Abstract:

In order to reveal stress recovery at Nojima fault after the 1995 Mw 6.9 Kobe earthquake, Japan, we carried out in-situ stress measurements by using core samples obtained from a vertical scientific drilling penetrated through the Nojima fault which ruptured during the Kobe earthquake. The drilling operations were conducted from 2016 to 2017, therefore the measured stress data using the core samples should be as the time of stress relief i.e. the same as the drilling time, ~22 years passed after the earthquake. We applied two core-based stress measurement methods called Anelastic Strain Recovery (ASR) and Diametrical Core Deformation Analysis (DCDA). Although we are continuously working on core re-orientation and discussions on how interpret the measured stress state, and have not reached conclusions, we believe that a useful data set of the current state of stress around Nojima fault has been obtained by using the two core-based stress measurement methods.

1 Introduction

Stress and earthquakes are known to be interrelated: stress triggers earthquakes and earthquakes alter the shear and normal stresses on the source faults. On the other hand, the stresses both on the fault plane and in the wall rock around the fault gradually build up in the interseismic period. We have, however, very limited knowledge on the quantitative relationship between stress change and elapsed time after earthquake occurrence. Therefore, in order to reveal stress recovery at Nojima fault after the 1995 Mw 6.9 Kobe earthquake, Japan, we carried out in-situ stress measurements by using core samples obtained from a vertical scientific drilling penetrated through the Nojima fault which ruptured during the Kobe earthquake (also called Hyogo-ken Nanbu earthquake). The drilling operation and ASR measurements were conducted from 2016 to 2017 when ~22 years passed after the earthquake. A recent stress measurement study using borehole breakout analyses suggested that the stress in the deep part of the same borehole as this study looks like to have started to recovery after the Kobe earthquake (Nishiwaki et al., 2018). We have conducted the ASR measurements for 19 core samples and DCDA for 13 core samples, respectively. The core reorientation works and discussion on how to interpret the measured stress state are still on going. Therefore, we only show two representative examples of ASR and DCDA preliminary results following the descriptions on the measurement methods in this paper. We believe that a useful data set of the current state of stress around Nojima fault has been obtained by using the two core-based stress measurement methods.

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