5:15 PM - 6:45 PM
[SGD02-P14] Numerical calculation of ground gravity changes due to postseismic deformation after the 2011 Tohoku earthquake
Keywords:Postseismic deformation, Viscoelastic deformation, Gravity change, The 2011 Tohoku earthquake
After great earthquakes, postseismic displacements and gravity changes are often observed due to afterslip, viscoelastic relaxation, and pore-elastic rebound. In the case of the 2011 earthquake off the Pacific coast of Tohoku (hereafter referred to as the Tohoku earthquake), the postseismic horizontal displacement of up to 120 cm was observed at the Tohoku region in 5 years after the main shock (Suito, 2017), and the postseismic gravity change of up to 40 microGal was also observed in 3 years (Okubo et al., 2017). Many studies have modeled physical processes of the postseismic deformation after the Tohoku earthquake (e.g., Sun et al., 2014; Yamagiwa et al., 2015; Fujiwara et al., 2022). However, most of these postseismic models have been obtained using the postseismic displacement data only, so they may not sufficiently explain the observed gravity changes. Therefore, the reproducibility of the postseismic gravity data should be discussed for the postseismic models, and a new model may need to be constructed in the future so as to consider the postseismic gravity changes as well as the postseismic displacements.
A few previous studies have discussed the postseismic ground gravity changes following the Tohoku earthquake. Takagi (2018) showed that the long-term absolute gravity changes observed after June 2014 (Okubo et al., 2017) can be explained by viscoelastic deformation of the Maxwell viscoelastic body. In his viscoelastic model, he set the asthenospheric viscosity to 5 * 1018 Pa s, which was determined by trial-and-error fitting to the observed ground gravity data. Tamura et al. (2023) derived the continuous time series of postseismic gravity change from the superconducting gravity data obtained at NAOJ Mizusawa in and after 2014. They also inferred the viscosity to be 2 * 1018 Pa s, by comparing the observed superconducting gravity change with the postseismic gravity change calculated in Takagi (2018).
These studies showed in common that the postseismic gravity changes of the Tohoku earthquake in and after 2014 can be attributed to the viscoelastic deformation of the Maxwell viscoelastic body, but there are still some unexplained questions about postseismic gravity changes. For example, Takagi (2018) did not reproduce the observed gravity changes before 2014 using his viscoelastic model accurately, probably because physical mechanisms behind the ground gravity changes during this period are not quantitatively clarified. Afterslip can complicate the time series of the postseismic gravity change in a few years after the Tohoku earthquake; in fact, Takagi (2018) showed that the sign of the gravity change caused by afterslip may become different from that caused by viscoelastic deformation. In addition, the viscosity value inferred by Tamura et al. (2023) was 2.5 times smaller than that determined by Takagi (2018), so an appropriate viscoelastic structure should be modeled to explain the multiple gravity time series.
Therefore, we are motivated to construct a postseismic deformation model to explain the ground gravity changes observed after the Tohoku earthquake. We here calculate the postseismic ground gravity changes numerically using the software developed by Zhou and Wang (2023), which considers self-gravitation, stratification, and surface sphericity. In this calculation, we test several patterns of viscoelastic structures and coseismic slip distributions to assess their effects for the resultant postseismic gravity changes. We then compare the calculated gravity changes with the absolute gravity data obtained at five observation points (Mizusawa, Esashi, Sendai, Honjo, and Tsukubane) in the Tohoku and Kanto regions (Takagi, 2018; Tamura et al., 2023). In this presentation, we will show some numerical results of gravity changes, and discuss which mechanisms contributed to the postseismic gravity changes in the case of the Tohoku earthquake.
A few previous studies have discussed the postseismic ground gravity changes following the Tohoku earthquake. Takagi (2018) showed that the long-term absolute gravity changes observed after June 2014 (Okubo et al., 2017) can be explained by viscoelastic deformation of the Maxwell viscoelastic body. In his viscoelastic model, he set the asthenospheric viscosity to 5 * 1018 Pa s, which was determined by trial-and-error fitting to the observed ground gravity data. Tamura et al. (2023) derived the continuous time series of postseismic gravity change from the superconducting gravity data obtained at NAOJ Mizusawa in and after 2014. They also inferred the viscosity to be 2 * 1018 Pa s, by comparing the observed superconducting gravity change with the postseismic gravity change calculated in Takagi (2018).
These studies showed in common that the postseismic gravity changes of the Tohoku earthquake in and after 2014 can be attributed to the viscoelastic deformation of the Maxwell viscoelastic body, but there are still some unexplained questions about postseismic gravity changes. For example, Takagi (2018) did not reproduce the observed gravity changes before 2014 using his viscoelastic model accurately, probably because physical mechanisms behind the ground gravity changes during this period are not quantitatively clarified. Afterslip can complicate the time series of the postseismic gravity change in a few years after the Tohoku earthquake; in fact, Takagi (2018) showed that the sign of the gravity change caused by afterslip may become different from that caused by viscoelastic deformation. In addition, the viscosity value inferred by Tamura et al. (2023) was 2.5 times smaller than that determined by Takagi (2018), so an appropriate viscoelastic structure should be modeled to explain the multiple gravity time series.
Therefore, we are motivated to construct a postseismic deformation model to explain the ground gravity changes observed after the Tohoku earthquake. We here calculate the postseismic ground gravity changes numerically using the software developed by Zhou and Wang (2023), which considers self-gravitation, stratification, and surface sphericity. In this calculation, we test several patterns of viscoelastic structures and coseismic slip distributions to assess their effects for the resultant postseismic gravity changes. We then compare the calculated gravity changes with the absolute gravity data obtained at five observation points (Mizusawa, Esashi, Sendai, Honjo, and Tsukubane) in the Tohoku and Kanto regions (Takagi, 2018; Tamura et al., 2023). In this presentation, we will show some numerical results of gravity changes, and discuss which mechanisms contributed to the postseismic gravity changes in the case of the Tohoku earthquake.