Tsunami early warning using earthquake rupture duration and P-wave dominant period: the importance of length and depth of faulting

After an earthquake, rapid, real-time assessment of hazards such as ground shaking and tsunami potential is important for early warning and emergency response. Tsunami potential depends on seafloor displacement, which is related to the length, L, width, W, mean slip, D, and depth, z, of earthquake rupture. Currently, the primary discriminant for tsunami potential is the centroid-moment tensor magnitude, M-w(CMT), representing the seismic potency LWD, and estimated through an indirect, inversion procedure. The obtained M-w(CMT) and the implied LWD value vary with the depth of faulting, assumed earth model and other factors, and is only available 30 min or more after an earthquake. The use of more direct procedures for hazard assessment, when available, could avoid these problems and aid in effective early warning. Here we present a direct procedure for rapid assessment of earthquake tsunami potential using two, simple measures on P-wave seismograms-the dominant period on the velocity records, T-d, and the likelihood that the high-frequency, apparent rupture-duration, T-0, exceeds 50-55 s. T-0 can be related to the critical parameters L and z, while T-d may be related to W, D or z. For a set of recent, large earthquakes, we show that the period-duration product TdT0 gives more information on tsunami impact and size than M-w(CMT) and other currently used discriminants. All discriminants have difficulty in assessing the tsunami potential for oceanic strike-slip and backarc or upper plate, intraplate earthquake types. Our analysis and results suggest that tsunami potential is not directly related to the potency LWD from the 'seismic' faulting model, as is assumed with the use of the M-w(CMT) discriminant. Instead, knowledge of rupture length, L, and depth, z, alone can constrain well the tsunami potential of an earthquake, with explicit determination of fault width, W, and slip, D, being of secondary importance. With available real-time seismogram data, rapid calculation of the direct, period-duration discriminant can be completed within 6-10 min after an earthquake occurs and thus can aid in effective and reliable tsunami early warning.