Strong inhomogeneity beneath Quaternary volcanoes revealed from the peak delay analysis ofS-wave seismograms of microearthquakes in northeastern Japan

Abstract

SUMMARY High‐frequency S‐wave envelopes of microearthquakes well reflect the medium inhomogeneity of the Earth. Defining the peak delay time as the time lag from the direct S‐wave onset to the maximum amplitude arrival of its envelope, we use this quantity to evaluate the strength of multiple forward scattering and diffraction due to random inhomogeneities along the seismic ray path. Analysing peak delay times of many microearthquakes occurred along the subducting Pacific Plate for 2–4, 4–8, 8–16 and 16–32 Hz frequency bands, we find a clear path dependence of the peak delay time in relation to the distribution of Quaternary volcanoes in northeastern Japan. Peak delay times of less than 2 s are usually observed at most of the stations in the study area, but large peak delay times of more than 5 s are observed in the backarc side stations for the case that S wave propagates beneath Quaternary volcanoes. The large peak delay times are inferred to be generated at a depth of 20–60 km beneath Quaternary volcanoes by considering ray paths under a 1‐D velocity structure. These strongly inhomogeneous regions are located at low‐velocity and high Vp/Vs regions revealed from tomography, which suggests the inhomogeneity may be related to dykes and melts of ascending magma.

S-wave envelope broadening characteristics of microearthquakes in the Canary Islands

“…6 Data show a linear trend of t p and t q as commonly observed in logarithmic scale plots for other regions (e.g., Saito et al 2002;Takahashi et al 2007). This means that, in spite of the scatter of the data, both t p and t q increase according to a power of hypocentral distance for all the studied frequency bands and focal depth ranges.…”

“…After Sato (1989) two parameters are used to quantify the non-uniform scattering structure of the earth by the analysis of envelope broadening: the peak delay time, t p , and the envelope duration, t q . The peak delay time is the time lag from the S-wave onset to the maximum amplitude arrival, and it is known to be a good measure of scattering strength (e.g., Gusev and Abubakirov 1999a,b;Takahashi et al 2007). The envelope duration is defined as the time difference between the S-wave onset and when the envelope decreases to half of its maximum amplitude.…”

“…A method based on the radiative transfer theory has also been developed to simulate the envelope broadening caused by forward scattering and to reconstruct the distribution of the scattering properties of the medium (Gusev and Abubakirov 1999a,b;Petukhin and Gusev 2003). Several observations on Swave envelope broadening have been reported: Obara and Sato (1995) analyzed the peak delay and duration times in the Kanto-Tokai region of Japan, and found that the envelope broadening in the backarc side of the volcanic front is larger than in the forearc side; Takahashi et al (2007) found a path dependence of the Swave peak delay times in relation to the distribution of Quaternary volcanoes in northeastern Japan; and Tripathi et al (2010) characterized the S-wave broadening effect based on the signal duration (e.g., Petukhin and Gusev 2003) in northeastern Japan.…”

Velocity increase in the uppermost oceanic crust of subducting Philippine Sea plate beneath the Kanto region due to dehydration inferred from high-frequency trapped P waves

“…In general, as the frequency increases, the seismic wave scattering due to small-scale velocity heterogeneities along the propagation path becomes more dominant and distorted seismograms characterized by frequency-dependent pulse broadening and peak delay are observed (Sato et al 2012;Takahashi et al 2007). However, the pulse-like trapped P waves were observed irrespective of the frequency, while the seismograms became more complicated with increasing frequency.…”

Heterogeneity of the Japanese islands as inferred from transverse component analyses of teleseismic P-waves observed at a seismic station network, Hi-net

“…Multi-lapse time window analyses for S-waves and their coda from local earthquakes clarified detailed spatial distributions of intrinsic and scattering Q of the crust in Japan (Carcore and Sato, 2009). Peak delay times of Swaves propagating in the medium characterized by von Kerman type spectrum of fluctuations are also used to evaluate the small-scale heterogeneity beneath the Tohoku region, Japan (Takahashi et al, 2009). Their spatial resolutions are high enough to compare the elastic parameters related with the small-scale heterogeneity to the tectonic settings at the target regions, such as volcanoes, active seismic faults, and so on.…”