FY 2007
M_{TSU}: Recovering seismic moments from tsunameter records
Okal, E.A., and V.V. Titov
Pure Appl. Geophys., 164(2–3), doi: 10.1007/s00024-006-0180-5, 355–378 (2007) |
We define a new magnitude scale, M_{TSU}, allowing the quantification of the seismic moment M_{0} of an earthquake based on recordings of its tsunami in the far field by ocean-bottom pressure sensors (``tsunameters'') deployed
in ocean basins, far from continental or island shores which are known to affect profoundly and in a nonlinear fashion the
amplitude of the tsunami wave. The formula for M_{TSU}, M_{TSU} = log_{10} M_{0} − 20 = log_{10} X (ω) + C_{D}^{TSU} + C_{S}^{TSU} + C_{0}, where X (ω) is the spectral amplitude of the tsunami, C_{D}^{TSU} a distance correction and C_{S}^{TSU} a source correction, is directly adapted from the mantle magnitude M_{m} introduced for seismic surface waves by Okal and Talandier. Like M_{m}, its corrections are fully justified theoretically based on the representation of a tsunami wave as a branch of the Earth's
normal modes. Even the locking constant C_{0}, which may depend on the nature of the recording (surface amplitude of the tsunami or overpressure at the ocean floor) and
its units, is predicted theoretically. M_{TSU} combines the power of a theoretically developed algorithm, with the robustness of a magnitude measurement that does not take
into account such parameters as focal geometry and exact depth, which may not be available under operational conditions in
the framework of tsunami warning. We verify the performance of the concept on simulations of the great 1946 Aleutian tsunami
at two virtual gauges, and then apply the algorithm to 24 records of 7 tsunamis at DART tsunameters during the years 1994–2003.
We find that M_{TSU} generally recovers the seismic moment M_{0} within 0.2 logarithmic units, even under unfavorable conditions such as excessive focal depth and refraction of the tsunami
wave around continental masses. Finally, we apply the algorithm to the JASON satellite trace obtained over the Bay of Bengal
during the 2004 Sumatra tsunami, after transforming the trace into a time series through a simple ad hoc procedure. Results are surprisingly good, with most estimates of the moment being over 10^{29} dyn-cm, and thus identifying the source as an exceptionally large earthquake. |