ALBERT

Description: 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.

Description: We investigate a releasing stepover between the Casa Loma and Claremont strands of the northern San Jacinto fault zone to evaluate the late Quaternary structural evolution of the fault zone, and to assess the likelihood of a rupture jumping across the stepover. Our new cone penetration test (CPT) and trench observations along the Claremont fault at Mystic Lake indicate that the main strand of the Claremont fault has jumped nearly a half kilometer westward into the San Jacinto releasing stepover during the late Quaternary. Multiple faults are inferred from the CPT data within a small sag at the northeast side of the stepover that cuts through younger stratigraphy to the west of the basin-bounding fault near Mystic Lake. Previous seismic-reflection data also suggest the presence of a young fault that cuts basin strata beneath the middle of Mystic Lake farther west of our study area. Numerous tectono-geomorphic features observed in satellite and Light Detection and Ranging Digital Elevation Model (LiDAR DEM) imagery are interpreted to delineate the location of the currently active faults, including a zone of faults that cut across the basin from the northern end of the Casa Loma fault to the Claremont fault. Seismicity observations suggest the presence of many faults within the stepover zone. Finally, new paleoseismic data from the Mystic Lake site suggest that some late Holocene earthquakes may have jumped the stepover. All of these observations suggest that the San Jacinto stepover, which has been used as the primary basis for segmenting the northern San Jacinto fault zone, is being bypassed and that the fault zone may now be capable of larger earthquakes than previously expected.

Description: We systematically mapped (scales 〉1:500) the surface rupture of the 4 April 2010 Mw (moment magnitude) 7.2 El Mayor-Cucapah earthquake through the Sierra Cucapah (Baja California, northwestern Mexico) to understand how faults with similar structural and lithologic characteristics control rupture zone fabric, which is here defined by the thickness, distribution, and internal configuration of shearing in a rupture zone. Fault zone thickness and master fault dip are strongly correlated with many parameters of rupture zone fabric. Wider fault zones produce progressively wider rupture zones and both of these parameters increase systematically with decreasing dip of master faults, which varies from 20° to 90° in our dataset. Principal scarps that accommodate more than 90% of the total coseismic slip in a given transect are only observed in fault sections with narrow rupture zones (〈25 m). As rupture zone thickness increases, the number of scarps in a given transect increases, and the scarp with the greatest relative amount of coseismic slip decreases. Rupture zones in previously undeformed alluvium become wider and have more complex arrangements of secondary fractures with oblique slip compared to those with pure normal dip-slip or pure strike-slip. Field relations and lidar (light detection and ranging) difference models show that as magnitude of coseismic slip increases from 0 to 60 cm, the links between kinematically distinct fracture sets increase systematically to the point of forming a throughgoing principal scarp. Our data indicate that secondary faults and penetrative off-fault strain continue to accommodate the oblique kinematics of coseismic slip after the formation of a thoroughgoing principal scarp. Among the widest rupture zones in the Sierra Cucapah are those developed above buried low angle faults due to the transfer of slip to widely distributed steeper faults, which are mechanically more favorably oriented. The results from this study show that the measureable parameters that define rupture zone fabric allow for testing hypotheses concerning the mechanics and propagation of earthquake ruptures, as well as for siting and designing facilities to be constructed in regions near active faults.

Description: We present new data from the Mystic Lake paleoseismic site along the Claremont segment of the northern San Jacinto fault zone. The site is located within a sag formed between two fault strands that pass through the eastern side of Mystic Lake in the San Jacinto Valley. Trenches excavated across the sag exposed faulted and folded lacustrine and alluvial strata that record at least seven ground-rupturing earthquakes during the past 1600 yr. Evidence for past surface deformation includes upward terminating faults with associated fissure fills, folding, angular unconformities, and pinching of strata against a paleoscarp. All of the event horizons occur at the tops of paleosols and are overlain by massive lacustrine clay units. We interpret this pattern to represent development of soils at the surface between earthquakes that are buried when fault rupture causes subsidence and renewed filling of the depression with lacustrine sediments. The ages of the events are constrained by 50 radiocarbon dates determined from detrital charcoal. The recurrence interval for the past seven events ranges from 159 to 210 yr, and the most recent event occurred sometime between A.D. 1738 and 1850 based on radiocarbon ages trimmed by historical data. Some of the event ages at Mystic Lake overlap in time with events recorded at Hog Lake on the Clark strand of the San Jacinto fault zone to the south, suggesting that these events may have jumped the San Jacinto Valley releasing stepover, or that events on one fault triggered closely timed events on the adjacent fault.

