Showing papers on "Fault (geology) published in 2009"

Coseismic reverse- and oblique-slip surface faulting generated by the 2008 Mw 7.9 Wenchuan earthquake, China

Abstract: The Mw 7.9 Wenchuan, China, earthquake ruptured two large thrust faults along the Long-menshan thrust belt at the eastern margin of the Tibetan Plateau. This earthquake generated a 240-km-long surface rupture zone along the Beichuan fault and an additional 72-km-long surface rupture zone along the Pengguan fault. Maximum vertical and horizontal offsets of 6.5 m and 4.9 m, respectively, were measured along the Beichuan fault. A maximum vertical offset of 3.5 m was measured along the Pengguan fault. Coseismic surface ruptures, integrated with aftershocks and industry seismic profiles, show that two imbricate structures have ruptured simultaneously, resulting in the largest continental thrust event ever documented. Large oblique thrusting observed during this earthquake indicates that crustal shortening is an important process responsible for the high topography in the region, as everywhere along the edge of Tibetan Plateau.

Slip maxima at fault junctions and rupturing of barriers during the 2008 Wenchuan earthquake

Abstract: The disastrous 12 May 2008 Wenchuan earthquake in China took the local population as well as scientists by surprise Although the Longmen Shan fault zone—which includes the fault segments along which this earthquake nucleated—was well known, geologic and geodetic data indicate relatively low (<3 mm yr−1) deformation rates Here we invert Global Positioning System and Interferometric Synthetic Aperture Radar data to infer fault geometry and slip distribution associated with the earthquake Our analysis shows that the geometry of the fault changes along its length: in the southwest, the fault plane dips moderately to the northwest but becomes nearly vertical in the northeast Associated with this is a change in the motion along the fault from predominantly thrusting to strike-slip Peak slip along the fault occurs at the intersections of fault segments located near the towns of Yingxiu, Beichuan and Nanba, where fatalities and damage were concentrated We suggest that these locations represent barriers that failed in a single event, enabling the rupture to cascade through several fault segments and cause a major moment magnitude (Mw) 79 earthquake Using coseismic slip distribution and geodetic and geological slip rates, we estimate that the failure of barriers and rupture along multiple segments takes place approximately once in 4,000 years The devastating Wenchuan earthquake in 2008 struck along a fault zone that showed low rates of deformation Analysis of GPS and InSAR data suggests that, as structural barriers failed during a single earthquake, the rupture cascaded across multiple fault segments, which may explain the high magnitude of the event

Tectonic evolution of the South Tianshan orogen and adjacent regions, NW China: geochemical and age constraints of granitoid rocks

Abstract: Geochemical and geochronological evidence was obtained from granitoids of the South Tianshan orogen and adjacent regions, which consist of three individual tectonic domains, the Kazakhstan–Yili plate, the Central Tianshan Terrane and the Tarim plate from north to south. The Central Tianshan Terrane is structurally bounded by the Early Paleozoic ‘Nikolaev Line–North Nalati Fault’ and Late Paleozoic ‘Atbashy–Inyl’chek–South Nalati–Qawabulak Fault’ zones against the Kazakhstan–Yili and Tarim plates, respectively. The meta-aluminous to weakly peraluminous granitic rocks, which are exposed along the Kekesu River and the Bikai River across the Central Tianshan Terrane, have a tholeiitic, calc-alkaline or high-potassium calc-alkaline composition (I-type). Geochemical trace element characteristics and the Y versus Rb–Nb or Y versus Nb discrimination diagrams favor a continental arc setting for these granitoid rocks. SHRIMP U–Pb and LA-ICP-MS U–Pb zircon age data indicate that the magmatism started at about 480 Ma, continued from 460 to 330 Ma and ended at about 275 Ma. The earlier magmatism (>470 Ma) is considered to be the result of a simultaneous southward and northward subduction of the Terskey Ocean beneath the northern margin of the Tarim plate and the Kazakhstan–Yili plate, respectively. The later magmatism (460–330 Ma) is related to the northward subduction of the South Tianshan Ocean beneath the southern margin of the Kazakhstan–Yili–Central Tianshan plate. The dataset presented here in conjunction with previously published data support a Late Paleozoic tectonic evolution of the South Tianshan orogen, not a Triassic one, as recently suggested by SHRIMP U–Pb zircon dating for eclogites.

