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Earthquake source properties from pseudotachylite

N. M. Beeler, Giulio Di Toro and Stefan Nielsen
Earthquake source properties from pseudotachylite
Bulletin of the Seismological Society of America (October 2016) 106 (6): 2764-2776


Earthquake-radiated motions contain information that can be interpreted as source displacement and therefore related to stress drop. Except in a few notable cases, these displacements cannot be easily related to the absolute stress level or the fault strength, or attributed to a particular physical mechanism. In contrast, paleoearthquakes recorded by exhumed pseudotachylite have a known dynamic mechanism whose properties constrain the coseismic fault strength. Pseudotachylite can be used to directly address a discrepancy between seismologically measured stress drops, which are typically a few MPa, and much larger dynamic stress drops expected from thermal weakening during slip at seismic speeds in crystalline rock (Mckenzie and Brune, 1972; Sibson, 1973; Lachenbruch, 1980; Mase and Smith, 1987; Rice, 2006), and as have been observed in laboratory experiments at high slip rates (Di Toro, Hirose, Nielsen, Pennacchioni, et al., 2006). This places pseudotachylite-derived estimates of fault strength and inferred crustal stress within the context and bounds of naturally observed earthquake source parameters: apparent stress, stress drop, and overshoot, including consideration of fault-surface roughness, off-fault damage, fracture energy, and the strength excess. The analysis, which assumes stress drop is related to corner frequency as in the Madariaga (1976) source model, is restricted to earthquakes of the Gole Larghe fault zone in the Italian Alps, where the dynamic shear strength is well constrained by field and laboratory measurements. We find that radiated energy is similar to or exceeds the shear-generated heat and that the maximum strength excess is approximately 16 MPa. These events have inferred earthquake source parameters that are rare, for instance, a low percentage of the global earthquake population has stress drops as large, unless fracture energy is routinely greater than in existing models, pseudotachylite is not representative of the shear strength during the earthquake that generated it, or the strength excess is larger than we have allowed.

ISSN: 0037-1106
EISSN: 1943-3573
Serial Title: Bulletin of the Seismological Society of America
Serial Volume: 106
Serial Issue: 6
Title: Earthquake source properties from pseudotachylite
Affiliation: U. S. Geological Survey, Cascades Observatory, Vancouver, WA, United States
Pages: 2764-2776
Published: 20161025
Text Language: English
Publisher: Seismological Society of America, Berkeley, CA, United States
References: 101
Accession Number: 2016-103537
Categories: SeismologyStructural geology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: Includes appendices
Illustration Description: illus. incl. 1 table
N46°00'00" - N46°45'00", E11°30'00" - E12°15'00"
Secondary Affiliation: University of Manchester, GBR, United KingdomDurham University, GBR, United Kingdom
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Seismological Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201650
Program Name: USGSOPNon-USGS publications with USGS authors
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