Slip Rate Variations Research Articles

Slow and Go: Pulsing slip rates on the creeping section of the San Andreas Fault

AbstractRising and falling slip rates on the creeping section of the San Andreas Fault have been inferred from variations of recurrence intervals of characteristically repeating microearthquakes, but this observation has not previously been confirmed using modern geodetic data. Here we report on observations of this “pulsing” slip obtained from advanced multitemporal interferometric synthetic aperture radar (InSAR) data, confirmed using continuous GPS sites of the Plate Boundary Observatory. The surface deformation time series show a strong correlation to the previously documented slip rate variations derived from repeating earthquakes on the fault interface, at various spatial and temporal scales. Time series and spectral analyses of repeating earthquake and InSAR data reveal a quasiperiodic pulsing with a roughly 2 year period along some sections of the fault, with the earthquakes on the fault interface lagging behind the far‐field deformation by about 6 months. This suggests a temporal delay between the pulsing crustal strain generated by deep‐seated shear and the time‐variable slip on the shallow fault interface, and that at least in some places this process may be cyclical. There exist potential impacts for time‐dependent seismic hazard forecasting in California and, as it becomes better validated in the richly instrumented natural laboratory of the central San Andreas Fault, the process used here will be even more helpful in characterizing hazard and fault zone rheology in areas without California's geodetic infrastructure.

Thermal evolution of the Hengshan extensional dome in central South China and its tectonic implications: New insights into low-angle detachment formation

The Hengshan massif is an exhumed, mid-crustal, plutonic–metamorphic dome formed during Cretaceous crustal extension in the Jiangnan orogenic belt, central South China. Multiple thermochronometers (mica 40Ar/39Ar, apatite fission track and zircon (U–Th)/He) are applied to its footwall along a slip-parallel transect to quantify its thermal history and cooling rate, and the slip magnitude, rate, initial geometry and kinematic evolution of the low-angle Hengshan detachment fault. Our thermochronological data, in conjunction with previous ages, indicate that (1) footwall rocks cooled from ~700°C to ~60°C in less than 60Myr (136–80Ma) at variable rates ranging from ~50°C/Myr to ~13°C/Myr, (2) the Hengshan detachment fault accommodated ~8–12km of total slip at variable slip rates from 0.14 to 1mm/yr during tectonic exhumation, (3) the footwall has been tilted ~26°–50° to the east since slip began, indicating that the low-angle Hengshan detachment fault initiated at a steep dip and was passively rotated to a more gentle orientation during subsequent normal slip. This study provides compelling evidence supporting that the low-angle detachment fault in the extensional dome can be generated by the reactivation and passive rotation of an initially steep reverse fault during normal slip. In addition, our thermochronological data constrain the time of extension in the Hengshan dome between 136 and 80Ma, which implies that the back-arc extension within South China associated with the rollback of the Paleo-Pacific slab might have lasted until at least 80Ma.

Spine growth and seismogenic faulting at Mt. Unzen, Japan

AbstractThe concluding episode of activity during the recent eruption of Mt. Unzen (October 1994 to February 1995) was characterized by incremental spine extrusion, accompanied by seismicity. Analysis of the seismic record reveals the occurrence of two dominant long‐period event families associated with a repeating, nondestructive source mechanism, which we attribute to magma failure and fault‐controlled ascent. We obtain constraints on the slip rate and distance of faulting events within these families. That analysis is complemented by an experimental thermomechanical investigation of fault friction in Mt. Unzen dacitic dome rock using a rotary‐shear apparatus at variable slip rates and normal stresses. A power density threshold is found at 0.3 MW m−2, above which frictional melt forms and controls the shear resistance to slip, inducing a deviation from Byerlee's frictional law. Homogenized experimentally generated pseudotachylytes have a similar final chemistry, thickness, and crystal content, facilitating the construction of a rheological model for particle suspensions. This is compared to the viscosity constrained from the experimental data, to assess the viscous control on fault dynamics. The onset of frictional melt formation during spine growth is constrained to depths below 300 m for an average slip event. This combination of experimental data, viscosity modeling, and seismic analysis offers a new description of material response during conduit plug flow and spine growth, showing that volcanic pseudotachylyte may commonly form and modify fault friction during faulting of dome rock. This model furthers our understanding of faulting and seismicity during lava dome formation and is applicable to other eruption modes.

