TITLE:
Decadal Forecasts of Large Earthquakes along the Northern San Andreas Fault System, California: Increased Activity on Regional Creeping Faults Prior to Major and Great Events
AUTHORS:
Lynn R. Sykes
KEYWORDS:
San Andreas and Hayward Faults, California, Fault Creep, Forecasts, Double-Difference Relocations
JOURNAL NAME:
International Journal of Geosciences,
Vol.15 No.2,
February
29,
2024
ABSTRACT: The three largest
earthquakes in northern California since 1849 were preceded by increased
decadal activity for moderate-size shocks along surrounding nearby faults.
Increased seismicity, double-difference precise locations of earthquakes
since 1968, geodetic data and fault offsets for the 1906 great shock are used
to re-examine the timing and locations of possible future large earthquakes.
The physical mechanisms of regional faults like the Calaveras, Hayward and Sargent, which exhibit creep, differ
from those of the northern San Andreas, which is currently locked and is not
creeping. Much decadal forerunning
activity occurred on creeping faults. Moderate-size earthquakes along those faults became more frequent as
stresses in the region increased in the latter part of the cycle of stress
restoration for major and great earthquakes along the San Andreas. They may be
useful for decadal forecasts. Yearly to decadal forecasts, however, are based
on only a few major to great events. Activity
along closer faults like that in the two years prior to the 1989 Loma Prieta shock
needs to be examined for possible yearly forerunning changes to large
plate boundary earthquakes. Geodetic observations
are needed to focus on identifying creeping faults close to the San
Andreas. The distribution of moderate-size earthquakes increased significantly
since 1990 along the Hayward fault but not
adjacent to the San Andreas fault to the south of San Francisco compared
to what took place in the decades prior to the
three major historic earthquakes in the region. It is now clear from a
re-examination of the 1989 mainshock that the increased level of moderate-size
shocks in the one to two preceding decades occurred on nearby East Bay faults.
Double-difference locations of small earthquakes provide structural
information about faults in the region, especially their depths. The northern San Andreas fault is divided into
several strongly coupled segments based on differences in seismicity.