Microtremor array surveys and development of the velocity model in the Hakodate Plain, Hokkaido, Japan

The Hakodate Plain in the southern part of the Oshima Peninsula, Hokkaido, Japan, is a sedimentary basin surrounded by mountains. The vertical displacements caused by active faults along the western margin of the Hakodate Plain increased the basin depth in the western part of the Hakodate Plain. Small-to-large-sized microtremor array surveys were conducted at five sites in the Hakodate Plain to estimate the S-wave velocity structure down to the seismic bedrock for each site to develop a detailed velocity structure model. A new three-dimensional velocity structure model of the Hakodate Plain was developed by integrating the results of microtremor array surveys and other existing geophysical explorations data. This three-dimensional velocity model was modeled as a stack of homogeneous isotropic layers to facilitate its incorporation into the present nation-wide three-dimensional velocity model for ground motion prediction. The bottom depth of the Quaternary sediments is deep along the western margin of the Hakodate Plain. The total thickness of the Quaternary and Neogene sedimentary layers reaches 2.9 km in the western Hakodate Plain. The proposed velocity model was validated by gravity anomaly modeling and ground motion simulation of a moderate-sized inland earthquake. The location of the low-gravity anomaly around the coastline of the Hakodate Bay was improved using the new model. The numerical ground motion simulation using FDM also demonstrated that the amplification and long duration observed in the western part of the Hakodate Plain were reproduced effectively using this velocity model. The spatial variation in long-period ground motion amplifications (period > 1 s) is discussed based on numerical simulations utilizing our three-dimensional velocity model. The highest amplifications at periods of 4 and 5 s were expected in the southwestern Hakodate Plain. The amplification at a period of 3 s was relatively high near the western margin of the plain. Conversely, the spatial characteristics below 2 s were quite complex due to interference of the seismic wavefield inside the basin structure. Variation due to the source location was also relatively high in the shorter period range. Graphical abstract