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A phase coherence approach to identifying co-located earthquakes and tremor

Hawthorne, J. C. and Ampuero, J.-P. (2017) A phase coherence approach to identifying co-located earthquakes and tremor. Geophysical Journal International, 209 (2). pp. 623-642. ISSN 0956-540X. http://resolver.caltech.edu/CaltechAUTHORS:20170509-134742865

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Abstract

We present and use a phase coherence approach to identify seismic signals that have similar path effects but different source time functions: co-located earthquakes and tremor. The method used is a phase coherence-based implementation of empirical matched field processing, modified to suit tremor analysis. It works by comparing the frequency-domain phases of waveforms generated by two sources recorded at multiple stations. We first cross-correlate the records of the two sources at a single station. If the sources are co-located, this cross-correlation eliminates the phases of the Green’s function. It leaves the relative phases of the source time functions, which should be the same across all stations so long as the spatial extent of the sources are small compared with the seismic wavelength. We therefore search for cross-correlation phases that are consistent across stations as an indication of co-located sources. We also introduce a method to obtain relative locations between the two sources, based on back-projection of interstation phase coherence. We apply this technique to analyse two tremor-like signals that are thought to be composed of a number of earthquakes. First, we analyse a 20 s long seismic precursor to a M 3.9 earthquake in central Alaska. The analysis locates the precursor to within 2 km of the mainshock, and it identifies several bursts of energy—potentially foreshocks or groups of foreshocks—within the precursor. Second, we examine several minutes of volcanic tremor prior to an eruption at Redoubt Volcano. We confirm that the tremor source is located close to repeating earthquakes identified earlier in the tremor sequence. The amplitude of the tremor diminishes about 30 s before the eruption, but the phase coherence results suggest that the tremor may persist at some level through this final interval.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1093/gji/ggx012DOIArticle
https://academic.oup.com/gji/article-lookup/doi/10.1093/gji/ggx012PublisherArticle
ORCID:
AuthorORCID
Ampuero, J.-P.0000-0002-4827-7987
Additional Information:© 2017 The Authors. Published by Oxford University Press on behalf of The Royal Astronomical Society. Accepted 2017 January 13. Received 2016 November 7; in original form 2016 May 17. Published: 15 January 2017. Seismic data for the Nenana earthquake were obtained via the IRIS DMC from stations in the Alaska Regional Network, run by the Alaska Earthquake Center and the University of Alaska Fairbanks (doi:10.7914/SN/AK) and from one station in the Global Seismograph Network (doi:10.7914/SN/IU) Seismic data from Redoubt volcano was obtained from stations run by the Alaska Volcano Observatory. Seismic data used for testing noise was obtained via the IRIS DMC from stations in the Southern California Seismic Network (doi:10.7914/SN/CI), the Plate Boundary Observatory borehole seismic network, operated by UNAVCO, and the ANZA regional network (doi:10.7914/SN/AZ) This research was supported by NSF grant EAR-1015698, by a Caltech Division of Geological and Planetary Sciences postdoctoral fellowship to JCH, and by the Southern California Earthquake Center, which is funded by National Science Foundation Cooperative Agreement EAR-82081033462 and U.S. Geological Survey Cooperative Agreement G12AC20038.
Group:Seismological Laboratory
Funders:
Funding AgencyGrant Number
NSFEAR-1015698
Caltech Division of Geological and Planetary SciencesUNSPECIFIED
Southern California Earthquake Center (SCEC)UNSPECIFIED
NSFEAR-82081033462
USGSG12AC20038
Subject Keywords:Fourier analysis, Earthquake source observations, Volcano seismology, Rheology and friction of fault zones
Record Number:CaltechAUTHORS:20170509-134742865
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20170509-134742865
Official Citation:J.C. Hawthorne, J.-P. Ampuero; A phase coherence approach to identifying co-located earthquakes and tremor. Geophys J Int 2017; 209 (2): 623-642. doi: 10.1093/gji/ggx012
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:77308
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:12 May 2017 22:56
Last Modified:12 May 2017 22:56

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