Short wave-length variation of seismic velocity anomalies associated with stagnant slab

When mineral physicists discuss mantle properties associated with stagnant slab referring to seismological studies, they focus on long wave-length characteristics provided by seismic tomography studies. In this sense the gap between seismological studies and mineral physics is still large to explore mantle properties. On the other hand, triplicated regional body waves are highly sensitive to the transition zone structure and have ample information regarding seismic velocity anomaly and the discontinuity depth associated with stagnant cold slab. We present results of regional waveform modeling using data from deep focus earthquakes (Mw≥5.5, depth≥300 km) that occurred in the Kurile to Izu-Bonin subduction zones during the period of 1990 to 2005. Here the range of regional distance is considered up to 32 deg where triplication can be observed from a deep focus event due to the high velocity anomaly (HVA) and depression of the discontinuity depth. After examining the data quality, regional waveform data of about 50 events were selected for analysis. The body waves which sampled the region with apparent stagnant slab strongly, can be modeled by model M3.11 (HVA in the transition zone with depression of the discontinuity to 690 km, Tajima and Grand, 1998), model M2.0 (HVA similar to M3.11 but without broad depression of the discontinuity) or a model with their slight modifications if not by iasp91 (Kennett and Engdahl, 1991). Structural variation in the transition zone has been determined in terms of seismic model M3.11 for the Kuriles subduction zone, M2.0 for the region beneath the Japan sea and the northeastern rim of the Eurasian plate or others. The structural boundaries are sharp and distinct showing steep gradient of velocity anomaly between the regions of different structure. The variation of the discontinuity depths indicates lateral variation of temperature beneath the flattened slab. We also found anomalously broadened P waveforms that propagated through the vicinity of structural boundaries. We postulate that the broadened P waves are SV-to-P converted waves in a narrow zone of low velocity anomaly (or a high Poisson's ratio of ~ 0.32) associated with stagnant slab.