Calico Fault Research Articles

Evidence for a prehistoric multifault rupture along the southern Calico fault system, Eastern California Shear Zone, USA

Abstract Geomorphic mapping and paleoseismologic data reveal evidence for a late Holocene multifault surface rupture along the Calico-Hidalgo fault system of the southern Eastern California Shear Zone (ECSZ). We have identified ~18 km of continuous surface rupture along the combined Calico and Hidalgo faults in the vicinity of Hidalgo Mountain in the southern Mojave Desert. Based on the freshness of geomorphic fault features and continuity of surface expression, we interpret this feature to reflect a simultaneous paleorupture of both faults. Displacement along the paleorupture is defined by 39 field measurements to be generally pure right-slip with a mean offset of 2.3 m. Scaling relationships for this offset amount imply that the original surface rupture length may have been ~82 km (corresponding to a M7.4 earthquake) and that much of the rupture trace was erased by subsequent erosion of sandy and unconsolidated valley alluvium. Eight luminescence ages from a paleoseismic trench across the paleorupture on the Hidalgo fault bracket the timing of the most recent rupture to 0.9–1.7 ka and a possible penultimate event at 5.5–6.6 ka. This timing is generally consistent with the known earthquake clusters in the southern ECSZ based on previous paleoseismic investigations. The ages of these earthquakes also overlap with the age brackets of the most recent events on the Calico fault 42 km to the north and the Mesquite Lake fault 40 km to the south from earlier work. Based on these age constraints and the expected surface rupture length, we propose that the Calico fault system experienced a major, multifault rupture that spanned the entire length of the fault system between the historical Landers and Hector Mine ruptures but preceded these events by ~1–2 k.y. Coulomb stress change modeling shows that the Calico paleorupture may have delayed the occurrence of the Landers-Hector Mine cluster by placing their respective faults in stress shadows and may have also prevented a triggered event from occurring on the Calico fault following the historic events. This work implies that closely spaced ruptures in complex shear zones may repel each other and thereby stretch out the duration of major earthquake clusters. These results also suggest that complex multifault ruptures in the ECSZ may not follow simple, repeatable patterns.

A new geological slip rate estimate for the Calico Fault, eastern California: implications for geodetic versus geologic rate estimates in the Eastern California Shear Zone

ABSTRACTAccurate estimation of fault slip rate is fundamental to seismic hazard assessment. Previous work suggested a discrepancy between short-term geodetic and long-term geologic slip rates in the Mojave Desert section of the Eastern California Shear Zone (ECSZ). Understanding the origin of this discrepancy can improve understanding of earthquake hazard and fault evolution. We measured offsets in alluvial fans along the Calico Fault near Newberry Springs, California, and used several techniques to date the offset landforms and determine a slip rate. Our preferred slip rate estimate is 3.2 ± 0.4 mm/yr, representing an average over the last few hundred thousand years, faster than previous estimates. Seismic hazard associated with this fault may therefore be higher than previously assumed. We discuss possible biases in the various slip rate estimates and discuss possible reasons for the rate discrepancy. We suggest that the ECSZ discrepancy is an artefact of limited data, and represents a combination of faster slip on the Calico Fault, off-fault deformation, unmapped fault strands, and uncertainties in the geologic rates that have been underestimated. Assuming our new rate estimate is correct and a fair amount (40%) of off-fault deformation occurs on major ECSZ faults, the summed geologic rate estimate across the Mojave section of the ECSZ is 10.5 ± 3.1 mm/yr, which is equivalent within uncertainties to the geodetic rate estimate.

A kinematic model for the evolution of the Eastern California Shear Zone and Garlock Fault, Mojave Desert, California

The Eastern California shear zone in the Mojave Desert, California, accommodates nearly a quarter of Pacific–North America plate motion. In south-central Mojave, the shear zone consists of six active faults, with the central Calico fault having the fastest slip rate. However, faults to the east of the Calico fault have larger total offsets. We explain this pattern of slip rate and total offset with a model involving a crustal block (the Mojave Block) that migrates eastward relative to a shear zone at depth whose position and orientation is fixed by the Coachella segment of the San Andreas fault (SAF), southwest of the transpressive “big bend” in the SAF. Both the shear zone and the Garlock fault are assumed to be a direct result of this restraining bend, and consequent strain redistribution. The model explains several aspects of local and regional tectonics, may apply to other transpressive continental plate boundary zones, and may improve seismic hazard estimates in these zones.

