The Landers earthquake, which had a moment magnitude (M(w)) of 7.3, was the largest earthquake to strike the contiguous United States in 40 years. This earthquake resulted from the rupture of five major and many minor right-lateral faults near the southern end of the eastern California shear zone, just north of the San Andreas fault. Its M(w) 6.1 preshock and M(w) 6.2 aftershock had their own aftershocks and foreshocks. Surficial geological observations are consistent with local and far-field seismologic observations of the earthquake. Large surficial offsets (as great as 6 meters) and a relatively short rupture length (85 kilometers) are consistent with seismological calculations of a high stress drop (200 bars), which is in turn consistent with an apparently long recurrence interval for these faults.

Geomorphic features offset along the central and eastern Garlock fault record the amount of surface slip associated with prehistoric earthquakes. Along the easternmost 90 km of the fault, the smallest offsets cluster around 2–3 m of left‐lateral slip, apparently associated with the most recent earthquake on this portion of the fault. Larger offsets along this part of the fault, especially in Pilot Knob Valley, cluster around values consistent with 2–4 m of slip in each of the past several events. Farther west, south of El Paso Mountains, offset geomorphic features suggest that each of the past two earthquakes on this stretch of the Garlock fault was produced by about 7 m of slip, whereas the third event back was produced by about 4 m of slip. Vertical displacements of geomorphic features range from 0% to 30% of the left‐lateral offsets. Within Pilot Knob Valley (along the southern side of the Slate Range), vertical displacements are consistently up on the northern side, whereas within the Avawatz Mountains both north‐ and south‐side‐up vertical displacements are present. On the basis of the geomorphic offsets, the geometry of the Garlock fault, and the precedents set by historical strike‐slip earthquakes elsewhere, a number of different rupture patterns are plausible. These range from rupture of the entire Garlock fault in a single event with a maximum magnitude of about Mw=7.8, to separate rupture of the western segment and of the central and eastern segments combined, with approximate magnitudes Mw≤1.1 and Mw=7.5, respectively, to separate rupture of even shorter segments, producing earthquakes of magnitudes Mw=6.6 to Mw=7.5. In conjunction with available slip rates for the Garlock fault, the geomorphic offsets suggest that average recurrence intervals are probably within the range of 600–1200 years south of El Paso Mountains, about 200–750 years in Searles Valley, about 200–1300 years in Pilot Knob Valley, and about 200–3000 years near Leach Lake and in the Avawatz Mountains.

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Pronounced variations in fault slip rate revealed by new measurements along the Garlock fault have basic implications for understanding how faults store and release strain energy in large earthquakes. Specifically, dating of a series of 26.0 +3.5 /-2.5 m fault offsets with a newly developed infrared-stimulated luminescence method show that the fault was slipping at >14.0 +2.2 /-1.8 mm/yr, approximately twice as fast as the long-term average rate, during a previously documented cluster of four earthquakes 0.5-2.0 ka. This elevated late Holocene rate must be balanced by periods of slow or no slip such as that during the c. 3300-yr-long seismic lull preceding the cluster. Moreover, whereas a comparison of paleoseismic data and stress modeling results suggests that individual Garlock earthquakes may be triggered by periods of rapid San Andreas fault slip or very large-slip events, the "on-off" behavior of the Garlock suggests a longer-term mechanism that may involve changes in the rate of elastic strain accumulation on the fault over millennial time scales. This inference is consistent with most models of the geodetic velocity field, which yield slip-deficit rates that are much slower than the average latest Pleistocene-early Holocene (post-8-13 ka) Garlock slip rate of 6.5±1.5 mm/yr. These observations indicate the occurrence of millennia-long strain "super-cycles" on the Garlock fault that may be associated with temporal changes in elastic strain accumulation rate, which may in turn be controlled by variations in relative strength of the various faults in the Garlock-San Andreas-Eastern California Shear Zone fault system and/or changes in relative plate motion rates. 1. Introduction The degree to which fault loading and strain release rates are constant in time and space is one the most fundamental, unresolved issues in modern tectonics. Analysis of faults reveals a wide range of behaviors, including: (1) relatively regular timing of earthquakes on some large strikeslip faults (e.g.,

[1] We present the first results from a dense network of 36 campaign and 46 continuous GPS stations located in the Eastern Transverse Ranges Province (ETR), a transition zone between the southernmost San Andreas fault (SSAF) and eastern California shear zone (ECSZ). We analyzed the campaign data together with available data from continuous GPS stations for the period 1994-2009. We used the GPS velocity estimates to constrain elastic block models to investigate fault-loading rates representing four hypotheses characterized by different fault-block geometries. Fault-block scenarios include blocks bounded by the east-striking left-lateral Pinto Mountain, Blue Cut, and Chiriaco faults of the ETR; blocks bounded by a right-lateral north-northwest striking structure (the "Landers-Mojave earthquake line") that cuts obliquely across the ETR and mapped Mojave Desert faults; and combinations of these end-member hypotheses. Each model implies significantly different active fault geometries, block rotation rates, and slip rates for ETR and ECSZ structures. All models suggest that SSAF slip rate varies appreciably along strike, generally consistent with rates derived from tectonic geomorphology and paleoseismology, with a maximum of ∼23 mm/yr right-lateral along the southernmost Coachella Valley strand, decreasing systematically to <10 mm/yr right-lateral through the San Gorgonio Pass region. Slip rate estimates for the San Jacinto fault are ∼12 mm/yr for all models tested. All four models fit the data equally well in a statistical sense. Qualitative comparison among models and consideration of geologic slip rates and other independent data reveals strengths and weaknesses of each model.