Effective Elastic Thickness Beneath the Japanese Islands, and its Relationship to Seismogenic Layer and Magnetic Crustal Thickness

We estimated `lithospheric` thicknesses from different sources. The effective elastic thickness of the lithosphere (Te) is estimated from the coherence between Bouguer gravity and topography. Te reflects the integrated brittle, elastic and ductile strength of the lithosphere over geological time scales, and depends on many geophysical properties, such as thermal structure, crustal composition, and plate geometry. The bottom depth of the magnetized crust (Zb), Curie-isotherm depth, determined from the spectral analysis of residual magnetic anomalies, is interpreted as a proxy for lithospheric thermal structure. Local maximum depth of earthquakes (D90), defined as the depth above which 90% of the earthquakes occur, is interpreted to mark the onset of plastic behavior. We showed spatial distributions of these three kinds of `lithospheric` thicknesses in square windows of 2.125o x 2.125o beneath the Japanese Islands. These values represent the average values over a wide area and may not characterize detailed lithospheric structure, while these spatial variations show similar trends. Because both Te and D90 are indicative of strength, variations in Te correlate with those in D90, both of them having similar values, although Te is usually smaller than D90. On the other hand Zb provides information on the thermal properties of the lithosphere, both variations in Te and D90 are broadly correlated with those in Zb. These values may have the potential to provide information on mechanical and thermal properties beneath the Japanese Islands.