Decadal Seismicity before Great Earthquakes—Strike-Slip Faults and Plate Interiors: Major Asperities and Low-Coupling Zones ()
ABSTRACT
Decadal forerunning
seismic activity is examined for very large, shallow earthquakes along
strike-slip and intraplate faults of the world. It includes forerunning shocks
of magnitude Mw ≥ 5.0 for 21 mainshocks of Mw 7.5 to 8.6 from 1989 to 2020.
Much forerunning activity occurred at what are interpreted to be smaller
asperities along the peripheries of the rupture zones of great mainshocks at
transform faults and subduction zones. Several great asperities as ascertained
from forerunning activity agree with the areas of high seismic slip as determined by others using geodetic,
mapping of surface faulting, and finite-source seismic modeling. The
zones of high slip in many great earthquakes were nearly quiescent beforehand
and are identified as the sites of great asperities. Asperities are strong,
well-coupled portions of plate interfaces. Different patterns of forerunning
activity on time scales of up to 45 years are attributed to the sizes and
spacing of asperities (or lack of). This permits at least some great asperities
along transform faults to be mapped decades before they rupture in great
shocks. Rupture zones of many great mainshocks along transform faults are
bordered either along strike, at depth or regionally by zones of lower plate
coupling including either fault creep forerunning activity,
aftershocks and/or slow-slip events. Forerunning activity to transforms in
continental areas is more widespread spatially than that adjacent to oceanic
transforms. The parts of the San Andreas fault themselves that ruptured in
great California earthquakes during 1812, 1857 and 1906 have been very quiet
since 1920; moderate to large shocks have been concentrated on their
peripheries. The intraplate shocks studied, however, exhibited few if any
forerunning events, which is attributed to the short period of time studied
compared to their repeat times. The detection of forerunning and precursory
activities for various time scales should be sought on the peripheries of great
asperities and not just along the major faults themselves. This paper
compliments that on decadal forerunning activity to great and giant earthquakes
along subduction zones.
Share and Cite:
Sykes, L. (2021) Decadal Seismicity before Great Earthquakes—Strike-Slip Faults and Plate Interiors: Major Asperities and Low-Coupling Zones.
International Journal of Geosciences,
12, 784-833. doi:
10.4236/ijg.2021.129044.