Japan Geoscience Union Meeting 2015

Presentation information

International Session (Oral)

Symbol S (Solid Earth Sciences) » S-TT Technology & Techniques

[S-TT13] Recent Advances in Exploration Geophysics (RAEG2015)

Thu. May 28, 2015 11:00 AM - 12:45 PM 102A (1F)

Convener:*Hitoshi Mikada(Kyoto University), Junichi Takekawa(Graduate School of Science, Kyoto University), Junzo Kasahara(University of Shizuoka, Faculty of Earth Scieces), Yoshihisa Iio(Disater Prevention Research Institute, Kyoto University), Yasuo Ogawa(Volcanic Fluid Research Center Tokyo Institute of Technology), Nobukazu Seama(Department of Earth and Planetary Sciences, Graduate School of Science, Kobe University), Tatsuya Sato(Geothermal Energy Research & Development Co., LTD.), Eiichi Asakawa(JGI, Incorporated), Chair:Toshinori Kimura(Japan Agency for Marine-Earth Science and Technology), Kyosuke Okamoto(Railway Technical Research Institute)

12:30 PM - 12:45 PM

[STT13-13] Effect of local heterogeneous conditions on growth curves of P-wave

*Kyosuke OKAMOTO1, Seiji TSUNO1 (1.Railway Technical Research Institute)

Keywords:heterogeneities, Born approximation, EEW, scattering, growth curve of P-wave, B-delta method

In the Earthquake Early Waning (EEW) system, magnitude and epicentral location of an earthquake are estimated within several seconds after P-wave arrival. According to the Odaka (2003), the equation Bt*exp(-At) is fitted to the early portion of P-waves. In the equation, A and B are constants and t represented time. From the B value, epicentral distances are estimated using an empirical relationship. The B value decreases as the epicentral distance increases due to the geometrical spreading of seismic waves. As the amplitude of P-waves decrease by travelling through the medium, the gradient of the early portion of P-waves also decrease. So that, the B value of a given epicentral distance marks the specific value and the epicentral distance can be obtained from the B value. However in practice, the B values of different earthquakes having the same epicentral distance show quiet different values. The gap in the B values is sometimes by several hundred times. One of the causes giving major effect on the B value is seismic attenuation due to heterogeneities in subsurface. Seismic waves propagating in heterogeneous medium attenuate due to seismic scattering and absorption. Tsukada et al. (2004) conducted a theoretical calculation on broadening of scalar wave envelopes based on three-dimensional von Karman type random medium. They found that the gap in the B values of different earthquakes having the same epicentral distance is accountable by difference in the scattering condition.
In this study, we theoretically calculate P waves in heterogeneous medium considering the conversion waves between P and S waves, which were ignored in the preceding studies while they give major effect on P-wave envelopes. We show that variation in condition of heterogeneities (in concrete correlation distance) changes the B value. In other word, regional conditions of heterogeneities make the different B values of a given epicentral distance. Then we compare the theoretically calculated B value with one calculated from earthquakes in 1996 - 2011 observed by the K-, KiK-net in Japan. We calculate a regional parameter of heterogeneities (correlation distance) in Japan which can account the fluctuation in the observed B values. As a result, it is found that the spatial distribution of the correlation distance (Figure 1) shows similar tendency with that of a parameter reflecting local heterogeneities, like coda-Q.
In this study we hypothesize that variation in the condition of heterogeneities makes the B value fluctuate significantly and verify it. If we treat the fluctuation in the B values by considering the local condition of heterogeneities, estimation on epicentral distances becomes more accurate. It will improve accuracy of the EEW. Furthermore, B values, that is, initial curve of P-waves can be an exploration tool for local heterogeneities. The early portion of P-wave has a strong advantage that the Born approximation can be used. Complex heterogeneities can be treated by the simple equation.