Showing papers on "Fault (geology) published in 1991"

The Mechanics of Earthquakes and Faulting

Abstract: This essential reference for graduate students and researchers provides a unified treatment of earthquakes and faulting as two aspects of brittle tectonics at different timescales. The intimate connection between the two is manifested in their scaling laws and populations, which evolve from fracture growth and interactions between fractures. The connection between faults and the seismicity generated is governed by the rate and state dependent friction laws - producing distinctive seismic styles of faulting and a gamut of earthquake phenomena including aftershocks, afterslip, earthquake triggering, and slow slip events. The third edition of this classic treatise presents a wealth of new topics and new observations. These include slow earthquake phenomena; friction of phyllosilicates, and at high sliding velocities; fault structures; relative roles of strong and seismogenic versus weak and creeping faults; dynamic triggering of earthquakes; oceanic earthquakes; megathrust earthquakes in subduction zones; deep earthquakes; and new observations of earthquake precursory phenomena.

Lateral extrusion in the eastern Alps, PArt 2: Structural analysis

Abstract: The late Oligocene-Miocene tectonic style of the Alps is variable along strike of the orogen. In the Western and Central Alps, foreland imbrication, backthrusting, and backfolding dominate. In the Eastern Alps, strike-slip and normal faults prevail. These differences are due to lateral extrusion in the Eastern Alps. Lateral extrusion encompasses tectonic escape (plane strain horizontal motion of tectonic wedges driven by forces applied to their boundaries) and extensional collapse (gravitational spreading away from a topographic high in an orogenic belt). The following factors contributed to the establishment of lateral extrusion in the Eastern Alps: (1) a rigid foreland, (2) a thick crust created by indentation and earlier collision, (3) a decrease in strength in the crust due to thermal relaxation, (4) a crustal thickness gradient from the Eastern Alps to the Carpathians, and, possibly, (5) a disturbance of the lithospheric root. Northward indentation by the Southern Alps causes thickening in and in front of the indenter and tectonic escape. Gravitational spreading attenuates crustal thickness differences. Indentation structures occur in the western Eastern Alps and comprise folds, thrusts, and strike-slip faults. These structures pass laterally into spreading structures, which encompass transtensional and normal faults in the eastern Eastern Alps. The overall structural pattern is dominated by escape structures, namely, sets of strike-slip faults that bound serially extruding wedges. Structural complexity arises from (1) interference of major fault sets, (2) accommodation of displacement differences between the Eastern Alps and their fore- and hinterland, (3) displacement transfer from the Eastern Alps toward the Carpathians which act as a lateral unconstrained margin, and (4) crustal decoupling, which partitions extrusion into brittle upper plate and ductile lower plate deformation. The kinematics of lateral extrusion is approximated by an extrusion-spreading model proposed for nappe tectonics.

Lateral extrusion in the eastern Alps, Part 1: Boundary conditions and experiments scaled for gravity

Abstract: Lateral extrusion encompasses extensional collapse (gravitational spreading away from a topographic high in an orogenic belt) and tectonic escape (plane strain horizontal motion of wedges driven by forces applied to their boundaries). In the Eastern Alps it resulted from (1) an overall northerly compression (Apulia against Eurasia), (2) a strong foreland (Bohemian massif), (3) lack of constraint along a lateral boundary (Carpathian region), and (4) a previously thickened, gravitationally unstable, thermally weakened crust (Eastern Alpine orogenic belt). Six indentation experiments reproduce lateral extrusion at lithospheric scale. The models have two to four lithospheric layers, with a Mohr/Coulomb rheology for the upper and a viscous rheology for the lower crust. The lithosphere rests upon a low-viscosity asthenosphere. A broad indenter, a narrow deformable area, and a weakly constrained eastern margin fullfill as closely as possible conditions in the Eastern Alps. Indentation produces both thickening in front of the indenter and escape of triangular wedges. Lateral variations in crustal thickness become attenuated by gravitational spreading. The overall fault pattern includes domains of reverse, strike-slip, oblique normal, and pure normal faults. Strike-slip faults in conjugate sets develop serially. The narrow width of the deformable area and the strength of the foreland determine the angles between the sets. Gravitational spreading produces a rhombohedral pattern of oblique and pure normal faults along the unconstrained margin. Opposite the unconstrained margin, the indenter front shows thrusts and folds intersecting with the conjugate strike-slip sets. A triangular indenter favors spreading. High velocity of indentation favors escape. High confinement limits lateral motion, inhibits spreading, and favors thickening. Lateral extrusion in the Eastern Alps is best modeled by (1) a weak lateral confinement, (2) a broad and straight indenter, (3) a narrow width of the deformable area, and (4) a rigid foreland. Crustal thickening, lateral escape, and gravitational spreading all contribute to the overall deformation.

