Geologic observations of surface fault rupture associated with the Ridgecrest M6.4 and M7.1 earthquake sequence by the Ridgecrest Rupture Mapping Group

Katherine J. Kendrick, Sinan O. Akciz, Stephen J. Angster, Jean-Philippe Avouac, Jeffrey L. Bachhuber, Scott E. Bennett, Kelly Blake, Stephan Bork, Benjamin A. Brooks, Paul Burgess, Colin Chupik, Timothy Dawson, Michael J. DeFrisco, Jaime Delano, Stephen DeLong, James F. Dolan, Christopher B. DuRoss, Todd Ericksen, Erik Frost, Ryan D. Gold, Nicholas A. Graehl, Elizabeth K. Haddon, Alexandra E. Hatem, Janis L. Hernandez, Christopher Hitchcock, Kenneth W. Hudnut, Rich D. Koehler, Ozgur Kozaci, Tyler Ladinsky, Christopher M. Madugo, Maxime Mareschal, Devin McPhillips, Chris Milliner, Alex E. Morelan, Johanna Nevitt, Brian Olson, Salena E. Padilla, Jason R. Patton, Belle Philibosian, Alexandra Pickering, Ian Pierce, Daniel J. Ponti, Cynthia Pridmore, Carla Rosa, Nathaniel Roth, Katherine M. Scharer, Gordon G. Seitz, Ellie Spangler, Brian J. Swanson, Kate Thomas, Jessica Thompson Jobe, Jerry A. Treiman, Alana M. Williams, & Michael E. Oskin

Submitted August 15, 2019, SCEC Contribution #9779, 2019 SCEC Annual Meeting Poster #217

Surface rupture associated with the July 2019 Ridgecrest earthquake sequence initiated with a M6.4 earthquake on 4 July 2019 that ruptured along a NE-striking, sinistral fault zone. The M7.1 earthquake, the largest of the sequence, followed on 5 July 2019 along a steeply-dipping, dextral, NW-striking fault zone; rupture from this earthquake intersected the NE portion of the M6.4 rupture. Here we present our geologic observations, including horizontal and vertical slip measurements made along these two fault zones. The surface rupture zones occur along a ~ 15 km long, NE-striking fault, and a ~ 50 km long, NW-striking fault. Both surface ruptures occurred along a zone of discontinuous fault segments in the Little Lake and Airport Lake fault zones, as well as on unnamed faults of the Coso Basin, all within the southern Walker Lane Belt.

Although these fault strands were previously mapped, a continuous, through-going fault was not previously recognized. Offset geologic features and preserved tectonic geomorphic features are present along both of the July 2019 ruptures and constitute evidence of prior late-Quaternary faulting.
Along the M6.4 rupture we observed a maximum left-lateral displacement of > 1m, negligible vertical displacement, and a 1km left stepover NE of the M7.1 rupture. To the SW, ruptures are right-stepping en-echelon and splay into several strands. The dominant NE-trending fault strands are crossed by multiple right-lateral, left-stepping en-echelon, NW-trending subsidiary fault strands.

The M7.1 rupture has a distributed rupture pattern, with slip occurring across two or more major, NW-trending fault strands along much of the rupture length. These fault strands are, in places, separated by up to 2.5km. Slip occurred along steeply-dipping faults that range from NW to NE in strike. Although NW-trending right-lateral rupture is the dominant style of faulting and is associated with the largest displacements, we also observed kinematics that involved left-lateral, transtensional and extensional fault motion. The M7.1 rupture is characterized by >3m right-lateral displacement and up to ~1 m west-side-down vertical separation of China Lake Playa. The maximum right-lateral slip we measured to date is > 5m on a single fault strand.
The complexity of the rupture pattern increases towards the rupture ends, as the slip amount diminishes and is distributed across multiple segments. Both ruptures display significant variability along strike.

Key Words
Ridgecrest Earthquake, surface mapping

Citation
Kendrick, K. J., Akciz, S. O., Angster, S. J., Avouac, J., Bachhuber, J. L., Bennett, S. E., Blake, K., Bork, S., Brooks, B. A., Burgess, P., Chupik, C., Dawson, T., DeFrisco, M. J., Delano, J., DeLong, S., Dolan, J. F., DuRoss, C. B., Ericksen, T., Frost, E., Gold, R. D., Graehl, N. A., Haddon, E. K., Hatem, A. E., Hernandez, J. L., Hitchcock, C., Hudnut, K. W., Koehler, R. D., Kozaci, O., Ladinsky, T., Madugo, C. M., Mareschal, M., McPhillips, D., Milliner, C., Morelan, A. E., Nevitt, J., Olson, B., Padilla, S. E., Patton, J. R., Philibosian, B., Pickering, A., Pierce, I., Ponti, D. J., Pridmore, C., Rosa, C., Roth, N., Scharer, K. M., Seitz, G. G., Spangler, E., Swanson, B. J., Thomas, K., Thompson Jobe, J., Treiman, J. A., Williams, A. M., & Oskin, M. E. (2019, 08). Geologic observations of surface fault rupture associated with the Ridgecrest M6.4 and M7.1 earthquake sequence by the Ridgecrest Rupture Mapping Group. Poster Presentation at 2019 SCEC Annual Meeting.


Related Projects & Working Groups
Ridgecrest Earthquakes