Just how similar are individual earthquakes within repeating sequences at Parkfield?

Rachel E. Abercrombie, Xiaowei Chen, & Jiewen Zhang

Submitted August 15, 2019, SCEC Contribution #9876, 2019 SCEC Annual Meeting Poster #182

The discovery of repeating sequences of earthquakes on the San Andreas at Parkfield has led to significant advances in our understanding of fault mechanics, and earthquake rupture dynamics. But just how similar are the repeating events? How similar are their rupture areas? Do they nucleate in the same place and rupture in the same direction? Sequences analyzed to date are too small to see resovable spatial variation.

We select 3 sequences of earthquakes within the borehole High Resolution Seismic Network (HRSN), large enough to be well-recorded, but small enough to have multiple repeats.

The sequences include multiple collocated events well-recorded by the HRSN between 2001 and 2016: Sequence 2 (M~2.7) includes 8 events, Sequence 9 (M~2.3), 9 events, and Sequence 5 (M~2), 11 events since 2001. We use P waves and empirical Green’s functions (EGFs) with high cross-correlation to remove the path effects and obtain spectral ratios and source time functions (STFs) for each event. The spectral ratios and source time functions within each sequence are extremely similar at each station.

The source spectra of Sequence 2 (largest M) exhibit no detectable temporal variation and no response to the M6 earthquake. The source spectra of Sequence 5 (smallest M) exhibit a decrease in high frequency energy following the M6 event, that recovers with time, consistent with a decrease in stress drop then gradual healing. Sequence 9 shows a possible small response to the M6 (similar to Sequence 5).

We calculate source directivity following Abercrombie et al. (2017b). For each event in turn, we stretch the time-axis of STFs at each station in turn to find the best correlation between pairs of stations, and so the azimuthal variation in duration. The individual events in each sequence all show the same directivity. Earthquakes in Sequence 2 rupture unilaterally to the NW, those in Sequence 9 rupture unilaterally to the SE and those of Sequence 5 are less well resolved.

All the above analyses used all available EGFs at each station over the entire time period. We also test using exactly the same stations and EGFs for each event within a sequence, and selecting in time to ensure that choices of stations and EGFs, do not affect the results.

Key Words
earthquake, directivity, source

Citation
Abercrombie, R. E., Chen, X., & Zhang, J. (2019, 08). Just how similar are individual earthquakes within repeating sequences at Parkfield?. Poster Presentation at 2019 SCEC Annual Meeting.


Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)