5:15 PM - 6:30 PM
[SSS11-P10] Determination of 3-D subsurface structure model in central Tottori, Japan
Keywords:central Tottori prefecture, 3-D subsurface structure model, gravity survey, microtremor survey, seismic observation
In central Tottori prefecture, there was the 2016 earthquake in central Tottori prefecture. This earthquake caused strong ground motion over a wide area and caused damage to buildings. Information on the underground structure is required to examine the causes of damage caused by this earthquake and evaluate the ground motion. In this area, microtremor, gravity observations, and temporary aftershock (strong ground motion) observations were carried out in central Tottori prefecture. Based on these data, an underground structure model has been obtained. In this study, new gravity observations were made mainly in the Hojo area in central Tottori, where many damages occurred. The underground structure was estimated by extending the underground structure model based on microtremor and aftershock observations to a two-dimensional or three-dimensional model using the new and the existing gravity data.
The analysis is as follows. First, a three-dimensional analysis was performed using a two-layer density structure model. The densities of the surface layer and the base layer of the two-layer model were set to 2.2g/cm^3 and 2.5g/cm^3, respectively. 8-seismic observation points with underground velocity structure models and 2-outcrop points of the basement were set as points to determine the depth to the basement. The depth to the basement controlled at the seismic station was the depth to the upper surface of the layer corresponding to Vs = 1700 m/s to 2200 m/s. As a result, it was possible to grasp the distribution of the basement surface in the target area, and it was found that the depth of the basement was 640 m at the deepest part.
Next, we assumed a three-layer density structure model and performed a two-dimensional analysis using gravity anomalies. The density of each layer was set to 2.0 g/cm^3 for the first layer, 2.3g/cm^3 for the second layer, and 2.5g/cm^3 for the third layer. In the velocity structure model, the first layer corresponds to Vs = 100 to 500 m / s layer, the second layer corresponds to Vs = 550 to 150m/s layer, and the third layer corresponds to Vs = 1700 to 3000m/s. The depth of the boundary between the first and second layers is based on the results of microtremor observation, and the depth of the boundary between the second and third layers is based on the results of three-dimensional analysis of the two-layer model. Under these conditions, the density structure model was estimated by trial and error to explain the gravity anomaly. As a result, the undulations of the layer boundaries of each layer could be grasped, and there were areas where the base shapes of the first and second layers were significantly different.
In the future, we plan to try a method of integrating the model of tremor and seismic motion and the model of gravity anomaly and expanding it to a three-dimensional structure. Furthermore, we will investigate the relationship between the undulations of the base surface of the obtained base structure and the damage distribution caused by the earthquake.
The analysis is as follows. First, a three-dimensional analysis was performed using a two-layer density structure model. The densities of the surface layer and the base layer of the two-layer model were set to 2.2g/cm^3 and 2.5g/cm^3, respectively. 8-seismic observation points with underground velocity structure models and 2-outcrop points of the basement were set as points to determine the depth to the basement. The depth to the basement controlled at the seismic station was the depth to the upper surface of the layer corresponding to Vs = 1700 m/s to 2200 m/s. As a result, it was possible to grasp the distribution of the basement surface in the target area, and it was found that the depth of the basement was 640 m at the deepest part.
Next, we assumed a three-layer density structure model and performed a two-dimensional analysis using gravity anomalies. The density of each layer was set to 2.0 g/cm^3 for the first layer, 2.3g/cm^3 for the second layer, and 2.5g/cm^3 for the third layer. In the velocity structure model, the first layer corresponds to Vs = 100 to 500 m / s layer, the second layer corresponds to Vs = 550 to 150m/s layer, and the third layer corresponds to Vs = 1700 to 3000m/s. The depth of the boundary between the first and second layers is based on the results of microtremor observation, and the depth of the boundary between the second and third layers is based on the results of three-dimensional analysis of the two-layer model. Under these conditions, the density structure model was estimated by trial and error to explain the gravity anomaly. As a result, the undulations of the layer boundaries of each layer could be grasped, and there were areas where the base shapes of the first and second layers were significantly different.
In the future, we plan to try a method of integrating the model of tremor and seismic motion and the model of gravity anomaly and expanding it to a three-dimensional structure. Furthermore, we will investigate the relationship between the undulations of the base surface of the obtained base structure and the damage distribution caused by the earthquake.