Fault zone fabric and fault weakness

Abstract: Geological and geophysical evidence suggests that some crustal faults are weak compared to laboratory measurements of frictional strength. Explanations for fault weakness include the presence of weak minerals, high fluid pressures within the fault core and dynamic processes such as normal stress reduction, acoustic fluidization or extreme weakening at high slip velocity. Dynamic weakening mechanisms can explain some observations; however, creep and aseismic slip are thought to occur on weak faults, and quasi-static weakening mechanisms are required to initiate frictional slip on mis-oriented faults, at high angles to the tectonic stress field. Moreover, the maintenance of high fluid pressures requires specialized conditions and weak mineral phases are not present in sufficient abundance to satisfy weak fault models, so weak faults remain largely unexplained. Here we provide laboratory evidence for a brittle, frictional weakening mechanism based on common fault zone fabrics. We report on the frictional strength of intact fault rocks sheared in their in situ geometry. Samples with well-developed foliation are extremely weak compared to their powdered equivalents. Micro- and nano-structural studies show that frictional sliding occurs along very fine-grained foliations composed of phyllosilicates (talc and smectite). When the same rocks are powdered, frictional strength is high, consistent with cataclastic processes. Our data show that fault weakness can occur in cases where weak mineral phases constitute only a small percentage of the total fault rock and that low friction results from slip on a network of weak phyllosilicate-rich surfaces that define the rock fabric. The widespread documentation of foliated fault rocks along mature faults in different tectonic settings and from many different protoliths suggests that this mechanism could be a viable explanation for fault weakening in the brittle crust.

The 2009 L'Aquila (central Italy) MW6.3 earthquake: Main shock and aftershocks

Abstract: [1] A M w 6.3 earthquake struck on April 6, 2009 the Abruzzi region (central Italy) producing vast damage in the L'Aquila town and surroundings. In this paper we present the location and geometry of the fault system as obtained by the analysis of main shock and aftershocks recorded by permanent and temporary networks. The distribution of aftershocks, 712 selected events with M L > 2.3 and 20 with M L > 4.0, defines a complex, 40 km long, NW trending extensional structure. The main shock fault segment extends for 15―18 km and dips at 45° to the SW, between 10 and 2 km depth. The extent of aftershocks coincides with the surface trace of the Paganica fault, a poorly known normal fault that, after the event, has been quoted to accommodate the extension of the area. We observe a migration of seismicity to the north on an echelon fault that can rupture in future large earthquakes.

Interactions between deformation and fluids in the frontal thrust region of the NanTroSEIZE transect offshore the Kii Peninsula, Japan: Results from IODP Expedition 316 Sites C0006 and C0007

Abstract: Integrated Ocean Drilling Program (IODP) Expedition 316 Sites C0006 and C0007 examined the deformation front of the Nankai accretionary prism offshore the Kii Peninsula, Japan. In the drilling area, the frontal thrust shows unusual behavior as compared to other regions of the Nankai Trough. Drilling results, integrated with observations from seismic reflection profiles, suggest that the frontal thrust has been active since ∼0.78–0.436 Ma and accommodated ∼13 to 34% of the estimated plate convergence during that time. The remainder has likely been distributed among out-of-sequence thrusts further landward and/or accommodated through diffuse shortening. Unlike results of previous drilling on the Nankai margin, porosity data provide no indication of undercompaction beneath thrust faults. Furthermore, pore water geochemistry data lack clear indicators of fluid flow from depth. These differences may be related to coarser material with higher permeability or more complex patterns of faulting that could potentially provide more avenues for fluid escape. In turn, fluid pressures may affect deformation. Well-drained, sand-rich material under the frontal thrust could have increased fault strength and helped to maintain a large taper angle near the toe. Recent resumption of normal frontal imbrication is inferred from seismic reflection data. Associated decollement propagation into weaker sediments at depth may help explain evidence for recent slope failures within the frontal thrust region. This evidence consists of seafloor bathymetry, normal faults documented in cores, and low porosities in near surface sediments that suggest removal of overlying material. Overall, results provide insight into the complex interactions between incoming materials, deformation, and fluids in the frontal thrust region.

