Sally F. McGill

Near-field investigations of the Landers earthquake sequence, April to July 1992.

The Landers earthquake, which had a moment magnitude (Mw) 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 Mw 6.1 preshock and Mw 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.

Primary Surface Rupture Associated with the Mw 7.1 16 October 1999 Hector Mine Earthquake, San Bernardino County, California

The M w 7.1 Hector Mine earthquake occurred within the Mojave Desert portion of the eastern California shear zone and was accompanied by 48 km of dextral surface rupture. Complex northward rupture began on two branches of the Lavic Lake fault in the northern Bullion Mountains and also propagated southward onto the Bullion fault. Lesser amounts of rupture occurred across two right steps to the south. Surface rupture was mapped using postearthquake, 1:10,000-scale aerial photography. Field mapping provided additional detail and more than 400 fault-rupture observations; of these, approximately 300 measurements were used to characterize the slip distribution. En echelon surface rupture predominated in areas of thick alluvium, whereas in the bedrock areas, rupture was more continuous and focused within a narrower zone. Measured dextral offsets were relatively symmetrical about the epicentral region, with a maximum displacement of 5.25 ± 0.85 m.[*][1] Vertical slip was a secondary component and was variable, with minor west-side-down displacements predominating in the Bullion Mountains. Field and aerial photographic evidence indicates that most of the faults that ruptured in 1999 had had prior late-Quaternary displacement, although only limited sections of the rupture show evidence for prior Holocene displacement. [1]: #fn-1

Surficial Offsets on the Central and Eastern Garlock Fault Associated With Prehistoric Earthquakes

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.

Holocene slip rate of the Central Garlock Fault in southeastern Searles Valley, California

A Late Pleistocene shoreline at the overflow level of Searles Lake has been displaced 82 to 106 m (preferred value is 90 m) in a left-lateral sense and 2.5 m (net) north side up along the Garlock fault, at the southeastern corner of Searles Valley. Previously published radiocarbon dates from both surface and subsurface strata indicate that the most recent highstand of Searles Lake ended sometime between 10,000 and 13,800 14C years ago. The maximum slip rate of the Garlock fault in southeastern Searles Valley is thus 11 mm/14C yr. If part of the offset of the shoreline occurred during older lakestands, then the slip rate may be somewhat less. A channel incised after the most recent highstand, however, is offset about 68 m, indicating that the minimum slip rate is 5 mm/14C yr. Subjective evaluation of the constraints on the offset and on the age of the shoreline yields a preferred rate of 6–8 mm/14C yr at this site. Assuming Bard et al.s (1990) recent calibration of the radiocarbon time scale, the calibrated slip rate of the Garlock fault is between 4 and 9 mm/yr with a preferred value of 5–7 mm/yr. This estimate is similar to a previous estimate of the Holocene slip rate and is slightly less than an estimate derived from modelling of geodetic data. Extension north of the Garlock fault in Indian Wells and Searles valleys contributes no more than 3 mm/yr left slip to the Garlock fault.

Slip rate of the western Garlock fault, at Clark Wash, near Lone Tree Canyon, Mojave Desert, California

The precise tectonic role of the left-lateral Garlock fault in southern California has been controversial. Three proposed tectonic models yield significantly different predictions for the slip rate, history, orientation, and total bedrock offset as a function of distance along strike. In an effort to test these models, we present the first slip-rate estimate for the western Garlock fault that is constrained by radiocarbon dating. A channel (referred to here as Clark Wash) incised into a Latest Pleistocene alluvial fan has been left-laterally offset at least 66 ± 6 m and no more than 100 m across the western Garlock fault, indicating a left-lateral slip rate of 7.6 mm/yr (95% confidence interval of 5.3–10.7 mm/yr) using dendrochronologically calibrated radiocarbon dates. The timing of aggradational events on the Clark Wash fan corresponds closely to what has been documented elsewhere in the Mojave Desert, suggesting that much of this activity has been climatically controlled. The range-front fault, located a few hundred meters northwest of the Gar-lock fault, has probably acted primarily as a normal fault, with a Holocene rate of dip-slip of 0.4–0.7 mm/yr. The record of prehistoric earthquakes on the Garlock fault at this site, though quite possibly incomplete, suggests a longer interseismic interval (1200–2700 yr) for the western Garlock fault than for the central Garlock fault. The relatively high slip rate determined here indicates that the western and central segments of the Garlock fault show similar rates of movement that are somewhat faster than rates inferred from geodetic data. The high rate of motion on the western Garlock fault is most consistent with a model in which the western Garlock fault acts as a conjugate shear to the San Andreas fault. Other mechanisms, involving extension north of the Garlock fault and block rotation at the eastern end of the fault may be relevant to the central and eastern sections of the fault, but they cannot explain a high rate of slip on the western Garlock fault.

Latest Pleistocene and Holocene slip rate for the San Bernardino strand of the San Andreas fault, Plunge Creek, Southern California: Implications for strain partitioning within the southern San Andreas fault system for the last ∼35 k.y.

