Highlights

A bayesian approach to the real time estimation of magnitude from the early P- and S-wave displacement peaks

Journal of Geophysical Research, 2008, vol. 113, B12302, doi:10.1029/2007JB005386.
M. Lancieri, A. Zollo

Abstract.

It has been shown that the initial portion of P- and S-wave signals can provide information about the final earthquake magnitude in a wide magnitude range. This observation opens the perspective for the real-time determination of source parameters.
In this paper we describe a probabilistic evolutionary approach for the real time magnitude estimation which can have a potential use in earthquake early warning. The technique is based on empirical prediction laws correlating the low-frequency peak ground displacement measured in a few seconds after the P and/or S phase arrival and the final event magnitude. The evidence for such a correlation has been found through the analysis of 256 shallow crustal events in the magnitude range Mjma 4-7.1 located over the entire Japanese archipelago. The peak displacement measured in a 2 seconds window from the first P-phase arrival correlates with magnitude in the range M = [4 − 6.5]. While a possible saturation effect above M = 6.5 is observed, it is no more evident in an enlarged window of 4 seconds.
The scaling of S peaks with magnitude is instead observed also at smaller time lapses (i.e., 1-sec) after the first S-arrival. The different scaling of P-and S-peaks with magnitude when measured in a 2 seconds window will be explained in terms of different imaged rupture surface by early portion of body wave signals. We developed a technique to estimate the probability density function (PDF) of magnitude, at each time step after the event origin. The predicted mag-nitude value corresponds to the maximum of PDF, while its uncertainty is given by the 95% confidence bound. The method has been applied to the 2007 (Mjma = 6.9) Noto-Hanto and 1995 (Mjma = 7.3) Kobe earthquakes. The results of this study can be summarized as follows: (1) The probabilistic algorithm founded on
the predictive model of peak displacement versus final magnitude is able to provide a fast and robust estimation of the final magnitude. (2) The information available after a few seconds from the first detection of the P phase at the network can be used to predict the peak ground motion at a given regional target with uncertainties which are comparable to those derived from the attenuation law. (3) The near-source S phase data can be used jointly with P data for regional early warning purposes, thus increasing the accuracy and reliability of magnitude estimation.

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