Detailed 3D source fault representations for the 2019 Ridgecrest earthquake sequence

Andreas Plesch, John H. Shaw, Zachary E. Ross, & Egill Hauksson

Published August 15, 2019, SCEC Contribution #9709, 2019 SCEC Annual Meeting Poster #278

We present a new 3D source fault representation for the 2019 M6.4 and M7.1 Ridgecrest earthquake sequence, which includes the Eastern and Southern Little Lake faults as part of the regional Little Lake fault zone (Wills, 1988). These representations are based on relocated hypocenter catalogs expanded by template matching and focal mechanisms for M5 and larger events provided by the SCEDC. Following the approach of Riesner et al. (2017), we generate reproducible 3D fault geometries by integrating hypocenter, nodal plane, and surface rupture trace constraints. We used the SW-NE striking nodal plane of the July 4th, 2019 M6.4 event to constrain the initial representation of the Southern Little Lake fault both in terms of location and orientation. The Eastern Little Lake fault was constrained by the four nodal planes of the M5 and larger events aligned with the main trend for orientation, and the July 5th, 2019 M7.1 hypocenter for fault location. The approach follows a defined workflow which allows for assigning weights to a variety of geometric constraints. These, main constraints have a weight of 1 relative to the weight of individual hypocenters (10^-4), ensuring that small aftershocks are applied as weaker constraints. The resulting fault planes can be considered averages of the hypocentral locations respecting nodal plane orientations. For the final representation we added detailed, field mapped rupture traces as strong (weight of 1) surface constraints. Hypocentral distance to fault distributions were then analyzed for peaks which lead to the introduction of fault branches in the Eastern Little Lake fault. The resulting fault representations are generally smooth but non-planar and dip steeply.

The Eastern Little Lake fault representation is truncated at bounding faults - the Airport Lake fault to the north and the Garlock fault to the South, which is consistent with the aftershock pattern. The terminations of the Southern Little Lake fault representation are controlled by aftershock distribution. As a result, the two faults intersect each other at nearly right angles. The base of the faults is defined by the SCEC Community Fault Model (CFM)’s regional seismogenic thickness surface.
These new 3d fault representations are available as triangulated surface representations, and are being added to the CFM (Nicholson et al., this meeting), and to derived products such as a CFM trace map and viewer (Mei et al., this meeting).

Key Words
Ridgecrest, 3d, CFM, structure, faults, mesh

Citation
Plesch, A., Shaw, J. H., Ross, Z. E., & Hauksson, E. (2019, 08). Detailed 3D source fault representations for the 2019 Ridgecrest earthquake sequence. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Ridgecrest Earthquakes