Springs Fault Research Articles

Paleoseismic Investigation of the Thousand Springs Fault, Northwestern Basin and Range, Oregon

ABSTRACT Earthquake recurrence intervals, surface-rupture extents, and interactions between faults provide insight into how faults behave and are critical for seismic hazard mitigation and earthquake forecasting. Investigating the paleoseismology of spatially related faults can reveal strain distribution and whether faults rupture as a system or independently. Summer Lake basin, a graben in the northwestern Basin and Range with four active faults (three of which have prior paleoseismic investigations), provides an opportunity to investigate fault interactions. To expand the paleoseismic record, two trenches were excavated across the previously undocumented Thousand Springs fault, exposing a normal fault zone that offsets a sequence of deep- to shallow-water lake sediments, sand dunes containing reworked Mazama ash, and other Cascades-sourced tephra. Tephra units were correlated to known units by their physical characteristics, stratigraphic sequence, glass chemistry, and two new radiocarbon dates from the uppermost lake sediments. Using trench exposures, measured vertical separations through auguring, colluvial wedges, and extrapolated offsets based on a constant sedimentation rate, we identified at least five surface-rupturing earthquakes with a total offset of 3.4 + 2/−1 m in the past ∼65 ka. The oldest event (EH5) occurred at 63.8 ± 1.5 ka, event horizon 4 at 36.2 ± 12.7 ka (which could be more than one event), and event horizon 3 at 24.6 ± 0.3 ka. Event horizon 2, a warping event at our site, is likely more than one event and occurred between 7.5 and 10 ka; and the most recent event (EH1+), most likely more than one event, occurred between 3.3 and 7.7 ka. Several events correlate, within error, with events on other faults in the Summer Lake basin, suggesting that (1) the faults generally rupture together as a system, (2) the most recent earthquake may have ruptured all faults in the region, and (3) fault rupture is influenced by the rapid regression of Lake Chewaucan (∼13 ka).

Integrated geologic and geophysical modeling across the Bartlett Springs fault zone, northern California (USA): Implications for fault creep and regional structure

Abstract The rate and location at depth of fault creep are important, but difficult to characterize, parameters needed to assess seismic hazard. Here we take advantage of the magnetic properties of serpentinite, a rock type commonly associated with fault creep, to model its depth extent along the Bartlett Springs fault zone, an important part of the San Andreas fault system north of the San Francisco Bay, California (western United States). We model aeromagnetic and gravity anomalies using geologic constraints along 14 cross sections over a distance of 120 km along the fault zone. Our results predict that the fault zone has more serpentinite at depth than inferred by geologic relationships at the surface. Existing geodetic models are inconsistent and predict different patterns of creep along the fault. Our results favor models with more extensive creep at depth. The source of the serpentinite appears to be ophiolite thrust westward and beneath the Franciscan Complex, an interpretation supported by the presence of antigorite, a high-temperature serpent ine mineral stable at depth, in fault gouge near Lake Pillsbury.

Pliocene–Pleistocene hydrology and pluvial lake during Marine Isotope Stages 5a and 4, Deep Springs Valley, western Great Basin, Inyo County, California

AbstractDeep Springs Valley (DSV) is a hydrologically isolated valley between the White and Inyo mountains that is commonly excluded from regional paleohydrology and paleoclimatology. Previous studies showed that uplift of Deep Springs ridge (informal name) by the Deep Springs fault defeated streams crossing DSV and hydrologically isolated the valley sometime after eruption of the Pleistocene Bishop Tuff (0.772 Ma). Here, we present tephrochronology and clast counts that reaffirms interruption of the Pliocene–Pleistocene hydrology and formation of DSV during the Pleistocene. Paleontology and infrared stimulated luminescence (IRSL) dates indicate a freshwater lake inundated Deep Springs Valley from ca. 83–61 ka or during Late Pleistocene Marine Isotope Stages 5a (MIS 5a; ca. 82 ka peak) and 4 (MIS 4; ca. 71–57 ka). The age of pluvial Deep Springs Lake coincides with pluvial lakes in Owens Valley and Columbus Salt Marsh and documents greater effective precipitation in southwestern North America during MIS 5a and MIS 4. In addition, we hypothesize that Deep Springs Lake was a balanced-fill lake that overflowed into Eureka Valley via the Soldier Pass wind gap during MIS 5a and MIS 4. DSV hydrology has implications for dispersal and endemism of the Deep Springs black toad (Anaxyrus exsul).

