Upper mantle velocity structure in the western Pacific rim estimated from short-period recordings at Matsushiro Seismic Array System

Abstract

We investigate lateral variation of the upper mantle P-wave velocity structure in the western Pacific rim using travel times of P phases from regional shallow events. Onsets of direct and later arrivals are manually picked up from short-period waveforms at Matsushiro Seismic Array System in central Japan by consulting the correlation among the waveforms at seven array stations. For the path along Kurile and Alaska seismic zones, observed first arrivals of shallow events (d = 5-50 km) are well explained by a standard travel time table for Japan. Observations provide no strong evidence of a distinct low-velocity zone in the uppermost mantle, and triplicated branches are adequately explained by a slight modification of model 12 by Nakanishi (1989, 1999). Observed travel times of direct P phases from events along the eastern and western Philippine Sea are slightly larger at around 15• and slightly smaller at around 25• , and the upper mantle is slower and the transition zone and uppermost lower mantle faster in the eastern and western Philippine Sea than in the northwestern Pacific. Such differences could reflect a high temperature in the upper mantle, and accumulation of subducted material at the base of transition zone as a result of the past subduction.

Subsurface Structure in Japan Based on P and S waves Travel Time Analysis Using Genetic Algorithm in Japan Seismological Network

“…The results from data processing of local earthquakes in Japan in order to obtain the structure of the crust and upper mantle as well as the P and S wave velocity respectively in the layer are shown in Table 1 which can be plotted as Figure 4. P wave velocity has value of 5,57 km/s based on sn12c model (Kato & Nakanishi, 2000) and ARC-TR (Fukao, 1977) located at 0-15 km depth, similarly issued by the Preliminary model, Jeffrey model, and CIT11CS3 model (Kanamori, 1967). This means the results on the lower crust layer close to the limit of these models.…”

Variation of teleseismic short-period waveforms at Matsushiro Seismic Array System

“…Despite the small diameter of MSAS, correlations among the observed waveforms are not high, which clearly shows the influence of the receiver characteristics at each station (Kato and Nakanishi, 2000). Our approach is to identify the relative receiver characteristics among the array stations in frequency domain.…”

Evidence for an elevated 410 km discontinuity below the Luzon, Philippines region and transition zone properties using seismic stations in Taiwan and earthquake sources to the south

“…Since that time, numerous studies of this region have been published, often using refracted P waves (e.g. Simpson et al, 1974;Walck, 1984;Kaiho and Kennett, 2000;Kato and Nakanishi, 2000;Simon et al, 2002), sometimes refracted S waves (e.g. Ibrahim and Nuttli, 1967;Simon et al, 2003) or surface waves (e.g.…”