Relationships of the Seismicity at the Alaska Subduction Zone to Metamorphism and the Deep Fluid Regime
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elationships of the Seismicity at the Alaska Subduction Zone to Metamorphism and the Deep Fluid Regime Margarita A. Nikitinaa, *, Mikhail V. Rodkina, b, c, and Ivan G. Shmakova aInstitute
of Earthquake Prediction Theory and Mathematical Geophysics, Russian Academy of Sciences, Moscow, 117997 Russia b Institute of Marine Geology and Geophysics, Far Eastern Branch, Russian Academy of Sciences, Yuzhno-Sakhalinsk, 693022 Russia c Oil and Gas Research Institute RAS, Russian Academy of Sciences, Moscow, 119333 Russia *e-mail: [email protected] Received May 18, 2019; revised June 25, 2019; accepted July 4, 2020
Abstract—We examined the spatial distribution of intermediate-depth earthquakes in southern Alaska and the adjacent Aleutian Islands in relation to the deep fluid regime and volcanism. The distribution of earthquake hypocenters is studied in two variables, the depth of focus and the distance from the top of the slab. This approach shows that seismic activity is concentrated at 10–15 and 20–25 km from the top of the downgoing plate. Clusters of seismicity such as these cannot be due to slippage along the boundary between the continental block and the slab. In addition, extended structures of sharply increased seismic activity were found. Their location fits certain quasi-linear relations between pressure and temperature, and can mark metamorphic fronts in the subducted plate. Besides, the spatial earthquake distribution has a peak that appears to be related to active recent volcanism. It can be caused by active dehydration reactions in the subduction plate. These empirical findings testify in favor of the fluid-metamorphic model of earthquakes origin. Keywords: intermediate-depth earthquakes, subduction zone, Alaska, metamorphism, dehydration reactions, fluid-metamorphic model of seismicity DOI: 10.1134/S1069351320060063
Deep earthquakes have been known since 1928, and a few models of such earthquakes were proposed over several recent decades (Wadati, 1928; Kalinin, 1982, 1986, 1989; Rodkin, 1993; Kirby, 1996; Davies, 1999; Abers, 2000; Hacker, 2003; Okazaki, 2016; Kuz’min, 2019; Rodkin, 2017). However, despite numerous studies, the nature of deep earthquakes is still unclear. The deep earthquakes are usually divided into two types: intermediate-depth earthquakes whose hypocenters are at depths of 70 to 300 km, and deep ones, deeper than 300 km (Miiamura, 1972). There are a few hypotheses for the occurrence of deep earthquakes. The leading ones include shear instability because of release of deep fluids from water-containing minerals, thermal shear instability, and the model of polymorphic phase transitions, this model has several variants (Kalinin, 1989; Peacock, 1996; Yamasaki, 2003; Okazaki, 2016; Rodkin, 2017; Kuz’min, 2019; etc.).
Yang, 1995; Fliedner, 2000; Rondenay, 2008; Cole, 2009). The available evidence clarifies the positions of earthquake hypocenters, the top of the downgoing plate, and the location of island-arc volcanism of different ages (Zhao, 1995; Cole, 2009). The heterogeneity in the spatial di
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