Global Positioning System constraints on fault slip rates in southern California and northern Baja, Mexico

Abstract

We use Global Positioning System (GPS) estimates of horizontal site velocity to constrain slip rates on faults comprising the Pacific-North America plate boundary in southern California and northern Mexico. We enlist a simple elastic block model to parameterize the distribution and sum of deformation within and across the plate boundary. We estimate a Pacific-North America relative plate motion rate of 49 _+ 3 mm/yr (one standard deviation), consistent with NUVEL-1A estimates. We are able to resolve robust slip rate estimates for the southernmost San Andreas, San Jacinto, and Elsinore faults (26 _+ 2, 9 +_ 2, and 6 _+ 2 mm/yr, respectively) and for the Imperial and Cerro Prieto faults (35 _+ 2 and 42 _+ 1 mm/yr, respectively), accounting for about 86% of the total plate motion. The remaining 14% appears to be accommodated to the west of these fault systems, probably via slip along the San Clemente fault and/or the San Miguel, Vallecitos, Rose Canyon, and Newport-Inglewood fault systems. These results are highly consistent with paleoseismic estimates for slip rates implying that off-fault strain accumulation within the deforming zone of the plate boundary is largely elastic. We estimate that the seismically quiescent, southernmost San Andreas fault has incurred about 8.2 m of slip deficit over the last few hundred years, presumably to be recovered during a future large earthquake. Introduction Sometime around 30 Ma, the ancestral East Pacific Rise collided with the trench that then separated the North America and Farallon plates [Atwater, 1970] bringing the Pacific and North America plates into contact. Although the details of the early evolution of this boundary are hazy, by about 20 Ma the existence of a right-lateral proto-San Andreas transform near the present-day continental borderland is probable [e.g., Sedlock and Hamilton, 1991]. Eastward migration of the proto-San Andreas transform to its present location and the associated rifting of the Gulf of California appear to be as recent as 5 Ma. The NUVEL-1A global plate motion model [DeMets et al., 1990, 1994] predicts Pacific-North America relative motion across southern California of 46 _+ 1 mm/yr. This estimate represents plate motions averaged over the last 3 Myr. Recently, however, DeMets [1995] compared seafloor spreading rates in the Gulf of California (used in the calculation of the NUVEL-1A rate) with the NUVEL-iA closure fitting rate (CFR) for Pacific-North America relative motion (estimated in the absence of these spreading rate data) and found that the NUVEL-1A model may underestimate the true rate by as much as 8-9%. That is, the true rate of Pacific-North America relative motion may be closer to 50 mm/yr. It is the ongoing motion of these plates that drives the contemporary accumulation of elastic strain within the plate boundary and is therefore ultimately responsible for earthquakes throughout the region. •Now at the Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts. While the San Andreas fault accommodates a significant frac...

Late Quaternary slip rate gradient defined using high‐resolution topography and 10Be dating of offset landforms on the southern San Jacinto Fault zone, California

“…This, in turn, suggests a complex kinematic evolution, which may explain apparent discrepancies between slip rate estimates obtained from geologic and geodetic data. Our results are at odds with combined InSAR and GPS data, which suggest much higher slip rates for the southern SJFZ [Fialko, 2006;Lundgren et al, 2009], but consistent with GPS block models, as well as elastic and viscoelastic models of crustal deformation in this region [Bennett et al, 1996;Meade and Hager, 2005;Becker et al, 2005;Fay and Humphreys, 2005]; the large differences of these geodetically derived slip rates may result from differences in modeling approaches, or temporal and spatial coverage of the geodetic data. Seismic hazard studies commonly rely on long-term Quaternary rates to infer short-term hazard.…”

“…The southern San Andreas and the San Jacinto fault zones are the two principal structures, together accommodating ∼35 mm/yr, that is ∼80%, of the Pacific-North America (PA-NA) relative plate motion in this region [King and Savage, 1983;DeMets and Dixon, 1999;Bennett et al, 1996;Fialko, 2006]. Geodetically derived slip rate estimates are on the order of 10-20 mm/yr for both of these fault zones, but only 2-6 mm/yr for the Elsinore fault zone [Johnson et al, 1994;Bennett et al, 1996;Meade and Hager, 2005;Becker et al, 2005;Fay and Humphreys, 2005].…”

“…Block models of GPS data from the southern SJFZ indicate slip rates of 9-15 mm/yr (Table 1) [Bennett et al, 1996;Becker et al, 2005;Meade and Hager, 2005], which is consistent with a 14-15 mm/yr slip rate estimate inferred from elastic and viscoelastic models of crustal deformation (Table 1) [Fay and Humphreys, 2005]. In contrast, the results of a study by Fialko [2006], combining interferometric satellite synthethic aperture radar (InSAR) data with an elastic deep slipping SJFZ suggest a slip rate of 21 ± 1 mm/yr along the southern part of the fault zone, although this rate probably includes the strain accommodated by folding and NE-striking cross-faults.…”

Slip tapers at the tips of faults and earthquake ruptures

“…Inversions of geodetic and seismic data indicated that the slip at depth in the northern part of both events were somewhat higher than the surface values, about a meter for the 1940 and about half that for the 1979 event, but the deep slip at the southern end is similar to the surface displacement profile [King and Thatcher, 1998]. The potential slip accumulated on the Imperial fault between 1940 and 1979 is shown by the horizontal line, based on the estimate of 35 ± 2 mm/yr from GPS data [Bennett et al, 1996]. Hence the 1940 stress-drop had by 1979 been recovered in the northern end of the rupture but not in the southern end; the slip deficit there indicating that a residual stress-drop existed in that region in 1979.…”

Interseismic strain accumulation and the earthquake potential on the southern San Andreas fault system

“…Previous studies have attempted to estimate the contemporaneous slip rates on the southern SAF system by fitting elastic halfspace models to rather sparse ground-based geodetic measurements from a dozen of monuments spanning a ∼100 km long profile across the fault 8,9 . Also, InSAR (Interferometric Synthetic Aperture Radar) data collected in the near field of the geologically mapped fault trace were used to detect the presence and extent of fault creep 4 .…”