Axial Trend Research Articles

Aftershock sequence analysis of 19 May, 2009 earthquake of Lunayyir lava flow, northwest Saudi Arabia

Aftershock sequence of 19th May, 2009 Lunayyir earthquake (Mw 5.7) has been recorded by deploying seismic stations immediately after the occurrence of the main shock. Analysis of this sequence clarified that; the major part of cumulative seismic moment released during the first hours after the occurrence of the main shock; their orientation is north west (NW) parallel to the Red Sea axial trend along Najd faulting system; it is clustered at two depths; from 5-10 and 15-25 km beneath Lunayyir area; illustrated successive periods of maxima and minima; decayed rapidly following the relation of n(t)=37.28 t-0.6. Fault plane solutions for five large events indicate normal faulting is the major mechanism. These findings are in agreement with the north east (NE) (transform fault) trend runs crossing the Red Sea into Shield area where the activity has been initiated and travelled through the Lunayyir area. While the NW Najd faulting system runs parallel to the Red Sea axial trend and intersected with NE fault trend underlying Lunayyir area. Accordingly, it can be concluded that, the recent earthquake activities at Lunayyir lava flow area are related to the present-day Red Sea tectonics. Monitoring earthquake activities through installing permanent seismic network around the area is highly recommended. Key words: aftershock sequence, fault plane solution, stress, and Lunayyir.

Parametric and non-parametric statistical approaches to the determination of paleostress from dilatant fractures: Application to an Early Miocene dike swarm in central Japan

Several methods have been proposed for determining paleostress states from orientations of dilatant fractures such as dikes and veins. Recently a stochastic inversion method was invented to objectively estimate the principal stress axes and the stress ratio. Whether a fracture is dilated or not is controlled by the balance of the fluid pressure and the normal stress acting on it. The magnitude of normal stress depends on the fracture orientation, which causes anisotropic orientation distribution of dilatant fractures. The inversion method assumes that the orientation distribution of fractures can be approximated by a Bingham distribution, an exponential probability distribution on the unit sphere, of which symmetric axes are interpreted as the principal stress axes. However, it is unknown if the exponential type of distribution function is suitable or not. Here, we examine the distribution functions and propose two improved methods. One method uses the shifted power-law function as the shape of probability distribution, which is more flexible than the Bingham distribution and is applicable to various shapes of orientation distributions. Furthermore, an index of the driving fluid pressure can be estimated with a confidence interval. The other is a non-parametric (distribution-free) method, which can avoid the a priori assumption on the shape of distribution function without significantly losing accuracy or precision. The new methods were applied to an Early Miocene dike swarm formed during the back-arc opening of the Japan Sea. A normal-faulting stress regime with the minimum principal stress axis trending roughly perpendicular to the arc was obtained from the dikes. A moderately high stress ratio and a high fluid pressure were also estimated.

Geophysical and Geological investigation of the Delga proposed Dam Site, Qala Diza, Sulaimani City, Kurdistan Region, NE-Iraq

The use of two geophysical methods and geological study in the investigation of the
 geological setting is aimed at evaluating the feasibility of the area for establishing a dam
 foundation and reservoir. 2D resistivity tomography was conducted on abutments while
 GPR was used on the river terraces in the trough of the valley. The framework and
 mutual relations of the geophysics, stratigraphy, lithology, structure, hydrologeology and
 geotechnique are used to study feasibility of a segment of river valley for dam building.
 The study is aided by detail and uptodate geological maps and geological cross sections
 which are all base on field data and geophysical measurements. This proposed dam is
 located at the southeast Delga village and about 20.0 km upstream of Dokan reservoir on
 little Zab River and about 19.5 km from the Iraqi-Iranian border. The dam site and
 surround areas are located in the Imbricated Zone which characterized by southwest
 overturning of the folds. The dam site is located on one of the overturned syncline in which
 Shiranish and Kometan Formations are exposed and the foundation and abutments of the
 dam are located on latter formation. The trend of the dam axis is N80W and the dip of the
 strata of the Kometan Formation is nearly parallel to the dam axis. This structure relation
 with dam axis improves water tightness and stability of the dam abutments against sliding
 and collapse. The 2D resistivity tomography and GPR survey showed that there are
 underground week zones (cavern and fractured zones) in restricted limited area. The site
 is water tight as the river descended inside fresh Kometan and Shiranish Formation.

