Pure and Applied Geophysics

, Volume 172, Issue 3–4, pp 791–804 | Cite as

New Tsunami Forecast Tools for the French Polynesia Tsunami Warning System

Part I: moment tensor, slowness and seismic source inversion
Article

Abstract

This paper presents the tsunami warning tools, which are used for the estimation of the seismic source parameters. These tools are grouped under a method called Preliminary Determination of Focal Mechanism_2 (PDFM2), that has been developed at the French Polynesia Warning Center, in the framework of the \(SEISCOMP3\) system, as a plug-in concept. The first tool determines the seismic moment and the focal geometry (strike, dip, and slip), and the second tool identifies the ”tsunami earthquakes” (earthquakes that cause much bigger tsunamis than their magnitude would imply). In a tsunami warning operation, initial assessment of the tsunami potential is based on location and magnitude. The usual quick magnitude methods which use \(P\) waves, work fine for smaller earthquakes. For major earthquakes these methods drastically underestimate the magnitude and its tsunami potential because the radiated energy shifts to the longer period waves. Since French Polynesia is located far away from the subduction zones of the Pacific rim, the tsunami threat is not imminent, and this luxury of time allows to use the long period surface wave data to determine the true size of a major earthquake. The source inversion method presented in this paper uses a combination of surface waves amplitude spectra and P wave first motions. The advantage of using long period surface data is that there is a much more accurate determination of earthquake size, and the advantage of using P wave first motion is to have a better constrain of the focal geometry than using the surface waves alone. The \(PDFM2\) method routinely gives stable results at \(T_0 + 45\) minutes, with \(T_0\) being the origin time of an earthquake. Our results are then compared to the Global Centroid Moment Tensor catalog for validating both the seismic moment and the source geometry. The second tool discussed in this paper is the slowness parameter \(\theta,\) and is the energy-to-moment ratio. It has been used to identify tsunami earthquakes, which are characterized by having unusual slow rupture velocity and release seismic energy that has been shifted to longer periods and, therefore, have low \(\theta\) values. The slow rupture velocity would indicate weaker material and bigger uplift and, thus, bigger tsunami potential. The use of the slowness parameter \(\theta\) is an efficient tool for monitoring the near real-time identification of tsunami earthquakes.

Keywords

Moment tensor Source inversion Slowness Tsunami warning 

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Copyright information

© Springer Basel 2014

Authors and Affiliations

  1. 1.CEA, DASE, Laboratoire de GeophysiquePapeeteFrench Polynesia

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