Synthetic Faults Research Articles

Reassessing the Rupture Process of the 2009 L’Aquila Earthquake (Mw 6.3) on the Paganica Fault and Investigating the Possibility of Coseismic Motion on Secondary Faults

The kinematic rupture of the L’Aquila earthquake on the Paganica fault is reassessed with a highly comprehensive set of data. This very complete dataset is also used to explore the possibility that secondary faults may be involved in the coseismic process. The main characteristics of the slip distribution of the Paganica normal fault found by previous studies are confirmed: an initial propagation around and up‐dip of the hypocenter, associated with a high velocity rupture (>3.5 km/s); a main asperity centered about 6–7 km southeast of the hypocenter, rupturing in the subsequent stage, with a maximum slip of nearly 1.5 m; a total rupture measuring about 15 km along strike and 12 km along dip; and an overall low average velocity rupture (<2 km/s). The statement that the main features of the slip distribution are confirmed by different studies (based on distinct inversion schemes) is by itself an achievement, knowing that many past studies provided strongly discrepant slip maps for common events. Moreover, we focus specifically on the existence of slip at shallow depth and its relations with the other known active faults in the vicinity of the Paganica fault. We find that (1) as found in previous studies, shallow slip near the ground surface up‐dip of the hypocenter is incompatible with the available Synthetic Aperture Radar interferograms and (2) the only secondary fault that is likely to have been activated during the mainshock is the San Gregorio synthetic fault located 2–3 km southwest of the Paganica fault. Online Material: Figures of waveform fitting using different velocity models, time versus distance distribution of slip from the single‐fault model, variations in the geometry of the Paganica fault, geometry of complex faults models and geodetic data modeling, uncertainty on the slip value and the rupture time, effect of noise on resolution tests, quantification of the improvement on the GPS data fitting with the preferred two faults model with respect to the single‐fault model, and a posteriori modeling of leveling data.

Diapiric shale and coast-perpendicular, fault-related subbasins, south Texas Gulf Coast

Coast-perpendicular shale ridges are rare structural features worldwide, and their origin remains a subject of debate. We studied some coast-perpendicular shale ridges and faults within a minibasin located onshore in Refugio County in the Texas Gulf Coast. We used 3D seismic data, visualization tools, and seismic attributes to examine the geometry of coast-perpendicular diapiric structures associated subbasins (SBs) and faults, and coast-parallel listric faults. Our results indicated that the minibasin is subdivided into four SBs by five diapiric shale ridges that intrude through the fault heaves of down-to-the-basin (synthetic) and coast-perpendicular faults. Three of the SBs are oriented perpendicular to the coast, whereas the fourth has a curvilinear form trending northeast–southwest–southeast. Of the five diapiric shale ridges, three are coast-perpendicular. The other two are curvilinear to the coast. All five diapiric shale ridges are associated with coast-perpendicular faults that bound the flanks of the ridges. On the basis of our mapping results, we deduced that the origin of the coast-perpendicular faults in the study area are related to the coalescing of en echelon synthetic faults, as well as the coalition of synthetic and antithetic fault planes. We inferred that the origin of the shale diapirs is related to vertical loading and, possibly, local southwest–northeast lateral compression of interbedded, overpressured, shale-prone intervals. The coast-perpendicular faults within the Frio formed as a result of reactivation of the Eocene-Vicksburg coast-perpendicular faults. Synthetic faults dominate the pattern within the SB in the north where shale ridges are broad, whereas antithetic faults dominate the pattern in the south where shale ridges are narrow.