Short-term variations in slip rate and size of prehistoric earthquakes during the past 2000 years on the northern San Jacinto fault zone, a major plate-boundary structure in southern California

Most of the displacement across the North American−Pacific plate boundary in southern California is accommodated by the San Jacinto and the southern San Andreas fault zones. If and how the rate of displacement across these fault zones varies along strike and through time are still being resolved. Here, we present four calculations of late Holocene slip rate and average slip per event from the Claremont fault of the northern San Jacinto fault zone that show variations in strain distribution over the past 2000 yr and illustrate how plate-boundary displacement is distributed between the San Jacinto and southern San Andreas fault zones. We calculate a slip rate of 12.8–18.3 mm/yr and an average slip per event of 2.5 m from two measurements of streams offset by 9–11 earthquakes in the past 1500–2000 yr. Faster slip rates of 21–30 mm/yr and an average slip per event of 2.7–3 m were determined from measurements of a stream and a buried channel that were offset by three earthquakes in the past 400–500 yr. The 2000 yr slip rate is similar to the range in slip rates reported for the adjacent San Bernardino section of the San Andreas fault zone, suggesting that the northern San Jacinto accommodates a similar amount of displacement as the San Andreas fault zone at the same latitude. The rate is also slightly faster (by ∼2–3 mm/yr) than reported slip rates from the central San Jacinto fault zone to the southeast. A slip rate of 15 ± 2 mm/yr is within the range of uncertainty for almost all the geologic and geodetic data for the entire length of the San Jacinto fault zone and may be the best approximation for long-term average slip rate of the fault zone. Alternatively, 2–3 mm/yr of slip along the northern San Jacinto fault zone may be accommodated to the south along the lesser-studied Hot Springs, Thomas Mountain, Buck Ridge, and Santa Rosa faults, the lateral slip rates of which are not well known nor included in typical estimates of slip rate along the central San Jacinto fault zone. We infer that the faster slip rate over the past 500 yr is due to a cluster of earthquakes along the Claremont fault between A.D. 1400 and A.D. 1850 and larger-than-average surface displacement of 3 m or more during the third event back. The 3 m or more measurement of displacement in this event corresponds to rupture lengths that are slightly longer than the total length of the Claremont fault, and previously published paleoseismic data indicate that this event occurred coincident in time with an event on the adjacent Clark fault. We propose that this combination of slip per event data and paleoseismic data from adjacent fault strands is strong evidence for rupture through the releasing step over that separates these two segments of the San Jacinto fault zone.

Paleoseismological and morphological evidence of slip rate variations along the North Tabriz fault (NW Iran)

Northwest Iran is characterized by a high level of historical and instrumental seismicity related to the ongoing convergence between the Arabian and Eurasian plates. In this region, the main right-lateral strike-slip fault known as the North Tabriz fault (NTF) forms the central portion of a large crustal fault system called the Tabriz fault system (TFS). The NTF is a major seismic source along which at least three strong and destructive earthquakes have occurred since 858AD. The two most recent destructive seismic events occurred in 1721AD and 1780AD, rupturing the SE and NW fault segments, respectively. This paper reports paleoseismological and quantitative geomorphologic investigations on the SE segment of the NTF, between the cities of Bostanabad and Tabriz. These observations help to improve our understanding of the seismic hazard for Tabriz city and its surrounding areas. Our field investigations revealed evidence of successive faulting events since the Late Quaternary. Paleoseismic investigations indicate that since 33.5kyr, the SE segment of the NTF has experienced at least three major (M>7.5) seismic events, including the 1721AD earthquake (M=7.6–7.7). Along the NW segment of the fault, however, our results suggest that the amount of strong (M~7.5) seismic events during the same period is significantly greater than along the SE segment. One possible explanation of such a difference in seismic activity is that the Late Quaternary–Holocene coseismic slip rate is decreasing along the NTF from the northwest to the southeast. This explanation contradicts the former hypothesis of a constant slip rate along the whole length of the NTF. In addition, more distributed deformation along several parallel fault branches, in a wider fault zone of the SE segment of the NTF may be considered as additional evidence for the estimation of lower rate of deformation along the fault segment. Such a slip distribution pattern can explain the existence of smaller (~300m) Pliocene–Quaternary cumulative dextral offsets along the SE fault segment than the measured cumulative offsets along the NW segment (~800m) of the NTF.

