Evaluation of surface-rupturing fault width by statistical analysis from the 2016 Kumamoto, Japan, earthquake: Implications in regulating fault avoidance zone

The 2016 Mw 7.0 Kumamoto earthquake produced surface ruptures along the Futagawa and Hinagu fault zones. This earthquake provides us a significant opportunity to understand surface rupture complexity and then improve the concept of the fault avoidance zone that prevents the damage of human life and property from fault displacement.

The objective of this study is to quantitatively analyze the separation distance between interpreted active fault (IF) and coseismic surface rupture (SR) and then to seek the essential parameters that brought the separation distance between IF and SR. In conclusion, we found that the width of existing regulations is not satisfactory to reduce the damages from surface rupture displacement in this area.

The surveyed faults are the Futagawa-Hinagu fault zone and Idenokuchi fault. Right-lateral slip up to 2.2 m occurred along the Futagawa fault, whereas nomal slip occurred along the Idenokuchi fault at the 2016 event (Kumahara et al., 2016; Shirahama et al., 2016). The IF traces are from a map interpreted by Prof. Chida (Nakata & Imaizumi, 2002) and the SR traces are from Kumahara et al. (2016). After these traces were imported to ArcGIS, only the SR traces were converted to point data every 10ｍ. We then measured the distances of perpendicular lines from the SR point data to the IF traces, and summarized as the quartile-based statistical estimates. The separation distances on the first, second, and third quartile points are following value: approximately 23 m, 75 m, 184 m on the Futagawa-Hinagu fault, approximately 28m, 157m, 466 m on the Idenokuchi fault.

The first quartile of both faults is 20-30 m on the separation distance, however wider gaps between IF and SR were found along the Idenokuchi fault rather than the Futagawa-Hinagu fault .

The reason is considered following: the comparatively low separation distance on Futagawa -Hinagu fault indicates the SR trace as conjugated faults occurred cross and near IF traces, whereas the high separation distance on Idenokuchi fault indicates the SR traces as antithetic fault occurred away from the IF traces.

If we had applied the widths of 15 m or 20 m as the fault avoidance zones adopted in California, New Zealand, Taiwan, Italy, and Tokushima Prefecture in Japan, their regulations would not have been effective to avoid the significant destruction due to the fault slip.