Jonathan R. Childs

Active tectonics of the Seattle fault and central Puget sound, Washington - Implications for earthquake hazards

We use an extensive network of marine high-resolution and conventional industry seismic-reflection data to constrain the location, shallow structure, and displacement rates of the Seattle fault zone and crosscutting high-angle faults in the Puget Lowland of western Washington. Analysis of seismic profiles extending 50 km across the Puget Lowland from Lake Washington to Hood Canal indicates that the west-trending Seattle fault comprises a broad (4–6 km) zone of three or more south-dipping reverse faults. Quaternary sediment has been folded and faulted along all faults in the zone but is clearly most pronounced along fault A, the northernmost fault, which forms the boundary between the Seattle uplift and Seattle basin. Analysis of growth strata deposited across fault A indicate minimum Quaternary slip rates of about 0.6 mm/yr. Slip rates across the entire zone are estimated to be 0.7–1.1 mm/yr. The Seattle fault is cut into two main segments by an active, north-trending, high-angle, strike-slip fault zone with cumulative dextral displacement of about 2.4 km. Faults in this zone truncate and warp reflections in Tertiary and Quaternary strata and locally coincide with bathymetric lineaments. Cumulative slip rates on these faults may exceed 0.2 mm/yr. Assuming no other crosscutting faults, this north-trending fault zone divides the Seattle fault into 30–40-km-long western and eastern segments. Although this geometry could limit the area ruptured in some Seattle fault earthquakes, a large event ca. A.D. 900 appears to have involved both segments. Regional seismic-hazard assessments must (1) incorporate new information on fault length, geometry, and displacement rates on the Seattle fault, and (2) consider the hazard presented by the previously unrecognized, north-trending fault zone.

New seismic images of the Cascadia subduction zone from cruise SO108 — ORWELL

Abstract In April and May 1996, a geophysical study of the Cascadia continental margin off Oregon and Washington was conducted aboard the German R/V Sonne. This cooperative experiment by GEOMAR and the USGS acquired wide-angle reflection and refraction seismic data, using ocean-bottom seismometers (OBS) and hydrophones (OBH), and multichannel seismic reflection (MCS) data. The main goal of this experiment was to investigate the internal structure and associated earthquake hazard of the Cascadia subduction zone and to image the downgoing plate. Coincident MCS and wide-angle profiles along two tracks are presented here. The plate boundary has been imaged precisely beneath the wide accretionary wedge close to shore at ca. 13 km depth. Thus, the downgoing plate dips more shallowly than previously assumed. The dip of the plate changes from 2° to 4° at the eastern boundary of the wedge on the northern profile, where approximately 3 km of sediment is entering the subduction zone. On the southern profile, where the incoming sedimentary section is about 2.2 km thick, the plate dips about 0.5° to 1.5° near the deformation front and increases to 3.5° further landwards. On both profiles, the deformation of the accretionary wedge has produced six ridges on the seafloor, three of which represent active faulting, as indicated by growth folding. The ridges are bordered by landward verging faults which reach as deep as the top of the oceanic basement. Thus the entire incoming sediment package is being accreted. At least two phases of accretion are evident, and the rocks of the older accretionary phase(s) forms the backstop for the younger phase, which started around 1.5 Ma ago. This documents that the 30 to 50 km wide frontal part of the accretionary wedge, which is characterized by landward vergent thrusts, is a Pleistocene feature which was formed in response to the high input of sediment building the fans during glacial periods. Velocities increase quite rapidly within the wedge, both landward and downward. At the toe of the deformation front, velocities are higher than 4.0 km/s, indicating extensive dewatering of deep, oceanic sediment. Further landward, considerable velocity variation is found, which indicates major breaks throughout the accretionary history.

Seismic survey probes urban earthquake hazards in Pacific Northwest

A multidisciplinary seismic survey earlier this year in the Pacific Northwest is expected to reveal much new information about the earthquake threat to U.S. and Canadian urban areas there. A disastrous earthquake is a very real possibility in the region. The survey, known as the Seismic Hazards Investigation in Puget Sound (SHIPS), engendered close cooperation among geologists, biologists, environmental groups, and government agencies. It also succeeded in striking a fine balance between the need to prepare for a great earthquake and the requirement to protect a coveted marine environment while operating a large airgun array.

