Earthquake source properties from pseudotachylite

Giulio Di Toro, N. M. Beeler, Stefan Nielsen

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    Earthquake-radiated motions contain in formation 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, the fault strength, or attributed to a particular physical mechanism. In contrast paleo-earthquakes recorded by exhumed pseudotachylite have a known dynamic mechanism whose properties constrain the co-seismic 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 [Sibson, 1973; McKenzie and Brune, 1969; Lachenbruch, 1980; Mase and Smith, 1986; Rice, 2006] as have been observed in laboratory experiments at high slip rates [Di Toro et al., 2006a]. This note 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 by 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 well21
    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 ~16 MPa. These events have inferred earthquake source parameters that are rare, for instance a few percent 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 unless the strength excess is larger than we have allowed.
    Original languageEnglish
    JournalBulletin of the Seismological Society of America
    Early online date31 Oct 2016
    Publication statusPublished - 2016


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