Acoustic Emission Events Interpreted in Terms of Source Directivity
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Pure and Applied Geophysics
Acoustic Emission Events Interpreted in Terms of Source Directivity PETR KOLA´Rˇ,1
MATEˇJ PETRUZˇA´LEK,2 TOMA´Sˇ LOKAJI´CˇEK,2 JAN SˇI´LENY´,1 1 PETRA ADAMOVA´, and ALENA BOUSˇKOVA´1
Abstract—During inversion for the source mechanisms of laboratory acoustic emission events, relatively high misfit values (expressed as the RMS of the inverted equations) were observed. Our experiment was performed on Westerly Granite. A processed set of data consisting of 2405 acoustic emission events was used and a semi-homogeneous velocity model was considered. A correction for sensor radiation patterns and individual sensor constants was taken into account, and an acausal attenuation model was assumed. Source mechanisms were inverted for the moment tensor. The application of a more sophisticated medium model improved inversion quality only for some events. Introducing the source directivity, a standard approach for earthquakes with magnitudes larger than approximately 4, increased the number of successfully inverted events. Directivity was introduced using a Haskell source model; optionally unilateral and bilateral versions of the source were considered. Lower values of RMS for the Haskell source model were considered to justify the directivity approach. This formalism enables us to select the preferable fault from the two nodal planes within the fault plane solution. The rupture directions were observed to tend to the dip direction of the preferred fault. They were found to be preferably subparallel to slip directions for acoustic emissions with a significant DC component. The source time functions retrieved from the seismograms are in agreement with finite source theory. Keywords: Source directivity, Haskell source, acoustic emission, source mechanism inversion.
1. Introduction A possible extension to a point seismic source approximation is a moving source model (Lay and Wallace 1995). One of the most popular representations is the Haskell source model (HS) (Haskell 1964) (Fig. 1). The HS consists of a rectangular area that is
1
Institute of Geophysics of the Czech Academy of Sciences, Prague, Czech Republic. E-mail: [email protected] 2 Institute of Geology of the Czech Academy of Sciences, Prague, Czech Republic.
ZUZANA JECHUMTA´LOVA´,1
successively ruptured using a constant rupture velocity. The rupture front spreads along the longer side of the rectangle. Spreading can occur in one direction (an unilateral HS) or in two opposite directions (a bilateral HS). The most important feature of the HS, or any other finite source, is its directivity, i.e. the dependence of its radiation on the angle between the direction of rupture propagation and the direction of radiation (Stein and Wysession 2003). Such dependence is often observed for large earthquakes and, in fact, was the main motivation for introducing moving source models in seismology. According to the classical approach, larger lithospheric earthquakes should be suitable for unilateral Haskell finite seismic source modeling (Liu and Liu 2012)
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