Nearby Faults Research Articles

Initiation and evolution of boundary-wall faults along the Mid-Atlantic Ridge, 25–29°N

Abstract The major faults that bound the median valley on slow spreading ridges form a staircase that migrates upwards and outwards from the valley floor like a slow escalator. Each boundary wall fault is generated at the margin of the median valley floor at about 2 km from the spreading axis. Boundary wall faults that appear to be single scarps on the relatively low resolution Seabeam multibeam echosounder maps are shown by high resolution TOBI sidescan sonar images in some cases to be single faults, but in others to be complex anastomosing fault zones. The major faults in such zones are often scalloped in plan view. There is no systematic relation between fault zone geometry and segment type. We conclude that during the growth of boundary wall faults the strain is first accommodated over a wide area, in which an array of small faults dissects the volcanic surface. However, with time, and with continued extension, the strain becomes organized into increasingly narrower zones of deformation until a major boundary wall fault develops. Faults grow by propagation and linkage with other nearby faults. The early stages of linkage include tilted ramps, but at later stages brittle faults cut across the ramps, and eventually become part of a scalloped fault trace. In some places, linkage is through zones of small-scale diffuse faulting, rather than through a single fault. Fault capture and linkage can occur over lateral distances (perpendicular to the mean azimuth of the faults) of up to 1.5 km. This deformation takes place within a fault growth window fixed relative to the spreading axis, through which the lithosphere passes as seafloor spreading continues. Most of the growth of each boundary fault takes place before the next fault is initiated. We conclude that the location and size of the narrow fault growth window is controlled by the changing mechanical properties of the ocean lithosphere as it spreads away from the axis. Its inner boundary represents the position at which a normal fault reaches the sea floor after originating at the base of the lithosphere at the spreading axis. Its outer boundary is interpreted as the point at which the strength of the boundary wall fault, increasing as the lithosphere ages and thickens, becomes greater than the strength of the lithosphere at the spreading axis, when a new fault is generated. The process of fault generation at mid-ocean ridges has relevance to fault generation in other extensional environments.

95/04436 Properties and structure of hard coals from the borehole Niedobezyce IG-1 in the Rybnik Coal District, Upper Silesian Coal Basin, their petrog

On 20 July 2017 (22:31:10 UTC), an Mw6.6 earthquake took place offshore Kos Island, in the southeastern (SE) Aegean Sea, producing severe damage and loss of life in the city of Kos and several smaller cities and villages both in Kos Island (Greece) and in the Bodrum peninsula (Turkey). All available seismological data until the end of October 2017 were gathered from seismological stations located in Greece and Turkey for the relocation process. The relocated aftershocks are clustered at least in three distinctive patches, creating a zone reaching a total length of about 40 km, elongated in a nearly east-west direction, and are mainly concentrated at depths 8–15 km, with the main shock hypocenter placed at ~13 km, implying a seismogenic layer of 7 km thickness, indicative for normal faulting earthquakes with Mmax ~ 6.5. The aftershock fault plane solutions are predominantly normal faulting in response to the north–south extension of the back arc Aegean area and consistent with the broader regional stress field. We also applied the satellite radar interferometry (InSAR) technique to define the coseismic surface displacements, revealing the range of deformation on the Island and the surrounding mainland. We combined this deformation field along with the available vectors of displacement measured by the Global Navigation Satellite System (GNSS) technique with the seismological data to determine the fault geometry and rupture process. The resulted variable slip model indicates a rather elliptical rupture area of 306 km2, with the coseismic slip ranging between 0.5 and 2.3 m. The peak moment release occurred in the depth interval of 9–11 km, consistent with the depth distribution of seismicity in the study area. We used the variable slip model to calculate Coulomb stress changes and investigate possible triggering due to stress transfer to the nearby fault segments.

