Geology topics

Katherine M. Scharer

Publications and source records attributed to Katherine M. Scharer.

At least 37 records · Page 2Linked to original sources

USGS2021

The San Andreas fault paleoseismic record at Elizabeth Lake: Why are there fewer surface-rupturing earthquakes on the Mojave section?

The structural complexity of active faults and the stress release history along the fault system may exert control on the locus and extent of individual earthquake ruptures. Fault bends, in particular, are often invoked as a possible mechanism for terminating earthquake ruptures. However, there are few records available to examine how these factors may influence the along‐fault recurrence of earthquakes. We present a new paleoearthquake chronology for the southern San Andreas fault at Elizabeth Lake and integrate this record with existing paleoearthquake records to examine how the timing and frequency of earthquakes vary through a major restraining bend. This restraining bend features a mature, throughgoing right‐lateral strike‐slip fault, two major fault intersections, proposed subsurface fault dip changes, and a > 200    km "> > 200 km long section of fault misaligned with the regional plate motion. The Frazier Mountain, Elizabeth Lake, Pallett Creek, Wrightwood, and Pitman Canyon paleoseismic sites are located on this relatively linear surface trace of the San Andreas fault between fault bends. Our paleoseismic investigations at Elizabeth Lake document 4–5 earthquakes, since ∼ 1100    C . E . "> ∼ 1100 C . E . , similar to the number of earthquakes recorded at Pallett Creek. In contrast, the Frazier Mountain and Wrightwood sites each record 8–9 earthquakes during this same time period. Differences in earthquake frequency demonstrate that fewer earthquakes rupture the central portion of the restraining bend than occur near the fault bends and intersections. Furthermore, the similarity of earthquake records from the Bidart Fan paleoseismic site northwest of the restraining bend and the Frazier Mountain paleoseismic site suggests that the broad, 30° curve of the Big Bend section of the San Andreas fault exerts less influence on fault rupture behavior than the 3D geometry of the Mojave sections of the fault.

California↗

USGS2021

A revised position for the primary strand of the Pleistocene-Holocene San Andreas fault in southern California

The San Andreas fault has the highest calculated time-dependent probability for large-magnitude earthquakes in southern California. However, where the fault is multistranded east of the Los Angeles metropolitan area, it has been uncertain which strand has the fastest slip rate and, therefore, which has the highest probability of a destructive earthquake. Reconstruction of offset Pleistocene-Holocene landforms dated using the uranium-thorium soil carbonate and beryllium-10 surface exposure techniques indicates slip rates of 24.1 ± 3 millimeter per year for the San Andreas fault, with 21.6 ± 2 and 2.5 ± 1 millimeters per year for the Mission Creek and Banning strands, respectively. These data establish the Mission Creek strand as the primary fault bounding the Pacific and North American plates at this latitude and imply that 6 to 9 meters of elastic strain has accumulated along the fault since the most recent surface-rupturing earthquake, highlighting the potential for large earthquakes along this strand.

California↗

USGS2020

Documentation of Surface Fault Rupture and Ground‐Deformation Features Produced by the 4 and 5 July 2019 Mw 6.4 and Mw 7.1 Ridgecrest Earthquake Sequence

The M w "> M w Mw 6.4 and M w "> M w Mw 7.1 Ridgecrest earthquake sequence occurred on 4 and 5 July 2019 within the eastern California shear zone of southern California. Both events produced extensive surface faulting and ground deformation within Indian Wells Valley and Searles Valley. In the weeks following the earthquakes, more than six dozen scientists from government, academia, and the private sector carefully documented the surface faulting and ground‐deformation features. As of December 2019, we have compiled a total of more than 6000 ground observations; approximately 1500 of these simply note the presence or absence of fault rupture or ground failure, but the remainder include detailed descriptions and other documentation, including tens of thousands of photographs. More than 1100 of these observations also include quantitative field measurements of displacement sense and magnitude. These field observations were supplemented by mapping of fault rupture and ground‐deformation features directly in the field as well as by interpreting the location and extent of surface faulting and ground deformation from optical imagery and geodetic image products. We identified greater than 68 km of fault rupture produced by both earthquakes as well as numerous sites of ground deformation resulting from liquefaction or slope failure. These observations comprise a dataset that is fundamental to understanding the processes that controlled this earthquake sequence and for improving earthquake hazard estimates in the region. This article documents the types of data collected during postearthquake field investigations, the compilation effort, and the digital data products resulting from these efforts.