Persistent slip rate discrepancies in the eastern California (USA) shear zone

Abstract Understanding fault slip rates in the eastern California shear zone (ECSZ) using GPS geodesy is complicated by potentially overlapping strain signals due to many sub-parallel strike-slip faults and by inconsistencies with geologic slip rates. The role of fault system geometry in describing ECSZ deformation may be investigated with total variation regularization, which algorithmically determines a best-fitting geometry from an initial model with numerous faults, constrained by a western United States GPS velocity field. The initial dense model (1) enables construction of the first geodetically constrained block model to include all ECSZ faults with geologic slip rates, allowing direct geologic-geodetic slip rate comparisons, and (2) permits fault system geometries with many active faults that are analogous to distributed interseismic deformation. Beginning with 58 ECSZ blocks, a model containing 10 ECSZ blocks is most consistent with geologic slip rates, reproducing five of 11 within their reported uncertainties. The model fits GPS observations with a mean residual velocity of 1.5 mm/yr. Persistent geologic-geodetic slip rate discrepancies occur on the Calico and Garlock faults, on which we estimate slip rates of 7.6 mm/yr and <2 mm/yr, respectively, indicating that inconsistencies between geology and geodesy may be concentrated on or near these faults and are not due to pervasive distributed deformation in the region. Discrepancies may in part be due to postseismic relaxation following the A.D. 1992 Mw 7.3 Landers and 1999 Mw 7.1 Hector Mine earthquakes. Otherwise, resolving geologic-geodetic discrepancies would require as much as 11.4 mm/yr of off-fault deformation within <10 km of the main ECSZ faults, with ∼5 mm/yr concentrated near the Calico fault.

Local-scale cross-correlation of seismic noise from the Calico fault experiment

Most studies of seismic noise cross-correlation (NCC) have focused on regional/continental scale imaging using empirical surface-wave Green’s functions extracted from primary (0.05–0.08 Hz) and secondary (0.1–0.16 Hz) microseisms. In this work, we present the NCC results at higher frequencies (>0.5 Hz) from 6 months seismic noise recorded by a local array (~4 km aperture) deployed along the Calico fault in the Mojave Desert, California. Both fast and slow propagating waves are observed from the NCC record-sections. We compare the NCCs from sensor pairs that share a common sensor with the records of a borehole shot located very close to this common sensor. The result shows a good match of the slow surface-wave arrivals, indicating that the NCC method is able to recover unbiased surface-wave Green’s functions at local scales. The strong body-wave NCC component is caused by the P waves generated offshore California. Along a SW–NE profile across the fault, we observe apparent P-wave arrivals and their reflections, which can be explained by a low-velocity-zone (LVZ) along the Calico fault. We calculate the LVZ width to be ~ 2.3 km, and the P-wave velocity reduction within the LVZ to be ~35 %. These estimates are consistent with other evidence for a relatively wide LVZ along the Calico fault.

Seismic structures of the Calico fault zone inferred from local earthquake travel time modelling

Author Posting. © The Authors, 2011. This article is posted here by permission of John Wiley & Sons for personal use, not for redistribution. The definitive version was published in Geophysical Journal International 186 (2011): 760-770, doi:10.1111/j.1365-246X.2011.05055.x.

Paleoseismologic evidence for multiple Holocene earthquakes on the Calico fault: Implications for earthquake clustering in the Eastern California shear zone

