Source characteristics of large deep earthquakes: Constraint on the faulting mechanism at great depths

Source parameters for eight large deep earthquakes are inferred from broadband data of the Global Seismograph Network. We completed an unprecedented data set composed of 14 largest deep earthquakes (M_w ranging from 7.0 to 8.3) from six different subduction zones studied with identical source parameter inversion methods. The study confirms earlier findings that deep earthquake source properties vary with the temperature of the subducting plate. Events in cold slabs show high aftershock activity, high rupture velocity, and high seismic efficiency. In contrast, earthquakes in warm slabs generally have low aftershock productivity, low rupture velocity, and low seismic efficiency. However, the 23 June 1991 Argentina event has source properties that are rather characteristic for earthquakes in cold slabs, and that differ from those of other large South American events. We suggest that two types of deep earthquakes occur in South America, and we interpret these results in the context of the shear instability model for the mechanism of deep earthquakes. In this model, shear failure occurs through a thermal runaway mechanism within a shear zone at great depth, and low seismic efficiency might indicate significant energy dissipation due to heating and possible melt production. The 1991 Argentina event, located in a pocket of deep seismicity within the slab core, may represent a shear instability with little melting as would be the case for large deep earthquakes in cold subduction zones, whereas the other known South American large deep events such as 1963 Peru, 1970 Colombia and 1994 Bolivia, are located in relatively aseismic regions and may involve significant melting within warmer regions of the slab.