North-northwest Direction Research Articles

Rupture of the 2004 Sumatra-Andaman earthquake inferred from direct P-wave imaging

The Sumatra-Andaman earthquake on December 26, 2004 is the first well recorded gigantic earthquake (moment magnitude Mw 9.3) by modern broadband seismic and Global Positioning System networks. The rich seismic and geodetic recordings have documented unprecedented details about the earthquake rupture, coseismic and postseismic deformations. This is a report of detailed images of the rupture process using the first-arriving compressional waves recorded by the China National Digital Seismic Network (CNDSN). An improved imaging condition was employed to account for the sparse distribution of the CNDSN stations. The resulting images are consistent with the major rupture features reported by previous seismic and geodetic studies. It is found that the earthquake rupture initiated at offshore of northwestern Sumatra and propagated in the north northwest direction at a speed of 2.7 ± 0.2 km/s. The rupture continued for at least 420 s and extended about 1200–1300 km along the Andaman trough with two bursts of seismic energy.

Detection of Nonthermal X-ray structures near the Galactic Center with Chandra

We have discovered a number of nonthermal X-ray features within central 40 pc region of the Galactic center by analysing 600-ksec observations of Chandra archival data. Most of the detected X-ray structures exhibit small-scale knot-like morphologies and their spectra are well reproduced by single hard power-law with photon indices of 1-2. Among them, the most outstanding features are the three X-ray knots which are aligned on a straight line from the potition of Sgr A* to north-northwest direction. The X-ray properties of these knots lead us to suspect that they are X-ray jets ejected from Sgr A* in the recent past. In addition, we have obtained an indication that the summed flux of nonthermal diffuse X-rays within 30 pc of the GC seems to be smoothly connected to the 20-100 keV flux detected with INTEGRAL IBIS/ISGRI. These results suggest that the origin of GC hard X-rays (or High energy Gamma-rays) is not (or partly) from the Galactic nucleus.

Rayleigh-Wave Group-Velocity Studies beneath the Indian Ocean

The fundamental mode Rayleigh wave generated by 16 earthquakes, which occurred in the Indian Ocean and were recorded at 14 seismic stations of Asia, Africa, and Australia, are analyzed to estimate the Rayleigh-wave group velocity at periods of 10 to 60 sec by using the multiple filter analysis technique. In addition to this, Rayleigh-wave group-velocity data available in the literature across different propagation paths of the Indian Ocean forming a dense distribution of seismic wave traverses have been considered for tomographic studies. The group-velocity distribution maps have been obtained at periods of 10, 20, 30, 40, 50, and 60 sec for the Rayleigh wave. The minimum value of the group velocity of 2.0 km/sec is centered near the Rodriguez triple junction (25° S, 70° E) at 10 sec and higher value (3.4–4.0 km/sec) at 20 sec and greater period. As we move in the north-northwest direction from the Rodriguez triple junction, the Rayleigh-wave group-velocity isolines at 10 and 20 sec are also increasing in the northern direction in both the east and west sides of the ridge axis. Another minimum value of group-velocity isolines (1.9 km/sec) is centered near (25° S, 90° E) at 10 sec, and there is a gradual increase in the northern direction across the Ninetyeast Ridge. These observations are in accordance with the increase in the age of the ocean floor and there is a gradual increase in the group-velocity isolines. This can be interpreted as due to the hot uprising convection current, the material shows low shear-wave velocity along the ridge axis, and farther away from the ridge axis the material has become cooler with higher shear-wave velocity. The inversion of Rayleigh-wave group-velocity across the Ninetyeast Ridge shows 120-km-thick lvz (low-velocity zone) of shear-wave velocity of 4.38–4.78 km/sec at a depth of 40 km from the water surface, whereas other parts of the Indian Ocean show 44-km-thick lvz with shear-wave velocity of 4.12–4.17 km/sec at a depth of 40 km from the water surface. The high shear-wave velocity below Moho (4.19–4.75 km/sec) beneath the Ninetyeast Ridge indicates the presence of a cold and dense lithosphere beneath it.

