Slip Partitioning Research Articles

Updated Segmentation Model and Cummulative Offset Measurement of the Aceh Segment of the Sumatran Fault System in West Sumatra, Indonesia

Sumatran fault in western Indonesia is one of the largest strike-slip fault in the world. The fault was formed as a result of the slip partitioning of the oblique convergence between the Indo-Australian and Eurasian plate along the Sunda trench. The right-lateral movement of the fault is accomodated by 19 fault segments that dissects the entire Sumatra island. We study the Aceh fault segment, which is located at the northernmost parts of the fault. The Aceh fault segment spans 250 km long passing through three districts: West Aceh, Pidie Jaya, and Aceh Besar and is affecting a total of ~546.143 population in the area. The current segmentation model assumes that Aceh fault segment acts as a single fault segment, which would generate closer to a M8 earthquake. This estimation is inconsistent with the ~M6-7 historical earthquake data. We conduct a detailed active fault mapping using the ~8 m resolution digital elevation model of DEMNAS and the sub-m DEM data from UAV-based photogrammetry to resolve the segmentation model of this fault. Our study indicate that the Aceh fault segment can be divided into 8 subsegments: Beutong, Kuala Tripa, Geumpang, Mane, Tangse, Jantho, Indrapuri, and Pulo Aceh. The fault kinematics identified in the field is consistent with right-lateral faulting. We measured cumulative displacement of geomorphic features (channels and ridges) ranging from 12.7 to 1931 m at some area. Findings of our study provide better estimation of the fault geometry and the maximum magnitude of potential earthquake along the Aceh fault segment as well as recommendation of prospective sliprate study sites. These informations are important for the development of seismic hazard analysis of the area.

Merzifon–Esençay splay fault within the North Anatolian Fault system: segmentation and timing of the last two surface faulting events on Esençay segment

North Anatolian Fault (NAF) as a seismically active intracontinental plate boundary is characterized by splay faults in a half fishbone geometry, reaching into the Anatolian plate. Recent crustal deformations within Central Anatolia Tectonic Province are accommodated by dextral and sinistral strike-slip splay fault systems. Sinistral splays extend conjugate to the NAF and subparallel to the East Anatolian Fault (EAF), while the dextral splays are subparallel to the NAF. Sungurlu–Ezinepazar (SEF) and Merzifon–Esencay (MEF) faults are two major dextral splays bifurcating from Niksar pull-apart basin, which distinguish eastern and central NAF. Geodetic and geological data suggest a total of 3 mm/year slip rate along these splays in a setting of slip partitioning with the main strand of the NAF. These two splays undertake 12–13% of the lateral motion within a broad deformation zone of the NAF where the MEF accommodates 1/3 of this amount. 227 km-long MEF is divided into six fault segments, which are separated from each other’s by releasing and restraining jogs. Cumulative offsets up to 3.0 km on Kizilirmak River valley and tributaries of Yesilirmak River since latest Pliocene-Early Pleistocene (~ 3 Ma) suggest an approximately 1 mm/year long-term geological slip rate on the MEF. Paleoseismological findings from Esencay segment reveal two discrete surface faulting events within the last 7 ka with a return period of maximum 2200 and minimum 1320 years. The lapse time since the last event is about 3700 years. This implies ~ 3.7 m of strain accumulation on the Esencay segment for present. The segments have the potential to produce induvial earthquakes with magnitudes varying between Mw 6.8 and Mw 7.2. Existing information imply that major fault splays have the potential to produce multi-segment large earthquakes within the NAF zone in addition to well-known Yedisu and Central Marmara seismic gaps.