An experimental study of the flow of gas along synthetic faults of varying orientation to the stress field: Implications for performance assessment of radioactive waste disposal

AbstractCritical stress theory states that fault transmissivity is strongly dependent upon orientation with respect to the stress tensor. This paper describes an experimental study aimed at verifying critical stress theory using a bespoke angled shear rig designed to examine the relationship between gas flows along a kaolinite‐filled synthetic fault as a function of fault dip. A total of 22 gas injection experiments were conducted on faults oriented 0°, 15°, 30°, and 45° to horizontal; both with and without active shear. Gas flow was seen to be complex; repeat gas injection testing showed a consistent gas entry pressure but considerably different, nonrepeatable, gas peak or breakthrough pressure. Gas flow occurred along discrete, dilatant pathways. The physics governing the pressure at which these features formed was repeatable; however, permeability was dependent on the number, distribution, and geometry of the resultant pathways. The nonrepeatable gas response suggests that the number of pathways was dependent on very subtle variations in gouge properties. No fault orientations were seen to exhibit nonflow characteristics, although critical stress theory predicted that two of the investigated fault angles should be effective seals. However, a small variation in gas entry pressure was seen with fault angle as a result of varying normal and shear stress acting on the gouge material. Shear was seen to enhance gas movement by reducing gas entry pressure and increased permeability once gas became mobile. Therefore, in kaolinite gouge‐filled faults, shear is not an effective self‐sealing mechanism to gas flow.

An Abrupt Transition in the Mechanical Response of the Upper Crust to Transpression along the Queen Charlotte Fault

The Queen Charlotte Fault (QCF) is a major strike‐slip fault that forms the boundary between the Pacific and North American plates from 51° to 58° N. Near 53.2° N, the angle of oblique convergence predicted by the Mid‐Ocean Ridge VELocity (MORVEL) interplate pole of rotation decreases from >15° in the south to <15° in the north. South of 53.2° N, the convergent component of plate motion results in the formation of a 40 km wide terrace on the Pacific plate west of QCF and earthquakes with thrust mechanisms (including the 2012 Haida Gwaii earthquake sequence) are observed. North of 53.2° N, in the primary rupture zone of the M 8.1 strike‐slip earthquake of 1949, the linear terrace disappears, and topography of the continental slope west of the QCF is characterized by a complex pattern of ridges and basins that trend obliquely to the primary trace of the QCF. Deformation within the Pacific plate appears to occur primarily through strike‐slip faulting with a minor thrust component on secondary synthetic faults. The orientations of these secondary faults, as determined from seismic reflection and bathymetric data, are consistent with the reactivation of faults originally formed as ridge‐parallel normal faults and as thrust faults formed parallel to the QCF south of the bend at 53.2° N and subsequently translated to the north. We suggest that an oblique convergence angle of 15° represents a critical threshold separating distinct crustal responses to transpression. This result is consistent with theoretical and analog strain models of transpressive plate boundaries. The sharpness of this transition along the QCF, in contrast to purely continental transform boundaries, may be facilitated by the relatively simple structure of oceanic crust and the presence of pre‐existing, optimally oriented faults in the young Pacific plate.

Effect of stress state on slow rupture propagation in synthetic fault gouges

Slow slip events (SSEs) in subduction zones are known to proceed so sluggishly that the associated slow ruptures do not generate any detectable radiating seismic waves. Moreover, they propagate at speeds at least four orders of magnitude slower than regular earthquakes. However, the underlying physics of slow slip generation has yet to be understood. Here, we carry out laboratory studies of unstable slip along simulated fault zones of lizardite/chrysotile (liz/ctl) and antigorite (i.e., low- and high-temperature serpentine phases, respectively) and olivine, under varying conditions of normal stress, with the aim of better understanding the influence of stress state on the process of slow rupture along the plate interface. During a single unstable slip, we clearly observe a slow rupture phase that is often followed by an unstable, high-speed rupture. We find that lower fault-zone friction coefficients (μ values from 0.7 down to 0.5) lead to increasing degree of the slow rupture mode, and also that the slow rupture velocities (V r = 0.07 to 5.43 m/s) are largely consistent with those of short-term SSEs observed in nature. Our findings suggest that the generation of SSEs is facilitated by conditions of low normal stress and low fault-zone strength along the plate interface, which may be weakened by metamorphic reactions that result in the production of hydrous phases (e.g., serpentine) and/or the direct involvement of fluid itself, leading to a reduction in effective normal stress.