Deep-focus earthquakes and recycling of water into the earth's mantle

Abstract: For more than 50 years, observations of earthquakes to depths of 100 to 650 kilometers inside Earth have been enigmatic: at these depths, rocks are expected to deform by ductile flow rather than brittle fracturing or frictional sliding on fault surfaces Laboratory experiments and detailed calculations of the pressures and temperatures in seismically active subduction zones indicate that this deep-focus seismicity could originate from dehydration and high-pressure structural instabilities occurring in the hydrated part of the lithosphere that sinks into the upper mantle Thus, seismologists may be mapping the recirculation of water from the oceans back into the deep interior of our planet

The Appalachian-Ouachita rifted margin of southeastern North America

Abstract: Promontories and embayments along the late Precambrian-early Paleozoic Appalachian-Ouachita continental margin of south-eastern North America are framed by a northeast-striking rift system offset by northwest-striking transform faults. Inboard from the continental margin, basement fault systems have two sets of orientation; one is northeast parallel with rift segments, and the other is northwest parallel with transform faults. Late Precambrian clastic and volcanic syn-rift rocks overlie Precambrian basement rocks along the Appalachian Blue Ridge. Lower Cambrian sandstone at the base of a transgressive passive-margin succession over-steps the rift-fill successions and basement rocks, defining the time of transition from an active rift to a passive margin along the Blue Ridge. Locally thick Early Late Cambrian and older sedimentary rocks fill downthrown blocks of the intracratonic Mississippi Valley-Rough Creek-Rome graben system and Birmingham basement fault system. These basement fault systems, which indicate north-west-southeast extension like the Blue Ridge rift, are overstepped by Upper Cambrian strata. The northwest-striking Southern Oklahoma fault system is interpreted to be a transform fault that propagated into the continent from the Ouachita rift. Early and Middle Cambrian rift-related igneous rocks along the fault system and adjacent Precambrian basement are overstepped by Upper Cambrian sandstone. The differences in age of rift-related rocks suggest a spreading-center shift at the beginning of the Cambrian Period from the Blue Ridge rift to the Ouachita rift southwest of the Alabama-Oklahoma transform fault. From Early to Early Late Cambrian, a small component of extension propagates north-eastward to form the intracratonic fault systems northeast of the transform fault, but most of the extension of the Ouachita rift was transformed along the Alabama-Oklahoma transform fault to the Mid-Iapetus Ridge outboard from the Blue Ridge passive margin.

Field study of a highly active fault zone: The Xianshuihe fault of southwestern China

