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Garlock Fault

The Garlock Fault is a major left-lateral strike-slip fault zone in southern California, extending approximately 250 kilometers (160 miles) eastward from its junction with the San Andreas Fault near Frazier Park to the Avawatz Mountains, where it terminates against the Death Valley Fault system.^[1]^[2] It forms the northern boundary of the Mojave Desert block, separating it from the Basin and Range province to the north, and plays a key role in accommodating right-lateral shear deformation within the Eastern California Shear Zone as part of the broader Pacific-North American plate boundary.^[1]^[3] Geologically, the fault trends east-west with a near-vertical dip and consists of multiple en echelon segments, including the prominent western, central, and eastern sections, which exhibit varying degrees of Holocene activity and associated folding.^[4] The fault's left-lateral slip rate varies along its length, ranging from 2 to 11 millimeters per year, with an average of about 7 mm/yr based on geologic and geodetic measurements, reflecting ongoing tectonic strain accumulation.^[2]^[5] Paleoseismic studies indicate a history of multiple surface-rupturing earthquakes, with the most recent major events occurring around 1050 AD in the western segment and 1500 AD in the central segment near Johannesburg; recurrence intervals for large ruptures (magnitudes 6.8–7.6) are estimated at 200–3,000 years, depending on the segment.^[1]^[2] The Garlock Fault's significance extends to regional seismic hazard assessment, as it has shown aseismic creep and triggered slip during nearby events, such as the 1992 Landers and 2019 Ridgecrest earthquakes, potentially influencing rupture propagation along adjacent faults like the San Andreas.^[6]^[7] Its interaction with surrounding structures, including north-south shortening north of the fault at its western junction, underscores its role in the complex kinematics of southern California's fault network.^[8]

Physical Description

Location and Extent

The Garlock Fault is a prominent left-lateral strike-slip fault in southern California, trending generally northeast-southwest over a length of approximately 250 km (160 mi).^[9] This orientation aligns it as a key structural feature within the region's tectonic framework, where it accommodates lateral motion between major crustal blocks.^[10] The fault's path forms a broad arc, transitioning from a more easterly strike in its central portions to a pronounced northeast direction toward its endpoints.^[3] The fault's western terminus is located at the complex triple junction with the San Andreas Fault and the Big Pine Fault, near Frazier Mountain in the eastern Transverse Ranges.^[9] From this point, it extends eastward along the northern margin of the Mojave Desert before curving with variations in strike through the western Mojave region.^[8] The central segment continues eastward for about 90 km, passing through areas like the Slate Range, before the eastern segment reaches its terminus near the Avawatz Mountains and the southern end of Death Valley, where it intersects the Death Valley Fault Zone and associated structures such as the Owl Lake Fault.^[9] Geographically, the Garlock Fault serves as a sharp divide between the Mojave Desert block to the south and the Basin and Range province to the north, marking the transition from compressional tectonics in the desert block to extensional deformation in the northern region.^[2] This boundary is evident in the contrasting topography, with the fault tracing the base of the Sierra Nevada and Panamint Mountains to the north while bounding the flatter Mojave terrain to the south.^[10]

Surrounding Terrain

The Garlock Fault demarcates a stark topographic transition, with its northern side bordering arid structural basins of the southern Basin and Range province, such as Indian Wells Valley and Searles Valley, featuring coalescing alluvial fans and dry lake beds like China Lake.^[11] The region to the north includes scattered mountain ranges like the El Paso Mountains and Argus Mountains, under an arid climate with low annual precipitation and sparse vegetation dominated by drought-tolerant shrubs and grasses, which aligns along fault traces due to minor groundwater variations.^[12] To the south, the fault lies at the northern edge of the Mojave Desert, encompassing expansive desert basins and scattered mountain ranges including the El Paso Mountains and Rand Mountains.^[12]^[13] The terrain features Joshua tree woodlands in lower elevations, transitioning to creosote bush scrub across broad, flat basins like Koehn Lake, all under the same arid conditions that limit vegetation cover to scattered desert-adapted species.^[14]^[15] The fault trace crosses Antelope Valley and Fremont Valley, structural depressions where it controls local hydrology by acting as a barrier to groundwater flow, creating fault-controlled valleys bounded by scarps up to several meters high.^[16]^[17] In Antelope Valley, it forms the northwestern boundary, directing recharge southward into playa lakes like Rosamond and Rogers, while in Fremont Valley, it separates consolidated rocks from alluvium, influencing basin recharge dynamics.^[18] Human elements are prominent near the fault, with populated areas such as Lancaster and Palmdale in Antelope Valley relying on groundwater extraction that has lowered levels by up to 300 feet since the 1930s, and Edwards Air Force Base situated adjacent to the trace in the northern Mojave.^[19] The fault also traverses remote desert expanses with a legacy of mining, including historic borax sites near Boron and Kramer, where early 20th-century operations left remnants of mills and pits amid the arid basins.^[17]