California↗

USGS2020

A maximum rupture model for the southern San Andreas and San Jacinto Faults California, derived from paleoseismic earthquake ages: Observations and limitations

Paleoseismic rupture histories provide spatiotemporal models of earthquake moment release needed to test numerical models and lengthen the instrumental catalog. We develop a model of the fewest and thus largest magnitude earthquakes permitted by paleoseismic data for the last 1,500 years on the southern San Andreas and San Jacinto Faults, California, USA. The largest geometric complexity appears to regulate the system: Only two ruptures break the San Gorgonio Pass region, followed by episodes of ruptures that could bridge the northern San Jacinto Fault and the San Andreas Fault. When tested against independent data on slip per event, the model produces comparable values indicating the end‐member model does not underpredict rupture rates. Rupture of >85% of the fault length in the historic period between 1800 and 1857 and the subsequent quiescence is similar to epochs of activity in the prehistoric model, suggesting that regional clustering of seismicity could be a trait of the system.

California↗

USGS2020

Surface displacement distributions for the July 2019 Ridgecrest, California earthquake ruptures

Surface rupture in the 2019 Ridgecrest, California, earthquake sequence occurred along two orthogonal cross faults and includes dominantly left‐lateral and northeast‐striking rupture in the M w "> M w 6.4 foreshock and dominantly right‐lateral and northwest‐striking rupture in the M w "> M w 7.1 mainshock. We present > 650 "> > 650 field‐based, surface‐displacement observations for these ruptures and synthesize our results into cumulative along‐strike displacement distributions. Using these data, we calculate displacement gradients and compare our results with historical strike‐slip ruptures in the eastern California shear zone. For the M w "> M w 6.4 rupture, we report 96 displacements measured along 18 km of northeast‐striking rupture. Cumulative displacement curves for the rupture yield a mean left‐lateral displacement of 0.3–0.5 m and maximum of 0.7–1.6 m. Net mean vertical displacement based on the difference of down‐to‐the‐west (DTW) and down‐to‐the‐east (DTE) displacement curves is close to zero (0.02 m DTW). The M w "> M w 6.4 displacement distribution shows that the majority of displacement occurred southwest of the intersection with the M w "> M w 7.1 rupture. The M w "> M w 7.1 rupture is northwest‐striking and 50 km long based on 576 field measurements. Displacement curves indicate a mean right‐lateral displacement of 1.2–1.7 m and a maximum of 4.3–7.0 m. Net vertical displacement in the rupture averages 0.3 m DTW. The M w "> M w 7.1 displacement distributions demonstrate that maximum displacement occurred along a 12‐km‐long portion of the fault near the M w "> M w 7.1 epicenter, releasing 66% of the geologically based seismic moment along 24% of the total rupture length. Using our displacement distributions, we calculate kilometer‐scale displacement gradients for the M w "> M w 7.1 rupture. The steepest gradients ( ⁠ ∼ 1 – 3    m / km "> ∼ 1 – 3 m / km ⁠ ) flank the 12‐km‐long region of maximum displacement. In contrast, gradients for the 1992 M w "> M w 7.3 Landers and 1999 M w "> M w 7.1 Hector Mine earthquakes are < 0.6    m / km "> < 0.6 m / km . Our displacement distributions are important for understanding the influence of cross‐fault rupture on M w "> M w 6.4 and 7.1 rupture length and displacement and will facilitate comparisons with distributions generated remotely and at broader scales.

California↗

USGS2020

Kinematics of fault slip associated with the July 4-6 2019 Ridgecrest, Californai earthquakes sequence