Rejuvenation of the eastern Mediterranean passive continental margin in northern and central Sinai: new data from the Themed Fault

AbstractThe Themed Fault marks the southernmost border of the Early Mesozoic passive continental margin of north Sinai. This 200-km long fault transects the northern part of the Tih Plateau that supposedly occupies a tectonically stable area. Post-Middle Eocene–pre-Early Miocene rejuvenation of this fault proceeded by right-lateral wrenching and represents a newly recognized phase of deformation in the history of north and central Sinai. The minimum estimate for the strike-slip movement on this fault is about 300–750 m. To the north of the Themed Fault is a narrow fault belt (Sinai hinge belt) that marks the boundary between a tectonically unstable crustal block to the north (the north Sinai fold belt area) and a tectonically stable crustal block to the south, the main part of the Tih plateau area.Four phases of dextral wrenching rejuvenated the faults of the Early Mesozoic passive continental margin in northern Egypt; one of them affected the Themed Fault. The oldest (Dl) deformation is early Late Senonian and is related to the closure of Neotethys and the Eastern Mediterranean basin. The D1 deformation proceeded by pure wrenching in the north Western Desert of Egypt. In contrast, it proceeded by transpression in north Sinai due to the irregular plate boundary and the relationship of this boundary to the slip vectors. D2 deformation (post-Middle Eocene–pre-Early Miocene) is clear in the Themed Fault area although reported herein for the first time; it is related to continued closure of the Eastern Mediterranean basin and proceeded by pure wrenching. D3 deformation (Late Oligocene–Early Miocene) proceeded by divergent wrenching in the north Eastern Desert and is kinematically related to the transfer of slip from the nearby faults of the Suez rift. D4 deformation (post-Early Miocene to Recent) affected the Sinai hinge belt by pure wrenching and is probably related to the left-lateral slip on the Dead Sea Transform and the related drag of the eastern edges of the fault blocks of this hinge belt. Recent seismic activity in the Sinai hinge belt perhaps indicates that the D4 deformation has continued to the present time, although morphological expression of recent tectonic movement is lacking. In contrast, the Themed Fault is seismically quiet at present.

Causes and consequences of variations in faulting style at the Mid‐Atlantic Ridge

Both volcanism and faulting contribute to the rugged topography that is created at the Mid‐Atlantic Ridge (MAR) and preserved off‐axis in Atlantic abyssal hill terrain. Distinguishing volcanic from fault‐generated topography is essential to understanding the variations in these processes and how these variations are affected by the three‐dimensional pattern of mantle upwelling, ridge segmentation, and offsets. Here we describe a new quantitative method for identifying fault‐generated topography in swath bathymetry data by measuring topographic curvature. The curvature method can distinguish large normal faults from volcanic features, whereas slope methods cannot because both faults and volcanic constructs can produce steep slopes. The combination of curvature and slope information allows inward and outward facing fault faces to be mapped. We apply the method to Sea Beam data collected along the MAR between 28° and 29°30′N. The fault styles mapped in this way are strongly correlated with their location within the ridge segmentation framework: long, linear, small‐throw faults occur toward segment centers, while shorter, larger‐throw, curved faults occur toward ends; these variations reflect those of active faults within the axial valley. We investigate two different physical mechanisms that could affect fault interactions and thus underlie variations in abyssal hill topography at the MAR. In the first model only one fault is active at a time on each side of the rift valley. Each fault grows while migrating away from the volcanic center due to dike injection; extension across the fault causes a flexural rotation of nearby inactive faults. The amount of stress necessary to displace the fault increases as the fault grows. When reaching a critical size the fault stops growing as fault activity jumps inward as a new fault starts its growth near the rift valley. This model yields a realistic terracelike morphology from the rift valley floor into the rift mountains; the relief is caused by the net rotation accumulated in the lithosphere from the active faults (e.g., 10° reached 20 km from the active fault). Fault spacing is controlled by lithospheric thickness, fault angle, and the ratio of amagmatic to magmatic extension. We hypothesize that this mechanism may be dominant toward ridge segment offsets. An alternative model considers multiple active faults; each fault relieves stresses as it grows and inhibits the growth of nearby faults, causing a characteristic fault spacing. Such fault interactions would occur in a region of necking instability involving deformation over an extended area. This mode of extension would drive a feedback mechanism that would act to regulate the size of nearby faults. We hypothesize that this mechanism may be active in the relatively weak regions of strong mantle upwelling near segment midpoints, causing the homogeneous abyssal hill fabric in these regions.