Constraints for the recent tectonics of the El Salvador Fault Zone, Central America Volcanic Arc, from morphotectonic analysis

Abstract We have used hypsometric analysis to improve our understanding of the current tectonic deformation and structure of El Salvador Fault Zone; a N90°E oriented strike-slip fault zone that extends 150 km through El Salvador, Central America. Our results indicate an important amount of transtensive strain along this fault zone, providing new data to understand the tectonic evolution of the Salvadorian volcanic arc. We have defined kilometric scale tectonic blocks and its relative vertical movements, length of segments with homogeneous vertical motions and lateral relay of active structures. We have identified and quantified slip-rate variations along-strike of the El Triunfo fault within El Salvador Fault Zone, ranging from 4.6 mm/year in its central parts to 1 mm/year towards the tips of the fault. This study supports the hypothesis of a recent rotation in the maximum shortening direction, and the accommodation of the current deformation through the reactivation of pre-existing structures inherited from a previous tectonic regime.

Temporal variations in Holocene slip rate along the central Garlock fault, Pilot Knob Valley, California

Average geologic slip rates along the central Garlock fault, in eastern California, are thought to have been relatively steady at 5–7 mm/yr since at least the Late Pleistocene, yet present-day rates inferred from geodetic velocity fields are indistinguishable from zero. We evaluate the possibility of non-steady slip over millennial timescales using displaced Late Holocene alluvium along the central Garlock fault in Pilot Knob Valley. Truncation of a Late Holocene alluvial fan deposit against a shutter ridge requires a minimum of 30–37 m of displacement since deposition of the fan; maximum allowable displacement is 43–50 m. The extent of soil development atop the fan surface and optically stimulated luminescence ages bracket fan deposition between 3.5 and 4.5 ka. Together, these data require that slip rates during the Late Holocene were ∼7–14 mm/yr, with a preferred rate of ∼11–13 mm/yr. Our results, in conjunction with previous estimates of displacement over the past ∼15 ka, require significant temporal variations in strain release along the Garlock fault and confirm previous suggestions that interactions among fault systems in eastern California give rise to alternating periods of fault activity and quiescence.

Temporal variations in Holocene slip rate along the central Garlock fault, Pilot Knob Valley, California

Temporal variations in Holocene slip rate along the central Garlock fault, Pilot Knob Valley, California

Viscous roots of active seismogenic faults revealed by geologic slip rate variations

Viscous flow in the deep crust and uppermost mantle can contribute to the accumulation of strain along seismogenic faults in the shallower crust1. It is difficult to evaluate this contribution to fault loading because it is unclear whether the viscous deformation occurs in localized shear zones or is more broadly distributed2. Furthermore, the rate of strain accumulation by viscous flow has a power law dependence on the stress applied, yet there are few direct estimates of what the power law exponent is, over the long term, for active faults. Here we measure topography and the offset along fault surfaces created during successive episodes of slip on seismically active extensional faults in the Italian Apennines during the Holocene epoch. We show that these data can be used to derive a relationship between the stress driving deformation and the fault strain rate, averaged over about 15 thousand years (kyr). We find that this relationship follows a well-defined power law with an exponent in the range of 3.0–3.3 (1σ). This exponent is consistent with nonlinear viscous deformation in the deep crust and, crucially, strain localization promoted by seismogenic faulting at shallower depths. Although we cannot rule out some distributed deformation, we suggest that fault strain and thus earthquake recurrence in the Apennines is largely controlled by viscous flow in deep, localized shear zones, over many earthquake cycles.

Fault slip rate variability on 104–105yr timescales for the Salsomaggiore blind thrust fault, Northern Apennines, Italy