Change in stress field after the 2011 great Tohoku-Oki earthquake

Temporal change in the stress field after the 2011 great Tohoku-Oki earthquake was observed by using stress tensor inversions of earthquakes near the source region. In the upper plate (hanging wall) before the Tohoku-Oki earthquake, the maximum compressive stress (σ1) axis was oriented in the direction of plate convergence, specifically, in the direction of an estimated large near-trench slip area. The stress field in the upper plate was completely transformed after the earthquake, with the minimum compressive stress (σ3) axis being oriented in the direction of the large near-trench slip area, even near the trench axis. In the near-coast area within the Pacific plate (i.e., in the footwall), the σ1 axis was oriented in the direction of plate convergence prior to the earthquake, as within the upper plate; however, the orientations of the principal stress axes in this area showed no change after the earthquake. After the earthquake, the σ3 axis was oriented in the plate-convergence direction in areas within the Pacific plate below the central to near-trench portions of the mainshock rupture area. These observations provide evidence for large near-trench slip at very shallow depths during the mainshock rupture. Moreover, the trend of the σ1 axis in the upper plate before the Tohoku-Oki earthquake (i.e., in the direction of an estimated large near-trench slip area) strongly suggests that strain accumulation is possible even in the near-trench area at very shallow depths. The observed temporal change in the stress field of the upper plate associated with the Tohoku-Oki earthquake indicates differential stress magnitudes as small as 5–15MPa.

Stress fluctuation during thrust-related folding: Boltaña anticline (Pyrenees, Spain)

Abstract A common feature of thrust-related anticlines developing in thrust wedges is the presence of extensional structures paralleling the fold axial trend (i.e. longitudinal structures). These form in response to hinge-perpendicular stretching and indicate that the minimum stress component orients parallel to the regional shortening direction, that is, the maximum and minimum stress components locally invert. Under the assumption that the regional stress component paralleling the shortening direction is almost constant during folding, such an inversion requires a large local stress drop. In the thrust-related Boltaña anticline, folding was both predated and accompanied by the development of longitudinal extensional faults. Meso-scale contractional structures are only rarely found. Where contractional structures are present, cross-cutting relationships indicate that these structures episodically developed within a mainly extensional framework which was established during flexural bending of the foredeep and continued during thrust-related folding. We conclude that the coexistence of extensional and compressional structures relates to a stress fluctuation, with normal faulting and discontinuous fold growth occurring during ‘extensional’ and ‘compressional’ stages, respectively.

Fracture patterns in the Zagros fold-and-thrust belt, Kurdistan Region of Iraq

Fracture data have been collected in the Kurdistan Region of Iraq, which is a poorly accessible and unexplored area of the Zagros. Pre to early folding NE–SW striking extensional fractures and NW–SE striking contractive elements represent the older set affecting the exposed multilayer of the area. These latter structures are early syn-folding and followed by folding-related mesostructural assemblages, which include elements striking parallel to the axial trend of major folds (longitudinal fractures). Bedding perpendicular joints and veins, and extensional faults belonging to this second fracture set are located in the outer arc of exposed anticlines, whilst longitudinal reverse faults locate in the inner arcs. Consistently, these elements are associated with syn-folding tangential longitudinal strain. The younger two sets are related to E–W extension and NNE–SSW to N–S shortening, frequently displaying reactivation of the older sets. The last shortening event, which is described along the entire Zagros Belt, probably relates with the onset of N–S compression induced by the northward movement of the Arabian plate relative to the Eurasian Plate. In comparison between the inferred palaeostrain directions and the kinematics of recent GPS measurements, we conclude that the N–S compression and the partitioning into NW–SE trending folds and NW to N trending strike-slip faults likely remained unchanged throughout the Neogene tectonic history of the investigated area.