Discovery of the Nalunaq Gold Deposit, Kirkespirdalen, SW Greenland

Results of mineral exploration of the first major gold deposit discovered in far southwestern Greenland are presented. An 800-m-long quartz Main Vein, containing abundant visible gold, was discovered in August 1992 and sampled at 20-m intervals. Geologic mapping and geophysical surveys were under way in 1993 to locate the sources of the deposit, to explore along the trend of the Main Vein (where mineralized extensions are expected to exist), and to determine the pattern of a nearby high-angle fault. Gold is distributed irregularly in discrete domains of the Main Vein and in smaller veins within its hanging wall. The main gold occurrences are found in granulated crush zones, with fine-grained irregular quartz grains within a much coarser-grained matrix. Other than gold there are very few minerals, the most important being lollingite. Microscopic examination of gravity concentrations during metallurgical testing revealed that free gold varies in size from 5-380 microns. Good recovery appears probable throug...

Palinspastic reconstruction of southeastern California and southwestern Arizona for the Middle Miocene

A paleogeographic reconstruction of southeastern California and southwestern Arizona at 10 Ma has been made based on available geologic and geophysical data. Clockwise rotation of 39° has been reconstructed in the eastern Transverse Ranges, consistent with paleomagnetic data from late Miocene volcanic rocks, and with slip estimates for left‐lateral faults within the eastern Transverse Ranges and NW‐trending right‐lateral faults in the Mojave Desert. This domain of rotated rocks is bounded by the Pinto Mountain fault on the north. The model requires that the western part of the late Miocene Pinto Mountain fault was a thrust fault, which gained displacement to the west, because of the absence of evidence for rotation of the San Bernardino Mountains or for significant right slip faults within the San Bernardino Mountains. The Squaw Peak thrust system of Meisling and Weldon (1989) may be a western continuation of this fault system. The Sheep Hole fault bounds the rotating domain on the east. East of this fault an array of NW‐trending right slip faults and south‐trending extensional transfer zones has produced a basin and range physiography while accumulating up to 16 km of right slip. This maximum is significantly less than the 37.5 km of right slip required in this region by a recent reconstruction of the central Mojave Desert (Dokka and Travis, 1990a). Geologic relations along the southern boundary of the rotating domain are poorly known, but this boundary is interpreted to involve a series of curved strike slip faults and linking normal faults that accommodated noncoaxial extension. Quaternary movement on the Pinto Mountain and nearby faults is unrelated to the rotation of the eastern Transverse Ranges, and a hiatus during part of Pliocene time followed the deformation which produced the rotation. The reconstructed Clemens Well fault in the Orocopia Mountains, proposed as a major early Miocene strand of the San Andreas fault, projects eastward towards Arizona, where early Miocene rocks and structures are continuous across its trace, making large displacements on this structure unlikely. The model predicts a 14° clockwise rotation and 32% extension during late Miocene and early Pliocene time along a NW‐trending line parallel to the present trace of the San Andreas fault. Palinspastic reconstructions of the San Andreas system based on this proposed reconstruction may be significantly modified from current models.

Stress inversion methods: are they based on faulty assumptions?

Stress inversion methods employed by structural geologists for estimating a regional stress tensor from populations of faults containing slickenlines rely on the basic assumption that slip on each fault plane occurs in the direction of maximum resolved regional shear stress. This premise ignores directional differences in fault compliance caused by fault shape, the Earth's surface or frictional anisotropy of the fault itself. It is also assumed that the regional stress field is homogeneous in space and time. Thus, perturbations in the local stress field caused by such things as material heterogeneities near the fault and mechanical interaction with nearby faults are not considered. Regional stresses may exercise dominant control on the slip direction; however local factors may perturb this field. We show how differences in fault compliance and local stress perturbations can result in a measureable difference between the direction of resolved shear stress and the direction of fault slip. Numerical modeling of common fault geometries in an elastic half space provides a means for evaluating the magnitude of this difference. We illustrate a few examples of geological circumstances under which the inversion techniques should be reliable, and a few where errors related to violations of the basic assumptions exceed those inherent to the data gathering and inverse techniques.

