Blind Fault Research Articles

The 1994–2004 Al Hoceima (Morocco) earthquake sequence: Conjugate fault ruptures deduced from InSAR

We study the May 26, 1994 (Mw = 6.0) and February 24, 2004 (Mw = 6.4) earthquakes that affected the Al Hoceima region of northern Morocco. These events are the two strongest earthquakes recorded in this region. Yet, the exact location, kinematics and relationships between these earthquakes are poorly known since neither of them produced surface ruptures. Using Synthetic Aperture Radar interferometry (InSAR) we mapped the surface displacement field of the two earthquakes to characterize their seismic source parameters. Analysis of the interferograms constructed from ascending and descending orbits for both earthquakes and subsequent elastic modeling using slip inversions on triangular fault patches suggest that the two mainshocks occurred on blind conjugate strike–slip faults; the 1994 earthquake being associated with N23° E trending left-lateral fault and the 2004 earthquake with N45° W trending right-lateral fault. This result contradicts previous inferences on the kinematics, location and rupture geometry of the earthquakes deduced from conventional analyses of seismic waveforms and aftershocks distribution. The InSAR analysis reveals the fragmentation of the Rif Mountain throughout a complex network of conjugate blind faults consistent with the transpression tectonics along the plate boundary in North Africa. Although the two earthquakes took place in the Rif thrust-and-fold belt, the late Quaternary deformation indicates E–W extension in agreement with the NW–SE and NE–SW trending conjugate strike–slip faulting.

Restraining segments and reactivation of the Santa Monica–Dume–Malibu Coast fault system, offshore Los Angeles, California

A regional west striking system of surface and blind faults transects northern metropolitan Los Angeles, separating the Santa Monica Mountains from two deep sedimentary basins. The surface faults include the Santa Monica, Dume, and Malibu Coast faults. The three‐dimensional (3‐D) geometries of these faults and deformed dated strata were examined in order to determine how oblique shortening is accommodated, how structural relief grows along a mountain front, and how block translation is related to block rotation. Industry seismic reflection, well, and outcrop data were used to construct digital structure‐contour maps of several of these faults and three stratigraphic horizons that intersect them. These maps, swath bathymetry, digital elevation models, and seismicity were incorporated into a 3‐D digital database. Modeling included new approaches to determining strike‐slip displacement by separate analyses of shortening and structural relief in a restraining double bend. Stratigraphic thicknesses indicate Miocene extension across the Santa Monica and Dume faults, which have been reactivated as a single or linked moderately dipping arcuate fault that accommodates one coherent block motion via left‐lateral and left‐reverse displacement. Modeling indicates 5 km (+8/−1 km) of left‐lateral displacement on part of the Santa Monica–Dume fault and ∼11° of clockwise rotation of the Santa Monica Mountains during the last ∼4 ± 1 m.y. Modern displacement rates modeled from GPS data are similar to our modeled post‐∼4 Ma rates of westward escape and clockwise rotation of the Santa Monica Mountains.

2-D deformation of two welded half-spaces due to a blind dip-slip fault

The solution of two-dimensional problem of an interface breaking long inclined dip-slip fault in two welded half-spaces is well known. The purpose of this note is to obtain the corresponding solution for a blind fault. The solution is valid for arbitrary values of the fault-depth and the dip angle. Graphs showing the variation of the displacement field with the distance from the fault, for different values of fault depth and dip angle are presented. Contour maps showing the stress field around a long dip-slip fault are also obtained

Coseismic Fault Rupture Detection and Slip Measurement by ASAR Precise Correlation Using Coherence Maximization: Application to a North–South Blind Fault in the Vicinity of Bam (Iran)