English

A three-dimensional seismic reflection survey carried out in Odo Field in the central swamp of the Niger Delta Basin has been interpreted with respect to exploration for hydrocarbons. The Odo Field is marked by a large southwest heading listric growth fault with numerous synthetic and antithetic faults. The two picked seismic events, horizons 1 and 2 indicate four closures each, which are hydrocarbon bearing. The faults in the field thus support hydrocarbon accumulations which are seen to be mainly fault/dip closures. The expectations calculated for the observed fault closures suggest that several in-field appraisal prospects exist. The seismic attributes (amplitude maps) furthermore suggest that horizon 1 is mainly gas bearing. Key words: Field, faults, Niger Delta, seismic, hydrocarbon

Structural analysis of a salt-cored transfer zone in the South Timbalier Block 54, offshore Gulf of Mexico: Implications for restoration of salt-related extensional structures

The style of faulting in offshore Louisiana, Gulf of Mexico, is characterized by short, arcuate regional and counterregional growth faults, which commonly form complex transfer zones above shallow, Miocene level salt bodies. South Timbalier Block 54 (ST54) constitutes one such area where a basinward-dipping regional and a landward-dipping counterregional growth fault form a convergent transfer zone. Structural interpretation using three-dimensional (3-D) seismic and well data reveals that the eastern and western flanks of the structure contain salt in the footwalls of the main regional and counterregional faults. The salt rises to a much shallower stratigraphic level in the central part of the transfer zone, forming a symmetric salt diapir. Secondary antithetic and synthetic faults adjacent to the two main faults and extending into the transfer zone accommodate slip between the main faults. Kinematic restoration of a series of north–south-trending cross sections across the structure show that upslope evacuation of salt is the result of sediment loading and growth fault movement, and the location of the transfer zone is possibly controlled by the allochthonous salt. The entire area is characterized by down-to-basin movement, with the major regional and counterregional faults displaying footwall and hanging-wall fixed deformation, respectively. The presence of the crestal graben above the salt high and the timing of maximum salt evacuation from the flanks suggest that active or reactive diapirism occurred during part of the deformation history. A 3-D structural model using depth-converted horizons, balanced cross sections, and well tops accurately portrays the subsurface structure. Understanding the evolution of the structure in ST54 provides insight on similar structures in other areas in offshore Louisiana and the relationship between salt evacuation and transfer zone development.

Stratigraphy, facies, and depositional environments of the Paleogene sediments in Cairo–Suez district, Egypt

The study of eight stratigraphic sections along Cairo–Suez district, between Gebel Ataqa and Gebel Mokattam, reveals a thick Lower Eocene Ypresian carbonate section, approximately 210 m thick, at Gebel Abu Treifiya just to the west of Gebel Ataqa. It is represented by the Minia Formation rich in Nummulites praecursor, Orbitolites pharaonum Schwager, Alveolina frumentiformis Schwager, and one of the precursors of Nummulites gizehensis group. The Minia Formation is a fairly clear, warm, and shallow marine facies. The Middle Eocene Lutetian sediments are totally missing in all studied sections probably reflecting instability in deposition echoed in the active block movements the area witnessed since the Paleozoic. The Bartonian sea transgressed over the area depositing Gebel Hof Formation at the base, Observatory Formation in the middle, and Qurn Formation at the top. The Gebel Hof Formation was deposited in an open-marine environment, passed upward into shallow marine, neritic to reefal facies; for the Observatory and Qurn formations, the former changed laterally into a sheltered lagoon facies, Sannor Formation, rich in Somalina stefaninii Silvestri, Dictyoconus aegyptiensis Chapman, and Idalina cuvillieri Bignot and Strougo. The Upper Eocene sediments, Maadi Formation, are mainly represented by a carbonate–clastic section showing shallowing-upward cycles resulted by the retreating of the sea shoreline northward during the Late Eocene, with high supply of terrigenous sediments. The lower carbonate cycle of the Maadi Formation was deposited in a restricted platform and tidal flats, whereas the upper siliciclastic cycle containing Carolia placunoides banks was probably deposited in a winnowed platform edge. These depositional environments were developed in grabens, ramps, and footslopes of the down-faulted Middle Eocene blocks, i.e., syn-tectonic deposition. The fluviatile Oligocene sediments were highly controlled by the structural and topographic lows, where a substantial thickness was deposited, Gebel Ahmer Formation, occupying several grabens and gently sloping areas between many synthetic faults.

