Seismic velocity constraints on the material properties that control earthquake behavior at the Quebrada‐Discovery‐Gofar transform faults, East Pacific Rise

Abstract

.[1] Mid-ocean ridge transform faults (RTFs) vary strongly along strike in their ability to generate large earthquakes. This general observation suggests that local variations in material properties along RTFs exert a primary control on earthquake rupture dynamics. We explore these relationships by examining the seismic structure of two RTFs that have distinctly different seismic coupling. We determine the seismic velocity structure at the Gofar and Quebrada faults on the East Pacific Rise (EPR) using P wave traveltime tomography with data from two active-source wide-angle refraction lines crossing the faults. We image low-velocity zones (LVZs) at both faults, where P wave velocities are reduced by as much as 0.5-1.0 km/s ($10-20%) within a several kilometer wide region. At the Gofar fault, the LVZ extends through the entire crust, into the seismogenic zone. We rule out widespread serpentinization as an explanation for the low velocities, owing to the lack of a corresponding signal in the locally measured gravity field. The reduced velocities can be explained if the plate boundary region is composed of fault material with enhanced fluid-filled porosity (1.5-8%). Local seismic observations indicate that the high-porosity region lies within a $10 km long portion of the fault that fails in large swarms of microearthquakes and acts as a barrier to the propagation of large (M $ 6.0) earthquakes. Tomographic images of fault structure combined with observed earthquake behavior suggest that EPR transform segments capable of generating large earthquakes have relatively intact gabbro within the seismogenic zone, whereas segments that slip aseismically or via earthquake swarms are composed of highly fractured, ≥2 km wide damage zones that extend throughout the crust.

The relationship between seismicity and fault structure on the Discovery transform fault, East Pacific Rise

“…A one‐dimensional version of the P‐wave velocity model developed by Roland et al . [] for the Gofar transform fault was used. The Vp/Vs ratio of 1.87 was obtained by fitting a linear least squares regression to differential S‐wave versus P‐wave arrival times for the two stations located on the fault trace (D01 and D07; Figures and ).…”

Crust and Lithospheric Structure - Seismic Structure of Mid-Ocean Ridges

“…Only a few seismic studies have examined their structure (e.g., McGuire et al, 2012;Roland et al, 2012;van Avendonk et al, 1998van Avendonk et al, , 2001, and these studies are mainly limited to imaging at crustal depths. Transform faults are long-lasting features that can offset ridges by one hundred or more kilometers, though usually less, and leave large fracture zones in their wakes.…”

Seismic constraints on the processes and consequences of secondary igneous evolution of Pacific oceanic lithosphere

“…The true path is determined to be the global minimum path time, following Fermat's principle. As in previous studies [Moser et al, 1992;Van Avendonk et al, 1998;Roland et al, 2012], the grid search is limited to the forward propagation direction, termed the forward star, which we limit at a minimum angle of 0.1° between search directions. We consider 12 nodes in the x-direction and 12 nodes in the z-direction during the graph method search, and choose a grid spacing of 100 m in both the vertical and horizontal directions.…”