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Geology topics

Robert C. Jachens

Publications and source records attributed to Robert C. Jachens.

At least 19 recordsLinked to original sources
USGS2016

Structure of the Hat Creek graben region: Implications for the structure of the Hat Creek graben and transfer of right-lateral shear from the Walker Lane north of Lassen Peak, northern California, from gravity and magnetic anomalies

Interpretation of magnetic and new gravity data provides constraints on the geometry of the Hat Creek Fault, the amount of right-lateral offset in the area between Mt. Shasta and Lassen Peak, and confirmation of the influence of pre-existing structure on Quaternary faulting. Neogene volcanic rocks coincide with short-wavelength magnetic anomalies of both normal and reversed polarity, whereas a markedly smoother magnetic field occurs over the Klamath Mountains and its Paleogene cover. Although the magnetic field over the Neogene volcanic rocks is complex, the Hat Creek Fault, which is one of the most prominent normal faults in the region and forms the eastern margin of the Hat Creek Valley, is marked by the eastern edge of a north-trending magnetic and gravity high 20-30 km long. Modeling of these anomalies indicates that the fault is a steeply dipping (~75-85°) structure. The spatial relationship of the fault as modeled by the potential-field data, the youngest strand of the fault, and relocated seismicity suggests that deformation continues to step westward across the valley, consistent with a component of right-lateral slip in an extensional environment. Filtered aeromagnetic data highlight a concealed magnetic body of Mesozoic or older age north of Hat Creek Valley. The body’s northwest margin strikes northeast and is linear over a distance of ~40 km. Within the resolution of the aeromagnetic data (1-2 km), we discern no right-lateral offset of this body. Furthermore, Quaternary faults change strike or appear to end, as if to avoid this concealed magnetic body and to pass along its southeast edge, suggesting that pre-existing crustal structure influenced younger faulting, as previously proposed based on gravity data.

California, Nevada↗
USGS2015

A summary of the late Cenozoic stratigraphic and tectonic history of the Santa Clara Valley, California

The late Cenozoic stratigraphic and tectonic history of the Santa Clara Valley illustrates the dynamic nature of the North American–Pacific plate boundary and its effect on basin and landscape development. Prior to early Miocene time, the area that became Santa Clara Valley consisted of eroding Franciscan complex basement structurally interleaved in places with Coast Range ophiolite and Mesozoic Great Valley sequence, and locally overlapped by Paleogene strata. During early to middle Miocene time, this landscape was flooded by the sea and was deformed locally into deeper depressions such as the Cupertino Basin in the southwestern part of the valley. Marine deposition during the middle and late Miocene laid down thin deposits in shallow water and thick deeper-water deposits in the Cupertino Basin. During this sedimentation, the San Andreas fault system encroached into the valley, with most offset partitioned onto the San Andreas fault southwest of the valley and the southern Calaveras–Silver Creek–Hayward fault system in the northeastern part of the valley. A 6-km-wide right step between the Hayward and Silver Creek faults formed the 40-km-long Evergreen pull-apart basin along the northeastern margin of the valley, leaving a basement ridge between it and the Cupertino Basin. The Silver Creek fault was largely abandoned ca. 2.5 Ma in favor of a compressional left step between the Calaveras and Hayward fault, although some slip continued to at least mid-Quaternary time. Gravity, seismic, stratigraphic, and interferometric synthetic aperture radar (InSAR) data indicate no other major San Andreas system faults within the central block between the present-day range-front faults bounding the valley and the Silver Creek fault. Sometime between 9 and 4 Ma (9 and 1 Ma for the central block), the area rose above sea level, and a regional surface of erosion was carved into the Mesozoic and Tertiary rocks. Alluvial gravels were deposited on this surface along the margins of the valley beginning ca. 4 Ma, but they may not have prograded onto the central block until ca. 1 Ma, because no older equivalents of the Pliocene–Quaternary Santa Clara gravels have been found there. Thus, either the central block was high enough relative to the surrounding areas that Santa Clara gravels were never deposited on it, or any Santa Clara gravels deposited there were stripped away before ca. 1 Ma. Analysis of alluvium on the central block implies a remarkably uniform, piston-like, subsidence of the valley of ∼0.4 mm/yr since ca. 0.8 Ma, possibly extending north to northern San Francisco Bay. Today, the central block continues to subside, the range-front reverse faults are active, and the major active faults of the San Andreas system are mostly outside the valley.

