Role of H₂O in Generating Subduction Zone Earthquakes

Abstract

A dense nationwide seismic network and high seismic activity in Japan have provided a large volume of highquality data, enabling high-resolution imaging of the seismic structures defining the Japanese subduction zones. Here, the role of H 2 O in generating earthquakes in subduction zones is discussed based mainly on recent seismic studies in Japan using these high-quality data. Locations of intermediate-depth intraslab earthquakes and seismic velocity and attenuation structures within the subducted slab provide evidence that strongly supports intermediate-depth intraslab earthquakes, although the details leading to the earthquake rupture are still poorly understood. Coseismic rotations of the principal stress axes observed after great megathrust earthquakes demonstrate that the plate interface is very weak, which is probably caused by overpressured fluids. Detailed tomographic imaging of the seismic velocity structure in and around plate boundary zones suggests that interplate coupling is affected by local fluid overpressure. Seismic tomography studies also show the presence of inclined sheet-like seismic low-velocity, high-attenuation zones in the mantle wedge. These may correspond to the upwelling flow portion of subduction-induced secondary convection in the mantle wedge. The upwelling flows reach the arc Moho directly beneath the volcanic areas, suggesting a direct relationship. H 2 O originally liberated from the subducted slab is transported by this upwelling flow to the arc crust. The H 2 O that reaches the crust is overpressured above hydrostatic values, weakening the surrounding crustal rocks and decreasing the shear strength of faults, thereby inducing shallow inland earthquakes. These observations suggest that H 2 O expelled from the subducting slab plays an important role in generating subduction zone earthquakes both within the subduction zone itself and within the magmatic arc occupying its hanging wall.

Hypocenter Migration and Seismicity Pattern Change in the Yamagata‐Fukushima Border, NE Japan, Caused by Fluid Movement and Pore Pressure Variation

“…Two typical forms of spatially and temporally clustered seismicity exist: mainshock-aftershock sequences and earthquake swarms. It has been suggested that a reduction in frictional strength due to elevated pore pressure plays an important role in earthquake generation (e.g., Hasegawa, 2017;Hubbert & Rubey, 1959;Miller, 2013;Nur & Booker, 1972;Rice, 1992;Sibson, 1992), both for mainshock-aftershock sequences (e.g., Nur & Booker, 1972) and for earthquake swarms (Nur, 1974;Parotidis et al, 2005;Yamashita, 1999). Mainshock-aftershock sequences are characterized by the decay of seismicity rate in proportion to the reciprocal of time (Omori law), and events are often assumed to be triggered by static stress changes from the mainshock (e.g., King et al, 1994) and other preceding earthquakes (Ogata, 1988).…”

Temporal Changes in Stress Drop, Frictional Strength, and Earthquake Size Distribution in the 2011 Yamagata‐Fukushima, NE Japan, Earthquake Swarm, Caused by Fluid Migration

“…There have been a number of studies suggesting that reduction in frictional strength due to pore fluid pressure plays an important role in earthquake generation (e.g., Fletcher & Sykes, 1977;Hasegawa, 2017;Hasegawa et al, 2005;Hubbert & Rubey, 1959;Iio et al, 2002;Nur & Booker, 1972;Ohtake, 1974;Sibson, 1992;Urata et al, 2013;Zoback & Harjes, 1997). Earthquakes induced by fluid injection often show migration of the hypocenter, which can be explained by fluid diffusion (e.g., Shapiro et al, 2002;Shapiro, Huenges, & Borm, 1997).…”

Fault‐Valve Behavior Estimated From Intensive Foreshocks and Aftershocks of the 2017 M 5.3 Kagoshima Bay Earthquake Sequence, Kyushu, Southern Japan

“…The fluid behavior in the Earth interior may affect not only earthquake swarms but also foreshock-mainshock-aftershock sequences (e.g., Hasegawa, 2017;Hubbert & Rubey, 1959;Nur & Booker, 1972;Rice, 1992;Sibson, 1992). Sibson (1992) and Sibson et al (1988) suggested the pore pressure cycle, due to overpressurized fluids rising from the deeper portion of the fault, controls the earthquake cycle.…”