Title: Resolving the scattering and intrinsic attenuation structure of Southern California

Abstract:

Characterizing scattering and absorbing properties and the power spectrum of crustal heterogeneity is a fundamental problem for informing strong ground motion estimates at high frequencies, where scattering and attenuation effects are critical. We perform a comprehensive study of local earthquake coda waves in Southern California to constrain scattering and intrinsic attenuation structures. We analyze data from 1195 spatially distributed earthquakes from 1981 to 2013 at source depths of 10 to 15 km and epicentral distances from 0 to 250 km with magnitudes larger than 1.8. We stack envelope functions from 28,127 vertical component and 27,521 transverse component seismograms, filtered from 2 to 4 Hz. We model these observations using a particle-based Monte Carlo algorithm that includes intrinsic attenuation as well as both P and S wave scattering and both single and multiple scattering events. We find that spatially averaged coda wave behavior for Southern California can be explained only with models containing an increase in scattering strength and intrinsic attenuation within the uppermost crust, i.e., they are poorly fit with half-space models of constant scattering strength. A reasonable fit to our data is obtained with a two-layer model, composed of a shallow crustal layer with strong wide-angle scattering and high P and S intrinsic attenuation and a deeper layer with weaker scattering and lower intrinsic attenuation (top 5.5 km: αQf=250, βQf=125, heterogeneity correlation length a = 50 m, fractional velocity heterogeneity ε = 0.4; lower crust: αQf=900, βQf=400, a = 2 km, ε = 0.05).

題目：いわき地域周辺の電気比抵抗構造

要旨:
福島県いわき地域では、2011年3月11日の東北太平洋沖地震の後、活発な地震活動が起きており、2011年4月11日には福島県浜通りを震源とするマグニチュード7.0の地震が発生した。地震の発生に地殻内の流体が重要な役割を果たしていることが明らかになってきており、地殻流体の存在に敏感である電気比抵抗に基づいた地殻構造推定がいわき地域周辺で実施されてきた。その結果、4月11日の浜通りの地震の震源域周辺では、地震の多くが高比抵抗域で起きていること、震源直下には低比抵抗域が存在することが明らかになっている。本発表では、はじめに電気比抵抗の特性と比抵抗構造探査手法を概説した上で、いわき地域周辺で明らかになっている電気比抵抗構造をレビューする。2011年4月11日の地震の震源周辺のローカルな比抵抗構造に加えて、いわき市から新潟市に至る島弧横断測線の比抵抗構造およびその南の那須岳、高原山を含むエリアの3次元比抵抗構造についても紹介する。

Title: Earthquake swarms and magma transport in the mantle sill complex beneath Hawai‘i

Abstract:

The deep magmatic architecture of the Hawaiian volcanic system is central to understanding the transport of magma from the upper mantle to the individual volcanoes. We leverage advances in earthquake monitoring with deep learning algorithms to image the structures underlying a major mantle earthquake swarm of nearly 200,000 events that rapidly accelerated after the 2018 Kīlauea caldera collapse. At depths of 36 to 43 kilometers, we resolve a 15-km-long collection of near-horizontal sheeted structures that we identify as a sill complex. These sills connect to the lower depths of Kīlauea’s plumbing by a 25-kilometers-long belt of seismicity. These findings implicate the mantle sill complex as a nexus for magma transport beneath Hawai‘i and furthermore indicate widespread magmatic connectivity in the volcanic system. The sill complex further presents an opportunity to elucidate interactions between stress and magma transport processes in the mantle. We invert for full moment tensors of sill earthquakes and identify predominantly shear mechanisms with persistent tensile faulting components. Slip occurs in-plane with the sill structures. Pressure axes are radially oriented about a point near Mauna Loa, consistent with a stress field generated by a flexural load. Together, these observations suggest that magma flux through the sill structures generates seismicity by increasing pore pressure and promoting slip. Our results suggest that stress changes in mantle structures may enable fluctuations in magma supply rates to the surface over short timescales.

Title: Improving hazard resilience by high-resolution 3D seismic tomography of subduction zones in Japan and New Zealand

Abstract:

Resolving subduction zone structure in three-dimensions, at high-resolution, and at scales comparable to the rupture area of the largest earthquakes represents a significant scientific challenge, but is important for understanding the impact of crustal scale architecture on seismic and tsunami hazard at subduction zones. In this presentation, I will describe two projects where we have overcome this challenge by harnessing three-decades of marine-seismic data to obtain high-resolution, regional constraints on the crustal structure of Nankai Trough, SW Japan and the Hikurangi margin, New Zealand. In both settings I will describe how crustal-scale architecture may be impacting the dimensions and slip-behavior of the seismogenic zone. I will conclude by describing new research involving the application of these technique to the NE Japan trench.

題目：内陸地震と水を考える ‐いわき地域の地震活動を例として‐