Slip Partitioning in the 2016 Alboran Sea Earthquake Sequence (Western Mediterranean)

A MW=5.1 earthquake on January 21st 2016 marked the beginning of a significant seismic sequence in the southern Alboran Sea, culminating in a MW=6.3 earthquake on January 25th, and continuing with further moderate magnitude earthquakes until March. We use data from 35 seismic broadband stations in Spain, Morocco and Portugal to relocate the seismicity, estimate seismic moment tensors, and isolate regional apparent source time functions for the main earthquake. Relocation and regional moment tensor inversion consistently yield very shallow depths for the majority of events. We obtain 50 moment tensors for the sequence, showing a mixture of strike-slip faulting for the foreshock and the main event and reverse faulting for the major aftershocks. The leading role of reverse focal mechanisms among the aftershocks may be explained by the geometry of the fault network. The mainshock nucleates at a bend along the left-lateral Al-Idrisi fault, introducing local transpression within the transtensional Alboran Basin. The shallow depths of the 2016 Alboran Sea earthquakes may favour slip-partitioning on the involved faults. Apparent source durations for the main event suggest a ~21 km long, asymmetric rupture that propagates primarily towards NE into the restraining fault segment, with fast rupture speed of ~3.0 km/s. Consistently, the inversion for laterally variable fault displacement situates the main slip in the restraining segment. The partitioning into strike-slip rupture and dip-slip aftershocks confirms a non-optimal orientation of this segment, and suggests that the 2016 event settled a slip deficit from previous ruptures that could not propagate into the stronger restraining segment.

Primary on fault paleoseismic evidences from trench investigation along the Bathubasti fault, South Andaman, India

Continued structural descent of the Indian lithosphere below the Sunda-Andaman trench produced several destructive large to great earthquakes in the region of Andaman and Nicobar Islands. The pattern of strain accumulation and its release in the context of great plate boundary mega thrust earthquakes and slip partitioning along active faults in the Andaman and Nicobar region during these events is unclear, owing to lack of on-fault primary paleoseismic evidences, structural complexity associated with subduction processes and lack of primary surface ruptures of large to huge magnitude earthquakes. In this context, preliminary paleoseismic trench investigations were performed across a 6-m high fault scarp at Bathubasti (South Andaman) to understand the slip partitioning and rupture dynamics of subduction zone earthquakes. Ground-penetrating radar (GPR) profiles and the trench exposure across the scarp suggest progressive deformation and several episodes of co-seismic scarp generation where Andaman Flysch Sandstone (Oligocene) is in fault contact with the late Holocene sediments. A total of eight stratigraphic units (a to g) including three fault strands (F0, F1, and F2) were observed and mapped in the trench stratigraphy. OSL ages constrain that event 1 occurred along F0 around 2312 ± 302 years BP and thrust unit (g) Andaman Flysch -bedrock over unit (f) paleosols. Event 2 has deformed unit (C)-medium sand bed and the basal portion of unit (b)-fine sand bed along F1 and F2 around 422 ± 57 years BP. Given the scarp height of 6 m and with, a vertical displacement of approximately 3 m of unit (g)- bed rock along low-angle fault (θ = 18°) implies an average slip rate of 4.12 mm/year, shortening rate of 3.88 mm/year, and an uplift rate of 4.54 mm/year.

The January 23, 2018 M7.9 Kodiak earthquake, Alaska: A consequence of slip partitioning in the outer rise region

The January 23, 2018 M7.9 earthquake ruptured conjugate set of strike-slip faults in the outer rise of the Alaska subduction zone, southeast of Kodiak Island. The complex nature of this earthquake is evident from the earthquake focal mechanism and aftershock relocation patterns which revealed rupture of multiple north-south and east-west trending conjugate fault planes. Here, we suggest that this earthquake occurred as a result of two different modes of interaction process. The stress triggering process reveals (i) the co-seismic and post-seismic deformation of the M9.2 1964 Alaska megathrust earthquake increased Coulomb failure stress of ∼10 kPa and ∼0.05 kPa respectively in the 2018 Kodiak earthquake source region, and (ii) the overall deformation is being accommodated in the outer rise region due to slip partitioning resulting from the oblique convergence between the Pacific and North American plates along the Alaska Aleutian megathrust.