Tectonics and Geology

Fault Mechanism

The Garlock Fault operates primarily as a left-lateral strike-slip fault, facilitating the accommodation of oblique convergence between the Pacific and North American plates through dextral shear distributed across the broader fault system.^[20] This motion transfers deformation from the San Andreas Fault system to the west into the extensional regime of the Basin and Range province to the north, forming a critical boundary within the Eastern California Shear Zone (ECSZ) and Walker Lane belt.^[21] The fault accommodates approximately 10-15% of the total relative plate motion, estimated at around 5-7 mm/yr of left-lateral slip in its central segments, contributing to the overall ~25% of plate boundary deformation handled by the ECSZ.^[22] Structurally, the Garlock Fault exhibits a steeply dipping geometry, with dips ranging from 70° to 80° to the north in its eastern sections, transitioning to near-vertical in other areas.^[4] The fault trace consists of long, straight segments arranged in an en echelon pattern, separated by complex bends that give rise to subsidiary faults.^[23] At these bends and restraining or releasing step-overs, the primary strike-slip motion incorporates secondary components of thrusting in compressional zones to the west and normal faulting in extensional zones to the east, reflecting local adjustments to the regional shear.^[24] The fault's interactions underscore its role in the tectonic framework, linking directly to the southern San Andreas Fault at its western terminus near the Transverse Ranges, where it helps accommodate the eastward extrusion of the Mojave block.^[25] To the east, it connects with the Panamint Valley Fault, facilitating slip transfer into the Death Valley region and integrating with the broader ECSZ network of dextral faults.^[20] This connectivity positions the Garlock as a transverse structure that partitions deformation between the stable craton to the south and the distributed extension to the north.^[26]

Formation and Evolution

The Garlock Fault initiated around 9–11 million years ago in the late Miocene, coinciding with the onset of Basin and Range extension and the development of the San Andreas transform fault system. This timing is evidenced by the offset of Miocene sedimentary and volcanic rocks, such as those in the Cudahy Camp Formation and El Paso basin, which indicate the fault's emergence along pre-existing northwest-trending crustal fabrics under regional extensional stresses.^[27]^[28]^[10] Over its evolution, the fault has accommodated left-lateral (sinistral) strike-slip motion through slip partitioning influenced by interactions with adjacent structures, including the Big Pine fault and Kern Canyon fault, as well as volcanic activity in the Big Pine volcanic field. Early development involved growth from an initial eastern segment westward, with cumulative displacement estimated at 48–64 km based on offset Miocene volcanic centers like the Lava Mountains–Summit Range. The fault's sinistral motion reflects differential extension between the Sierra Nevada block to the north and the Mojave Desert block to the south, with partitioning modulated by conjugate shear from the Eastern California Shear Zone and ongoing Basin and Range extension.^[29]^[24] Long-term slip rates over the Quaternary period average 5–11 mm/yr, determined from displaced alluvial fans, offset stream channels, and cosmogenic nuclide dating of faulted landforms. Late Pleistocene rates show variations, with some segments recording 2–4 mm/yr based on offset features in Pilot Knob Valley and adjacent areas. Holocene rates exhibit further segmentation: the western segment slips at 1.6–3.3 mm/yr, as measured from displaced alluvium at Oak Creek Canyon; the central segment reaches up to 10 mm/yr in recent millennia, evidenced by cosmogenic dating of offset fans in the Slate Range and El Paso Mountains; and the eastern segment displays variable rates around 1 mm/yr due to interactions with the Eastern California Shear Zone. These rates highlight temporal and spatial variability driven by fault interactions and regional tectonics.^[30]^[25]^[4]^[31]^[10]

