Stochastic Green's Function: Estimation of Duration and Master Envelope Due to the Propagation Path Effect

Abstract eng:
Average envelopes of small earthquakes represent a convenient basis for the construction of semi-empirical stochastic Green’s functions, used for prediction of future strong ground motions. Petukhin and Gusev (2003) proposed method for estimation of the average envelopes at rock sites based on assumption of self-similarity of envelopes and applied it for Kamchatka. Self-similarity means that average envelopes for small earthquakes have similar shapes and controlled by two scaling parameters: amplitude and duration. It is also assumed that the high-frequency envelope at a site could be represented as the convolution of source, path and site envelopes. For records of small earthquakes, source envelope is the δ-function. Path envelope can be estimated by the generalization of observed envelopes at bedrock sites. In this work, we continue study of the average envelope shapes and estimate average envelopes of the wavelet coefficients for a region adjoining to the Tonankai earthquake for bedrock sites (path effect). First, we slightly modified methodology. (1) To estimate the duration scaling parameter, instead of using the RMS-duration in earlier work, here we use the envelope-delay time Tenv (also known as group-delay time). The new duration parameter is less affected by the coda waves or by the asymmetry of record. (2) In order to select “bedrock” sites, i.e. sites having negligible duration site corrections, Tenv site corrections were estimated by the regression analysis of the observed Tenv values. Second, we divided studied region adjoining to the source region of the Tonankai earthquake, Japan, into three large zones according to tectonical and geological structure: accretion prism (AP), middle zone (MZ), and fault zone along the Hanaore-Nojima fault system (FZ). Third, to study depth dependence of the average envelopes we also analyzed records from deep events, having paths cross to the Lower Crust (LC). In this work we estimate the dependence of scaling parameter Tenv vs. hypocentral distance R, and the master envelope shapes for the bedrock sites. The results show strong regional dependence of the relationship Tenv(R). The smallest Tenv values are observed in the LC zone. For AP zone Tenv values are larger than in other zones; they increase with distance at R < 60km and than stabilize at R > 60km. Master envelopes are more robust: they are practically the same for all studied zones.

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