An alluvial succession on the northeast side of the San Bernardino strand of the San Andreas fault includes distinctive aggradational and degradational features that can be matched with correlative features on the southwest side of the fault. Key among these are (1) a terrace riser on the northeast side of the fault that correlates with an offset channel wall on the southwest side of the fault and forms a basis for slip estimates for the period ca. 35 ka to the present, and (2) a small alluvial fan on the southwest side of the fault that has been matched with its most likely source gullies on the northeast side of the fault and forms a basis for slip estimates for the last 10.5 k.y. Slip-rate estimates for these two separate intervals are nearly identical. The rate for the older feature is most likely between 8.3 and 14.5 mm/yr, with a 95% confidence interval of 7.0–15.7 mm/yr. The rate for the younger feature is most likely between 6.8 and 16.3 mm/yr, with a 95% confidence interval of 6.3–18.5 mm/yr. These rates are only half the previously published slip rate for the San Andreas fault 35 km to the northwest in Cajon Pass, a rate that traditionally is extrapolated southeastward along the San Bernardino section of the fault. Results from Plunge Creek suggest that about half of the 25 mm/yr rate at Cajon Pass transfers southeastward to the San Jacinto fault, as proposed by other workers on the basis of regional geologic relations. These results indicate that the discrepancy between latest Quaternary slip rates and present-day rates of strain accumulation across the San Bernardino section of the San Andreas fault from geodesy can be largely explained by slip transfer between faults, leading to spatial variation in rate along the San Andreas fault. Nonetheless, the latest Pleistocene and Holocene slip rate at Plunge Creek is still somewhat faster than rates inferred for the San Bernardino section of the San Andreas fault based on elastic block modeling of geodetic data and may be more appropriate than those rates for hazard estimation.

Paleoseismology of the San Andreas Fault at Plunge Creek, near San Bernardino, Southern California

We have documented the stratigraphy and structure of several trenches across the San Bernardino strand of the San Andreas fault at the Plunge Creek site, near San Bernardino, southern California. The most recent faulting event exposed in the trenches (event W) appears to have occurred between about A.D. 1440 and A.D. 1660, if the radiocarbon dates are taken at face value. Two of the trenches reveal suggestive evidence for an older faulting event (event R), which postdates A.D. ∼1220. Because the age control at Plunge Creek is based on radiocarbon dating of detrital-charcoal samples, we must consider all of the radiocarbon ages as maximum estimates of the depositional ages for the layers from which the samples were collected. Thus, event W is not strictly constrained to predate A.D. 1660. We use ecological arguments to infer that the detrital-charcoal samples at the Plunge Creek site probably overestimate the depositional ages of the sedimentary layers by about 1 ± 1 fire-cycle (i.e., by about 70 ± 70 years). An independent estimate, based on extrapolation of sedimentation rates to the ground surface, suggests a similar value (0–95 years) for the lag time between the calibrated radiocarbon date of a sample and the depositional date of the layer from which it was collected. After applying an estimated correction (70 years) for the inherited ages of the detrital-charcoal samples, the date of event W is most likely between A.D. 1510 and A.D. 1730, with a preferred date of about A.D. 1630. The preferred date for event R is about A.D. 1450. Given the range of allowable dates, event W probably correlates with either the second-youngest earthquake (A.D. ∼1690) or the third youngest earthquake (A.D. ∼1600) documented at the nearest paleoseismic site to the northwest (Pitman Canyon). Comparison with the paleoseismic record at other sites along the southern San Andreas fault suggests a rupture length of 85–190 km for event W, implying a magnitude of M 7.3–7.7. Event R probably correlates with the fourth-youngest event at Pitman Canyon (∼A.D. 1450). Prehistoric earthquakes within several decades of this date have been documented all along the ∼450-km length of the southern San Andreas fault, suggesting (though by no means requiring) that event R could potentially have been a very large earthquake ( M ∼8.2). If our interpretation is correct, surface rupture from the youngest earthquake at Pitman Canyon (A.D. 1812) apparently died out between Pitman Canyon and Plunge Creek. Previous estimates of the probability of future earthquakes have assumed that the entire length of the San Bernardino strand slipped during the 1812 earthquake. Our results suggest that the southeastern half of this section of the fault did not slip in 1812 and that strain has been accumulating on this portion of the fault for at least the past 3 centuries. Depending on the slip rate, accumulated strain sufficient to generate 4.2–7.5 m of right-lateral slip may currently be stored on the southeastern half of the San Bernardino strand of the San Andreas fault.