Abstract Documenting fault slip rate variability on intermediate (103–105 yr) timescales is crucial for understanding the discrepancies between short-term (101–102 yr) and long-term (106 yr) patterns of deformation. A major obstacle in bridging this gap has been the inability to document multi-Myr records of fault slip with 104–105 yr resolution. Here we present a 3 Myr long record of thrust fault slip with 40 kyr resolution by inverse modeling of Late Pliocene–Early Pleistocene growth strata exposed on the forelimb of the Salsomaggiore blind thrust anticline in the Northern Apennines, Italy. We augment geological data with seismic reflection and well data to construct structural models of the Salsomaggiore anticline and forelimb growth strata. We show that the deformation of the growth strata was due to slip on both a deep, late-stage, thick-skinned reverse fault and the shallow, thin-skinned Salsomaggiore thrust. We show that the thick-skinned fault slipped at a steady rate of 1.4 (+/− 0.7) mm/yr since its activation between 1.0 and 1.8 Ma, while the shallower thin-skinned Salsomaggiore thrust has slipped at rates that varied between ≤ 0.1 and 6 mm/yr over the last 3 Myr. The Salsomaggiore thrust exhibits a high frequency slip rate variability at 40–500 kyr timescales that is likely related to strain partitioning on connected imbricate thrusts. A major deceleration in Salsomaggiore slip rate between 1.0 and 1.8 Ma is coincident with orogenic wedge thickening due to the initiation of thick-skinned reverse faulting following the clastic infilling of the Po foreland basin during the Middle Pleistocene.

Slip-rates along the Chaman fault: Implication for transient strain accumulation and strain partitioning along the western Indian plate margin

The Chaman fault in Western Pakistan marks the western collision boundary between the Indian and Eurasian plates and connects the Makran subduction zone to the Himalayan convergence zone. Geomorphic-scale slip-rates along an active strand of the Chaman fault are added to the sporadic data set of this poorly investigated transform system. Field investigations coupled with high-resolution GeoEye-1 satellite data of an alluvial fan surface (Bostankaul alluvial fan) show ~ 1150 m left-lateral offset by the fault since the formation of the alluvial fan surface. A weighted mean 10Be exposure age of 34.8 ± 3 kyr for the Bostankaul alluvial surface yields a slip-rate of 33.3 ± 3.0 mm/yr. This rate agrees with the geologically defined slip-rates along the Chaman fault, but is approximately twice as large as that inferred from the decade-long global positioning system measurements of 18 ± 1 mm/yr. The contrast in geomorphic and geodetic slip-rates along the Chaman fault, like other major intra-continental strike–slip faults, has two major implications: 1) the geodetic rates might represent a period of reduced displacement as compared to the averaged Late Pleistocene rate because of transient variations in rates of elastic strain accumulation; or 2) strain partitioning within the plate boundary zone. While strain partitioning could be the reason of slip-rate variations within the western Indian plate boundary zone, transient strain accumulation could explain contrasting slip-rates along the Chaman fault at this stage in its poorly understood seismic cycle.

Continuous Evaluation of the Road Skid Resistance with ViaFriction

A newly developed skid resistance measurement system ViaFriction is studied, which can detect the skid resistance under conditions with a variable slip rate, just like the braking process with an anti-lock braking system (ABS). Until now there is still no evaluation framework on the testing results from this test system. In the scope of this paper, the impact from the temperature of the ambient air, the road surface, the tire and the water for the test as well as the influence of the driving speed of the test vehicle on the testing results of the ViaFriction was identified and transferred to a correction function. This total correction algorithm for the measurement results of the ViaFriction allows a comparative analysis of the friction coefficients at different speeds and under different testing temperatures.

