Anthony Lomax - ALomax Scientific, Mouans-Sartoux, France. www.alomax.net, anthony@alomax.net
Alberto Michelini - Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy
Published in Geophysical Journal International, first published on-line Jan. 20, 2011, DOI: 10.1111/j.1365-246X.2010.04916.x. (PDF)
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 and size 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.
Further information:
http://www.alomax.net/projects/early-est
http://www.alomax.net/posters/duration-exceedance
Lomax, A. and A. Michelini, 2009B. Tsunami early warning using earthquake rupture duration, Geophys. Res. Lett., 36, L09306, doi:10.1029/2009GL037223
Lomax, A. and A. Michelini, 2009A. Mwpd: A duration-amplitude procedure for rapid determination of earthquake magnitude and tsunamigenic potential from P waveforms, Geophys. J. Int., 176, 200–214, doi:10.1111/j.1365-246X.2008.03974.x
Video presentation (mostly in Italian): http://www.youtube.com/watch?v=StOjg8WyoU4&list=PL85287C0A1F6AD36C&index=4&feature=plpp_video
Example: Mw9 Tohoku, Japan Earthquake and Tsunami, 11 March 2011, 05:46 UTC (PDF)
Example: Tsunamigenic and non-tsunamigenic earthquakes with similar magnitudes
Evolution for real-time data available 10 min after OT of the period-duration product TdT0 calculation for: (upper) 2006.07.17, Mw7.7, T0=180 s, tsunami importance It=19, Indonesia tsunami earthquake, and (lower) 2009.03.19, Mw7.6, T0=39 s, tsunami importance It=1, Tonga Islands interplate thrust. Blue lines show P-arrival times for each station; red, yellow or green horizontal bars show the station exceedance levels, l50, starting at its first reported time (about 60 s after the corresponding P time). Histogram shows l50 values at 600s; the median (50 percentile) and bounds (20 and 80 percentile), respectively, for L50 are indicated by solid and dotted white lines on the main plot and as a colored diamond and error bar. Red indicates l50(or L50)≥1 (likely that T0>50 s and It≥2); yellow indicates 0.7≤l50(or L50)<1 (possible that T0>50 s and It≥2); green indicates l50(or L50)≤0.7 (unlikely that T0>50 s or It≥2). For both events the L50 values have stabilized by 4-6 min after OT.