The 2019 Ridgecrest, California, earthquake sequence produced observable crustal deformation over much of central and southern California, as well as surface rupture over several tens of kilometers. To obtain a detailed picture of the fault slip involved in the 4 July M 6.4 foreshock and 6 July M 7.1 mainshock, we combine strong‐motion seismic waveforms with crustal deformation observations to obtain kinematic and static slip models of both events. We sample the regional seismic wavefield for both the foreshock and mainshock with three‐component records from 31 stations of the California Integrated Seismic Network. The deformation observations include Global Positioning System (GPS), Interferometric Synthetic Aperture Radar (InSAR), and borehole strainmeter recordings of the dynamic strain field. These data collectively constrain the kinematic coseismic slip distributions of the events, with measurements variously observing coseismic slip from one event (e.g., seismic waveforms, kinematic solutions from continuous GPS, and strainmeter time series) or coseismic slip from both events combined (InSAR). We find that the foreshock ruptured two separate faults, one with left‐lateral strike slip on a northeast–southwest‐trending fault and the other with right‐lateral strike slip on an orthogonal fault, with unilateral rupture propagation along both. The mainshock ruptured a series of northwest–southeast‐trending faults with right‐lateral strike slip concentrated in the uppermost 6 km with exceptionally low‐rupture velocity averaging 1.0 – 1.5    km / s ">

California↗

USGS2020

Airborne lidar and electro-optical imagery along surface ruptures of the 2019 Ridgecrest earthquake sequence, Southern California

Surface rupture from the 2019 Ridgecrest earthquake sequence, initially associated with the M 6.4 foreshock, occurred on July 4 on a ~17 km long, northeast-southwest oriented, left-lateral zone of faulting. Following the M 7.1 mainshock on July 5 (local time), extensive northwest-southeast-oriented, right-lateral faulting was then also mapped along a ~50 km long zone of faults, including sub-parallel splays in several areas. The largest slip was observed in the epicentral area, and crossing the dry lakebed of China Lake to the southeast. Surface fault rupture mapping by a large team, reported elsewhere, was used to guide the airborne data acquisition reported here. Rapid rupture mapping allowed for accurate and efficient flight line planning for the high-resolution lidar and aerial photography. Flight line planning trade-offs were considered to allocate the medium (25 pulses per square meter, or ppsm) and high resolution (80 ppsm) lidar data collection polygons. The National Center for Airborne Laser Mapping (NCALM) acquired the airborne imagery with a Titan multispectral lidar system and DiMAC aerial digital camera, and USGS acquired GPS ground control data. This effort required extensive coordination with the Navy as much of the airborne data acquisition occurred within their restricted airspace at the China Lake Ranges.

Calilfornia↗

USGS2020

Late Quaternary slip rates on the Sierra Madre fault zone and paleoseismic evidence on the size and frequency of past ruptures

The Sierra Madre fault zone is a south-vergent, active reverse fault that accommodates shortening between basins on the northern margin of the Los Angeles region and the San Gabriel Mountains. The preservation of late Quaternary alluvial fill and fan surfaces in the hanging wall of the fault provides evidence of long-term uplift. Surface rupture from the 1971 Mw 6.6 San Fernando earthquake and evidence of large prehistoric displacements from trenching investigations emphasize the ongoing hazard posed by the fault system to the region. This one-day field trip visits some of the key locations near Pasadena and San Fernando, California, where slip rates have been determined from cosmogenic and luminescence dating of abandoned surfaces dating to 50–70, ca. 30, and ca. 12 ka and surface offsets measured from lidar and pre-development topographic maps. Another stop is the site of a paleoseismic trench, which provided key evidence on the timing and displacement of past ruptures on the fault. In combination, results from these field investigations converge on a slip rate for the eastern ~100 km of the fault zone of 1–2 mm/yr, which matches or exceeds the rates for other reverse faults in southern California. This rate, in combination with trenching data that show no evidence of post–mid Holocene ruptures along the central and eastern portions of the fault, indicate the fault zone poses a significant seismic hazard to the region.

California↗

USGS2020

EERI earthquake reconnaissance report: 2019 Ridgecrest earthquake sequence

The Ridgecrest Earthquake Sequence began the morning of 4 July 2019 with an M6.4 earthquake at 10:33 a.m., closely following several small foreshocks. The epicenter of this event was roughly 11 miles (18 km) east-northeast of Ridgecrest (Figure 1) within the Naval Air Weapons Station China Lake (NAWS-CL). Seismic and geologic data established that the M6.4 earthquake occurred primarily along a steeply dipping northeast-trending strike-slip fault with left-lateral slip. This earthquake and preliminary reports of damage in Ridgecrest and Trona and associated ground cracking triggered a response by earthquake scientists and engineers throughout the region. A California Earthquake Clearinghouse was established in Ridgecrest to help coordinate the scientific response effort and to share data.

