Teleseismic estimates of radiated seismic energy:: The E/M0 discriminant for tsunami earthquakes

We adapt the formalism of Boatwright and Choy for the computation of radiated seismic energy from broadband records at teleseismic distances to the real-time situation when neither the depth nor the focal geometry of the source is known accurately. The analysis of a large data set of more than 500 records from 52 large, recent earthquakes shows that this procedure yields values of the estimated energy, E-E, in good agreement with values computed from available source parameters, for example as published by the National Earthquake Information Center (NEIC), the average logarithmic residual being only 0.26 units. We analyze the energy-to-moment ratio by defining Theta = log(10) (E-E/M-0). For regular earthquakes, this parameter agrees well with values expected from theoretical models and from the worldwide NEIC catalogue. There is a one-to-one correspondence between values of Theta that are deficient by one full unit or more, and the so-called "tsunami earthquakes", previously identified in the literature as having exceedingly slow sources, and believed due to the presence of sedimentary structures in the fault zone. Our formalism can be applied to single-station measurements, and its coupling to automated real-time; measurements of the seismic moment using the mantle magnitude M-m should significantly improve real-time tsunami warning.