Evolution and petroleum geology of Amlia and Amukta intra-arc summit basins, Aleutian Ridge

Abstract Amlia and Amukta Basins are the largest of many intra-arc basins formed in late Cenozoic time along the crest of the Aleutian Arc. Both basins are grabens filled with 2–5 km of arc-derived sediment. A complex system of normal faults deformed the basinal strata. Although initial deposits of late Micocene age may be non-marine in origin, by early Pliocene time, most of the basinfill consisted of pelagic and hemipelagic debris and terrigenous turbidite deposits derived from wavebase and subaerial erosion of the arcs crestal areas. Late Cenozoic volcanism along the arc commenced during or shortly after initial subsidence and greatly contributed to active deposition in Amlia and Amukta Basins. Two groups of normal faults occur: major boundary faults common to both basins and ‘intra-basin’ faults that arise primarily from arc-parallel extension of the arc. The most significant boundary fault, Amlia-Amukta fault, is a south-dipping growth fault striking parallel to the trend of the arc. Displacement across this fault forms a large half-graben that is separated into the two depocentres of Amlia and Amukta Basins by the formation of a late Cenozoic volcanic centre, Seguam Island. Faults of the second group reflect regional deformation of the arc and offset the basement floor as well as the overlying basinal section. Intra-basin faults in Amlia Basin are predominantly aligned normal to the trend of the arc, thereby indicating arc-parallel extension. Those in Amukta basin are aligned in multiple orientations and probably indicate a more complex mechanism of faulting. Displacement across intra-basin faults is attributed to tectonic subsidence of the massif, aided by depositional loading within the basins. In addition, most intra-basin faults are listric and are associated with high growth rates. Although, the hydrocarbon potential of Amlia and Amukta Basins is difficult to assess based on existing data, regional considerations imply that an adequate thermal history conducive to hydrocarbon generation has prevailed during the past 6-5 my. The possibility for source rocks existing in the lower sections of the basins is suggested by exposures of middle and upper Miocene carbonaceous mudstone on nearby Atka Island and the implication that euxinic conditions may have prevailed during the initial formation of the basins. Large structures have evolved to trap migrating hydrocarbons, but questions remain concerning the preservation of primary porosity in a sedimentary section rich in reactive volcaniclastic debris.

Seismic-reflection signature of cretaceous continental breakup on the Wilkes Land margin, Antarctica

The passive (rifted) continental margin of Wilkes Land, Antarctica, is characterized on seismic reflection records by (i) in the south, a block-faulted sequence of highly stratified continental beds overlain by two distinct unconformities; (ii) a transitional, greatly thinned continental crust overlain by material interpreted to be flood basalt; and (iii) in the north, oceanic crust with a boundary ridge at its edge. The Mohorovičić discontinuity can be followed across the continent-ocean boundary and shows a progressive thinning of continental crust to a minimum of 2.5 kilometers at its northern edge.

Acquiring Marine Data in the Canada Basin, Arctic Ocean

Despite the record minimum ice extent in the Arctic Ocean for the past 2 years, collecting geophysical data with towed sensors in ice-covered regions continues to pose enormous challenges. Significant parts of the Canada Basin in the western Arctic Ocean have remained largely unmapped because thick multiyear ice has limited access even by research vessels strengthened against ice [Jackson et al., 1990]. Because of the resulting paucity of data, the western Arctic Ocean is one of the few areas of ocean in the world where major controversies still exist with respect to its origin and tectonic evolution [Grantz et al., 1990; Lawver and Scotese, 1990; Lane, 1997; Miller et al., 2006].

Scientific teams analyze earthquake hazards of the Cascadia Subduction Zone

Scientists from GEOMAR, the U.S. Geological Survey (USGS), and Oregon State University recently collected seismic reflection and wide-angle seismic data for the continental margin off the Oregon and Washington coasts—a subduction zone that poses a significant earthquake hazard to populated areas of the Pacific Northwest. Geologic findings indicate that great earthquakes (magnitude 8 to 9) have occurred within the Cascadia subduction zone and that an earthquake of this magnitude could some day devastate urban areas of the Pacific Northwest. One such quake may have rocked this region as recently as 300 years ago, and the debate concerning the imminence of the next one continues [Heaton and Kanamori, 1984; Atwater, 1992; Hyndman and Wang, 1993; Wang et al., 1995; Satake et al., 1996]. Potential earthquake source regions along the interplate decollement are unexplored. In fact, the geometry of the downgoing Juan de Fuca plate beneath Oregon and Washington has been poorly understood because of the paucity of shallow (<30 km) earthquakes.

Hydrocarbon Potential of Aleutian Basin, Bering Sea

The Aleutian basin is the deep-water (more than 3,000 m) basin that lies north of the Aleutian Islands adjacent to the Bering Sea continental shelf. The basin, about the size of Texas, is underlain by a flat-lying sequence of mostly Cenozoic sediment 2 to 9 km thick that rests on an igneous oceanic crustal section. Prior to 1974, marine investigations in the Aleutian basin were directed at understanding the basins regional geologic and geophysical framework; more recent investigations by the U.S. Geological Survey have been aimed at assessing the basins hydrocarbon potential. Preliminary results suggest that the four major requirements for hydrocarbon accumulations may be present--structural and stratigraphic traps, source rocks, reservoir beds, and an adequate thermal nd sedimentation history. The recent energy-resource studies indicate that: (1) numerous structural features (gentle folds, diapirs, basement ridges) are present in the central and eastern parts of the basin; (2) acoustic features called VAMPs (velocity amplitude features) are common (over 350 identified) in the central basin; these features may be caused by pockets of gases and possibly other hydrocarbons that have been trapped in the sedimentary section; (3) the sedimentary section consists of diatomaceous sediment overlying indurated mudstones; high porosities (58 to 85%) and good permeabilities (10 to 35 md) in the diatomaceous sediment suggest that it is a potential reservoir unit and the thick section of underlying mudstone may contain the source beds; (4) concentrations of organic gases, primarily metha e, in the upper 1 to 3 m beneath the seafloor are very small, increase with depth, and are highest in areas near VAMPs; (5) the thermal gradient and the sediment thickness are sufficiently large to allow hydrocarbon maturation at depth, if suitable organic material is present. Our initial results suggest that the Aleutian basin deserves further exploration as a site for possible hydrocarbon accumulations.