California↗

USGS2019

The current unlikely earthquake hiatus at California’s transform boundary paleoseismic sites

Paleoseismic and historical earthquake records used to quantify earthquake recurrence rates can also be used to test the likelihood of seismically quiescent periods. At principal paleoseismic sites in California on the San Andreas, San Jacinto, Elsinore, and Hayward faults, no ground‐rupturing earthquake has occurred in the last 100 yr, yet this interval is about three times the average interearthquake period for the ensemble of sites. We examine long paleoseismic records from these faults, as they carry most of the transform fault slip on the plate boundary, to see if the current hiatus has any precedent in the last 1000 yr. The selection of sites is designed to sample fault sections unlikely to have ruptured together, so their conditional probabilities of a hiatus can be combined as independent events. We find a 100‐yr hiatus is not predicted by common time‐dependent or time‐independent recurrence models. Paleoearthquake dating uncertainties can allow long open intervals at individual sites or subsets of sites, but do not explain the observed gap in the ensemble. After approximately removing redundancies in the full paleoearthquake record, the time‐independent probability of the current 100‐yr gap is of order 0.3%. This raises several questions. Do we live in a statistically exceptional time? Or does some wide‐scale effect modulate earthquake occurrence among sites over longer timescales? Finally, how should we understand seismic hazard estimates in California if the recurrence models on which they rely seem, at minimum, incomplete? Whether due to a statistical anomaly, some longer‐term modulation of earthquake occurrence, or another cause, our results emphasize that the hiatus of the last century has been exceptional.

California↗

USGS2019

Late Quaternary slip rate of the Central Sierra Madre fault, southern California: Implications for slip partitioning and earthquake hazard

The Sierra Madre fault system accommodates contraction within a large restraining bend area of the San Andreas fault along the northern margin of the Los Angeles metropolitan area in Southern California. Reverse slip along this fault system during earthquakes controls growth of the San Gabriel Mountains and poses a significant seismic hazard to the region. Here, we measure the late Quaternary slip rate of the Central Sierra Madre fault (CSMF) using analysis of high-resolution topography combined with cosmogenic 10Be surface exposure dating and post-IR IRSL geochronology. We mapped terrace and fan surfaces from three drainages that cross the CSMF and correlate them based on soil development and geomorphic position. Cosmogenic nuclide and luminescence ages are consistent amongst the three prominent surfaces offset ~5 to 28 m across the fault zone. We devised a new strategy to estimate surface ages, incorporating data from two dating methods at three locations, refined by inset age relationships, that yields surface age estimates of 54 +21/-13 ka, 36 ± 8 ka, and 12 ± 4 ka. Estimated slip for these geomorphic markers is more uncertain than the measured vertical separation due to uncertainties in fault dip and ranges from 7.5 +5.4/-3.1 m to 58.5 +46.3/-14.4 m. Incremental dip-slip rate estimates from different age surfaces and locations overlap within uncertainty, with median values ranging from 0.7 to 1.1 mm/yr. The average slip rate for all three generations of markers is 1.1 +1.2/-0.4 mm/yr. This late Quaternary slip rate for the CSMF is slower than estimates based on interseismic geodetic data, and emphasizes the importance of contraction distributed across multiple structures south of the Sierra Madre fault when assessed against the geodetic shortening budget. Despite being the central portion of the broader Sierra Madre fault system, the CSMF has a slip rate similar to or lower than neighboring sections, suggesting that slip transfer onto other nearby faults control the along-strike pattern of deformation rate. Paleoseismic evidence indicates that the last earthquake on the CSMF was in the early Holocene, and the slip rate we estimate suggests that the accumulated elastic strain could produce many meters of slip in future earthquakes.

California↗

USGS2019

Preliminary report on engineering and geological effects of the July 2019 Ridgecrest earthquake sequence

The Ridgecrest Earthquake sequence included a foreshock event on July 4 2019 (M6.4) and a M7.1 mainshock event on July 5 2019. These events occurred in the Eastern California Shear Zone, near Indian Wells Valley, south of China Lake and west of Searles Valley. GEER has partnered with several organizations to collect perishable data and document the important impacts of these events, including the US Geological Survey, the California Geological Survey, the US Navy, the Southern California Earthquake Center, and local utilities. Critical geotechnical features of this event are extensive left-lateral (M6.4 event) and right-lateral (M7.1 event) surface ruptures over fault segments of variable complexity and width as well as across extensional and compressive step-over zones. We also document lifeline performance at fault crossings (gas, water, electrical), mainshock slip and afterslip, liquefaction and lateral spreading features, and liquefaction effects on structures. These effects are documented using field (ground) mapping and aerial imagery that will support subsequent development of high-resolution digital elevation models. Over 750 ground motions were recorded from the foreshock and mainshock alone, with many additional aftershock records. The data demonstrate significant impacts of site response and rupture directivity on ground motion attributes.