Weighted local and global regressive mapping: A new manifold learning method for machine fault classification

This article studies if machine faults can be effectively determined in a reduced dimensional space. When faults occur in machines, machine vibration signals will deviate from its normal signal pattern. Such changes can be reflected in the features constructed from the machine signals. In this article, 13-dimension feature data set is constructed to represent different health conditions of machines, and unsupervised learning algorithms are introduced to deal with feature data sets for feature extraction and fault classification. A weighted local and global regressive mapping (WLGRM) algorithm is proposed for machine fault classification. Two synthetic fault data sets and two experimental data sets are employed to validate the effectiveness of the proposed approach. Comparative analysis with other unsupervised learning algorithms, such as local and global regressive mapping, locality preserving projection, Isomap, principal component analysis, and Sammon mapping, are reported. The results show that different machine faults can be classified, the degree of fault severity can be captured, and WLGRM can achieve better performance than other algorithms in most cases of machine fault classification.

Fault Diagnosis Expert System of Turbine Generator Sets Based on Rule Reasoning and Case Reasoning

A synthetic fault diagnosis expert system for turbine generator sets based on rule based reasoning and cases based reasoning is built in this paper. The structure of synthetic fault diagnosis expert system is discussed. The rule base and case base for the fault diagnosis of expert system is established based on the domain expert knowledge and relevant fault cases of turbine generator sets. Both the inference flow and case retrieval strategy of diagnosis system are discussed in detail. Finally the expert system is verified by a given application example.

Geomorphology and structural geology of Saturnalia Fossae and adjacent structures in the northern hemisphere of Vesta

Vesta is a unique, intermediate class of rocky body in the Solar System, between terrestrial planets and small asteroids, because of its size (average radius of ∼263km) and differentiation, with a crust, mantle and core. Vesta’s low surface gravity (0.25m/s2) has led to the continual absence of a protective atmosphere and consequently impact cratering and impact-related processes are prevalent. Previous work has shown that the formation of the Rheasilvia impact basin induced the equatorial Divalia Fossae, whereas the formation of the Veneneia impact basin induced the northern Saturnalia Fossae. Expanding upon this earlier work, we conducted photogeologic mapping of the Saturnalia Fossae, adjacent structures and geomorphic units in two of Vesta’s northern quadrangles: Caparronia and Domitia. Our work indicates that impact processes created and/or modified all mapped structures and geomorphic units. The mapped units, ordered from oldest to youngest age based mainly on cross-cutting relationships, are: (1) Vestalia Terra unit, (2) cratered highlands unit, (3) Saturnalia Fossae trough unit, (4) Saturnalia Fossae cratered unit, (5) undifferentiated ejecta unit, (6) dark lobate unit, (7) dark crater ray unit and (8) lobate crater unit. The Saturnalia Fossae consist of five separate structures: Saturnalia Fossa A is the largest (maximum width of ∼43km) and is interpreted as a graben, whereas Saturnalia Fossa B-E are smaller (maximum width of ∼15km) and are interpreted as half grabens formed by synthetic faults. Smaller, second-order structures (maximum width of <1km) are distinguished from the Saturnalia Fossae, a first-order structure, by the use of the general descriptive term ‘adjacent structures’, which encompasses minor ridges, grooves and crater chains. For classification purposes, the general descriptive term ‘minor ridges’ characterizes ridges that are not part of the Saturnalia Fossae and are an order of magnitude smaller (maximum width of <1km vs. maximum width of ∼43km). Shear deformation resulting from the large-scale (diameter of <100km) Rheasilvia impact is proposed to form minor ridges (∼2km to ∼25km in length), which are interpreted as the surface expression of thrust faults, as well as grooves (∼3km to ∼25km in length) and pit crater chains (∼1km to ∼25km in length), which are interpreted as the surface expression of extension fractures and/or dilational normal faults. Secondary crater material, ejected from small-scale and medium-scale impacts (diameters of <100km), are interpreted to form ejecta ray systems of grooves and crater chains by bouncing and scouring across the surface. Furthermore, seismic shaking, also resulting from small-scale and medium-scale impacts, is interpreted to form minor ridges because seismic shaking induces flow of regolith, which subsequently accumulates as minor ridges that are roughly parallel to the regional slope. In this work we expand upon the link between impact processes and structural features on Vesta by presenting findings of a photogeologic, structural mapping study which highlights how impact cratering and impact-related processes are expressed on this unique, intermediate Solar System body.

