An extension of the E/M0 tsunami earthquake discriminant Θ to regional distances

Abstract

The ‘slowness’ parameter Θ, the logarithmic ratio of the estimated energy radiated by an earthquake EE to its moment M0, is a robust indicator of tsunami earthquakes when calculated from waveforms recorded at teleseismic epicentral distances (35° < Δ < 80°). However, Θ values calculated from waveforms recorded at regional epicentral distances (5° < Δ < 35°) are unreliable. This is because the necessary use of a differentiable traveltime T(Δ) curve smoothed through transition zone caustic distances leads to the systematic overestimation of EE, and hence of Θ, for waveforms recorded at or near these distances. Using a data set comprised of 67 global oceanic dip‐slip earthquakes occurring in the last 20 yr that includes six recognized tsunami earthquakes, we empirically develop a Θ correction for waveforms recorded at regional epicentral distances. Application of the correction to our data set allows the recovery of Θ values fully consistent with those calculated using only teleseismic waveforms. The incorporation of this correction into existing tsunami warning algorithms has the potential to significantly advance near‐field warning efforts.

Rapid earthquake rupture duration estimates from teleseismic energy rates, with application to real-time warning

“…[15] Because TACER reliably estimates durations in real time, it is useful for early earthquake information and potentially tsunami warning in the case of very large and tsunami earthquakes. While the energy methods are more problematic at distances less than 25°due to triplication effects, a recent study by Ebeling and Okal [2012] suggests that energy calculations can be made much closer by applying a distancebased empirical correction that reduces the final energy calculation by up to an order of magnitude at distances less than 10°. Thus, the combination of such a correction with a systematic evaluation of near-field station duration estimates may yield an improved rapid near-field duration result.…”