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Showing papers on "Fault (geology) published in 2005"


Journal ArticleDOI
TL;DR: In this article, the authors considered the East African rift system (EARS) as an intra-continental ridge system, comprising an axial rift, and the structural organization in three branches, the overall morphology, lithospheric cross-sections, the morphotectonics, the main tectonic features, and volcanism in its relationships with the tectonics.

659 citations


Journal ArticleDOI
TL;DR: In this article, the authors derived a detailed horizontal velocity field for the southeast borderland of the Tibetan Plateau using GPS data collected from the Crustal Motion Observation Network of China between 1998 and 2004.
Abstract: [1] We derive a detailed horizontal velocity field for the southeast borderland of the Tibetan Plateau using GPS data collected from the Crustal Motion Observation Network of China between 1998 and 2004. Our results reveal a complex deformation field that indicates that the crust is fragmented into tectonic blocks of various sizes, separated by strike-slip and transtensional faults. Most notably, the regional deformation includes 10–11 mm/yr left slip across the Xianshuihe fault, � 7 mm/yr left slip across the Anninghe-Zemuhe-Xiaojiang fault zone, � 2 mm/yr right slip across a shear zone trending northwest near the southern segment of the Lancang River fault, and � 3 mm/yr left slip across the Lijiang fault. Deformation along the southern segment of the Red River fault appears not significant at present time. The region south and west of the XianshuiheXiaojiang fault system, whose eastward motion is resisted by the stable south China block to the east, turns from eastward to southward motion with respect to south China, resulting in clockwise rotation of its internal subblocks. Active deformation is detected across two previously unknown deformation zones: one is located � 150 km northwest of and in parallel with the Longmenshan fault with 4–6 mm/yr right-slip and another is continued south-southwestward from the Xiaojiang fault abutting the Red River fault with � 7 mm/yr left slip. While both of these zones are seismically active, the exact locations of faults responsible for such deformation are yet to be mapped by field geology. Comparing our GPS results with predictions of various models proposed for Tibetan Plateau deformation, we find that the relatively small sizes of the inferred microblocks and their rotation pattern lend support to a model with a mechanically weak lower crust experiencing distributed deformation underlying a stronger, highly fragmented upper crust.

584 citations


Journal ArticleDOI
TL;DR: In this article, the authors presented equations for the estimation of horizontal strong ground motions caused by shallow crustal earthquakes with magnitudes Mw ≥ 5 and distance to the surface projection of the fault less than 100km.
Abstract: This article presents equations for the estimation of horizontal strong ground motions caused by shallow crustal earthquakes with magnitudes Mw ≥ 5 and distance to the surface projection of the fault less than 100km. These equations were derived by weighted regression analysis, used to remove observed magnitude-dependent variance, on a set of 595 strong-motion records recorded in Europe and the Middle East. Coefficients are included to model the effect of local site effects and faulting mechanism on the observed ground motions. The equations include coefficients to model the observed magnitude-dependent decay rate. The main findings of this study are that: short-period ground motions from small and moderate magnitude earthquakes decay faster than the commonly assumed 1/r, the average effect of differing faulting mechanisms is not large and corresponds to factors between 0.8 (normal and odd) and 1.3 (thrust) with respect to strike-slip motions and that the average long-period amplification caused by soft soil deposits is about 2.6 over those on rock sites. Disappointingly the standard deviations associated with the derived equations are not significantly lower than those found in previous studies.

533 citations


Journal ArticleDOI
TL;DR: The Andaman Sea is an active backarc basin lying above and behind the Sunda subduction zone where convergence between the overriding Southeast Asian plate and the subducting Australian plate is highly oblique.

530 citations


Journal ArticleDOI
07 Apr 2005-Nature
TL;DR: In this article, the authors show that plate unbending with distance from the top of an axial high reproduces the observed dip directions and offsets of faults formed at fast-spreading centres.
Abstract: Abyssal-hill-bounding faults that pervade the oceanic crust are the most common tectonic feature on the surface of the Earth. The recognition that these faults form at plate spreading centres came with the plate tectonic revolution. Recent observations reveal a large range of fault sizes and orientations; numerical models of plate separation, dyke intrusion and faulting require at least two distinct mechanisms of fault formation at ridges to explain these observations. Plate unbending with distance from the top of an axial high reproduces the observed dip directions and offsets of faults formed at fast-spreading centres. Conversely, plate stretching, with differing amounts of constant-rate magmatic dyke intrusion, can explain the great variety of fault offset seen at slow-spreading ridges. Very-large-offset normal faults only form when about half the plate separation at a ridge is accommodated by dyke intrusion.