Strong earthquake clustering around the eastern Tibetan Plateau after the 2008 MW7.9 Wenchuan earthquake

After the 2008 MW7.9 Wenchuan earthquake, the eastern Tibetan Plateau experienced a series of MW>6.0 earthquakes, including the 2013 MW6.6 Lushan, 2014 MW6.1 Kangding and 2017 MW6.5 Jiuzhaigou events. Based on available constraints, we build a three-dimensional viscoelastic finite element model to calculate Coulomb failure stress caused by these strong earthquakes. In this model, the geometry and slip vector of the initial rupture zone of each earthquake are used to better evaluate the earthquake-related stress projection. Considering reasonable ranges of viscosities for the crust and upper mantle in different tectonic units, numerical results show that after the Wenchuan earthquake, the coseismic Coulomb failure stress change at the hypocenters of the subsequent earthquakes increased to approximately +0.012–+0.040, +0.01–+0.03, and +0.008–+0.015 MPa, respectively. With viscoelastic relaxation of the lower crust and upper mantle, the Coulomb failure stress change at the hypocenters of these earthquakes accumulated to about +0.014–+0.042, +0.016–+0.036, and +0.003–+0.007 MPa just before their occurrence. This suggests that the Wenchuan earthquake indeed triggered or hastened the occurrence of the Lushan, Kangding and Jiuzhaigou events, supporting that strong earthquake clustering around the eastern Tibetan Plateau could be related to stress interaction between the seismogenic faults. Besides, ~94% and ~6% of the stress increase around (and before the occurrence of) the Kangding earthquake were contributed by the Wenchuan event and the Lushan event, respectively; the positive Coulomb failure stress change at the Jiuzhaigou earthquake hypocenter was related to coseismic slip partitioning of the Wenchuan earthquake. This means that stress interaction among the earthquakes could be controlled by the combined effect of stress of the previous events and by the complexity of earthquake ruptures. Thus, in researches on the earthquake-triggering mechanism, special attentions should be paid on both details of the rupture model and multiple factors of previous earthquakes.

Onset of slip partitioning under oblique convergence within scaled physical experiments

Abstract Oblique convergent margins host slip-partitioned faults with simultaneously active strike-slip and reverse faults. Such systems defy energetic considerations that a single oblique-slip fault accommodates deformation more efficiently than multiple faults. To investigate the development of slip partitioning, we record deformation throughout scaled experiments of wet kaolin over a low-convergence (<30°), obliquely slipping basal dislocation. The presence of a precut vertical weakness in the wet kaolin impacts the morphology of faults but is not required for slip partitioning. The experiments reveal three styles of slip partitioning development delineated by the order of faulting and the extent of slip partitioning. Low-convergence angle experiments (5°) produce strike-slip faults prior to reverse faults. In moderate-convergence experiments (10°–25°), the reverse fault forms prior to the strike-slip fault. Strike-slip faults develop either along existing weaknesses (precut or previous reverse-slip faults) or through the coalescence of new echelon cracks. The third style of local slip partitioning along two simultaneously active dipping faults is transient while global slip partitioning persists. The development of two active fault surfaces arises from changes in off-fault strain pattern after development of the first fault. With early strike-slip faults, off-fault contraction accumulates to produce a new reverse fault. Systems with early lobate reverse faults accommodate limited strike-slip and produce extension in the hanging wall, thereby promoting strike-slip faulting. The observation of persistent slip partitioning under a wide range of experimental conditions demonstrates why such systems are frequently observed in oblique convergence crustal margins around the world.

Segmentation and termination of the surface rupture zone produced by the 1932 Ms 7.6 Changma earthquake: New insights into the slip partitioning of the eastern Altyn Tagh fault system

AbstractThe 1932 Ms 7.6 earthquake struck the active Changma fault in the NE Tibetan Plateau, and produced a distinct surface rupture along the fault zone. However, the segmentation and termination of the surface rupture zone are still unclear. In this paper, the active tectonic analyses of multiple satellite images complemented by field investigations present the 120-km-long surface rupture zone, which can be divided into five discrete first-order segments, ranging from 14.4 to 39.56 km in length, linked by step-overs. Our results also indicate that the 1932 rupture zone could jump across step-overs 0.3–4.5 km long and 2.2–5.4 km wide in map view, but was terminated by a 6.3-km-wide restraining step-over at the eastern end. The left-lateral slip rates along the mid-eastern and easternmost segments of the Changma fault are 3.43 ± 0.5 mm/yr and 4.49 ± 0.5 mm/yr since 7–9 ka, respectively. The proposed tectonic models suggest that the slip rates on the Changma fault are similar to the slip rate on the eastern segment of the Altyn Tagh fault system near the junction point with the Changma fault. These results imply that the Changma fault plays a leading role in the slip partitioning of the easternmost segment of the Altyn Tagh fault system.