Using the phase differences between satellite radar images recorded before and after an earthquake, interferometry allows mapping the projection along the line of sight (LOS) of the ground displacement. Acquisitions along multiple LOS theoretically allow deriving the complete deformation vector; however, due to the orbit inclination of current radar satellites, precision is poor in the north-south direction. Moreover, large deformation gradients (e.g., fault ruptures) prevent phase identification and unwrapping and cannot be measured directly by interferometry. Subpixel correlation techniques using the amplitude of the radar images allow measuring such gradients, both in slant-range and in azimuth. In this letter, we use a correlation technique based on the maximization of coherence for a radar pair in interferometric conditions, using the complex nature of the data. In the case of highly coherent areas, this technique allows estimating the relative distortion between images. Applied to ASAR images acquired before and after the December 26, 2003 Bam earthquake (Iran), we show that the near-field information retrieved by this technique is useful to constrain geophysical models. In particular, we confirm that the major gradients of ground displacement do not occur across the known fault scarp but approximately 3 km west of it, and we also estimate directly the amplitude of right lateral slip, while retrieving this value from interferometry requires passing through the use of a model for the earthquake fault and slip.

Style and sequence of deformation during extensional fault-propagation folding: examples from the Hammam Faraun and El-Qaa fault blocks, Suez Rift, Egypt

Kilometre-scale fault-parallel folds are identified adjacent to normal faults in the Oligo-Miocene Suez Rift, Egypt and are interpreted to have formed in response to fault-propagation folding above upward propagating blind faults. The geometry, scale and distribution of secondary structures within the folds and their cross-cutting relationships with the master faults allow the style and sequence of deformation during fault-propagation folding to be established and suggest that during the initial stages of folding, the proto-footwall underwent extension which was accommodated by layer-parallel slip in encasing mudstone horizons and linked normal faulting and block rotation in carbonate and sandstone units. The proto-hanging wall also contains dominantly extensional normal faults although locally, where the master fault had a convex-into-the-footwall map-view trace, reverse faulting and fracturing occurred. Secondary structures adjacent to the master fault were not all active simultaneously, but initiated and died at different stages during the evolution of the fault-propagation fold. The results of this study confirm many key predictions of numerical and physical analogue models but also highlight several important controls on the evolution of fault propagation folds in extensional settings which existing models cannot capture, such as the influence of the map-view trace of the propagating fault and lateral variations in cover stratigraphy lithology and strength on the style and magnitude of secondary deformation.

Propagation of normal faults to the surface in basalt, Koae fault system, Hawaii

Normal fault scarps of the Koae fault system on Kilauea volcano consistently display locally breached monoclines underlain by prominent cavities, deep gaping fissures on the footwall, finer fissures on the hanging wall, and buckles at the scarp base. Elastic analyses reveal that this assemblage forms as a fault propagates up towards the surface rather than down from it. Models of a planar blind normal fault with a dip exceeding 60° yield a monocline with a tensile stress concentration at the surface where gaping fissures occur, a stronger subsurface tensile stress concentration near the blind fault tip, where cavities occur, and a compressive stress concentration at the surface where buckles occur. The footwall fissures grow down from the tensile stress concentration at the surface and link with a fault as its scarp grows. In contrast, the cavities initiate at depth near the fault tip and propagate with it up towards the surface. The hanging wall fissures apparently open in response to slip on late-forming blind antithetic faults near the surface. Stoped blocks derived from footwall fissure walls help prop the footwall fissures open as a normal fault breaches the surface. The fissures, cavities, and scarp rubble provide highly conductive hydraulic pathways.

3-D seismic structure of the source area of the 1993 Latur, India, earthquake and its implications for rupture nucleations

The Latur earthquake ( M w 6.1) of 29 September 1993 is a rare stable continental region (SCR) earthquake that occurred on a previously unknown blind fault. In this study, we determined detailed three-dimensional (3-D) P- and S-wave velocity ( V p, V s) and Poisson's ratio ( σ) structures by inverting the first P- and S-wave high-quality arrival time data from 142 aftershocks that were recorded by a network of temporary seismic stations. The source zone of the Latur earthquake shows strong lateral heterogeneities in V p, V s and σ structures, extending in a volume of about 90 × 90 × 15 km 3. The mainshock occurred within, but near the boundary, of a low- V p, high- V s and low- σ zone. This suggests that the structural asperities at the mainshock hypocenter are associated with a partially fluid-saturated fractured rock in a previously unknown source zone with intersecting fault surfaces. This might have triggered the 1993 Latur mainshock and its aftershock sequence. Our results are in good agreement with other geophysical studies that suggest high conductivity and high concentration of radiogenic helium gas beneath the source zone of the Latur earthquake. Our study provides an additional evidence for the presence of fluid related anomaly at the hidden source zone of the Latur earthquake in the SCR and helps us understand the genesis of damaging earthquakes in the SCR of the world.