335 citations


Journal ArticleDOI
01 Sep 2005-Nature
TL;DR: Structural observations of the Punchbowl fault are quantified, and it is shown that the energy required to create the fracture surface area in the fault is about 300 times greater than seismological estimates would predict for a single large earthquake.
Abstract: Fracture energy is a form of latent heat required to create an earthquake rupture surface and is related to parameters governing rupture propagation and processes of slip weakening. Fracture energy has been estimated from seismological and experimental rock deformation data, yet its magnitude, mechanisms of rupture surface formation and processes leading to slip weakening are not well defined. Here we quantify structural observations of the Punchbowl fault, a large-displacement exhumed fault in the San Andreas fault system, and show that the energy required to create the fracture surface area in the fault is about 300 times greater than seismological estimates would predict for a single large earthquake. If fracture energy is attributed entirely to the production of fracture surfaces, then all of the fracture surface area in the Punchbowl fault could have been produced by earthquake displacements totalling <1 km. But this would only account for a small fraction of the total energy budget, and therefore additional processes probably contributed to slip weakening during earthquake rupture.

323 citations


Journal ArticleDOI
04 Aug 2005-Nature
TL;DR: This work shows that the discrepancy between laboratory and seismological results can be resolved if thermal pressurization of the pore fluid is the slip-weakening mechanism, and indicates that a planar fault segment with an impermeable and narrow slip zone will become very unstable during slip and is likely to be the site of a seismic asperity.
Abstract: An earthquake occurs when a fault weakens during the early portion of its slip at a faster rate than the release of tectonic stress driving the fault motion. This slip weakening occurs over a critical distance, D(c). Understanding the controls on D(c) in nature is severely limited, however, because the physical mechanism of weakening is unconstrained. Conventional friction experiments, typically conducted at slow slip rates and small displacements, have obtained D(c) values that are orders of magnitude lower than values estimated from modelling seismological data for natural earthquakes. Here we present data on fluid transport properties of slip zone rocks and on the slip zone width in the centre of the Median Tectonic Line fault zone, Japan. We show that the discrepancy between laboratory and seismological results can be resolved if thermal pressurization of the pore fluid is the slip-weakening mechanism. Our analysis indicates that a planar fault segment with an impermeable and narrow slip zone will become very unstable during slip and is likely to be the site of a seismic asperity.

309 citations


Journal ArticleDOI
TL;DR: In this paper, the location of the hypocenter within the fault and the correlation between hypocenter location and regions of large slip was analyzed. But the authors did not consider the effect of the number of faults in the fault.
Abstract: We use a database of more than 80 finite-source rupture models for more than 50 earthquakes ( M w 4.1–8.1) with different faulting styles occurring in both tectonic and subduction environments to analyze the location of the hypocenter within the fault and to consider the correlation between hypocenter location and regions of large slip. Rupture in strike-slip and crustal dip-slip earthquakes tends to nucleate in the deeper sections of the fault; subduction earthquakes do not show this tendency. Ratios of the hypocentral slip to either the average or the maximum slip show that rupture can nucleate at locations with any level of relative displacement. Rupture nucleates in regions of very large slip ( D ≥ 2/3 D max ) in only 16% of the events, in regions of large slip (1/3 D max D D max ) in 35% of the events, and in regions of low slip ( D ≤ 1/3 D max ) in 48% of the events. These percentages significantly exceed the percentages of fault area with very large (∼7%) and large (∼28%) slip. Ruptures that nucleate in regions of low slip, however, tend to nucleate close to regions of large slip and encounter a zone of very large slip within half the total rupture length. Applying several statistical tests we conclude that hypocenters are not randomly located on a fault but are located either within or close to regions of large slip.

260 citations


Journal ArticleDOI
TL;DR: The MARMARASCARPS cruise using an unmanned submersible (ROV) provides direct observations to study the fine-scale morphology and geology of those scarps, their distribution, and geometry as discussed by the authors.
Abstract: Earthquake scarps associated with recent historical events have been found on the floor of the Sea of Marmara, along the North Anatolian Fault (NAF). The MARMARASCARPS cruise using an unmanned submersible (ROV) provides direct observations to study the fine-scale morphology and geology of those scarps, their distribution, and geometry. The observations are consistent with the diversity of fault mechanisms and the fault segmentation within the north Marmara extensional step-over, between the strike-slip Ganos and Izmit faults. Smaller strike-slip segments and pull-apart basins alternate within the main step-over, commonly combining strike-slip and extension. Rapid sedimentation rates of 1?3 mm/yr appear to compete with normal faulting components of up to 6 mm/yr at the pull-apart margins. In spite of the fast sedimentation rates the submarine scarps are preserved and accumulate relief. Sets of youthful earthquake scarps extend offshore from the Ganos and Izmit faults on land into the Sea of Marmara. Our observations suggest that they correspond to the submarine ruptures of the 1999 Izmit (Mw 7.4) and the 1912 Ganos (Ms 7.4) earthquakes. While the 1999 rupture ends at the immediate eastern entrance of the extensional Cinarcik Basin, the 1912 rupture appears to have crossed the Ganos restraining bend into the Sea of Marmara floor for 60 km with a right-lateral slip of 5 m, ending in the Central Basin step-over. From the Gulf of Saros to Marmara the total 1912 rupture length is probably about 140 km, not 50 km as previously thought. The direct observations of submarine scarps in Marmara are critical to defining barriers that have arrested past earthquakes as well as defining a possible segmentation of the contemporary state of loading. Incorporating the submarine scarp evidence modifies substantially our understanding of the current state of loading along the NAF next to Istanbul. Coulomb stress modeling shows a zone of maximum loading with at least 4?5 m of slip deficit encompassing the strike-slip segment 70 km long between the Cinarcik and Central Basins. That segment alone would be capable of generating a large-magnitude earthquake (Mw 7.2). Other segments in Marmara appear less loaded.

