Multiscale off-fault brecciation records coseismic energy budget of principal fault zone

Abstract

Breccia and pulverized rock are typical textures in off-fault damage adjacent to a main seismogenic zone. Previously, by estimating the energy required to advance the rupture in this zone using particle size distribution at sub-millimeter/micrometer scales, we could constrain the energy budget during coseismic events. However, whether microscopic estimation is sufficient to capture surface energy fragmentation during an earthquake and the effect of measurement scale variation on calculation of co-seismic energy partitioning remained unclear. Here, we investigated the mechanism of coseismic off-fault damage based on field and microstructural observations of a well-exposed breccia body in Ichinokawa, Japan. We used in situ clast measurements coupled with thin-section analysis of breccia clasts to estimate the energy budget of the damage zone adjacent to the principal slip zone of the Median Tectonic Line (MTL). The total surface energy density and corresponding surface energy per unit fault for a width of ~ 500 m of the dynamical damage zone were estimated. The moment magnitude estimated based on surface energy was 5.8–8.3 Mw. In Ichinokawa, off-fault fragmentation is initiated by coseismic activity and is followed by fluid activity. Under dynamic fragmentation conditions, the scale is important to calculate the surface energy.