Triggered slip and afterslip in the 2019 Ridgecrest earthquakes: evidence for rotation of fault zone fabrics

Roger Bilham, & Castillo Bryan

Published August 15, 2019, SCEC Contribution #9846, 2019 SCEC Annual Meeting Poster #241 (PDF)

We report sub-cm triggered slip and afterslip from eighteen creepmeters in the epicentral region and at distances of up to 450 km from the Ridgecrest earthquake sequence. Eleven extensometers 300-450 km south and NE of the July 2019 Mw6.4 and Mw7.1 Ridgecrest earthquakes sampled 20 µm to 4.3 mm of triggered slip at one sample per minute during the passage of surface waves. At all locations triggered slip for the Mw7.1 mainshock exceeded slip triggered by the Mw6.4 foreshock. In most cases slip was complete between 1 minute samples, but at three sites slip persisted for 2-10 ten minutes following initial triggering. Triggered slip also occurred on numerous unmapped faults in the mezzocentral area and on the Garlock Fault to the south but seven creepmeters installed 2-10 days after the earthquakes recorded negligible afterslip. Slip across the western and southern ends of the main ruptures slowed from 20 µm/day to 10 µm/day two weeks after the mainshock with an exponential decay constant of 16±1 days, comparable to the decay in mezzocentral aftershock energy release for the same interval. Slip rates on triggered surface fractures of the Garlock fault were initially less than 5 µm/day with decaying rate.

Within 5 m of the surface trace of the San Andreas fault on Durmid Hill we observe a shear fabric consisting of 10-20 cm dimension phacoidal clasts with their long axes elongated NS within the N45W fault zone. The clasts are separated by polished and slickensided clay surfaces indicative of repeated differential motion. Triggered slip (≤0.8 mm) from the Ridgecrest Mw=7.1 earthquake at Salt Creek measured by two non-parallel creepmeters revealed fault zone contraction and fault parallel extension exceeding 25 µstrain. The Chiapas 2017 Mw8.1 earthquake triggered identical strain changes within the fault zone, as did a subsequent creep event. We interpret these strain changes as counter-clockwise rotation of clasts released by dextral slip on the fault. Between creep events we hypothesize that dextral shear strain in the fault zone causes clasts to rotate clockwise, possibly increasing fault-normal stress, which is released during a creep event. Similar shear fabrics have been observed in the SAFOD hole, within the North Anatolian fault, and in oceanic thrust zones, and may play a crucial role in the nucleation in creep events. The surface San Andreas fault provides a natural laboratory for the study of these processes.

Key Words
Creep, Triggered slip, Fault fabric, nucleation, aseismic slip

Citation
Bilham, R., & Bryan , C. (2019, 08). Triggered slip and afterslip in the 2019 Ridgecrest earthquakes: evidence for rotation of fault zone fabrics. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Ridgecrest Earthquakes