Report↗

USGS2018

Quantifying uncertainty in cumulative surface slip along the Cucamonga Fault, a crustal thrust fault in southern California

Studies of historic earthquake ground surface ruptures show that displacements along strike are spatially variable. As a result, latest Quaternary slip rates developed from a spatially restricted set of cumulative displacement measurements may not accurately represent fault velocity. Here we examine the uncertainties associated with slip on the Cucamonga Fault, which is part of a network of faults that have generated damaging historical earthquakes in and around Los Angeles, California. Numerous scarps along its ~25‐km length are well expressed on alluvial fans. We make 310 measurements of vertical separation across the scarps using lidar data. We show that the dispersion of the vertical separations cannot be explained by our best estimates of analytical uncertainties alone. Additional epistemic uncertainties are required. We find that the magnitude of the required epistemic uncertainty is typically larger than analytical uncertainty by a factor of 3 and typically about 22% of the maximum vertical separation. These relationships appear to hold at several spatial scales. We examine three potential sources of epistemic uncertainty and find that none among surface age uncertainty, fault dip, and anthropogenic landscape alteration is likely sufficient to explain the overdispersion of the data, which suggests differences in cumulative strain along the strike of the fault. We calculate a range of dip‐slip rates between 0.4 and 2.6 mm/year. In light of our results, we suggest that future thrust‐fault slip‐rate studies adopt an epistemic uncertainty of 22% of the maximum value for vertical separation measurements, unless there are sufficient data to demonstrate otherwise.

California↗

USGS2018

The San Andreas Fault System--Complexities along a major transform fault system and relation to earthquake hazards

The San Andreas Fault System is a 1300-km-long transform boundary that accommodates motion between the North American and Pacific Plates. New technologies and data reveal rich details about the present configuration of faults, distribution of strain and associated seismic hazard on this complex network of faults. This contribution provides a brief summary of the geologic history of the San Andreas Fault System, followed by an introduction to recent research that has changed understanding of the hazards along the main faults. Organized by region, we highlight a selection of recent research using new geodetic techniques, improved topographic data, advanced geochronologic methods, and high-resolution geophysics. The contribution ends with a review of the historic earthquakes on the San Andreas and San Jacinto Faults, comparing these to past ruptures interpreted from paleoseismic studies.

California↗

USGS2018

Characterizing the Los Angeles Aqueduct crossing of the San Andreas Fault for improved earthquake resilience

The five-mile-long Elizabeth Tunnel, which crosses the San Andreas fault (SAF) zone near Lake Hughes, California, is part of the Los Angeles Aqueduct (LAA) that delivers water from Owens Valley to the City of Los Angeles. Geologic characterization of the Elizabeth Tunnel alignment is focused on developing a better understanding of fault displacement hazards at the SAF crossing to support design of both short- and long-term strategies to increase the earthquake resilience of the City’s water supply system. The results of this study define a fault zone that is wider and more complex at the surface than at tunnel depth. A 750-ft transect of deep, angled core borings, located ~ 100 ft west of, and parallel to the tunnel alignment, penetrated a wide zone (>550 ft) of fault-damaged granitic and gneissic rocks containing a few thick fault zones that can be correlated confidently between borings as well as a multitude of very thin faults and shears throughout the rock mass. Fault rocks are predominantly defined as coherent cataclasites and ultracataclasites, as opposed to incoherent clay gouge, even at this relatively shallow sampling depth. Integrating geomorphic, geologic, petrographic, and geophysical data defines a steeply south-dipping SAF that splays upward and increases in complexity toward the surface, and provides the basis for engineering design considerations at this critical SAF crossing.