The case for seismic superiority of well‐engineered tall buildings

AbstractThe objective of this paper is to bring the lingering debate over whether more rigid or more flexible buildings fare better during major earthquakes to an end. Historical data, design experience, and ground motions obtained from simulation of a magnitude 7·1 blind fault rupture directly below downtown Los Angeles are utilized in this study. Assuming the same level of attention to design, detailing, and construction workmanship is applied to buildings of all heights resulting in well‐engineered buildings, the results indicate that well‐engineered tall buildings are substantially safer than shorter, stiffer, buildings when subjected to strong earthquake ground motions. This is true even in the case of strong ground motions containing significant directivity and near‐source velocity pulses. Copyright © 2005 John Wiley & Sons, Ltd.

The 2003 Bam, Iran, Earthquake: An Interpretation of the Strong Motion Records

On 26 December 2003, a destructive earthquake occurred in southeastern Iran, demolishing the city of Bam and vicinity. The highest intensity of shaking (VIII–IX) was observed in the city of Bam. The source of this shock was reported to have had a right-lateral strike-slip mechanism initiated in a blind fault in the north-south direction. A regional network consisting of 23 strong motion stations (SSA-2 Accelerograph), located within 1–290 km from the epicenter, registered the earthquake. The compact and pulse-shape arrivals of strong signals recorded at the Bam station strongly suggest that the rupture was initiated south of the city and propagated toward Bam. Based on the relative arrival times of the rupture front and the arrivals of P and S waves at this station, the velocity of rupture was estimated as 2.5±0.2 km/sec. Comparisons made between the attenuation curves constructed for this earthquake and those of the regional curves show that the effects of directivity caused significant deviations at near distances from the fault. This strong motion data yields estimates of source parameter values of 8.3×025(dyne-cm), 7.5 km, and 90 bars, respectively, for seismic moment, source radius, and stress drop.

Characteristics of Near-Fault Ground Motions by Dynamic Thrust Faulting: Two-Dimensional Lattice Particle Approaches

Two-dimensional lattice particle models have been used to simulate dynamic thrust faulting and its effects on near-fault ground motions. The lattice particle modeling approach has been demonstrated as an efficient way to model the dynamic rupture phenomena observed from a foam rubber experiment on dipping faults. We constructed a 42° dipping fault model to simulate near-fault ground motions under different site conditions in which different combinations of fault geometry (blind fault underlying a sedimentary layer and outcropping thrust models) and near-surface sedimentary layers were investigated. In particular, dynamic behaviors between a blind fault with an overlying sedimentary layer and an outcropping fault without a sedimentary layer were compared. In this simulation, a dynamic slip pulse accompanied by fault separation was initiated at the deepest part of the fault and propagated updip along the fault under a subshear rupture velocity. A strong asymmetrical ground-motion pattern on the hanging wall and footwall, caused by near-source rupture effects, was observed. Rupture directivity played an important role in determining the size and distribution of peak ground velocities and accelerations on the hanging wall and footwall. In the case of an outcropping thrust without a sedimentary layer, the hanging wall underwent a stronger ground motion, caused by the near-surface breakout phase, as the rupture reached the free surface. In the case of a blind thrust overlying a sedimentary layer, the peak ground particle velocity and acceleration could be much larger on the footwall. This is a result of the amplification effect of trapped long-period seismic energy in the sedimentary layer. The seismic energy emitted from the rupture-stopping phase was incident to the sedimentary layer and radiated under the rupture directivity effect. The radiated long-period seismic energy was trapped in the sedimentary layer, propagating away from the fault trace toward the footwall side. The numerical results show that, for an initial slip of 5 to 5.5 m, the horizontal peak ground velocity and acceleration could reach about 1.5 m/sec and 2 g , respectively, on the footwall for a blind thrust with a sedimentary layer. With the same initial slip level, the peak ground velocity and acceleration were about 1. 1 m/sec and 1.5 g , respectively, on the hanging wall for an outcropping thrust without a sedimentary layer. These results can partially explain the field observations of precarious rock distributions and overturned transformers in the vicinity of the White Wolf Fault during the 1952 M s 7.6 Kern County earthquake. Furthermore, the current simulation can be used for near-fault strong-motion prediction for large thrust faults in the Los Angeles Basin or similar tectonic settings around the world.

