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Stress drops on the Blanco oceanic transform fault from interstation phase coherence

Joshua R. Williams, Jessica C. Hawthorne, Sebastian Rost and Tim J. Wright
Stress drops on the Blanco oceanic transform fault from interstation phase coherence
Bulletin of the Seismological Society of America (June 2019) 109 (3): 929-943

Abstract

Oceanic transform faults display a wide range of earthquake stress drops, large aseismic slip, and along-strike variation in seismic coupling. We use and further develop a phase coherence-based method to calculate and analyze stress drops of 61 M> or =5.0 events between 2000 and 2016 on the Blanco fault, off the coast of Oregon. With this method, we estimate earthquake rupture extents by examining how apparent source time functions (ASTFs) vary between stations. The variation is caused by the generation of seismic waves at different locations along the rupture, which arrive at different times depending on station location. We isolate ASTFs at a range of stations by comparing seismograms of collocated earthquakes and then use the interstation ASTF coherence to infer rupture extent and stress drop. We examine how our analysis is influenced by various factors, including poor trace alignment, relative earthquake locations, focal mechanism variation, azimuthal distribution of stations, and depth phase arrivals. We find that as alignment accuracy decreases or distance between earthquakes increases, coherence is reduced, but coherence is unaffected by focal mechanism variation or depth phase arrivals for our dataset. We calibrate the coherence-rupture extent relationship based on the azimuthal distribution of stations. We find the phase coherence method can be used to estimate stress drops for offshore earthquakes, but is limited to M> or =5.0 earthquakes for the Blanco fault due to poor trace alignment accuracy. The median stress drop on the Blanco fault is 8 MPa (with 95% confidence limits of 6-12 MPa) for 61 earthquakes. Stress drops are a factor of 1.7 (95% confidence limits 0.8-3.5) lower on the more aseismic northwest segment of the Blanco fault. These lower stress drops could be linked to reduced healing time due to higher temperatures, which reduce the depth of the seismogenic zone and shorten the seismic cycle.


ISSN: 0037-1106
EISSN: 1943-3573
Coden: BSSAAP
Serial Title: Bulletin of the Seismological Society of America
Serial Volume: 109
Serial Issue: 3
Title: Stress drops on the Blanco oceanic transform fault from interstation phase coherence
Affiliation: University of Leeds, School of Earth and Environment, Leeds, United Kingdom
Pages: 929-943
Published: 201906
Text Language: English
Publisher: Seismological Society of America, Berkeley, CA, United States
References: 54
Accession Number: 2019-062012
Categories: Seismology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sketch maps
N42°00'00" - N45°00'00", W131°00'00" - W126°00'00"
Secondary Affiliation: University of Oxford, GBR, United Kingdom
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Abstract, Copyright, Seismological Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201915
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