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-CG 固体地球科学複合領域・一般

[S-CG48] Science of slow earthquakes: Toward unified understandings of whole earthquake process

2019年5月29日(水) 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:井出 哲(東京大学大学院理学系研究科地球惑星科学専攻)、廣瀬 仁(神戸大学都市安全研究センター)、氏家 恒太郎(筑波大学生命環境系)、波多野 恭弘(東京大学地震研究所)

[SCG48-P04] Evaluation of Magnitude and Characteristics of Seismicity of Low Frequency Earthquakes in Parkfield

*麻生 未季1,3麻生 尚文2井出 哲1 (1.東京大学大学院理学系研究科、2.東京工業大学理学院、3.応用アール・エム・エス株式会社)

キーワード:サンアンドレアス断層、パークフィールド、地震活動、波形スタック、低周波地震、振幅評価

Over a million tectonic low frequency earthquakes (LFEs) have been found in Parkfield along the San Andreas Fault, as 88 families by Shelly and Hardebeck [2010]. Thanks to enormous event detection, interesting characteristics focusing on their event amplitudes have been reported such as the spatial variation along the fault, the exponential frequency distribution for each family, and the relationship with recurrence time [Shelly and Hardebeck, 2010; Shelly, 2010]. However, the relative size of families has not been quantified accurately, because we have to consider various factors such as radiation pattern depending on focal mechanisms, attenuation during wave propagation, and site amplification effects. For quantitative comparison of scale among differently located events, here we estimate the absolute magnitude of each event considered those effects.

We estimate the seismic moment Mo [Nm] and moment magnitude Mw of all LFE events in the catalog of Shelly [2017]. We first stack seismograms of over one million events in the catalog to improve the signal-to-noise (S/N) ratio. Then, we evaluate the absolute amplitude of the original seismograms using the stacked waveforms. Specifically, we evaluate seismic moment Mo by assuming focal mechanisms of each family, attenuation quality factor, site effect estimated by Aso et al. [2019, submitted to JpGU meeting, “Variations of Focal Mechanisms of Low Frequency Earthquake in Parkfield”], and the source time function estimated by Thomas et al. [2016]. Note that we consider the frequency-limited seismic moment Mo, though it is not exactly the same as the original definition of seismic moment measured at a lower frequency than the minimum frequency contained in one phenomenon.

As previously reported by reported by Shelly and Hardebeck [2010], LFEs in the southern part has larger magnitude. We also find that shallower events are smaller than deeper events.

Similarly, most of the frequency distribution for each family shows an exponential distribution which is also reported for LFEs in Cascadia [Sweet et al., 2014] and New Zealand [Chamberlain et al., 2014].

By comparing size and recurrence interval, the events during LFE burst (seconds interval) tend to be larger than the first event of burst, which occurs after hours-to-days quiescence. A similar tendency is found in tremor burst occurring with a longer time scale (~1-week-long) than that of LFEs (~1-minute-long) [Yabe and Ide, 2014]. These might suggest some temporal change in fault rheology and/or some characteristic spatiotemporal behavior such as Brownian slow earthquake processes [Ide, 2008].