California↗

USGS2017

Ground-rupturing earthquakes on the northern Big Bend of the San Andreas Fault, California, 800 A.D. to Present

Paleoseismic data on the timing of ground-rupturing earthquakes constrain the recurrence behavior of active faults and can provide insight on the rupture history of a fault if earthquakes dated at neighboring sites overlap in age and are considered correlative. This study presents the evidence and ages for 11 earthquakes that occurred along the Big Bend section of the southern San Andreas Fault at the Frazier Mountain paleoseismic site. The most recent earthquake to rupture the site was the M w 7.7–7.9 Fort Tejon earthquake of 1857. We use over 30 trench excavations to document the structural and sedimentological evolution of a small pull-apart basin that has been repeatedly faulted and folded by ground-rupturing earthquakes. A sedimentation rate of 0.4 cm/yr and abundant organic material for radiocarbon dating contribute to a record that is considered complete since 800 A.D. and includes 10 paleoearthquakes. Earthquakes have ruptured this location on average every ~100 years over the last 1200 years, but individual intervals range from ~22 to 186 years. The coefficient of variation of the length of time between earthquakes (0.7) indicates quasiperiodic behavior, similar to other sites along the southern San Andreas Fault. Comparison with the earthquake chronology at neighboring sites along the fault indicates that only one other 1857-size earthquake could have occurred since 1350 A.D., and since 800 A.D., the Big Bend and Mojave sections have ruptured together at most 50% of the time in M w ≥ 7.3 earthquakes.

California↗

USGS2017

Evidence for coseismic subsidence events in a southern California coastal saltmarsh

Paleoenvironmental records from a southern California coastal saltmarsh reveal evidence for repeated late Holocene coseismic subsidence events. Field analysis of sediment gouge cores established discrete lithostratigraphic units extend across the wetland. Detailed sediment analyses reveal abrupt changes in lithology, percent total organic matter, grain size, and magnetic susceptibility. Microfossil analyses indicate that predominantly freshwater deposits bury relic intertidal deposits at three distinct depths. Radiocarbon dating indicates that the three burial events occurred in the last 2000 calendar years. Two of the three events are contemporaneous with large-magnitude paleoearthquakes along the Newport-Inglewood/Rose Canyon fault system. From these data, we infer that during large magnitude earthquakes a step-over along the fault zone results in the vertical displacement of an approximately 5-km 2 area that is consistent with the footprint of an estuary identified in pre-development maps. These findings provide insight on the evolution of the saltmarsh, coseismic deformation and earthquake recurrence in a wide area of southern California, and sensitive habitat already threatened by eustatic sea level rise.

California↗

USGS2016

Earthquake geology and paleoseismology of major strands of the San Andreas fault system

The San Andreas fault system in California is one of the best-studied faults in the world, both in terms of the long-term geologic history and paleoseismic study of past surface ruptures. In this paper, we focus on the Quaternary to historic data that have been collected from the major strands of the San Andreas fault system, both on the San Andreas Fault itself, and the major subparallel strands that comprise the plate boundary, including the Calaveras-Hayward- Rogers Creek-Maacama fault zone and the Concord-Green Valley-Bartlett Springs fault zone in northern California, and the San Jacinto and Elsinore faults in southern California. The majority of the relative motion between the Pacific and North American lithospheric plates is accommodated by these faults, with the San Andreas slipping at about 34 mm/yr in central California, decreasing to about 20 mm/yr in northern California north of its juncture with the Calaveras and Concord faults. The Calaveras-Hayward-Rogers Creek-Maacama fault zone exhibits a slip rate of 10-15 mm/yr, whereas the rate along the Concord-Green Valley-Bartlett Springs fault zone is lower at about 5 mm/yr. In southern California, the San Andreas exhibits a slip rate of about 35 mm/yr along the Mojave section, decreasing to as low as 10-15 mm/yr along its juncture with the San Jacinto fault, and about 20 mm/yr in the Coachella Valley. The San Jacinto and Elsinore fault zones exhibit rates of about 15 and 5 mm/yr, respectively. The average recurrence interval for surface-rupturing earthquakes along individual elements of the San Andreas fault system range from 100-500 years and is consistent with slip rate at those sites: higher slip rates produce more frequent or larger earthquakes. There is also evidence of short-term variations in strain release (slip rate) along various fault sections, as expressed as “flurries” or clusters of earthquakes as well as periods of relatively fewer surface ruptures in these relatively short records. This is reflected by non-periodic coefficients of variation in earthquake recurrence of 0.4 to 0.7 for the various paleoseismic sites.

California↗