Complex Fault Geometry of the 2020 Mww 6.5 Monte Cristo Range, Nevada, Earthquake Sequence

AbstractOn 15 May 2020 an Mww 6.5 earthquake occurred beneath the Monte Cristo Range in the Mina Deflection region of western Nevada. Rapid deployment of eight temporary seismic stations enabled detailed analysis of its productive and slowly decaying aftershock sequence (p=0.8), which included ∼18,000 autodetected events in 3.5 months. Double-difference, waveform-based relative relocation of 16,714 earthquakes reveals a complex network of faults, many of which cross the inferred 35-km-long east–northeast-striking, left-lateral mainshock rupture. Seismicity aligns with left-lateral, right-lateral, and normal mechanism moment tensors of 128 of the largest earthquakes. The mainshock occurred near the middle of the aftershock zone at the intersection of two distinct zones of seismicity. In the western section, numerous subparallel, shallow, north-northeast-striking faults form a broad flower-structure-like fault mesh that coalesces at depth into a near-vertical, left-lateral fault. We infer the near-vertical fault to be a region of significant slip in the mainshock and an eastward extension of the left-lateral Candelaria fault. Near the mainshock hypocenter, seismicity occurs on a northeast-striking, west-dipping structure that extends north from the eastern Columbus Salt Marsh normal fault. Together, these two intersecting structures bound the Columbus Salt Marsh tectonic basin. East of this intersection and the mainshock hypocenter, seismicity occurs in a narrow, near-vertical, east-northeast-striking fault zone through to its eastern terminus. At the eastern end, the aftershock zone broadens and extends northwest toward the southern extension of the northwest-striking, right-lateral Petrified Springs fault system. The eastern section hosts significantly fewer aftershocks than the western section, but has more moment release. We infer that shallow aftershocks throughout the system highlight fault-fracture meshes that connect mapped fault systems at depth. Comparing earthquake data with surface ruptures and a simple geodetic fault model sheds light on the complexity of this recent M 6.5 Walker Lane earthquake.

Serpentinite‐Rich Gouge in a Creeping Segment of the Bartlett Springs Fault, Northern California: Comparison With SAFOD and Implications for Seismic Hazard

AbstractAn exposure of a creeping segment of the Bartlett Springs Fault (BSF), part of the San Andreas Fault system in northern California, is a ~1.5‐m‐wide zone of serpentinite‐bearing fault gouge cutting through Late Pleistocene fluvial deposits. The fault gouge consists of porphyroclasts of antigorite serpentinite, talc, chlorite, and tremolite‐actinolite, along with some Franciscan metamorphic rocks, in a matrix of the same materials. The Mg‐mineral assemblage is stable at temperatures above 250–300 °C. The BSF gouge is interpreted to have been tectonically incorporated into the fault from depths near the base of the seismogenic zone and to have risen buoyantly to the surface where it is now undergoing right‐lateral displacement. The ultramafic‐rich composition, frictional properties, and inferred mode of emplacement of the BSF serpentinitic gouge correspond to those of the creeping traces of the San Andreas Fault identified in the SAFOD (San Andreas Fault Observatory at Depth) drill hole. This suggests a common origin for creep at both locations. A tectonic model for the source of the ultramafic‐rich materials in the BSF is proposed that potentially could explain the distribution of creep throughout the northernmost San Andreas Fault system.

