Using wet kaolin experiments, we document the evolution of strain localization during strike-slip fault maturation under variable boundary conditions (pre-existing fault, depth of and distribution of ...basal shear). While the nature of the basal shear influences strain localization observed at the clay surface, similarities between experiments reveal a general conceptual model of strain accommodation. First, shear strain is accommodated as distributed shear (Stage 0), then by development of echelon faults (Stage I), then by interaction, lengthening and propagation of those echelon faults (Stage II) and, finally, by slip along through-going fault (Stage III). Stage II serves as a transitory period when the system reorganizes after sufficient strain localization. Here, active fault system complexity is maximized as faults link producing apparent rotation of active fault surfaces without material rotation. As the shear zone narrows, off-fault deformation decreases while fault slip and kinematic efficiency increases. We quantify kinematic efficiency as the ratio of fault slip to applied displacement. All fault systems reach a steady-state efficiency in excess of 80%. Despite reducing off-fault deformation, the through-going fault maintains <1.5 cm structural irregularities (i.e., stepovers), which suggests that small (<3 km) stepovers may persist along mature, efficient faults in the crust.
•A four-stage strain localization model for strike-slip faulting is proposed.•Mature strike-slip faults have persistent off-fault deformation.•Active fault complexity does not decrease monotonically.
Geologic slip rates are a time‐averaged measurement of fault displacement calculated over hundreds to million‐year time scales and are a primary input for probabilistic seismic hazard analyses, which ...forecast expected ground shaking in future earthquakes. Despite their utility for seismic hazard calculations, longer‐term geologic slip rates represent a time‐averaged measure of the tempo of strain release and do not measure variability across earthquake cycles. We have developed a numerical approach called STEPS (Slip Time Earthquake Path Simulations), which is built upon field‐based observations and explicitly incorporates realistic variations in displacement per event and variability in the recurrence interval between earthquakes. The STEPS approach, which simulates strain release through time, relies on representing earthquake cycles as stairsteps, rather than straight‐line paths, connecting per earthquake time‐displacement coordinates. We simulate earthquake histories based on these input constraints using two examples: the Carrizo section of the San Andreas fault and the Toe Jam Hill fault of the Seattle fault zone. We find that modeled slip rate distributions agree with slip rates reported for the sites of interest by the original investigators, while providing a slip rate distribution that reflects the variability of earthquake frequency and displacement. The STEPS approach provides an estimate of fault slip rate uncertainty based on a synthetic suite of plausible time‐displacement paths resulting from individual earthquakes, rather than measurement uncertainties associated with offset features. When considering this simulated earthquake behavior between measurements, the uncertainty associated with earthquake paths is greater than that calculated by the long‐term rate.
Plain Language Summary
The rate at which faults accommodate displacement at the Earth's surface during earthquakes may be variable through time. Such variability may result from differences between successive earthquakes, both in how much displacement occurs in each earthquake and the elapsed time between each earthquake. While this variability is challenging to measure, it may have a large impact for determining the activity of a fault and subsequently understanding seismic hazard posed by a fault. We have developed a set of calculations that provide an estimate of this earthquake cycle variability, which explore nuances of earthquake behavior and can combine what we know about past earthquakes into an understanding of when the next earthquake could occur.
Key Points
Numerical models interpolate ground‐rupturing earthquakes between dated offset features observed in the field
Short‐term slip rate uncertainty is estimated from variability in earthquake behavior, rather than only from measurement uncertainty
Test case model results from San Andreas and Toe Jam Hill faults confirm and augment preexisting calculated rates
We develop a model for the evolution and activity of the Garlock fault that combines elements of three previously proposed mechanisms: (1) conjugate slip to the San Andreas fault, (2) extension in ...the Basin and Range, and (3) bending from oblique shear in the eastern California shear zone (ECSZ). Conjugate slip is greatest in the west and decreases eastward. Conversely, extension‐induced slip increases westward from the eastern termination of the fault, reaching a maximum at and to the west of the intersection with the Sierra Nevada frontal fault. Oroclinal bending provides only a small contribution to Garlock slip that increases eastward from the east‐central segment. These spatiotemporally complex loading patterns may explain alternating periods of fault activity along the Garlock and neighboring faults. Moreover, these complex kinematic relationships demonstrate that the Garlock fault acts as an efficient mechanical bridge linking slip on the northern ECSZ and San Andreas fault that may have delayed or even obviated the long‐hypothesized development of a new Pacific‐North America plate boundary along the ECSZ‐Walker Lane.
Key Points
Garlock fault slip is driven by a combination of conjugate shear, intracontinental transform, and oroclinal bending
Specific contributions to Garlock fault slip from different drivers are determined using geologic slip rates
Garlock fault is a mechanical bridge that stabilizes the primary plate boundary configuration
The 6 February 2023 Mw 7.8 Pazarcik and subsequent Mw 7.5 Elbistan earthquakes generated strong ground shaking that resulted in catastrophic human and economic loss across south-central Türkiye and ...northwest Syria. The rapid characterization of the earthquakes, including their location, size, fault geometries, and slip kinematics, is critical to estimate the impact of significant seismic events. The U.S. Geological Survey National Earthquake Information Center (NEIC) provides real-time monitoring of earthquakes globally, including rapid source characterization and impact estimates. Here, we describe the seismic characterization products generated and made available by the NEIC over the two weeks following the start of the earthquake sequence in southeast Türkiye, their evolution, and how they inform our understanding of regional seismotectonics and hazards. The kinematics of rupture for the two earthquakes was complex, involving multiple fault segments. Therefore, incorporating observations from rupture mapping was critical for characterizing these events. Dense local datasets facilitated robust source characterization and impact assessment once these observations were obtained and converted to NEIC product input formats. We discuss how we may improve the timeliness of NEIC products for rapid assessment of future seismic hazards, particularly in the case of complex ruptures.
