. The literature agrees that the transform zone is overall ENE in Reykjanes, and considers the transform zone to be E-W in South Iceland (Figure 1). A recent multidisciplinary analysis at the scale of South Iceland, however, demonstrates that the transform zone likely trends ENE also in South Iceland [88], which explains why the internal permeable Riedel shears have the same strike-ranges and overall motions in Areas 2 to 7. Despite slight fluctuations, the principal stresses compatible with a regional ENE shear zone and its internal Riedel shears would be a NE Ϭ₁ and a NW Ϭ₃, applicable to all areas.

Another point to address is the dextral motion of the WNW and NW sets in Areas 5 and 7 (Figures 11(e)-(g)). In Area 5, the NW dextral permeable fracture

that ruptured alongside the main N-S dextral source fault during the 2000 earthquakes could be a splay of this latter, both compatible with the same stress field. In Area 7 within the Reykjanes oblique-rift, however, the WNW and NW permeable sets are more complex and their origin is still a matter of debates in the literature. The NW set could be a splay of the WNW set, or vice versa, but these fractures are dextral, highly frequent and well developed compared to the typical sinistral Riedel shears (P). Thus, they likely represent a separate system. Similar to the WNW dextral permeable shear fractures in Area 1, they require an additional set of stresses with a northerly Ϭ₁ and an E-W Ϭ₃ in Reykjanes (Figure 11(g)).