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The rupture extent of low frequency earthquakes near Parkfield, CA

Hawthorne, Jessica C. and Thomas, Amanda M. and Ampuero, Jean-Paul (2019) The rupture extent of low frequency earthquakes near Parkfield, CA. Geophysical Journal International, 216 (1). pp. 621-639. ISSN 0956-540X.

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The low frequency earthquakes (LFEs) that constitute tectonic tremor are often inferred to be slow: to have durations of 0.2–0.5 s, a factor of 10–100 longer than those of typical M_W 1–2 earthquakes. Here we examine LFEs near Parkfield, CA in order to assess several proposed explanations for LFEs’ long durations. We determine LFE rupture areas and location distributions using a new approach, similar to directivity analysis, where we examine how signals coming from various locations within LFEs’ finite rupture extents create differences in the apparent source time functions recorded at various stations. We use synthetic ruptures to determine how much the LFE signals recorded at each station would be modified by spatial variations of the source–station traveltime within the rupture area given various possible rupture diameters, and then compare those synthetics with the data. Our synthetics show that the methodology can identify interstation variations created by heterogeneous slip distributions or complex rupture edges, and thus lets us estimate LFE rupture extents for unilateral or bilateral ruptures. To obtain robust estimates of the sources’ similarity across stations, we stack signals from thousands of LFEs, using an empirical Green’s function approach to isolate the LFEs’ apparent source time functions from the path effects. Our analysis of LFEs in Parkfield implies that LFEs’ apparent source time functions are similar across stations at frequencies up to 8–16 Hz, depending on the family. The interstation coherence observed at these relatively high frequencies, or short wavelengths (down to 0.2–0.5 km), suggest that LFEs in each of the seven families examined occur on asperities. They are clustered in patches with sub-1-km diameters. The individual LFEs’ rupture diameters are estimated to be smaller than 1.1 km for all families, and smaller than 0.5 km and 1 km for the two shallowest families, which were previously found to have 0.2-s durations. Coupling the diameters with the durations suggests that it is possible to model these M_W 1–2 LFEs with earthquake-like rupture speeds: around 70 per cent per cent of the shear wave speed. However, that rupture speed matches the data only at the edge of our uncertainty estimates for the family with highest coherence. The data for that family are better matched if LFEs have rupture velocities smaller than 40 per cent per cent of the shear wave speed, or if LFEs have different rupture dynamics. They could have long rise times, contain composite sub-ruptures, or have slip distributions that persist from event to event.

Item Type:Article
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URLURL TypeDescription Paper
Hawthorne, Jessica C.0000-0002-4117-2082
Ampuero, Jean-Paul0000-0002-4827-7987
Additional Information:© 2018 The Author(s). Published by Oxford University Press on behalf of The Royal Astronomical Society. Accepted 2018 October 12. Received 2018 September 27; in original form 2018 July 13. We used seismic waveform data from the Berkeley Parkfield High Resolution Seismic Network (HRSN), provided via the Northern California Earthquake Data Center and the Berkeley Seismological Laboratory (doi: 10.7932/NCEDC), as well as seismic waveform data from the Plate Boundary Observatory (PBO) borehole seismic network, operated by UNAVCO and funded by NSF grant EAR-0732947. The PBO data was obtained via IRIS. The fault traces shown in Fig. 2 were obtained from the USGS and California Geological Survey fault and fold database, accessed from in 2016. We are grateful to David Shelly for providing an earlier version of his Parkfield LFE catalog. We thank the editor and reviewers for comments that improved the paper.
Group:Seismological Laboratory
Funding AgencyGrant Number
Subject Keywords:Transient deformation; Earthquake source observations; Rheology and friction of fault zones; Continental tectonics: strike-slip and transform
Issue or Number:1
Record Number:CaltechAUTHORS:20180904-092451160
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Official Citation:Jessica C Hawthorne, Amanda M Thomas, Jean-Paul Ampuero, The rupture extent of low frequency earthquakes near Parkfield, CA, Geophysical Journal International, Volume 216, Issue 1, January 2019, Pages 621–639,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:89349
Deposited By: Tony Diaz
Deposited On:04 Sep 2018 16:56
Last Modified:03 Oct 2019 20:15

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