FIA trends along the Precambrian Rocky Mountains: a new approach to timing continental docking

AbstractA similar succession of Foliation Inflection/Intersection Axis (FIAs) trends preserved within porphyroblasts is present in two areas separated by 200 km along the Rocky Mountains. The Precambrian rocks in Central Colorado and Northern New Mexico were affected by deformation and metamorphism from ∼1506 to 1366 Ma. A succession of five FIAs trending W–E, SSW–NNE, NNW–SSE, NW–SE and WSW–ENE is distinguished in Central Colorado and dated at 1506 ± 15 Ma, 1467 ± 23 Ma, 1425 ± 18 Ma, not dated and 1366 ± 20 Ma respectively. To the south in Northern New Mexico, a succession of five FIAs trending SSW–NNE, WNW–ESE, NNW–SSE, NW–SE and WSW–ENE is distinguished and dated at 1482 ± 48 Ma, 1448 ± 12 Ma, 1422 ± 35 Ma, not dated and 1394 ± 22 Ma. The excellent correlation of the sequence of FIA trends and their ages between regions reveals a sixfold‐FIA succession across the region with the first developed FIA set in Central Colorado not present in Northern New Mexico and the third FIA set in the region not present in Central Colorado. Preferential partitioning of W–E trending deformation into the Central Colorado region ∼1506 ± 15 Ma was followed by SSW–NNE trending deformation that affected both regions at 1470 ± 20 Ma. However, preferential partitioning of WNW–ESE trending deformation into Northern New Mexico at 1448 ± 12 Ma left Central Colorado unaffected. Both regions were then affected by the three remaining periods of orogenesis, the first trending NNW–SSE at 1424 ± 15 Ma followed by one trending NW–SE that has not yet been dated, and then one trending WSW–ENE at 1390 ± 19 Ma. This suggests that the Yavapai terrane was tectonized at ∼1506 Ma, prior to amalgamation with the Mazatzal terrane ∼1470 Ma. Subsequent orogenesis was initially partitioned preferentially into the Mazatzal terrane, but the following three periods of tectonism affected both terranes in a similar manner.

Stress states in the Zagros fold-and-thrust belt from passive margin to collisional tectonic setting

The present-day Zagros fold-and-thrust belt of SW-Iran corresponds to the former Arabian passive continental margin of the southern Neo-Tethyan basin since the Permian–Triassic rifting, undergoing later collisional deformation in mid–late Cenozoic times. In this paper an overview of brittle tectonics and palaeostress reconstructions of the Zagros fold-and-thrust belt is presented, based on direct stress tensor inversion of fault slip data. The results indicate that, during the Neo-Tethyan oceanic opening, an extensional tectonic regime affectedthe sedimentary cover in Triassic–Jurassic times with an approximately N–S trend of the σ3 axis, oblique to the margin, which was followed by some local changes to a NE–SW trend during Jurassic–Cretaceous times. The stress state significantly changed to thrust setting, with a NE–SW trend of the σ1 axis, and a compressional tectonic regime prevailed during the continental collision and folding of the sedimentary cover in Oligocene–Miocene times. This compression was then followed by a strike-slip stress state with an approximately N–S trend of the σ1 axis, oblique to the belt, during inversion of the inherited extensional basement structures in Pliocene–Recent times. The brittle tectonic reconstructions, therefore, highlighted major changes of the stress state in conjunction with transitions between thin- and thick-skinned structures during different extensional and compressional stages of continental deformation within the oblique divergent and convergent settings, respectively.

New play type, southern Bonaparte Basin-Petrel Sub-basin—WA-442-P and NT/P81 exploration permits

Recent investigation, including mapping re-processed seismic data, suggests there is deeper hydrocarbon potential in the WA-442-P and NT/P81 exploration permits beneath the Early Carboniferous Tanmurra Formation horizon. Earlier interpretation of the area showed tilted fault blocks commonly thought of as economic basement in the vicinity of the Turtle and Barnett oil fields and extending to the northwest to connect with the Berkley Platform. The deep-gas play type is structural and is believed to be two nested three-way dip anticlines developed against a large bounding fault to the northeast, with axial trends northwest to southeast, and axial plane curving towards the northeast for the deeper structure. This play type is believed to be associated with structural compression and movement along the master fault with incremental re-activation most recently during the Cainozoic as recorded in overlying sediments. The Nova Structure and the deeper Super Nova structure have closures of about 450 and 550 km2, respectively. The sediments beneath the Nova horizon are believed to be of Devonian Frasnian-Famennian age but have not been drilled offshore in the Southern Bonaparte Basin (Petrel Sub-basin). Earlier work suggests that there are two petroleum systems present in the southern Bonaparte Basin, a Larapintine source from Early Palaeozoic Devonian to Lower Carboniferous source rocks, and a transitional Larapintine/Gondwana system sourced from Lower Carboniferous to Permian source rocks. Hydrocarbon charge for the structures is most likely from the Larapintine source rock intervals or yet to be identified older intervals associated with the salt deposition during the Ordovician and Silurian. Independent estimates place close to 7 TCF (trillion cubic feet) of gas in the Nova Structure. New 3D seismic data acquisition is planned over the structures to better define the geology and ultimately delineate well locations.

