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Earthquakes generated from bedding plane-parallel reverse faults above an active wedge thrust, Seattle fault zone

Harvey M. Kelsey, Brian L. Sherrod, Alan R. Nelson and Thomas M. Brocher
Earthquakes generated from bedding plane-parallel reverse faults above an active wedge thrust, Seattle fault zone
Geological Society of America Bulletin (November 2008) 120 (11-12): 1581-1597

Abstract

A key question in earthquake hazard analysis is whether individual faults within fault zones represent independent seismic sources. For the Seattle fault zone, an upper plate structure within the Cascadia convergent margin, evaluating seismic hazard requires understanding how north-side-up, bedding-plane reverse faults, which generate late Holocene fault scarps, interact with the north-vergent master-ramp thrust and overlying backthrust of the fault zone. A regional uplift at A.D. 900-930 involved an earthquake that nucleated at depth and included slip on both the master-ramp thrust and the back-thrust. This earthquake also included slip on some of the <6-km-deep north-side-up, bedding-plane reverse faults. At locales where the north-side-up reverse faults intersect the Puget Sound coast, an earthquake a few centuries earlier than the A.D. 900-930 regional uplift only uplifted areas within hundreds of meters north of the reverse faults. We infer that the bedding-plane reverse faults are seismogenic because shore platforms near the reverse faults have been abruptly uplifted during earthquakes when other shorelines in the Seattle fault zone were unaffected. Faults of the Seattle fault zone therefore can both produce regional uplift earthquakes, with or without surface displacement on the reverse faults, and produce earthquakes that rupture the bedding-plane reverse faults causing fault scarps and uplift localized to hundreds of meters north of these faults. This latter type of earthquake has occurred at least twice and perhaps three times in the late Holocene, and all these earthquakes preceded the regional coseismic uplift of A.D. 900-930. To account for the paleoseismic observations, we propose that the Seattle fault zone is a wedge thrust, with the leading edge being a fault-bend, wedge thrust fold. The active axial surface of the wedge thrust fold is pinned at the tip of the wedge, and a steeply north-dipping sequence of Tertiary sediment forms the south limb of the wedge thrust fold. Some of these steeply north-dipping, bedding-plane surfaces are seismogenic reverse faults that produce scarps. Earthquakes on the wedge thrust produce the regional coseismic uplift events, and earthquakes within the fault-bend fold cause the local uplift earthquakes. Thus, bedding-plane faults can rupture during earthquakes when the wedge thrust does not rupture but instead continues to accumulate seismic energy.


ISSN: 0016-7606
EISSN: 1943-2674
Coden: BUGMAF
Serial Title: Geological Society of America Bulletin
Serial Volume: 120
Serial Issue: 11-12
Title: Earthquakes generated from bedding plane-parallel reverse faults above an active wedge thrust, Seattle fault zone
Affiliation: Humboldt State University, Department of Geology, Arcata, CA, United States
Pages: 1581-1597
Published: 200811
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 58
Accession Number: 2009-001615
Categories: SeismologyStructural geology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sects., geol. sketch map
N47°25'00" - N47°55'00", W123°00'00" - W122°30'00"
N47°06'00" - N47°46'00", W122°32'60" - W121°04'60"
Secondary Affiliation: U. S. Geological Survey, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 200902
Program Name: USGSOPNon-USGS publications with USGS authors
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