SHEAR AND COMPRESSIONAL WAVE STRUCTURE OF THE EAST PACIFIC RISE, 9-DEGREES-10-DEGREES-N

We use shear and compressional arrivals recorded by 45 ocean bottom se ismograph receivers to model regional structure of the 9 degrees-10 de grees N region of the East Pacific Rise. Modeling indicates that shear conversion occurs at the base of layer 2A, producing Ps (energy that travels through oceanic crust as P and converts to S at the base of la yer 2A on upgoing path) and pS (energy that travels as P through layer 2A and converts to S at the base of layer 2A on downgoing path) arriv als. The travel times of these arrivals require a shear wave velocity within layer 2A of 0.4-0.8 km/s (Poisson's ratio of 0.46-0.49). Wavefo rm inversion was used to model the Poisson's ratio structure of layers 2B and 3 for nine instruments with good shear arrivals. Poisson's rat io within layer 2B was highly variable, with values as low as 0.24 and a mean value of 0.263. Some of this variability might be due to later al variability in layer 2A structure. The mean Poisson's ratio of laye r 3 was 0.271. According to the cracking model of Shaw [1994], the low Poisson's ratios within the upper portion of layer 2B indicate that t hick cracks (aspect ratio a = 0.1) extend to depths of similar to 1.5- 1.7 km in this region. Two-dimensional travel time modeling indicates that the southwestern portion of our study area is associated with ano malously low seismic velocities, with layer 2B and layer 3 velocities reduced by up to 11% from the regional values. Cracking theory suggest s that these low velocities could be caused by porosities of 0.3-5.5%, depending on the crack aspect ratio; a maximum porosity of 1.5% is pr edicted from our one Poisson's ratio measurement in this area. The ano malous velocities are located near the western discordant zone left by the 9 degrees 03'N overlapping spreading center. We suggest that shea ring associated with on-axis rotation of the overlap basins is respons ible for the low seismic velocities. The pattern of anomalies indicate s that faulting extends to distances of 10-15 km from the basins.