Outer Rise Seismicity and Dynamics: Implications for Plate Rheology and Seamount Loading at the Tonga Subduction Zone

We are investigating the locations and fault plane solutions of outer rise earthquakes at the Tonga subduction zone in order to understand the distribution of strain in the bending oceanic plate. With simple subduction zone plate bending, intraplate outer rise earthquakes are due to tensional failure at shallow depths and compressional failure at deeper depths. The tension-to-compression transition depth is an important constraint for the study of downgoing plate stresses, interplate coupling along the subduction interface, and the rheology of the lithosphere in the outer rise region. However, the subduction of large bathymetric features may increase seismic coupling and cause important perturbations on the plate rheology. Using a 1994 three-month ocean bottom seismograph deployment across the Tonga subduction zone at about -19° latitude, more than 70 local earthquakes were relocated to less than 10 km error using either a multiple-event hypocenter determination algorithm or the HYPOELLIPSE program. Due to the strong lateral heterogeneity in the region, different structural models were used for stations in the Pacific plate, Tongan arc, and Lau basin. Regional waveform inversion for the largest events using local broadband land stations yielded focal mechanisms for some earthquakes. An interesting feature in the distribution of events is a swarm of 15 earthquakes that occurred almost exclusively over a two-day period under the Capricorn Seamount directly beneath normal faults seen with high-resolution sea beam sidescan sonar bathymetry. The events in this cluster with obtainable fault plane solutions have thrust-faulting mechanisms. Another cluster of 7 mostly normal-faulting events not associated with any bathymetric features offers a good basis for comparison with the Capricorn events. Comparison of the focal mechanisms for outer rise earthquakes at various locations along strike helps illustrate the effects of the seamount loading on the plate bending at the trench. To explore the rheological effects of seamount loading at subduction zones, yield strength envelopes (YSE) assuming an inelastic rheology (brittle/elastic/ductile) were constructed using the method of Mueller, et al. [1996]. Since forcing an isolated buoyant seamount into a trench will induce compression seaward of the seamount and tension downdip of the seamount, the effects of varying the history of plate curvature and inplane force were modeled in order to match YSE stress profiles with the depths and stress states inferred from the earthquakes in this study.