Frequency dependent eddy current models for nonlinear iron cores

Frequency dependent representations of eddy currents in laminated cores of power transformers are developed. One representation is based on a continued fraction expansion for the frequency dependent magnetizing impedance while the other is based on a discretization of flux distribution within the lamination. Both models are low order and reproduce the frequency characteristics of the transformer magnetizing branch up to 200 kHz with less than 5% error. The importance of modeling the frequency dependence of eddy currents for the calculation of the transient recovery voltage across a low voltage transformer breaker interrupting a nearby fault is demonstrated. >

A GIS-Based Assessment of Earthquake Property Damage and Casualty Risk: Salt Lake County, Utah

This paper offers a probabilistic assessment of expected property damage and casualty risk due to the earthquake ground shaking hazard affecting Salt Lake County, Utah (population = 725,600). Salt Lake County is bisected by a segment of the Wasatch Fault. It is also at risk from twenty-one other nearby fault segments. Findings are based on (1) a microzonation of the earthquake ground shaking hazard, (2) an inventory of buildings by value, structural frame type and use, (3) earthquake damage functions defining the performance of buildings as a function of ground shaking intensity, (4) data on the density of residential and employee populations, and (5) earthquake casualty functions defining casualty risk as a function of building damage. The analysis is supported by the algebraic combination of digital map layers within a vector-based geographic information system. Triangular irregular network models show the expected distributions of casualties. Hazard mitigation policy implications are also considered.

Implications of paleomagnetic data on Miocene extension near a major accommodation zone in the Basin and Range Province, northwestern Arizona and southern Nevada

Paleomagnetic data from volcanic and crystalline rocks elucidate the evolution of a major Miocene accommodation zone in the northern Colorado River extensional corridor. The accommodation zone is a 10‐km‐wide belt of intermeshing conjugate normal faults that facilitates reversals in the dominant tilt direction of fault blocks and dip direction of major normal fault systems. Tilt‐corrected means (e.g., N = 28 sites, D = 353.4°, I = 61.3°, α95 = 6.7°, k = 17.4) from Miocene volcanic strata overlap expected Miocene directions at the 95% confidence level. These data and geologic relations suggest that at exposed structural levels the accommodation zone did not facilitate distributed strike‐slip displacement between opposing tilt block domains. Vertical axis rotations are probably negligible in most of the corridor, as the Miocene structural grain generally mimics that in the zone. Discrepancies between characteristic remanent magnetizations (ChRM) in crystalline rocks and expected directions are therefore attributed to rotations about horizontal (i.e., tilting or flexing) axes. ChRMs from Cretaceous and Miocene intrusions suggest 50°–90° of tilting of large crystalline terranes on either side of the accommodation zone. The magnitude of tilting inferred from the paleomagnetic data is similar to that of Tertiary strata in nearby fault blocks, implying that these crystalline terranes are parts of highly tilted fault blocks rather than flexed lower plate rocks. Major low‐angle normal faults that bound highly tilted parts of these crystalline terranes probably nucleated at steep dips and were rotated to gentle dips by block tilting. Paleomagnetic data indicative of negligible tilting (e.g., Miocene intrusions, northern Black Mountains crystalline terrane, N = 13 sites, D = 359°, I = 55°, α95 = 9°, k = 24) and geologic relations imply that lower plate rocks may surface in both the east and west tilted domains 35–50 km away from the zone. The trend toward shallower structural levels, with respect to Miocene extension, and tapering of highly extended terrane toward the accommodation zone imply that the magnitude of upper crustal extension decreases toward the zone. Temporal patterns of major extension, especially an apparently continuous northward younging across both the east and west tilted domains, further suggest that the accommodation zone served as a long‐lived rupture barrier between conjugate normal fault systems rather than as a short‐term boundary between opposing systems that propagated toward and converged at the zone.

Triggered earthquakes and deep well activities

Earthquakes can be triggered by any significant perturbation of the hydrologic regime. In areas where potentially active faults are already close to failure, the increased pore pressure resulting from fluid injection, or, alternatively, the massive extraction of fluid or gas, can induce sufficient stress and/or strain changes that, with time, can lead to sudden catastrophic failure in a major earthquake. Injection-induced earthquakes typically result from the reduction in frictional strength along preexisting, nearby faults caused by the increased formation fluid pressure. Earthquakes associated with production appear to respond to more complex mechanisms of subsidence, crustal unloading, and poroelastic changes in response to applied strains induced by the massive withdrawal of subsurface material. As each of these different types of triggered events can occur up to several years after well activities have begun (or even several years after all well activities have stopped), this suggests that the actual triggering process may be a very complex combination of effects, particularly if both fluid extraction and injection have taken place locally. To date, more than thirty cases of earthquakes triggered by well activities can be documented throughout the United States and Canada. Based on these case histories, it is evident that, owing to preexisting stress conditions in the upper crust, certain areas tend to have higher probabilities of exhibiting such induced seismicity.

