Abstract
The reactivation mechanisms of coseismic surface ruptures associated with the 2011 Mw 6.7 Fukushima-ken Hamadori earthquake in Japan are investigated using in-situ controlled hydraulic injections in subsurface boreholes. Two fault segments were selected for reactivation studies, one across a coseismic rupture, the Shionohira site, and one across a non-coseismically ruptured segment, the Minakami-kita site. A series of water injections in sealed sections of boreholes set across the fault progressively bring the fault to rupture by a step-by-step decrease of the effective normal stress clamping the fault. While the fault is rupturing during these hydraulic stimulations, borehole displacements, fluid pressure and injection flowrate are continuously monitored. Then, the tests were analyzed using fully coupled hydromechanical modeling. The model was calibrated on field data, and a parametric study was conducted to examine the modes of fault reactivation. Coseismic surface rupture of the Shionohira fault showed a pure dilatant slip response to hydraulic tests, while the tectonically un-activated Itozawa fault (South) indicated a complex hybrid response to tests related to both a higher frictional and cohesive strengths of the fault. The analysis of the induced Shionohira slip event showed that it is reasonably modeled as a Coulomb rupture with an eventual dependency of friction on slip velocity, in good accordance with laboratory-derived rate-and-state friction data on the Shinohira gouge samples. In contrast, the Itozawa fault reactivation mechanism appears dominated by tensile failure with limited Coulomb shear failure. Thus, the applied protocol proves to be able to isolate significant differences in fault physical properties and rupture mechanisms between two segments of the same fault system, opening perspectives to better assess near-surface rupture effects, and therefore the safety of structures such as dams or nuclear waste repositories subject to large earthquakes.
Highlights
In earthquake geology and mechanics, the characterization of sur face deformation is crucial, especially when the coseismic rupture rea ches the surface from the seismogenic depth (Anastasopoulos et al, 2007)
The mechanism of fault rupture propagation to ground surface mainly depends on crustal geology and tectonics, field studies after earthquakes highlighted the effects of near-surface de posits, if they exist, on the width and architecture of the outcropping fault rupture
Three SIMFIP tests were completed in the injection borehole (Fig. 4b), respectively at [7–11.5 m] in the intact sandstone, at [12.8–17.3 m] across the activated principal shear zone (PSZ) and at [16.4–23.5 m] in the old inactive fault zone
Summary
In earthquake geology and mechanics, the characterization of sur face deformation is crucial, especially when the coseismic rupture rea ches the surface from the seismogenic depth (Anastasopoulos et al, 2007). In 2018 and 2019, we conducted hydraulic tests in shallow 20–30 m deep boreholes cross-cutting one activated of the Shionohira fault and one inactivated segment of the Itozawa fault (South), respectively at the Shionohira and at the Minakami-kita sites (Fig. 1). A borehole probe developed by LBNL (Lawrence Berkeley National Laboratory, Berkeley, US) was deployed, and the Step Rate Injection Method for Fracture InSitu Properties (SIMFIP, Guglielmi et al, 2014) was applied in bore hole sealed-sections set across the fault zone. Allow testing different fault thickness (up to ~5 m), soft and/or hard rock materials and at increased pressure/depth conditions up to 40 MPa. In this study, the SIMFIP three-dimensional displacement sensor which is a 0.2 m long and 0.1 m diameter pre-calibrated aluminum cage is integrated in the mandrel straddling the two 1 m long probe's inflatable rubber packers which are sliding sleeves (Fig. 2a). The irreversible displacement produced by the test which is the displacement value at the end of the test when borehole pressure has returned to its initial pre-test value
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