Shear and compressional wave structure of the East Pacific Rise, 9°–10°N

Abstract

We use shear and compressional arrivals recorded by 45 ocean bottom seismograph receivers to model regional structure of the 9°–10°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 layer 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) arrivals. 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). Waveform inversion was used to model the Poisson's ratio structure of layers 2B and 3 for nine instruments with good shear arrivals. Poisson's ratio 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 lateral variability in layer 2A structure. The mean Poisson's ratio of layer 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 thick cracks (aspect ratio α = 0.1) extend to depths of ∼ 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 anomalously low seismic velocities, with layer 2B and layer 3 velocities reduced by up to 11% from the regional values. Cracking theory suggests 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 predicted from our one Poisson's ratio measurement in this area. The anomalous velocities are located near the western discordant zone left by the 9°03′N overlapping spreading center. We suggest that shearing associated with on‐axis rotation of the overlap basins is responsible for the low seismic velocities. The pattern of anomalies indicates that faulting extends to distances of 10–15 km from the basins.