INTERMEDIATE EARTHQUAKE SWARMS AS INDICATORS FOR SLAB TEAR

The geometry of a subduction zone is influenced by features and processes, such as slab-rollback, subduction of relict seafloor fabric and sea mounts, and the concave or convex nature of the trench axis (creating a compressive or extensional regime, respectively). The process of slab tear generates seismic swarms of complex mechanisms. These intermediate depth seismic swarms likely indicate the location of the point of tear propagation within the slab.

Slab tearing has been documented in convergent plate boundaries around the world, including in the Tonga trench, Mariana trench, Kamchatka, Calabrian, Izu-Bonin and the Philippine Sea slabs, the South Sandwich region, and the Cocos plate, as well as, from our recent , well-constrained observations, in the northeast Caribbean. Most of the previous suggestions of slab tearing result from less-than-ideal observational geometries, due to the strong heterogeneities of subduction zones and limitations of land-based seismic instruments. However, in 2007 we deployed five ocean bottom seismographs in an attempt to augment constraints on the seismic swarms that occur repeatedly offshore Puerto Rico, and were able to record two such swarms during the six-month OBS deployment. Our observations, therefore, offer the strongest constraints to date on such swarms, and their interpretation sheds light on the process of lithospheric disintegration during subduction--here and in similar tectonic settings elsewhere.

Analysis of stresses from focal mechanisms and mantle flow directions from anisotropy studies suggest that this corner of the subducting North American Plate is tearing to accommodate the curvature of the plate boundary. The large frequency of intermediate-depth seismic swarms that have remained in this corner of the plate boundary suggests that they result from active tear propagation in the slab. This tearing may indicate the eastward propagation of the Puerto Rico Trench, already the deepest part of the Atlantic Ocean, and that slab tearing is an essential component of slab-rollback and, ultimately, the evolution of plate boundaries.