Abstract: The Xianshuihe fault of western Sichuan Province, China, is one of the world's most active faults, having produced 4 earthquakes during this century of magnitude ≥7 along a 350-km length of the fault. At least 8 such events have occurred since 1725. In the more limited 150-km-long segment including Luhuo and Daofu, major earthquakes in 1904, 1923, 1973, and 1981 (M = 7, 7½, 7.6, 6.9) were associated with overlapping surficial fault ruptures and with individual left-lateral displacements as large as 3.6 m. Field studies indicate that this high degree of activity is typical of the fault's longer-term history. The Holocene left-lateral slip rate on the north-western segment of the fault has been 15 ± 5 mm/yr, decreasing to about 5 mm/yr on its southeastern segment, based on radiometrically dated offset stream-channel and terrace deposits and on offset glacial moraines. Physiographic features of active faulting are fully as diagrammatic as those of California's San Andreas fault, mainly because of high-altitude preservation and the absence of cultural modification on this eastern margin of the Tibetan Plateau. Detailed en echelon tensional and pushup features resulting from surface ruptures in 1973, 1955, 1923, and 1893 can still be recognized today, and new data have been collected bearing on the offsets and fault-rupture lengths during these and other events. The locations and magnitudes of historic earthquakes suggest that the characteristic earthquake model may apply to the Xianshuihe fault. Obvious geometric segmentation of the fault has controlled the initiation and termination of ruptures in some events, whereas segmentation control for others remains obscure. Based on the historic record, repeat times estimated from slip rates, and current seismic gaps, two segments are particularly likely sites for M = 7+ earthquakes in the near future: the 65-km-long segment between Daofu and Qianning, and the 135-km-long segment bracketing Kangding. Continuing creep has been documented along some segments of the fault, and this, together with the high degree of activity and other unique attributes, makes the Xianshuihe fault one of the most promising sites in the world for earthquake prediction and hazard-evaluation studies.

Geology of the Haiyuan Fault Zone, Ningxia‐Hui Autonomous Region, China, and its relation to the evolution of the Northeastern Margin of the Tibetan Plateau

Abstract: The Haiyuan area, located along the northeastern margin of the Tibetan Plateau in north central China, provides a laboratory for studying how the plateau has grown in late Cenozoic time. Rocks in the area range from pre-Silurian (Precambrian?) to Recent; the pre-Silurian and Cenozoic rocks form the most extensive outcrops. The pre-Silurian rocks consist of amphibolite- and greenschist-grade metasedimentary and metaigneous rocks unconformably overlain by Silurian and Devonian red beds. All of these rocks are intruded by granodiorite of unknown age. Cenozoic rocks consist of 2.6–3.0 km of Eocene to Miocene red beds that were deposited over an extensive area in this part of China. Pliocene conglomerate contains clasts from all older formations and is interpreted to have been derived from highlands developed during the beginning of Cenozoic deformation in the Haiyuan area. Except for the widespread loess deposits, Quaternary rocks reflect deposition in local tectonic environments. The oldest Cenozoic structures in the Haiyuan area are folds and small thrust faults that generally strike N30°–45°W and involve mostly pre-Quaternary rocks. These structures and all the Quaternary rocks are cut by the Haiyuan left-lateral strike-slip (left-slip) fault zone that generally trends N60°–65°W and is nearly vertical. At the western end of the mapped area a fault zone, which strikes N75°–90°W, forms a left-stepping transfer zone that connects with another segment of the Haiyuan fault zone, which continues N60°–65°W west into Gansu Province. A small basin, the Salt Lake Basin, is marked by active faults in the area of the transfer zone and is interpreted as a pull-apart basin along the left-slip Haiyuan fault zone. At its eastern end the Haiyuan fault zone has an irregular surface trace; east of Luzigou an active fault striking N35°–45°W branches to the south. This southern branch appears to be a younger fault and now accommodates most of the left-slip deformation that formerly occurred on the easternmost part of the Haiyuan fault zone. This younger fault connects through a left-stepping transfer zone to a parallel fault, the Xiaokou fault, that can be traced into the Liupan Shan about 60 km to the southeast. The Laohuyaoxian Basin is interpreted as a very young pull-apart basin in the area of the transfer zone. Matching different geological features across the Haiyuan fault zone yields a total left-slip offset of between 10.5 and 15.5 km, and the best constrained offsets yield 12.9–14.8 km. If left slip began near the end of the Pliocene time or earliest Pleistocene time, it indicates an average slip rate between 5 and 10 mm/yr. Progressively smaller offsets can be determined on progressively younger geological features, but dates for these younger features are too imprecise to constrain slip rates through time. Surface ruptures that formed during the 1920 Haiyuan earthquake (M = 8.7) show mostly left-slip displacement with magnitudes of more than 10 m in some places. Active faulting in the region suggests the Tibetan Plateau may be extending to the northeast in time. In the Haiyuan area, deformation probably began in Pliocene time, compared with a likely earlier initiation to the southwest; thus deformation began about 40–45 m.y. after collision between India and Asia. Formation of the low ranges to the northeast of the Haiyuan area, however, may have developed at different times and deformation may not have propagated regularly to the northeast. A total displacement of 10.5–15.5 km on the Haiyuan fault zone indicates that this fault zone does not accommodate large-scale eastward lateral transfer of continental fragments in the northern Tibetan Plateau.