256 citations


Journal ArticleDOI
TL;DR: In this article, the authors presented a neotectonic map of Taiwan based on structural and geomorphic expression of active faults and folds both in the field and on shaded relief maps prepared from a 40m resolution digital elevation model, augmented by geodetic and seismologic data.
Abstract: The disastrous effects of the 1999 Chi-Chi earthquake in Taiwan demonstrated an urgent need for better knowledge of the island's potential earthquake sources. Toward this end, we have prepared a neotectonic map of Taiwan. The map and related cross sections are based upon structural and geomorphic expression of active faults and folds both in the field and on shaded relief maps prepared from a 40-m resolution digital elevation model, augmented by geodetic and seismologic data. The active tandem suturing and tandem disengagement of a volcanic arc and a continental sliver to and from the Eurasian continental margin have created two neotectonic belts in Taiwan. In the southern part of the orogen both belts are in the final stage of consuming oceanic crust. Collision and suturing occur in the middle part of both belts, and postcollisional collapse and extension dominate the island's northern and northeastern flanks. Both belts consist of several distinct neotectonic domains. Seven domains (Kaoping, Chiayi, Taichung, Miaoli, Hsinchu, Ilan, and Taipei) constitute the western belt, and four domains (Lutao-Lanyu, Taitung, Hualien, and Ryukyu) make up the eastern belt. Each domain is defined by a distinct suite of active structures. For example, the Chelungpu fault (source of the 1999 earthquake) and its western neighbor, the Changhua fault, are the principal components of the Taichung Domain, whereas both its neighboring domains, the Chiayi and Miaoli Domains, are dominated by major blind faults. In most of the domains the size of the principal active fault is large enough to produce future earthquakes with magnitudes in the mid-7 values.

241 citations


Journal ArticleDOI
TL;DR: In this paper, a structural and stratigraphic framework for the southernmost Red Sea passive margin using new and existing 40 Ar/39 Ar age data along 6 transects is presented.
Abstract: The process of strain localization as rifting proceeds to continental breakup is readily observed along the Oligocene-Recent southern Red Sea rift, yet much of the Red Sea margin in Ethiopia remains unmapped. Rifting initiated above or near a mantle plume, which is marked by the Eo-Oligocene Ethiopia-Yemen fl ood basalt province. Objectives of this fi eld, remote sensing, and geochronology study are to establish a structural and stratigraphic framework for the southernmost Red Sea passive margin using new and existing 40 Ar/ 39 Ar age data along 6 transects. We present new sketch geological maps and cross sections to document the timing of extension in relation to magmatism and its variation along strike. These new data are integrated with plate kinematic, geological, and geophysical data to present a model for evolution of the southern Red Sea margin. Faults commonly marked by eruptive centers initiated between 29 and 26 Ma, coincident with rifting in the Gulf of Aden. The Red Sea rift terminated at 10°N until linkage of the Main Ethiopian rift and southern Red Sea occurred at ca. 11 Ma. Rifting progressed in three distinct stages; each new phase saw a marked change in the style of volcanism and a narrowing of the locus of extension. Stage 1 rhyolites were emplaced from 29 to 26 Ma in basins bounded by a steep border fault system. Between 25 and 20 Ma, strain localized to narrow zones of basaltic fi ssural eruptions and minor faulting. Stage 2 faults and eruptive centers are located ~50 km to the east of the border faults, and they comprise fl ows spanning at least 16‐7 Ma. After ca. 7 Ma, the locus of strain again migrated eastward (Stage 3). Strain in Stage 3 was largely accommodated by dike injection. Plate reconstructions predict high stretching factors (β ~3) in the southern Red Sea, suggesting that Stages 2 and 3 mark the onset of formation of crust transitional between oceanic and continental.

