ABSTRACT
A small amplitude precursor before an impulsive P arrival is consistently observed from intermediate-depth subduction zone earthquakes in local, regional, and teleseismic distances. This small amplitude precursor has been identified as a P-wave train from the particle motion and polarization analysis. Time separations between the two P arrivals can be a few to several seconds which appear to be proportional to the traveling length of the seismic waves inside the subducting slab. A preliminary two-dimensional crustal and upper mantle velocity model including a 60° dipping slab was designed to investigate the relationships between the propagation of high-frequency seismic waves and the internal structure of the subducting slab. Synthetic P-wave seismograms at surface stations of various distances from the trench axis assuming earthquake sources at various depths and different parts of the subducting slab were calculated using a two-dimensional finite-element method. Although it is still very preliminary, results of the modeling suggest that earthquakes at intermediate-depth showing two P arrivals are most likely located in the upper portion and not in the central or lower portions of the subducting slab. The small amplitude first P arrival can be interpreted as a P wave train traveling deeper into the central portion of the slab where seismic wave velocity is higher than that in the upper and lower portions of the slab. The impulsive large amplitude second P arrivals may represent a P wave train traveling along the upper portion of the slab where velocity and thermal gradients are the highest. Basically the second P waves are traveling along a well-developed wave guide near the upper portion of the slab where velocity is slower than that towarding the inner portion of the slab. The muhipathing effect becomes apparent only when P waves are traveling beyond certain length inside the slab along up-dip direction such that the arrival times of the two P wave trains become separable.
