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Repeating Earthquakes With Remarkably Repeatable Ruptures on the San Andreas Fault at Parkfield
Author(s) -
Abercrombie Rachel E.,
Chen Xiaowei,
Zhang Jiewen
Publication year - 2020
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2020gl089820
Subject(s) - san andreas fault , geology , seismology , sequence (biology) , borehole , attenuation , directivity , nucleation , fault (geology) , aftershock , magnitude (astronomy) , paleontology , physics , telecommunications , genetics , astronomy , computer science , antenna (radio) , optics , biology , thermodynamics
We calculate rupture directivity and velocity for earthquakes in three well‐recorded repeating sequences (2001–2016) on the San Andreas Fault at Parkfield using P waves from borehole recordings and the empirical Green's function method. The individual events in each sequence all show the same directivity; the largest magnitude sequence ( M ~ 2.7, 8 events) ruptures unilaterally NW (at ~0.8 Vs ), the second sequence ( M ~ 2.3, 9 events) ruptures unilaterally SE, and the smallest magnitude sequence ( M ~ 2, 11 events) is less well resolved. The highly repetitive rupture suggests that geometry or material properties might control nucleation of small locked patches. The source spectra of the M ~ 2.7 sequence exhibit no detectable temporal variation. The smaller M sequences both exhibit a decrease in high‐frequency energy following the M 6 earthquake that recovers with time. This could indicate a decrease in stress drop, an increase in attenuation, or a combination of the two, followed by gradual healing.