Seismicity

Past Earthquakes

The Garlock Fault has not produced any major instrumental earthquakes in the historical record, with no surface-rupturing events observed since the advent of seismographic monitoring in the early 20th century.^[2] The largest seismic event associated with the fault in historic times was likely the shaking from the 1857 Fort Tejon earthquake (M7.9) on the adjacent San Andreas Fault, which interacted with the Garlock through stress transfer but did not trigger rupture along it.^[32] Paleoseismic investigations reveal at least six to eight surface-rupturing earthquakes along the Garlock Fault during the Holocene epoch, based on trench excavations and offset geomorphic features across its segments, with recent studies suggesting up to 10 or more events over the past ~10,000 years.^[33]^[34]^[35] These events exhibit an average recurrence interval of approximately 1,000 to 2,000 years, with individual intervals varying irregularly due to clustering patterns.^[35] Average left-lateral slip per event ranges from 4 to 7 meters, consistent with the fault's capacity for magnitude ~7 earthquakes assuming full-length ruptures.^[36] Notable prehistoric events include a central segment rupture between A.D. 1450 and 1640, estimated at magnitude ~7 based on slip and rupture length, documented through faulted sediments and radiocarbon dating at the El Paso Peaks site.^[33] On the western segment, paleoseismic trenches at Twin Lakes reveal evidence for multiple events, including offsets from ruptures around A.D. 625–1525 (encompassing circa A.D. 1050) and post-A.D. 1450, indicating significant horizontal displacement.^[34] Eastern segment activity appears linked to ruptures on the nearby Owens Valley Fault, with stress interactions suggesting possible coseismic triggering during late Holocene events, as inferred from Coulomb modeling of fault connectivity.^[37] Rupture patterns on the Garlock Fault exhibit super-cycle behavior, characterized by clusters of activity separated by extended lulls of 3,000–4,000 years with minimal or no slip, as evidenced by multi-millennial slip rate variations from offset dating and trench stratigraphy.^[38] For instance, four earthquakes occurred between approximately 500 and 2,000 years ago, doubling the long-term slip rate during that active phase before a prior quiescent period. Recent refinements confirm these temporal variations, with fast slip phases up to 13–14 mm/yr alternating with slower periods.^[39]

Current Status and Risks

The 2019 Ridgecrest earthquake sequence, comprising a magnitude 6.4 foreshock on July 4 and a magnitude 7.1 mainshock on July 5, significantly influenced the Garlock Fault by triggering a large swarm of over 4,000 small earthquakes (M > 0) along its central portion during the first three weeks following the events. This activity included partial creep along approximately 20–25 km of the fault, with InSAR measurements detecting up to 3.2 cm of line-of-sight displacement in the central segments.^[40] The sequence also imparted positive Coulomb stress changes, estimated at up to 1 bar, which promoted this aseismic slip and brought the fault closer to failure.^[40] Probabilistic assessments immediately following the 2019 sequence indicated a 2.3% chance of a magnitude 7.5 or greater rupture on the Garlock Fault within the subsequent 12 months, a roughly 100-fold increase from pre-sequence levels. Ongoing monitoring of the Garlock Fault relies on the U.S. Geological Survey's (USGS) extensive seismic networks, which continuously track microseismicity and aftershock patterns across Southern California, including real-time detection of events as small as magnitude 1.0. Interferometric synthetic aperture radar (InSAR) data from satellites like Sentinel-1 have further revealed aseismic slip in the central segments, with persistent deformation observed for over 178 days post-Ridgecrest, highlighting the fault's dynamic response to regional stress perturbations.^[40] These tools enable detailed analysis of creep rates and stress accumulation, informing updates to seismic hazard models. The Garlock Fault poses substantial hazards due to its capability to generate earthquakes of magnitude 7 or greater, with USGS scenarios modeling a potential magnitude 7.7 event across its full extent. A major rupture would threaten the densely populated Antelope Valley region, encompassing cities like Lancaster and Palmdale with a combined population of approximately 475,000 as of the 2020 census, potentially causing widespread structural damage from intense ground shaking. Critical infrastructure at risk includes the California Aqueduct, which supplies water to much of Southern California and crosses the fault, as well as Edwards Air Force Base, a key military installation in the northern Mojave Desert. Sedimentary basins in the surrounding valleys could amplify shaking through site effects, increasing the intensity of ground motion and liquefaction potential in low-lying areas. Fault interactions exacerbate these risks, as Coulomb stress transfer from prior events like the 1992 magnitude 7.3 Landers earthquake and the 1999 magnitude 7.1 Hector Mine earthquake has loaded locked segments of the Garlock Fault, promoting strain accumulation and raising the likelihood of future ruptures.^[41] These nearby shocks, occurring within 50–100 km, imparted static stress increases of 0.02–0.05 MPa to the Garlock, contributing to its current heightened state of readiness alongside the more recent Ridgecrest influences.^[41]

