Chihiro Kinoshita

Shallow crustal permeability enhancement in central Japan due to the 2011 Tohoku earthquake

Pore pressure decreased at the Kamioka mine in central Japan after the Tohoku earthquake (M9.0) on 11 March 2011, which can be attributed to a permeability increase. We focus on the Earths tidal response before and after the earthquake to evaluate rock permeability change through hydraulic diffusivity change. If we assume a constant elastic modulus, hydraulic diffusivity is found to increase from 3.3 to 6.7 m2/s after the Tohoku earthquake. We also analyzed data before and after the 2007 Noto Hanto (M6.9) and 2008 Suruga Bay (M6.5) earthquakes, which yield no significant tidal response changes. We examined the amount of dynamic and static stress changes caused by these earthquakes and show that it is difficult to attribute the permeability enhancement solely to dynamic stress, and static stress change may also affect the permeability enhancement.

Near‐field observations of an offshore Mw 6.0 earthquake from an integrated seafloor and subseafloor monitoring network at the Nankai Trough, southwest Japan

An Mw 6.0 earthquake struck ~50 km offshore the Kii Peninsula of southwest Honshu, Japan on 1 April 2016. This earthquake occurred directly beneath a cabled offshore monitoring network at the Nankai Trough subduction zone and within 25–35 km of two borehole observatories installed as part of the International Ocean Discovery Programs NanTroSEIZE project. The earthquakes location close to the seafloor and subseafloor network offers a unique opportunity to evaluate dense seafloor geodetic and seismological data in the near field of a moderate-sized offshore earthquake. We use the offshore seismic network to locate the main shock and aftershocks, seafloor pressure sensors, and borehole observatory data to determine the detailed distribution of seafloor and subseafloor deformation, and seafloor pressure observations to model the resulting tsunami. Contractional strain estimated from formation pore pressure records in the borehole observatories (equivalent to 0.37 to 0.15 μstrain) provides a key to narrowing the possible range of fault plane solutions. Together, these data show that the rupture occurred on a landward dipping thrust fault at 9–10 km below the seafloor, most likely on the plate interface. Pore pressure changes recorded in one of the observatories also provide evidence for significant afterslip for at least a few days following the main shock. The earthquake and its aftershocks are located within the coseismic slip region of the 1944 Tonankai earthquake (Mw ~8.0), and immediately downdip of swarms of very low frequency earthquakes in this region, illustrating the complex distribution of megathrust slip behavior at a dominantly locked seismogenic zone.

Changes in Physical Properties of the Nankai Trough Megasplay Fault Induced by Earthquakes, Detected by Continuous Pressure Monitoring

One primary objective of Integrated Ocean Drilling Program (IODP) Expedition 365, conducted as part of the NanTroSEIZE project, was to recover a temporary observatory, termed the “GeniusPlug“ emplaced to monitor formation, pore fluid pressure and temperature within a major splay fault that branches from the main plate interface, at a depth of ~400 m below sea floor (mbsf). The instruments were installed in Dec. 2010 and recovered in April 2016, yielding 5.3 years record of formation pressure and temperature within fault zone. Here, we use the pressure timeseries, and in particular the response to ocean tidal loading, to evaluate changes in physical properties of fault zone induced by several regional earthquakes. To accomplish this, we quantify: (1) the amplitude of the formation’s response to tidal loading, defined in terms of a tidal loading efficiency, governed primarily by the formation and fluid elastic properties; (2) the phase lag between the ocean tidal signal and the measured response in the observatory, which is governed by a combination of formation hydraulic diffusivity and the relative compressibilities of the formation and sensing volume; and (3) pressure steps associated with earthquakes, identified in formation pressure after removal of the tidal signal. We observe essentially no phase lag, but in for many events we detect both pressure steps and transient decreases in loading efficiency. To reveal the cause of these changes, we investigate the effects of static and dynamic crustal strains. Changes in theoretical static volumetric strain and the associated expected pressure step for each event are calculated based on Okada (1992), and using a conversion from volumetric strain to pore pressure based on formation properties defined by laboratory experiments. We find that, there is no clear correlation between observed pressure steps and predicted static volumetric strain; furthermore, the predicted pressure steps are ten to hundreds of times smaller than observed. As a proxy for dynamic strains, we calculate the integrated “pressure energy density” over a 30 minute window for each event, and show a positive correlation with both step changes in pressure and changes in loading efficiency. Most of the detected changes represent pressure increases and loading efficiency decreases. We speculate that disruption of grain contacts and subsequent pore collapse induced by dynamic strain produces changes of hydraulic properties in the fault zone. Alternatively, these changes could reflect exsolution of gas from pore fluids that drives pore pressures up while simultaneously reducing loading efficiency by increasing the compressibility of pore-filling fluids. Finally, the observed amplitude response and negligible phase lag of the formation pressure response to ocean tidal loading, taken together, allow an estimate of the minimum hydraulic diffusivity of splay fault of 8.9×10 m/s. U03-P02 JpGU-AGU Joint Meeting 2017