The importance of small-scale faulting in regional extension

Abstract: A RECURRING observation in many studies of extensional basins has been that the amount of extension visible on normal faults (for example, on seismic reflection profiles) is significantly less than the amount of extension indicated by crustal thickness and thermal subsidence1–5. One mechanism suggested to account for this discrepancy is small-scale faulting, with offsets too small to be resolved seismically6,7. But earthquake studies8–10 indicate that small faults are responsible for only a small fraction of the total seismic moment in an active area. Scholz and Cowie11 have recently attempted to extend this approach to the total strain at the end of a finite deformation interval by combining scaling laws describing the distributions of fault lengths and displacements. Here we present fault displacement data that directly conflict with Scholz and Cowie's conclusions, and imply that up to 40% of the extension may be missed by summing fault offsets on basin profiles. The fault population at the end of a long deformation interval may differ substantially from that responsible for the earthquake population at any one time.

The active tectonics of the eastern Himalayan syntaxis and surrounding regions

Abstract: Source parameters of 53 moderate-sized earthquakes, obtained from the joint inversion of regional and teleseismic distance long-period body waves, provide the data set for an analysis of the style of deformation and kinematics in the region of the Eastern Himalayan Syntaxis. Focal mechanisms of Eastern Himalayan events show oblique thrust, consistent with the N-NE directed movement of the Indian plate as it underthrusts a boundary that strikes at an oblique angle to the direction of convergence. Earthquakes near the Sagaing fault show strike-slip mechanisms with right-lateral slip. Earthquakes on its northern splays, however, indicate predominant thrusting, evidence that the dextral motion on the Sagaing fault, which accommodates a portion of the lateral motion between India and southeast Asia, terminates in a zone of thrust faulting at the Eastern Himalayan Syntaxis. Remaining motion between India and southeast Asia is accommodated in a zone of distributed shear in east Burma and Yunnan, manifested by strike-slip and oblique normal faulting, east-west extension, crustal thinning, and clockwise rotation of crustal blocks. We determined strain rates throughout the region with a moment tensor summation using 25 years (modern) and 85 years (modern and historic) of earthquake data. We matched the observed strains with a fifth-order polynomial function, and from this we determined both the velocity field and rotations with respect to a specified region. Velocities calculated relative to south China stationary show that the entire area, extending from 20°N–36°N, within deforming Asia (Yunnan, western Sichuan, and east Tibet), constitutes a distributed dextral shear zone with clockwise rotations up to 1.7°/m.y., maximum in the region of the Eastern Syntaxis proper. Integrated strains across this zone, relative to south China stationary, show 38 mm/yr ± 12mm/yr of north-directed motion at the Himalaya. Remaining plate motion, relative to south China fixed, must be taken up by the underthrusting of India beneath the lesser Himalaya, strike-slip motion on the Sagaing fault, and intraplate NE directed shortening within NE India as well as NE directed shortening within the Eastern Syntaxis proper. 10 mm/yr ± 2 mm/yr of relative right-lateral motion between India and southeast Asia is absorbed in the region between the Sagaing and Red River faults (94°E–100°E). It is the clockwise vorticity (relative to south China) associated with the deformation in Yunnan, east Tibet, and western Sichuan that provides the relative north-directed motion of 38 ± 12 mm/yr at the Himalaya. Not all of the deformation is accommodated in right-lateral shear between India and south China and between east Tibet and south China; velocity gradients exist that are parallel to the trend of the shear zone. Relative to a point within western Sichuan (32°N, 100°E), the velocity field shows that the Yunnan crust is moving S-SE at rates of 8–10 mm/yr. Relative to south China, there is no eastward expulsion of crustal material beyond the eastern margin of the Tibetan plateau.

