Anomalously Early First Arrivals to the J-Array fromTeleseismic Events

Abstract

An analysis of hundreds of seismograms from a seismic array (J-Array) in the Japanese archipelago shows travel-time anomalies on the order of 4-10 s for P-waves from teleseismic events in some source-receiver geometries. The source-receiver pairs exhibiting such large anomalies are 1) Philippine to Indonesia-Kanto, Japan, 2) Kamchatka-Southern Hokkaido to Tohoku, Japan, and 3) Taiwan to Ryukyu-Shikoku to Kinki, Japan. A simple ray-path consideration indicates that the rays responsible for these anomalies are those incident to the base of the Izu-Bonin slab from below at near critical angles and those propagating laterally within the Kurile slab and the Ryukyu slab along their strikes. The travel times of the first arrivals are calculated for slab structural models using the method of Sakai (1992), which traces progressive expansion of a wavefront away from the source based on the Huygens' principle. This method has an advantage over conventional ray tracing methods in its robustness against the complexity of a laterally heterogeneous velocity structure. A slab model is specified by its thickness and velocity contrast with the surrounding mantle. Numerical experiments show that the effect of slab thickness is different from the effect of velocity contrast, and that their effects are separable. With an important assumption that the IASP91 model approximates the surrounding mantle, the observed negative anomalies can be consistently explained by a 50-km thick slab with a velocity contrast of 5% for the Pacific plate and by a 30-km thick slab with a velocity contrast of 5% for the Philippine Sea plate, although the observed records exhibit negative anomalies that are unable to be explained by the mere introduction of a high-velocity slab.