下記のとおり地震研究所談話会を開催いたしますので、ご案内申し上げます。今回は、コロナウィルス感染対策として、地震研究所の会場での開催は行いません。WEB会議システムを利用した参加のみとなります。参加に必要な設定URL・PWDについては、参加をご希望される方宛に別途ご連絡をいたしますので、共同利用担当宛（k-kyodoriyo@eri.u-tokyo.ac.jp)お問い合わせください。

なお、お知らせする設定URL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３年９月１０日（金）午後１時３０分～　インターネット　WEB会議

1. 13:30-13:45

演題：父島空振記録から推定する福徳岡ノ場2021年8月噴火に伴う浅海爆発過程

著者：○市原美恵・西田　究・金子隆之

2. 13:45-14:00

演題：Rheology of granular materials; a frictional approach for a liquid-like material

著者：○Franco TAPIA･Mie ICHIHARA、 Elisabeth GUAZZELLI (Universite de Paris-Diderot)、

Olivier POULIQUEN (IUSTI, Aix-Marseille Universite)

要旨: Short overview of how local-frictional granular rheology is an interesting tool to the understanding of heterogeneous flows.

3. 14:00-14:15

演題：日本海溝・千島海溝海側における熱流量分布の特徴：海洋地殻内の流体循環による熱輸送

 【所長裁量経費成果報告】

著者：○山野　誠・木下正高、川田佳史（海洋研究開発機構）、佐々木肯太（構造計画研究所）

要旨: 日本海溝と千島海溝のアウターライズにおける熱流量分布の特徴を比較する。また、日本海溝海側の正断層近傍での熱流量測定の結果を報告する。

4. 14:15-14:30

演題：Petrophysical and hydrogeological study of sediment inputs to subduction zones

著者: 〇Jade DUTILLEUL、Yves GERAUD･Sylvain BOURLANGE (University of Lorraine)

要旨: Examples of relationships between earthquake and tsunami risks and petrophysical and hydrogeological properties of input sedimentary sections at the North Sumatra and Hikurangi margins, from core and logging data of the International Ocean Discovery Program.

○発表者

※時間は質問時間を含みます。

※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研究支援チーム

E-mail：k-kyodoriyo@eri.u-tokyo.ac.jp

※次回の談話会は令和３年１０月２２日（金）午後１時３０分～です。

Lithosphere dynamics and deformation of the central North China Craton: Constraints from new seismic images

Abstract:
The North China Craton, which experienced distinct diachronous lithospheric thinning in the late Mesozoic and Cenozoic, provides a classic example of lithospheric reactivation to mantle upwelling in the world. In this talk, I will focus on the central North China Craton, which is a transition zone between the relatively stable Ordos Block and the Huabei Basin suffering from cratonic destruction. Firstly, I will talk about the techniques we used in imaging the crust and mantle structure, such as ambient noise tomography, array-based two plane surface wave tomography, and joint inversion of surface wave and receiver function. Then I will present the new seismic images derived from ambient noise and earthquake data, which help understanding the continental rifting and lithospheric modification in this region. Specifically, different rifting mechanisms between the southern and northern segments of the Fenhe-Weihe Rift is revealed by our isotropic and anisotropic images.

Deciphering subduction polarity reversal from the Cretaceous Andaman Ophiolite, SE Asia

Abstract:
Switching of subduction zone polarity (subduction polarity reversal) is believed to be a key mechanism to initiate new subduction zone system, however inferring it from rock record is challenging. Ophiolites are widely studied to unravel the evolutionary history of intra-oceanic subduction, from the juvenile stages to subduction maturation. Here we present an integrated geochemical and geochronological synthesis of various rock types of Andaman Ophiolite to evaluate the feasibility of a previously speculated process of subduction initiation via polarity reversal when the former (Woyla) arc collided with the Sundaland continent. The Andaman Ophiolite Suite is exposed on the Andaman and Nicobar Islands in the outer arc ridge of the Sunda trench of the India-Eurasia subduction system. The Ophiolite Suite is made up of incoherent Penrose sequence including contrasting crustal (especially volcanic) and mantle rocks along with underlying metamorphic sole and mélange, and an overlying Palaeogene and younger forearc sedimentary sequence. Detailed field observation indicated that the existing geochemical and geochronological results on the Andaman Ophiolite and its metamorphic sole are difficult to reconcile in a single tectonic setting but may rather record different stages in a longer evolution. They may contain crustal and mantle rocks that formed during juvenile stages of intra-oceanic subduction, modifying the pre-existing oceanic lithosphere within which subduction started, and in which a magmatic arc formed upon subduction maturation. To this end, we provide new, and review existing geochemical as well as geochronological constraints on the formation of its crustal rocks, as well as the evolution of its mantle portion. We identify mafic magmatic rocks and cogenetic plagiogranites that are consistent with formation in a magmatic arc, whereas other magmatic rocks, as well as metamorphic sole protoliths, have characteristics indicative of a back-arc origin. Three new, and two previous zircon U/Pb ages of arc magmatic rocks give a 99–93 Ma age range, but we also identify an inherited ~105 Ma age. This latter age coincides with Ar/Ar cooling ages of the Andaman metamorphic sole, and with a plagioclase xenocryst age from recent Barren Island volcanics east of Andaman. The geochemical and geochronological constraints of the Andaman ophiolites are straightforwardly explained in the context of the regional kinematic history: (1) The original lithosphere formed in the back-arc basin of the Woyla intra-oceanic arc that collided in the mid-Cretaceous with Sumatra; (2) Subduction initiation and SSZ ophiolite formation within this basin occurred around or slightly before 105 Ma; (3) This was followed by arc magmatism between 99 and 93 Ma upon subduction maturation. This study thus directly contradicts the previously inferred ~95 Ma age of the Andaman ophiolite and the concurrent subduction initiation as well as provides new insights into the possible petrological manifestation of subduction polarity reversal scenario.