Chute channel dynamics in large, sand‐bed meandering rivers

ABSTRACTMeander bends of many large, sand‐bed meandering rivers are partitioned by chute channels that convey permanent flow, and co‐exist with the mainstem for decades. As a first step toward understanding the dynamics and morphodynamic implications of these ‘bifurcate meander bends’, this study applied binary logistic regression analysis to determine whether it is possible to predict chute initiation based on attributes of meander bend character and dynamics. Regression models developed for the Strickland River, Papua New Guinea, the lower Paraguay River, Paraguay/Argentina, and the Beni River, Bolivia, revealed that the probability of chute initiation at a meander bend is a function of the bend extension rate (the rate at which a bend elongates in a direction perpendicular to the valley axis trend). Image analyses of all rivers and field observations from the Strickland suggest that the majority of chute channels form during scroll–slough development. Rapid extension is shown to favour chute initiation by breaking the continuity of point bar deposition and vegetation encroachment at the inner bank, resulting in widely‐spaced scrolls with intervening sloughs that are positively aligned with primary over‐bar flow. The rivers plot in order of increasing chute activity on an empirical meandering‐braided pattern continuum defined by potential specific stream power (ωpv) and bedload calibre (D50). Increasing stream power is considered to result in higher bend extension rates, with implications for chute initiation. In addition, chute stability is shown to depend on river sediment load relative to flow discharge (Qs/Q), such that while the Beni may plot in the region of highly braided rivers by virtue of a high potential specific stream power, the formation of stable chute channels is suppressed by the high sediment load. This tendency is consistent with previous experimental studies, and results in a planform that is transitional between single‐thread meandering and braided. Copyright © 2011 John Wiley & Sons, Ltd.

Numerical Simulation on the Microstructure of Small Electroslag Remelting Ingot

The technological parameters determine the metal pool profile and the grain size and orientation in the electroslag remelting process, and thus affect the quality of the final ingot. The microstructure in the small electroslag casting process was simulated by CA method by means of the software MATLAB. Along with the remelting process, the columnar grain begins to grow to certain angle with axles, the shape of the metal pool gets closer to paraboloid shape. Improving the slag pool temperature or increasing the coefficient of heat transfer at the bottom or decreasing the coefficient of heat transfer at the side can lead the axial trend of grain growing to increase and the depth of the pool to shallow, otherwise can lead the trend of grain growing to radial direction to increase and the depth of the pool to deep.

The fracture pattern of the Sant Corneli Bóixols oblique inversion anticline (Spanish Pyrenees)

In this work we present and discuss the macro and mesostructural deformation patterns of the Late Cretaceous Sant Corneli Bóixols oblique inversion anticline (Spanish Pyrenees). This E–W striking and south-verging structure developed by inversion of an Early Cretaceous extensional fault system, formed by WNW–ESE striking faults (oriented perpendicular to the Early Cretaceous stretching direction) and transfer faults. During inversion, the shortening direction was oriented NNW–SSE, i.e. oblique to both basin margin and inherited extensional deformation structures. A pre to early-folding layer-parallel shortening (LPS) stage caused the strike-slip reworking of inherited extensional structures and the development of extensional and contractional structures oriented parallel and perpendicular to the shortening direction, respectively. Few faults clearly developed during the late stages of fold growth, in response to a stress field having principal axes parallel to those of the layer-parallel shortening event, i.e. oblique to the fold axis. WSW–ENE striking bedding-perpendicular joints and veins postdate the LPS pattern and can be associated with a syn-folding layer-parallel stretching. Analogously, few reverse faults having a top-to-the crest shear sense are associated with flexural-slip folding. Both layer-parallel stretching and shear sense provided by flexural-slip related elements are not perpendicular to the fold axial trend. All these assemblages were overprinted by a set of post-folding extensional structures oriented oblique to the strike of the anticline.Presented data show that syn-inversion fractures in the Sant Corneli Bóixols Anticline are neither parallel nor perpendicular to the fold axial trend, and their development has been controlled by inherited extensional structures and the regional shortening direction.

