Crustal structure and cumulative fault offset in the 2019 Ridgecrest earthquake region from new gravity data

Victoria E. Langenheim, Elizabeth K. Haddon, Brandon Dean, Laurie Zielinksi, & Jacob E. Anderson

Published September 10, 2023, SCEC Contribution #12973, 2023 SCEC Annual Meeting Poster #092

We investigate with new gravity data the cumulative dextral slip and bedrock influence along and at the ends of the 2019 M7.1 Ridgecrest earthquake rupture. Previous work based on old gravity data indicates the rupture zone is characterized by thin basin fill except at the northwest and southeast ends of the rupture. We collected ~1600 new gravity measurements along the rupture and along the Garlock Fault. These new gravity data reveal small gravity lows above the rupture, indicating in some cases undulations in the bedrock surface. However, such gravity lows above Mesozoic bedrock suggest lower densities caused by either fracturing or more felsic bedrock. The rupture appears to coincide with small-amplitude magnetic potential lows, which is more consistent with more felsic compositions. This may also apply to the 2019 M6.4 northeast-striking ruptures which are parallel to the edges of a more felsic pluton defined by gravity and magnetic anomalies. About 15 km SE from the NW end of the M7.1 rupture, a deeper pocket of basin fill is located in an apparent right step in the rupture, a possible pull-apart basin. Using the length of the basin as a proxy for long-term offset, this local gravity low suggests ~5 km of offset. Magnetic anomalies another 15 km farther to the SE along the M7.1 rupture suggest 1-2 km of cumulative offset. Even farther SE at the SE end of the M7.1 rupture, new gravity data suggest <1 km offset of the northern margin of a basin 2-3 km deep. If true, this suggests a long-term pattern of either NW increasing offset away from the Garlock Fault or distributed, nascent faulting at the SE terminus of the rupture (Paxton Ranch fault) north of the Garlock Fault.

The new gravity data also sharpen delineation of basins bounded by the Garlock Fault zone. The basin at the SE end of the 2019 M7.1 rupture is bounded on the south by the Garlock Fault and coincides with a 10-mGal gravity gradient. This basin margin is imaged by a seismic-reflection profile but not by recently published seismic tomography although both the tomography and gravity anomalies indicate denser, high-velocity crust north of the basin. The basin is ~ 30 km long. Along strike to the west, the Garlock Fault forms a transtensional left step marked by a narrow 40-km long gravity low, consistent with a pattern of non-uniform cumulative sinistral offset along the Garlock Fault.

Key Words
crustal structure, gravity, Ridgecrest earthquakes

Citation
Langenheim, V. E., Haddon, E. K., Dean, B., Zielinksi, L., & Anderson, J. E. (2023, 09). Crustal structure and cumulative fault offset in the 2019 Ridgecrest earthquake region from new gravity data. Poster Presentation at 2023 SCEC Annual Meeting.

Related Projects & Working Groups
San Andreas Fault System (SAFS)