Journal ArticleDOI
TL;DR: In this paper, a detailed field and petrographic analysis of fault textures in two dissected rhyolitic conduits in Iceland preserve evidence for shallow seismogenic faulting within rising magma during the emplacement of highly viscous lava flows, which may shed light on the origin of long-period and hybrid volcanic earthquakes at active volcanoes.
Abstract: It is proposed that fault textures in two dissected rhyolitic conduits in Iceland preserve evidence for shallow seismogenic faulting within rising magma during the emplacement of highly viscous lava flows. Detailed field and petrographic analysis of such textures may shed light on the origin of long-period and hybrid volcanic earthquakes at active volcanoes. There is evidence at each conduit investigated for multiple seismogenic cycles, each of which involved four distinct evolutionary phases. In phase 1, shear fracture of unrelaxed magma was triggered by shear stress accumulation during viscous flow, forming the angular fracture networks that initiated faulting cycles. Transient pressure gradients were generated as the fractures opened, which led to fluidisation and clastic deposition of fine-grained particles that were derived from the fracture walls by abrasion. Fracture networks then progressively coalesced and rotated during subsequent slip (phase 2), developing into cataclasite zones with evidence for multiple localised slip events, fluidisation and grain size reduction. Phase 2 textures closely resemble those formed on seismogenic tectonic faults characterised by friction-controlled stick-slip behaviour. Increasing cohesion of cataclasites then led to aseismic, distributed ductile deformation (phase 3) and generated deformed cataclasite zones, which are enriched in metallic oxide microlites and resemble glassy pseudotachylite. Continued annealing and deformation eventually erased all structures in the cataclasite and formed microlite-rich flow bands in obsidian (phase 4). Overall, the mixed brittle-ductile textures formed in the magma appear similar to those formed in lower crustal rocks close to the brittle-ductile transition, with the rheological response mediated by strain-rate variations and frictional heating. Fault processes in highly viscous magma are compared with those elsewhere in the crust, and this comparison is used to appraise existing models of volcano seismic activity. Based on the textures observed, it is suggested that patterns of long-period and hybrid earthquakes at silicic lava domes reflect friction-controlled stick-slip movement and eventual healing of fault zones in magma, which are an accelerated and smaller-scale analogue of tectonic faults.

Journal ArticleDOI
TL;DR: In this article, the authors studied bending-related faults in the incoming oceanic plate along segments of Middle America and Chile subduction zones and its relationship to intermediate-depth intraslab seismicity and slab geometry.
Abstract: [1] We have studied faulting associated with bending of the incoming oceanic plate along segments of Middle America and Chile subduction zones and its relationship to intermediate-depth intraslab seismicity and slab geometry. Multibeam bathymetry shows that bending-related faulting forms patterns made of sets of faults with orientations ranging from parallel to almost perpendicular to the trench axis. These fault patterns may change along a single subduction zone within along-strike distances of several hundred kilometers or less. Where available, near-trench intraplate earthquakes show normal-fault focal mechanisms consistent with mapped bending-related normal faults. The strike of bending-related faults in the incoming oceanic plate is remarkably similar to the strike of the nodal planes of intermediate-depth earthquakes for each segment of the study areas. This similarity in strike is observed even for faults oriented very oblique to the trench and slab strikes. Thus, in the studied subduction zones, results strongly support that many intraslab earthquakes do not occur along the planes of maximum shear within the slab and that much intermediate-depth seismicity occurs by reactivation of faults formed by plate bending near the trench. Furthermore, a qualitative relationship between trench faulting and intraslab seismicity is indicated by segments of the incoming plate with pervasive bend-faulting that correspond to segments of the slabs with higher intermediate-depth seismicity.

Journal ArticleDOI
TL;DR: In this article, the authors examined the spatial arrangement of fractures in the damage zones of a segment of the large-scale Moab Fault (45 km in length), SE Utah, and found that the structural pattern across the fault zone is strongly asymmetric.

Journal ArticleDOI
TL;DR: In this article, geophysical and geological data from the Southern Alps were used to explore the relationship between plate motions and crustal structure on the geomorphology, exhumation state, and deformation style of rocks uplifted along amajor oblique-slip fault.
Abstract: We use geophysical and geological data from the Southern Alps to explore the relationship between plate motions and crustal structure on the geomorphology, exhumation state, and deformation style of rocks uplifted along amajor oblique-slip fault. A ∼50-km-long segment of the Southern Alps has a higher uplift rate, more relief, deeper exhumation, and a narrower width than surrounding regions. There, the delaminated, east-tilted crust of the Pacific Plate yields the youngest, late Cenozoic thermochronometric ages. Contours for fission-track, Ar/Ar and K-Ar ages on several different minerals define an asymmetrically nested pattern of ages that increase away from the western side of the central Southern Alps. Eleven new 4 0 Ar/ 3 9 Ar samples of hornblende from the hanging wall of the Alpine fault indicate that lower crustal rocks exhumed from >500 °C in the late Cenozoic are confined to a 20-km-long culmination at the southern end of the central Southern Alps. Ages as low as 3-5 Ma imply time-integrated vertical exhumation rates as high as ∼6-9 mm/yr. This is the only part of this 5-8 Ma range that may have achieved exhumational steady state. Remnant plugs of the original crustal hanging wall ramp are apparently preserved outside the central Alps, implying <70 km of fault convergence there. 4 0 Ar/ 3 9 Ar age trends for hornblende near the Alpine fault suggest that horizontal surfaces in the lower crust in the Pacific Plate have been overturned by reverse-slip ductile shearing across a zone of distributed deformation that extends ∼2 km beyond the ∼1-km-thick, basal mylonite zone. At the broadest, orogen scale, higher uplift rates throughout the central Southern Alps may be related to a rheologically controlled increase in the convergent velocity of points to the east of the Alpine fault, associated with a strengthening of the Pacific crust. At a more local scale, maximum rates of uplift are inferred to occur near Franz-Josef and Fox Glaciers because the Alpine fault steepens at depth. Structural data suggests that its footwall ramp is curved, and that the fault's dip steepens by 15-20° relative to its attitude farther to the south. This 10-20-km-long restraining bend may enhance local rates of rock uplift near Franz Josef and Fox Glaciers. Contemporary normal stress and shear resistance may also be increased on this part of the Alpine fault, helping to explain the central region's quiet historic seismicity and apparently strongly locked nature.

