Segmentation of the Blanco Transform Fault Zone from earthquake analysis: Complex tectonics of an oceanic transform fault

Abstract

The Blanco Transform Fault Zone (BTFZ) forms the ∼350 km long Pacific-Juan de Fuca plate boundary between the Gorda and Juan de Fuca ridges. Nearby broadband seismic networks provide a unique framework for a detailed, long-term seismotectonic study of an entire oceanic transform fault (OTF) system. We use regional waveforms to determine 129 earthquake source parameters; combined with 28 Harvard moment tensors, they represent the largest waveform derived OTF source parameter data set. Joint epicenter determination removes the northeasterly routine location bias. Projecting seismicity onto the BTFZ, we determine along-fault seismic slip rate variations. Earthquake source parameters and morphology indicate several transform segments separated by extensional step overs. The eastern segment from Gorda Ridge to Gorda Depression is a pull-apart basin. The longest transform (∼150 km) following Blanco Ridge from the Gorda to Cascadia depression is seismically very active, seismically fully coupled, has a wider seismic zone (∼9 km) than other BTFZ transform segments and accommodates the largest (Mw 6.4-6.5) BTFZ earthquakes. Interpretation of Cascadia Depression as spreading ridge is supported by plate motion parallel normal faulting T axes. Spreading is currently tectonic; 9 km deep earthquakes indicate a deep source for intermittent intrusives and rapid postemplacement cooling. A short transform connects to the pull-apart Surveyor Depression. Widely spread seismicity along the western BTFZ reflects complex morphology indicating ongoing plate boundary reorganization along short, narrow width subparallel faults. Seismic coupling is low in extensional (± 15%) compared to transform areas (35-100%), implying different mechanical roperties. Centroid depth variations are consistent with seismic slip cutoff near 600°C. Copyright 2008 by the American Geophysical Union.