Submerged reefal deposits near a present‐day northern limit of coral reef formation in the northern Ryukyu Island Arc, northwestern Pacific Ocean

AbstractHigh‐resolution single‐channel seismic reflection surveys were carried out in the northern Ryukyu Island Arc during an NT05‐14 cruise of the R/V Natsushima. The survey area is located northeast of Amami Oshima and southwest of Kikai Shima immediately south of the current northern limit of coral reef formation in the northwestern Pacific Ocean. The main purpose of the surveys was to ascertain whether coral reefs formed during glacial periods and, if so, to determine their three‐dimensional distribution. We collected 784 km of high‐resolution single‐channel seismic data during the cruise. The acoustic basement is clearly observed under the Amami Spur, off Amami Oshima. The seismic profiles show two anticlines, each with an axis trending northeast to southwest. Thin Quaternary stratified sediment overlies the acoustic basement in the northwestern and southeastern slope areas, as well as between the anticlinal axes. The stratified sediment can be divided into several sedimentary units. The mound‐shaped reflections are found within the stratified sedimentary units in the central to eastern parts of the spur. Because the mound‐shaped reflections are characterized by strong reflections and chaotic internal structures in profiles, they are considered biogenic ‘reefs’ or banks consisting of coarse‐grained bioclasts. They overlie the acoustic basement or stratified sediment and reach 15 m in thickness and 400 m in width. In contrast, irregularly shaped topographic highs were detected at the shelf edge southwest off Kikai Shima, which are likely to be remnants of coral reefs formed during the last glacial period. Our seismic data indicate probable coral reef formation at low stands during glacial stages, such as the Last Glacial Maximum, even in the northern Central Ryukyus. The occurrence of drowned reefs may indicate that their growth rate was not sufficient to keep up with a rapid rise in sealevel after a glacial period.

Bed‐thickness and grain‐size trends in a small‐scale proglacial channel–levée system; the Carboniferous Jejenes Formation, Western Argentina: implications for turbidity current flow processes

AbstractPreserved in Quebrada de las Lajas, near San Juan, Argentina, is an ancient subaqueous proglacial sedimentary succession that includes a small‐scale (ca 50 m thick and ca 200 m wide) channel–levée system with excellent exposure of the channel axis and levée sediments. Coeval deposition of both the channel axis and the levées can be demonstrated clearly by lateral correlation of individual beds. The channel axis consists predominantly of a disorganized, pebble to boulder conglomerate with a poorly sorted matrix. The channel axis varies from 10 to 20 m wide and has a total amalgamated thickness of around 50 m. Beds fine gradationally away from the cobble–boulder conglomerates of the channel axis within a few metres, transitioning to well‐organized pebble to cobble conglomerates and sandstones of the channel margin. Within 60 m outboard of the channel axis in both directions, perpendicular to the trend of the channel axis, the mean grain size of the beds in the levées is silt to fine‐grained sand. Deposits in this channel–levée system are the product of both debris flows (channel axis) and co‐genetic turbidity currents (channel margins and levées). Bed thicknesses in the levées increase for up to 10 to 25 m away from the channel axis, beyond which bed thicknesses decrease with increasing distance. The positions of the bed thickness maxima define the levée crests, and the thinning beds constitute the outer levée slopes. From these relationships it is clear that the levée crest migrated both away from and toward the channel axis, and varied in height above the channel axis from 4 to 5 m (undecompacted), whereas the height of the levée crest relative to the distal levée varied from 4·5 to 10 m, indicating that the channel was at times super‐elevated relative to the distal levée. Bed thickness decay on the outside of the levée crest can be described quite well with a power‐law function (R2 = 0·85), whereas the thickness decay from the levée crest toward the channel axis follows a linear function (R2 = 0·78). Grain‐size changes are quite predictable from the channel margin outward, and follow logarithmic (R2 = 0·77) or power‐law (R2 = 0·72) decay curves, either of which fit the data quite well. This study demonstrates that, in at least this case: (i) levée thickness trends can be directly related to channel‐flow processes; (ii) individual bed thickness changes may control overall levée geometry; and (iii) levée and channel deposits can be coeval.

