Active Pacific North America plate boundary tectonics as evidenced by seismicity in the oceanic lithosphere offshore Baja California, Mexico

Egill Hauksson, Hiroo Kanamori, Joann M. Stock, Marie-Helene Cormier, & Mark R. Legg

Published 2014, SCEC Contribution #1745

Pacific Ocean crust west of southwest North America was formed by Cenozoic seafloor spreading between the large Pacific plate and smaller microplates. The eastern limit of this seafloor, the continent-ocean boundary, is the fossil trench along which the microplates subducted and were mostly destroyed in Miocene time. The Pacific-North America plate boundary motion today is concentrated on continental fault systems well to the east, and this region of oceanic crust is generally thought to be within the rigid Pacific plate. Yet, the December 14, 2012 Mw6.3 earthquake that occurred about 275 km west of Ensenada, Baja California, Mexico is evidence for continued tectonism in this oceanic part of the Pacific plate. The preferred mainshock centroid depth of 20 km was located close to the bottom of the seismogenic thickness of the young oceanic lithosphere. The focal mechanism, derived from both teleseismic P-wave inversion and W-phase analysis of the mainshock waveforms, and the 12 aftershocks of M~3-4 are consistent with normal faulting on northeast striking nodal planes, which align with surface mapped extensional tectonic trends such as volcanic features in the region. Previous Global Positioning System (GPS) measurements on offshore islands in the California Continental Borderland had detected some distributed Pacific and North America relative plate motion strain that could extend into the epicentral region. The release of this lithospheric strain along existing zones of weakness is a more likely cause of this seismicity than current thermal contraction of the oceanic lithosphere or volcanism. The mainshock caused weak to moderate ground shaking in the coastal zones of southern California, USA and Baja California, Mexico but the tsunami was negligible.

Citation
Hauksson, E., Kanamori, H., Stock, J. M., Cormier, M., & Legg, M. R. (2014). Active Pacific North America plate boundary tectonics as evidenced by seismicity in the oceanic lithosphere offshore Baja California, Mexico. Geophysical Journal International, 196(3), 1619-1630. doi: 10.1093/gji/ggt467.