Fluid and deformation regime of an advancing subduction system at Marlborough, New Zealand

Abstract: Newly forming subduction zones on Earth can provide insights into the evolution of major fault zone geometries from shallow levels to deep in the lithosphere and into the role of fluids in element transport and in promoting rock failure by several modes. The transpressional subduction regime of New Zealand, which is advancing laterally to the southwest below the Marlborough strike-slip fault system of the northern South Island, is an ideal setting in which to investigate these processes. Here we acquired a dense, high-quality transect of magnetotelluric soundings across the system, yielding an electrical resistivity cross-section to depths beyond 100 km. Our data imply three distinct processes connecting fluid generation along the upper mantle plate interface to rock deformation in the crust as the subduction zone develops. Massive fluid release just inland of the trench induces fault-fracture meshes through the crust above that undoubtedly weaken it as regional shear initiates. Narrow strike-slip faults in the shallow brittle regime of interior Marlborough diffuse in width upon entering the deeper ductile domain aided by fluids and do not project as narrow deformation zones. Deep subduction-generated fluids rise from 100 km or more and invade upper crustal seismogenic zones that have exhibited historic great earthquakes on high-angle thrusts that are poorly oriented for failure under dry conditions. The fluid-deformation connections described in our work emphasize the need to include metamorphic and fluid transport processes in geodynamic models.

Structural development of a major late Cenozoic basin and transpressional belt in central Iran: The Central Basin in the Qom-Saveh area

Abstract: The Central Basin of the Iran Plateau is between the geologically better-known regions of the Zagros and Alborz Mountains. Hydrocarbon exploration in the Central Basin has revealed the details of the late Eocene–Holocene evolution of the basin based on seismic reflection data, geological field work, basin modeling, and satellite interpretation. The multistage basin history commenced with broad sag-type subsidence and isolated normal faults during Oligocene–early Miocene time. It evolved to an extensional or transtensional basin in the early-middle Miocene, with as much as 4–5 km of Upper Red Formation section being deposited in some parts of the basin during this stage. The upper part of the Upper Red Formation is associated with a change to transpressional deformation, with the development of thrusts and folds. This latest (probably middle and/or late Miocene–Holocene) deformation is transpressional, and includes a mixture of basement-involved strike-slip and thrust faults and thin-skinned folding and thrusting detached on Oligocene evaporites. Local detachment levels higher in the stratigraphy also exist. Subsidence in mini-foredeep basins and strike-slip fault bounded basins occurred during this stage, and several kilometers of Upper Red Formation were deposited in the main depocenters. Northwest-southeast– to north-northwest–south-southeast– striking dextral strike-slip to compressional faults dominate the area, with subordinate east-west and north-south fault orientations also present. These different fault sets combine in places to form major strike-slip duplex geometries. The Eocene volcanic belt (Urumieh-Dokhtar zone) along the southern margin of the basin forms a chain of massifs as much as 3 km high, the outcrops of which were exhumed by movement along major thrusts from 5–6 km depth between the middle Miocene and present day. The Central Basin–Urumieh-Dokhtar zone forms a distinctive transpressional belt that underwent a minimum of 38 km shortening between the late Miocene and Holocene. The Central Basin and the Zagros and Alborz Mountains all indicate that the onset of widespread crustal shortening in Iran occurred late (latest early Miocene or later), relative to the initial collision of the Arabia Peninsula with Eurasia during the late Eocene or early Oligocene. Uplift of the Central Basin surface from approximately sea level to 900–1000 m occurred during the middle or late Miocene, after deposition of the marine Qom Formation.