Pervasive lower-crustal seismic anisotropy in Southern California: Evidence for underplated schists and active tectonics

Understanding lower-crustal deformational processes and the related features that can be imaged by seismic waves is an important goal in active tectonics. We demonstrate that teleseismic receiver functions calculated for broadband seismic stations in Southern California reveal a signature of pervasive seismic anisotropy in the lower crust. The large amplitudes and small move-out of the diagnostic converted phases, as well as the broad similarity of data patterns from widely separated stations, support an origin primarily from a basal crustal layer of hexagonal anisotropy with a dipping symmetry axis. We conducted neighborhood algorithm searches for depth and thickness of the anisotropic layer and the trend and plunge of the anisotropy symmetry (slow) axis for 38 stations. The searches produced a wide range of results, but a dominant SW-NE trend of the symmetry axis emerged. When the results are divided into crustal blocks and restored to their pre–36 Ma locations, the regional-scale SW-NE trend becomes more consistent, although a small subset of the results can be attributed to NW-SE shearing related to San Andreas transform motion. We interpret this dominant trend as a fossilized fabric within schists, created from top-to-the-SW sense of shear that existed along the length of coastal California during pretransform, early Tertiary subduction or from shear that occurred during subsequent extrusion. Comparison of receiver-function common conversion point stacks to seismic models from the active Los Angeles Regional Seismic Experiment shows a strong correlation in the location of anisotropic layers with “bright” reflectors, further affirming these results.

Stress field in the 2008 Iwate-Miyagi earthquake (M7.2) area

We determined the focal mechanism solutions (FMS) of 191 crustal earthquakes as well as the stress tensor in the source area of the 2008 Iwate-Miyagi earthquake (2008 IMEQ, M7.2) that occurred in the central portion of northeast (NE) Japan. The FMS and the stress tensors were determined by using both 1-D and 3-D velocity models, which exhibit almost the same results. The differences caused by the use of 1-D and 3-D models can be neglected when compared with the differences due to the different methods, which indicates that the FMS and the stress tensor determined with a 1-D model are accurate enough to study the crustal stress field in the study region. The obtained P axis (σ1) trends WNW-ESE subhorizontally, and the T axis (σ3) is oriented subvertically in a NNE-SSW belt perpendicular to σ1. The σ1 orientation is consistent with the motion of the Pacific plate relative to NE Japan, which indicates that the plate boundary forces dominate the intraplate stress regime. Both temporal and spatial variations of the stress field in the IMEQ source area are detected, which may be induced by the stress rotation accompanying the main shock and its aftershocks. The seismogenic faults in the study area are estimated to be very weak, which argues against the concept of strong crust. The faults may be weakened by the high-temperature magma and the fluids in the lower crust and uppermost mantle that intrude upward into the shallower crust.

Geometry, kinematics and fracture pattern of the Bangestan anticline, Zagros, SW Iran

AbstractThrust-related anticlines in the Zagros Simply Folded Belt provide excellent exposed analogue structures for fractured reservoirs located in the more external sectors of the belt. In these structures it is possible to study the fracture network attributes and understand their relationships to the folding process, thus gathering fundamental information for fracture modelling in reservoirs. In this work we analyse the mesoscopic deformation pattern of the NW–SE-trending Bangestan anticline (SW Zagros, Iran) and discuss its relationship to the kinematic evolution of the hosting structure. The deformation pattern mostly includes extensional structures and pressure solution cleavages striking parallel to the fold axial trend (i.e. longitudinal), transversal extensional structures, and N–S- and E–W-striking extensional structures (oriented oblique to the fold axis). With the aid of deep wells and a transversal reflection seismic profile, we constructed a balanced cross-section of the anticline and propose a kinematic evolution pathway constrained by the mesoscopic deformation pattern. Longitudinal and transversal deformation structures developed before and/or in the very early stages of fold growth. During this stage, the Bangestan anticline grew as a set of unconnected décollement anticlines involving the Cambrian to Pliocenic sedimentary cover. In a later stage, inherited basement faults were reactivated with a right-lateral strike-slip component and the previously developed anticlines propagated laterally up to their complete linkage and thrust breakthrough. This produced the right-lateral strike-slip reactivation of longitudinal joints and the development of N–S- and E–W-striking extensional structures, which were also frequently reworked as strike-slip faults.

