Seismic measurements to reveal short-­term variations in the elastic properties of the Earth crust

Abstract

Since the late the late ’60s-early ’70s era seismologists started developed theories that included variations of the elastic property of the Earth crust and the state of stress and its evolution crust prior to the occurrence of a large earthquake. Among the others the theory of the dilatancy (Scholz et al., 1973): when a rock is subject to stress, the rock grains are shifted generating micro-cracks, thus the rock itself in- creases its volume. Inside the fractured rock, fluid saturation and pore pressure play an important role in earthquake nucleation, by modulating the effective stress. Thus measuring the variations of wave speed and of anisotropic parameter in time can be highly informative on how the stress leading to a major fault failure builds up. In 80s and 90s such kind of research on earthquake precursor slowed down and the priority was given to seismic hazard and ground motions studies, which are very important since these are the basis for the building codes in many countries. Today we have dense and sophisticated seismic networks to measure wave-fields characteristics: we archive continuous waveform data recorded at three components broad-band seismometers, we almost routinely obtain high resolution earthquake locations. Therefore we are ready to start to systematically look at seismic-wave propagation properties to possibly reveal short-term variations in the elastic properties of the Earth crust. In active fault areas and volcanoes, tectonic stress variation influences fracture field orientation and fluid migration processes, whose evolution with time can be monitored through the measurement of the anisotropic parameters (Piccinini et al., 2006) and of the relative velocity variations through the ambient seismic noise cross-correlation analysis (Campillo, 2006). Through the study of S waves anisotropy it is therefore potentially possible to measure the presence, migration and state of the fluid in the rock traveled by seismic waves, thus providing a valuable route to understanding the seismogenic phenomena and their precursors (Crampin & Gao, 2010). Variations of anisotropic parameter and of the ratio between the compressional (P-wave) and the shear (S-wave) seismic velocities, the Vp/Vs (Nur, 1972) have been recently observed and measured during the preparatory phase of a major earthquake (Lucente et al. 2010). The seismic noise cross-correlation analysis has shown remarkable results in studying strong earthquakes (Brenguier et al., 2008). The sudden decrease of relative velocity variations occurring at the same time of the mainshock has been interpreted as due to the modification of the coseismic stress field (Zaccarelli et al., 2011). This study has been developed by the Research Unit 2 inside the “INGV-DPC S3-Project 2012-2013: Short term earthquake prediction and preparation”. The RU 2 is composed by two Work Packages (WP1 and WP2), aimed to study the variations of seismic waves velocities using different techniques: the cross-correlation of seismic noise (WP1) in the area Po Plain before and after the 2012 Emilia seismic sequence, and as well as the seismic activity of the last few years in the Pollino region (southern Apennines) and the anisotropy of S waves (WP2) to all events recorded during the ongoing seismic sequence in the Pollino area and to compare their temporal trends to other seismic observable as the ratio Vp / Vs.