Abstract
Postseismic Global Navigation Satellite System (GNSS) time series followed by megathrust earthquakes can be interpreted as a result of afterslip on the plate interface, especially in its early phase. Afterslip is a stress release process accumulated by adjacent coseismic slip and can be considered a recovery process for future events during earthquake cycles. Spatio-temporal evolution of afterslip often triggers subsequent earthquakes through stress perturbation. Therefore, it is important to quantitatively capture the spatio-temporal evolution of afterslip and related postseismic crustal deformation and to predict their future evolution with a physics-based simulation. We developed an adjoint data assimilation method, which directly assimilates GNSS time series into a physics-based model to optimize the frictional parameters that control the slip behavior on the fault. The developed method was validated with synthetic data. Through the optimization of frictional parameters, the spatial distributions of afterslip could roughly (but not in detail) be reproduced if the observation noise was included. The optimization of frictional parameters reproduced not only the postseismic displacements used for the assimilation, but also improved the prediction skill of the following time series. Then, we applied the developed method to the observed GNSS time series for the first 15 days following the 2003 Tokachi-oki earthquake. The frictional parameters in the afterslip regions were optimized to A–B ~ O(10 kPa), A ~ O(100 kPa), and L ~ O(10 mm). A large afterslip is inferred on the shallower side of the coseismic slip area. The optimized frictional parameters quantitatively predicted the postseismic GNSS time series for the following 15 days. These characteristics can also be detected if the simulation variables can be simultaneously optimized. The developed data assimilation method, which can be directly applied to GNSS time series following megathrust earthquakes, is an effective quantitative evaluation method for assessing risks of subsequent earthquakes and for monitoring the recovery process of megathrust earthquakes.
Highlights
Large megathrust earthquakes have been recurrently observed in many subduction zones (Ando 1975)
For year-long triggering, Uchida et al (2009) showed that the 2004 M 7.1 Kushiro-oki earthquake, located ~ 100 km east of the 2003 Tokachi-oki earthquake, could be advanced by the eastward propagation of the afterslip of the Tokachi-oki earthquake. These examples show that the triggering of subsequent earthquakes due to the spatio-temporal evolution of afterslip can often occur, whether the stress perturbations of afterslip can directly trigger subsequent earthquakes highly depends on the amplitude of the perturbation and the level of accumulated stress on possible source regions
2003 Tokachi‐oki earthquake and the postseismic Global Navigation Satellite System (GNSS) observations This study focuses on the postseismic deformation following the 2003 Tokachi-oki earthquake, which was a megathrust earthquake of M 8.0 that occurred on September 25, 2003 (UT) off the Tokachi region in northeastern Japan
Summary
Introduction Large megathrust earthquakes have been recurrently observed in many subduction zones (Ando 1975). Stress release due to afterslip further yields a stress perturbation in the surrounding areas that can be inferred as spatio-temporal evolution or propagation of afterslip Such stress perturbations due to afterslip sometimes trigger subsequent earthquakes in different time scales (Murakami et al 2006; Pollitz et al 2006; Miyazaki and Larson 2008; Uchida et al 2009; Ohta et al 2012). For year-long triggering, Uchida et al (2009) showed that the 2004 M 7.1 Kushiro-oki earthquake, located ~ 100 km east of the 2003 Tokachi-oki earthquake, could be advanced by the eastward propagation of the afterslip of the Tokachi-oki earthquake These examples show that the triggering of subsequent earthquakes due to the spatio-temporal evolution of afterslip can often occur, whether the stress perturbations of afterslip can directly trigger subsequent earthquakes highly depends on the amplitude of the perturbation and the level of accumulated stress on possible source regions
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