Marianas Trench stress regime
[Emry, Wiens & Garcia-Castellanos, 2014, JGR, pdf here]
Faulting within the Pacific plate at the Mariana Trench: Implications for plate interface coupling and subduction of hydrous mineral.
We investigate faulting within the incoming Pacific plate at the Mariana subduction trench to understand stresses within the bending plate, regional stresses acting upon the plate interface, and the extent of possible faulting-induced mantle serpentinization. We determine accurate depths by inverting teleseismic P and SH waveforms for earthquakes occurring during 1990–2011 with Global Centroid Moment Tensor (GCMT) solutions. For earthquakes with Mw 5.0+, we determine centroid depths and source time functions and refine the fault parameters. Results from Central Mariana indicate that all earthquakes are extensional and occur at centroid depths down to 11 km below the Moho. At the Southern Mariana Trench, extensional earthquakes continue to 5 km below the Moho. One compressional earthquake at 34 km below the seafloor suggests stronger plate interface coupling in the Southern Mariana Trench. In addition, we model the stress distribution within the Pacific plate along two bathymetric profiles extending seaward from the Mariana subduction trench axis to better understand whether our earthquake depth solutions match modeled scenarios for plate bending under applied external forces. Results from our flexure models match the locations of extensional and compressional earthquakes and suggest that the Pacific plate at Southern Mariana is experiencing larger, compressional stresses, possibly due to greater interplate coupling. Additionally, we conclude that if extensional faulting promotes the infiltration of water into the subducting plate mantle, then the top 5–15 km of the Pacific plate mantle are partially serpentinized, and a higher percentage of serpentinization is located near the Central Mariana trench where extensional events extend deeper.
Stress distribution predicted in the Tonga trench (blue means extension; red means compression). Circles show the observed earthquake hypocenter distribution: black circles are compressive shocks and white circles are tensional. The size of the circles is proportional to earthquake magnitude. Dotted regions correspond to the predicted occurrence of brittle and ductile failure: black dots for compressive yielding and white for tensional yielding.
Cartoon showing the forces acting on a subducting plate. Bending is caused by an applied moment M, vertical force Fz and horizontal force Fx acting at the trench axis (the arrows show the positive sense). Far enough from the trench, the plate preserves its reference depth d0. The mechanical response of the plate is controlled by the yield stress envelope (Fig.3) and the mechanical thickness H of the lithosphere (depth at which the strength reduces to less than 10 MPa).