Felt index, source parameters and ground motion evaluation for earthquakes at Mt. Vesuvius

&lt;p&gt;Results of non-instrumental surveys carried out on recent and past seismicity at Vesuvius have been retaken in order to propose new analyses regarding source mechanisms and causative faults. We present the results of the October 9, 1999, earthquake, the most intense event since the 1944 eruption. The intensity was evaluated by utilizing integer values of the MCS Scale and the felt index as a continuous parameter. Values of magnitude and attenuation determinated by applying macroseismic models to data, and compared to instrumental ones, were utilized to assess the “size” of the historical Vesuvian earthquakes. A magnitude of M = 5.1±.3 was considered for the A.D. 62 earthquake, the largest one of the area that preceded the A.D. 79 famous eruption. By using the macroseismic field of October 9, 1999, the source mechanism of the earthquake was obtained, and synthetic isoseisms and causative fault of the A.D. 62 are also proposed.&lt;/p&gt;

Piston Engine Fuel Injector Synthetic Clearance Overrun Fault and its Control

To import a certain aviation piston engine fuel injector for an object, a synthetic clearance fit and its control was studied. A certain type of general aviation aircraft piston engine with fuel injector has synthetic clearance overrun fault after long time using, causing flight control difficult, and seriously affect the working properties of engine, in the condition of idle state more obvious, easy to cause unsafe events such as engine parking. Send for repair by a large number of fuel injectors and its related parts fit clearance measurement and use effect evaluation, might bring negative influence for the mechanism analysis and effective repair and control scheme are put forward..

Initiation and growth of strike-slip faults within intact metagranitoid (Neves area, eastern Alps, Italy)

Exhumed strike-slip faults exposed in the Neves area of the Tauern Window (eastern Alps, Italy) formed in the lower brittle crust under hydrous conditions within intact metagranitoids, where any precursor fractures were already healed due to earlier amphibolite-facies metamorphism. Faults initiated as newly formed en-echelon fractures delineating shear bands, with segmentation occurring over scales of 10 –3 to 10 2 m. Due to the initial en-echelon pattern, stepovers between fault segments were almost invariably contractional during subsequent slip accumulation. In initial low-slip (centimeter to decimeter) stages, synthetic slip on the segments was associated with the development of a set of antithetic faults in the contractional stepovers, oriented at an angle of 30°–45° to the bounding faults. Slip on this antithetic set did not fully compensate for the decrease in slip on the main faults, implying an additional deformation mechanism within the stepovers (e.g., block rotation and/or out-of-plane movement). In larger faults with slip on the order of a few meters, the antithetic faults within the stepover were crosscut by synthetic sigmoidal faults that connected the overstepped fault segments and accommodated most of the subsequent displacement transfer. This second stage of evolution involving fault linkage is well documented in the Mesule fault, which has a current maximum offset of ∼10 m. In the Mesule fault, the total horizontal slip summed along all secondary faults within the stepover accounts almost entirely for the net slip decrease toward the tips of the overstepping faults. However, the boundary faults remain nearly straight in spite of the sigmoidal ramp-like geometry of the connecting faults. Since the adjacent blocks are little deformed and there is no evidence for appreciable volume loss or block rotation in the stepover, significant out-of-plane movement is implied, although it is difficult to quantify. The Neves area provides unusually detailed field constraints on fault initiation, linkage, and displacement accumulation within nonbedded and relatively isotropic granitoid rocks at the base of the brittle crust, where neither a free upper surface nor substantial volume change (e.g., by veining and pressure solution) was a controlling factor in accommodating fault linkage and displacement transfer.