Seismic response of the fractured and faulted granite of Soultz-sous-Forêts (France) to 5 km deep massive water injections

Abstract: SUMMARY The European Enhanced Geothermal System (EGS, formerly Hot Dry Rock, HDR) programme of Soultz-sous-Foris organized around three wells drilled to a depth of about 5000 m. Hydraulic stimulations were performed in the wells in 2000 (GPK2 well), 2003 (GPK3 well) and 2004 and 2005 (GPK4 well). The stimulation of GPK2 induced more than 700 seismic events with a magnitude greater than 1.0. The seismicity depicts a dense, homogeneous cloud, without any apparent structure. Medium-size earthquakes represent more than 80 per cent of the cumulative seismic moment. The b-value of the Gutenberg and Richter law is larger than 1.2. The injectivity has been increased by a factor 20. These characteristics indicate that the stimulation reactivated a 3-D dense network of fractures. The stimulation of GPK3 induced only about 250 events with a magnitude greater than 1.0 but with a greater proportion of large events, up to 2.9. The hypocentres form clear structures identified as large faults, the b-value is about 0.9 and the large events (M > 2.0) account for the greater part of the cumulative seismic moment. The injectivity of the well, which was already high before the stimulation, remained unchanged. The stimulation of GPK4 was achieved in two stages. This stimulation produced even less induced events, making the interpretation difficult. The differences between the seismic response of GPK2 and GPK3 are due to the presence of large faults cut by GPK3 or in its close vicinity and reached by the injected water. Once a seismic event occurs on a fault, a sequence of earthquakes is triggered and the seismicity behaves, for a large part, independent of the injected flow rate. The stimulations also show some evidence that creeping could be a major source of deformation, if not the main one. The future EGS programme will have to drill wells in zones free of large faults to avoid poor hydraulic performance and inconvenience to the population.

Low Quaternary slip rate reconciles geodetic and geologic rates along the Altyn Tagh fault, northwestern Tibet

Abstract: For more than two decades the slip rate along the active, left-slip Altyn Tagh fault of northwestern Tibet has been disputed, with millennial rates reported to be as much as three times faster than those determined geodetically. This problem is significant because the total offset, plate-boundary length, and age of the Altyn Tagh fault make it the most important single structure accommodating India-Asia convergence north of the Himalayas. Here we show that the central Altyn Tagh fault slipped at only 14–9 mm/a over the past 4–6 ka by tightly bracketing the age of a displaced fluvial terrace riser at Yuemake (88.51°E, 38.19°N). This result contradicts previous latest Quaternary rates and is consistent with those derived from geodetic, paleoseismic, and geologic measurements, and thus resolves the long-standing dispute over the latest Quaternary slip rate along the longest active strike-slip fault in Tibet.

Structural and diagenetic control of fluid migration and cementation along the Moab fault, Utah

Abstract: The Moab fault, a basin-scale normal fault that juxtaposes Jurassic eolian sandstone units against Upper Jurassic and Cretaceous shale and sandstone, is locally associated with extensive calcite and lesser quartz cement. We mapped the distribution of fault-related diagenetic alteration products relative to the fault structure to identify sealing and conductive fault segments for fluid flow and to relate fault–fluid-flow behavior to the internal architecture of the fault zone. Calcite cement occurs as vein and breccia cement along slip surfaces and as discontinuous vein cement and concretions in fault damage zones. The cement predominates along fault segments that are composed of joints, sheared joints, and breccias that overprint earlier deformation bands. Using the distribution of fault-related calcite cement as an indicator of paleofluid migration, we infer that fault-parallel fluid flow was focused along fault segments that were overprinted by joints and sheared joints. Joint density, and thus fault-parallel permeability, is highest at locations of structural complexity such as fault intersections, extensional steps, and fault-segment terminations. The association of calcite with remnant hydrocarbons suggests that calcite precipitation was mediated by the degradation and microbial oxidation of hydrocarbons. We propose that the discontinuous occurrence of microbially mediated calcite cement may impede, but not completely seal, fault-parallel fluid flow. Fault-perpendicular flow, however, is mostly impeded by the juxtaposition of the sandstone units against shale and by shale entrainment. The Moab fault thus exemplifies the complex interaction of fault architecture and diagenetic sealing processes in controlling the hydraulic properties of faults in clastic sequences.