Seismicity increase after the construction of the world's tallest building: An active blind fault beneath the Taipei 101

Seismic activity began to slightly increase during the construction but rose sharply upon the completion of the tallest building in the world, the Taipei 101, which stands at 508 m in the Taipei basin, where local seismicity had historically very low. Besides an increase in both seismic energy and the number of micro‐earthquakes, two felt earthquakes astonishingly occurred beneath the completed building. The focal mechanism of the larger felt earthquake and its aftershocks are indicative of an active blind normal fault just beneath the building. Estimations of the vertical loading of the Taipei 101 show that local stress on its foundation increased at least 4.7 bars, making it seem likely that the increased seismicity was a direct product of the loading of the mega‐structure. Further investigations in unison with continuous seismic monitoring must be conducted because the safety of the high‐rise building in the Taipei basin be comprehensively assessed.

Neotectonics of the Somogy hills (Part II): Evidence from seismic sections

The Somogy hills are located in the Pannonian Basin, south of Lake Balaton, Hungary, above several important tectonic zones. Analysis of industrial seismic lines shows that the pre-Late Miocene substratum is deformed by several thrust faults and a transpressive flower structure. Basement is composed of slices of various Palaeo-Mesozoic rocks, overlain by sometimes preserved Paleogene, thick Early Miocene deposits. Middle Miocene, partly overlying a post-thrusting unconformity, partly affected by the thrusts, is also present. Late Miocene thick basin-fill forms onlapping strata above a gentle paleo-topography, and it is also folded into broad anticlines and synclines. These folds are thought to be born of blind fault reactivation of older thrusts. Topography follows the reactivated fold pattern, especially in the central-western part of the study area. The map pattern of basement structures shows an eastern area, where NE–SW striking thrusts, folds and steep normal faults dominate, and a western one, where E–W striking thrusts and folds dominate. Folds in Late Neogene are also parallel to these directions. A NE–SW striking linear normal fault and associated N–S faults cut the highest reflectors. The NE–SW fault is probably a left-lateral master fault acting during–after Late Miocene. Gravity anomaly and Pleistocene surface uplift maps show a very good correlation to the mapped structures. All these observations suggest that the main Early Miocene shortening was renewed during the Middle and Late Miocene, and may still persist. Two types of deformational pattern may explain the structural and topographic features. A NW–SE shortening creates right-lateral slip along E–W faults, and overthrusts on NE–SW striking ones. Another, NNE–SSW shortening creates thrusting and uplift along E–W striking faults and transtensive left-lateral slip along NE–SW striking ones. Traces of both deformation patterns can be found in Quaternary exposures and they seem to be consistent with the present day stress orientations of the Pannonian Basin, too. The alternation of stress fields and multiple reactivation of the older fault sets is thought to be caused by the northwards translation and counter-clockwise rotation of Adria and the continental extrusion generated by this convergence.

Neotectonic architecture of Taiwan and its implications for future large earthquakes

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

Lessons on the calculation of static stress loading from the 2003 Bingol, Turkey earthquake

