Analysis of Apparent Stress for Subduction Zone Earthquakes Along the Nicoya Peninsula , Costa Rica

VOLUME NSF MARGINS Program, SEIZE Workshop, Mt. Hood, Oregon, 2008. Analysis of Apparent Stress for Subduction Zone Earthquakes Along the Nicoya Peninsula, Costa Rica Susan L. Bilek, sbilek@nmt.edu Jana Stankova-Pursley, janas@nmt.edu W. Scott Phillips, wsp@lanl.gov Andrew V. Newman, anewman@gatech.edu Subduction zones around the world release much of the world’s seismic energy in mega-thrust earthquakes. This energy release varies likely in response to variable strength of the mega-thrust zone. Here we examine the seismic energy along the megathrust at the Middle America Trench, focusing on small magnitude earthquakes at the northwestern margin of Costa Rica, Central America. This area is interesting because seismicity patterns vary along strike of the subduction zone, and there is heterogeneous topography on the incoming Cocos plate that could affect the strength of the interface. We compute source spectra and seismic moment from coda amplitude measurements for about 260 well-located earthquakes from the Nicoya Peninsula portion of the 1999-2001 CR-SEIZE project. Using the coda wave amplitudes provides more stability for magnitude estimates because we minimize effects of energy distortion from crustal heterogeneity. We use the source spectra to examine the source scal ing relationships between the large and the small events, and to explore possible spatial distribution of apparent stress. We find that apparent stress is larger for events along the northern and central portions of the Nicoya Peninsula than those in the southern portion of the peninsula, coinciding with the changes in interface dip as well as the subduction of the Fisher seamount group at the southern tip of Nicoya. These results suggest that the apparent stress is not constant within this dataset, and these imply that the subduction zone interface in the northern and central portions of the peninsula is either stronger or experiences higher friction along its surface than in the southern region. NSF MARGINS Program, SEIZE Workshop, Mt. Hood, Oregon, 2008. The Strength of Antigorite Deformed at High Temperatures Linda J. Chernak (Linda_Chernak@Brown.edu) Greg Hirth (Greg_Hirth@Brown.edu) Experiments have been conducted on intact cores of antigorite serpentine in a Griggstype apparatus at 550°C, 1.5 GPa, and a constant strain rate of ~10-5/s. In all experiments, despite choosing samples with limited impurities and despite the orientation of the foliation, the samples fractured at 45! to ! 1 and underwent frictional sliding. The fracture strength of the sample where fracture and slip occurred parallel to the foliation was ~100 200 MPa less than the fracture strength when foliation was at a 45! angle to the fracture. In addition, in the 45! oriented sample, the fracture appears to have initiated upon loading; it is possible that this is related to the mechanical anisotropy of the sample owing to the foliation. The results from our deformation experiments can be compared to results from antigorite deformed using the D-DIA apparatus at high temperature and pressure (Hilairet et al. 2007). Hilairet et al. (2007) deformed sintered powders of antigorite at 1 and 4 GPa, 300 500°C, and constant strain rates of ~10-4/s to 10-6/s and were able to determine flow law parameters for serpentine to fit the following power law: where is the strain rate, A is a material constant, ! is the differential stress, n is the stress exponent, E is the activation energy, V is the activation volume, R is the ideal constant, and P and T are the pressure and temperature of deformation. We have used the parameters determined by their deformation experiments to predict the strength of antigorite at our experimental conditions of 1.5 GPa and 550°C using the equation below: The strengths predicted by the flow law of Hilairet et al. (2007), ~550 – 850 MPa depending on which data set was used, are significantly lower than the fracture strength of antigorite in this study, which was 690 1365 MPa. The disagreement between the strength values predicted by the flow law of Hilairet et al. (2007) and the measured fracture strengths of our samples, indicate that additional experiments need to be conducted in the Griggs Rig to constrain strength measurements with our apparatus. We will conduct experiments on sintered powders of antigorite to more directly compare our measurements to those of Hilairet et al. (2007). Accurate strength data for serpentine is necessary in order to determine flow law parameters and, ultimately, calculate a viscosity for serpentine that can be incorporated subduction zone models.