Journal ArticleDOI
TL;DR: In this article, surface mapping of syntectonic sediments, interpretation of industry reflection-seismic profiles, gravity data, and isotopic age dating are employed to reconstruct the tectonic evolution of the southern Altiplano (∼20-22°S) between the cordilleras defining its margins.
Abstract: [1] Employing surface mapping of syntectonic sediments, interpretation of industry reflection-seismic profiles, gravity data, and isotopic age dating, we reconstruct the tectonic evolution of the southern Altiplano (∼20–22°S) between the cordilleras defining its margins. The southern Altiplano crust was deformed between the late Oligocene and the late Miocene with two main shortening stages in the Oligocene (33–27 Ma) and middle/late Miocene (19–8 Ma) that succeeded Eocene onset of shortening at the protoplateau margins. Shortening rates in the southern Altiplano ranged between 1 and 4.7 mm/yr with maximum rates in the late Miocene. Summing rates for the southern Altiplano and the Eastern Cordillera, we observe an increase from Eocene times to the late Oligocene to some 8 mm/yr, followed by fluctuation around this value during the Miocene prior to shutoff of deformation at 7–8 Ma and transfer of active shortening to the sub-Andean fold and thrust belt. Shortening inverted early Tertiary graben and half graben systems and was partitioned in three fault systems in the western, central, and eastern Altiplano, respectively. The east vergent fault systems of the western and central Altiplano were synchronously active with the west vergent Altiplano west flank fault system. From these data and from section balancing, we infer a kinematically linked western Altiplano thrust belt that accumulated a minimum of 65 km shortening. Evolution of this belt contrasts with the Eastern Cordillera, which reached peak shortening rates (8 mm/yr) in between the above two stages. Hence local shortening rates fluctuated across the plateau superimposed on a general trend of increasing bulk rate with no trend of lateral propagation. This observation is repeated at the shorter length and time scales of individual growth structures that show evidence for periods of enhanced local rates at a timescale of 1–3 Myr. We interpret this irregular pattern of deformation to reflect a plateau-style of shortening related to a self-organized state of a weak crust in the central South American back arc with a fault network that fluctuated around the critical state of mechanical failure. Tuning of this state may have occurred by changes in plate kinematics, during the Paleogene, initially reactivating crustal weak zones and by thermal weakening of the crust with active magmatism mainly in the Neogene stage.

Journal ArticleDOI
TL;DR: In this article, the authors used multiple seismic attributes and neural networks to enhance fluid migration pathways, including subtle features that are not detectable using single attributes only, and provided information about which faults and fault segments are sealing or leaking.
Abstract: A new and efficient method for fault seal analysis using seismic data is presented. It uses multiple seismic attributes and neural networks to enhance fluid migration pathways, including subtle features that are not detectable using single attributes only. The method may be used as a first estimate of fault seal or to calibrate results from other techniques. The results provide information about which faults and fault segments are sealing or leaking. Fluid flow along individual faults appears to be focused along zones of weakness, and fault seal research should thus be focused on ¢nding such weak locations within fault zones, a task that is best done using three-dimensional (3D) seismic data. Under certain conditions, it is suggested that fluids migrate along fault planes by a diapiric fluid flow mechanism. The results assist in calibrating the bulk hydraulic properties of faults and rock formations and can be used in basin modelling.

Journal ArticleDOI
TL;DR: In this article, the authors examined the Cape Egmont Fault in offshore New Zealand, using good quality seismic-reflection data tied to wells and seabed bathymetry.