The 2003 Bingol earthquake ( M w = 6.4) occurred very close to a region along the east Anatolian fault zone which was identified in 2002 as posing a particularly high seismic risk. This damaging earthquake occurred on a conjugate right-lateral blind fault that was inconsistent with the stress-change field calculated for preceding large earthquakes in the region. In this paper, four reasons which might be responsible for this stress discrepancy are identified and investigated individually. Firstly, co-seismic stress changes are considered. The time frame of the previous stress calculations is extended to include the large earthquakes in the 1780s which were not included in the earlier study. A sensitivity analysis is then conducted on the more recent events since 1822 to examine the effect of errors in their location and sizes. The possibility of the occurrence of a small magnitude event close to the 2003 epicentre is considered. We argue that, barring the occurrence of a low-probability, unmodelled local event, the Bingol earthquake was unlikely to have been triggered by co-seismic stress transfer from any known sequence of previous earthquakes. Finally we examine and modify the secular loading model used in the 2002 study and show that loading which is properly constrained by regional GPS data produces a positive stress change on the 2003 rupture. As a result of our examination of the stressing history of the Bingol hypocentre we argue that it is through a combination of historical seismology, guided and constrained by structural geology, directed paleoseismology in which the locations and extent of historical events are confirmed, and stress modelling which has been informed by detailed GPS data, that an integrated seismic hazard program might have the best chance of success.

Three-dimensionalVPandVSstructural models associated with the active subduction and collision tectonics in the Taiwan region

SUMMARY 3-D VP and VS models for the crust and upper mantle beneath the Taiwan area have been determined using selected high-resolution earthquake data from an island-wide seismic network and two local seismic arrays. Lateral structural variations in the upper crust, as also evident from surface geology, are responsible for the observed large traveltime residuals or station corrections. Prior shallow velocity information inferred from traveltime residuals and joint hypocentral determination (JHD) station corrections for the uppermost crust is essential to facilitate a reliable tomographic inversion. A finite-difference method, that is efficient and accurate for a highly heterogeneous velocity structure, is applied to calculate P- and S-wave traveltimes from the source to receiving stations. All earthquakes in the Taiwan Central Weather Bureau’s catalogue are then relocated using the resultant 3-D VP and VS models. The depth of the Moho varies significantly, especially along the east‐west direction. In the western Coastal Plain and Western Foothills the depth of the Moho is around 35 km, which deepens gradually eastward, reaches a maximum depth of ∼55 km beneath the eastern Central Mountain Range, shallows up rapidly beneath the Longitudinal Valley and Coastal Range, and merges with the thin Philippine Sea Plate offshore of eastern Taiwan. In central Taiwan, the Central Mountain Range is bounded to the east and west by two steeply westward dipping active faults from the upper crust to a depth of about 30 km. Therefore, the uplifted and thickened Central Mountain Range serves as a backstop for the converging Eurasian and Philippine Sea plates. The crust beneath the Central Mountain Range is characterized by a brittle, high-velocity and seismically active upper crust (<15 km) and a ductile, low-velocity and aseismic mid-to-lower crust (below 15 km), most probably due to the high geothermal activity from the excess heat supplied from the hot upper mantle beneath the thin oceanic crust to the east, from the surrounding hotter upper mantle beneath the thickened continental crust, and from shear heating during active collision. The collision zone in eastern Taiwan is characterized by an active and steeply eastward dipping seismic zone along a region of low VP and high VP/VS ratio near the Taitung region in southeastern Taiwan. It transforms into an active westward steeply dipping seismic zone along a transition zone between the high VP and VS oceanic crust and the low VP and VS continental crust near Hualien region in central eastern Taiwan. There is no apparent seismicity within many sedimentary basins imaged from the tomographic inversion. However, af ew basins are either bounded on one side by an active fault or are characterized by blind faults beneath. The geometry of the subduction zone in northeastern Taiwan can be clearly imaged from the relocated earthquake locations. Behind the subduction, a region of low VP and high VP/VS ratio at depths of 5 to 10 km can be identified beneath the Tatun-Chilung volcano group indicating a potential magma reservoir. Two steeply dipping linear seismic zones in the volcano region may mark the upward escape paths of the magmatic materials in the region.

