Potential for a large earthquake near Los Angeles inferred from the 2014 La...

Donnellan, A., L. G. Ludwig, J. W. Parker, J. Rundle, J. Wang, M. Pierce, G. Blewitt, and S. Hensley (2015), Potential for a large earthquake near Los Angeles inferred from the 2014 La Habra earthquake, Earth and Space Science, 2, 378-385, doi:10.1002/2015EA000113.

Tectonic motion across the Los Angeles region is distributed across an intricate network of strike-slip and thrust faults that will be released in destructive earthquakes similar to or larger than the 1933 M6.4 Long Beach and 1994 M6.7 Northridge events. Here we show that Los Angeles regional thrust, strike-slip, and oblique faults are connected and move concurrently with measurable surface deformation, even in moderate magnitude earthquakes, as part of a fault system that accommodates north-south shortening and westerly tectonic escape of northern Los Angeles. The 28 March 2014 M5.1 La Habra earthquake occurred on a northeast striking, northwest dipping left-lateral oblique thrust fault northeast of Los Angeles. We present crustal deformation observation spanning the earthquake showing that concurrent deformation occurred on several structures in the shallow crust. The seismic moment of the earthquake is 82% of the total geodetic moment released. Slip within the unconsolidated upper sedimentary layer may reflect shallow release of accumulated strain on still-locked deeper structures. A future M6.1–6.3 earthquake would account for the accumulated strain. Such an event could occur on any one or several of these faults, which may not have been identified by geologic surface mapping. 1. The 2014 M 5.1 La Habra Earthquake The M5.1 La Habra earthquake occurred on 28 March 2014 at a depth of ~5.85 km (33.9225°N, 117.9352°W) beneath suburban La Habra at the northeastern margin of the Los Angeles basin [Wright, 1991] in southern California. Global Positioning System (GPS) geodesy and interferometric synthetic aperture radar (InSAR) show that the northern Los Angeles region is shortening at a rate of 4.5 ± 1 mm/yr in a north-south direction [Argus et al., 2005]. The style of tectonic deformation in the region is influenced by northwest trending rightlateral strike-slip faults associated with the Peninsular Ranges and San Andreas plate boundary fault system, and north-south shortening along north dipping thrust faults, often associated with oblique left-lateral motion and east-west trending folds of the Transverse Ranges [Yeats, 2004]. Seismic hazard assessments for the region have focused on major faults such as the strike-slip Newport-Inglewood fault that caused the destructive 1933 Mw 6.4 Long Beach earthquake [Hauksson and Gross, 1991] and the blind thrust system that generated the 1994 M6.7 Northridge earthquake [Walls et al., 1998]. The La Habra earthquake sequence occurred between the right-lateral strike-slip Whittier fault and the Puente Hills thrust fault, above a regional decollement [Yang and Hauksson, 2011]. A surprising amount of damage occurred, despite the moderate main shock magnitude and peak ground accelerations of 0.7 g and 0.35 g northeast and southeast of and near the epicenter (USGS: Peak ground acceleration for La Habra earthquake,

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Earth Surface & Interior Program (ESI)