Journal ArticleDOI
05 May 2005-Nature
TL;DR: It is shown that lithospheric rebound caused by regression of Lake Bonneville and deglaciation of adjacent mountain ranges provides a feasible mechanism for the high Holocene rates of faulting in the Wasatch region, implying that climate-controlled changes in loads applied to Earth's surface may exert a fundamental control on the slip history of individual normal faults.
Abstract: Geologic and palaeoseismological data document a marked increase in the slip rates of the Wasatch fault and three adjacent normal faults in the Basin and Range Province during the Late Pleistocene/Early Holocene epochs. The cause of this synchronous acceleration of fault slip and the subsequent clustering of earthquakes during the Holocene has remained enigmatic, although it has been suggested that the coincidence between the acceleration of slip and the shrinkage of Lake Bonneville after the Last Glacial Maximum may indicate a causal relationship. Here we use finite-element models of a discrete normal fault within a rheologically layered lithosphere to evaluate the relative importance of two competing processes that affect fault slip: postglacial unloading (the removal of mass), which decreases the slip rate, and lithospheric rebound, which promotes faster slip. We show that lithospheric rebound caused by regression of Lake Bonneville and deglaciation of adjacent mountain ranges provides a feasible mechanism for the high Holocene rates of faulting in the Wasatch region. Our analysis implies that climate-controlled changes in loads applied to Earth's surface may exert a fundamental control on the slip history of individual normal faults.

Journal ArticleDOI
TL;DR: The Walker Lane is composed of discontinuous sets of right-slip faults that are located to the east and strike approximately parallel to the San Andreas fault system as mentioned in this paper, which accounts for most relative Pacific-North American transform plate motion.
Abstract: [1] Deformation across the San Andreas and Walker Lane fault systems accounts for most relative Pacific–North American transform plate motion. The Walker Lane is composed of discontinuous sets of right-slip faults that are located to the east and strike approximately parallel to the San Andreas fault system. Mapping of active faults in the central Walker Lane shows that right-lateral shear is locally accommodated by rotation of crustal blocks bounded by steep-dipping east striking left-slip faults. The left slip and clockwise rotation of crustal blocks bounded by the east striking faults has produced major basins in the area, including Rattlesnake and Garfield flats; Teels, Columbus and Rhodes salt marshes; and Queen Valley. The Benton Springs and Petrified Springs faults are the major northwest striking structures currently accommodating transform motion in the central Walker Lane. Right-lateral offsets of late Pleistocene surfaces along the two faults point to slip rates of at least 1 mm/yr. The northern limit of northwest trending strike-slip faults in the central Walker Lane is abrupt and reflects transfer of strike-slip to dip-slip deformation in the western Basin and Range and transformation of right slip into rotation of crustal blocks to the north. The transfer of strike slip in the central Walker Lane to dip slip in the western Basin and Range correlates to a northward broadening of the modern strain field suggested by geodesy and appears to be a long-lived feature of the deformation field. The complexity of faulting and apparent rotation of crustal blocks within the Walker Lane is consistent with the concept of a partially detached and elastic-brittle crust that is being transported on a continuously deforming layer below. The regional pattern of faulting within the Walker Lane is more complex than observed along the San Andreas fault system to the west. The difference is attributed to the relatively less cumulative slip that has occurred across the Walker Lane and that oblique components of displacement are of opposite sense along the Walker Lane (extension) and San Andreas (contraction), respectively. Despite the gross differences in fault pattern, the Walker Lane and San Andreas also share similarities in deformation style, including clockwise rotations of crustal blocks leading to development of structural basins and the partitioning of oblique components of slip onto subparallel strike-slip and dip-slip faults.

Journal ArticleDOI
TL;DR: In this paper, the authors present a new interpretation for the evolution of the Kazerun Fault Zone based on field evidence and a review of various published and unpublished data, arguing that at the surface, the fault zone is delineated by four north-south trending segments and that marked differences occur in the distribution of deformation on either side of the fault segments in the Late Tertiary.
Abstract: [1] The Kazerun Fault is a N-S trending fault zone, which obliquely truncates the NW-SE trending Zagros Fold-Thrust Belt. This active fault zone is an ancient structural lineament which has controlled the structure, sedimentation, and subsidence of the central part of the Zagros since the Early Cambrian and has had an influence on the formation of the hydrocarbon system of the belt. The Kazerun Fault limited the distribution of the Cambrian Hormuz salt (the major decollement horizon that separates the Precambrian basement from the thick sedimentary cover) to the west. Its later reactivation with other N-S trending fault zones (e.g., the Izeh Fault Zone) during the Cretaceous resulted in major sedimentary thickness and facies variations along the belt. This study presents a new interpretation for the evolution of the Kazerun Fault Zone based on field evidence and a review of various published and unpublished data. It is argued that at the surface, the fault zone is delineated by four north-south trending segments and that marked differences occur in the distribution of deformation on either side of the fault segments in the Late Tertiary. During this time these segments acted as transfer faults (lateral ramps) linking different segments of the Zagros deformation fronts.