Geosciences of the Himalaya–Karakoram–Tibet orogen

The Himalaya–Karakoram–Tibet orogen, a product of India–Eurasia collision *55 Ma back, continues to attract global attention for its many geoscientific uniqueness. This orogen is the most exciting ‘natural laboratory’ to study continental dynamics and its evolution. During past few decades, phenomenal progresses have been made in uplift and extrusion models, dynamic metamorphism, magmatism, climate–tectonics interaction, etc. This thematic volume on the geosciences of the Himalaya–Karakoram– Tibet consists of eighteen papers and two Geosites. In the first paper, Kirby and Harkins correlated variation in slip rate along the Kunlun fault with topography of the Anyemaqen Shan Mountain at the eastern Tibetan plateau. Their work indicates that fault terminations might be associated with crustal thickening of the plateau and demonstrate that the upper crustal deformation of the Tibetan plateau to be pervasive and dispersed throughout the crustal blocks rather than localized along narrow fault zones. Robinson and Pearson compiled an orogen-wide correlation of footwall and hanging wall units of the Ramgarh and Munsiari thrust sheets. They considered the Ramgarh–Munsiari thrust sheet as a single ‘major orogenscale fault’ system. They hypothesized that the extensional shear within the South Tibetan Detachment was triggered possibly by ‘slip transfer’ from the Main Central Thrust into the Ramgarh–Munsiari Thrust during the Miocene. Thakur reviewed the tectonics of the Siwalik range of the Himalaya. He concluded in-sequence deformation and critical taper mechanism acted in this terrain. Presuming the crustal channel flow model to be correct, Mukherjee estimated a viscosity of 10–10 Pa s, and a Prandtl number of 10–10 for the Greater Himalayan Crystallines. Moharana et al., described the Munsiari Thrust from Kumaun Himalaya to consist of a core and a damage zone and described their structural geology in detail. Mukherjee described a ‘basal detachment’ of extensional ductile shear from the base of the Greater Himalayan Crystallines at Bhagirathi section of the Indian Himalaya and explained it in terms of a shifting crustal channel flow. Ubiquitous backthrusts within this terrain were another new finding and were explained by southward subduction of Eurasian plate below the Indian plate. Based on balanced crosssection studies, Khanal and Robinson estimated slip along major Himalayan faults from the Budhi-Gandhaki river section of central Nepal. Sen and Collins inferred dextral transpression and changing angle of convergence of the formation of the Ladakh magmatic arc. They also reported Late Eocene S-type granite magmatism in its central part. Mathew et al. discuss the tectonothermal evolution of the Arunachal Himalaya. They obtained isothermal decompression for the Greater Himalayan Sequence and isobaric cooling from the Main Central Thrust region. Jayangondaperumal et al. reevaluated co-seismic slip of the Himalayan Frontal Thrust. The estimated shortening around the Himalayan Frontal Thrust would lead a bigger earthquake in the western part of Indian Himalaya. Shah presented a geomorphologic study from Kashmir Basin in India, identified faults from remote sensing studies, and predicted a future earthquake of Mw 7.6. Srivastava et al. S. Mukherjee (&) Department of Earth Sciences, Indian Institute of Technology Bombay, Mumbai, India e-mail: soumyajitm@gmail.com

Pre- and postseismic slow slip surrounding the 2011 Tohoku-oki earthquake rupture

Slow (aseismic) slip that accommodates part of the long-term plate motion on subduction megathrusts is thought to be strongly related to the occurrence of large earthquakes on the same fault zone. However, the temporal evolution and spatial distribution of the aseismic slip before major earthquakes and of accelerated postseismic afterslip are largely unconstrained. We estimate cumulative offsets of small repeating earthquakes that are interpreted to reflect the in situ aseismic slip history on the subduction zone offshore northeastern Japan. These data reveal contrasting aseismic slip patterns between the coseismic rupture area of the Mw 9.0 Tohoku-oki earthquake and surrounding portions of the subduction thrust. The rupture area is characterised by low and variable slip rates before 2008, and the slip stopped almost completely after the earthquake. The region surrounding the rupture area exhibited higher aseismic fault slip rates before the earthquake and clear postseismic slip of up to 1.6m within 9 months following the main shock. The frictional fault properties and complete relief of ambient stress in the central rupture zone of the main shock probably control the observed distribution. The postseismic slip shows a more abrupt increase in the region closer to the source, suggesting outwards propagation of afterslip. Small but distinct increases in the slip rate in the ~3yr before the earthquake near the area of large coseismic slip suggests preseismic unfastening of the locked area in the last stage of the earthquake cycle.

Pulsed deformation and variable slip rates within the central Himalayan thrust belt