Space geodesy as a tool for measuring ice surface velocity in the Dome C region and along the ITASE traverse

AbstractDome C was chosen by the European Project for Ice Coring in Antarctica (EPICA) as the site for the drilling of a deep ice core. This paper presents results from geodetic surveys of ice velocities (absolute and relative) at Dome C and along a transect to Terra Nova Bay. The purpose of the surveys was to provide accurate data for the study of ice dynamics, particularly a strain network comprising 37 poles surveyed in 1995 and again in 1999. Data indicate that the ice surface at the poles closest to the topographic summit moves horizontally by up to a few mm a–1 in a direction consistent with downslope motion of the ice sheet, while 25 km from the summit it moves up to 211 mma–1. The EPICA drilling site yields an interpolated velocity of about 15 ±10mma–1 in a north-northwesterly direction. Analysis of the velocity field and surface topography reveals that the surface flow centre is nearly co-located with the dome summit, and that both are in a steady-state condition. The measured horizontal velocities are consistent with the remote-sensing result and provide accurate ground-truth control for flow mapping. Seven snow–firn cores, up to 53m deep, were drilled during the Terra Nova Bay–Dome C traverse. Submerged velocity systems were installed at the borehole and measured using the global positioning system (GPS). First results show a steady-state condition. Measured (horizontal) ice velocities increase from the summit of the ice sheet to the coast, reaching about 28 ma–1 at site GPS2A.

Geostatistical Characteristics of Hydraulic Conductivity

The aquifer which forms the main water supply for the municipalities of Malmö and Lund owes its value to the deposition of coarse sands during the deglaciation of the late Weichselian ice. Spatial statistical analysis based on construction of the variogram has been applied to the hydraulic conductivity of this aquifer. This has identified an anisotropic distribution of the geohydrological properties with the least continuity striking in a north-northwest direction, consistent with the believed flow direction of the Weichselian melt-water streams which deposited the coarse sands. The application of geostatistical estimation (kriging) to model the distribution of hydraulic conductivity within the aquifer has honoured the depositional controls, notably the increased hydraulic conductivity towards the ice-proximal parts of the deposit.

The 2000 Western Tottori Earthquake—Seismic activity revealed by the regional seismic networks—

On October 6, 2000, the 2000 Western Tottori Earthquake (Mjma 7.3) occurred in the western Tottori prefecture area, southwestern Japan. It initiated at a depth of 12 km at the bottom of the seismogenic zone, which was derived from aftershock distribution. The aftershocks extend over a 35 km length in a north-northwest direction. Spatial and temporal distribution of the aftershocks exhibits local characteristics in the fault region. The northern part consists of earthquake clusters while the southern part consists of a rather simple lineament of aftershocks, and the spreading and decaying rate of the aftershocks is slower in the northern part. This contrast is possibly due to the heterogeneity of the fault system and probably affected the rupture process of the mainshock. Two swarm sequences occurred in the surrounding region after the mainshock. One initiated 48 hours after the mainshock 25 km southwest of the main aftershock distribution. The other started 20 hours after the mainshock northeast of the mainshock on the southeast flank of Daisen volcano. These activities are probably induced seismicity due to stress changes in the focal region. Pre-seismic swarm activities occurred in the focal region from 1989 and deep low-frequency earthquakes were observed since 1999. It is important to understand the relationship between these possible precursory phenomena and the occurrence of the mainshock.

Source processes and seismotectonic implications of the 1995 and 1996 Cyprus, Eastern Mediterranean region, earthquakes

The teleseismic body waves of two earthquakes were inverted for their sources to obtain a rupture process for each. The February 23, 1995 ( M s=5.8) and October 9, 1996 ( M s=6.8) events took place to the northwest and southwest of Cyprus. Both the earthquakes generated complex body waveforms which were inverted using the method of Kikuchi and Kanamori (1991). The inversion results suggest that the 1995 and 1996 events were multiple events associated with subevents taking place on different faults located nearby the source regions. Three subevents were necessary to explain the waveforms of the 1995 earthquake. The first subevent showed a predominantly normal faulting mechanism while the second subevent, located 20 km to the north of the first subevent, had a strike-slip mechanism with dip-slip component. Despite the difference in mechanisms both the subevents had slip vectors oriented nearly E–W. The last subevent was located in the proximity of the first subevent and had a normal faulting mechanism with a slip vector in N–S direction. The different depths and slip-vector directions of the first and third subevents may suggest strain partitioning with depth in the source region of the 1995 event. The complex waveforms of the 1996 earthquake were modeled with three subevents, too. Combining the hypocentral distribution of the aftershocks with the inversion results revealed that the first subevent took place on a fault plane dipping about 20° to the east-southeast. The mechanism of the first subevent, together with the azimuthal pulse-width distribution suggest that the sense of motion on the fault plane was right-lateral strike-slip and unilateral rupture propagation. We estimate from the major subevent a slip rate of 10 mm/year for the African plate. The second and third subevents were located 35 km to the southeast of the main shock and had a predominantly normal faulting mechanism with slip vectors in north-northwest direction. The rupture process analysis of the 1995 and 1996 events suggests that the region to the west of Cyprus experiences extensional tectonics.

