An extension of the E/M ₀ tsunami earthquake discriminant Θ to regional distances

The 'slowness' parameter Θ, the logarithmic ratio of the estimated energy radiated by an earthquake E ^E to its moment M ₀, 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 E ^E, 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.