Triple junction kinematics accounts for the 2016 Mw 7.8 Kaikoura earthquake rupture complexity

The 2016, moment magnitude (Mw) 7.8, Kaikoura earthquake generated the most complex surface ruptures ever observed. Although likely linked with kinematic changes in central New Zealand, the driving mechanisms of such complexity remain unclear. Here, we propose an interpretation accounting for the most puzzling aspects of the 2016 rupture. We examine the partitioning of plate motion and coseismic slip during the 2016 event in and around Kaikoura and the large-scale fault kinematics, volcanism, seismicity, and slab geometry in the broader Tonga-Kermadec region. We find that the plate motion partitioning near Kaikoura is comparable to the coseismic partitioning between strike-slip motion on the Kekerengu fault and subperpendicular thrusting along the offshore West-Hikurangi megathrust. Together with measured slip rates and paleoseismological results along the Hope, Kekerengu, and Wairarapa faults, this observation suggests that the West-Hikurangi thrust and Kekerengu faults bound the southernmost tip of the Tonga-Kermadec sliver plate. The narrow region, around Kaikoura, where the 3 fastest-slipping faults of New Zealand meet, thus hosts a fault-fault-trench (FFT) triple junction, which accounts for the particularly convoluted 2016 coseismic deformation. That triple junction appears to have migrated southward since the birth of the sliver plate (around 5 to 7 million years ago). This likely drove southward stepping of strike-slip shear within the Marlborough fault system and propagation of volcanism in the North Island. Hence, on a multimillennial time scale, the apparently distributed faulting across southern New Zealand may reflect classic plate-tectonic triple-junction migration rather than diffuse deformation of the continental lithosphere.

Late Quaternary slip rate of the Central Sierra Madre fault, southern California: Implications for slip partitioning and earthquake hazard

The Sierra Madre fault system accommodates contraction within a large restraining bend area of the San Andreas fault along the northern margin of the Los Angeles metropolitan area in southern California. Reverse slip along this fault system during earthquakes controls growth of the San Gabriel Mountains and poses a significant seismic hazard to the region. Here, we measure the late Quaternary slip rate of the Central Sierra Madre fault (CSMF) using analysis of high-resolution topography combined with cosmogenic 10Be surface exposure dating and post-IR IRSL geochronology. We mapped terrace and fan surfaces from three drainages that cross the CSMF and correlate them based on deposit character and geomorphic position. Cosmogenic nuclide and luminescence ages are consistent amongst the three prominent surfaces offset ∼5 to 28 m across the fault zone. Age estimates for these surfaces are 53 +21/−15 ka, 35 ± 9 ka, and 12±5 ka based on data from two dating methods at three locations, refined by inset age relationships. Estimated slip for these geomorphic markers is more uncertain than the measured vertical separation due to uncertainties in fault dip and ranges from 7.5 +5.4/−3.1 m to 58.5 +46.3/−14.4 m. Incremental dip-slip rate estimates from different age surfaces and locations overlap within uncertainty, with median values ranging from 0.6 to 1.1 mm/yr. The average slip rate for all three generations of markers is 1.1 +1.1/−0.4 mm/yr. This late Quaternary slip rate for the CSMF is slower than estimates based on interseismic geodetic data, and emphasizes the importance of contraction distributed across multiple structures south of the Sierra Madre fault when assessed against the geodetic shortening budget. Despite being the central portion of the broader Sierra Madre fault system, the CSMF has a slip rate similar to or lower than neighboring sections, suggesting that slip transfer onto other nearby faults control the along-strike pattern of deformation rate. Paleoseismic evidence indicates that the last earthquake on the CSMF was in the early Holocene, and the slip rate we estimate suggests that the accumulated elastic strain could produce many meters of slip in future earthquakes.

