Identification of the Mechanisms of the Seasonal Crustal Deformation

在日本东北部的连续全球定位系统（GPS）站点坐标的时间序列中，经常发现明显的周期性变化与年频率。它们显示出几毫米的最大弧正常收缩以及1-2厘米的最大下沉，都在3月。在日本东北部，主干山脉的西侧会下大雪，3月最深可达数米。通过比较由GPS数据估算的载荷分布和观测到的积雪深度，我们发现，积雪引起的地表载荷是GPS站点年位移的主要原因，沿日本列岛主干山脉西侧的雪载荷沿着扰动了震间应变积累，并可能季节性地影响日本的地震活动。日本东北部的板内地震发生在与积雪带平行的逆断层上。在日本中部和西南部，它们出现在与雪盖平行或垂直的走滑断层上。雪荷载增强了这些断层处的压缩，使库仑破坏应力降低了几千帕。这是大到足以调制几十千帕/年的长期应力积累。过去发生在冬季积雪地区的7.0级以上破坏性内陆地震，往往在春夏多发，而在无雪地区则没有这种变化。虽然由于过去地震的数量有限，其统计意义并不强，但它表明春季解冻增强了积雪下的地震活动。众所周知的近海板间地震活动的年变化不能用雪荷载来解释，而可能是由其他机制驱动的，如海洋荷载的变化。这需要洋底压力计的观测来证实。

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.