Subsurface Structure of Long Valley Caldera Imaged With Seismic Scattering and Intrinsic Attenuation

AbstractWe image seismic intrinsic ( ) and scattering ( ) attenuations in Long Valley Caldera, California, by analyzing more than 1,700 vertical component waveforms from local earthquakes. Observed energy envelopes are fit to the diffusion model and seismic attenuation images are produced using two‐dimensional space weighting functions. Low intrinsic and low scattering attenuation anomalies in the center south of the caldera correspond to the location of an earthquake swarm in 2014. We identify high intrinsic and high scattering attenuation anomalies in the fluid‐rich western and eastern areas of the caldera. From a comparison with other geophysical images (magnetotellurics and seismic tomography) we attribute these anomalies to a hydrothermal system (high attenuation). Average to high attenuation values are also observed at the adjacent Mammoth Mountain (southwest of the caldera) and may also have a hydrothermal origin. High intrinsic attenuation at low frequencies to the west of the Hartley Springs Fault may be produced by the magmatic system that produced the Inyo Craters. Seismic attenuation imaging provides insights into subsurface structures that are complementary to velocity and conductivity images.

Integrated geophysical investigations of Main Barton Springs, Austin, Texas, USA

Barton Springs is a major discharge site for the Barton Springs Segment of the Edwards Aquifer and is located in Zilker Park, Austin, Texas. Barton Springs actually consists of at least four springs. The Main Barton Springs discharges into the Barton Springs pool from the Barton Springs fault and several outlets along a fault, from a cave, several fissures, and gravel-filled solution cavities on the floor of the pool west of the fault.Surface geophysical surveys [resistivity imaging, induced polarization (IP), self-potential (SP), seismic refraction, and ground penetrating radar (GPR)] were performed across the Barton Springs fault and at the vicinity of the Main Barton Springs in south Zilker Park. The purpose of the surveys was two-fold: 1) locate the precise location of submerged conduits (caves, voids) carrying flow to Main Barton Springs; and 2) characterize the geophysical signatures of the fault crossing Barton Springs pool.Geophysical results indicate significant anomalies to the south of the Barton Springs pool. A majority of these anomalies indicate a fault-like pattern, in front of the south entrance to the swimming pool. In addition, resistivity and SP results, in particular, suggest the presence of a large conduit in the southern part of Barton Springs pool. The groundwater flow-path to the Main Barton Springs could follow the locations of those resistivity and SP anomalies along the newly discovered fault, instead of along the Barton Springs fault, as previously thought.

Long Valley Caldera and the UCERF Depiction of Sierra Nevada Range-Front Faults

Long Valley caldera lies within a left-stepping offset in the north-northwest-striking Sierra Nevada range-front normal faults with the Hilton Creek fault to the south and Hartley Springs fault to the north. Both Uniform California Earthquake Rupture Forecast (UCERF) 2 and its update, UCERF3, depict slip on these major range-front normal faults as extending well into the caldera, with significant normal slip on overlapping, subparallel segments separated by ∼10 km. This depiction is countered by (1) geologic evidence that normal faulting within the caldera consists of a series of graben structures associated with postcaldera magmatism (intrusion and tumescence) and not systematic down-to-the-east displacements consistent with distributed range-front faulting and (2) the lack of kinematic evidence for an evolving, postcaldera relay ramp structure between overlapping strands of the two range-front normal faults. The modifications to the UCERF depiction described here reduce the predicted shaking intensity within the caldera, and they are in accord with the tectonic influence that underlapped offset range-front faults have on seismicity patterns within the caldera associated with ongoing volcanic unrest.

Identifying block structure in the Pacific Northwest, USA

AbstractWe have identified block structure in the Pacific Northwest (west of 116°W between 38°N and 49°N) by clustering GPS stations so that the same Euler vector approximates the velocity of each station in a cluster. Given the total number k of clusters desired, the clustering procedure finds the best assignment of stations to clusters. Clustering is calculated for k = 2–14. In geographic space, cluster boundaries that remain relatively stable as k is increased are tentatively identified as block boundaries. That identification is reinforced if the cluster boundary coincides with a geologic feature. Boundaries identified in Northern California and Nevada are the Central Nevada Seismic Belt, the west side of the Northern Walker Lane Belt, and the Bartlett Springs Fault. Three blocks cover all of Oregon and Washington. The principal block boundary there extends west‐northwest along the Brothers Fault Zone, then north and northwest along the eastern boundary of Siletzia, the accreted oceanic basement of the forearc. East of this boundary is the Intermountain block; its eastern boundary undefined. A cluster boundary at Cape Blanco subdivides the forearc along the faulted southern margin of Siletzia. South of Cape Blanco, the Klamath Mountains‐Basin and Range block, extends east to the Central Nevada Seismic Belt and south to the Sierra Nevada‐Great Valley block. The Siletzia block, north of Cape Blanco, coincides almost exactly with the accreted Siletz terrane. The cluster boundary in the eastern Olympic Peninsula may mark permanent shortening of Siletzia against the Intermountain block.

