Constraints on Slab Morphology in Southwest Japan from Low Frequency Earthquakes

English Abstract:

Slab morphology below 20 km depth in southwest Japan has been previously mapped using receiver functions and double-difference seismic tomography where a low-Vs, high-Vp/Vs layer is associated with the subducting oceanic crust. Low frequency earthquakes associated with deep tectonic tremor afford an alternative and more direct means of mapping depth to top of plate in the depth range of 35-45 km depth range as these events are usually inferred to represent shear slip on the plate interface. We are compiling catalogues of LFE detections and LFE templates along the Nankai subduction zone from Kyushu east to the Tokai district using waveform data obtained from multiple seismograph networks in southwest Japan. The improved signal to noise ratio of LFE templates over individual LFE detections enables precise measurement of both P and S wave times and estimation of accurate hypocentral depths. We will present a preliminary map of plate depth contours between 35-45 km depth for SW Japan from LFE templates for comparison with estimates based on tomographic and receiver function imaging of the low velocity layer. This comparison should enable insight into the spatio-structural relation between regular seismicity, slow earthquakes and the subducting oceanic plate.