Description: We present the results of intensive field investigations of the scarp associated with the 23 February 1892 earthquake in northern Baja California. Newly recognized additional offsets suggest the rupture was about 58 km in length, twice as long as previous estimates. Slip produced in the 1892 event varied from purely dextral slip near the international border to roughly 1:1 oblique-normal slip farther south along the 2–4-km-deep portion of the Laguna Salada basin. The portion of the 1892 rupture with oblique-normal slip comprises a number of short, poorly organized, and discontinuous fault scarps with heights that vary in concert with their strike. Slip was linked farther south to a short, purely normal fault that forms a large releasing bend at the southern termination of the fault zone. Given the distribution of slip along the earthquake and a likely range of locking depths, we conclude the 1892 earthquake was between M w  7.1–7.3 in magnitude, consistent with previous estimates from macroseismic observations. The length of the Laguna Salada fault that ruptured in 1892 also accommodated minor normal sense displacement along much of its length in the recent 2010 M w  7.2 El Mayor–Cucapah earthquake, which guided the remapping effort. Online Material: Table of displacement measurements with uncertainty, location, waypoint number, soil unit designation, and the strike, dip, and type of feature measured.

Description: We measured offsets on tectonically displaced geomorphic features along 80 km of the Clark strand of the San Jacinto fault (SJF) to estimate slip-per-event for the past several surface ruptures. We identify 168 offset features from which we make over 490 measurements using B4 light detection and ranging (LiDAR) imagery and field observations. Our results suggest that LiDAR technology is an exemplary supplement to traditional field methods in slip-per-event studies. Displacement estimates indicate that the most recent surface-rupturing event (MRE) produced an average of 2.5–2.9 m of right-lateral slip with maximum slip of nearly 4 m at Anza, a Mw 7.2–7.5 earthquake. Average multiple-event offsets for the same 80 kms are ~5.5??m, with maximum values of 3 m at Anza for the penultimate event. Cumulative displacements of 9–10 m through Anza suggest the third event was also similar in size. Paleoseismic work at Hog Lake dates the most recent surface rupture event at ca. 1790. A poorly located, large earthquake occurred in southern California on 22 November 1800; we relocate this event to the Clark fault based on the MRE at Hog Lake. We also recognize the occurrence of a younger rupture along ~15–20??km of the fault in Blackburn Canyon with ~1.25??m of average displacement. We attribute these offsets to the 21 April 1918 Mw 6.9 event. These data argue that much or all of the Clark fault, and possibly also the Casa Loma fault, fail together in large earthquakes, but that shorter sections may fail in smaller events.Online Material: Topographic contour maps and hillshades generated from B4 LiDAR data, corresponding field photographs, and data tables comparing LiDAR-based and field-based slip measurements for individual geomorphic features along the Clark fault.

Description: Estimating the potential for the occurrence of large earthquakes on slow-slip-rate faults in continental interiors, away from plate boundaries, is possible only if the long-term geological record of past events is available. However, our knowledge of strong earthquakes appears to be incomplete for thrust faults flanking large actively growing mountain ranges, such as those surrounding Tibet and the Andes Mountains. We present a paleoseismic study of a prominent fault scarp at the west flank of the Andes in Santiago, Chile. The evidence demonstrates recurrent faulting with displacement of ~5 m in each event. With two large earthquake ruptures within the past 17–19 k.y., and the last event occurring ~8 k.y. ago, the fault appears to be ripe for another large earthquake (moment magnitude, M w 7.5). These results emphasize the potential danger of intraplate continental faults, particularly those associated with youthful mountain fronts.