Dynamic rupture models of earthquakes on the Bartlett Springs Fault, Northern California

AbstractThe Bartlett Springs Fault (BSF), the easternmost branch of the northern San Andreas Fault system, creeps along much of its length. Geodetic data for the BSF are sparse, and surface creep rates are generally poorly constrained. The two existing geodetic slip rate inversions resolve at least one locked patch within the creeping zones. We use the 3‐D finite element code FaultMod to conduct dynamic rupture models based on both geodetic inversions, in order to determine the ability of rupture to propagate into the creeping regions, as well as to assess possible magnitudes for BSF ruptures. For both sets of models, we find that the distribution of aseismic creep limits the extent of coseismic rupture, due to the contrast in frictional properties between the locked and creeping regions.

Structural controls on the emission of magmatic carbon dioxide gas, Long Valley Caldera, USA

AbstractWe present a degassing study of Long Valley Caldera that explores the structural controls upon emissions of magmatic carbon dioxide gas. A total of 223 soil gas samples were collected and analyzed for stable carbon isotopes using a field‐portable cavity ring‐down spectrometer. This novel technique is flexible, accurate, and provides sampling feedback on a daily basis. Sampling sites included major and minor volcanic centers, regional throughgoing faults, caldera‐related structures, zones of elevated seismicity, and zones of past and present hydrothermal activity. The classification of soil gases based on their δ13C and CO2 values reveals a mixing relationship among three end‐members: atmospheric, biogenic, and magmatic. Signatures dominated by biogenic contributions (~4 vol %, −24‰) are found on the caldera floor, the interior of the resurgent dome, and areas associated with the Hilton Creek and Hartley Springs fault systems. With the introduction of the magmatic component (~100 vol %, −4.5‰), samples acquire mixing and hydrothermal signatures and are spatially associated with the central caldera and Mammoth Mountain. In particular, they are concentrated along the southern margin of the resurgent dome where the interplay between resurgence‐related reverse faulting and a bend in the regional fault system has created a highly permeable fracture network, suitable for the formation of shallow hydrothermal systems. This contrasts with the south moat, where despite elevated seismicity, a thick sedimentary cover has formed an impermeable cap, inhibiting the ascent of fluids and gases to the surface.

Slip rates and spatially variable creep on faults of the northern San Andreas system inferred through Bayesian inversion of Global Positioning System data

AbstractFault creep, depending on its rate and spatial extent, is thought to reduce earthquake hazard by releasing tectonic strain aseismically. We use Bayesian inversion and a newly expanded GPS data set to infer the deep slip rates below assigned locking depths on the San Andreas, Maacama, and Bartlett Springs Faults of Northern California and, for the latter two, the spatially variable interseismic creep rate above the locking depth. We estimate deep slip rates of 21.5 ± 0.5, 13.1 ± 0.8, and 7.5 ± 0.7 mm/yr below 16 km, 9 km, and 13 km on the San Andreas, Maacama, and Bartlett Springs Faults, respectively. We infer that on average the Bartlett Springs fault creeps from the Earth's surface to 13 km depth, and below 5 km the creep rate approaches the deep slip rate. This implies that microseismicity may extend below the locking depth; however, we cannot rule out the presence of locked patches in the seismogenic zone that could generate moderate earthquakes. Our estimated Maacama creep rate, while comparable to the inferred deep slip rate at the Earth's surface, decreases with depth, implying a slip deficit exists. The Maacama deep slip rate estimate, 13.1 mm/yr, exceeds long‐term geologic slip rate estimates, perhaps due to distributed off‐fault strain or the presence of multiple active fault strands. While our creep rate estimates are relatively insensitive to choice of model locking depth, insufficient independent information regarding locking depths is a source of epistemic uncertainty that impacts deep slip rate estimates.

