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Seismotectonics of the 20 August 1999 Red Rock Valley, Montana, earthquake

Michael C. Stickney and David R. Lageson
Seismotectonics of the 20 August 1999 Red Rock Valley, Montana, earthquake
Bulletin of the Seismological Society of America (August 2002) 92 (6): 2449-2464

Abstract

On 20 August 1999, a magnitude 5.3 earthquake occurred in south-western Montana, ending a 25-year hiatus for magnitude 5+ seismicity in Montana. This earthquake occurred in the central part of the Red Rock Valley, a northwest-trending graben bounded by late Pleistocene and Holocene faults. A focal depth of 12.4 km and a normal-faulting focal mechanism suggest that this earthquake resulted from continued graben development, although not along graben-bounding faults mapped at the surface. The 1999 Red Rock Valley earthquake occurred near the northern end and within the footwall block of the northeast-dipping Red Rock fault. We deployed a temporary network close to the mainshock epicenter and located 65 aftershocks over 3 days, including a magnitude 4.0 aftershock on 26 August that allowed determination of P-wave travel-time delays for regional seismograph stations. Using these station delays to improve relative hypocenter locations, we recomputed hypocenter locations for >1000 Red Rock Valley area earthquakes that occurred since 1989. Relocated hypocenters avoid the Holocene portion of the Red Rock fault but do surround it. The mainshock epicenter location is close to that of a magnitude 5.0 earthquake in 1965.


ISSN: 0037-1106
EISSN: 1943-3573
Coden: BSSAAP
Serial Title: Bulletin of the Seismological Society of America
Serial Volume: 92
Serial Issue: 6
Title: Seismotectonics of the 20 August 1999 Red Rock Valley, Montana, earthquake
Affiliation: Montana Tech of the University of Montana, Montana Bureau of Mines and Geology, Butte, MT, United States
Pages: 2449-2464
Published: 200208
Text Language: English
Publisher: Seismological Society of America, Berkeley, CA, United States
References: 33
Accession Number: 2003-010765
Categories: Seismology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables, geol. sketch maps
N44°45'00" - N44°45'00", W111°15'00" - W111°15'00"
Secondary Affiliation: Montana State University, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 200304
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