Tsunami Early Warning Using Earthquake Rupture Duration and P-Wave Dominant-Period: The Importance of Length and Depth of Faulti

Tsunami Early Warning Using Earthquake Rupture Duration and P-Wave Dominant-Period: The Importance of Length and Depth of Faulting Comparison of centroid-moment tensor magnitude, M CMT, (left column), apparent source duration, T0, (centre column) and teleseismic, period-duration, TdT0, (right column) with tsunami importance, It, (upper row) and representative tsunami amplitude at 100km, At, (lower row). Vertical red lines show the target It≥2 threshold; horizontal red lines show the optimal cutoff values for the M , M CMT,T0 and TdT0 discriminants; quadrants containing correctly identified tsunamigenic and non-tsunamigenic events are labelled “OK”; quadrants containing incorrectly identified events are labelled “Missed” and “False”. The At and T0 axes use logarithmic scaling. Diagonal lines show possible linear relationships between At and M0 (lower left), T0 (lower centre) and TdT0 (lower right). Event labels show earthquakes type for non interplate-thrust events with It≥2 (I– interplate-thrust; T–tsunami earthquake (blue); O–outer-rise intraplate; B–back- arc or upper-plate intraplate; So–strike- slip oceanic, S–strike-slip continental, R–reverse-faulting).
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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 sea floor 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, MwCMT, representing the product LWD, and estimated through an indirect, inversion procedure. The obtained MwCMT 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, Td, and the likelihood that the high-frequency, apparent rupture-duration, T0, exceeds 50-55 sec. Td and T0 can be related to the critical parameters L, W, D and z. For a set of recent, large earthquakes, we show that the period-duration product TdT0 gives more information on tsunami impact, It, and size, At, than MwCMT and other currently used discriminants. All discriminants have difficulty in assessing the tsunami potential for oceanic strike-slip and back-arc, intraplate earthquake types. Our analysis and results suggest that tsunami potential is not directly related to the product LWD from the “seismic” faulting model, as is assumed with the use of the MwCMT 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.
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References
</p><p>Lomax, A., and A. Michelini (2009), Tsunami early warning using earthquake rupture duration, Geophys. Res. Lett., 36, L09306.
</p><p>This work Submitted to Geophys. J. Int., 2 Apr 2010.
</p><p>
Acknowledgements: This work is supported by the 2007-2009 Dipartimento della Protezione Civile S3 project. The IRIS DMC (http://www. iris.edu) provided access to waveforms used in this study; we thank all those who install, operate and maintain seismic stations worldwide.</p>

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