Paleoseismologic data from trenches excavated across the Calico fault in the Eastern California shear zone reveal evidence for four surface ruptures during the past ~9000 yr. Twelve optically stimulated luminescence dates constrain the timing of these surface ruptures, which are defi ned by the geometry of growth strata, fi ssure fi lls, and upward fault terminations, to 0.6‐2.0 ka, 5.0‐5.6 ka, 5.6‐6.1 (or possibly 7.3) ka, and 6.1 (or 7.3) to 8.4 ka. Geomorphologic mapping of the 8 km section of the fault extending southward from the trenches reveals two sets of displacements that record the slip from the past two or three surface ruptures. The slip caused by the most recent event was ~2.0 m, while the cumulative slip during the penultimate (and possibly the antepenultimate) event was ~4.5 m. The ages of the paleoearthquakes coincide with periods of clustered moment release identifi ed previously on other faults in the Eastern California shear zone at 0‐1.5 ka, 5‐6 ka, and ca. 8‐9.5 ka, with two Calico fault surface ruptures occurring during the 5‐6 ka Eastern California shear zone cluster. These data strongly reinforce earlier suggestions that earthquake recurrence in the Eastern California shear zone is highly clustered in time and space. Such seismic clustering suggests that at least some regional fault networks undergo distinct periods of systemwide accelerated seismic moment release that may be driven by feedbacks between fault-loading rate and earthquake activity.

Deformation processes adjacent to active faults: Examples from eastern California

Major seismogenic faults are embedded within narrow zones of inelastic off‐fault deformation (OFD), where both distributed displacement and modification of rock properties occur. Active distributed displacement may affect slip rate estimates, seismic energy radiation and geodynamic models. This study addresses the role of OFD in the displacement history and mechanical behavior of seismogenic faults, by multisite study of deformed geologic features adjacent to 30–60 km long active strike‐slip faults of <10 km of dextral displacement in the Mojave Desert, eastern California. We find that distributed displacement accommodates 0 to ∼25% of the total displacement over zones of one to two kilometers width. Displacement occurs mostly within 100–200 m of faults and decreases nonlinearly away from the main fault. We show evidence for distributed displacement through simple shear in the form of parallel secondary faults and progressive deformation of linear markers adjacent to the Calico fault. We also find evidence for shear via rotation and progressive fragmentation adjacent to the Harper Lake fault. Analysis of block dimensions show that blocks tend to decrease in size toward faults and that cumulative length of secondary faults is longer than the main fault by at least a factor of 10. Based on crosscutting relationships and the relationship of OFD to geophysically imaged compliant zones around active faults, we argue that distributed displacement is an active process and suggest that zones of diminished rigidity near faults may be at least in part driven by secondary faulting during the rupture propagation along the main fault.

Three-dimensional models of elastostatic deformation in heterogeneous media, with applications to the Eastern California Shear Zone

SUMMARY We present a semi-analytic iterative procedure for evaluating the 3-D deformation due to faults in an arbitrarily heterogeneous elastic half-space. Spatially variable elastic properties are modelled with equivalent body forces and equivalent surface traction in a ‘homogenized’ elastic medium. The displacement field is obtained in the Fourier domain using a semi-analytic Green function. We apply this model to investigate the response of 3-D compliant zones (CZ) around major crustal faults to coseismic stressing by nearby earthquakes. We constrain the two elastic moduli, as well as the geometry of the fault zones by comparing the model predictions to Synthetic Aperture Radar inferferometric (InSAR) data. Our results confirm that the CZ models for the Rodman, Calico and Pinto Mountain faults in the Eastern California Shear Zone (ECSZ) can explain the coseismic InSAR data from both the Landers and the Hector Mine earthquakes. For the Pinto Mountain fault zone, InSAR data suggest a 50 per cent reduction in effective shear modulus and no significant change in Poisson's ratio compared to the ambient crust. The large wavelength of coseismic line-of-sight displacements around the Pinto Mountain fault requires a fairly wide (∼1.9 km) CZ extending to a depth of at least 9 km. Best fit for the Calico CZ, north of Galway Dry Lake, is obtained for a 4 km deep structure, with a 60 per cent reduction in shear modulus, with no change in Poisson's ratio. We find that the required effective rigidity of the Calico fault zone south of Galway Dry Lake is not as low as that of the northern segment, suggesting along-strike variations of effective elastic moduli within the same fault zone. The ECSZ InSAR data is best explained by CZ models with reduction in both shear and bulk moduli. These observations suggest pervasive and widespread damage around active crustal faults.