The velocity field along the San Andreas Fault in central and southern California

Abstract: The velocity field within a 100-km-broad zone centered on the San Andreas fault between the Mexican border and San Francisco Bay has been inferred from repeated surveys of trilateration networks in the 1973–1989 interval. The velocity field has the appearance of a shear flow that remains parallel to the local strike of the fault even through such major deflections as the big bend of the San Andreas fault in the Transverse Ranges of southern California. Across-strike profiles of the fault-parallel component of velocity exhibit the expected sigmoidal shape, whereas across-strike profiles of the fault-normal component of velocity are flat and featureless. No significant convergence upon the fault is observed even along the big bend sector of the fault. Simple dislocation models can explain most of the features of the observed velocity field, but those explanations are not unique. About 35 mm/yr of relative plate motion is accounted for within the span of the trilateration networks. Geologic studies indicate that the secular slip rate on the San Andreas fault is about 35 mm/yr. The agreement between these two estimates implies that most of the strain accumulation is elastic and will be recovered in subsequent earthquakes. The relative motion observed across the San Andreas fault (35 mm/yr) plus that observed across the Eastern California shear zone (8 mm/yr) accounts for most (43 mm/yr) of the observed North America-Pacific relative plate motion (47 mm/yr).

Flexural extension of the upper continental crust in collisional foredeeps

Abstract: Normal faults on the outer slopes of trenches and collisional foredeeps reveal that high-amplitude lithospheric flexure can result in inelastic extensional deformation of the convex side of a flexed plate. This process, which we call "flexural extension," differs fundamentally from rifting in that the lower lithosphere contracts while the upper lithosphere extends. In the Taconic foreland of New York, a >100-km-wide zone of brittle failure propagated ahead of the convergent plate boundary, rupturing the upper crust to an estimated depth of 15-20 km. Dip-slip displacement on normal faults in the Taconic and Arkoma foredeeps produced water depths like those in the closest modern analogue, the Timor Trough. Structural evidence does not support common illustrations of flexural normal faults as planar-irrotational structures which simply die out at shallow crustal depths. Instead, the surface geology shows that flexural normal faulting must be rotational with respect to the enveloping surface of the flexed plate. This toppled domino geometry implies the presence at depth of a detachment or zone of distributed ductile simple shear where fault displacement and block rotation are accommodated.

Circum-Pacific seismic potential: 1989–1999

Abstract: The seismic potential for 96 segments of simple plate boundaries around the circum-Pacific region is presented in terms of the conditional probability for the occurrence of either large or great interplate earthquakes during the next 5, 10, and 20 years (i.e., 1989–1994, 1989–1999 and 1989–2009). This study represents the first probabilistic summary of seismic potential on this scale, and involves the comparison of plate boundary segments that exhibit varying recurrence times, magnitudes, and tectonic regimes. Presenting these data in a probabilistic framework provides a basis for the uniform comparison of seismic hazard between these differing fault segments, as well as accounting for individual variations in recurrence time along a specific fault segment, and uncertainties in the determination of the average recurrence time.

Current Sierra Nevada-North America motion from very long baseline interferometry:Implications for the kinematics of the western United States

