Title: Understanding the co-to-post-seismic transition through geodetic observations and slip imagery

Abstract:
Earthquake occurrence is mainly controlled by the spatio-temporal evolution of stresses in the Earth’s crust, primarily caused by the large-scale movement of tectonic plates. However, postseismic transient processes strongly contribute to stress redistribution after an earthquake and to the generation of catastrophic seismic sequences. It is thus critical to better understand them. Our project tackled this critical question by combining geodetic and seismological observations of the phenomena with numerical modelling. The novelty of the project, I will present, lay in the period over which the problem was addressed: the very early postseismic phase (minutes to days) for which few studies had been conducted. Indeed, the existence of suitable data for documenting this transition period between fast and slow slip was limited, and the methodologies to analyze these data and to provide accurate and robust slip imagery also needed to be improved or developed.
I will present a summary of the results we obtained based on our ability to accurately resolve deformation over a wide range of frequencies, amplitudes and spatial scales as well as to analyze and interpret the observations with a new approach to optimize the information contained in the surface observations or kinematic slip inversion from noisy surface ones. Our results on very early postseismic deformation allow for a better understanding of the complex spatial and temporal evolution of the deformation and hopefully by continuing to study this phase, a better assessment of fault slip over a seismic cycle, physical parameters controlling fault slip and ruptures and seismic hazard.

題目：東北日本の地質構造・地殻構造・活構造

要旨:
東北日本では、沈み込み帯巨大地震である2011年3月11日東北太平洋沖地震(M 9.0)の前後の期間に、2011年4月11日福島県浜通りの地震(M 7.0)に代表されるM6-7級の上部地殻内の地震等の活発な地殻活動が上盤側プレートで発生してきた。このような地殻活動については、地殻内流体の寄与を重視するモデルや観測が提示されている。本発表では、現在の地殻活動・テクトニクスを理解する観点から、東北日本の主に中新世以降の構造発達・テクトニクス・地殻構造・活構造についてレビューを行う。これを踏まえて、東北日本の地形・地質構造発達の理解に重要な数万年から100万年スケールの長期間地殻変動および地殻構造について、現状の理解と課題について議論する。

Title: Subduction Initiation along the Macquarie Ridge Complex, Southwest Pacific Ocean

Abstract:

How tectonic plate subduction starts is a first-order question in Earth science. The Macquarie Ridge Complex (MRC), extending 1600 km between New Zealand’s South Island and the Australia-Pacific-Antarctic triple junction, arguably constitutes the global type example for initiating plate subduction. The MRC marks the boundary between oceanic portions of the great Australian and Pacific plates, is situated proximal to the pole of rotation between these two plates, and consists of four ridge and trench/trough segments, of enigmatic alternating polarity, along its strike. The largest intraoceanic earthquake of the 20th Century, a Mw 8.2 event, occurred along the MRC in 1989, and a Mw 8.1 event occurred in 2004. Evidence is accumulating that subduction is initiating along the southern (Hjort) and northern (Puysegur) portions of the MRC, with strain partitioned between transform motion and compression along its entire length. The intervening McDougall and Macquarie segments are ambiguous with respect to subduction initiation. The MRC has the steepest sustained topography of any mountain range on Earth, with vertical relief of ~6,000 m over a horizontal distance of <25 km. Along the Macquarie segment, which includes Macquarie Island, the only subaerial expression of the MRC aside from some nearby islets and rocks, are prominent mass wasting deposits likely associated with the MRC’s high seismicity and steep topography. Such mass wasting is potentially tsunamigenic. Interpretations of marine geophysical data from the Macquarie Ridge segment of the MRC that were acquired on research voyages in 1994, 1996, 2020, and 2021 are the focus of this presentation.

Title: Assessing the Impact of Very Early Postseismic Slip on Fault Slip Budget

Abstract:

Earthquakes are very often, if not always, followed by a relaxation phase, the post-seismic phase. During this time, the fault continues to slip both seismically and aseismically, contributing to the overall earthquake slip budget. Since the intensity of the post-seismic phase is known to decrease exponentially with time, monitoring the very early stage of the post-seismic phase is key to an accurate assessment of the fault slip budget. To access the first hours of the post-seismic phase, we have adopted an innovative processing strategy for the GNSS data to obtain position time series with high temporal resolution (30 seconds) and starting from 5 minutes after the earthquakes. In this talk, I will first give an overview of the results we have obtained from the analysis of the very early stage of the post-seismic phase, with a focus on how this affects the estimation of the fault slip budget. Then I will present the project that motivates my 3-month visit to the ERI.