Incremental slip rates of the Clarence fault, a dextral fault in the Marlborough fault system of South Island, New Zealand, varied by a factor of 4–5 during Holocene–latest Pleistocene time, as ...revealed by geomorphic mapping and luminescence dating of faulted fluvial landforms at the Tophouse Road site. We used high‐resolution lidar microtopographic data and field surveys to map the fine‐scale geomorphology and precisely restore the offset features. We dated the offsets using a stratigraphically informed protocol for infrared stimulated luminescence dating. These data show that incremental slip rates varied from ~2.0 to 9.6 mm/year, averaged over multiple earthquakes and millennial timescales. Comparison to incremental slip rates of the nearby Awatere fault suggests that these faults may behave in coordinated (and anticorrelated) fashion. This study adds to a growing body of evidence suggesting that incremental slip rate variation spanning multiple earthquake cycles may be more common than previously recognized.
Plain Language Summary
Faults are commonly assumed to accommodate relative tectonic motions (slip) at constant rates when averaged over several large earthquakes. Testing this assumption requires documenting how far a fault has slipped at several different points in time over the past several millennia. In this study, we examine a rare instance in which four independent markers of fault slip over time are recorded in the landscape along the Clarence fault ‐ a major fault in the Marlborough fault system of South Island, New Zealand. We use high‐resolution digital topographic laser scans (lidar) to document the fine‐scale topography of the site, and luminescence dating to determine the ages at which the sediments constituting the landforms were last exposed to sunlight. Together, these markers show that, far from being constant over time, the Clarence fault at Tophouse Road has sped up and slowed by by a factor of four or five over the past eleven thousand years or so. This type of study needs to be conducted on other faults to examine whether they have exhibited similar behavior. Such phenomena can provide insights into the processes controlling tectonic plate behavior over timescales of hundreds to thousands of years.
Key Points
Geomorphic analysis of lidar data and a luminescence dating protocol accurately constrain the offset history of a faulted terrace flight
Clarence fault incremental slip rates, spanning millennia and multiple earthquakes, varied by a factor of 4–5 since latest Pleistocene time
Comparison of incremental slip rate variability for the Clarence fault and Awatere fault may suggest coordinated system‐level slip behavior
Abstract
The 6 February 2023 Kahramanmaraş, Turkey (Türkiye), earthquake sequence produced > 500 km of surface rupture primarily on the left-lateral East Anatolian (~345 km) and Çardak (~175 km) ...faults. Constraining the length and magnitude of surface displacement on the causative faults is critical for loss estimates, recovery efforts, rapid identification of impacted infrastructure, and fault displacement hazard analysis. To support these efforts, we rapidly mapped the surface rupture from satellite data with support from remote sensing and field teams, and released the results to the public in near-real time. Detailed surface rupture mapping commenced on 7 February and continued as high-resolution (< 1.0 m/pixel) optical images from WorldView satellites (2023 Maxar) became available. We interpreted the initial simplified rupture trace from subpixel offset fields derived from Advanced Land Observation Satellite2 and Sentinel-1A synthetic aperture radar image pairs available on 8 and 10 February, respectively. The mapping was released publicly on 10 February, with frequent updates, and published in final form four months postearthquake (Reitman, Briggs, et al., 2023). This publicly available, rapid mapping helped guide fieldwork and constrained U.S. Geological Survey finite-fault and loss estimate models, as well as stress change estimates and dynamic rupture models.
Abstract
The 2020 moment magnitude (Mw) 6.5 Stanley, Idaho, earthquake raised questions about the history and extent of complex faulting in the northwestern Centennial Tectonic Belt (CTB) and its ...relation to the Sawtooth normal fault and Eocene Trans-Challis fault system (TCFS). To explore faulting in this area, we excavated a paleoseismic trench across the Sawtooth fault along the western margin of the CTB, and compared an early Holocene (9.1 ± 2.1 ka, 1σ) rupture at the site with lacustrine paleoseismic data and fault mapping in the 2020 epicentral region. We find: (1) a history of partial to full rupture of the Sawtooth fault (Mw 6.8–7.4), (2) that shorter ruptures (Mw≤6.9) are likely along distributed and discontinuous faults in the epicentral region, (3) that this complex system that hosted the 2020 earthquake is not directly linked to the Sawtooth fault, (4) that the northeast-trending TCFS likely plays a role in controlling fault length and rupture continuity for adjacent faults, and (5) that parts of the TCFS may facilitate displacement transfer between normal faults that accommodate crustal extension and rotation. Our results help unravel complex faulting in the CTB and imply that relict structures can help inform regional seismic hazard assessments.
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