One and two-dimensional Triplicated waveform modeling beneath northeast Asia

Northeast Asia has long been investigated by geoscientists for its critical importance where the Pacific plate is subducting beneath the Eurasian continent accompanying by complicated mantle processes. In our study, taking advantages of frequent occurrence of deep earthquakes(>300km) in the subduction zone and dense seismic arrays in mainland China and Japan, we conducted both 1-Dimensional (1-D) and 2-Dimensional (2-D) triplicated forward waveform modeling on several selected deep events in the Japan-kuril and Izu areas. Triplicated waveform or Triplication which consists of refracted and reflected wave caused by the 410km/660km discontinuity, is very sensitive to the mantle transition zone velocity structure compared with the most commonly adopted methods such as receiver functions and seismic tomography. From 1-D modeling, we obtained the 1-D P and SH wave velocity structure model in the research area, which has an average P wave velocity 1% with maximum 2.1% and an average SH wave velocity 2% with maximum 3.9% higher than the global Iasp91 model in the mantle transition zone, consistent with the tomographic image of a fast stagnant slab in the region. In addition, we found slightly low velocities at approximately 560 km depth. However, the distribution and properties of such a low velocity feature cannot be well constrained based purely on the simple 1-D waveform modeling. In our 2-D simulations, we constructed a series of models and adopted the SEM method to investigate the sensitivity of triplicated waveforms on the slab structure in the mantle transition zone. In particular, we tested a model with a low velocity metastable olivine wedge (mow) added in the slab in the case that seismic waves travel roughly along the trending direction of the steep slab beneath the Izu Arc. Our preliminary result shows that the synthetic seismograms with the mow model display waveform variations distinctly different from the observations, suggesting that a metastable olivine wedge either may not exist or is too small/insignificant to be detected seismically in the study area. We also utilized deep events in the Kuril region with which seismic waves sample mostly the stagnant slab part beneath Northeast China. We thus gained stronger constraints on the extending range and velocity features of the stagnant slab in that region.