California↗
USGS2015

Structural superposition in fault systems bounding Santa Clara Valley, California

Santa Clara Valley is bounded on the southwest and northeast by active strike-slip and reverse-oblique faults of the San Andreas fault system. On both sides of the valley, these faults are superposed on older normal and/or right-lateral normal oblique faults. The older faults comprised early components of the San Andreas fault system as it formed in the wake of the northward passage of the Mendocino Triple Junction. On the east side of the valley, the great majority of fault displacement was accommodated by the older faults, which were almost entirely abandoned when the presently active faults became active after ca. 2.5 Ma. On the west side of the valley, the older faults were abandoned earlier, before ca. 8 Ma and probably accumulated only a small amount, if any, of the total right-lateral offset accommodated by the fault zone as a whole. Apparent contradictions in observations of fault offset and the relation of the gravity field to the distribution of dense rocks at the surface are explained by recognition of superposed structures in the Santa Clara Valley region.

California↗
USGS2015

Physical subdivision and description of the water-bearing sediments of the Santa Clara Valley, California

A thick Quaternary alluvial section fills a sedimentary basin beneath the Santa Clara Valley, California, located within the San Andreas Fault system at the south end of San Francisco Bay. This section consists of an upper sequence about 1,000 feet thick containing eight sedimentary cycles and a lower fine-grained unit as thick as several hundred feet. Together these constitute the Quaternary Santa Clara Basin. The section overlies an irregular unconformity with more than 1,200 feet of relief cut into the underlying bedrock. This stratigraphy is determined through study of new wells and seismic reflection profiles, together with a sample of the many thousands of water wells in the valley. It represents a major change and improvement in understanding of the basin, particularly with regard to the upper cyclic sequence, which forms a large groundwater system that is an important resource in the San Francisco Bay region. Each of the eight sedimentary cycles consists of a coarse-grained bottom interval overlain by a fine-grained top, with the coarse bottom forming a permeable sheet that is more or less continuous around the basin and the fine top forming a similarly extensive, relatively impermeable confining layer. This stratigraphic organization contrasts with most previous views, which have considered the coarse sediment in the basin to occur as scattered, discrete lenses and (or) sinuous channel sands, all embedded in a predominantly fine-grained section. Temperature logs in several wells demonstrate that the fine cycle tops do limit vertical movement of groundwater, although this may not be the case where those tops are thin to perhaps locally absent around parts of the basin margin. Age control has been obtained from previous work, in which the sedimentary cycles were correlated with the marine oxygen isotope record and the ages of two deeper Quaternary unconformities were estimated, and from detailed paleomagnetic study of cores from the new wells by E.A. Mankinen. Despite careful search of the cores, very few fossils were found, and none that are helpful in subdividing the section. No tephra (volcanic ash) was recovered, and the few carbon samples found and dated radiometrically are limited to the upper 120 feet of the section. The upper cyclic section ranges in age from 0 to somewhat older than 718 thousand years (ka), and the lower fine-grained section lies between unconformities with estimated ages of 950 and 1500 ka. Reflections in the seismic profiles indicate that layering in the basin is subparallel to the ground surface, and this fact, together with the continuous stratigraphic detail provided by geophysical logs of the new wells, allows the confident interwell correlation required to delineate the sedimentary cycles. The sequence of layers within any one cycle tends to persist laterally between the wells in the dataset, which are spaced 1 to 3 km apart, with most changes occurring gradually. The eight cycles, in contrast, tend to differ from each other in the details of their internal organization. Maps and cross sections show the elevations of cycle boundaries and the underlying bedrock surface, the varying thicknesses of the cycles and of their fine tops and coarse bottoms, and the aggregate thickness of coarse layers in those bottom intervals. Coarse sediment is more abundant toward some parts of the basin margin and in the southern part of the basin. Cycle boundary surfaces are relatively smooth, and their shapes are consistent with having been intercycle topographic surfaces. The underlying bedrock surface has a relief of more than 1,200 feet and deepens toward the center of the basin and the west edge of the fault-bounded Evergreen Basin, which is concealed beneath the east side of the Quaternary basin. The absence of consistent abrupt changes in thicknesses or boundary elevations across the basin or in cross section indicates that the interior of the basin is largely unfaulted, with the Silver Creek strand of the San Andreas system at the west edge of the Evergreen Basin being the sole exception. The east and west margins of the Santa Clara Basin, in contrast, are marked by reverse and thrust fault systems.

