Monitoring medium changes in an intraplate setting with coda wave interferometry

Abstract

This paper presents the application of coda wave interferometry (Snieder et al., 2002) in the case of an intraplate seismic sequence of events located in Sao Caetano, Pernambuco state, North-East of Brazil in which we report velocity changes related to a 3.7 mR event in the area. Firstly, we use synthetic data for a homogeneous and isotropic medium. In our model, the ray path between source and receivers has scatters, which effects are calculated using the technique from Groenenboom and Snieder (1995). The synthetic data shows that minute change in the relative position of the scatters or the source provokes changes in the coda-wave part and that the P-wave early arrival part is not changed. In the real data set, we first discriminated clusters among our data using P phase cross-correlation. Events with high P-phase correlation coefficient mean that they are collocated. Then from this cluster, we cross-correlated the P phase and the coda wave of every data with a master event separately. We also observe in the real data that whilst the P-wave early part’s correlation coefficient remains constant, the coda wave’s correlation coefficient exhibit variations. These variations can be interpreted as a result of a change in the medium between the occurrence of the (major) 3.7 mR seismic event. Here, we present a case in which this methodology was successfully applied to detect subtle medium subsurface changes in an intraplate setting. We report that even a small 3.7 mR microseismic event can cause velocity changes in the medium. We also point out that – to the best of our knowledge – examples of velocity changes monitoring for such small magnitude events were not previously reported in the literature.