Imaging subducted high velocity slabs beneath the sea of Okhotsk using depth phases

Abstract:

A recent study of a shallow Kuril subduction zone event displays significant waveform multi-pathing for paths propagating down the slab towards Europe(Zhan,Zhongwen 2014). Relatively fast structures (5%) are invoked to simulate such observations requiring numerical methods to capture such proportional distortions. Here, we present results from the reverse direction that is the effects on depth phases of deep events propagating up the slab. In particular the Mw6.7 Sea of Okhotsk deep earthquake occurred at a depth of 640 km is believed to be near the bottom of the slab structure and produced an abundance of depth phases. Differential travel time sP-P analysis shows a systematic decrease of up to 5 seconds from Europe to Australia and then to Pacific which is indicative of a dipping high velocity layer above the source region. Multiple simulations using WKM(An upgraded variation of the traditional WKBJ method) and finite difference methods were conducted in an effort to assess the effects of sharp structure on the whole wave-field. Results obtained from analytical methods, by the WKM code become questionable compared against the finite difference method due to its inability to handle the diffraction phases which become crucial in complex structures.

In this example, seismicity clustered within a 45 degree dipping benioff zone at shallow depth but became blurred beyond 400 km. Finite difference simulations showed that a slab shapped structure that follows the benioff zone at shallow depth and steepens beyond 400 km produces a model that can account for the sP-P differential travel times of our 5s for oceanic paths.