California↗
USGS2014

Gravity survey and interpretation of Fort Irwin and vicinity, Mojave Desert, California

In support of a hydrogeologic study of the groundwater resources on Fort Irwin, we have combined new gravity data with preexisting measurements to produce an isostatic residual gravity map, which we then separated into two components reflecting (1) the density distribution in the pre-Cenozoic basement complex and (2) the distribution of low-density Cenozoic volcanic and sedimentary deposits that lie on top of the basement complex. The second component was inverted to estimate the three-dimensional distribution of Cenozoic deposits by using constraints from geology, drillholes, and time-domain electromagnetic soundings. In most of the base, the Cenozoic deposits are no more than 300 m thick, except in the basins with more than 500 m of fill beneath Coyote Lake, Red Pass Lake, west of Nelson Lake, west of Superior Lake, Bicycle Lake, and in the vicinity of Nelson Lake.

California↗
USGS2014

Aeromagnetic data, processing, and maps of Fort Irwin and vicinity, California

Aeromagnetic data help provide the underpinnings of a hydrogeologic framework for Fort Irwin by locating inferred structural features or grain that influence groundwater flow. Magnetization boundaries defined by horizontal-gradient analyses coincide locally with Cenozoic faults and can be used to extend these faults beneath cover. These boundaries also highlight the structural grain within the crystalline rocks and may serve as a proxy for fracturing, an important source of permeability within the generally impermeable basement rocks, thus mapping potential groundwater pathways through and along the mountain ranges in the study area.

California↗
USGS2014

Subsurface geometry of the San Andreas-Calaveras fault junction: Influence of serpentinite and the Coast Range Ophiolite

While an enormous amount of research has been focused on trying to understand the geologic history and neotectonics of the San Andreas-Calaveras fault (SAF-CF) junction, fundamental questions concerning fault geometry and mechanisms for slip transfer through the junction remain. We use potential-field, geologic, geodetic, and seismicity data to investigate the 3-D geologic framework of the SAF-CF junction and identify potential slip-transferring structures within the junction. Geophysical evidence suggests that the San Andreas and Calaveras fault zones dip away from each other within the northern portion of the junction, bounding a triangular-shaped wedge of crust in cross section. This wedge changes shape to the south as fault geometries change and fault activity shifts between fault strands, particularly along the Calaveras fault zone (CFZ). Potential-field modeling and relocated seismicity suggest that the Paicines and San Benito strands of the CFZ dip 65° to 70° NE and form the southwest boundary of a folded 1 to 3 km thick tabular body of Coast Range Ophiolite (CRO) within the Vallecitos syncline. We identify and characterize two steeply dipping, seismically active cross structures within the junction that are associated with serpentinite in the subsurface. The architecture of the SAF-CF junction presented in this study may help explain fault-normal motions currently observed in geodetic data and help constrain the seismic hazard. The abundance of serpentinite and related CRO in the subsurface is a significant discovery that not only helps constrain the geometry of structures but may also help explain fault behavior and the tectonic evolution of the SAF-CF junction.

California↗
USGS2014

Geophysical framework of the Peninsular Ranges batholith—Implications for tectonic evolution and neotectonics

The crustal structure of the Peninsular Ranges batholith can be divided geophysically into two parts: (1) a western mafic part that is dense, magnetic, and characterized by relatively high seismic velocities (>6.25 km/s), low heat flow (<60 mW/m 2 ), and relatively sparse seismicity, and (2) an eastern, more felsic part that is less dense, weakly magnetic, and characterized by lower seismic velocities (<6.25 km/s), high heat flow (>60 mW/m 2 ), and abundant microseismicity. Potential-field modeling indicates that the dense, mafic part of the batholith extends to depths of at least 20 km and likely to the Moho. The magnetic anomalies of the western part of the batholith extend south beyond the spatially extensive exposures of the batholith to the tip of the Baja California peninsula, which suggests that the mafic part of the batholith projects beneath Cenozoic volcanic cover another 400 km. The linearity and undisrupted nature of the magnetic belt of anomalies suggest that the western part of the batholith has behaved as a rigid block since emplacement of the batholith. The batholith may have influenced not only the development of the Gulf of California oblique rift, but also strike-slip faulting along its northern margin, and transtensional faulting along its western margin, likely because it is thermally and mechanically more resistant to deformation than the surrounding crust.

