Changes in stress state and seismicity pattern responsible for the 2019
Ridgecrest, California, earthquakes
Abstract
Monitoring stress state in the Earth’s crust plays a crucial role in our
understanding of an earthquake’s mechanism, especially how earthquake
ruptures nucleate, as well as in calculating the distribution of
hazards. Crustal deformation due to the 2019 Ridgecrest earthquakes,
which occurred near the town of Ridgecrest, California, that culminated
in a preceding earthquake of magnitude (M) 6.4 and a subsequent M7.1
event, caused stress perturbation in nearby regions. However,
implications of future seismic activity are still unclear. Here we
analyze the occurrence of small earthquakes compared to larger
ones—the b-values, showing how the nucleation area for both the M6.4
and M7.1 earthquakes had low b-values before these events occurred, and
mid-to-high b-values thereafter. The slip distribution of the M7.1 event
is also well correlated with the b-value map. Additionally, the time and
local-dependent variations in b-values of the Ridgecrest earthquakes are
linked with estimates of changes to Coulomb stress. The main conclusion
is that the b-value mapping provide insight into the stress state in the
fault zone, which is likely closely related to the nucleation and
evolution of earthquakes in the sequence. The combined approach of
stress-change and b-value analyses to the post-M7.1-quake sequence shows
an area that is currently being stressed near the Garlock fault that
hosted past large earthquakes. The b-values are not as low as those
immediately before the M6.4 and M7.1 events, but contribute the most
recent values in a decreasing trend of the b-value. Together with
geodetic and seismological observations, monitoring the spatial and
temporal distribution of b-value would contribute to seismic hazards in
the Eastern California Shear Zone.