日本地球惑星科学連合2019年大会

講演情報

[E] ポスター発表

セッション記号 S (固体地球科学) » S-SS 地震学

[S-SS07] Rigorous Seismicity Modelling and Hypothesis Testing

2019年5月27日(月) 17:15 〜 18:30 ポスター会場 (幕張メッセ国際展示場 8ホール)

コンビーナ:庄 建倉(統計数理研究所)、Schorlemmer Danijel(GFZ German Research Centre for Geosciences)、Matt Gerstenberger(GNS Science)、鶴岡 弘(東京大学地震研究所)

[SSS07-P08] Distributions of focal mechanisms in background seismicity and earthquake clusters in Japan

*庄 建倉1,2蒔田 恵理2 (1.統計数理研究所、2.総合大学院大学)

キーワード:地震機構、確率分布、地震クラスター、常時地震、ETASモデル

This study tries to find the probability distributions of focal mechanisms in background seismicity and earthquake clusters in Japan by analyzing F-net data. To describe the probability distribution, each focal mechanism is discomposed as the result of a rotation about a particular pole starting from a reference focal mechanism. In this study, DC4 symmetry is considered, i.e., either nodal-plane can be the fault plane and the two sides of the fault plane are distinguishable.

To separate the background and clustering seismicity, we fit the space-time ETAS model (e.g., Zhuang et al, 2011) to the earthquakes with magnitude 4.2+ from 1997 to 2017 in the F-net catalog, and use the stochastic declustering method to estimate the background probabilities that each event is a background event and the triggered probabilities that each event is triggered by other events.

For the background events, the reference focal mechanism is taken as the average focal mechanism in each area, calculated by the method in Kagan (2014). The ration angles between a background event and the mean focal mechanisms at (x, y) is reconstructed by

ζ(Δ)\sum_j φjI(Rotation angle betwen j and the mean focal mechanism is within (Δ-δ, Δ+δ)),

where j runs over all background events near x,y, δ is a small number, and φj is the background probability estimated from fitting the space-time ETAS model. For the triggered events, the reference focal mechanism is naturally taken the focal mechanism of the event which triggers it. The rotation angles between the parent event and its direct offspring are reconstructed by

ξ(Δ)\sum_{i,j} ρi,j I(Rotation angle betwen i and j is within (Δ-δ, Δ+δ)) .

where i and j run over all the event pairs in the catalog, ρij is the probability that event j is triggered by event i estimated based on the ETAS model.

We find that the rotation poles are almost uniformly distributed. The reconstructed results for the rotation angles are shown in Figure 1.

Figure 1 (a) mean focal mechanisms of background seismicity in Japan. (b) Reconstructed distribution of rotation angles for focal mechanisms in the background seismicity and triggered seismicity.

References:
Zhuang J., (2011) , Next-day earthquake forecasts for the Japan region generated by the ETAS model. Earth Planets Space, 63, 207-216. doi:10.5047/eps.2010.12.010.
Kagan Y.Y. and Jackson D. D. (2014) Statistical earthquake focal mechanism forecasts. Geophys. J. Int. 197, 620–629. doi: 10.1093/gji/ggu015