Reduction of friction on geological faults by weak-phase smearing

Most common crustal rock types display friction coefficients of 0.6 or higher, but some faults must be frictionally weak as they slip when the stress state is unfavourably-oriented (i.e. the resolved shear stress is low for a given normal stress across the fault surface). A role for low-friction minerals and high pore fluid pressures, either separately or in combination, is frequently invoked to explain such slip, but volume fractions of dispersed weak phases often seem to be present in fault gouge in amounts too small to produce significant mechanical weakening. By means of mechanical tests on synthetic fault gouge and microstructural study of run products, we show that the effective area of an embedded weak phase (graphite) on a slip plane can be substantially increased by mechanical smearing, and that the enlarged area of the weak phase on the slip plane follows a linear mixing law. This allows a relatively small volume fraction of the initially dispersed weak phase to have a disproportionately large effect, provided the smearing is concentrated into a narrow planar slip zone or into an interconnected network of them.

Radiography of a normal fault system by 64,000 high‐precision earthquake locations: The 2009 L'Aquila (central Italy) case study

AbstractWe studied the anatomy of the fault system where the 2009 L'Aquila earthquake (MW 6.1) nucleated by means of ~64 k high‐precision earthquake locations spanning 1 year. Data were analyzed by combining an automatic picking procedure for P and S waves, together with cross‐correlation and double‐difference location methods reaching a completeness magnitude for the catalogue equal to 0.7 including 425 clusters of similar earthquakes. The fault system is composed by two major faults: the high‐angle L'Aquila fault and the listric Campotosto fault, both located in the first 10 km of the upper crust. We detect an extraordinary degree of detail in the anatomy of the single fault segments resembling the degree of complexity observed by field geologists on fault outcrops. We observe multiple antithetic and synthetic fault segments tens of meters long in both the hanging wall and footwall along with bends and cross fault intersections along the main fault and fault splays. The width of the L'Aquila fault zone varies along strike from 0.3 km where the fault exhibits the simplest geometry and experienced peaks in the slip distribution, up to 1.5 km at the fault tips with an increase in the geometrical complexity. These characteristics, similar to damage zone properties of natural faults, underline the key role of aftershocks in fault growth and co‐seismic rupture propagation processes. Additionally, we interpret the persistent nucleation of similar events at the seismicity cutoff depth as the presence of a rheological (i.e., creeping) discontinuity explaining how normal faults detach at depth.

Structural characteristics of the Altar Basin, Northwest Sonora, Mexico

Eight two-dimensional, multichannel seismic reflection lines were acquired, processed, and interpreted to study the structure of the Altar Basin, which is part of the Salton Trough tectonic province. We identified two basin-bounding zones characterized by different degrees of strain: the Cerro Prieto–Altar deformation zone (CPADZ) and the Altar–Caborca deformation zone (ACDZ). The CPADZ is bounded on the west by the Cerro Prieto fault and on the east by the Altar fault. To the north, the strike of both faults changes slightly from a NW to more NNW direction. In the CPADZ, the thickness of the crust decreases southward towards the Gulf of California, and is associated with a deformation-developing fault. The CPADZ has a rotation component orientating these faults in an oblique direction to the Cerro Prieto fault, whereas within the ACDZ, a geometric coherence of synthetic and antithetic faults exists, creating horsts and graben striking N37° W. The Altar fault is recognized by basement interruption, with a vertical component of ∼1 km, striking at N37° W and dipping 83° SW. On the northeastern side of the Altar Basin, the basement configuration shows that the minimum time of basement record (∼0.4 s of two-way travel time) and the time curve gradient decrease in the NE–SW direction. The depocentre is ∼6 km deep in the central-west portion of the basin. We identified a graben between the Rosario and Tinajas Altas mountains (Rosario Basin). The extension–connection of the Altar and Rosario basins to the south is not well defined; nevertheless, these basins could represent the link between the Colorado River and the Gulf of California during the late Miocene, whereas this link was abandoned in the Pliocene as subsidence migrated towards the northwest into the Cerro Prieto and Laguna Salada basins.

