Dr. Mathilde Raduguet, a researcher from Institut des Sciences de la Terre, Grenoble, France who is visiting MIT EAPS as a Fullbright Scholar, presents "Investigating short-term transient deformations in subduction zones with two examples: early post-seismic deformation following Tohoku (Japan) and Slow Slip Events in Mexico" as part of the EAPS Seminar in Geophysics.

"I will present two recent works that focus on the study of small-scale transient deformations using (mainly) GNSS time series, and what constraints these studies can provide on the mechanical behavior of subduction zones interfaces. The first example concerns the analysis of the early post-seismic phase following the Mw9 2011 Tohoku earthquake. Constraining the deformation during the early postseismic period, typically a few minutes to a few hours/days after a major event can bring strong constraints on the postseismic mechanisms. This period is characterized by high deformation rate, yet standard daily GNSS processing cannot capture it. In this study, we use one month of high-rate (30s) GNSS time series to investigate the deformation at short time scale following Tohoku. Our result show that the temporal evolution of the early postseismic signal near the Tohoku mainshock can be explained by an Omori-like Transient Brittle Creep mechanism with a p-value around 0.75. This temporal evolution differs from the rate-strengthening afterslip for which p=1. A secondary afterslip zone, located South of the main one close to the Ibaraki-Oki aftershock, has a distinct temporal evolution with a p value around 1. To interpret these p-values, we develop a simple numerical model that qualitatively reproduced p values lower than one. The second example focusses on the tracking of small slow slip events in Mexico using seismic tremors. We use a tectonic tremor catalog to investigate, over a 10-year-period (2009-2019), how tremor activity can help to characterize the dynamic of transient events over shorter time scales than accessible with GNSS data only. Decomposing the daily GNSS time series relative to tremor burst time, we identify short-term SSEs, lasting ~8-9 days, in the region of Guerrero (where they had been observed previously), but also for the first time in the Oaxaca region. We also show that during the long-term SSEs, most (80%) of the slip occurs at tremor time during bursts of slip. These results confirm the observations previously made in Guerrero using LFE catalogs in 2005-2007. Our investigation over a longer time period, and larger spatial extend (including Oaxaca), precises the detailed spatio-temporal characteristics of transient events in Mexico."

Seminar in Geophysics 12.571 is a class for EAPS graduate students in geophysics. All members of the MIT Community are welcome to join for presentations by guest speakers which will take place approximately every two weeks during the spring semester on Tuesday mornings 10am in 54-209 and on Zoom.