Subsurface damage zone of the Calico Fault viewed by fault-zone trapped waves from teleseismic earthquakes

Yong-Gang Li

Published October 2016, SCEC Contribution #7164

The fault-zone trapped waves (FZTWs) are observed at a square seismic array
consisting of 40 intermediate-period stations and 60 short-period stations deployed in a 1.5 km by 5.5 km
grid adjacent to the Calico Fault (CF) in Mojave Desert, California for teleseismic earthquakes. In the
previous study, FZTWs generated by explosions and local earthquakes, traveltimes inverse and InSAR
observations have imaged a low-velocity compliant zone along the CF within which seismic velocities
are reduced to the maximum of ~40-50% in the center of a ~1-km-wide fault core zone extending to at
least 5-6 km depth (Cochran, et al., 2009). In order to obtain further constraints on the deep portion of the
CF below the depth coverage of local earthquakes, we innovatively use the FZTWs recorded at this
seismic array atop the CF compliant zone for teleseismic earthquakes. This type of FZTWs has not been
used before, but they appear to have great promise for providing unprecedented constrains of the depth
extension of fault-zone damage structure because these FZTWs arise from seismic waves incident at the
fault bottom at deep level. We examined the data from 26 M≥6 teleseismic earthquakes occurring at
distances of ~40o-80o great circles to the Calico array site during 2006. We identify this type of FZTWs
at dominant frequencies 0.5-2 Hz, which show much larger amplitudes and longer wavetrains (~6-8s)
starting ~5-s after the first-arrival P-waves at stations located within the ~1-km-wide CF compliant zone
than those registered at farther stations out of the zone for teleseismic earthquakes. We synthesize these
FZTWs formed by S-waves converted from the first-arrival P waves at the Moho (~30-km depth beneath
Mojave Desert) and entering the bottom of the CF compliant zone. Combined with the previous velocity
model of the CF, the results from observations and simulations of these FZTWs indicate that the
compliant zone with velocity reduction of 40-50% along the CF likely extends to ~8-km depth.

Key Words
Fault-Zone Trapped Waves, Teleseismic Earthquakes, Moho Discontinuity, Fault Compliant Zone and Depth Extension

Citation
Li, Y. (2016). Subsurface damage zone of the Calico Fault viewed by fault-zone trapped waves from teleseismic earthquakes. Academic Perspective, 12, 7-23.

Related Projects & Working Groups
Fault and Rupture Mechanics, seismology