Abstract
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 2012 December 14 M_w 6.3 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 main shock 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 main shock 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 main shock caused weak to moderate ground shaking in the coastal zones of southern California, USA, and Baja California, Mexico, but the tsunami was negligible.
Highlights
In 2012 December an Mw 6.3 earthquake occurred within oceanic lithosphere of the eastern Pacific Plate
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
The Mw 6.3 earthquake of 2012 December 14 may be the largest earthquake recorded since 1900 in the eastern Pacific oceanic lithosphere, to the west of the continental shelf of the southwestern United States and Baja California, Mexico. This area was previously considered to be aseismic, and having no significant shear strain rate. This earthquake was located far offshore, about 225 km away from the nearest seismic station, it was felt in the coastal communities in southern California and Baja California, Mexico
Summary
In 2012 December an Mw 6.3 earthquake occurred within oceanic lithosphere of the eastern Pacific Plate. It occurred in an unusual tectonic setting, near a fossil trench that juxtaposes Miocene oceanic lithosphere and submerged, thinned continental lithosphere of the California Continental Borderland. This region is hundreds of kilometres away from the edge of the Pacific Plate (Fig. 1). Nicholson et al 1994; Bohannon & Geist 1998) Both global and regional seismic networks in California, USA, and Baja California, Mexico, recorded the 2012 earthquake sequence. During the previous 80 yr, only three M3 events had been recorded in the epicentral area (Fig. 2)
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