Crustal structure and earthquake source faults on the backarc side of northern Honshu, Japan

Abstract

The backarc side of northern Honshu is marked by an arc-parallel fold-and-thrust belt. Historical and instrumental records suggest that this belt is the site of frequent, shallow, damaging crustal earthquakes. To improve our estimates of strong ground motions and to evaluate the risk of damaging earthquakes, it is important to identify the locations of seismogenic source faults beneath the thick cover of Neogene sediments in this region. Based on deep seismic reflection profiling and the distribution of aftershocks associated with damaging earthquakes, we assessed the relationship between an earthquake source fault and geological structure at several areas in northern Honshu. Reverse movement along Miocene normal faults is dominant along the fore-arc side of the Miocene backarc rift system. Source faults dip at moderate to high angles, reflecting their origin. On the backarc side, a shallow detachment is developed within upper Miocene mudstone, and thin-skinned tectonics are dominant within the sediment cover. The relationship between the deep-seated source fault in the Niigata sedimentary basin and the sedimentary cover remains a topic of controversy, mainly because of the occurrence of >6 km of sediments within the basin and the possible existence of a ductile mudstone layer within the lower part of the succession. Given the great thickness of the sedimentary cover, deep seismic profiling is required to understand the nature of the source fault.