Journal ArticleDOI
TL;DR: The Ramgarh thrust is one of the major fault systems of the Himalayan thrust belt in Nepal and northern India and can be traced along strike the entire length of a Himalaya in Nepal as discussed by the authors.
Abstract: [1] The Ramgarh thrust is one of the major fault systems of the Himalayan thrust belt in Nepal and northern India. The Ramgarh thrust sheet is ∼0.2–2.0 km thick and can be traced along strike the entire length of the Himalaya in Nepal. The fault generally places the oldest Paleoproterozoic rocks in the Lesser Himalayan series upon younger Lesser Himalayan rocks or lower Miocene foreland basin deposits. Regional balanced cross sections suggest that the Ramgarh thrust had at least ∼120 km of initial south vergent displacement. Subsequently, the frontal part of the thrust experienced further slip as the roof thrust for a large duplex in underlying Lesser Himalayan rocks. Ramgarh hanging wall strata are greenschist-grade phyllite, quartzite, and augen gneiss, all of which locally exhibit phyllonitic and mylonitic fabrics that indicate a top-to-the-south sense of shear. Structural fabrics in the Ramgarh thrust sheet are generally parallel to the fabrics in rocks above and below the thrust sheet. Regional and local mapping of the Ramgarh thrust in Nepal demonstrates that the fault always places a hanging wall flat upon a footwall flat, except where local lateral ramps complicate its geometry. Similarly, the structurally overlying Main Central thrust always places a hanging wall flat in Greater Himalayan series rocks upon the regionally flat Ramgarh thrust sheet. These geometric relationships preclude kinematic and thermal models that elevate Greater Himalayan and lower Lesser Himalayan rocks along high-angle thrust ramps in the vicinity of the present traces of the Ramgarh and Main Central thrust faults. Instead, the corresponding footwall ramps for these thrusts must be located more than 100 km north of the current trace of the Main Central thrust. The present steep dips of the Ramgarh and Main Central thrust sheets can be attributed to tilting during emplacement of structurally lower thrust sheets within a large antiformal duplex that occupies most of the Lesser Himalayan zone. The Ramgarh thrust sheet overlaps a bed length of at least 100 km in lower Miocene foreland basin deposits, indicating that a significant amount of displacement on the thrust must have occurred after ∼15 Ma. Growth of the Lesser Himalayan duplex and additional slip on the frontal part of the Ramgarh thrust occurred from ∼12 to 5 Ma. The presence of a major greenschist-grade metasedimentary thrust sheet composed of Lesser Himalayan rocks directly below the Main Central thrust suggests that the famous “inverted metamorphism” in this region is a result of structural inversion. Similarly, the concept of a broad zone of intense shear strain related exclusively to emplacement of the Main Central thrust sheet is probably invalid in Nepal.

Journal ArticleDOI
TL;DR: In this paper, the authors used centroid moment tensor analysis for one of the larger events suggests that the source depth is very shallow and the focal mechanism is the reverse faulting.
Abstract: Anomalous seismic events were observed after the occurrence of the foreshock (Mw=7.2) and the main shock (Mw=7.5) of the 2004 off the Kii peninsula earthquakes. These anomalous events are characterized by very low-frequency energy of around 10 seconds with almost no higher-frequency energy and are considered the same as the very low-frequency (VLF) earthquakes discovered by Ishihara (2003) in some places along the Nankai trough, southwest Japan. The VLF seismic activity is mainly coincident with the aftershock area of the 2004 off the Kii peninsula earthquakes; however a minor activity was also excited in the southern Kii channel area. The VLF seismograms sometimes include higher-frequency wave trains with amplitudes much smaller than that of regular aftershocks. This indicates that VLF earthquakes have different source properties from the regular earthquakes. The centroid moment tensor analysis for one of the larger events suggests that the source depth is very shallow and the focal mechanism is the reverse faulting. These features suggest that the event occurs on the well-developed reverse fault system in the large accretionary prism near the Nankai trough. The swarm activity of VLF earthquakes might be considered as a chain-like occurrence of slips on the reverse fault system and thus the signature of a dynamic deformation process in the accretionary prism.

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TL;DR: In this article, the authors used deformed marker surfaces to define the three-dimensional deformation field associated with their surface expression and to map displacement and length on ∼40 fault segments.

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TL;DR: The Gole Larghe Fault is an exhumed paleoseismic fault crosscutting the Adamello tonalites (Italian Southern Alps) as mentioned in this paper, which accommodates ∼ 1100 m of dextral strike-slip over a fault thickness of 550 m. Displacement is partitioned into three hierarchically different sets of discrete subparallel cataclastic horizons.

