Linking fault roughness and earthquake behavior

Morgan T. Page, Elizabeth S. Cochran, Nicholas J. van der Elst, Zachary E. Ross, & Daniel Trugman

Submitted September 11, 2022, SCEC Contribution #12126, 2022 SCEC Annual Meeting Poster #019

It remains an open question how earthquake behavior may be influenced by the physical properties of a fault. The 2016-2019 Cahuilla, California earthquake swarm allows for high-resolution mapping of fault roughness, b-values, and stress drop. The sequence comprises 18,250 earthquakes (M-0.27 to M4.4) that ruptured across a 4-km by 4-km fault plane at 4-9 km depth. We first estimate roughness using 2-D fault profiles – 150 m-wide bins in the along-strike and along-dip directions of the fault plane. Roughness is estimated from the residuals to a line fit to the profiles. We find that the fault is, on average, 50% rougher in the slip perpendicular direction than parallel to slip. Next, 3D mapping of fault roughness, estimated from residuals to a plane fit to moving subsets of events, suggests roughness varies by a factor of 6 over length scales of ~500 m. Similarly, b-values determined using the b-positive method vary by a factor of 2 across the fault. In general, we find that b-values are weakly positively correlated with fault roughness. However, following the largest earthquake (M4.4), we observe a distinct population of earthquakes with low b-values occurring in an area of high roughness values. While stress drops also vary across the fault, we find no correlation between roughness and stress drops. We infer that earthquake sequence behavior is partially controlled by fault roughness, especially at low driving stresses.

Key Words
Roughness, b-Values, swarms, Cahuilla

Page, M. T., Cochran, E. S., van der Elst, N. J., Ross, Z. E., & Trugman, D. (2022, 09). Linking fault roughness and earthquake behavior. Poster Presentation at 2022 SCEC Annual Meeting.

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