Activity of crustal faults and the Xolapa sliver motion in Guerrero\u2013Oaxaca forearc of Mexico, from seismic data

Oblique convergent margins often host forearc slivers separated by the subduction interface and a trench parallel strike-slip fault system in the overriding plate. Mexican oblique subduction setting led to the formation of a forearc sliver and accomodation of part of the slip at the bounding system of strike-slip faults. The Xolapa sliver is, on average, a sim 105-km-wide crustal block located along the coast of Guerrero and Oaxaca states of Mexico, and is limited by a sim 650-km-long La Venta-Chacalapa fault zone. Two types of datasets, local catalog and Global CMT compilation, are used to estimate the motion of the Xolapa sliver using the rigid block model that describes the phenomenon of slip partitioning. According to the results obtained from local and Global CMT catalogs for selected subduction thrust earthquakes, the forearc sliver moves southeastwards with respect to the fixed North America plate at the rate of 10 ± 1 mm/year and 5.6 ± 0.8 mm/year, respectively. These velocities in general agree with the values obtained from long-term GPS observations (5–6 mm/year). The origin of the inconsistency between local and teleseismic estimates is attributed to a difference in the double couple focal mechanism parameters for two types of datasets. Convergence obliquity changes from 10.42{^circ } and the rate of 58 mm/year to 13.29{^circ } at the rate of 68 mm/year along the Guerrero and Oaxaca coast increasing from northwest to southeast; therefore, the Xolapa sliver is supposed to be stretched. However, the slip vector azimuths of thrust subduction earthquakes tend to approach plate convergence vectors southeastwards along the coast; so, we assume that this may produce the forearc block compression.

Slip partitioning along an idealized subduction plate boundary at deep slow slip conditions

Below the base of many subduction seismogenic zones, the plate interface periodically slips at rates 1 to 2 orders of magnitude faster than tectonic plate velocities. A number of competing hypotheses exist to explain the mechanisms for these slow slip events (SSEs), but they remain incompletely tested because we do not know how deformation is partitioned across the lithologically complex plate boundary interface. We use the deepest exposure of the Arosa zone, a ∼520 m-thick exhumed subduction interface, as a case study to evaluate the partitioning of strain between lithologic units throughout the SSE cycle. We review and synthesize published constitutive relations for the five lithologic units present to express shear stress as a function of deformation rate. We use these results to predict (1) the shear stress across the plate boundary as a function of slip velocity and (2) the partitioning of deformation among the different lithologic units for SSE and aseismic creep velocities. We conduct this analysis for pore fluid pressures from hydrostatic to near-lithostatic. Our results show that, at pore fluid pressure close to hydrostatic, aseismic creep and SSE velocities occur by viscous deformation of calcareous and quartzose units. However, once the pore fluid pressure increases above 80% of lithostatic, plate boundary slip migrates from the calcareous and quartzose rocks during aseismic creep to frictional deformation of talc schist during slow slip. This result is insensitive to differences in the thicknesses of metasedimentary units that may be present along subduction plate boundaries and, therefore, may apply to subduction plate boundaries in general.

Strain Accommodation in the Daliangshan Mountain Area, Southeastern Margin of the Tibetan Plateau

AbstractThe Xianshuihe‐Xiaojiang fault system (XXFS) plays a crucial role in accommodating relative motions of the south China block and the expanding Tibetan Plateau. Compared to the narrow southern and northern segments of the XXFS, the central section (also referred to as the Daliangshan mountain area) is broader and dissected by a network of faults. Geologic studies suggest that three major NNW trending faults may dominate slip partitioning of more recent faults in this region. However, due to accessibility issues, sparse geodetic studies have constrained geodetic fault slip rates with contradictory results. In this study, we use 17 new Global Navigation Satellite System sites combined with existing solutions to investigate slip partitioning of the region. We derive a tectonic velocity solution, develop a block kinematic model constrained by Euler pole clustering technique, and calculate geodetic fault slip rates to compare with geologic fault slip rates. We confirm that the Anninghe‐Daliangshan sliver (ADS) is a distinct, rigid block, and we resolve a new, rigid Daliangshan‐Yingjing‐Mabian block (DYB). Kinematic modeling predicts long‐term slip rates that suggest both the ADS and the DYB partition (~10.0–12.0 mm/year) relative motion between the Tibetan Plateau and south China block in the central XXFS, reconciling geologic observations. We suggest that both the continuum and microplate deformation patterns are present in our study area, of which the local tectonics is better explained by the microplate model, but scale of the kinematics makes its motion more consistent with the continuum model.

