Physical model of deep lowfrequency earthquakes occurred beneath Japan Island Arc
Project/Area Number  14340127 
Research Category 
GrantinAid for Scientific Research (B)

Allocation Type  Singleyear Grants 
Section  一般 
Research Field 
固体地球物理学

Research Institution  The University of Tokyo 
Principal Investigator 
TAKEO Minoru The University of Tokyo, Earthquake Research Institute, Professor, 地震研究所, 教授 (00197279)

Project Period (FY) 
2002 – 2004

Project Status 
Completed(Fiscal Year 2004)

Budget Amount *help 
¥7,100,000 (Direct Cost : ¥7,100,000)
Fiscal Year 2004 : ¥2,100,000 (Direct Cost : ¥2,100,000)
Fiscal Year 2003 : ¥2,400,000 (Direct Cost : ¥2,400,000)
Fiscal Year 2002 : ¥2,600,000 (Direct Cost : ¥2,600,000)

Keywords  Lowfrequency earthquake / KM_2OLangevin equation / New algorithm / Analysis of time series / 震源過程 
Research Abstract 
Since the 1970's, a few earthquakes with depths ranging 20 km to 40 km have been observed just beneath the volcanic front in the northeastern part of Japan Island Arc. Almost all of the deep lowfrequency earthquakes (DLF) recognized up to now have had magnitudes less than 2.5, so that we have little information concerning deep lowfrequency earthquakes except for their waveforms and the locations of their hypocenters. We develop a new algorithm to estimate an averaged KM_2OLangevin matrix function for multiple local time series excited by the same source dynamics, and propose the new concept of "Average Dissipation Spectrum". This algorithm was applied in the analysis of DLF occurred in Akita prefecture on 11 July 2001, and we succeeded in separating the characteristics of the source vibration system and the source excitation process into the averaged dissipation term and the fluctuation term, respectively. The continuous seismic tremor called the deep lowfrequency tremor (LFT) was f
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ound very recently in the Southwest Japan forearc, along the the subducting Philippine Sea Plate. We applied the method of average dissipation spectrum to the LFT data and obtained the characteristic frequency and decay rate of the LFT waveforms. We confirm the several peaks in the dominant frequency range 15Hz of the LFT. The decay rate of the LFT is about 0.20 for the data of 2003 and 0.15 to 0.18 for the data of 2002. The major parts of the coda waves of DLF satisfy the stationary property, and the causality values for the linear and odddegree nonlinear transformations are relatively higher than those for the evendegree nonlinear transformations. This quantitative property is common among all DLF. A nonlinear process to explain many characteristics of volcanic tremors proposed by Julian (1994) can produce the two kinds of characteristics of causality values represented in DLF and in the deep lowfrequency tremor. Of course, this model is not a solitary physical process causing the two kinds of causality value characteristics. However, this physical process is a valid candidate for the source model that is common to DLF and the deep lowfrequency tremor. Less

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Research Products
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