Exploring Lesser Antilles subduction zone locking through modeling of cGPS and earthquake seismology data.

Friday, 19 December 2014
Machel Higgins, University of the West Indies, Kingston, Jamaica, John C Weber, Grand Valley State Univ, Allendale, MI, United States and Richard E A Robertson, Univ West Indies, St Augustine, Trinidad and Tobago
We are undertaking a study to better determine the locking characteristics of Lesser Antilles subduction zone using new cGPS data from 20 stations in the Lesser Antilles volcanic islands and outboard islands in the forearc sliver. The new data come from the SRC, IGS and IPGP cGPS networks. Each site we use has a minimum of 3 years of data, and raw site velocities have average uncertainties in the horizontal on order ≤ 2 mm/yr. Eventually, we also hope to incorporate vertical velocities, which have slightly larger uncertainties, into the model along with earthquake slip vectors from the Harvard CMT catalog. We model the cGPS data using Defnode, which performs inverse modeling using the Okada (1985; 1992) method to determine the elastic slip distribution along block (CA plate, NA plate, SA plate, forearc sliver) boundaries. Free parameters are the distribution of locking ratios of the fault representing the subduction zone, and deformation in each block. Our model’s weakness is that it is constrained by data from islands with a limited distribution in trying to model a large area. Using the SRC earthquake catalogue, the subduction zone was gridded and we summed the total seismic moment for each grid cell, using only M>3.5 earthquakes with low RMS values. The gridded seismic moment rates were then averaged over the length of time of the complete catalog. Several models were then produced with Defnode and a solution optimization technique that estimated the locking ratio distribution. The models’ resulting seismic moment rates are then compared against those gleaned from the SRC catalogue. Previous works have suggested that subducting ridges on the NA plate may indeed be locked, this work also tries to identify the locking ratio of these ridges.