Preliminary results of three-dimensional stress orientation determined by anelastic strain recovery (ASR) measurements of core samples retrieved from IODP Expedition 343

Integrated Ocean Drilling Program (IODP) Expedition 343, Japan Trench Fast Drilling Project (JFAST) penetrated to ~850 meter below seafloor (mbsf) in a water depth of 6890 m and passed through the plate boundary fault of the overriding North American Plate and the subducting Pacific plate witch. The fault locates at ~820 mbsf and is preliminarily considered to be the source fault of the 2011 Tohoku-oki Mw 9.0 earthquake. Area of JFAST drilling site (C0019) was in the largest coseismic slip zone where the fault slipped more than 50 m during the earthquake. Hole C0019E dedicated to coring retrieved a total of 21 cores having a total of 51 m long cores from both the hanging wall and the footwall of the plate boundary fault. To determine three-dimensional stress state after the huge earthquake, we collected four whole round core samples and measured anelastic strain recovery (ASR) also called 'relaxation' of the core samples onboard D/V Chikyu. The principle idea behind the ASR method is that stress-induced elastic strain is released first instantaneously (i.e., as time-independent elastic strain), followed by a more gradual or time-dependent recovery of anelastic strain. The ASR method takes advantage of the time-dependent strain and has been successfully applied in IODP expeditions (e.g. Byrne et al., 2009; Yamamoto et al., 2013). The four core samples used for ASR measurements were taken from C0019E-1R1 (~177 mbsf), C0019E-5R1 (~697 mbsf)), C0019E-13R1 (~802 mbsf) and C0019E-19R2 (~828 mbsf). The three core samples at shallower depths were in the hanging wall of the fault; and the deepest one was in the footwall. We started ASR measurements approximate three hours after the core was 'on deck', that is approximate six hours from the in situ stress was released, and keep the measurements for about two weeks. The anelastic strains measured in nine directions including six independent directions were extensional; all of the curves varied smoothly and similarly with increasing time. The magnitudes of the anelastic strains were dependent on the measured directions, had a typical value of several hundred microstrains measured for about two weeks. To convert core orientation into geographic coordinate system, we conducted paleomagnetic measurements in shore based laboratory after the ASR measurements. Stepwise alternating field demagnetization on natural remanent magnetization revealed that three core samples from 177, 697 and 828 mbsf record paleomagnetic inclinations that are consistent with the latitude of the sample recovery site (C0019). Thus it is considered that declination can be used to convert core orientation for these three samples. In contrast, paleomagnetic inclination determined for core sample from 802 mbsf was not consistent with the latitude of the sample recovery site and conversion of the core orientation was not successful. Preliminarily, in-situ stress state after the Tohoku-oki earthquake determined by ASR at the four depths showed either in or close to the normal faulting stress regime. In addition, the maximum horizontal stress orientations by ASR were good consistent with those determined by borehole breakouts in Hole C0019B (Lin et al., 2013) at the two shallower depths where both breakout data and core orientation correction are available.