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Static stress change models for known or hypothesized faults in the Hispaniola and Puerto Rico subduction segments due to (a) slip on a patch of the Puerto Rico subduction zone, and (b) slip on a patch of the Hispaniola subduction zone. An open arrow denotes slip direction and the patches are marked by dashed rectangles. Heavy black lines are simplified fault traces. Colored-filled rectangles below the fault traces represent the Coulomb stress change on vertical fault planes associated with these fault traces assuming left-lateral strike slip motion on the fault planes. Earthquakes used to determine average slip parameters are shown as "beach balls".
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Strike-slip faults in the forearc region of a subduction zone often present
significant seismic hazard because of their proximity to population centers. We explore
the interaction between thrust events on the subduction interface and strike-slip faults
within the forearc region using 3-D models of static Coulomb stress change. Model
results reveal that subduction earthquakes with slip vectors subparallel to the trench axis
enhance the Coulomb stress on strike-slip faults adjacent to the trench, but reduce the
stress on faults farther back in the forearc region. In contrast, subduction events with slip
vectors perpendicular to the trench axis enhance the Coulomb stress on strike-slip faults
farther back in the forearc, while reducing the stress adjacent to the trench. A significant
contribution to Coulomb stress increase on strike-slip faults in the back region of the
forearc comes from “unclamping” of the fault, i.e., reduction in normal stress due to
thrust motion on the subduction interface. We argue that although Coulomb stress
changes from individual subduction earthquakes are ephemeral, their cumulative effects
on the pattern of lithosphere deformation in the forearc region are significant.
We use the Coulomb stress models to explain the contrasting deformation pattern between two
adjacent segments of the Caribbean subduction zone. Subduction earthquakes with slip
vectors nearly perpendicular to the Caribbean trench axis is dominant in the Hispaniola
segment, where the strike-slip faults are more than 60 km inland from the trench. In
contrast, subduction slip motion is nearly parallel to the Caribbean trench axis along the
Puerto Rico segment, where the strike-slip fault is less than 15 km from the trench. This
observed jump from a strike-slip fault close to the trench axis in the Puerto Rico segment
to the inland faults in Hispaniola is explained by different distributions of Coulomb stress
in the forearc region of the two segments, as a result of the change from the nearly trench
parallel slip on the Puerto Rico subduction interface to the more perpendicular subduction
slip beneath Hispaniola. The observations and modeling suggest that subduction-induced
strike-slip seismic hazard to Puerto Rico may be smaller than previously assumed, but the
hazard to Hispaniola remains high.
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U.S. Geological Survey