Relocation of Small EarthquakesiIn Western Sichuan, China and Its Implications for Active Tectonics

We relocated 13367 small earthquakes that occurred in western Sichuan between 1992 and 2002 using a double-difference (DD) earthquake location algorithm to improve the relative location precision. The relocated microseismicity forms highly organized structures that correlate with the surface faulting well: showing the flower structure across the simple strike-slip fault, and the organized but dispersed structure across the pull-apart basin and thrust fault. Beneath thrust fault, there exists an aseismic layer. The seismicity shows segmentation characteristics on active fault zones. Other significant tectonic features that were previously obscured by routine location errors are also revealed by the relocated seismicity. Some blind faults are delineated by lineages of seismicity that are suggestive of faulting structures. A series of large voids in seismicity appear with dimensions of tens of kilometers on the Xianshuihe-Anninghe-Zemuhe fault zone that have been aseismic over the 10-year time interval, suggesting that these segments may be locked and storing strain energy for release in future large earthquakes. A 5km thick aseismic layer appears in most places in western Sichuan plateau at depths of 15~20km. The crustal strength envelopes are calculated for the western Sichuan plateau based on the results from hightemperature and high-pressure experiments. The result shows that the granite at depths of 14 to 19km appears to be ductile, which is in good agreement with the thickness and depth range of the aseismic layer, suggesting that the aseismic layer may be the result of the ductile deformation from the granite in the upper crust.

A note on 2-D lithospheric deformation due to a blind strike-slip fault

Analytical solution for the problem of a surface-breaking long strike-slip fault in an elastic layer overlying an elastic half-space is well known. The purpose of this note is to obtain the corresponding solution for a blind fault. Since the solution is valid for arbitrary values of the fault-depth and the dip angle, the effects of these two important fault parameters can be studied numerically. The variation of the parallel displacement and shear stress with the distance from the fault is studied numerically for different values of the fault-depth and dip angle.

Horizontal coseismic deformation of the 2003 Bam (Iran) earthquake measured from SPOT‐5 THR satellite imagery

On 26 December 2003, a destructive Mw 6.5 earthquake occurred in Kerman province, Iran, killing more than 30,000 people around the ancient city of Bam. We map the fault zone and determine the slip distribution using sub‐pixel correlation of images taken by the SPOT‐5 THR sensor before and after the main shock. Our results show that the surface displacement occurred on a blind fault oriented N174 and located South of Bam. A 15 km along‐strike profile depicts a distribution with a maximum displacement of 1.2 ± 0.15 m near the centre of the fault. The mean slip that we infer (∼0.77 ± 0.05 m) is much higher than the ground displacement measured in the field (∼0.2 m). We suggest that field‐measured displacements are underestimated due to slip accommodation within an 500 m wide shear zone. Furthermore, we demonstrate for the first time the relevancy of 2.5 m SPOT‐5 THR images for sub‐pixel ground displacement measurements.

Three terrane-defining thrusts of the Kumaun Himalaya: controversies on position and nomenclature