Forward modeling reconstructions and data derived from the Himalayan thrust belt and the foreland basin of far western Nepal tie the erosional unroofi ng and associated deposition to the kinematics and age of fault motion. We reproduce the deformation identifi ed at the surface through a forward-propagating, linked fold-and-thrust belt‐foreland basin system. This approach permits estimates of the magnitude of erosion at each time step and the extent, depth, and age of the associated foreland basin. The model reconstructions reveal that the units that supplied the sediment to the foreland basin changed through time: 25‐13 Ma, erosion of the Tethyan Himalaya; ca. 12 Ma, fi rst exposure of the Greater Himalaya; ca. 11 Ma, fi rst exposure of the Lesser Himalaya. In our model, exposure of Greater Himalaya and Lesser Himalaya rock is associated with the formation of a thrust ramp that cuts through 7 km of footwall Lesser Himalaya stratigraphy and translates >7 km of Lesser Himalaya rock over the ramp, forming a Lesser Himalaya duplex. An increase in structural relief focuses erosion over the region of the ramp and facilitates exposure of Greater Himalaya and Proterozoic Lesser Himalaya rocks. As the Lesser Himalaya ramp propagates southward, more Lesser Himalaya thrust sheets are incorporated into the Lesser Himalaya duplex. Although uniquely dating thrust events is challenging, these model reconstructions allow us to associate time steps with an age of deposition or exhumation. What emerges is a tempo of deformation that varies with time, marked by periods of rapid shortening during propagation of the Main Central thrust, Ramgarh thrust, and middle stages of the development of the Lesser Himalaya duplex (~25‐30 mm/yr). After emplacement of the Ramgarh thrust, early and late stages of Lesser Himalaya duplex development are marked by periods of slow shortening (~13‐14 mm/yr). Although longterm and modern (geodetic) rates of deformation agree at ~20 mm/yr, rates of shortening through time have varied from 4 to 33 mm/yr.

Static stress interactions in extensional earthquake sequences: An example from the South Lunggar Rift, Tibet

An extensional earthquake sequence occurred in 2004–8 across a graben in the South Lunggar Rift on the Tibetan Plateau. We use InSAR data to determine the location, fault geometry and slip distribution of these earthquakes and to test whether the sequence is compatible with static stress triggering. The Mw 6.2 and 6.3 earthquakes in 2004 and 2005 both ruptured west‐dipping faults on the east side of a graben. In 2008, a Mw 6.7 earthquake ruptured a pair of east‐dipping fault segments on the other side of the graben, offset from the earlier ruptures. We compute first‐order dislocation models of stress change and demonstrate that the order and spatial configuration of this sequence of events is compatible with triggering by static stress transfer. A continuation of the sequence would be most likely to occur on the northern extension of the 2008 rupture, although variable slip rate along the rift may mean that the sequence has run its course. The InSAR data for the 2008 earthquake also reveal slip on a fault that cuts the graben at a highly oblique angle. We suggest that this is a release fault accommodating differential throw in the hanging wall, and associate the deformation with a Mw 6.0 aftershock. Activity on such a release fault has not been directly imaged before. The Zhongba sequence is one of several examples of recent clustered normal‐fault earthquakes on the Plateau, and may be an example of phase‐locking of similar faults.

Relationships between fault geometry, slip rate variability and earthquake recurrence in extensional settings

Field observations and modelling indicate that elastic interaction between active faults can lead to variations in earthquake recurrence intervals measured on timescales of 102-104 yr. Fault geometry strongly influences the nature of the interaction between adjacent structures as it controls the spatial redistribution of stress when rupture occurs. In this paper, we use a previously published numerical model for elastic interaction between spontaneously growing faults to investigate the relationships between fault geometry, fault slip rate variations and the statistics of earthquake recurrence. These relationships develop and become systematic as a long-term consequence of stress redistribution in individual rupture events even though on short timescales earthquake activity appears to be stochastic. We characterize fault behaviour using the coefficient of variation (CV) of earthquake recurrence intervals and introduce a new measure, slip-rate variability (SRV) that takes into account the size and time ordering of slip events. CV generally increases when the strain is partitioned on more than one fault but the relationship between long-term fault slip rate (SRmean) and CV is poorly defined. In contrast, SRV increases systematically where faulting is more distributed and SRmean is lower. To first order, SRV is inversely proportional to SRmean. We also extract earthquake recurrence statistics and compare these to previously published probability density functions used in earthquake forecasting. The histograms of earthquake recurrence vary systematically as a function of fault geometry and are best characterized by a Weibull distribution with fitting parameters that vary from site to site along the fault array. We explain these phenomena in terms of a time-varying, geometrical control on stress loading of individual faults arising from the history of elastic interactions and compare our results with published data on SRV and earthquake recurrence along normal faults in New Zealand and in the Italian Apennines. Our results suggest that palaeoseismic data should be collected and analysed with structural geometry in mind and that information on SRV, CV and SRmean should be integrated with data from earthquake catalogues when evaluating seismic hazard.