Seismic and geodetic evidence for extensive, long-lived fault damage zones

During earthquakes, slip is often localized on preexisting faults, but it is not well understood how the structure of crustal faults may contribute to slip localization and energetics. Growing evidence suggests that the crust along active faults undergoes anomalous strain and damage during large earthquakes. Seismic and geodetic data from the Calico fault in the eastern California shear zone reveal a wide zone of reduced seismic velocities and effective elastic moduli. Using seismic traveltimes, trapped waves, and interferometric synthetic aperture radar observations, we document seismic velocities reduced by 40%‐ 50% and shear moduli reduced by 65% compared to wall rock in a 1.5-km-wide zone along the Calico fault. Observed velocity reductions likely represent the cumulative mechanical damage from past earthquake ruptures. No large earthquake has broken the Calico fault in historic time, implying that fault damage persists for hundreds or perhaps thousands of years. These fi ndings indicate that faults can affect rock properties at substantial distances from primary fault slip surfaces, and throughout much of the seismogenic zone, a result with implications for the amount of energy expended during rupture to drive cracking and yielding of rock and development of fault systems.

Structural and stratigraphic evolution of the Calico Mountains: Implications for early Miocene extension and Neogene transpression in the central Mojave Desert, California

New geologic mapping, structural data, and 40 Ar/ 39 Ar geochronology document early Miocene sedimentation and volcanism and Neogene deformation in the Calico Mountains, located in a complexly deformed region of California9s central Mojave Desert. Across most of the Calico Mountains, volcaniclastic sediments and dacitic rocks of the Pickhandle Formation accumulated rapidly between ca. 19.4 and 19 Ma. Overlying fine-grained lacustrine beds (here referred to as the Calico Member of the Barstow Formation) are bracketed between ca. 19 and 16.9 Ma, and are thus older than the type section of the Barstow Formation in the Mud Hills. Several 17.1–16.8 Ma calc-alkaline dacite domes intrude the Calico Member and represent a previously unrecognized volcanic episode in this region. In the southern Calico Mountains, the Calico fault (part of the Eastern California shear zone) forms a west-northwest–striking, transpressional restraining bend with ∼3 km of right-lateral slip and perhaps 1 km of reverse (north side up) throw distributed on two main fault strands. Part of the Calico fault appears to have originated as an early Miocene normal fault that unroofed metavolcanic basement rocks in the footwall and created a hanging-wall basin in which Pickhandle Formation strata accumulated. This extensional slip must have largely ceased prior to deposition of the Calico Member, which unconformably overlies the Pickhandle Formation north of the Calico fault and directly overlies metavolcanic rocks south of the Calico fault. Deposition of the Pickhandle Formation and at least part of the Calico Member was coeval with rapid unroofing of the central Mojave metamorphic core complex, yet extension in the Calico Mountains is minor and is overprinted by dextral faulting and transpression. Calico Member beds north of the Calico fault are intensely folded into numerous east-west–trending, upright anticlines and synclines that represent 25%–33% (up to ∼0.5 km) north-south shortening. Folds are detached along the base of the Calico Member and thrust over the Pickhandle Formation, which dips homoclinally ∼15–30°S to SE. The geometry and distribution of folds are most compatible with localized transpression between the Calico Member and the Pickhandle Formation within a positive flower structure. Transpressional folding and faulting in the Calico Mountains postdate the ca. 17 Ma dacite intrusions and appear to be largely restricted to the area along the Calico fault restraining bend.

Correction to “Slip rate of the Calico fault: Implications for geologic versus geodetic rate discrepancy in the Eastern California Shear Zone”

[1] In the paper “Slip rate of the Calico fault: Implications for geologic versus geodetic rate discrepancy in the Eastern California Shear Zone” by Michael Oskin, Lesley Perg, Dylan Blumentritt, Sujoy Mukhopadhyay, and Alexander Iriondo (Journal of Geophysical Research, 112, B03402, doi:10.1029/2006JB004451, 2007), erosion rates were miscalculated from 10Be concentrations of exhumed quartz monzonite boulders on the F alluvial fan at the Sheep Springs wash study area. The measured concentrations could be interpreted as steady state erosion rates between 1.6 and 1.9 cm/kyr through alluvium with bulk density of 2.0 g/cm3. At the corrected rates it would take >500 kyr to remove a 10-m-thick alluvial fan deposit. Thus these rates are consistent with preservation of older fan remnants in the study area. These slow alluvial fan erosion rates are also consistent with denudation rates of alluvial fans in the western Mojave Desert [Matmon et al., 2006]. In addition, the exposure age of quartz monzonite boulder sample CC04-025 in Table 3, from the B alluvial fan, is corrected to 59.4 ± 1.6 ka. These corrections do not affect our calculations of the slip rates for the Calico fault, which are based on cosmogenic dating of the better preserved K alluvial fan surface and on stratigraphic constraints that bound the age of the B alluvial fan.