A model for seismicity rates observed during the 1982–1984 unrest at Campi Flegrei caldera (Italy)

We consider the space–time distribution of seismicity during the 1982–1984 unrest at Campi Flegrei caldera (Italy) where a correlation between seismicity and rate of ground uplift was suggested. In order to investigate this effect, we present a model based on stress transfer from the deformation source responsible for the unrest to potential faults. We compute static stress changes caused by an inflating source in a layered half-space. Stress changes are evaluated on optimally oriented planes for shear failure, assuming a regional stress with horizontal extensional axis trending NNE-SSW. The inflating source is modeled as inferred by previous studies from inversion of geodetic data with the same crustal model here assumed. The magnitude of the regional stress is constrained by imposing an initial condition of “close to failure” to potential faults. The resulting spatial distribution of stress changes is in agreement with observations. We assume that the temporal evolution of ground displacement, observed by a tide-gauge at Pozzuoli, was due mainly to time dependent processes occurring at the inflating source. We approximate this time dependence in piecewise-linear way and we attribute it to each component of average stress-change in the region interested by the observed seismicity. Then we evaluate the effect of a time dependent stressing rate on seismicity, by following the approach indicated by Dieterich (1994) on the basis of the rate- and state-dependent rheology of faults. The seismicity rate history resulting from our model is in general agreement with data during the period 1982–1984 for reasonable values of unconstrained model-parameters, the initial value of the direct effect of friction and the reference shear stressing rate. In particular, this application shows that a decreasing stressing-rate is effective in damping the seismicity rate.

Transverse Tectonics in the Sikkim Himalaya: Evidence from Seismicity and Focal-Mechanism Data

Abstract In the present study, about 356 local earthquakes in the region of the Sikkim Himalaya have been accurately located and analyzed using 2181 P travel times and 2161 S travel times from a network of 11 broadband seismic stations operated by the National Geophysical Research Institute during January 2006 to November 2007. Further refinement of the hypocentral parameters using the hypoDD relocation program resulted in 198 well-constrained locations. Interestingly, this study reveals several characteristic features that distinguish Sikkim from the rest of the Himalaya. The seismicity distribution is found to be confined mostly between the main boundary thrust (MBT) and the main central thrust (MCT) but not quite associated with either. While the entire Himalayan front is generally characterized by shallow-angle thrust faulting, focal mechanisms in this region are predominantly of strike-slip type in conformity with a right-lateral strike-slip mechanism along the northwest-trending Tista and Gangtok lineaments. The P -axis trends of earthquake focal mechanisms are clearly oriented north-northwest, marking a clear transition from the ambient north-northeast trending direction of Indian plate motion with respect to the Eurasian plate all along the Himalayan front. Moderate-sized earthquakes occur down to 70 km depth in this region, compared to an average focal depth of 15–20 km in the rest of the Himalaya. Also, a high average crustal P velocity of 6.66 km/sec and a fairly low b value of 0.83±0.04 are obtained indicating the probability of occurrence of a higher magnitude earthquake in the future. A north–south section in the Sikkim region shows a relatively flat topography, unlike in the rest of the Himalayan mountain chain and suggestive of lower rates of convergence in the recent geologic past. It is proposed that crustal shortening in the Sikkim Himalaya has been substantially accommodated by transverse tectonics rather than underthrusting in recent times.

Interference between shallow and deep-seated structures in the Sicilian fold and thrust belt, Italy

Abstract: The fold and thrust belt in western Sicily is characterized by the presence and interference of shallow and deep-seated compressional structures, which were generated and developed at different structural levels. The shallow structures consist of imbricated thrusts and asymmetric folds, with a typical wavelength of 2 km, involving relatively thin deep-water units. These units are superimposed on thick platform carbonate units, along a wide and originally almost flat floor thrust. The axial trend of the folds is variable, as multi-phase folding often occurred, producing a characteristic interference pattern, reflecting continuous variations of the apparent transport direction during the emplacement (i.e. rotation of the allochthonous thrust sheets). The deep-seated structures consist of large, double-verging pop-up structures, with a typical wavelength of 5–10 km, involving thicker platform carbonate successions. The deep-seated structures are characterized by large folds, with vertical to overturned limbs, caused by high-angle, transpressive ramps that reactivated previous (i.e. Mesozoic) synsedimentary normal or transtensional faults. The floor thrust of the shallow structures was passively deformed by the subsequent growth of the underlying, younger deep-seated structures. Large clockwise rotations of the tectonic units occurred during the compressional deformation, and the amount of rotation is apparently related to the timing and amount of the tectonic transport.