Abstract
Abstract Local earthquakes observed at Sendai, Japan, were analyzed to confirm the validity of a method of polarization analysis using the spectral matrix of seismic wave and its application to seismic reflection studies of the crust using local earthquake data. Reflectors (Bright spots) are known below the Nagamachi-Rifu fault, which caused an M 5.0 class event in 1998. Polarization analysis was applied to earthquake data in and around the fault. Use of the Z-parameter, which is defined using the eigenvalues of the spectral matrix and a statistical value representing the confidence level for the detection of the arrival of polarized waves, allowed detection of linearly and elliptically polarized waves in coda waves. The Z-parameter was also used to image the reflectors by using a migration technique that assumes the P×P and S×S reflection waves travel through a multi-layered velocity structure. Distinct reflectors were detected at depths of around 10 km, 14 km, 17 km, 21–26 km, 35 km and 40 km, that is, from deeper than the fault and the Moho. This study demonstrated the feasibility of using the spectral matrix of three-component seismic signal to detect polarized waves and to image reflectors in the earth’s crust and upper mantle.
Highlights
In 1998, an earthquake of magnitude 5.0 occurred beneath Sendai in the Miyagi district of Japan
Local earthquakes observed at Sendai, Japan, were analyzed to confirm the validity of a method of polarization analysis using the spectral matrix of seismic wave and its application to seismic reflection studies of the crust using local earthquake data
I used data from only one observation station, and a simplified migration algorithm, because the purpose of the study was to confirm whether the Z -parameter derived from the spectral matrix can detect reflected seismic waves and to test the application of the method for seismic reflection studies based on local earthquake data
Summary
In 1998, an earthquake of magnitude 5.0 occurred beneath Sendai in the Miyagi district of Japan. Umino et al (2002) investigated the reflected waves in detail and estimated the depth and dip angle of the mid-crustal thin reflector. Hori et al (1999) revealed the existence of reflected P×P and S×S waves in the waveforms of aftershocks of the magnitude 5.0 event. They estimated its depth to be from 15 to 21 km beneath the magnitude 5.0 event, and suggested that the existence of melt material and water at that depth would provide a plausible explanation for the reflectors (bright spots). Bright spots can usually be identified from reflected and converted waves in coda wave, and the detection of such the waves is important to understand their location and physical properties and to elucidate their relationship to the earthquake occurrence
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