Detailed seismic imaging of the Mw 7.1 Ridgecrest earthquake rupture
zone from data recorded by dense linear arrays
Abstract
We analyze seismograms recorded by four arrays (B1-B4) with 100-m
station spacing and apertures of 4-8 km that cross the surface rupture
of the 2019 Mw7.1 Ridgecrest earthquake. The arrays extend from B1 in
the northwest to B4 in the southeast of the surface rupture. Delay times
between P-wave arrivals associated with ∼1200 local earthquakes
and four teleseismic events are used to estimate local velocity
variations beneath the arrays. Both teleseismic and local P waves
travel faster on the northeast than the southwest side of the fault for
~4.6% and ~7.5% beneath arrays B1 and
B4, but the velocity contrast is less significant at arrays B2 and B3.
We identify several 1- to 2-km-wide low-velocity zones with more
intensely damaged inner cores beneath each array. The damage zone at
array B4 generates fault-zone head, reflected, and trapped waves. An
automated detector, based on peak ground velocities and durations of
high-amplitude waves, identifies candidate fault-zone trapped waves
(FZTWs) in a localized zone for ~600 earthquakes.
Synthetic waveform modeling of averaged FZTWs, generated by
~30 events with high-quality signals, indicate that the
trapping structure at array B4 has a width of ∼300 m, depth of 3-5 km,
S-wave velocity reduction of ∼20% with respect to the
surrounding rock, Q-value of ∼30, and S-wave velocity
contrast of ~4% across the fault (faster on the
northeast side). The results show complex fault-zone internal structures
that vary along fault strike, in agreement the surface geology
(alternating playa and igneous rocks).