Slip rate of the Calico fault: Implications for geologic versus geodetic rate discrepancy in the Eastern California Shear Zone

Long‐term (105 years) fault slip rates test the scale of discrepancy between infrequent paleoseismicity and relatively rapid geodetic rates of dextral shear in the Eastern California Shear Zone (ECSZ). The Calico fault is one of a family of dextral faults that traverse the Mojave Desert portion of the ECSZ. Its slip rate is determined from matching and dating incised Pleistocene alluvial fan deposits and surfaces displaced by fault slip. A high‐resolution topographic base acquired via airborne laser swath mapping aids in identification and mapping of deformed geomorphic features. The oldest geomorphically preserved alluvial fan, unit B, is displaced 900 ± 200 m from its source at Sheep Springs Wash in the northern Rodman Mountains. This fan deposit contains the first preserved occurrence of basalt clasts derived from the Pipkin lava field and overlies Quaternary conglomerate deposits lacking these clasts. The 40Ar/39Ar dating of two flows from this field yields consistent ages of 770 ± 40 ka and 735 ± 9 ka. An age of 650 ± 100 ka is assigned to this fan deposit based on these ages and on the oldest cosmogenic 3He exposure date of 653 ± 20 ka on a basalt boulder from the surface of unit B. This assigned age and offset together yield a mid‐Pleistocene to present average slip rate of 1.4 ± 0.4 mm/yr. A younger fan surface, unit K, records 100 ± 10 m of dextral displacement and preserves original depositional morphology of its surface. Granitic boulders and pavement samples from this surface yield an average age of 56.4 ± 7.7 ka after taking into account minimal cosmogenic inheritance of granitic clasts. The displaced and dated K fans yield a slip rate of 1.8 ± 0.3 mm/yr. Distributed deformation of the region surrounding the fault trace, if active, could increase the overall displacement rate to 2.1 ± 0.5 mm/yr. Acceleration of slip rate from an average of 1.4 mm/yr prior to ∼50 ka to 1.8 mm/yr since ∼50 ka is possible, though a single time‐averaged slip rate of 1.6 ± 0.2 mm/yr satisfies the data. These rates are faster than any other paleoseismic or long‐term slip rate yet determined for other dextral faults in the Mojave Desert and imply that fault slip rates and earthquake productivity are heterogeneous across this portion of the ECSZ. Total displacement across the Calico fault diminishes northward as shear is distributed into folding and sinistral faults in the Calico Mountains. This pattern is consistent with an approximately threefold drop in geologic slip rate as the Calico fault steps over onto the Blackwater fault and demonstrates the significance of fault interaction for understanding the pattern of present‐day strain accumulation in the ECSZ.

Accelerated stress buildup on the southern San Andreas fault and surrounding regions caused by Mojave Desert earthquakes

A sequence of four Mw . 6 earthquakes, including the 1992 Mw 5 7.3 Landers and Mw 5 7.1 Hector Mine earthquakes, occurred in the Mojave Desert in the 1990s in close proximity to the southern San Andreas fault, inducing stress changes on several of its segments. We calculate that coseismic slip combined with postseismic relaxation of viscous lower crust and/or upper mantle has led to a Coulomb stress increase of 2.3‐3.5 bar on the San Bernardino Mountain segment of the southern San Andreas fault between 1992 and 2001, with a projected increase of 3.6‐4.9 bar by the year 2020. In comparison, the calculated coseismic stress increase is 1.8 bar for this segment. This accelerated buildup of stresses is predicted to bring the San Bernardino Mountain segment, which last ruptured more than 190 yr ago, closer to a potentially major rupture. Meanwhile we project a net stress decrease of as much as 23.5 bar between 1992 and 2020 for the western Coachella Valley segment if the fault is governed by low effective friction, or an increase of 1.5 bar if the fault is governed by high effective friction. Coulomb stresses are calculated to decrease on the Mojave segment by as much as 21 bar between 1992 and 2020. Accelerated stress buildup is also predicted to occur on parts of the San Jacinto, Elsinore, and Calico faults. The pattern of the observed post-Landers aftershock clustering and the calculated Coulomb stress buildup on the Calico fault is similar to that noted in the Hector Mine region prior to the 1999 Mw 5 7.1 earthquake. These results imply that the stress changes caused by an earthquake may still play a role in triggering future quakes in neighboring crust many years later through viscoelastic processes.