References

[1]
Quaternary Fault and Fold Database of the United States
**Key Facts about the Garlock Fault:**
[2]
Garlock fault zone - Southern California Earthquake Data Center
The Garlock fault zone is one of the most obvious geologic features in southern California, clearly marking the northern boundary of the area known as the ...
[3]
Insight into the evolution of the Garlock fault zone, California (USA ...
Mar 9, 2023 · The Garlock fault, a large continental strike-slip fault, exhibits the typical long straight fault segments separated by structurally complex ...
[4]
Quaternary Fault and Fold Database of the United States - USGS.gov
Jun 29, 2000 · East of the Owl Lake fault [70] the Garlock fault zone is delineated by two principal faults, the Mule Springs fault (sinistral-oblique slip to ...
[5]
Persistent slip rate discrepancies in the eastern California (USA ...
Sep 1, 2016 · Persistent geologic-geodetic slip rate discrepancies occur on the Calico and Garlock faults, on which we estimate slip rates of 7.6 mm/yr ...
[6]
Recent movement on the Garlock Fault as suggested by water level ...
Two types of creep events are plausible: (1) normal slip on an en echelon trace of the Garlock fault less than 300 m south of the well, with the north side up ...
[7]
Internal Structure of the Central Garlock Fault Zone From Ridgecrest ...
Jan 9, 2023 · Along the northern edge of the Mojave Desert, the Garlock fault intersects the San Andreas fault and is the second largest (∼300 km long) fault ...
[8]
Kinematic evolution of the junction of the San Andreas, Garlock, and ...
During this deformation, the crust north of the Garlock in the vicinity of the junction undergoes north-south shortening, while the fault junction migrates ...
[9]
Quaternary Fault and Fold Database of the United States
- **Location**: Garlock Fault, USA.
[10]
A Model for the Initiation, Evolution, and Controls on Seismic ...
Jun 27, 2018 · The sinistral Garlock fault extends for ~255 km in a broad NE to EW arc that spans more than half the width of California (Figure 1). Along its ...Missing: survey | Show results with:survey
[11]
[PDF] The Owens Valley Fault Zone, Eastern California, and Surface ...
Our report further establishes that Owens Valley has at least two major independent but subparallel active faults only a few kilometers apart: The strike-slip ...
[12]
[PDF] 4.7 Geology and Soils - California Public Utilities Commission
The Sierra Nevada is a major north-south trending mountain range that rises steeply on the west side of the Owens Valley, and which forms the western boundary ...<|separator|>
[13]
[PDF] bedrock geologic map of the el paso mountains in the garlock and el ...
1). Numerous surficial features that indicate youthful activity along the left-lateral Garlock Fault are present along the south edge of the mountain range.
[14]
[PDF] iii.4 geology and soils - Bureau of Land Management
Oct 1, 2015 · This chapter describes the regulatory setting and the affected environment for geology and soils, including regional topography, geology, ...Missing: terrain | Show results with:terrain
[15]
[PDF] /3 /7P - NASA Technical Reports Server
Sep 15, 1973 · about halfway between the San Andreas and Garlock faults. On ... the Joshua Tree or Tree Yucca, Yucca brevifolia (Jaeger variation; Jaeger.
[16]
[PDF] Areal . Geology of the Western Mojave Desert California
Near Garlock :fault northeast of Summit Diggings, about 1,900 feet o:f strata referred to Bedrock. Spring( n Formation exposed in anticline, but almost.Missing: AFB | Show results with:AFB
[17]
Groundwater - Antelope Valley - California Water Science Center
The Fremont Valley groundwater basin is about 523 square miles and is bounded by the Garlock fault zone to the northwest, the Bissell Hills to the east, and by ...
[18]
[PDF] geohydrology of the antelope valley area, california, and
Borate is mined in several locations near Boron and Kramer. Erosional debris of the borate deposits, including the accessory mineral realgar, a sulfide of.
[19]
[PDF] Fremont Valley Groundwater Basin
The basin is bounded on the northwest by the Garlock fault zone against impermeable crystalline rocks of the El Paso Mountains and the Sierra Nevada.
[20]
[PDF] Section 3.9 Geology, Soils, Seismicity, and Paleontological Resources
Since the 1930s, groundwater levels near the City of. Lancaster have declined as much as 300 feet, and at Edwards Air Force Base, groundwater levels declined ...
[21]
https://nbmg.unr.edu/staff/faulds/33_AGS22_Faulds_and_Henry_%28Walker_Lane%29_final.pdf
[22]