Faulted terrace risers place new constraints on the late Quaternary slip rate for the central Altyn Tagh fault, northwest Tibet

The active, left-lateral Altyn Tagh fault defines the northwestern margin of the Tibetan Plateau in western China. To clarify late Quaternary temporal and spatial variations in slip rate along the central portion of this fault system (85°–90°E), we have more than doubled the number of dated offset markers along the central Altyn Tagh fault. In particular, we determined offset-age relations for seven left-laterally faulted terrace risers at three sites (Kelutelage, Yukuang, and Keke Qiapu) spanning a 140-km-long fault reach by integrating surficial geologic mapping, topographic surveys (total station and tripod–light detection and ranging [T-LiDAR]), and geochronology (radiocarbon dating of organic samples, 230 Th/U dating of pedogenic carbonate coatings on buried clasts, and terrestrial cosmogenic radionuclide exposure age dating applied to quartz-rich gravels). At Kelutelage, which is the westernmost site (37.72°N, 86.67°E), two faulted terrace risers are offset 58 ± 3 m and 48 ± 4 m, and formed at 6.2–6.1 ka and 5.9–3.7 ka, respectively. At the Yukuang site (38.00°N, 87.87°E), four faulted terrace risers are offset 92 ± 12 m, 68 ± 6 m, 55 ± 13 m, and 59 ± 9 m and formed at 24.2–9.5 ka, 6.4–5.0 ka, 5.1–3.9 ka, and 24.2–6.4 ka, respectively. At the easternmost site, Keke Qiapu (38.08°N, 88.12°E), a faulted terrace riser is offset 33 ± 6 m and has an age of 17.1–2.2 ka. The displacement-age relationships derived from these markers can be satisfied by an approximately uniform slip rate of 8– 12 mm/yr. However, additional analysis is required to test how much temporal variability in slip rate is permitted by this data set.

Slip rate variations on faults during glacial loading and post-glacial unloading: implications for the viscosity structure of the lithosphere

Abstract: Many active faults in extensional and contractional tectonic settings experienced a slip rate increase after the last glacial period when the volume of nearby glaciers and lakes decreased. This post-glacial slip rate increase is caused by transient stresses that are generated by the unloading-induced rebound and superimposed on the tectonic background stress field. As the latter is different for normal and thrust faults, the response to loading and unloading should depend on the fault type. Here we use finite-element models including a fault in rheologically layered lithosphere to explore the conditions under which both normal and thrust faults experience a post-glacial slip rate increase. The results show that a post-glacial slip rate increase occurs on normal faults if the lower crust is stronger than the lithospheric mantle, whereas thrust faults accelerate if the lower crust is weaker than the lithospheric mantle. These findings imply that the response of faults to mass fluctuations on the Earth's surface may provide constraints on the rheological stratification of the lithosphere. We use our results to make predictions on the viscosity structure of the Basin-and-Range Province and northern Scandinavia, where palaeoseismological data document a pronounced increase in seismicity as a result of post-glacial unloading and rebound.

Slip-rate variability and distributed deformation in the Marmara Sea fault system

The slip rate along a fault controls the accumulation of strain that is eventually released during an earthquake. Estimates from a three-dimensional geomechanical model of the slip rate on the main Marmara fault near Istanbul, Turkey reconcile geodetic and geological observations and indicate smaller values and higher variability than previously thought. The slip rate along a fault controls the accumulation of strain that is eventually released during an earthquake1. Along a 150-km-long stretch of the North Anatolian fault near Istanbul, Turkey, strain has been building up2 since the last large earthquake in 1766. Estimates of the geodetic slip rates along the main Marmara fault vary widely, ranging between 17 and 27.9 mm yr−1 (refs 2, 3, 4, 5). This slip rate is difficult to quantify because of the lack of satellite observations offshore and the complexity of the submarine fault system that includes the main Marmara fault2,6,7. Here we estimate the right-lateral slip rate on the main Marmara fault using a three-dimensional geomechanical model that incorporates these structural complexities. From our simulations we infer slip rates between 12.8 and 17.8 mm yr−1; our estimates are smaller and more variable than previous results, primarily because of slip partitioning and internal deformation. Our model results reconcile geodetic observations and geological fault slip rates8,9,10, which had been considered conflicting previously. We suggest that the inferred variability in slip rate on the main Marmara fault favours segmented release of seismic moment during consecutive events over the failure of the whole seismic gap in one large earthquake.