Three of the five thrust fault systems defining the boundaries of four lithotectonic terranes of the Himalaya and involved in controversies related to their positions and nomenclature, are the objects of discussion in this paper. Youngest of the five terrane-defining faults, the Himalayan Frontal Fault (HFF) is a series of reverse faults that demarcates the boundary of the Siwalik front of the Himalayan province with the alluvial expanse of the Indo-Gangetic Plains. Over large tracts, it is either concealed under younger sediments or has as yet not reached the ground surface and is therefore a blind fault. The nature of this frontal fault varies along its length. Where the hidden ridges of the Indo-Gangetic basement impinge the Himalaya, the mountain front is ruptured and the HFF is repeatedly reactivated. In the sectors intervening these ridges, it is not expressed on the surface, but the ground of the adjoining Indo-Gangetic Plain is sinking, the rivers are shifting their courses and large tracts of land are waterlogged and characterized by The Vaikrita Thrust is the plane that marks pronounced metamorphic break and abrupt change in style and orientation of structures within the succession of crystalline rocks that build the bulk of the snowy ranges in the Kumaun Himalaya. Not only is there a jump of pressure of the order of 4 kb and a temperature rise of &gt;200 oC, but also is there a conspicuous change of neodymium isotope value across the tectonic plane that separates the low-grade metarnorphics in the lower part from the high-grade metamorphic rocks of the upper part of the Great Himalayan succession. The Vaikrita Thrust is therefore recognised as the Main Central Thrust (MCT). While the basal low-grade metamorphic assemblage comprises 1900±100 Ma old highly tectonised porphyritic granite characterised by low initial strontium isotope ratio, the upper high -grade metamorphic group is intruded by 20±1 Ma old anatectic granites characterised by garnet, kyanite, sillimanite and cordierite, and a high but variable value of strontium isotope ratio. Moreover, the anatectic Lower Miocene granites are singularly absent in the succession of the Lesser Himalayan nappes. Probably it is this thrust that has flexed downwards the plane of decoupling and displacement between the under thrusting Indian plate and the overlying Himalayan mass.&#x0D; The terrane-defining fault between the high-grade metamorphics with granitic rocks of the Himadri (i.e., Great Himalaya) and the Tethyan sedimentary pile was recognised as the Malari Thrust Fault in the northern Kumaun Himalaya in the early seventies, as the South Tibetan Detachment System in southern Tibet adjoining north-eastern Nepal in the early eighties and as the Zanskar Shear Zone in north-western Himachal Pradesh in the late eighties. Not only is there an abrupt change in the metamorphic grade across the tectonic plane, but also an attenuation and wholesale elimination of some lithostratigraphic formations of the hanging wall besides the difference in the style of deformation. Exhibiting predominant dip-slip movement in the central sector of the Himalayan arc, the Trans-Himadri Fault (T-HF) was formed as a consequence of the Tethyan sedimentary cover detaching from its rigid foundation of the basement complex that was squeezed up following blocking or slowing down of tectonic movements related to India-Asia convergence. The sedimentary cover lagging behind the thrust­ up basement complex slid down and toppled over northward and gave rise to back-folds and back-thrusts. In Kashmir, western Himachal Pradesh, central Nepal and north-western Bhutan, in sharp contrast, there was very strong compression, so that the Tethyan sedimentary rocks-along with a slice of the low-grade metamorphic basement in the hanging wall advanced southwards across the Himadri and were emplaced as nappes and klippen south of the Main Central Thrust. The T-HF movement occurred approximately around 20.9 Ma, although the movement had started quite earlier in some places. Quaternary reactivation resulted in river ponding and development of huge lakes such as the Garbyang palaeolake in the Kali valley and the ~ 40,000 year-old Goting palaeolake in the Western Dhauli Valley.

Geochemical signatures of large active faults: The example of the 5 February 1783, Calabrian earthquake (southern Italy)

Five large earthquakes shook southern Calabria in February–March 1783. We focused on the first shock (Me 6.9), which occurred on 5 February in the Gioia Tauro Plain. Most investigators attribute the event to a W-dipping, high-angle fault running at the base of the Aspromonte crystalline bedrock on the ESE side of the Plain (Aspromonte Fault). Other workers contend that the earthquake was generated by an E-dipping, low-angle blind fault (Gioia Tauro Fault) similar to the adjacent Messina Straits Fault. In 1999–2000 we carried out four geochemical surveys in the Gioia Tauro Plain with the aim of contributing to this debate with an independent line of evidence. We sampled 240 groundwater sites and measured a suite of in-situ physical and chemical parameters. Our goal was to gain new insight into the seismogenic source by identifying geochemical anomalies associated with the deepening of the hydrological circuits due to the presence of enhanced faulting/fracturing. The deep-fluid signatures are mainly represented by temperature, salinity, total carbon and radon anomalies. We identified three zones of dominant deep fluid discharge: the Nicotera-Galatro area (along the Nicotera-Galatro portion of the NW-trending Nicotera-Gioiosa Jonica lineament), a small NW-SE trending area between Gioia Tauro and Seminara, and the coastline between Rosarno and Palmi. This latter sector locates just above the upper edge of the hypothesised Gioia Tauro Fault. Most of the geochemical anomalies are recorded around Rosarno, at the intersection between the Gioia Tauro Fault and the Nicotera-Gioiosa Jonica lineament. In contrast, no evidence of groundwater deepening and active fracturing was found along the Aspromonte Fault. Based on our new findings we updated the concepts of Geochemically Active Fault Zone and Geochemical Interaction Fault Zone in view of the modern understanding of the hydro-mechanical properties of fault zones and the faulting mechanisms promoting fracture permeability in the crust.