Title: Seismological evidence for hidden crustal fluid diffusion transients in California

Abstract:
Earthquake swarms are primarily a consequence of aseismic driving processes that occur transiently, but the details of these processes are still unclear. We use deep learning algorithms to produce a significantly enhanced catalog of seismicity for Southern California and better resolve the details of swarms. Near the San Jacinto fault zone, a major strike-slip fault system, we identify nearly one hundred swarms that have durations in the range 0.5-6 years. These swarms are not directly driven by tectonic processes yet constitute about 1/4 of the total seismicity. We find that about half of the swarms exhibit ultra-slow diffusive migration patterns, along with propagating backfronts, in which a shut-down of seismicity propagates away from the source region. These observations are all consistent with expectations for natural fluid injection processes. Furthermore, we find evidence that permeability along the fault zone increases exponentially with time elapsed. The chronology of the swarms indicates that these aseismic driving processes were active in some part of the region throughout 2008–2020. The observations challenge common views about the nature of swarms, which would characterize any one of these sequences as anomalous. The regional prevalence of these sequences suggests that transient fluid injection processes play a key role in crustal fluid transport.

Title: Seismic radiation from regions sustaining brittle rock damage

Abstract:

Analytical results indicate that dynamic changes of elastic moduli in source volumes produce “damage-related-radiation” associated with tensorial products of the changes of elastic moduli and the elastic strain components at the source. Decreasing elastic moduli (as produced by brittle deformation of low-porosity rocks and explosions) increase the radiation to the surrounding medium, while increasing moduli (which may be produced during the formation of compaction bands in porous rocks) decrease the radiation to the bulk. A tensorial decomposition analysis indicates that the damage-related-radiation has a large isotropic component. While the total radiation may still be dominated by shear, the dynamic dilation associated with the isotropic radiation can significantly reduce the heat generation near the rupture front and produce a pulse-type rupture and rock fragmentation. Numerical simulations confirm these expectations. The damage-related-radiation involves small length scales (proportional to the rupture width) and is therefore difficult to observe in the far field. However, analyses of earthquake waveforms using seismic data recorded close to earthquake ruptures reveal elevated P-wave radiation and appreciable explosive isotropic source terms (e.g., 5-15% of the total event potencies). Geological studies of deeply exhumed faults in dry granulite rocks show evidence of pulverization and delayed heating in earthquake rupture zones consistent with strong dynamic dilation at the rupture fronts. The high potential importance of damage-related-radiation to the local physics of earthquake ruptures motivates additional theoretical, experimental, and observational research.

 

下記のとおり地震研究所談話会を開催いたします。

ご登録いただいたアドレスへ、開催当日午前中にURL・PWDをお送りいたします。
なお、お知らせするzoomURLの二次配布はご遠慮ください。また、著作権の問題が
ありますので、配信される映像・音声の録画、録音を固く禁じます。

記

　　　　日　　時：　令和4年12月9日(金)　午後１時３０分～　
　　　　開催方法：　インターネット　WEB会議

1. 13:30-13:45
   演題：北海道北部の褶曲・衝上断層帯の構造・テクトニクスと地震活動
   著者：○石山達也、松原 誠（防災科学技術研究所）
   要旨：変動地形および地質・反射法の構造解析、地震波トモグラフィ等に基づき、北海道北部に発達する褶曲・衝上断層帯のテクトニクス・構造・地震活動の特徴について議論する。
2. 13:45-14:00
   演題：箱根火山深部で発生した調和型微動とその物理モデル
   著者：○行竹洋平・尾崎智紀・市原美恵
3. 14:00-14:15
   演題：細く貫入しやすい温度プローブの開発と琵琶湖湖底での熱流量測定への応用【所長裁量経費成果報告】
   著者：○山野 誠、後藤秀作（産業技術総合研究所）、濱元栄起（埼玉県環境科学国際センター）
   要旨：海底熱流量測定の精度・能率の向上を目指す、細くセンサー数が多い温度プローブの開発状況と、琵琶湖湖底での湧水活動調査に使用した結果を報告する。

○発表者

※時間は質問時間を含みます。
※既に継続参加をお申し出いただいてる方は、当日zoomURLを自動送信いたします。
※談話会のお知らせが不要な方は下記までご連絡ください。

〒113－0032　東京都文京区弥生1－1－1　
東京大学地震研究所 共同利用担当
E-mail：k-kyodoriyo(at)eri.u-tokyo.ac.jp

※次回の談話会は令和5年1月20日(金)　午後１時３０分～です。

Title: What Causes Seismic Anisotropy at Volcanoes and Geothermal Areas, and can it be used for Monitoring?

 

Abstract:

Seismic anisotropy may reveal the state of stress in the crust, and its temporal changes have been attributed to deformation, seismicity, magmatic activity and geothermal extraction.  We review crustal anisotropy in volcanic and geothermal regions from articles published through the end of 2019.  We provide a database of anisotropy measurements to test hypotheses about the origin of anisotropy and about its utility for monitoring magmatic unrest or geothermal production. The majority of the articles (~100) examined shear-wave splitting, and were about evenly divided between causes related entirely to regional stress, local stress or structure. Delay times (a measure of anisotropy strength) increased with period and with depth in the two sets, but with much scatter.

Time variations in shear wave splitting were examined in 29 studies, but few of these presented statistical tests.  Changes were reported in delay times and fast azimuths, and were mostly reported to vary with the occurrence of eruptions or intrusions, seismicity or tremor rate changes, or deformation changes such as GNSS, tilt or strain measurements There is a clear need for studies that examine statistical relationships between anisotropy and other parameters to test monitoring capabilities.

Title:

The use of Computer Vision for Structural Health Monitoring Applications in Hight Seismic Countries

 

Abstract:

Structural Health Monitoring (SHM) is a process were by continuous monitoring and analysis of its response data it is possible to automatically detect initial damage, identify modal properties or characterize the general behavior of a system. The applications of SHM in a high seismic environment have been successful but challenging. I will present the results of three office buildings, two adobe churches and one wharf structure. In one of the building, damages were clearly identify producing significant changes during and after a severe earthquake, the effect on modal properties as a functions of response amplitude are shown to vary nonlinearly even without the presence of damage. In the wharf structure the effect of the environment is particularly strong due to variations of sea level (tide) and loading, a simple but successful scheme was used, given the operators practical tools for action. One of the main limitations of SHM is the cost of instrumentations and its maintenance.  An option is to complement the standard sensors with massive use of relative low-cost web camaras. The use of videos for motion tracking vibrations is possible. Examples will be presented for laboratory and real structures.