California↗
USGS2013

Previously unrecognized regional structure of the Coastal Belt of the Franciscan Complex, northern California, revealed by magnetic data

Magnetic anomalies provide surprising structural detail within the previously undivided Coastal Belt, the westernmost, youngest, and least-metamorphosed part of the Franciscan Complex of northern California. Although the Coastal Belt consists almost entirely of arkosic graywacke and shale of mainly Eocene age, new detailed aeromagnetic data show that it is pervasively marked by long, narrow, and regularly spaced anomalies. These anomalies arise from relatively simple tabular bodies composed principally of magnetic basalt or graywacke confi ned mainly to the top couple of kilometers, even though metamorphic grade indicates that these rocks have been more deeply buried, at depths of 5&ndash;8 km. If true, this implies surprisingly uniform uplift of these rocks. The basalt (and associated Cretaceous limestone) occurs largely in the northern part of the Coastal Belt; the graywacke is recognized only in the southern Coastal Belt and is magnetic because it contains andesitic grains. The magnetic grains were not derived from the basalt, and thus require a separate source. The anomalies defi ne simple patterns that can be related to folding and faulting within the Coastal Belt. This apparent simplicity belies complex structure mapped at outcrop scale, which can be explained if the relatively simple tabular bodies are internally deformed, fault-bounded slabs. One mechanism that can explain the widespread lateral extent of the thin layers of basalt is peeling up of the uppermost part of the oceanic crust into the accretionary prism, controlled by porosity and permeability contrasts caused by alteration in the upper part of the subducting slab. It is not clear, however, how this mechanism might generate fault-bounded layers containing magnetic graywacke. We propose that structural domains defined by anomaly trend, wavelength, and source reflect imbrication and folding during the accretion process and local plate interactions as the Mendocino triple junction migrated north, a hypothesis that should be tested by more detailed structural studies.

California↗
USGS2012

Evolution of the Rodgers Creek–Maacama right-lateral fault system and associated basins east of the northward-migrating Mendocino Triple Junction, northern California

The Rodgers Creek–Maacama fault system in the northern California Coast Ranges (United States) takes up substantial right-lateral motion within the wide transform boundary between the Pacific and North American plates, over a slab window that has opened northward beneath the Coast Ranges. The fault system evolved in several right steps and splays preceded and accompanied by extension, volcanism, and strike-slip basin development. Fault and basin geometries have changed with time, in places with younger basins and faults overprinting older structures. Along-strike and successional changes in fault and basin geometry at the southern end of the fault system probably are adjustments to frequent fault zone reorganizations in response to Mendocino Triple Junction migration and northward transit of a major releasing bend in the northern San Andreas fault. The earliest Rodgers Creek fault zone displacement is interpreted to have occurred ca. 7 Ma along extensional basin-forming faults that splayed northwest from a west-northwest proto-Hayward fault zone, opening a transtensional basin west of Santa Rosa. After ca. 5 Ma, the early transtensional basin was compressed and extensional faults were reactivated as thrusts that uplifted the northeast side of the basin. After ca. 2.78 Ma, the Rodgers Creek fault zone again splayed from the earlier extensional and thrust faults to steeper dipping faults with more north-northwest orientations. In conjunction with the changes in orientation and slip mode, the Rodgers Creek fault zone dextral slip rate increased from ∼2–4 mm/yr 7–3 Ma, to 5–8 mm/yr after 3 Ma. The Maacama fault zone is shown from several data sets to have initiated ca. 3.2 Ma and has slipped right-laterally at ∼5–8 mm/yr since its initiation. The initial Maacama fault zone splayed northeastward from the south end of the Rodgers Creek fault zone, accompanied by the opening of several strike-slip basins, some of which were later uplifted and compressed during late-stage fault zone reorganization. The Santa Rosa pull-apart basin formed ca. 1 Ma, during the reorganization of the right stepover geometry of the Rodgers Creek–Maacama fault system, when the maturely evolved overlapping geometry of the northern Rodgers Creek and Maacama fault zones was overprinted by a less evolved, non-overlapping stepover geometry. The Rodgers Creek–Maacama fault system has contributed at least 44–53 km of right-lateral displacement to the East Bay fault system south of San Pablo Bay since 7 Ma, at a minimum rate of 6.1–7.8 mm/yr.