Magnetite lava flows in the Pleito-Melon District of the Chilean iron belt

(1982) concluded that this sequence was deposited in a volcanic island-arc setting. In the Pleito-Mel(n district the Bandurrias Group is composed principally ofandesitic lavas and volcanic breccias with intercalated rhyolitic tuffs and stratiform iron ores; the Bandurrias volcanics continues to the east of the area shown in Figure 2. An intrusive complex with tonalitic, granodioritic, and dioritic units crops out to the west and east of the volcanic sequence (Fig. 2). This complex has up to now been regarded as Tertiary in age (Moscoso et al., 1982), but our stratigraphic data indicate that parts of it may be Cretaceous and constitute a basement for the Bandurrias sequence. In the Berenguela area (Fig. 3) the diorite is younger than the Bandurrias andesites, which are recrystallized. A large number of rhyolitic dikes and a few andesitic ones cut extrusive as well as intrusive rocks, and some of the dikes may be feeders of the extrusions. Structures There are three fault systems in the district, all of them part of the Atacama megafault (cf. Thiele and Pincheira, 1987). The most important systems follow north-northeast and northwest to north-northwest directions and the third one consists of secondary east to east-northeast faults. The north-northeast system is longitudinal and seems to be the oldest. It has produced downfaulted blocks, for example in the E1 Mel(n area (Fig. 3), and many of the iron ores in the district are associated with it. The north

Reevaluation of Stevens Sand Potential--Maricopa Depocenter, Southern San Joaquin Basin, California

During the upper Miocene in the Southern San Joaquin basin surrounding highlands contributed coarse material to a deep marine basin dominated by fine grained silicious bioclastic deposition. these coarse deposits became reservoirs isolated within the silicious Antelope Shale Member of the Monterey Formation. In the southern Maricopa depocenter these Stevens sands are productive at Yowlumne, Landslide, Aqueduct, Rio Viejo, San Emidio Nose, Paloma, and Midway-Sunset fields, and are major exploration targets in surrounding areas. In the ARCO Fee lands area of the southern Maricopa depocenter, Stevens sands occur as rapidly thickening lens-shaped bodies that formed as channel, levee, and lobe deposits of deep-marine fan systems. These fans were fed from a southerly source, with apparent transport in a north-northwesterly direction. Sands deflect gently around present-day structural highs indicating that growth of structures influenced depositional patterns. Correlations reveal two major fan depositional intervals bounded by regional N, O, and P chert markers. Each interval contains numerous individual fan deposits, with many lobes and channels recognizable on three-dimensional seismic data. In addition to these basinal sand plays presently being evaluated, ARCO is pursuing a relatively new trend on Fee lands along the southern basin margin, where correlation to mountain data reveals Stevensmore » sands trend into the steeply dipping beds of the mountain front. This area, the upturned Stevens,' has large reserve potential and producing analogies at Metson, Leutholtz, Los Lobos, and Pleito Ranch fields.« less

Pnobservation and interpretation in Washington

abstractThe travel-time characteristics of Pn waves in the state of Washington were investigated using regional earthquakes as seismic sources and stations of the state-wide University of Washington telemetered network as receivers. There is a significant difference in upper mantle velocity between eastern Washington (8.20 ± 0.02 km/sec) and western Washington (7.79 ± 0.02 km/sec). Modifications of the time-term method were used to find the dependence of P velocity on distance and azimuth to the source. Pn velocity in eastern Washington depends on the distance to the source, implying a velocity gradient with depth in the upper mantle. No significant distance dependence was found for western Washington. Azimuthal anisotropy was found for Pn with a northwest direction of maximum velocity in eastern Washington and a north-northwest direction in western Washington. Assuming no lateral crustal velocity gradients, the Pn refracting interface dips to the west approximately 0.8° in eastern Washington and to the southeast approximately 3.2° in western Washington. Contour maps of the station time terms indicate significant variations in crustal structure across the state. The combination of low Pn velocity and small Moho dip beneath western Washington suggests that the subducting lithospheric slab underlying the continental margin lies below the level of transition to upper mantle velocity over much of this region.

