Identification of the Mechanisms of the Seasonal Crustal Deformation

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Distinct periodic variations with annual frequencies are often found in the time series of continuous Global Positioning System (GPS) site coordinates in northeastern Japan. They show maximum arc-normal contraction of a few millimeters as well as maximum subsidence of 1-2 centimeters, both in March. In northeastern Japan, it snows heavily on the western flank of the backbone range, attaining the maximum depth of up to a few meters in March. We showed that the surface loads caused by the snow are largely responsible for the annual displacement of GPS sites, by comparing the load distribution estimated from the GPS data and the observed snow depths.Snow load along the western flank of the backbone range of the Japanese Islands perturbs the interseismic strain buildup, and may seasonally influence the seismicity in Japan. Intraplate earthquakes in northeastern Japan occur on reverse faults striking parallel with the snow-covered zone. In central and southwestern Japan, they occur on strike-slip faults striking either parallel with, or perpendicular to the snow cover. The snow load enhances compression at these faults, reducing the Coulomb failure stress by a few kPa. This is large enough to modulate the secular stress buildup of a few tens of kPa/yr. Past destructive inland earthquakes with magnitudes >7.0 that occurred in regions covered with snow in winter, tend to occur more in spring and summer than in autumn and winter, while those in the snow-free regions do not show such variation. Although its statistical significance is not strong due to limited number of past earthquakes, it suggests that the spring thaw enhances seismicity beneath the snow cover. Well-known annual variation of offshore interplate seismicity cannot be explained by the snow load, and might be driven by other mechanisms such as change in ocean loads. This needs substantiation by ocean bottom pressure gauge observations.