South Belridge Field, Borderland Basin, U.S., San Joaquin Valley

South Belridge is a giant field in the west San Joaquin Valley, Kern County. Cumulative field production is approximately 700 MMBO and 220 BCFG, with remaining recoverable reserves of approximately 500 MMBO. The daily production is nearly 180 MBO from over 6100 active wells. The focus of current field development and production is the shallow Tulare reservoir. Additional probable diatomite reserves have been conservatively estimated at 550 MMBO and 550 BCFG. South Belridge field has two principal reservoir horizons; the Mio-Pliocene Belridge diatomite of the upper Monterey Formation, and the overlying Plio-Pleistocene Tulare Formation. The field lies on the crest of a large southeast-plunging anticline, sub-parallel to the nearby San Andreas fault system. The reservoir trap in both the Tulare and diatomite reservoir horizons is a combination of structure, stratigraphic factors, and tar seals; the presumed source for the oil is the deeper Monterey Formation. The diatomite reservoir produces light oil (20-32{degree} API gravity) form deep-marine diatomite and diatomaceous shales with extremely high porosity (average 60%) and low permeability (average 1 md). In contrast, the shallow ({lt}1000 ft (305 m) deep) overlying Tulare reservoir produces heavy oil (13-14{degree} API gravity) from unconsolidated, arkosic, fluviodeltaic sands of high porosity (average 35%)more » and permeability (average 3000 md). The depositional model is that of a generally prograding fluviodeltaic system sourced in the nearby basin-margin highlands. More than 6000 closely spaced, shallow wells are the key to steamflood production from hundreds of layered and laterally discontinuous reservoir sands which create laterally and vertically discontinuous reservoir flow units.« less

Regional fluid and metal mobility in the Dalradian metamorphic belt, Southern Grampian Highlands, Scotland

A prominent set of veins was formed during post-metamorphic deformation of the Caledonian Dalradian metamorphic belt. These veins are concentrated in dilational zones in fold hinges, but apophyses follow schistosity and fold axial surface fractures. The veins are most common in the cores of regional structures, especially the Dalradian Downbend and consist of quartz, calcite, chlorite and metallic sulphides and oxides. Metals, including gold, have been concentrated in the veins. The fluid which formed the veins was low salinity (1–5 wt% NaCl and KCl) CO2-bearing (3–16 wt% CO2) water of metamorphic origin. The fluid varies slightly in composition within and between samples, but is essentially uniform in composition over several hundred km2. Vein formation occurred at about 350±50 °C and 200–300 MPa pressure. Further quartz mineralization occurred in some dilational zones at lower temperatures (160–180 °C). This later mineralization was accompanied by CO2 immiscibility. Dilution and oxidation of the metamorphic fluid occurred due to mixing with meteoric water as the rocks passed through the brittle-ductile transition. A similar metamorphic fluid is thought to have been responsible for gold mineralization in the nearby Tyndrum Fault at a later stage in the Dalradian uplift.

