Title:  Periodic seismicity: from the laboratory to Nepal

Abstract:
Small transient stress perturbations are prone to trigger earthquakes and a better understanding of the dynamics of earthquake triggering by transient stress perturbations is essential in order to improve our understanding of earthquake physics and our consideration of seismic hazard. In the Earth’s crust, these transient stress changes can be caused by various sources (passing of seismic waves, forcing by tides, hydrological load, and other seasonal climatic loads).
At the laboratory scale, we describe an experimental fault system, which mimics the natural seismic cycle, at the laboratory scale. Periodic perturbations of stresses (of variable amplitude and frequency) are superimposed to a far field loading during the frictional shearing of a granular gouge. The experimental system is instrumented so that acoustic emissions (AEs), deformations and stresses, can be continuously recorded during the experiments. Experimental results show that both background seismicity and modulation increase with loading rate, increasing perturbation amplitude and period. The magnitude-frequency distribution of AEs is also computed and we show that the Gutenberg-Richter b-value oscillates with stress oscillations.
At the field scale, Nepalese seismicity has been reported as having seasonal variations. We here re-investigate this seasonal nature by analyzing Nepalese seismicity over a period of 20 years, as detected by the Nepalese national network, restricting ourselves to earthquakes located along the eastern and central sections of the Nepalese Main Himalayan Thrust.  In the light of our experimental observation of b-value dependency to transient stressing, we investigate potential Gutenberg-Richter b-value annual variations. We use GRACE and GRACE-FO large-scale mass distribution data to resolve the stress tensor variations generated at depth by transient surface loads and find that that mean annual b-value peaks when Coulomb stress reaches its recurring lowest value during the summer monsoon, suggesting a periodic clamping of the Main Himalayan Thrust.
Our observations may help complement our understanding of the influence of low inertia stress phenomena on the distribution of seismicity, such as observations of dynamic triggering and seismicity modulation by tides or hydrological loading.

題目：新学術分野「バイブロニクス」の地球物理学への展開

要旨：
「フォノン」は、原子が規則正しく並ぶ結晶における弾性波や熱キャリアである。そして、結晶で成立するフォノン輸送に関する学問はフォノニクスとして深く研究されてきた。近年の複雑化、複合化した材料・デバイスにおける熱マネジメントの重要性から、フォノンのみならず、多様な固体および界面におけるエネルギー輸送についての包括的な学術の確立が必要である。
無秩序系、メタ次元構造、高分子、界面など、複雑かつ多様な系や非従来の次元性をもつ構造における振動の伝播とエネルギー輸送を包括的に扱う学術分野「バイブロニクス」を確立する。本分野の構築により、統一的かつ有機的に固体におけるエネルギー輸送の記述が可能になる。物理、化学、材料工学、デバイス工学など、理学と工学の多様な分野の研究者を集め、フォノンエンジニアリングを拡張することで、量子技術、先端半導体デバイス、熱機能材料・デバイスなどへの応用展開の基盤となる学術分野を開拓する。
今回のセミナーでは、バイブロニクスについてコンセプトをご紹介し、地震波をバイブロンと捉えることで、本分野を地球物理学にも展開できないか議論させて頂ければ幸いである。

Title:  Physics-based modeling of earthquakes: fault geometry and fluid flow

Abstract:
As an earthquake modeler, I have focused on the effect of geometric complexity of the fault and fluid flow as a fundamental component of the earthquake generation process. I will present two projects in this seminar. First, by using an efficient and flexible computational method for solving earthquake sequences, I will show that a complex fault network can explain various characteristics of aftershocks. Second, I propose a new approach to calculate the fluid pressure and shear stress along the megathrust by considering fluid flow and dehydration reactions. In this model, near-lithostatic fluid pressure is achieved everywhere below the seismogenic zone.
I will then talk about my research plans for the next few years. I am developing a coupled earthquake cycle and fluid flow code that relaxes several assumptions in the previous work. This will allow us to better understand (1) earthquake swarms, such as in the Noto Peninsula, and (2) the mechanical/hydrological interaction between the megathrust (slow and fast) earthquakes and the intraplate earthquakes. I also discuss what kind of data is needed to inform/verify the models.