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The Mantle Magnitude M_m and the Slowness Parameter : Five Years of Real-Time Use in the Context of Tsunami Warning

¹ Pacific Tsunami Warning Center, NOAA
91-270 Fort Weaver Road
Ewa Beach, Hawaii 96706
 (S.A.W.)

² Department of Geological Sciences
Northwestern University
Evanston, Illinois 60201
 (E.A.O.)

We study a database of more than 119,000 measurements of the mantle magnitude M_m introduced by Okal and Talandier (1989), obtained since 1999 as part of the operational procedures at the Pacific Tsunami Warning Center. The performance of this method is significantly affected by the seismic instrumentation at the recording station, with the very-broadband STS-1 and KS54000 systems offering the lowest residuals between measured values of M_m and those predicted from the Harvard Centroid Moment Tensor (CMT) catalog, and also by the period at which spectral amplitudes are measured, with the best results between 70 and 250 sec. With such mild restrictions, estimates of seismic moments can be obtained in real time by retaining either the maximum value of M_m measured on each record, or its average over the various mantle frequencies, with the resulting residuals, on the order of 0.1 ± 0.2 moment magnitude units. M_m deficiencies in the case of the two large earthquakes of Peru (2001) and Hokkaido (2003) are attributed to azimuthal bias from an excess of stations (principally in North America) in directions nodal for the focal mechanism and directivity patterns. We further study a group of more than 3000 measurements of the energy-to-moment ratio introduced by Newman and Okal (1998), which allows the real-time identification of teleseismic sources violating scaling laws and, in particular, of so-called "tsunami earthquakes." The use of a sliding window of analysis in the computation of allows the separation of "late earthquakes," characterized by a delayed but fast moment release, from truly slow earthquakes. Many such events are recognized, notably on major oceanic and continental strike-slip faults.

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