Seismic velocity structure in the Hot Springs and Trifurcation areas of the San Jacinto fault zone, California, from double-difference tomography

We present tomographic images of crustal velocity structures in the complex Hot Springs and Trifurcation areas of the San Jacinto Fault Zone (SJFZ) based on double-difference inversions of earthquake arrival times. We invert for V P , V S and hypocentre location within 50 × 50 × 20 km 3 volumes, using 266969 P and 148249 S arrival times. We obtain high-fidelity images of seismic velocities with resolution on the order of a few kilometres from 2 to 12 km depth and validate the results using checkerboard tests. Due to the relatively large proportion of S-wave arrival times, we also obtain stable maps of V P /V S ratios in both regions. The velocity of the Trifurcation Area as a whole is lower than adjacent unfaulted material. We interpret a 4-km-wide low velocity zone with high V P /V S ratio in the trifurcation itself as related to fault zone damage. We also observe clear velocity contrasts across the Buck Ridge, Clark and Coyote Creek segments of the SJFZ. The Anza segment of the SJFZ, to the NW of the trifurcation area, displays a strong (up to 27 per cent) contrast of V S from 2 to 9 km depth. In the Hot Springs area, a low velocity zone between the Claremont and Casa Loma Strands narrows with depth, with clear velocity contrasts observed across both segments. A roughly 10-km-wide zone of low velocity and low V P /V S ratio at the NW tip of the Hot Springs fault is indicative of either unconsolidated sediments associated with the San Jacinto basin, or fluid-filled cracks within a broad deformation zone. High V P /V S ratios along the Anza segment could indicate a preferred nucleation location for future large earthquakes, while the across-fault velocity contrast suggests a preferred northwest rupture propagation direction for such events.

Geophysical signatures of Barton Springs (Parthenia, Zenobia and Eliza) of the Edwards Aquifer, Austin, Texas

Barton Springs is a major discharge site for the Barton Springs Segment of the Edwards Aquifer and is located in Zilker Park in Austin, Texas. Barton Springs actually consists of four springs: (1) The Main Barton Springs discharges into the Barton Springs pool from the Barton Springs Fault and several outlets along a fault and from a cave, several fissures, and gravel-filled solution cavities on the floor of the pool west of the fault. The thin-bedded unit on the southwest side of the fault is the regional dense member, and the lower Georgetown Formation of the Edwards Group is exposed on the northeast side of the fault. The offset of the fault is between 40 and 70 ft (12–21 m). (2) Old Mill Springs is located in the sunken gardens southeast of the Barton Springs Pool and is primarily fed by relatively mineralized groundwater from the Saline-Line Flow Route. (3) Eliza Springs is also located along the Barton Springs Fault north of Barton Springs pool. (4) The Upper Barton Springs is located upstream of the Barton Springs pool on the south bank. Surface geophysical surveys [resistivity imaging and natural potential (NP)] were performed over the first three springs (Main Barton, Old Mill and Eliza Springs). Conductivity (EM) surveys were conducted in some areas to distinguish utility lines. The purpose of the surveys was to: (1) locate the precise location of submerged conduits carrying flow to Main Barton Springs on the north and south banks of the Barton Springs pool; (2) characterize the hydraulic relation between the Main Barton, Old Mill and Eliza Springs; (3) determine the potential location of caves and active flow paths beneath the three springs; and (4) characterize the geophysical signatures of the fault crossing the Barton Springs pool. The geophysical surveys revealed three general types of anomalies. Resistivity results from the south of the Barton Springs swimming pool indicate presence of a thick, laterally extensive high conductivity layer above the pool elevation. This high conductivity layer is interpreted to be lateral clay deposits, either associated with the Del Rio clay or clay-rich alluvial deposits associated with Barton Creek. These clay layers appear to overlie the Edwards Aquifer south of the pool. Also south and east of the pool are cylindrical high conductivity anomalies that extend deeper than the elevation of the submerged cave observed in Barton Springs pool. These cylindrical high conductivity anomalies are also associated with NP anomalies, suggesting groundwater flow. One hypothesis is that the alignment of the high conductivity and NP anomalies corresponds to the Saline-Line Flow Route that is known to discharge primarily at Old Mill Springs, and is hydraulically connected to Main Barton Springs and Eliza Springs. This hypothesis is favored because the Saline-Line Flow Route carries relatively mineralized groundwater and is known to connect to Old Mill Springs and at some times Main Barton and Eliza Springs. There are likely several conduit paths to the pool from the southern part of Zilker Park. Flow paths to Barton Springs from the east may be localized within the uppermost leached and collapsed members of the Edwards Group, which is known for its extensive horizontal cave development. A third type of anomaly, generally found west and immediately adjacent to the Barton Springs Fault, southwest of Barton Springs pool are circular low conductive features associated with NP anomalies. These circular anomalies are interpreted to be groundwater flow conduits bearing less mineralized groundwater associated with the Sunset Valley and possibly Manchaca Flow Routes that are expected to cross that area. The surveys allow an opportunity to compare geophysical responses to directly observed features. The Barton Springs Fault appears to be associated with circular high- and low-resistivity anomalies. Eliza and Old Mill Springs also indicate significant resistivity and NP anomalies suggesting presence of submerged conduits and faults in the vicinity. Although it is obvious such conduits are present adjacent to major springs, the surveys allow examination of how such water-filled conduits appear using various geophysical methods. Known underground infrastructure was also included in the surveys to see how air and water-filled pipes responded. An air-filled train tunnel corresponded to a high-resistivity anomaly. Local utility lines crossing the site showed no significant resistivity anomaly but metal pipes were detectable with EM conductivity surveys.