The 2018 Mw6.4 Hualien earthquake: Dynamic slip partitioning reveals the spatial transition from mountain building to subduction

The Hualien earthquake of 6 February 2018 occurred on the east coast of Taiwan in the collision zone between the Eurasia and Philippine Sea plates. Even though an event with a moderate magnitude of Mw6.4, it caused widespread surface rupture in and around the city of Hualien, leading to significant loss of life and property damages. In this study, we investigate the rupture process of this earthquake using teleseismic, strong motion, GPS and SAR observations. We first employ the geodetic data to search for the geometry of multiple fault planes, followed by a multi-point source inversion using teleseismic and strong motion waveforms to determine the possible change in mechanism during the rupture process. We parameterize the fault model with three curved segments to represent the complicated fault system involved in the rupture process. An offshore fault was ruptured by the Hualien earthquake showing a curved geometry with the dip angle changing from shallow (∼45°) in the north to sub-vertical (∼80°) in the south. Evidence from tomographic velocity model indicates that the offshore fault is the boundary between the subducted Coastal Range and Meilun Tableland, which is also a plate interface. Finally, a joint inversion is carried out to constrain the complete rupture process. The inversion result reveals that the rupture initiated on the offshore fault interface and propagated unilaterally towards south-southwest. The rupture then jumped on to the sub-vertical Meilun Fault and reached the ground surface. Significant dynamic slip partitioning occurred during the rupture process near the Meilun Fault, with the majority of thrust and strike-slip motions occurring on the off-shore fault and the Meilun Fault, respectively. The afterslip model is inverted with SAR timeseries spanning 6 months after the mainshock. The afterslip model shows a strong complementary pattern with co-seismic slips. The comparison between the 1951 and 2018 Hualien earthquake shows that the rupture behavior of the Meilun Fault does not follow a repeating earthquake pattern: thrust motion may occur on the two side walls of the Meilun Tableland with a “teeterboard” uplift pattern in different earthquake cycles. The change in geometry and slip on the inter-plate fault appears to be related to the transition of the Coastal Range from mountain building to subduction.

Push-pull driving of the Central America Forearc in the context of the Cocos-Caribbean-North America triple junction

Different kinematic models have been proposed for the triple junction between the North American, Cocos and Caribbean plates. The two most commonly accepted hypotheses on its driving mechanism are (a) the North American drag of the forearc and (b) the Cocos Ridge subduction push. We present an updated GPS velocity field which is analyzed together with earthquake focal mechanisms and regional relief. The two hypotheses have been used to make kinematic predictions that are tested against the available data. An obliquity analysis is also presented to discuss the potential role of slip partitioning as driving mechanism. The North American drag model presents a better fit to the observations, although the Cocos Ridge push model explains the data in Costa Rica and Southern Nicaragua. Both mechanisms must be active, being the driving of the Central American forearc towards the NW analogous to a push-pull train. The forearc sliver moves towards the west-northwest at a rate of 12–14 mm/yr, being pinned to the North American plate in Chiapas and western Guatemala, where the strike-slip motion on the volcanic arc must be very small.

Statistical modelling of co-seismic knickpoint formation and river response to fault slip