Abstract: We use geodetic measurements from very long baseline interferometry to estimate the motion of the Sierra Nevadan microplate, which is composed of the Sierra Nevada and the Great Valley The motion of the Sierra Nevadan microplate relative to the North American plate is described by a right-handed rotation of 061°/my about lat 32°N, long 128°W This Euler pole lies only 10° southwest of the Sierra Nevadan microplate and predicts a significant counterclockwise rotation about a local vertical axis It further predicts a velocity of the eastern edge of the Sierra Nevada (at 380°N, 1193°W) relative to stable North America of 11 ±1 mm/yr toward N36° ±3°W (quoted uncertainties are plus or minus one standard error), which accounts for about one-fourth of the velocity between the Pacific and North American plates and is ∼25° clockwise of many prior estimates The velocity nearly parallels the boundary between the Sierra Nevada and the Great Basin, which implies that current motion within the Great Basin results in a rotational, noncoaxial deformation We use this velocity to estimate how motion is distributed across the broad deforming zone taking up Pacific-North America plate motion We find that the vector sum of strike slip along the San Andreas fault and motion of the Sierra Nevada relative to stable North America (taken up by deformation within the Great Basin) differs little from the Pacific-North America plate velocity The difference can be described at 36°N along the San Andreas fault by a vector of 6 mm/yr directed toward N20°W This vector resolves into components of 5 mm/yr parallel to the fault and 2 mm/yr perpendicular to the fault with 95% confidence intervals of 0 to 10 mm/yr and -1 to +5 mm/yr, respectively The component perpendicular to the fault is several times smaller than found in prior studies and places a small upper bound on fault-perpendicular shortening We conclude that motion previously inferred to be taken up by deformation other than strike slip along the San Andreas fault or deformation within the Great Basin is much smaller than previously thought

Geometric models of listric normal faults and rollover folds

Abstract: Existing geometric models allow master fault shapes to be constructed, given the shape and heave or displacement of a deformed marker horizon in the hanging wall. These models assist in projecting normal faults to depth where the fault geometry is poorly constrained by available seismic data. Currently, it is unclear which model best predicts the actual relation between rollover and fault geometries. To address this deficiency, the relative accuracies of the constant-heave, constant-displacement, constant-bed-length, slip-line, and inclined-shear constructions are evaluated using clay model analogs and seismic examples. Each of the geometric models predicts a different fault shape and depth to detachment for the same rollover shape, because different hanging-wall deformat on mechanisms are assumed. A construction is developed for determining the hanging-wall geometry from a known fault geometry. This construction is based on the inclined-shear model and geometric constraints imposed by material displacement paths viewed from different reference frames fixed to the hanging-wall and footwall blocks. In clay models, the inclined-shear (antithetic shear, (Greek) alpha = 20 degrees) and slip-line constructions produce the best agreement between actual and modeled fault and marker bed shapes in rollovers. For subsurface examples, the inclined-shear model (antithetic shear, (Greek) alpha = 20-40 degrees) and the constant-displacement model predict fault trajectories that are the most similar to the positions and shapes of the master-fault segments interpreted fr m offshore Norway and Gulf of Mexico seismic lines. The shear angle required in the inclined-shear model can often be estimated from the orientation of minor faults in the hanging wall. The shear angle may also be determined through iterative modeling in which the shear angle is systematically varied, and the resultant model fault shape is compared to the shape of the shallow segment of the master fault interpreted from the seismic record.

Source parameters of large earthquakes in the East Anatolian Fault Zone (Turkey)

Abstract: SUMMARY The East Anatolian Fault Zone accommodates most of the motion between the Arabian plate and the apparently little-deforming interior of central Turkey. The direction of overall slip across this zone is crucial to the determination of the slip rate on the North Anatolian Fault. We use long-period P- and SN-waveforms to determine the source parameters of the four largest earthquakes that occurred in, or near, the East Anatolian Fault Zone in the last 35 years. Only one of these actually involved left-lateral strike-slip motion on a NE-SW fault. But the other three, and the nearby 1975 Lice earthquake, all had steeply dipping nodal planes with a NNW strike: if these were the auxiliary planes then all the earthquakes had a slip vector direction within about 10" of 063". If this direction represents the Arabia-Turkey motion, then the slip rate on the North Anatolian Fault must be in the range 31 to 48 mm yr-', with a probable value of 38 mm yr-', and the overall slip rate across the East Anatolian Fault Zone must be about 29mmyr-' with a range of 25

