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[SSS03-11] Average Vp/Vs ratio below the sediment layer derived from local earthquake time picks of the ocean bottom seismometer records.
Keywords: Ocean Bottom Seismometer, Vp/Vs ratio estimate for event location, Wadati diagram, Event location , Off-Ibaraki region
A pursuit of the accuracy of event locations without a location bias is one of the fundamental subjects in seismology. Especially in subduction zones in the ocean, the accurate depth of earthquakes is critically important to understand the subduction process. For such accurate event location, the setup of the 2D or 3D velocity model is a key for both P- and S-wave. On one hand, for P-wave, the precise velocity structure is obtained in wide region around Japan using the active source seismic surveys such as the seismic tomography analysis or the refraction seismic survey using Ocean Bottom Seismometers (OBSs). On the other hand, for S-wave, the velocity structure of the entire model is rarely measured due to the lack of the active source S-wave velocity measurement technology. Hence, the reliable estimate of the S-wave velocity structure has been a major obstacle for accurate event location. In the sediment layer, some approaches are available to estimate the Vp/Vs ratio. However, the Vp/Vs ratio below the basement in the consolidated layer is not well constrained. In past studies, an empirical Vp/Vs ratio such as 1.73 has been used by assuming a Poisson solid which represents the average Vp/Vs ratio of the study area. The use of this empirical value may make the resultant event location more or less biased, hence difficult to quantitatively compare with the other geophysical survey results such as multi-channel seismic profiles.
In this study, we attempt to devise a simple and robust method to quantitatively estimate the Vp/Vs ratio below the sediment layer. The target situation of this study is the local natural earthquakes recorded in an OBS network. For this given condition, this study formulates and estimates the average Vp/Vs ratio below the basement assuming that the P-wave velocity structure and Vp/Vs ratio in the sediment layer are already known.
The formulation on the estimate of the Vp/Vs ratio below the basement was made based on the multi-layered Wadati-diagram (Kisslinger and Engdahl, 1973) to accommodate with the OBS measurement condition; sediment layer and a consolidated layer (2-layered Wadati-digram). Here, one technical consideration is that the conventional Wadati-diagram requires a long-offset between the source and the seismic stations. Hence, sometimes Wadati-diagram was applied in combination with the OBS and land seismic network (e.g. Yarce et al., 2019). However, since this study does not use the land seismic network, this requirement of the long-offset posed to the conventional Wadati-diagram has to be mitigated. After the formulation, this study succeeded to mitigate this condition so as to be applicable to the local OBS network by having simple assumptions of the ray-path coincidence between P- and S-wave and a vertical incidence in the sediment layer.
Thus formulated Wadati-diagram equation for the OBS network shows that this model has an intercept time related to the sediment layer which is not present in the conventional 1-layer Wadati-diagram model. This intercept time is evidently an OBS site dependent because of the locality of the sediment layer for each OBS. This suggests that the intercept term for each OBS has to be estimated as well for individual OBSs together with the average Vp/Vs ratio below the basement.
The method was applied and evaluated for OBS data set at off-Ibaraki region from 2010 to 2011 for over 20,000 events. Finally, the seismotectonic implication about the depth of the location of small earthquakes is discussed.
Reference
Kisslinger, C., & Engdahl, E. R. (1973). The interpretation of the Wadati diagram with relaxed assumptions. Bulletin of the Seismological Society of America, 63(5), 1723–1736.
Yarce, J. et al. (2019). Seismicity at the northern Hikurangi Margin, New Zealand, and investigation of the potential spatial and temporal relationships with a shallow slow slip event. Journal of Geophysical Research: Solid Earth, 124(5), 4751–4766.
In this study, we attempt to devise a simple and robust method to quantitatively estimate the Vp/Vs ratio below the sediment layer. The target situation of this study is the local natural earthquakes recorded in an OBS network. For this given condition, this study formulates and estimates the average Vp/Vs ratio below the basement assuming that the P-wave velocity structure and Vp/Vs ratio in the sediment layer are already known.
The formulation on the estimate of the Vp/Vs ratio below the basement was made based on the multi-layered Wadati-diagram (Kisslinger and Engdahl, 1973) to accommodate with the OBS measurement condition; sediment layer and a consolidated layer (2-layered Wadati-digram). Here, one technical consideration is that the conventional Wadati-diagram requires a long-offset between the source and the seismic stations. Hence, sometimes Wadati-diagram was applied in combination with the OBS and land seismic network (e.g. Yarce et al., 2019). However, since this study does not use the land seismic network, this requirement of the long-offset posed to the conventional Wadati-diagram has to be mitigated. After the formulation, this study succeeded to mitigate this condition so as to be applicable to the local OBS network by having simple assumptions of the ray-path coincidence between P- and S-wave and a vertical incidence in the sediment layer.
Thus formulated Wadati-diagram equation for the OBS network shows that this model has an intercept time related to the sediment layer which is not present in the conventional 1-layer Wadati-diagram model. This intercept time is evidently an OBS site dependent because of the locality of the sediment layer for each OBS. This suggests that the intercept term for each OBS has to be estimated as well for individual OBSs together with the average Vp/Vs ratio below the basement.
The method was applied and evaluated for OBS data set at off-Ibaraki region from 2010 to 2011 for over 20,000 events. Finally, the seismotectonic implication about the depth of the location of small earthquakes is discussed.
Reference
Kisslinger, C., & Engdahl, E. R. (1973). The interpretation of the Wadati diagram with relaxed assumptions. Bulletin of the Seismological Society of America, 63(5), 1723–1736.
Yarce, J. et al. (2019). Seismicity at the northern Hikurangi Margin, New Zealand, and investigation of the potential spatial and temporal relationships with a shallow slow slip event. Journal of Geophysical Research: Solid Earth, 124(5), 4751–4766.