Eastern Section-SSA 2012 Meeting Report

The 84th annual meeting of the Eastern Section of the Seismological Society of America (ES-SSA) was held on 28--30 October 2012 at the Inn at Virginia Tech in Blacksburg, Virginia. The meeting was hosted by the Virginia Tech Department of Geosciences and began with a visit on Sunday to the Department of Geosciences Museum led by Llyn Sharp followed by a field trip led by William Henika and Martin Chapman. The field trip, Ancient Faults, Groundwater Flow and Intraplate Earthquakes in the Roanoke Valley, involved 29 participants and examined structural, lithological, and hydrogeological characteristics along the Mowles Springs fault zone, a prominent Paleozoic--Mesozoic cross-structure that separates Paleozoic sedimentary rocks in the Valley and Ridge near Salem, Virginia, from Mesoproterozoic basement rocks in the nearby Blue Ridge. This northwest-trending fault zone has recently hosted a series of small earthquakes. The weather for the field trip was initially fine, but as the afternoon progressed, temperatures dropped, and the humidity increased with the approach of hurricane Sandy. Sandy arrived in the vicinity of Virginia on Monday; Blacksburg was less affected than regions further to the northeast, receiving rain, a dusting of snow, and high winds, and some attendees experienced difficulties traveling to or from the meeting.

Quaternary faulting in Queen Valley, California-Nevada: Implications for kinematics of fault-slip transfer in the eastern California shear zone–Walker Lane belt

New geologic map, tectonic, geomorphologic, and terrestrial cosmogenic nuclide (TCN) geochronologic data document the geometry, style, kinematics, and slip rates on late Quaternary faults within the Queen Valley, California-Nevada area. These data provide important insight into the kinematics of fault-slip transfer from the dextral White Mountains fault zone northward into the Mina defl ection. Queen Valley is an ~16-kmlong, NE-trending basin bounded to the south by the White Mountains and underlain by four major Pleistocene to Holocene alluvialfan surfaces. Four different fault types and orientations cut and offset all but the youngest surfaces: (1) The normal-slip Queen Valley fault, which consists of a set of NE-striking, NW- and SE-dipping normal fault scarps that cut across the SE side of the valley and offset all but the youngest surfaces; (2) discontinuous NE-striking, sinistral faults exposed on the north side of the valley; (3) the NW-striking dextral Coyote Springs fault, which merges into (4) a set of E-W‐striking thrust faults. Measured offsets across normal fault scarps developed within 10 Be TCN-dated surfaces yield minimum late Pleistocene horizontal extension rates of 0.1‐0.3 mm/yr. Documented fault geometries and slip orientations across Queen Valley suggest that fault-slip transfer models, such as the extensional displacement transfer, block rotation, and simple shear models, within the dextral fault system proposed for the eastern California shear zone‐ Walker Lane belt are not applicable to this part of the Mina defl ection. Rather, dextral fault slip is transferred by both a restraining westward step and a releasing eastward step. Restraining and releasing bends have been extensively documented at a range of scales in strike-slip fault tectonic settings globally, and they have been simulated in analog models; thus, it is not surprising to document both within the ~630-km-long dextral shear zone that makes up the northern eastern California shear zone‐Walker Lane belt. Our results, combined with published slip rates for the dextral White Mountain fault zone and the eastern sinistral Coaldale fault, suggest that transfer of dextral slip into the Mina defl ection is partitioned into three different components: horizontal extension along the Queen Valley fault, thrust faulting that merges into the dominantly dextral slip along the Coyote Springs fault, and dominantly sinistral slip along the Coaldale fault. A velocity vector diagram of fault-slip partitioning across Queen Valley predicts a small component of contraction across the Coyote Springs and western Coaldale faults. Contraction across the Mina defl ection is consistent with global positioning system data. An observed reduction in late Pleistocene fault-slip rates at the northern end of the eastern California shear zone and across the southwestern part of the Mina defl ection may be explained by distribution of slip across a much broader zone than generally thought.

