Variations in Earthquake Stress Drop on Gofar Transform Fault at the End of the 2008 and 2020 Seismic Cycles

Pamela A. Moyer, Margaret S. Boettcher, Jianhua Gong, Wenyuan Fan, Jeffery J. McGuire, Jessica M. Warren, Mark D. Behn, John A. Collins, Emily C. Roland, Christopher R. German, & Yajing Liu

Published September 11, 2022, SCEC Contribution #12477, 2022 SCEC Annual Meeting Poster #056 (PDF)

Poster Image: 
Mid-ocean ridge transform faults (RTFs) exhibit some of the most systematic and predictable slip behavior known in seismology. Abundant foreshocks precede the largest earthquakes on RTFs, which repeatedly re-rupture the same area of the fault on a regular basis. On the Gofar Transform Fault on the East Pacific Rise, magnitude ~6 earthquakes reoccur every ~5-6 years on the same rupture patch, while adjacent fault segments (barriers to the largest earthquake ruptures) host only small earthquakes. This long-term seismic behavior provides a unique opportunity to identify structural and material controls on earthquake nucleation and rupture extent, and to explore the effect of fault strength on transform fault seismicity.

In 2008 and 2020, ocean bottom seismometer (OBS) networks on Gofar successfully captured the end of two seismic cycles. In 2008, OBSs on the western segment of the Gofar system (G3) recorded an extensive foreshock sequence localized within a 10-km long rupture barrier, the anticipated Mw 6.0 mainshock and its aftershocks in a rupture patch, and an intense earthquake swarm in another rupture barrier. In 2020, OBSs on the eastern segment of the Gofar system (G1) recorded seismicity with a spatiotemporal distribution similar to that observed on G3, including an extensive foreshock sequence in a localized rupture barrier and the 2020 Mw 6.1 mainshock and its aftershocks in an adjacent rupture patch.

We use an empirical Green's function spectral ratio method to estimate the stress drops of earthquakes on Gofar. On G3, we found stress drops from 0.04 to 3.2 MPa for 138 earthquakes (2.3 ≤ Mw ≤ 4.0) and average stress drop ~2 times higher for earthquakes in the 2008 rupture patch compared to the rupture barriers. We also inferred an inverse correlation between stress drop and fault zone damage from reduced seismic velocity in the foreshock rupture barrier. On G1, we found stress drops from 0.2 to 26 MPa for 390 earthquakes (2.0 ≤ Mw ≤ 4.5) in the foreshock rupture barrier and 2020 rupture patch. While stress drops on G1 overlap the values we found on G3, average stress drop on G1 is ~1 order of magnitude greater than average stress drop on G3, suggesting a stronger fault system on G1 compared to G3. We continue to investigate stress drop variations on G1 and how our stress drops may be used to infer changes in transform fault strength along-strike and with time throughout the seismic cycle.

Key Words
stress drop, fault strength, rupture barriers, oceanic transform faults

Citation
Moyer, P. A., Boettcher, M. S., Gong, J., Fan, W., McGuire, J. J., Warren, J. M., Behn, M. D., Collins, J. A., Roland, E. C., German, C. R., & Liu, Y. (2022, 09). Variations in Earthquake Stress Drop on Gofar Transform Fault at the End of the 2008 and 2020 Seismic Cycles. Poster Presentation at 2022 SCEC Annual Meeting.


Related Projects & Working Groups
Seismology