Fault patterns at outer trench walls

Abstract: Profiles across subduction-related trenches commonly show normal faulting of the outer trench wall. Such faulting is generally parallel or sub-parallel to the trench and is ascribed to tension in the upper part of the oceanic plate as it is bent into the subduction zone. A number of authors have noted that outer trench wall faulting may involve re-activation of the oceanic spreading fabric of the subducting plate, even when the trend of this fabric is noticeably oblique to the extensional stress direction. However, one previous review of outer trench wall fault patterns questioned the occurrence of a consistent link between fault orientation and such controlling factors. This latter study predated the widespread availability of swath bathymetry and longrange sidescan sonar data over trenches. Based only on profile data, it was unable to analyse fault patterns with the accuracy now possible. This paper therefore re-examines the relationship between outer trench wall faulting and the structure of the subduction zone and subducting plate using GLORIA and Seabeam swath mapping data from several locations around the Pacific and Indian Oceans. The principal conclusions is that the trend of outer trench wall faults is almost always controlled by either the subducting slab strike or by the inherited oceanic spreading fabric in the subducting plate. The latter control operates when the spreading fabric is oblique to the subducting slab strike by less than 25–30°; in all other cases the faults are parallel to slab strike (and parallel or sub-parallel to the trench). Where the angle between spreading fabric and slab strike is close to 30°, two fault trends may coexist; evidence from the Aleutian Trench indicates a gradual change from spreading fabric to slab strike control of fault trend as the angle between the two increases from 25 to 30°. The only observed exception to the above ‘rule’ of fault control comes from the western Aleutian Trench, where outer trench wall faults are oblique to the slab strike, almost perpendicular to the spreading fabric, and parallel to the convergence direction. Re-orientation of the extensional stress direction due to right-lateral shear at this highly oblique plate boundary is the best explanation of this apparently anomalous observation.

Four thousand years of seismicity along the Dead Sea Rift

Abstract: A study of major earthquake occurrence along the Dead Sea transform (35.5°–36.5° E; 27.2°–37.5° N) during the past four millenia has been attempted. Geological, archaeological, biblical, historical, and seismological evidence were integrated in an effort to quantify the space-time distribution of seismicity in the said province. The overall earthquake activity in the conterminous Near East indicates a stable pattern and appeared to have been stationary over the examined time window. About 110 earthquakes in the magnitude range 6.7 ≤ ML ≤ 8.3 affected the area during the past 2500 years. Of these, 42 originated along the Dead Sea fault system itself, while 68 were imported from the Helenic-Cyprian arcs and the Anatolian-Elburz-Zagros fault systems. These events were responsible for the repeated destruction of many cultural centers. In the Dead Sea region proper, the major seismic activity since 2100 B.C.E. (Before Christian Era), has been confined to the vicinity of its eastern shore with extremal seismicity at its southern tip near the prehistorical site of Bab-a-Dara'a (31° 15'N, 35° 32'E). This may constitute the first solid evidence that the Biblical “cities of the Plain” (Sodom, Gommorah, etc.) were located there. Recent studies of earthquake deformations in the Lisan deposits near Bab-a-Dara'a, agree with our findings. At the present time, a magnitude 6¾ earthquake is pending at the northern edge of the Levant rift, with its average recurrence interval (83 years) exceeded by one standard deviation (32 years).

Half-graben basin filling models: new constraints on continental extensional basin development

Abstract: Three end-member models of half-graben development (detachment fault, domino-style, and fault growth) evolve differently through time and produce different basin-filling patterns. The detachment fault model incorporates a basin-bounding fault that soles into a subhorizontal detachment fault ; the change in the rate of increase in the volume of the basin during uniform fault displacement is zero. Younger strata consistently pinch out against older synrift strata rather than pre-rift rocks. Both basin-bounding faults and the intervening fault blocks rotate during extension in the domino fault block model ; a consequence of this rotation is that the change in the rate of increase of the volume of the basin is negative during uniform extension. Basin fill commonly forms a fanning wedge during fluvial sedimentation, whereas lacustrine strata tend to pinch out against older synrift strata. In the fault growth models, basins grow both wider and longer through time as the basin-bounding faults lengthen and displacement accumulates; the change in the rate of increase in basin volume is positive. Fluvial strata progressively onlap pre-rift rocks of the hanging wall block, whereas lacustrine strata pinch out against older fluvial strata at the centre of the basin but onlap pre-rift rocks along the lateral edges. These fundamental differences may be useful in discriminating among the three endmember models. The transition from fluvial to lacustrine deposition and hanging wall onlap relationships observed in numerous continental extensional basins are best explained by the fault growth models.