Displacement-length scaling for single-event fault ruptures: insights from Newberry Springs Fault Zone and implications for fault zone structure

Abstract The Newberry Springs Fault Zone experienced slip associated with the 1992 Landers earthquake in the Mojave Desert of California, USA. Detailed analysis of scaling relationships from single-event ground ruptures in the Newberry Springs Fault Zone mapped in the field shows an average maximum displacement to length ( D max / L ) relationship for fault segments (rupture lengths in the range of 100–1000 m) of 8×10 −5 –consistent with previously published D max / L ratios for normal fault earthquake ground ruptures (rupture lengths in the range of 1–100 km) of 7×10 −5 . To explore the ability of remote sensing (interferometric synthetic aperture radar or InSAR) to map small-displacement single-event fault ruptures and add constraints on segment displacements, we applied established interferometry methods with phase unwrapping to produce maps of line-of-sight displacement and displacement gradient. These maps highlight fault traces that experienced displacement during the time between collection of the synthetic aperture radar images. Comparison of published 1992 single-event ground rupture maps with mapping based on photogeologic interpretation of 1950s vintage aerial photographs indicates that most of the 1992 ruptures occurred as reactivation of pre-existing slip surfaces. In general, D max / L for total fault displacement is approximately 100 times D max / L for single-event ruptures. Evidence from the Newberry Springs Fault Zone indicates that, since the Pleistocene, at least 10–20 Landers-like slip events have occurred, reactivating the Newberry Springs Fault Zone. Evidence of wide damage zones and reactivation of individual segments developed in alluvial floodplain deposits, at relatively small (order of metres) fault displacements, supports a conceptual model of fault damage zone width being established early, during fault propagation. With continued displacement by accumulation of additional slip events, fault zone damage intensifies. The fault zone width may remain relatively stable, although the active portion of the fault zone will likely narrow as faulting continues and a throughgoing slip surface develops and accumulates the bulk of displacement.

GPS-derived strain in northwestern California: Termination of the San Andreas fault system and convergence of the Sierra Nevada–Great Valley block contribute to southern Cascadia forearc contraction

GPS-derived velocities (1993–2002) in northwestern California show that processes other than subduction are in part accountable for observed upper-plate contraction north of the Mendocino triple junction (MTJ) region. After removing the component of elastic strain accumulation due to the Cascadia subduction zone from the station velocities, two additional processes account for accumulated strain in northern California. The first is the westward convergence of the Sierra Nevada–Great Valley (SNGV) block toward the coast and the second is the north–northwest impingement of the San Andreas fault system from the south on the northern California coastal region in the vicinity of Humboldt Bay. Sierra Nevada–Great Valley block motion is northwest toward the coast, convergent with the more northerly, north–northwest San Andreas transform fault-parallel motion. In addition to the westward-converging Sierra Nevada–Great Valley block, San Andreas transform-parallel shortening also occurs in the Humboldt Bay region. Approximately 22 mm/yr of distributed Pacific–SNGV motion is observed inland of Cape Mendocino across the northern projections of the Maacama and Bartlett Springs fault zones but station velocities decrease rapidly north of Cape Mendocino. The resultant 6–10 mm/yr of San Andreas fault-parallel shortening occurs above the southern edge of the subducted Gorda plate and at the latitude of Humboldt Bay. Part of the San Andreas fault-parallel shortening may be due to the viscous coupling of the southern edge of the Gorda plate to overlying North American plate. We conclude that significant portions of the upper-plate contraction observed north of the MTJ region are not solely a result of subduction of the Gorda plate but also a consequence of impingement of the western edge of the Sierra Nevada–Great Valley block and growth of the northernmost segments of the San Andreas fault system.