Abstract. Most landscape evolution models adopt the paradigm of constant and uniform uplift. It results that the role of fault activity and earthquakes on landscape building is understood under simplistic boundary conditions. Here, we develop a numerical model to investigate river profile development subjected to fault displacement by earthquakes and erosion. The model generates earthquakes, including mainshocks and aftershocks, that respect the classical scaling laws observed for earthquakes. The distribution of seismic and aseismic slip can be partitioned following a spatial distribution of mainshocks along the fault plane. Slope patches, such as knickpoints, induced by fault slip are then migrated at a constant rate upstream a river crossing the fault. A major result is that this new model predicts a uniform distribution of earthquake magnitude rupturing a river that crosses a fault trace and in turn a negative exponential distribution of knickpoint height for a fully coupled fault, i.e. with only co-seismic slip. Increasing aseismic slip at shallow depths, and decreasing shallow seismicity, censors the magnitude range of earthquakes cutting the river towards large magnitudes and leads to less frequent but higher-amplitude knickpoints, on average. Inter-knickpoint distance or time between successive knickpoints follows an exponential decay law. Using classical rates for fault slip (15 mm year−1) and knickpoint retreat (0.1 m year−1) leads to high spatial densities of knickpoints. We find that knickpoint detectability, relatively to the resolution of topographic data, decreases with river slope that is equal to the ratio between fault slip rate and knickpoint retreat rate. Vertical detectability is only defined by the precision of the topographic data that sets the lower magnitude leading to a discernible offset. Considering a retreat rate with a dependency on knickpoint height leads to the merging of small knickpoints into larger ones and larger than the maximum offset produced by individual earthquakes. Moreover, considering simple scenarios of fault burial by intermittent sediment cover, driven by climatic changes or linked to earthquake occurrence, leads to knickpoint distributions and river profiles markedly different from the case with no sediment cover. This highlights the potential role of sediments in modulating and potentially altering the expression of tectonic activity in river profiles and surface topography. The correlation between the topographic profiles of successive parallel rivers cutting the fault remains positive for distance along the fault of less than half the maximum earthquake rupture length. This suggests that river topography can be used for paleo-seismological analysis and to assess fault slip partitioning between aseismic and seismic slip. Lastly, the developed model can be coupled to more sophisticated landscape evolution models to investigate the role of earthquakes on landscape dynamics.

Tsunamigenic Splay Faults Imply a Long‐Term Asperity in Southern Prince William Sound, Alaska

AbstractCoseismic slip partitioning and uplift over multiple earthquake cycles is critical to understanding upper‐plate fault development. Bathymetric and seismic reflection data from the 1964 Mw9.2 Great Alaska earthquake rupture area reveal sea floor scarps along the tsunamigenic Patton Bay/Cape Cleare/Middleton Island fault system. The faults splay from a megathrust where duplexing and underplating produced rapid exhumation. Trenchward of the duplex region, the faults produce a complex deformation pattern from oblique, south‐directed shortening at the Yakutat‐Pacific plate boundary. Spatial and temporal fault patterns suggest that Holocene megathrust earthquakes had similar relative motions and thus similar tsunami sources as in 1964. Tsunamis during future earthquakes will likely produce similar run‐up patterns and travel times. Splay fault surface expressions thus relate to plate boundary conditions, indicating millennial‐scale persistence of this asperity. We suggest structure of the subducted slab directly influences splay fault and tsunami generation landward of the frontal subduction zone prism.

Kinematics of the 2012 Ahar–Varzaghan complex earthquake doublet (Mw6.5 and Mw6.3)

On 2012 August 11, an earthquake doublet (Mw6.5 and Mw6.3), separated in time by 11 min, occur in the northwest of Iran. The hypocentres of these earthquakes are close (∼6 km) and located near the cities of Ahar and Varzaghan. The rupture process of both main shocks is retrieved by inverting the near-field strong motions data and using the elliptical subfault approximation method. Our calculations show that the two earthquakes are occurring on two distinct fault planes: the first main shock (M1) has nucleated at a depth of ∼8.5 km, and is located ∼4 km east of the eastern termination of the E-W trending surface rupture. The slip reaches the ground surface west of the hypocentre on an E-W striking fault (N88°E) that dips almost vertically (80°S). This earthquake exhibits a right-lateral strike-slip mechanism. The entire slip is imaged on a single patch that ruptures with an average speed of 2.4 km s−1. The rupture duration is ∼5.6 s and the earthquake releases a seismic moment of ∼8.41E + 18 N·m. The slip reaches the surface with a right-lateral dislocation value of ∼1 m, which is consistent with the observed surface rupture. About 11 min later, the second main shock (M2) nucleates ∼5 km to the west and 4 km to the north with respect to the hypocentre of the M1, and at a depth of ∼16.5 km. The M2 rupture evolves toward shallower depths and to the west on an ENE-WSW oriented fault plane (strike ∼256°) with a dip of ∼60° northward. The slip is essentially distributed on two distinct patches with strike-slip and reverse mechanisms, respectively. The first patch has a pure right-lateral strike-slip mechanism, and ruptures at a relatively fast speed of over 2.8 km s−1, and last for about 2.6 s until it reaches the second patch. The latter has a reverse mechanism (rake∼112°) and extends the rupture toward shallow depths, and to the west at a speed of ∼2.5 km s−1, and its rupture lasts for ∼2.5 s. The top of the slip distribution of M2 stops at a depth of ∼8 km. We observe that aftershocks surround the M1 and most of the M2 slip models. They are not distributed in the region of high slip (∼3.1 m) of M1. We show that the rupture of M2 is controlled by the static Coulomb stress changes caused by M1, with the maximum slip of M2 located in the positive Coulomb stress caused by M1. The M2 rupture stops where it reaches the area of high negative Coulomb stress change (over −10 bars). The cumulative Coulomb stress fields of both main shocks show a transfer of positive static Coulomb stress change of >0.1 bars on the eastern segment of the North Tabriz Fault. This segment did not rupture since the 1721 M∼7.6–7.7 event that has destroyed the city of Tabriz, and that currently hosts 2 million people. The occurrence of this earthquake doublet with different mechanisms reveals the slip partitioning of the oblique convergence regime of NW Iran on the Ahar–Varzaghan complex fault system.