Bottom and suspended particle sizes: Implications for modern sediment transport in Quinault submarine canyon

Electronic particle size analysis of suspended particulate matter and seabed sediments collected over a two-year field study of Quinault Canyon indicates that fine silts (5–9 μm modal diameter) dominate canyon sedimentation. These sediments are markedly finer than the coarse silts (20–40 μm modal diameter) characteristic of shelf sediments at the canyon head. Most of the suspensate delivered to the canyon is derived from the intermediate nepheloid layer (INL) over the slope, which in turn originates from the bottom nepheloid layer (BNL) on the shelf. Current meter data and particle sizes suggest that sediment accumulating in the upper head of the canyon ( <450 m ) is derived directly from the mid-shelf silt deposit. Sediment accumulating in lower portions of the canyon head and in the lower canyon ( >1000 m depth) is apparently resuspended from the outer shelf south of the canyon. Transport is dominantly across-canyon (rather than offshore) in a north-northwesterly direction. Storms strongly influence sediment transport, dramatically increasing suspensate modal sizes in the uppermost portions of the canyon head ( <450 m depth) and over the shelf break. Storms produce less significant changes in particle size distributions on the open slope ( <450 m ) and no apparent changes in the deeper portions of the canyon ( >1000 m ). Rapid settling of >30 μ m particles restricts sand and coarse silt accumulation primarily to the southern portion of the upper canyon head.

Sedimentology and Geochemistry of Recent Sediments off the Mouth of the Niagara River, Lake Ontario

Sediment thickness and bathymetric characteristics were derived from echo sounding, side scan sonar, and shallow seismic reflection recordings. Sediment samples were analyzed for particle size, major and trace elements, and for benthic organisms. The Niagara Bar has been separated into Inner and Outer Bar deposits and it is proposed that the Outer Bar was formed during an earlier, lower stage of Lake Ontario. Both deposits appear to be largely derived from reworked, local, shoreline materials. Trace element analyses indicate that the surficial sediments (prior to the 1972 Great Lakes Water Quality Agreement) were moderately enriched in Cd, Cr, Cu, Pb, and Zn, and greatly enriched in Hg. Normalized distributions of trace elements in the bar sediments show the presence of distinct dispersal patterns. These dispersals indicate that some of the suspended load passes out of the main outflow channel, across the bar, and toward west and north northwest directions. Particle-size and geochemical characteristics of the off-shore sediments indicate that they differ significantly from marginal facies and imply that local variations in the forms and type of peripheral inputs to the lake have little influence on deep basin deposits.

Sedimentological synthesis of permian fluviatile sediments of East Bokaro basin, Bihar, India

The early Permian Karharbari and Barakar formations of East Bokaro basin comprise the following lithofacies: Lithofacies A, consisting of monomictic cobble- and pebble-conglomerate including pebbly sandstone, with faint crossbeds; deposited mainly by high-velocity aqueous currents as channel-lag deposit or longitudinal bars. Lithofacies B, coarse to medium sandstone, profusely crossbedded; interpreted as channel facies formed by downcurrent migration of sand bars in low-sinuosity streams. Lithofacies C, mainly fine sandstone with interbedded siltstone, characterised by small-scale ripple-lamination; formed in a low-energy environment such as swale-fill and/or overbank deposits. Lithofacies D, including carbonaceous shale and coal, with lack-of-current structures, indicating quiet-water deposition; interpreted as backswamp and lacustrine deposits. The overall context of the Karharbari and Barakar assemblage with relative abundance of pebbly coarse sandstone in the former and fine clastics in the latter, the presence of fining-upward cycles, widespread development of tabular and trough crossbedding and sample to sample variation of foreset azimuths, all suggest a generally alluvial environment. Analysis of crossbedding dip azimuth and dimensional fabric suggest that the sediment milieu in either case consisted of streams flowing persistently from the south-southeast to north-northwest direction. It is inferred that the unidirectional system of streams flowing across the East Bokaro basin changes systematically in channel sinuosity through time.