A volcanotectonic cascade: Activation of range front faulting and eruptions by dike intrusion, Mono Basin‐Long Valley Caldera, California

Stratigraphic data suggest that during the North Mono‐Inyo eruption sequence of ∼1350 A.D. a series of strong earthquakes occurred near the end of the North Mono explosive phases and the beginning of the Inyo explosive phases. The temporal proximity of these events suggests the possibility of a causal relationship. Geological and geomorphic features of the Hartley Springs Fault are consistent with rupture of the fault during the eruption sequence. These features include steep central slope segments on several fault scarps and a fault scarp and stratigraphic offset since the deposition of ∼1200‐year‐old tephra. We hypothesize that the Inyo Dike, found by drilling underneath the main Inyo vents, neared the Hartley Springs Fault as it propagated southward from the Mono Basin circa 1350 A.D. We demonstrate that once the lateral distance between dike and fault was sufficiently small, the mechanical interaction between them could have triggered the slip observed on the fault. The slip, in turn, could have reduced the horizontal confining pressure in a region near the southern tip of the fault. The presence of the main Inyo vents in this region suggests that the reduction in confining stress was sufficient to allow magma to propagate to the surface. The results suggest that a volcanotectonic “cascade” of eruptions and earthquakes is a possible mechanism by which a large section of a range front or rift system can be activated because of the positive feedback provided by each element to continued activity.

Stress orientations in northern and central California: Evidence for the evolution of frictional strength along the San Andreas plate boundary system

We determined orientations of principal stresses around the San Andreas fault (SAF) system in the greater San Francisco Bay Area and regions farther north along the strike‐slip plate boundary. Stress orientations, as well as a ratio between principal stress magnitudes, were determined by inversions of ∼6000 earthquake fault plane solutions, divided into ∼100 groups based on the spatial distribution of seismicity with respect to major regional fault strands (i.e., the SAF, the Hayward‐Rodgers Creek‐Maacama fault zone, and the Calaveras‐Green Valley‐Bartlett Springs fault zone). The stress orientations, while spatially variable, reveal several features; to describe them, it is useful to distinguish between groups of events occurring on and off the major fault strands. First, for off‐fault groups, the angle between major fault strands and the maximum horizontal compression SH decreases systematically to the north. This contrasts with the high angles of SH (∼80°) found immediately adjacent to, as well as farther from, the creeping segment of the SAF in central California by Provost and Houston [2001]. Second, for on‐fault groups, the angle that SH makes with major fault strands changes little along strike, averaging 50° to 55° in the creeping segment, the Bay area, and the northernmost part of our study area. Third, as in the vicinity of the creeping segment of the SAF in central California, the majority of both the off‐fault groups, and the on‐fault groups, are in a strike‐slip, rather than thrust, tectonic regime. Finally, anomalous east‐west SH orientations are seen in the vicinity of Sutter Buttes. In the north, multiple strands of the strike‐slip fault system have accumulated little slip, dip relatively shallowly and are composed of short, complex en échelon segments, suggesting that they originated as thrust faults in the accretionary prism associated with the Farallon subduction and have been subsequently reactivated in a strike‐slip sense following the northward passage of the Mendocino triple junction. Our results, together with the geological context, suggest that the fault system is mechanically stronger with a greater effective frictional strength in the northern portion than in the creeping section of the SAF, with the Bay Area in an intermediate state. We interpret this situation to result from the evolution of the plate boundary toward lower effective frictional strength with increasing slip.