Stress in the Lithosphere and the Strength of Active Faults

Abstract: An air pollution control system for treating contaminated air generated by food processing apparatus. Moist air laden with impurities and odors during food treatment is withdrawn from a processing chamber, cooled to condense moisture and particulate matter out of the air, and returned to the processing chamber, without discharge into the atmosphere. The conditioned air is reheated, absorbing moisture from the food product and again fed into the system.

Philippine fault: A key for Philippine kinematics

Abstract: On the basis of new geologic data and a kinematic analysis, we establish a simple kinematic model in which the motion between the Philippine Sea plate and Eurasia is distributed on two boundaries: the Philippine Trench and the Philippine fault. This model predicts a velocity of 2 to 2.5 cm/yr along the fault. Geologic data from the Visayas provide an age of 2 to 4 Ma for the fault, an age in good agreement with the date of the beginning of subduction in the Philippine Trench. The origin of the Philippine fault would thus be the flip of subduction from west to east after the locking of convergence to the west by the collision of the Philippine mobile belt with the Eurasian margin.

Amount and style of late Cenozoic deformation in the Liupan Shan area, Ningxia autonomous region, China

Abstract: The structures of the Liupan Shan area are characterized by numerous active thrust and strike-slip faults that suggest thin-skinned deformation. The structural history of this area can be divided into three phases that probably overlap one another in time and are parts of a single protracted deformation. The oldest Cenozoic deformational phase occurred probably between late Pliocene and early Quaternary time and produced some of the folds and thrust faults in the Liupan Shan and Yueliang Shan. During this phase, deformation was the result of approximately N50°E shortening, and the amount of shortening seems to have been about 1–2 km. The second phase of deformation was dominated by left-lateral strike-slip faulting (left slip) on the N60°W striking Haiyuan fault zone and shortening on north-south trending structures; shortening was associated with a transfer of the left-slip displacement on the Haiyuan fault zone to shortening in areas farther east. Shortening occurred by thrust faulting in the Liupan Shan and Xiaoquan Shan and by folding in the Madong Shan. During this phase the orientation of shortening changed to N60°W. The average amount of shortening on the north-south trending folds in the Madong Shan is about 6.3–7.8 km. Most of the shortening on the Liupan Shan and Xiaoguan Shan thrust faults also occurred during this phase and amounted to a minimum of 4.8–6.3 km and 6.6–7.6 km, respectively, also with an orientation of N60°W. During the third phase of deformation, about 1–1.5 km of late Pleistocene to Recent left slip occurred on the Xiaokou fault, which was transferred into oblique left-slip thrusting in the Liupan Shan. At this time, deformation in the Madong Shan and Xiaoguan Shan ceased or was reduced to a very slow rate. The present, active left-slip on the Haiyuan fault zone is accommodated by shortening in the Liupan Shan area. The total displacement along the Haiyuan fault is essentially the same as the total amount of shortening in the Liupan Shan area. The sequence and interaction of strike-slip and thrust faults in the Liupan Shan area seem to apply to the folds and thrust faults farther north in the southern Ningxia region. Thus the northeastern margin of the Tibetan Plateau appears to grow by shortening oriented northeast and by left slip faulting that transfers material from farther to the west. The total left-slip in the entire southern Ningxia region probably is less than 20–25 km and may be absorbed by shortening within this region. Thus the eastward translation of crustal fragments in the northern part of the Tibetan Plateau may not extend east of southern Ningxia, and if large-scale eastward displacement has occurred, it must lie farther south.