A Rapid Energy-Duration Discriminant for Tsunami Earthquakes

"Tsunami earthquakes" are a class of rare, but devastating slow earthquakes that create tsunami waves much larger than expected from their initial magnitudes. Because these events rupture more slowly than standard "strong" earthquakes, their resultant shaking is much weaker, while they rupture much longer than similarly sized events. By utilizing global seismic data from recent large earthquakes recorded at teleseismic distances (30-80°), we determined the cumulative time history of earthquake energy release and unique rupture durations, TR. Using data for all earthquakes with MW >7.2, we identified a unique and robust discriminant for "tsunami earthquakes" based on their long duration and weak shaking. A simple logarithmic discriminant of the form: log E=a+b × log\ TD clearly separates normal strong shaking earthquakes from slow tsunami earthquakes. For broad-band energy estimates, EBB, b~2, and a=25 to 26.8, while for short-period energy (between 0.5 and 2 Hz), EHF, b~1.6, and a=25 to 27.5. In both cases, a nearly 102 difference exists between the weakest normal earthquake and the strongest-shaking tsunami earthquake. While broad-band energy results are more physically meaningful, the high-frequency results are preferred because they do not suffer significantly from later phases, yield a clearer discriminant, and can be performed with more widely available short-period instruments. The methodology described here clearly identified the 1992 MW 7.7 Nicaragua, the 1994 MW 7.8 and the 2006 MW 7.8 Java earthquakes as slow tsunami earthquakes, by their markedly small energy for duration. Because the method uses a real-time estimated-correction for focal solutions (described in Newman and Okal, [1998]), it can be applied in real-time allowing for a rapid tsunami earthquake assessment within ~10 minutes of an earthquake rupture, depending on size and station availability.