JpGU-AGU Joint Meeting 2020

Presentation information

[E] Oral

S (Solid Earth Sciences ) » S-SS Seismology

[S-SS08] Statistical seismology and underlying physical processes

convener:Yasuhiro Yoshida(Meteorological College, Japan Meteorological Agency)

[SSS08-06] Changes in stress state associated with the 2019 Ridgecrest,
California, earthquakes

*Kazuyoshi Nanjo1 (1.University of Shizuoka)

Keywords:Seismic modelling, Crustal deformation, Stress distribution, b values

The 2019 Ridgecrest earthquakes, which occurred near the town of
Ridgecrest, California, included a magnitude (M) 7.1 earthquake that
struck on 5 July 2019 (UTC) as well as active foreshocks and
aftershocks. A M6.4 earthquake preceded the M7.1 quake 34 hours later.
The broad context of the Ridgecrest earthquakes is that they occurred
under the current tectonic stress that created the Eastern California
Shear Zone (ECSZ).

Crustal deformation due to the occurrence of large earthquakes causes
stress perturbation in nearby regions. From the viewpoint of physics
of earthquakes, the probability of a subsequent large earthquake
depends on the stress conditions set up by the previous events and
long-term tectonic state. Given the tectonic stress of the ECSZ, an
investigation into the spatiotemporal state of stress along and near
the faults coseismically ruptured by the M7.1 and M6.4 quakes can play
a crucial role in understanding the distribution of post-seismic
hazards after these quakes. A Coulomb stress model can be used, but
such approaches have so far not been successful in forecasting
upcoming large earthquakes any better than statistical models. This is
partly due to the fact that the locations of potential faults,
essential inputs to the calculation of change in Coulomb stress, are

Here, we used an alternative statistics-based approach to infer the
changes in stress state, based on the b-value of the Gutenberg-Richter
law. The b-value is sensitive to differential stress, and its inverse
dependence on differential stress has been confirmed many times in
both laboratory and field studies. We found that the rupture
initiation from an area of low-b-values, indicative of high stress,
was common to both M6.4 and M7.1 quakes. The post-M7.1-quake sequence
revealedthat another low-b-value zone, which avoided its ruptured
area, fell into the remaining unruptured area. This shows that if a
high-likelihood future rupture were to occur, this might influence the
nearby Garlock fault that hosted large earthquakes for several
thousand years.