Periodic Slow Slip Events and Their Interactions with Megathrust Earthquakes on Northeast Japan Subduction Zone

Mostafa Khoshmanesh, Jennifer M. Weston, Manoochehr Shirzaei, & Naoki Uchida

Published August 8, 2018, SCEC Contribution #8332, 2018 SCEC Annual Meeting Poster #119

Slow-slip event (SSE), a temporarily accelerated aseismic slip lasting hours to months, changes the stress regime on the fault itself and surrounding areas. On northeast Japan subduction zone (NJSZ), SSEs on the shallow locked zone, which is characterized by velocity-weakening (VW) properties, are suggested to induce major seismic events, including the 2011 Mw9.0 Tohoku earthquake. Moreover, periodic SSEs on the northern area of the 2011 earthquake rupture zone are collocated with elevated seismicity. Here, we model the daily evolution of creep distribution on the NJSZ during the interseismic period of 1996-2003 using GPS and repeating earthquake observations. Our results indicate that episodic SSEs occur annually on the velocity-strengthening area, located downdip of the shallow locked segment, which is confined to depths between 30 and 50 km. We show that the time series of the total seismic moment on the shallow VW segment is correlated with the timing of the downdip periodic SSEs. Also, the majority of Mw5+ earthquakes on this VW segment occurred during the peak Coulomb stress caused by the downdip SSEs. This suggests a causal relationship between the transient and seismic events. Also, the SSEs are accompanied by an increase in the ratio of smaller to larger earthquakes on the VW zone, apparent in the increasing trend of Gutenberg-Richter b-value time series. We suggest that the VW zone of NJSZ is paved with individual asperities, which form localized high frictional strength contact areas of different size, embedded in the low strength creeping areas that often act as seismic barriers. Elevated shear stress on the VW zone imparted by the downdip SSEs may result in two partially coupled processes; 1) failure of smaller asperities, leading to the observed increase in the number of small earthquakes, 2) domino effect due to the efficient interaction between microearthquake ruptures, leading to much bigger seismic events. To further support this hypothesis, we extended the b-value analysis to the year 2011, and show that b-value has an increasing trend before the majority of Mw6+ earthquakes. Our observations and models provide new insight into triggering mechanism of megathrust earthquakes that can improve forecast models.

Khoshmanesh, M., Weston, J. M., Shirzaei, M., & Uchida, N. (2018, 08). Periodic Slow Slip Events and Their Interactions with Megathrust Earthquakes on Northeast Japan Subduction Zone. Poster Presentation at 2018 SCEC Annual Meeting.

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Tectonic Geodesy