Low Quaternary slip rate reconciles geodetic and geologic rates along the Altyn Tagh Fault

Abstract: For more than two decades the slip rate along the active, left-slip Altyn Tagh fault of northwestern Tibet has been disputed, with millennial rates reported to be as much as three times faster than those determined geodetically. This problem is significant because the total offset, plate-boundary length, and age of the Altyn Tagh fault make it the most important single structure accommodating India-Asia convergence north of the Himalayas. Here we show that the central Altyn Tagh fault slipped at only 14–9 mm/a over the past 4–6 ka by tightly bracketing the age of a displaced fluvial terrace riser at Yuemake (88.51°E, 38.19°N). This result contradicts previous latest Quaternary rates and is consistent with those derived from geodetic, paleoseismic, and geologic measurements, and thus resolves the long-standing dispute over the latest Quaternary slip rate along the longest active strike-slip fault in Tibet.

Paleoseismology of silent faults in the Central Apennines (Italy): the Mt. Vettore and Laga Mts. faults

Abstract: Paleoseismological analyses have been performed in the Central Apennines along faults showing geomorphological evidence of Late Quaternary activity and characterised by the absence of historical seismicity. Three trenches were made along the Mt. Vettore Fault, across a scarp on a Late Pleistocene-Holocene alluvial fan. The youngest displacement event (E1) occurred after 4155-3965 years BP and before the 6th-7th century A.D., a previous event (E2) occurred between 5940-5890/5795-5780 years BP and 4155-3965 years BP, while the oldest event (E3) occurred between 18.000-12.000 years BP and 5940-5890/5795-5780 years BP. One trench was excavated across the Laga Mts. Fault which gave evidence for two displacement events after 8320-8150 years BP. The minimum vertical slip rate estimated through the paleoseismological analysis of the Mt. Vettore Fault is 0.11-0.36 mm/yr, while the minimum slip rate along the Laga Mts. Fault is 0.12 mm/yr. The paleoseismologically inferred recurrence interval is not longer than 4690 years for the Mt. Vettore Fault and not longer than 7570 years for the Laga Mts. Fault, while the minimum elapsed times since the last activation are 1300 and 800 years for the two faults, respectively. The evaluation of the former elapsed time was based on paleoseismological data, while the estimation of the latter was based on the absence of historical earthquakes which may have been caused by the Laga Mts. Fault and on the completeness of the historical catalogues for the large magnitude events in the last eight centuries. Based on the length of the fault at the surface, earthquakes with M 6.5 and 6.6 may be expected from the activation of the Mt. Vettore and Laga Mts. faults, respectively.

Structural heterogeneity of the Longmenshan fault zone and the mechanism of the 2008 Wenchuan earthquake (Ms 8.0)