Geosphere↗
USGS2010

The Quaternary Silver Creek Fault Beneath the Santa Clara Valley, California

The northwest-trending Silver Creek Fault is a 40-km-long strike-slip fault in the eastern Santa Clara Valley, California, that has exhibited different behaviors within a changing San Andreas Fault system over the past 10-15 Ma. Quaternary alluvium several hundred meters thick that buries the northern half of the Silver Creek Fault, and that has been sampled by drilling and imaged in a detailed seismic reflection profile, provides a record of the Quaternary history of the fault. We assemble evidence from areal geology, stratigraphy, paleomagnetics, ground-water hydrology, potential-field geophysics, and reflection and earthquake seismology to determine the long history of the fault in order to evaluate its current behavior. The fault formed in the Miocene more than 100 km to the southeast, as the southwestern fault in a 5-km-wide right step to the Hayward Fault, within which the 40-km-long Evergreen pull-apart basin formed. Later, this basin was obliquely cut by the newly recognized Mt. Misery Fault to form a more direct connection to the Hayward Fault, although continued growth of the basin was sufficient to accommodate at least some late Pliocene alluvium. Large offset along the San Andreas-Calaveras-Mt Misery-Hayward Faults carried the basin northwestward almost to its present position when, about 2 Ma, the fault system was reorganized. This led to near abandonment of the faults bounding the pull-apart basin in favor of right slip extending the Calaveras Fault farther north before stepping west to the Hayward Fault, as it does today. Despite these changes, the Silver Creek Fault experienced a further 200 m of dip slip in the early Quaternary, from which we infer an associated 1.6 km or so of right slip, based on the ratio of the 40-km length of the strike-slip fault to a 5-km depth of the Evergreen Basin. This dip slip ends at a mid-Quaternary unconformity, above which the upper 300 m of alluvial cover exhibits a structural sag at the fault that we interpret as a negative flower structure. This structure implies some continuing strike slip on the Silver Creek Fault in the late Quaternary as well, with a transtensional component but no dip slip. Our only basis for estimating the rate of this later Quaternary strike slip on the Silver Creek Fault is to assume continuation of the inferred early Quaternary rate of less than 2 mm/yr. Faulting evident in a detailed seismic reflection profile across the Silver Creek Fault extends up to the limit of data at a depth of 50 m and age of about 140 ka, and the course of Coyote Creek suggests Holocene capture in a structural depression along the fault. No surface trace is evident on the alluvial plain, however, and convincing evidence of Holocene offset is lacking. Few instrumentally recorded earthquakes are located near the fault, and those that are near its southern end represent cross-fault shortening, not strike slip. The fault might have been responsible, however, for two poorly located moderate earthquakes that occurred in the area in 1903. Its southeastern end does mark an abrupt change in the pattern of abundant instrumentally recorded earthquakes along the Calaveras Fault-in both its strike and in the depth distribution of hypocenters-that could indicate continuing influence by the Silver Creek Fault. In the absence of convincing evidence to the contrary, and as a conservative estimate, we presume that the Silver Creek Fault has continued its strike-slip movement through the Holocene, but at a very slow rate. Such a slow rate would, at most, yield very infrequent damaging earthquakes. If the 1903 earthquakes did, in fact, occur on the Silver Creek Fault, they would have greatly reduced the short-term future potential for large earthquakes on the fault.

Open-File Report↗
USGS2007

Relatively simple through-going fault planes at large-earthquake depth may be concealed by the surface complexity of strike-slip faults

At the surface, strike-slip fault stepovers, including abrupt fault bends, are typically regions of complex, often disconnected faults. This complexity has traditionally led geologists studying the hazard of active faults to consider such stepovers as important fault segment boundaries, and to give lower weight to earthquake scenarios that involve rupture through the stepover zone. However, recent geological and geophysical studies of several stepover zones along the San Andreas fault system in California have revealed that the complex nature of the fault zone at the surface masks a much simpler and direct connection at depths associated with large earthquakes (greater than 5 km). In turn, the simplicity of the connection suggests that a stepover zone would provide less of an impediment to through-going rupture in a large earthquake, so that the role of stepovers as segment boundaries has probably been overemphasized. However, counter-examples of fault complexity at depth associated with surface stepovers are known, so the role of stepovers in fault rupture behaviour must be carefully established in each case.

