The seismic body wave tomography method has been improved and extended to adapt to a general velocity structure with a number of complexly shaped seismic velocity discontinuities (SVDs) and with three‐dimensional variations in the velocities in the modeling space. An efficient three dimensional ray tracing algorithm which iteratively uses the pseudobending technique and Snell's law is developed. The large and sparse system of observation equations is solved by using the LSQR algorithm. This method is applied to 18,679 arrival times from 470 shallow and intermediate‐depth earthquakes in order to study P and S wave tomographic images beneath northeastern Japan. In addition to first P and S wave arrivals, clear later arrivals of SP waves converted at the Moho and PS and SP waves converted at the upper boundary of the subducted Pacific plate (UBPP) are also used in the inversion. The UBPP, Conrad and Moho are taken as three SVDs, and their depth distributions obtained by previous studies are used. High‐resolution P and S wave tomographic images down to a depth of 200 km have been determined. Large velocity variations amounting to 6% for P wave and 10% for S wave are revealed in the crust and upper mantle. In the crust low‐velocity (low‐V) zones exist beneath active volcanoes. In the upper mantle the low‐V zones dip toward the west from the volcanic front. A high‐velocity (high‐ V) zone corresponding to the subducted Pacific plate is clearly delineated. Most earthquakes in the lower plane of the double‐planed deep seismic zone are found to occur in relatively high‐V areas. The obtained tomographic images are also found to explain other seismological observations well.

Abstract. We have determined a detailed three-dimensional P wave velocity structure of the Japan subduction zone to 500-km depth by inverting local, regional, and teleseismic data simultaneously. We used 45,318 P wave arrivals from 1241 shallow and deep earthquakes which occurred in and around the Japan Islands. The arrival times are recorded by the Japan University Seismic Network which covers the entire Japan Islands densely and uniformly. We also used 4211 travel time residuals from 100 teleseismic events which are read from seismograms recorded by seismic stations in northeastern Japan. In comparison with the previous results obtained from only local and regional events, the present result for the area around the lower plate boundary and the mantle below the plate is determined more reliably because of the addition of 7035 data from 100 teleseismic events and 41 very deep earthquakes. In the crust and uppermost mantle, low-velocity zones are clearly visible beneath active volcanoes. In the mantle wedge the low-velocity zones generally parallel with the slab and exist continuously to a depth of about 200-km, which is consistent with the petrological, geochemical and geodynamic studies. We consider that the existence of volcanism-related low-velocity anomalies in the mantle wedge is a general seismological characteristic of subduction zones, in light of all the available tomographic results for many subduction zones in the world. The Pacific slab beneath Japan is imaged more clearly than in previous studies as a high-velocity zone with a thickness of 80-90 km and a P wave velocity 4-6% higher than the normal mantle. Lower velocity anomalies are found in the mantle below the slab.

[1] On the basis of a waveform similarity analysis, we detected 321 earthquake clusters with very similar (cross-correlation coefficient >0.95) waveforms on the plate boundary in the northeastern Japan subduction zone. Most of them were not found within the subducting Pacific plate with a few exceptions. Moreover, even on the plate boundary, they were not located in the large moment release areas of large interplate earthquakes that occurred recently or in the areas where the plates are inferred to be strongly coupled from GPS data analyses. These observations suggest that these similar earthquakes are caused by repeating slips of small asperities with a dimension of around 0.1 to 1 km surrounded by stable sliding areas on the plate boundary. If the aseismic slip portion in these small asperities is negligible, we can estimate the cumulative amount of aseismic slip in the area surrounding each asperity. In other words, repeating earthquake data potentially can be used to estimate the spatiotemporal aseismic slip distribution on the plate boundary. We estimated the spatial distribution of slip rate on the plate boundary from repeating earthquake data. The scaling relation between seismic moment and seismic slip by Nadeau and Johnson [1998] is used for the estimation of the slip amount by each repeating earthquake cluster. Obtained spatial distribution is consistent with that estimated from GPS data on land.INDEX TERMS: 7209 Seismology: Earthquake dynamics and mechanics; 7230 Seismology: Seismicity and seismotectonics; 8150 Tectonophysics: Plate boundary-general (3040); KEYWORDS: repeating earthquake, asperity, aseismic slip Citation: Igarashi, T., T. Matsuzawa, and A. Hasegawa, Repeating earthquakes and interplate aseismic slip in the northeastern Japan subduction zone,

Abstract. We estimated both P (Vp) and S wave velocity (Vs) structures beneath northeasternThe present study has the following advantages over the previous study of Zhao e! al. [1992]. First, the number of seismic stations and the number of earthquakes used are considerably increased, and thus seismic rays cover the study area much more Data and MethodWe use P and S wave arrival time data from shallow and intermediate-depth earthquakes located during the period from October 1997 to July 1999 by the seismic network of Tohoku University that covers NE Japan. Data selection is based on the following criteria: (1) Table 2. In this section, we discuss the cause of these velocity anomalies beneath active volcanoes. Seismic velocity varies depending on many physical conditions. Composition, saturation condition, temperature, and ambient pressure play an important role in velocity variations. Composition may be more fundamental than the other factors because it is an intrinsic property of the local rocks. We think that the differences in velocity among layers as shown in Table 1 are caused by the differences in rocks composing each layer. Vp/V• ratio of a rock containing cracks saturated with melt is much higher than that of the intact rock. This is the case when the aspect ratio of cracks is the same for water and melt. Otherwise, the relationship will not be so simple. As velocity reduction mainly depends on the kind of fluids filling the inclusions, their volume fractions, and the shape of inclusions, we tried to calculate the velocity variations with various conditions to explain the velocity anomalies beneath active volcanoes presently obtained (Table 2)

Three‐dimensional seismic velocity structure in and around the Philippine Sea plate subducting beneath southwestern (SW) Japan is determined by applying double‐difference tomography method to arrival time data for earthquakes obtained by a dense nationwide seismic network in Japan. A region of low S wave velocity and high Vp/Vs of several kilometers in thickness is recognized immediately above the region of intraslab seismicity in a wide area from Tokai to Kyushu. This characteristic layer dips shallowly in the direction of slab subduction. Compared with the upper surface of the Philippine Sea slab based on seismic reflection and refraction surveys on seven survey lines, we interpret that the low‐Vs and high‐Vp/Vs layer corresponds to the oceanic crust of the Philippine Sea slab. On the basis of the position of the low‐Vs and high‐Vp/Vs layer and the precisely relocated hypocenter distribution of intraslab earthquakes, the upper surface of the Philippine Sea slab is reliably determined for the entire area of SW Japan. Nonvolcanic deep low‐frequency earthquakes that occurred associated with the subduction of the Philippine Sea slab are distributed along the isodepth contour of 30 km in SW Japan, except for the Tokai district where the depth of deep low‐frequency earthquakes becomes gradually deeper toward northeast.