[PDF] Tectonic influences on the spatial and temporal evolution of the ...
The Walker Lane – eastern California shear zone splays from the San Andreas fault system in southern California, shunting ~20% of the relative plate motion east ...
[23]
[PDF] A kinematic model for the evolution of the Eastern California Shear ...
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 ...
[24]
ges02483.1.pdf - GeoScienceWorld
Mar 9, 2023 · The Garlock fault, a large continental strike- slip fault, exhibits the typical long straight fault seg- ments separated by structurally ...
[25]
[PDF] Evolution of the central Garlock fault zone, California
Aug 26, 2014 · The Lava Mountains–Summit. Range (LMSR), located along the central. Garlock fault, is a Miocene volcanic center that holds the key to unraveling ...
[26]
[PDF] Slip rate of the western Garlock fault, at Clark Wash, near Lone Tree ...
The central and eastern sections of the fault may also have initiated as a northeast-striking conjugate fault and then have been rotated to their present ...
[27]
Inception of the eastern California shear zone and evolution of the ...
Jul 7, 2010 · 3.3. First, the Garlock Fault further impedes the relative plate motion, causing a broader velocity gradient across the plate boundary zone ( ...
[28]
Evidence for post-early Miocene initiation of movement on the ...
Jun 2, 2017 · This is the first documented occurrence of Miocene rocks offset by the Garlock fault. Because the Cudahy Camp rocks and their sources appear to ...
[29]
[PDF] The stratigraphie evolution of the El Paso basin, southern California
The stratigraphie evolution of the El Paso basin, southern California: Implications for the Miocene development of the Garlock fault and uplift of the ...
[30]
[PDF] Quaternary reactivation of the Kern Canyon fault system, southern ...
May 20, 2010 · In the Sierra Nevada east of the Kern Canyon fault, earthquakes occur on a combination of dextral strike-slip and normal faults, and to the.
[31]
Temporal variations in Holocene slip rate along the central Garlock ...
Mar 9, 2017 · Average geologic slip rates along the central Garlock fault, in eastern California, are thought to have been relatively steady at 5–7 mm/yr ...Geologic Setting · Late Holocene Displacement · Alluvial Chronology
[32]
[PDF] LATE HOLOCENE SLIP HISTORY OF THE CENTRAL GARLOCK ...
May 4, 2023 · The Garlock Fault is a left lateral strike slip fault at the northern edge of the Mojave Desert, it extends 250 km from the area of Fort Tejon ...
[33]
[PDF] GE111 – Garlock Fault Regional framework - CalTech GPS
SAF last earthquake 1857. ECSZ 1872-1992-1999. San Andreas Fault. Eastern. C ... Maduga, C.M., et al., New Paleo-earthquake Ages from the Western Garlock Fault: ...
[34]
Irregular recurrence of paleoearthquakes along the central Garlock ...
Jul 31, 2003 · Event evidence, in the form of buried fissures, scarps, folds, and upward terminations, has allowed us to identify six well-resolved earthquakes ...
[35]
[PDF] New Paleoearthquake Ages from the Western Garlock Fault
Abstract New paleoseismic investigations on the western segment of the Garlock fault at Twin Lakes, California, reveal evidence for up to six surface ...
[36]
paleoseismology of the central garlock fault in searles valley, california
Preliminary findings suggest at least 10 surface rupturing earthquake events occurred during the 10 k.y. time period exposed in the trench. To provide age ...
[37]
Quaternary Fault and Fold Database of the United States - USGS.gov
Jun 29, 2000 · Sections: This fault has 3 sections. Garlock fault zone is divided based on McGill (1992 #6053). The western Garlock fault section extends from ...
[38]
[PDF] Paleoseismology of the southern Panamint Valley fault
Coulomb failure function modeling of the Panamint Valley and Garlock faults reveals significant stress interactions between these two faults that may influence ...
[39]
Earthquake 'super-cycle' patterns on the Garlock fault - Phys.org
Apr 22, 2015 · Previous paleoseismic results show, however, that this cluster was preceded by a 3000-year lull of very small or no slip. This "on-off" ...
[40]
The July 2019 Ridgecrest, California, Earthquake Sequence ...
Oct 15, 2019 · 3.2 Garlock Fault: Triggered Slip. In addition to fault slip associated with the Mw6.4 and 7.1 events in the Ridgecrest sequence, we find ...<|separator|>
[41]
Transient Stress-Coupling Between the 1992 Landers and 1999 ...
Mar 3, 2017 · The predicted change in Coulomb stress near the hypocenter of the Hector Mine earthquake increases from 0.02 to 0.05 MPa during the 7-yr ...