Late Holocene episodic displacement on fault scarps related to interstratal dissolution of evaporites (Teruel Neogene Graben, NE Spain)

The Mio-Pliocene formations of the Turolian type section (Teruel Graben, Spain) have sagged due to dissolution of the underlying Triassic evaporites generating a 1.7 km long monocline and accompanying synform with 130 m of structural relief. The crest of the monocline is affected by a graben that counterbalances the shortening caused by passive bending in the adjacent syncline. This graben is controlled by a master synthetic fault and a swarm of antithetic and synthetic faults with a conspicuous geomorphic expression. After acquiring Ground Penetrating Radar (GPR) profiles, three trenches were excavated in depressions associated with uphill-facing fault scarps. To our knowledge, these are the first trenches excavated across faults caused by deep-seated dissolution of evaporites. The geometrical relationships observed in two trenches indicate late Holocene episodic displacement (as many as 3 events). Some of the parameters estimated for the investigated faults are clearly different from those expectable for tectonic faults in this intraplate area. They include high apparent vertical slip rates (0.6–1 mm/yr), low average recurrence of faulting events (1.2–2 ka) and very high displacement per event values (>65 cm) for surface ruptures less then 200 m long. Our findings suggest that considering evaporite collapse faults as creeping structures is not a reliable criterion to differentiate between tectonic faults (seismogenic) and dissolution-induced gravitational faults (nonseismogenic).

Epistemic Uncertainty in California-Wide Synthetic Seismicity Simulations

Abstract The generation of seismicity catalogs on synthetic fault networks holds the promise of providing key inputs into probabilistic seismic-hazard analysis, for example, the coefficient of variation, mean recurrence time as a function of magnitude, the probability of fault-to-fault ruptures, and conditional probabilities for foreshock–mainshock triggering. I employ a seismicity simulator that includes the following ingredients: static stress transfer, viscoelastic relaxation of the lower crust and mantle, and vertical stratification of elastic and viscoelastic material properties. A cascade mechanism combined with a simple Coulomb failure criterion is used to determine the initiation, propagation, and termination of synthetic ruptures. It is employed on a 3D fault network provided by Steve Ward (unpublished data, 2009) for the Southern California Earthquake Center (SCEC) Earthquake Simulators Group. This all-California fault network, initially consisting of 8000 patches, each of ∼12 square kilometers in size, has been rediscretized into patches, each of ∼1 square kilometer in size, in order to simulate the evolution of California seismicity and crustal stress at magnitude M ∼5–8. Resulting synthetic seismicity catalogs spanning 30,000 yr and about one-half million events are evaluated with magnitude-frequency and magnitude-area statistics. For a priori choices of fault-slip rates and mean stress drops, I explore the sensitivity of various constructs on input parameters, particularly mantle viscosity. Slip maps obtained for the southern San Andreas fault show that the ability of segment boundaries to inhibit slip across the boundaries (e.g., to prevent multisegment ruptures) is systematically affected by mantle viscosity.

Cenozoic wrench tectonics and oroclinal bending in SE Korea

Based on fault geometry, petrography, and geochronology of granitic rocks as well as palaeomagnetic data from the Gyeongsang Basin, two conjugate fault sets are explained as a reflection of NNE-trending right-lateral wrench tectonics. According to this interpretation, the Gaum and Yangsan fault sets correspond to antithetic faulting by R′-shear and synthetic faulting by R-shear, respectively; they have rotated clockwise and counterclockwise, respectively, due to NE–SW compression (shortening), as a result of a NNE-trending wrenching force (simple shear). During progressive deformation, NS- or NNW-trending strike–slip faulting by P-shear occurred in the Yeongyang sub-basin, and finally the Yangsan fault formed as a wrench fault bisecting the P-shear and R-shear directions. Extension of the faults (R-shear, striking ∼N22°E) generated by block rotation on the east side of the Yangsan fault (wrench fault, striking ∼N13°E) resulted in convex eastward deflections. We suggest that this was caused by oroclinal bending of the existing faults generated by block rotations in opposite directions and is inferred to have been closely related to the East Sea (i.e. Sea of Japan) opening.