Nature of the southwestern boundary of the central Mojave Tertiary province, Rodman Mountains, California

Tertiary volcanic and sedimentary rocks in the central Mojave Desert are largely confined to the Barstow-Bristol trough, an elongate tectonic depression filled with lower Miocene strata. The southwestern boundary of the trough is sharply defined and separates thick (∼2 km) Tertiary sequences from an area to the southwest that is virtually devoid of Tertiary rocks. Segments of this boundary have been interpreted as a buttress unconformity, as a right-slip sidewall fault that formed the lateral boundary of an early Miocene extensional terrane, and as a dip-slip fault caused by flexure of the North American plate above the subducted Mendocino fracture zone. The Silver Bell fault in the Rodman Mountains, central Mojave Desert, is the best exposed segment of the boundary, and relations exposed along it can be used to test these hypotheses. The Tertiary sequence north of the Silver Bell fault is at least 2.3 km thick and comprises at least 1.9 km of mafic lava flows overlain by and intercalated with coarse clastic rocks derived from the south side of the fault. Stratigraphic relations require faulting during volcanism and indicate that the minimum throw across the fault is at least equal to the thickness of the exposed Tertiary section. Tuff layers intercalated in the lower part of the clastic deposits are 23–24 m.y. old, indicating that faulting and volcanism were contemporaneous with extension and plutonism in the nearby central Mojave metamorphic core complex. Structural and stratigraphic data indicate that the Silver Bell fault is primarily a dip-slip fault. The Tertiary section is folded into a west-trending, upright to overturned syncline located near the southern edge of exposed Tertiary rocks. The syncline records late Cenozoic north-south shortening along the northwest-striking Calico fault, which in this area accommodated about 10 km of right slip and perhaps 1 km of reverse slip. Southwestward-dipping strata in the areally extensive north limb of the syncline resemble tilted strata in nearby highly extended areas and have been interpreted to reflect crustal extension. However, bedding dip does not decrease upward in the Tertiary section, paleocurrent directions trend obliquely updip, and northeast-dipping normal faults are rare. Stratal tilting postdated the accumulation of the exposed section, which therefore was not deposited during rotational normal faulting. The new data contradict interpretation of the Silver Bell fault as a major right-slip fault bounding the central Mojave extensional terrane, although they permit small-magnitude extension across the fault. The amount of early Miocene crustal extension in this area therefore is apparently small in spite of stratal tilting, and the lateral limit of large-magnitude crustal extension in the central Mojave block is farther northwest. The data are consistent with the fault having formed by failure during plate flexure above the subducted Mendocino fracture zone.

Displacements on late Cenozoic strike-slip faults of the central Mojave Desert, California

Field studies demonstrate that displacements on northwest-striking wrench faults of the central Mojave Desert are too small to support hypotheses suggesting large interior translations and associated rotation of the province during late Cenozoic time. The margin of an early Miocene structural belt provides the marker with which to establish lateral displacement on individual faults. Displacement values for faults are as follows: Lenwood fault = 1.5–3 km, Camp Rock Fault = 1.6–4.0 km, Calico fault = 8.2 km, and Rodman-Pisgah faults = 6.4–14.4 km. Cumulative displacement on all the major northwest-striking faults of the Mojave Desert is about 26.7–38.4 km. Most, if not all, regionally distributed right shear (presumably related to Pacific–North American plate interaction) developed in the central Mojave Desert after 20 m.y. B.P. Right shear was preceded by significant amounts of kinematically unrelated northeast-southwest crustal extension.