Crustal thermal regime and its relationship to seismogenic layer thickness: Comparisons among Japan, California, and Kamchatka

Temperature at bottom of seismogenic layer is one of the most important and still controversial information for geological phenomena. Spatial variations in the maximum depth of seismicity have been correlated with crustal temperature. However this correlation is shown at geographically restricted area. In order to overcome this spatial limitation, determination of the basal depth of magnetic layer, Curie point depth, based on spectrum analysis of magnetic anomaly data was applied to estimate regional thermal structure. This analysis is still controversial and this depth does not necessarily represent an isotherm, however, previous studies suggested that there was an inverse correlation between these depths and heat-flow measurements. Recently, the correlation between the basal depth of magnetized layer and the seismogenic layer showed at Japan [Tanaka and Ishikawa, 2005] and California [Ross et al., 2006]. The centroid of depth of magnetized layer also showed a good correlation with the seismogenic layer beneath Kamchatka region [Tanaka, 2007]. To address how thermal regime relates to seismogenic layer, we compare relationship between depths of magnetized and seismogenic layers at three active plate boundaries: Japan, California and Kamchatka. Relationship between depths of magnetized and seismogenic layers for each region is clear, however each region has own cluster; the basal depth of magnetic sources of California region are significantly deeper than those of Japan, and the centroid of magnetic layer of Kamchatka region are deeper than those of Japan. Previous work has attributed the concept of thermal structure as a fundamental parameter for determining the thickness of the seismogenic zone, which might be strain rate, lithology, stress-state, and pore fluid pressure dependent. Each region consists of different physiographic provinces; therefore another factors are required to explain why each region has its own clusters. The Pacific/North America plate boundary dominates right-lateral shear along the San Andreas Fault in California, whereas Japanese Islands are mainly located in east-west or northwest-southeast compressional area. This difference of tectonic setting may cause the cluster shift between Japan and California. The difference in slab depths beneath Kamchatka and Japan may explain the difference.