The Origin of Surface Cracks from the 2019 Ridgecrest Earthquake Sequence M6.4 and M7.1 Fault Zones: Primary Faulting, Triggered Slip or Shaking Related; Can we tell the difference?

Gordon G. Seitz, Maxime Mareschal, Timothy Dawson, & Chris Milliner

Published August 15, 2019, SCEC Contribution #9847, 2019 SCEC Annual Meeting Poster #222

Although large magnitude strike-slip earthquakes are often characterized by relatively simple ruptures, they generally also include rather complex zones of cracking resulting from various origins. Mapping of the 2019 Ridgecrest Earthquake Sequence has revealed a NE striking 18 km-long sinistral rupture resulting from the M6.4 July 4th event, that was cut by the subsequent conjugate NW striking dextral 50 km-long M7.1 July 5th event rupture with maximum offsets in the 4-meter (m) range. As observed in these and other large earthquakes, the ruptures commonly consist of a relatively narrow main rupture trace, generally less than 5 m in width, that accommodate most of the displacement. This is often clearly expressed along the highest displacement, m-scale portions of ruptures. In these recent earthquakes the surface rupture pattern includes several prominent splays that branch from the main surface trace and extend up to several kilometers (km) distances subparallel to the main trace. As kinematically expected, releasing or restraining fault bends and steps are associated with additional deformation. Extensional areas exhibit extensive distributed cracking, often expressed as a pervasive deformation fabric of discontinuous, fault subparallel cracks. Our field mapping was supplemented by Sentinel-1 and ALOS-2 interferometry processed by JPL ARIA web service, and pixel correlated imagery. This was especially valuable along low-displacement portions of the surface rupture, where surface expression was weak.

As much as we recognize various origins of surface cracks, our ability to correctly interpret the origin for specific cracks is challenging. During strong shaking events, triggered slip is commonly observed on faults, often at significant distances from source events. Here we are focusing on the surface fault zones directly associated with surface ruptures, generally the cracking area in a relatively narrow zone within a few km around and loosely connected to the main surface rupture trace. We will discuss various examples of surface cracking and possible criteria to help interpret the origins of cracking as primary faulting, triggered slip or shaking related.

Key Words
Ridgecrest Earthquake , Fault Mapping, Rupture Mapping, Triggered Slip

Citation
Seitz, G. G., Mareschal, M., Dawson, T., & Milliner, C. (2019, 08). The Origin of Surface Cracks from the 2019 Ridgecrest Earthquake Sequence M6.4 and M7.1 Fault Zones: Primary Faulting, Triggered Slip or Shaking Related; Can we tell the difference?. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Ridgecrest Earthquakes