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.

Title: Mineral dehydrations at subduction zones conditions – experimental evidence of tectonic tremors and high Vp/Vs ratio

Abstract:
Since their discovery (Obara 2002), the mechanics driving the generation of tectonic tremors has been debated. In hot subduction zones, tectonic tremors generally occur in relation with zones where large Vp/Vs ratios have been observed, in such a way that the presence of fluids (and fluid diffusion) is often invoked as an important aspect of their generation mechanisms. Yet, and to this day, tremors have never reproduced in the laboratory at subduction zones P–T conditions, in such way that their origin remains amongst the most elusive seismic phenomena observed in subduction zones.
Here, we employ a new High Pressure – High Temperature (HPT) experimental device designed to replicate the conditions typical of tremor-generating environments. Utilizing mixtures of antigorite and olivine as an analogue for the water-rich lithologies prevalent in subduction zones, we conducted experiments under hydrostatic (no deformation) pressure conditions (from 1.5 to 3.0 GPa) and temperatures between 750 and 800 °C, i.e. following a P-T path typical of hot subduction zones. During these experiments, we monitor the microseismicity (Acoustic Emissions- AEs), and observe a clear transition between earthquake-like AEs (below 1GPa and 500°C) and microseismic signals that are reminiscent of natural tremors. These tremor-like AEs follow both the Gutenberg-Richter relationship and a linear scaling between moment and duration, while their frequency x size ratio scales with that of natural tremors. Importantly, these signals, triggered upon mineral dehydration, are also continuously observed in the absence of a fluid phase, which is evidence that these tremor-like seismic waves are emitted by the solid phase deforming viscously, rather than by the percolating fluid phase.
Using the same apparatus, we also performed experiments where the evolution of P-wave velocities was monitored during pure antigorite dehydration. In these experiments, no tremor-like signal were recorded, while dehydration was accompanied by an important decrease in Vp. The softening of elastic properties upon dehydration can be related to fracturing processes at grain scale generated by water release, which supports the idea that dehydration stress transfer may be a reasonable model for both tremor and intermediate depth earthquake triggering. Finally, combining our experimental data and thermodynamic databases, we compute the elastic properties of dehydrating mineral assemblages and predict that, at subduction zone conditions, mineral dehydration signature must indeed that of a low Vp, but a large Vp/Vs ratio, which is compatible with seismological observations performed on subduction zones around the world.