2004 Volume 2004 Issue 24 Pages 39-48
In Japan, the empirical formula proposed by Matsuda (1975) mainly based on the length of historical surface fault ruptures, is generally applied to estimate the size of future earthquakes from the extent of existing active faults for seismic hazard assessment. Therefore validity of the fault extent and defining individual segment boundaries where propagating ruptures terminate are essential and crucial to the reliability for the accurate assessments.
It is, however, not likely for us to clearly identify behavioral earthquake segments by the observation of surface faulting during the historical period, because most of the active faults have longer recurrence intervals than 1000 years in Japan. Besides uncertainties of the datasets obtained from fault trenching studies are quite large for fault grouping/segmentation. This is why new methods or criteria should be applied for active fault grouping/segmentation, and one of the candidates may be geometric criterion of active faults.
Matsuda (1990) used“ five kilometer” as a critical distance for grouping/separation of neighboring active faults. On the other hand, Nakata and Goto (1998) proposed the geometric criteria such as (1) branching features of active fault traces and (2) characteristic pattern of vertical-slip distribution along the strike-slip fault traces as tools to predict rupture length of future earthquakes. The branching during the fault rupture propagation is regarded as an effective energy dissipation process and could result in final rupture termination. With respect to the characteristic pattern of vertical-slip distribution, especially with strike-slip components, the up-thrown sides along the faults are, in general, located on the fault blocks in the direction of relative strike-slip.
By applying these new geometric criteria to the high-resolution active fault distribution maps, the fault grouping/segmentation could be more practically conducted. We tested this model successfully on the active faults generated the 1943 Tottori earthquake, the Chojagahara-Yoshii fault zone in Chugoku district in Japan, as well as the active fault system in northern Luzon, the Philippines. Thus, we name this conceptual model as“ Morphometric Unit Model” and call the active faults grouped by the model as“ Packaged Active Faults” for individual seismogenic faults. Moreover, we come to know that active fault mapping with the model in mind enables us to find many new active fault traces (e. g., the Shigenobu fault along the MTL in Japan).