Development of San Leandro synform and neotectonics of the San Francisco Bay block, California

Abstract High-resolution, 24-channel seismic-reflection data show a stratified synform beneath south San Francisco Bay. These seismic-reflection data reveal an eastward-dipping bedrock surface that is about 40 m deep (subbottom) beneath the western south bay, and that reaches a maximum observed depth of 500–800 m (subbottom) below the eastern half of the south bay. An angular unconformity cuts both the synform and underlying bedrock. The age of the unconformity is unknown but may be Pleistocene, when these strata forming the synform were presumably exposed subaerially during lowered sea levels. The synformal strata, the unconformity, and some generally flat-lying and overlying strata are folded near the eastern shore of the bay. This folding may result from movement on the Hayward fault ( fault interactions and localized strain partitioning) or from compressional deformation in the East Bay Hills related to NE–SW (fault-normal) convergence between the Pacific and North American plates. In general, reflections from sediment overlying the unconformity are flat lying (except near the eastern shore of the bay), whereas reflections beneath the unconformity dip eastward. The overlying, flat sediment section fills a shallow basin that is coincident with an elliptical residual gravity low. This low appears to be related to the deeper sedimentary, synformal section based on the spatial correlation between the east-dipping reflections and the gravity anomaly. Projecting the east-dipping reflections to the center of the gravity low suggests that the total section of flat-lying and dipping reflections in the synform may exceed 1000 m. Modeling of the gravity low suggests a total low-density section, about 1.5 km thick, at the center of the synform relative to the surrounding bedrock of presumed Franciscan Complex.

Tectonic Evolution of Gulf of Anadyr and Formation of Anadyr and Navarin Basins

The Gulf of Anadyr is underlain by two major sedimentary basins, Kresta basin and East Anadyr trough, that trend east to southeast and contain, in places, more than 9 km (29,500 ft) of fill. The basins are flanked on the east and north by the Okhotsk-Chukotsk volcanic belt, a broad bedrock high composed of plutonic and volcanic rocks that extends from eastern Siberia along the inner Bering Sea shelf at least to St. Matthew Island. The East Anadyr trough extends onshore and connects with the larger Anadyr basin, which underlies the lowlands between the Koryak Range and Okhotsk-Chukotsk volcanic belt of eastern Siberia. New seismic reflection and refraction data reveal that Anadyr basin is separated from Navarin basin by Anadyr ridge, a southeast-northwest-trending bedrock high that is characterized by high-amplitude, short-wavelength magnetic anomalies. Anadyr ridge may be an offshore extension of the melange belt underlying the Koryak Range. Sonobuoy refraction data indicate that the velocity profile of strata in East Anadyr trough is similar to that in Navarin basin. Structurally, the basins are different: Navarin basin is complex and contains both compressional and extensional elements, whereas Anadyr basin is a simple, broad crustal sag semicircular in outline. Correlation of our reflection data from the offshore part of the Anadyr basin (including the East Anadyr trough and Kresta basin) with d illing data onshore allows us to differentiate three distinct sequences in the offshore portion of the basin. These sequences are separated by two strong reflectors, ^agr and s, and are tentatively identified with increasing depth as boundaries separating the Neogene and Paleogene, and Paleogene and Mesozoic, respectively. In the northeastern corner of the Gulf of Anadyr, across the Anaut uplift, shallow beds are folded and broken by faults that commonly offset the sea floor. Furthermore, earthquake epicenters recorded landward of Anadyr basin from Cape Navarin to the southwest and around the Chukotsk Peninsula to the northeast suggest recent tectonic movement in the Gulf of Anadyr near Cape Navarin and near Kresta Bay. We believe that Anadyr basin originally formed as a fore-arc basin during the Cretaceous as a result of underthrusting of the Kula plate beneath Siberia. The collision of the Kula plate with Siberia resulted in the formation of the Okhotsk-Chukotsk volcanic arc north of Anadyr basin and the Koryak melange belt south of the basin. Anadyr basin continued to subside during the Cenozoic and ajor uplift in the Koryak Range occurred in the late Miocene to Pliocene.