Investigation of Fateh Mishrif Fluid-Conductive Faults

Summary Adjacent well production histories indicated that several faults within theFateh Mishrif reservoir have fluid-conductive natures, Example historiesdisplayed high formation water cuts in upstructure wells that could beexplained only by aquifer influx along nearby faults. Subsequent data gatheredin these wellsincluding the results from pressure-transient tests, productionpressure-transient tests, production logging, and radioactive-tracersurveys-verified fluid flow paths. This paper demonstrates how specificdata-gathering paper demonstrates how specific data-gathering operations andrecent reservoir simulation studies revealed the effect of fluid flow alongfault planes on overall reservoir performance. The paper presents effects offluid-conductive presents effects of fluid-conductive faults ondevelopment-well placement, waterflood pattern balancing, and workoverevaluations. Introduction Fateh field is in the Arabian Gulf about 60 miles [97 kml offshore Dubai. The Fateh Mishrif reservoir, at 8,300 ft [2,530 m] subsea (ss), is a limestoneformation with an average thickness of 250 ft [76.2 m] formed during the MiddleCretaceous Age, Upthrust from an underlying salt dome created a dense networkof high-angle normal faults. Subsequent erosion resulted in the doughnut-shapedstructure shown in Fig. 1. The Mishrif is surrounded by an aquifer, but waterinflux is inhibited by a tarmat, or viscous oil layer, at the base of thereservoir. Fluid-conductive fault sections are indicated by dashed fault-plane lines inFig. 1 and designated according to nearby platform names. Little information isavailable to describe the actual physical characteristics of these faults, butFig. 2 provides a very simple conception. The faults are actually composed ofmultiple failure planes that do not necessarily extend throughout the lateraldirection of the faults. Before oil migration, water movement through the faultplanes resulted in calcite crystallization, as planes resulted in calcitecrystallization, as limited core data and drilling cuttings proved. After theoil reservoir formed, proved. After the oil reservoir formed, further faultmovement created crushed limestone sections into which the oil moved. Waterfrom the aquifer traveled along these fluid-filled areas as the reservoir wasproduced. Vertical fluid movement is restricted produced. Vertical fluidmovement is restricted because the Mishrif is bounded by shaly formations thatslightly deform to seal the faults. We identified five different fault sections as fluid conductive. Eachsection is discussed in detail according to three levels of recognition orstudy. First, a suspicion stage develops during which nearby well productiondata suggest fluid flow, usually water, along a highly conductive path. Forexample, a high formation water cut in a newly completed upstructure wellimplies flow along the fault. Next, an investigation stage begins, involvingsuch data-gathering activities as pressure-transient tests, radioactive-tracerstudies, and production logging, which serve to confirm the fluid-conductivefault suspicion. Finally, a management stage handles the fault's influence onreservoir production. Injection pattern balancing, production. Injectionpattern balancing, reservoir modeling for development planning, and wellworkovers are involved in planning, and well workovers are involved in thisfinal stage. Fault B Suspicion Stage. Fig. 3 illustrates the Fateh Mishrif structure in the FaultB area. Open well symbols designate offset wells pertinent to the study of thisfluid-conductive fault. Well completion dates are also provided. A fluid-conductive fault was suspected adjacent to Well B-5 becauseformation water was produced within 6 months of well completion in Feb. 1971. For comparison, offset Production Well B-6, which was completed 37 ft [11.3 m]lower, did not produce water until 1986. However, Well produce water until1986. However, Well B-4, positioned adjacent to the fault plane 200 ft [61 m]updip of Well B-5, has not shown any influence from the fault. Therefore, ahigh-conductivity path was presumed to extend from the aquifer to slightlypresumed to extend from the aquifer to slightly updip of Well B-5. Investigation Stage. A 1981 pressure-buildup (PBU) test on Well B-4indicated the presence of a barrier-i.e., a sealing fault. This barrier wasphysically possible because the total Mishrif thickness in Well B-4 is only 66ft [20.1 m], probably close to the fault-block offset. TheMiller-Dyes-Hutchinson (MDH) buildup plot displayed the characteristic doublingof slope, as seen in Fig. 4. With an intersection time of 26 hours, thedistance to linear discontinuity was calculated at 205 ft [62.5 m], whichcorresponds to the distance between Well B-4 and Fault B. Previous PBU surveys on Well B-5 indicated neither a sealing fault nor anextensive linear flow period. Extensive linear flow was apparent on PBU testsfor other producers adjacent to fluid-conductive faults, as described later inthis paper. We concluded that the Well B-5 completion interval does notintersect the fault directly, but is heavily influenced by fluid flow along thefault, as production data show. An extensive radioactive-tracer survey began in the Fateh Mishrif reservoirin Feb. 1979. JPT P. 1038

Empirical Earthquake Ground Motions for an Engineering Site with Fault Sources: Tooele Army Depot, Utah