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TL;DR: The boundary element code Poly3D as mentioned in this paper employs a set of planar triangular elements of constant displacement discontinuity to model fault surfaces to better approximate curved three-dimensional surfaces bounded by curved tiplines.
Abstract: The increasing quality of geodetic data (synthetic aperture radar interferometry [insar] dense Global Positioning System [gps] arrays) now available to geophysicists and geologists are not fully exploited in slip-inversion procedures. Most common methods of inversion use rectangular dislocation segments to model fault ruptures and therefore oversimplify fault geometries. These geometric simplifications can lead to inconsistencies when inverting for slip on earthquake faults, and they preclude a more complete understanding of the role of fault geometry in the earthquake process. We have developed a new three-dimensional slip-inversion method based on the analytical solution for an angular dislocation in a linear-elastic, homogeneous, isotropic, half-space. The approach uses the boundary element code Poly3D that employs a set of planar triangular elements of constant displacement discontinuity to model fault surfaces. The use of triangulated surfaces as discontinuities permits one to construct fault models that better approximate curved three-dimensional surfaces bounded by curved tiplines: shapes that commonly are imaged by three-dimensional reflection seismic data and inferred from relocated aftershock data. We demonstrate the method’s ability to model three-dimensional rupture geometries by inverting for slip associated with the 1999 Hector Mine earthquake. The resulting model avoids displacement anomalies associated with the overlapping rectangular dislocations used in previous models, improving the fit to the geodetic data by 32%, and honors the observed surface ruptures, thereby allowing more direct comparisons between geologic and geodetic data on slip distributions. Online Material : Hector Mine input files and file format description.

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TL;DR: In this article, the authors analyzed trapped wave data collected by stations within the San Jacinto fault zone near Anza, California and concluded that the shallow trapping structures are offset to the northeast from the surface trace of each fault branch.
Abstract: SUMMARY We analyse fault zone trapped waves, generated by ∼500 small earthquakes, for high-resolution imaging of the subsurface structure of the Coyote Creek, Clark Valley and Buck Ridge branches of the San Jacinto fault zone near Anza, California. Based on a small number of selected trapped waves within this data set, a previous study concluded on the existence of a lowvelocity waveguide that is continuous to a depth of 15‐20 km. In contrast, our systematic analysis of the larger data set indicates a shallow trapping structure that extends only to a depth of 3‐5 km. This is based on the following lines of evidence. (1) Earthquakes clearly outside these fault branches generate fault zone trapped waves that are recorded by stations within the fault zones. (2) A traveltime analysis of the difference between the direct S arrivals and trapped wave groups shows no systematic increase (moveout) with increasing hypocentral distance or event depth. Estimates based on the observed average moveout values indicate that the propagation distances within the low-velocity fault zone layers are 3‐5 km. (3) Quantitative waveform inversions of trapped wave data indicate similar short propagation distances within the low-velocity fault zone layers. The results are compatible with recent inferences on shallow trapping structures along several other faults and rupture zones. The waveform inversions also indicate that the shallow trapping structures are offset to the northeast from the surface trace of each fault branch. This may result from a preferred propagation direction of large earthquake ruptures on the San Jacinto fault.

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TL;DR: In this article, the geomorphological and structural data permitted to define the active faulting framework of the eastern Southern Alps (NE Italy) and all the active faults detected in the investigated area are thrust segments of the complex thrust system, which has been responsible for the latest building of the Eastern Southalpine chain.
Abstract: SUMMARY New geomorphological and structural data permitted to define the active faulting framework of the eastern Southern Alps (NE Italy). All the active faults detected in the investigated area are thrust segments of the complex thrust system, which has been responsible for the latest building of the Eastern Southalpine chain (ESC). Geomorphological investigations were performed to identify the surficial traces of recent fault activity, generally represented by gentle scarps connecting uplifted palaeolandscapes of Quaternary age with the flat and lower areas of the Venetian and Friulian plains. Surficial and subsurficial data (the latter from reflection seismic profiles) available for the investigated faults indicate that the thrusts have been responsible for the displacement of the entire wedge of Quaternary deposits. In the western sector of the investigated area, the six recognized fault segments represent portions of a 100-km-long thrust system, at the boundary between the Alpine relief and the plain areas. In the eastern sector, active tectonics is the result of parallel thrust segments, located both in the Alpine mountainous area and in the Friulian plain. The 3-D geometry of the active thrust segments has been derived from new structural surficial surveys and the interpretation of reflection seismic profiles for a total length of 1700 km. On this basis, we defined the geometry of 10 seismogenic sources whose dimensions are consistent with the occurrence of earthquakes with M≥ 6. The comparison between the source geometry and the highest intensity data point distribution of large historical earthquakes has permitted to make hypotheses on the association of past seismic events to specific seismogenic sources. This procedure indicated that no large historical events can be attributed to three sources (Montello-Conegliano, Arba-Ragogna, Medea). This may indicate an elapsed time since the last activation of more than eight centuries, based on the completeness of the historical catalogues. The available data define, therefore, sources (and related areas) for which a high level of seismic hazard may be invoked.

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TL;DR: In this article, the authors describe ductile and brittle deformation styles in western Yunnan and NE Myanmar, using field data and Landsat 7 imagery, and show that this complex area located at the northern termination of Sunda Plate (Three Rivers area) was wedged during the Tertiary between the left-lateral Ailao Shan/Chong Shan metamorphic belts to the east and the right-linear Shan scarp/Gaoligong metmorphic belt in west.

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TL;DR: In this article, the San Andreas and Walker-Lane faults are divided into subparallel strike-slips and dip-slip faults, respectively, and the relative right-lateral Pacific-North American transform motion across the western US is largely taken up by the San and Walker Lane fault systems.