Crosscorrelation of Earthquake Data Using Stationary Phase Evaluation: Insight into Reflection Structures of Oceanic Crust Surface in the Nankai Trough
Seismic interferometry (SI) has been recently employed to retrieve the reflection response from natural earthquakes. We perform experimental study to apply SI to Ocean Bottom Seismogram (OBS) records in the Nankai Trough, southwest Japan in order to reveal the relatively shallow geological boundaries including surface of oceanic crust. Although the local earthquakes with short raypath we use to retrieve reflection response are expected to contain the higher-frequency components to detect fine-scale structures by SI, they cannot be assumed as plane waves and are inhomogeneously distributed. Since the condition of inhomogeneous source distribution violates the assumption of SI, the conventional processing yields to the deteriorated subsurface images. Here we adopt the raypath calculation for stationary phase evaluation of SI in order to overcome this problem. To find stationary phase, we estimate the raypaths of two reflections: (1) sea-surface P-wave reflection and (2) sea-surface multiple P-wave reflection. From the estimated raypath, we choose the crosscorrelation traces which are expected to produce objective reflections considering the stationary phase points. We use the numerical-modeling data and field data with 6 localized earthquakes and show that choosing the crosscorrelation traces by stationary phase evaluation improves the quality of the reflections of the oceanic crust surface. 1. Introduction Among various seismic exploration methods using the body-wave of natural earthquakes [1, 2], seismic interferometry (SI) has been recently employed to retrieve the reflection response. Although the receiver function method [1] has been broadly used to image the Moho and mantle discontinuities, there is a study claiming that retrieving and migrating the reflection response using SI is superior to the receiver function method [3]. SI retrieves Green’s function between receivers by crosscorrelating wavefield [4, 5]. This theory requires the physical sources homogeneously distributed along the enclosed surface which surrounds the receivers [4]. There are several successful applications of SI to natural earthquakes. Abe et al. [3] and Tonegawa et al. [6] retrieved the crustal reflection response in central Japan using P coda and S coda, respectively. Ruigrok et al. [7] used P wave to retrieve reflection response using Laramie array in USA. Abe et al. [3] further showed the comparison of migrated images of SI and those using receiver function analysis. These applications focused on the teleseismic wavefields in which the epicentral distance is much longer
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