Basic Study on Causes of Cut-off Frequency <i>f<sub>max</sub></i> of Earthquake (Part 2): Estimation of Source Spectra Considering Bedrock Characteristics and Propagation Characteristics of the Observation Site

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  • KOBAYASHI Genyuu
    Standard and Research Department Secretariat of Nuclear Regulation Authority
  • MAMADA Yutaka
    Standard and Research Department Secretariat of Nuclear Regulation Authority

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Other Title
  • 地震の高域遮断周波数<i>f<sub>max</sub></i>の生成要因に関する基礎的検討（その2）
  • ―観測サイトの基盤特性と伝播経路特性を考慮した震源スペクトルの推定―

Abstract

<p>In order to understand spectral characteristics of source spectra in the high frequency band in detail, we estimated acceleration source spectra up to 70 Hz high frequency band of the 2016 Tottori-ken chubu earthquake (foreshock (Mj4.2), mainshock (Mj6.6) and largest aftershock (Mj5.0)) and the 2000 Tottori-ken seibu earthquake (four aftershocks (Mj2.5 to Mj3.0)) by using 1000 m deep borehole array recordings of hard rock site. The source spectra were calculated from the bedrock spectra at each depth of 100 m (Vs = 2.6 km/s), 300 m (Vs = 2.8 km/s) and 1000 m (Vs = 3.2 km/s). Based on a conventional method, the source spectra obtained by removing the propagation characteristics of the observation site from the bedrock spectra of each depth exhibit a tendency to decrease in the high frequency band above the corner frequency fc and have different spectral shapes. On the other hand, the source spectra obtained by removing "bedrock characteristics" defined by Kobayashi and Mamada (2018) in addition to the propagation characteristics exhibit flat frequency characteristics in the high frequency band above fc. In this case, there is little difference in the spectral shapes for each depth of seismic bedrock, and the source spectra can be stably estimated. The source spectra estimated by appropriately taking into account the bedrock characteristics and the propagation characteristics of the observation site show that the ω-2 source spectral model roughly holds up to high frequency band up to 70 Hz which observation accuracy is guaranteed.</p>

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