Abstract: We determined detailed 3-D images of P and S wave velocity (Vp, Vs) and Poisson's ratio (σ) in and around the Longmenshan (LMS) fault zone by using a large number of P and S wave arrival times from the aftershocks of the 2008 Wenchuan earthquake (Ms 8.0) and other local events. Our results show that the structure of the LMS fault zone north of the Wenchuan main shock is very different from that south of the main shock. The southern section of the LMS fault zone contains a broad zone with low-Vp, low-Vs, and high-σ anomalies, while the northern segment exhibits more scattered heterogeneities, corresponding to most of the aftershocks which occurred there. A prominent low-Vp, low-Vs, and high-σ anomaly exists directly beneath the Wenchuan main shock hypocenter, suggesting that in addition to compositional variations, fluid-filled fractured rock matrices may exist in the LMS fault zone, which may have influenced the generation of the large Wenchuan earthquake. Our tomographic results provide sound seismic evidence for the hypothesis that an upward intrusion of lower crustal flow occurred along the LMS fault zone. In addition, most small earthquakes before the 2008 Wenchuan main shock occurred around the Guanxian-Jiangyou fault, while the Wenchuan aftershocks were mainly concentrated on the Yingxiu-Beichuan fault, suggesting that the rupture process of the Wenchuan earthquake may belong to an out-of-sequence thrusting event, a suggestion which is in good agreement with the results from geological surveys and also quite similar to the rupture processes of the 1999 Chi-Chi earthquake (M 7.5) and the 2005 Kashmir earthquake (M 7.6). A few aftershocks occurred close to the blind Guangyuan-Dayi fault in the Sichuan basin, suggesting that this blind fault was also ruptured by the Wenchuan earthquake, consistent with geological surveys.

Travertine deposition and faulting: the fault-related travertine fissure-ridge at Terme S. Giovanni, Rapolano Terme (Italy)

Abstract: In this paper we describe an example of travertine fissure-ridge development along the trace of a normal fault with metre displacement, located in the eastern margin of the Neogene–Quaternary Siena Basin, in the Terme S. Giovanni area (Rapolano Terme, Italy). This morphotectonic feature, 250 m long, 30 m wide and 10 m high, formed from supersaturated hot waters (39.9°C) flowing from thermal springs aligned along the trace of the normal fault dissecting travertines not older than Late Pleistocene (24 ± 3 ka). A straight, continuous fissure with a maximum width of 20 cm occurs at the top of the ridge, along its crest. Hot fluids flow from cones mainly located at the extremities of the ridge, where travertine is depositing. The travertine fissure-ridge shows an asymmetrical profile since it buries the fault scarp. The difference in height of slopes corresponds to the vertical displacement of the normal fault. Fissuring of the recent travertine deposits along the strike of the crestal fissure, as well as recent hydrothermal circulation, lead us to believe that the Terme S. Giovanni normal fault may be currently active. On the whole, the Terme S. Giovanni fissure ridge can be defined as a travertine fault trace fissure-ridge, adding a helpful example for studying the relationship between faulting and travertine deposition.

Geometric and rheological asperities in an exposed fault zone

Abstract: [1] Earthquake dynamics are strongly affected by fault zone structure and fault surface geometry. Here we investigate the interplay of bulk deformation and surface topography using detailed structural analysis of a fault zone near Klamath Falls, Oregon, combined with LiDAR measurements of the fault surface. We find that the fault zone has a layered damage architecture. Slip primarily occurs inside a 1–20 mm wide band that contains principal slip surfaces with individual widths of ∼100 μm. The slip band sits atop a cohesive layer which deforms by granular flow. Several fault strands with total slips of 0.5–150 m also have cohesive layers with thicknesses increasing monotonically with slip. The thickness added to the cohesive layer per unit slip decreases with increasing displacement indicating that slip progressively localizes. The main fault is a continuous surface with 10–40 m long quasi-elliptical geometrical asperities, i.e., bumps. The bumps reflect variations of the thickness of the granular cohesive layer and can be generated by a pinch-and-swell instability. As the granular layer is rheological distinct from its surroundings, the asperities are both geometrical and rheological inhomogenities. Modeling slip along wavy faults shows that slip on a surface with a realistic geometry requires internal yielding of the host rock. Our observations suggest that the internal deformation processes in the fault zone include ongoing fracture, slip along secondary faults, and particle rotation. Granular flow is an important part of faulting in this locale. Slip surfaces localize on the border of the granular cohesive layer. The ongoing slip smoothes the surfaces and thus the structural and geometrical evolution of the granular layer creates a preference for continued of slip on the same surface. There is a feedback cycle between slip on the surface and the generation of the granular layer that then deforms and controls the locus of future slip.