Semi-quantitative Measure of Fluid Amount and Distribution in and Around the Fault Zone During and After a Large Seismic Slip, an Example From Kamishiro-Fault During 2014 North Nagano Earthquake

Abstract:

Exploring the relationship between mode of seismicity and distribution and amount fluids in and around the fault zone is one of the most important issues to understand the mechanism of earthquake generation. However, quantitative measure of amount and distribution of the fluids has been difficult target to estimate so far. We tried to overcome this barrier by using transient discharge of hot-spring fluids associated with a large earthquake.

On November 22, 2014 at 22:08, a large earthquake (M 6.7 depth 5.0 km) occurred at the Hakuba area, Northern part of Nagano Prefecture (Naganoken-Hokubu Earthquake). The Kamishiro Fault, striking NS and dipping moderately to the east, and one of the active traces of Itoigawa-Shizuoka Tectonic Line, was activated by the earthquake, and generated the surface displacements (reverse + left lateral, less than 0.5m) mainly at the southern trace of the fault. Eight hours after the main shock, hot spring suddenly gushed out at the northern part of Kamishiro Fault. We kept measuring the flow amount, electric conductivity, pH, chemical- and isotopic- characteristics of the spring-fluids for four months until transient spring was dried out. A clear trend of exponential decay of flow amount was observed (R² = 8.63) with time, and total volume of fluid sprang out is estimated to be 7300 m³. Using our fluid – porous fracture zone model and Darcy-Brinkman’s equation, we estimated fluid pressure right after the main shock at the focal depth. Based on the results of observation and calculation, we discuss the relationship between mode of seismicity and fluid condition in and around the activated fault zone.