Ages of Late Holocene earthquakes on the central Garlock fault near El Paso Peaks, California

A trench across the central Garlock fault within a small playa adjacent to El Paso Mountains revealed buried fissures and fault scarps that provide strong evidence for five surface rupturing earthquakes within the past 5 kyr. Weaker evidence suggested the possibility of three additional events within this same time period. These observations indicate an average recurrence interval of 700 to 1200 years. Nearly all 28 radiocarbon dates on samples of detrital charcoal, wood and shell are in correct stratigraphic order. These dates constrain the ages of the individual faulting events and indicate that the recurrence interval is not periodic but is quite irregular. Individual recurrence intervals range from 190 to 1545 years (if all eight events are actual earthquakes) or from 190 to 3405 years (if only the five well-documented events represent earthquakes). The two most recent earthquakes occurred within the past 550 years, with preferred ages of about A.D. 1790 and A.D. 1600. The A.D. 1790 event ruptured a narrower width of the fault zone, and may have been a smaller earthquake than the others. The radiocarbon dates from the trench also indicate that the sedimentation rate on the playa has fluctuated within the past 5 kyr. Low sedimentation rates (0.1 to 0.35 mm/yr) and infrequent floods (≤1 per century) prevailed from about 2000 B.C. to A.D. 100 and from A.D. 500 to 1300. During the remaining intervals within the past 5 kyr, the sedimentation rate averaged 1.0 to 1.7 mm/yr and major floods occurred two to four times per century. The changes in sedimentation rate and flood frequency may reflect climatic changes, or they may simply reflect changes in drainage patterns.

Short-term variations in slip rate and size of prehistoric earthquakes during the past 2000 years on the northern San Jacinto fault zone, a major plate-boundary structure in southern California

Most of the displacement across the North American−Pacific plate boundary in southern California is accommodated by the San Jacinto and the southern San Andreas fault zones. If and how the rate of displacement across these fault zones varies along strike and through time are still being resolved. Here, we present four calculations of late Holocene slip rate and average slip per event from the Claremont fault of the northern San Jacinto fault zone that show variations in strain distribution over the past 2000 yr and illustrate how plate-boundary displacement is distributed between the San Jacinto and southern San Andreas fault zones. We calculate a slip rate of 12.8–18.3 mm/yr and an average slip per event of 2.5 m from two measurements of streams offset by 9–11 earthquakes in the past 1500–2000 yr. Faster slip rates of 21–30 mm/yr and an average slip per event of 2.7–3 m were determined from measurements of a stream and a buried channel that were offset by three earthquakes in the past 400–500 yr. The 2000 yr slip rate is similar to the range in slip rates reported for the adjacent San Bernardino section of the San Andreas fault zone, suggesting that the northern San Jacinto accommodates a similar amount of displacement as the San Andreas fault zone at the same latitude. The rate is also slightly faster (by ∼2–3 mm/yr) than reported slip rates from the central San Jacinto fault zone to the southeast. A slip rate of 15 ± 2 mm/yr is within the range of uncertainty for almost all the geologic and geodetic data for the entire length of the San Jacinto fault zone and may be the best approximation for long-term average slip rate of the fault zone. Alternatively, 2–3 mm/yr of slip along the northern San Jacinto fault zone may be accommodated to the south along the lesser-studied Hot Springs, Thomas Mountain, Buck Ridge, and Santa Rosa faults, the lateral slip rates of which are not well known nor included in typical estimates of slip rate along the central San Jacinto fault zone. We infer that the faster slip rate over the past 500 yr is due to a cluster of earthquakes along the Claremont fault between A.D. 1400 and A.D. 1850 and larger-than-average surface displacement of 3 m or more during the third event back. The 3 m or more measurement of displacement in this event corresponds to rupture lengths that are slightly longer than the total length of the Claremont fault, and previously published paleoseismic data indicate that this event occurred coincident in time with an event on the adjacent Clark fault. We propose that this combination of slip per event data and paleoseismic data from adjacent fault strands is strong evidence for rupture through the releasing step over that separates these two segments of the San Jacinto fault zone.

Kinematic modeling of fault slip rates using new geodetic velocities from a transect across the Pacific-North America plate boundary through the San Bernardino Mountains, California

©2015. American Geophysical Union. All Rights Reserved. Campaign GPS data collected from 2002 to 2014 result in 41 new site velocities from the San Bernardino Mountains and vicinity. We combined these velocities with 93 continuous GPS velocities and 216 published velocities to obtain a velocity profile across the Pacific-North America plate boundary through the San Bernardino Mountains. We modeled the plate boundary-parallel, horizontal deformation with 5-14 parallel and one obliquely oriented screw dislocations within an elastic half-space. Our rate for the San Bernardino strand of the San Andreas Fault (6.5±3.6mm/yr) is consistent with recently published latest Quaternary rates at the 95% confidence level and is slower than our rate for the San Jacinto Fault (14.1±2.9mm/yr). Our modeled rate for all faults of the Eastern California Shear Zone (ECSZ) combined (15.7±2.9mm/yr) is faster than the summed latest Quaternary rates for these faults, even when an estimate of permanent, off-fault deformation is included. The rate discrepancy is concentrated on faults near the 1992 Landers and 1999 Hector Mine earthquakes; the geodetic and geologic rates agree within uncertainties for other faults within the ECSZ. Coupled with the observation that postearthquake deformation is faster than the pre-1992 deformation, this suggests that the ECSZ geodetic-geologic rate discrepancy is directly related to the timing and location of these earthquakes and is likely the result of viscoelastic deformation in the mantle that varies over the timescale of an earthquake cycle, rather than a redistribution of plate boundary slip at a timescale of multiple earthquake cycles or longer.