SPATIO-TEMPORAL MONITORING OF SEISMIC WAVE VELOCITIES IN THE UPPER CRUST

Abstract

Variations in seismic velocity, ratio of P -to S-wave speed (Vp/Vs), and seismic anisotropy were heralded in the 1970s and 1980s as proxies to examine the buildup of stress preceding large earthquakes. The idea is that high pressures could cause rocks to “dilate,” changing the elastic properties of the crust by increasing crack numbers and/or dimensions, thus affecting the seismic waves propagation velocities. Rock dilatancy causes the rock to undersaturate, which will strongly reduce Vp. but will have little effect on Vs. resulting in the drop of the Vp/Vs ratio (Sholz et al., 1973). Furthermore, the formation and propagation of cracks within the rock affects its anisotropic characteristic. Several studies reported changes between properties recorded before and after mainshock occurrences. A recent example is provided by Lucente et al. (2010), who reported some clear variations in the seismic wave propagation characteristics approaching a mainshock: the elastic properties of the crustal rocks in the fault region underwent a sharp change about a week before the 6 April 2009, Mw 6.3 l’Aquila earthquake. Back in the seventies, it was hoped that these kinds of studies would allow earthquake prediction to be “just around the corner” (Savage, 2010). Over the subsequent decades, this “corner” is progressively drifted away, nevertheless for seismologists, the understanding of the processes that preside over the earthquakes nucleation and the mechanics of faulting, represents a big step toward the ability to predict earthquakes. In this regard, the integration of the monitoring of the crustal proprieties variations into middle and long term forecasting tools could help in the definition of priority areas where risk reduction interventions are more urgent, with a consequent improvement in the emergency preparedness. In the framework of the guidelines defined in the general agreement DPC-INGV for the period 2012-2022, we formed a Research Unit (UR) with the aim to study the seismic property changes occurring around the fault zones to better understand the physics of the earthquake. Our final goal is to eventually provide effective, practical tools to be applied for monitoring purposes and decision making. The UR includes two Working Packages (WP) that will investigate the variation of seismic wave velocities through different approaches. The first WP will analyze the ambient seismic noise cross-correlations to estimate the relative velocity variations occurred in the Po Plain before and after the 2012 seismic sequence, and in the Pollino region (southern Apennines) shaken by multiple seismic sequences during the last years. The second WP will focus on the shear wave seismic anisotropy temporal fluctuation, through the application of a systematic study to all events recorded during the ongoing seismic sequence in the Pollino area.