References:

1.  Bandyopadhyay, D., Ghosh, B., Guilmette, C., Plunder, A., Corfu, F., Advokaat, E.L., Bandopadhyay, P.C., van Hinsbergen, D.J.J. (2021) Geochemical and geochronological record of the Andaman Ophiolite, SE Asia: From back-arc to forearc during subduction polarity reversal?, Lithos, vol-380-381, 105853. [DOI: 10.1016/j.lithos.2020.105853]
2. Plunder, A., Bandyopadhyay, D., Ganerød, M., Advokaat, E.L., Ghosh, B., Bandopadhyay, P.C., van Hinsbergen, D.J.J. (2020) History of subduction polarity reversal during arc-continent collision: constraints from the Andaman Ophiolite and its metamorphic sole, Tectonics, vol-39, e2019TC005762, [DOI: 10.1029/2019tc005762]
3. Bandyopadhyay, D., van Hinsbergen, D. J. J., Plunder, A., Bandopadhyay, P. C., Advokaat, E., Chattopadhaya, S., Morishita, T., and Ghosh, B. (2020) Andaman Ophiolite: An Overview. In: Ray, J. S.and Radhakrishna, M. (eds.), The Andaman Islands and Adjoining Offshore: Geology, Tectonics and Palaeoclimate, Springer International Publishing, Cham, pp.1-17. [DOI: 10.1007/978-3-030-39843-9_1]

Blending Physics-Based numerical simulations and seismic databases using Generative Adversarial Networks (GAN) for Earthquake Engineering and Structural Health Monitoring

Abstract:
A new strategy to blend the outcome of physics-based numerical simulations with massive seismic databases is proposed, in order to improve earthquake simulation and structural health monitoring. The approach relies on a set of adversarial learning techniques with a threefold purpose: (1) finding a reduced-dimensional non-linear representation of both synthetic and experimental data; (2) training a stochastic generator of fake experimental responses conditioned by the physics-based simulation results; (3) classify the damage class based on the structural response and predict the damaged response. This methodology is applied to earthquake ground motion prediction and structural damage assessment. Regional three-dimensional high-fidelity numerical models accounting for both extended sources and complex geology are still limited to a low-frequency range. Moreover, they are prone to significant uncertainties induced by a lack of data on small scale geological structures and rupture processes. Databases of broadband seismic signals recorded worldwide at seismological networks are used to retrieve some pieces of information on these small-scale data to generate realistic broadband signals from synthetic ones. On the other hand, given the structural response of an undamaged structure, engineers need to predict the eventual damage beforehand, based on a monitoring network. The proposed tool demonstrates outstanding performances in encoding seismic signals, together with efficient generation capabilities and cluster capabilities, provided that the physics-based results carry enough information to properly condition the stochastic generator and classify the damage state. In addition, the proposed method, fed only with raw data from both databases and numerical models, outperforms other random signal generators based on pre-existing expertise such as prescribed spectra and more or less complex phenomenological models.

Title: Segmentation and supercycles: A catalog of earthquake rupture patterns from well-studied faults worldwide

Abstract:
After more than 100 years of earthquake research, earthquake forecasting, which relies on knowledge of past fault rupture patterns, has become the foundation for societal defense against seismic natural disasters.  A concept that has come into focus more recently is that rupture segmentation and cyclicity can be complex, and that a characteristic earthquake model is too simple to adequately describe much of fault behavior.  Nevertheless, recognizable patterns in earthquake recurrence emerge from long, high resolution, spatially distributed chronologies.  Researchers now seek to discover the maximum, minimum, and typical rupture areas; the distribution, variability, and spatial applicability of recurrence intervals; and patterns of earthquake clustering in space and time.  The term “supercycle” has been used to describe repeating longer periods of elastic strain accumulation and release that involve multiple fault ruptures.  However, this term has become very broadly applied, lumping together several distinct phenomena that likely have disparate underlying causes.  I divide earthquake cycle behavior into four major classes that have different implications for seismic hazard and fault mechanics: 1) quasi-periodic similar ruptures, 2) clustered similar ruptures, 3) clustered complementary ruptures / rupture cascades, and 4) superimposed cycles.  “Segmentation” is likewise an ambiguous term; we identify “master segments” and “asperities” as defined by barriers to fault rupture.  These barriers may be persistent (rarely or never traversed), frequent (occasionally traversed), or ephemeral (changing location from cycle to cycle).  I present a catalog of the historical and paleoseismic evidence that currently exists for each of these types of behavior on major well-studied faults worldwide.  Due to the unique level of paleoseismic and paleogeodetic detail provided by the coral microatoll technique, the Sumatran Sunda megathrust provides one of the most complete records over multiple earthquake rupture cycles.  Long historical records of earthquakes along the South American and Japanese subduction zones are also vital contributors to our catalog, along with additional data compiled from subduction zones in Cascadia, Alaska, and Middle America, as well as the North Anatolian and Dead Sea strike-slip faults in the Middle East.  This compilation reveals that persistent and frequent barriers, rupture cascades, superimposed cycles, and quasi-periodic similar ruptures are common features of most major faults.  Clustered similar ruptures do not appear to be common, but broad overlap zones between neighboring segments do occur. Barrier regions accommodate slip through reduced interseismic coupling, slow slip events, and/or smaller more localized ruptures, and are frequently associated with structural features such as subducting seafloor relief or fault trace discontinuities.  This catalog of observations provides a basis for exploring and modeling root causes of rupture segmentation and cycle behavior. We expect that researchers will recognize similar behavior styles on other major faults around the world.