Late Cenozoic deformation in the Matanuska Valley, Alaska: Three-dimensional strain in a forearc region

The Matanuska Valley is located in the forearc terrane of the Alaska-Aleutian volcanic arc. A period of extensive faulting occurred in the western Matanuska Valley during the Neogene. Faulting was a response to intraplate strain caused by subduction of the Pacific plate beneath the convergent margin. The Neogene fault system consisted of three sets of faults: two sets of north-trending strike-slip faults and a set of east-northeast–trending reverse faults. Three fault sets were required to accommodate the general three-dimensional deformation. Crustal shortening in a north-northwest direction was accompanied by lateral and vertical extension. The direction of maximum horizontal shortening strain has apparently been constant since the Neogene, although there is no evidence for extensive faulting of Quaternary deposits in the area we have studied. The western Matanuska Valley is located only 50 km above the Benioff zone, and the predicted maximum principal shortening direction is north-northwest, on the basis of the slip-vector azimuth between the North American and Pacific plates and the nature of surficial ground displacements during the 1964 (M = 8.4) subduction zone earthquake. Focal mechanisms for earthquakes in the vicinity of the Benioff zone and in the lower part of the North American plate indicate west-northwest extension and north-northeast shortening. This means that the strain field at depths below 20 km may differ significantly from the near-surface strain field.

Deformation of the southeast part of the Columbia Plateau

Four structural elements north of the Olympic-Wallowa lineament in the southeast part of the Columbia Plateau (Washington, Idaho, and Oregon) are (1) the offlap of progressively younger basalt units from pre-basalt topographic highs; (2) east-west open folds associated with reverse faulting; (3) northwest-southeast, northeast-southwest, and north-south faults with predominantly vertical displacement; and (4) vertical north-northwest–south-southeast feeder dikes. These may be explained by (1) a regional east to west tilting of the plateau caused by the isostatic rise of older rocks on the eastern margin; (2) a stress regime with a horizontal maximum principal stress in a north-northwest–south-southeast direction, and a horizontal minimum principal stress in a west-southwest–east-northeast direction; and (3) reactivation of an older northwest-southeast, northeast-southwest, and north-south structural grain in the pre-Miocene basement. The stress regime is similar to that envisaged for the area southwest of the Olympic-Wallowa lineament, and the difference in the type of deformation on either side of that feature may be attributed to differences in the thickness of the crust across the ancient boundary.

Relation of the tectonics of eastern China to the India-Eurasia collision: Application of slip-line field theory to large-scale continental tectonics

The active tectonics of northeast China is dominated by strike-slip and normal faulting, with right-lateral motion on north-northeast-trending planes, left-lateral motion on west-northwest-trending planes, and extension in approximately a north-northwest direction. Southeast China, however, is relatively stable. We interpret both the style of deformation in northeast China and the stability of southeast China as consequences of the India-Eurasia collision and the presumably relatively small compressive stress transmitted across the island arcs of the western Pacific. The relationship of the stress field and deformation in eastern China to the India-Eurasia collision are analogous to those in a laterally bounded rigid-plastic medium indented by a rigid die (India).

The Vancouver Island earthquake of 5 July 1972

An earthquake occurred at 10 h 16 m 39 s GMT on 5 July 1972 in close proximity to the west coast of Vancouver Island, near 49.5 °N and 127.2 °W. Its magnitude (mb) was 5.7 and the hypocenter was near the middle of the crust. A well defined P nodal solution has a pressure axis with a north-south orientation and a tension axis with an east-west orientation. The preferred nodal plane suggests right lateral strike-slip faulting on a near vertical fault, striking in a north-northwest direction. The other nodal plane suggests that left lateral strike-slip faulting on a near vertical fault striking in an east-northeast direction is also a plausible solution. A field study with portable seismographs revealed that very few aftershocks were associated with this earthquake. The largest aftershock had a magnitude (ML) of 3.4. The distribution of intensity of the mainshock observed on Vancouver Island differs from that predicted by the intensity versus distance relation presently used for western Canada.