Empirical earthquake ground motions were developed for an engineering site at Tooele Army Depot near Tooele, Utah. Nearby fault sources were judged capable of generating earthquakes, as was the Wasatch fault. Comparisons were made between peak horizontal motions obtained from charts giving motions for Modified Mercalli intensity and charts giving motions for earthquake magnitude and distance. The latter motions were compared for charts by various authors. The motions that were generated include the effects of a major earthquake on the Wasatch fault at 50 km from the site, with M = 7.5, and a local earthquake at 21 km from the site, also of M = 7.5. The Wasatch earthquake for a soft site involves peak horizontal motions with acceleration = 380 cm/sec 2 , velocity =58 cm/sec and duration (≥0.05 g ) = 56 sec. The local earthquake has an acceleration = 1,100 cm/sec 2 , velocity = 180 cm/sec and duration (≥0.05 g ) = 54 sec. Appropriate time histories are recommended. Operating basis motions are also provided.

Source parameters of the October 1, 1987 Whittier Narrows Earthquake from crustal deformation data

Offsets in the regional strain field, generated by the October 1, 1987, Whittier Narrows earthquake, were recorded with large amplitudes on two deep‐borehole dilational strainmeters at distances of 46.7 and 65.5 km from the hypocenter and marginally on instruments at greater distances in the Parkfield area and at Pinon Flat, where laser extensometers also recorded small offsets. These data are insufficient to solve for the location and physical parameters of the earthquake, but by also using the measured elevation changes in the epicentral area, we are able to invert for source models consistent with all available observations of crustal deformation. The source models obtained indicate that slip extends to a depth of about 30 km, well below the recorded aftershock zone. The requirement for deeper (and presumably aseismic) slip derives from the large negative dilatation experienced by the nearest strainmeter (PUBS), but high‐frequency data from the same site exclude any significant slow component of moment release. By ignoring PUBS we obtain a moment of about 0.7 × 1018 N m on a fault that has a strike and dip of about N60°W and 40° down to the north, respectively, and extends to a depth of about 16 km. The most likely reason for the anomalous offset at PUBS appears to be sympathetic slip triggered on a nearby fault by the main shock. Precursive strain during the month prior to the earthquake is not apparent at the nanostrain level in the data from the closest instrument, but the event was accompanied by a change in strain rate at the two nearer sites.

Past and Possible Future Earthquakes of Significance to the San Diego Region

The potential for earthquakes that may significantly affect the San Diego, California, region is examined from the viewpoint of geology, seismic history, and strong ground motion. We have compiled the best available data on slip rates and recurrence times for all major faults in southern California and northern Baja California, identified possible fault segments that might rupture in single earthquakes and obtained repeat times for these events which are consistent with (or at least not contradicted by) trenching studies. The most important faults for San Diego's seismic hazard are the Rose Canyon fault, the Elsinore fault, and faults immediately offshore (Coronado Banks, San Diego trough). There have not been any major earthquakes on any of these nearby faults in historical time, but the geological evidence is clear that such events will eventually occur. San Diego is located on the western flank of the Peninsular Range batholith, and there is weak evidence that attenuation in this batholith might be lower than average for California. We combine the geological data with an attenuation model to obtain an estimate for the occurrence rate of various levels of peak ground acceleration in downtown San Diego from events with moment magnitude greater than about 6. We find that peak accelerations of 10% g to 20% g are expected about once every 100 years. There are considerable uncertainties in this estimate, but nevertheless, strong ground shaking from a magnitude 6.0 or greater earthquake near the populated area would not be a scientific surprise.

Slip on the Superstition Hills fault and on nearby faults associated with the 24 November 1987 Elmore Ranch and Superstition Hills earthquakes, southern California

AbstractAlignment arrays and creepmeters spanning several faults in southern California recorded slip associated with the 24 November 1987 Elmore Ranch and Superstition Hills earthquakes. No precursory slip had occurred on the Superstition Hills fault up to 27 October 1987, when the last measurement before the earthquakes was made. About 23 days before the earthquake, dextral creep events of about 13 mm and 0.5 mm may have occurred simultaneously on the Imperial and southern San Andreas faults, respectively, but the tectonic origin of the smaller event is questionable.Within 12 hr after the Superstition Hills earthquake, 20.9 cm of dextral slip occurred on the main fault trace at the Superstition Hills alignment array, and 39.8 cm of dextral slip was recorded over the entire 110-m width of the array. Despite this initial wide distribution of slip, nearly all of the postseismic slip is occurring on the main fault trace. As of 3 August 1988, the alignment array had recorded a total of 80.2 cm of dextral slip. As of 5 days after the earthquakes, 65 to 80 per cent of the total slip measured by the alignment array had occurred on discrete, mappable fractures.In addition, the two earthquakes triggered slip on the Coyote Creek fault, the southern San Andreas fault, and on the Imperial fault. Telemetered data from creepmeters on the southern San Andreas and Imperial faults indicate that triggered slip began there within 3 min or less of each of the two earthquakes. Additional triggered slip occurred on the Imperial fault beginning 3.5 hr after the second earthquake.