Topographic Loads Modified by Fluvial Incision Impact Fault Activity in the Longmenshan Thrust Belt, Eastern Margin of the Tibetan Plateau

AbstractWhether external or internal forces of the Earth control the behaviors of upper‐crustal faults in a fold‐and‐thrust belt has been debated for decades. The Longmenshan thrust belt (LTB) along the eastern margin of the Tibetan Plateau may provide insights into such a debate, as the central segment of the LTB has relatively uniform shortening strains yet various fluvial incision capability along the strike. This tectonic setting enables a better assessment of the effects of external forces on fault activity. We analyzed the variations of topography, fluvial incision intensity, coseismic slips, and coseismic landslides along the central LTB. The Longmen subsegment in the northern half has higher elevation and lower fluvial incision intensity than the Hongkou subsegment in the south. We calculated the topographic stresses on the faults ruptured during the 2008 Wenchuan earthquake and found topographic introduced normal stress increase may explain the coseismic slip partitioning onto two subfaults along the Longmen subsegment. Our results indicate that fluvial incision may have produced the along‐strike variations of the topography, which may further produce the different rupture behavior. In addition, the mean hillslope angle along the central LTB appeared to be at the critical condition prior to the 2008 Wenchuan earthquake and reduced significantly by the earthquake, indicating that geomorphic indices may vary with different stages in an earthquake cycle. Therefore, scrutinizing the mean hillslope angle and other geomorphic indices may help identify potential seismic hazards in an active fault system.

Corrugated megathrust revealed offshore from Costa Rica

Exhumed faults are rough, often exhibiting topographic corrugations oriented in the direction of slip; such features are fundamental to mechanical processes that drive earthquakes and fault evolution. However, our understanding of corrugation genesis remains limited due to a lack of in situ observations at depth, especially at subducting plate boundaries. Here we present three-dimensional seismic reflection data of the Costa Rica subduction zone that image a shallow megathrust fault characterized by corrugated, and chaotic and weakly corrugated topographies. The corrugated surfaces extend from near the trench to several kilometres down-dip, exhibit high reflection amplitudes (consistent with high fluid content/pressure) and trend 11–18° oblique to subduction, suggesting 15 to 25 mm yr−1 of trench-parallel slip partitioning across the plate boundary. The corrugations form along portions of the megathrust with greater cumulative slip and may act as fluid conduits. In contrast, weakly corrugated areas occur adjacent to active plate bending faults where the megathrust has migrated up-section, forming a nascent fault surface. The variations in megathrust roughness imaged here suggest that abandonment and then reestablishment of the megathrust up-section transiently increases fault roughness. Analogous corrugations may exist along significant portions of subduction megathrusts globally. Mature parts of the shallow megathrust beneath Costa Rica are characterized by striking corrugations that may channel fluids, according to seismic images. Nascent sections of the subduction zone plate boundary appear only weakly corrugated.