Turner Valley Limestone Oilfield Past, Present, Future

Abstract A review of the history of development of the field is presented along withthe production history over a 26-year period. The performance of the reservoirindicates that the major recovery mechanism has been a gas in solution driveaided to a small extent by down dip water intrusion and gravity drainage.Waterflood performance predictions, based on detailed study of the north end ofthe Turner Valley field will be presented. Water injection on the down dip edgeof the total field is in progress. Current results are difficult to evaluatebecause of relatively short period of injection and change in operatingconditions. The injection pattern and facilities are described. Potentialresponse areas are dealt with. History Introduction The Turner Valley Limestone Oilfield is located 30 miles southwest of the City of Calgary, Alberta. It extends in a north-northwest direction from Lsd.16. Section 5, Township 18, Range 2, W.5M in the south to Lsd. 8, Section 24, Township 21, Range 3, W.5M in the north. The oil reservoir is 23 miles long andvaries in width from a half to one and one half miles. Production is obtainedfrom the Madison Paleozoic limestone. There is a gas cap associated with theoil reservoir. Development and Production The discovery well for the crude oil zone came into production on June 16,1936, almost 26 years ago. This well, "Turner Valley Royalties No. 1", wasdrilled in the southeast quarter of Lsd. 13, Section 26, Township 18, Range 2,W.5M, which was downdip from gas production in the limestone. Initialproduction was 960 barrels in 24 hours. The development of the field progressed at a fairly constant rate over thenext ten-year period, during which 293 wells were drilled primarily on 40-acrespacing. The maximum number of completions in any one year was 42. A total of317 oil wells have been completed in the pool to date.

Recent Salina and Trenton Discoveries Reflect Trend Toward Deeper Formational Drilling in Michigan Basin: ABSTRACT

Good producers, large lost circulation zones, blowouts, and high proration have focused the attention of the petroleum industry on southern Michigan. The Trenton-Black River formation play in this area and the basal Salina-Niagaran plays in eastern and western Michigan have rejuvenated a declining oil industry in Michigan. Structurally, Michigan can be defined as a part of an intracratonic basin, flanked on the east and southeast by the Cincinnati arch, on the south and southwest by the Kankakee arch, on the west by the Wisconsin arch and peninsula and on the north and northeast by the Canadian shield. Previous to the present activity, all the oil production of any consequence had been associated with sediments of Devonian age. However, the present developments have been mainly in Silurian and Ordovician sediments. This activity has been more or less confined to the flanks of the structural basin where the older formations are at a relatively shallow depth. In eastern Michigan, St. Clair County, the Silurian production is from reef-type build-ups which occurred during late Niagaran deposition. Production has also been obtained from the overlying basal carbonates of the Salina group. In Western Michigan, Allegan, and Ottawa counties, the producing units are the basal carbonates of the Salina group, namely, in descending order, the A-2 dolomite and the A-1 dolomite. Unlike eastern Michigan, this production is associated with regional structures which appear to have been affected by the lowermost evaporite unit of the Salina group, the A-1 salt. The present Trenton-Black River development of Middle Ordovician carbonates is located in south-central Michigan on the southern flank of the Michigan basin. Production has been from a secondary dolomite confined to a fracture zone. Structurally, the fracture zone is directly associated with a shallow narrow depression which plunges in a north-northwesterly direction about 50 feet per mile. Approximately 25 linear miles has been partially proven productive with efforts being made to extend the trend and also to prove the inside acreage. End_of_Article - Last_Page 1251------------

Structural Trends off the California Coast: ABSTRACT

The sea bottom off the California coast shows a series of steep escarpments with a dominant trend in a north-northwesterly direction. The continental slope escarpment follows this direction and is not influenced by the large change in trend found on shore at Point Conception. Many of the submarine escarpments show offsets with the northerly portions set to the northeast. Also a number of the escarpments show slope reversals along their length. Directly outside the continental slope there are a series of oval mountains with trends in a north-northeasterly direction. These are probably volcanoes and from their trends suggest possible tensional phenomena. End_of_Article - Last_Page 1716------------