California↗
USGS2007

Preliminary Gravity and Magnetic Data of the Lake Pillsbury Region, Northern Coast Ranges, California

The Lake Pillsbury region is transected by the Bartlett Springs Fault zone, one of the main strike-slip faults of the San Andreas system north of San Francisco Bay, California. Gravity and magnetic data were collected to help characterize the geometry and offset of the fault zone as well as determine the geometry of the Gravelly Valley pull-apart basin and Potter Valley, an alluvial intermontane basin southwest of Lake Pillsbury. The Bartlett Springs fault zone lies at the base of a significant gravity gradient. Superposed on the gradient is a small gravity low centered over Lake Pillsbury and Gravelly Valley. Another small gravity low coincides with Potter Valley. Inversion of gravity data for basin thickness indicates a maximum thickness of 400 and 440 m for the Gravelly and Potter Valley depressions, respectively. Ground magnetic data indicate that the regional aeromagnetic data likely suffer from positional errors, but that large, long-wavelength anomalies, sourced from serpentinite, may be offset 8 km along the Bartlett Springs Fault zone. Additional gravity data collected either on the lake surface or bottom and in Potter Valley would better determine the shape of the basins. A modern, high-resolution aeromagnetic survey would greatly augment the ability to map and model the fault geometry quantitatively.

Open-File Report↗
USGS2005

Highly magnetic Upper Miocene sandstones of the San Francisco Bay area, California

[1] A high-resolution aeromagnetic survey of the San Francisco Bay area shows prominent positive anomalies over distinctive blue sandstones of Late Miocene age. The total-field survey was measured at a nominal height of 300 m above the land surface along flight lines spaced 0.5 km apart. Anomalies with amplitudes up to 200 nT correlate with sandstones of the San Pablo Group, and these anomalies are similar in strength to the magnetic signatures of serpentinites and basalts in the surveyed region. Andesitic sandstone of the Neroly Formation, the upper part of the San Pablo Group, has high magnetic susceptibility (0.013 SI units, volume) and relatively strong natural remanent magnetization (0.29 A/m). Total magnetization of the sandstone is two thirds induced and one third remanent magnetization. The presence of coarse-grained magnetite detritus, low coercivity of remanence, low thermal stability of remanence, and multidomain properties is consistent with the NRM being a viscous remanent magnetization that grew during the Brunhes normal-polarity chron. The strong magnetic signature of the Upper Miocene sandstones allows their delineation over distances as great as 100 km, through areas where they are concealed by landslides and younger deposits. The sandstones are important structural markers for understanding the complex folding and faulting associated with active fault systems in the San Francisco Bay area.

California↗
USGS2004

Gravity, magnetic, and high‐precision relocated seismicity profiles suggest a connection between the Hayward and Calaveras Faults, northern California

Gravity, magnetic, and seismicity data profiled across the Hayward Fault Zone were generated as part of ongoing studies to help determine the geologic and tectonic setting of the San Francisco Bay region. These data, combined with previous geophysical studies that indicate that the Hayward Fault Zone dips 75°NE near San Leandro and follows a preexisting structure, reveal a possible direct connection between the seismogenic portion of the Hayward and Calaveras Faults at depth. Although the relocated seismicity data are regional in nature, they suggest that the dip of the Hayward Fault Zone may vary from near vertical in the northwestern part of the fault to about 75°NE at San Leandro in the central part of the fault to about 50°NE in the southeastern part of the fault. Gravity and magnetic data, profiled across the Hayward Fault Zone, were processed using standard geophysical techniques. Cross sections of high‐precision relocated hypocenters were constructed along each profile from the northwestern to the southeastern end of the Hayward Fault Zone. Profiles and cross sections are referenced to Pinole Point, where the Hayward Fault enters San Pablo Bay, and are spaced 2.5 km apart. Topographic profiles shown on the seismicity cross sections were generated using U.S. Geological Survey (USGS) 7.5‐min, 30‐m digital elevation models. Relocation of seismicity data was accomplished using a regional double‐difference method. The double‐difference method incorporates ordinary travel time measurements and cross correlation of P and S wave differential travel time measurements. Relative locations between earthquakes have hypocentral errors of about 100 m horizontally and 250 m vertically. Absolute location uncertainties were not determined but are probably dramatically improved compared to the USGS's Northern California Seismic Network catalog data.

California↗
USGS2004

Cross-sections and maps showing double-difference relocated earthquakes from 1984-2000 along the Hayward and Calaveras faults, California

We present cross-section and map views of earthquakes that occurred from 1984 to 2000 in the vicinity of the Hayward and Calaveras faults in the San Francisco Bay region, California. These earthquakes came from a catalog of events relocated using the double-difference technique, which provides superior relative locations of nearby events. As a result, structures such as fault surfaces and alignments of events along these surfaces are more sharply defined than in previous catalogs.

California↗