Surface faulting along the Superstition Hills fault zone and nearby faults associated with the earthquakes of 24 November 1987

AbstractThe M 6.2 Elmore Desert Ranch earthquake of 24 November 1987 was associated spatially and probably temporally with left-lateral surface rupture on many northeast-trending faults in and near the Superstition Hills in western Imperial Valley. Three curving discontinuous principal zones of rupture among these breaks extended northeastward from near the Superstition Hills fault zone as far as 9 km; the maximum observed surface slip, 12.5 cm, was on the northern of the three, the Elmore Ranch fault, at a point near the epicenter. Twelve hours after the Elmore Ranch earthquake, the M 6.6 Superstition Hills earthquake occurred near the northwest end of the right-lateral Superstition Hills fault zone. Surface rupture associated with the second event occurred along three strands of the zone, here named North and South strands of the Superstition Hills fault and the Wienert fault, for 27 km southeastward from the epicenter. In contrast to the left-lateral faulting, which remained unchanged throughout the period of investigation, the right-lateral movement on the Superstition hills fault zone continued to increase with time, a behavior that was similar to other recent historical surface ruptures on northwest-trending faults in the Imperial Valley region.We measured displacements over 339 days at as many as 296 sites along the Superstition Hills fault zone, and repeated measurements at 49 sites provided sufficient data to fit with a simple power law. Data for each of the 49 sites were used to compute longitudinal displacement profiles for 1 day and to estimate the final displacement that measured slips will approach asymptotically several years after the earthquakes. The maximum right-lateral slip at 1 day was about 50 cm near the south-central part of the North strand of Superstition Hills fault, and the predicted maximum final displacement is probably about 112 cm at Imler Road near the center of the South strand of the Superstition Hills fault. The overall distributions of right-lateral displacement at 1 day and the estimated final slip are nearly symmetrical about the midpoint of the surface rupture. The average estimated final right-lateral slip for the Superstition Hills fault zone is about 54 cm. The average left-lateral slip for the conjugate faults trending northeastward is about 23 cm.The southernmost ruptured member of the Superstition Hills fault zone, newly named the Wienert fault, extends the known length of the zone by about 4 km. The southern half of this fault, south of New River, expressed only vertical displacement on a sinuous trace. The maximum vertical slip by the end of the observation period there was about 25 cm, but its growth had not ceased. Photolineaments southeast of the end of new surface rupture suggest continuation of the Superstition Hills fault zone in farmland toward Mexico.

Estimated Performance of Twitchell Island Levee System, Sacramento-San Joaquin Delta, under Maximum Credible Earthquake Conditions

Studies have shown that levee failure in the Sacramento-San Joaquin Delta can lead to salt water contamination of one of California9s most important surface sources of freshwater. This paper examines the seismic stability of one of the delta9s strategic levee systems for maintaining water quality: Twitchell Island9s Three Mile Slough levee. This levee will become unstable (factor of safety less than 1.00), due to low shear strength peat foundations beneath it, if the dead weight its suction-dredge placed sand toe berm is removed. The toe berm is highly susceptible to liquefaction from the Maximum Credible Earthquake (MCE) accelerations of several nearby faults based on the Simplified Liquefaction Procedure developed by Seed and others (1984), and field data presented in this paper. The Maximum Credible Earthquakes of several San Francisco Bay Area faults are capable of causing liquefaction of the toe berm and failure of the levee. A moderate earthquake of magnitude 6, centered within 14 mi of Twitchell Island, can also cause levee failure. Similarities between Twitchell Island and other delta islands suggest that the entire delta is vulnerable to even moderate earthquakes.