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Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

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At least 181 records · Page 10Linked to original sources

USGS2014

Marsh dieback, loss, and recovery mapped with satellite optical, airborne polarimetric radar, and field data

Landsat Thematic Mapper and Satellite Pour l'Observation de la Terre (SPOT) satellite based optical sensors, NASA Uninhabited Aerial Vehicle synthetic aperture radar (UAVSAR) polarimetric SAR (PolSAR), and field data captured the occurrence and the recovery of an undetected dieback that occurred between the summers of 2010, 2011, and 2012 in the Spartina alterniflora marshes of coastal Louisiana. Field measurements recorded the dramatic biomass decrease from 2010 to 2011 and a biomass recovery in 2012 dominated by a decrease of live biomass, and the loss of marsh as part of the dieback event. Based on an established relationship, the near-infrared/red vegetation index (VI) and site-specific measurements delineated a contiguous expanse of marsh dieback encompassing 6649.9 ha of 18,292.3 ha of S. alterniflora marshes within the study region. PolSAR data were transformed to variables used in biophysical mapping, and of this variable suite, the cross-polarization HV (horizontal send and vertical receive) backscatter was the best single indicator of marsh dieback and recovery. HV backscatter exhibited substantial and significant changes over the dieback and recovery period, tracked measured biomass changes, and significantly correlated with the live/dead biomass ratio. Within the context of regional trends, both HV and VI indicators started higher in pre-dieback marshes and exhibited substantially and statistically higher variability from year to year than that exhibited in the non-dieback marshes. That distinct difference allowed the capturing of the S. alterniflora marsh dieback and recovery; however, these changes were incorporated in a regional trend exhibiting similar but more subtle biomass composition changes.

Louisiana↗

USGS1999

Late 20th century increase in South Pole snow accumulation

A compilation of the 37-year history of net accumulation at the South Pole [ Mosley-Thompson et al ., 1995] suggests an increase in net annual accumulation since 1965. This record is sporadic and its quality is compromised by spatially restricted observations and nonsystematic measurement procedures. Results from a new, spatially extensive network of 236 accumulation poles document that the current 5-year (1992–1997) average annual net accumulation at the South Pole is 84.5 ± 8.9 mm water equivalent (w.e.). This accumulation rate reflects a 30% increase since the 1960s when the best, although not optimal, records indicate that it was 65 mm w.e. Identification of two prominent beta radioactivity horizons (1954/1955 and 1964/1965) in six firn cores confirms an increase in accumulation since 1965. Viewed from a longer perspective of accumulation provided by ice cores and a snow mine study, the net accumulation of the 30-year period, 1965–1994, is the highest 30-year average of this millennium. Limited data suggest this recent accumulation increase extends beyond the South Pole region and may be characteristic of the high East Antarctic Plateau. Enhanced accumulation over the polar ice sheets has been identified as a potential early indicator of warmer sea surface temperatures and may offset a portion of the current rise in global sea level.

Journal of Geophysical Research D: Atmospheres↗

USGS2001

Travel times of P and S from the global digital seismic networks: Implications for the relative variation of P and S velocity in the mantle

We present new data sets of P and S arrival times which have been handpicked from long-period vertical and transverse component recordings of the various global seismic networks. Using events which occurred from 1976 to 1994 results in ∼38,000 globally well-distributed measurements of teleseismic P and ∼41,000 measurements of S . These data are particularly useful for looking at the relative variation of S and P velocities in the lower mantle. We describe both the measurement techniques and the gross characteristics of the data sets. The size of our data sets allows us to exploit the internal consistency of the data to identify outliers using a summary ray analysis. Since the polarity of each arrival is also known, we can construct fault plane solutions and/or compare with polarities predicted by the Harvard centroid moment tensor solutions to further diagnose phase misidentification. This analysis results in ∼5% of the data being identified as outliers. An analysis of variance indicates that the S residual travel times are dominated by the effects of three-dimensional structure but the P data have comparable contributions from noise and source mislocation effects. The summary ray analysis reveals the basic character of lower mantle structure, and there are large-scale patterns in both the S and P data sets that correlate quite well with each other. This analysis suggests that on average, d ln v s / d ln v p is an increasing function of depth in the mantle going from a value of ∼1.7 at the top of the lower mantle to an apparent value of 4 near the base of the mantle. This latter extreme value of R seems to result mainly from data which sample one region in the lowermost mantle under the central Pacific, where large positive S residuals are associated with very small P residuals. Such an anomaly cannot be thermal in origin.

Journal of Geophysical Research B: Solid Earth↗

USGS1998

Non-double-couple earthquakes 2. Observations

Most studies assume that earthquakes have double-couple (DC) source mechanisms, corresponding to shear motion on planar faults. However, many well-recorded earthquakes have radiation patterns that depart radically from this model, indicating fundamentally different source processes. Seismic waves excited by advective processes, such as landslides and volcanic eruptions, are consistent with net forces rather than DCs. Some volcanic earthquakes also have single-force mechanisms, probably because of advection of magmatic fluids. Other volcanic earthquakes have mechanisms close to compensated linear vector dipoles and may be caused by magmatic intrusions. Shallow earthquakes in volcanic or geothermal areas and mines often have mechanisms with isotropic components, indicating volume changes of either explosive or implosive polarity. Such mechanisms are consistent with failure involving both shear and tensile faulting, which may be facilitated by high-pressure, high-temperature fluids. In mines, tunnels are cavities that may close. Deep-focus earthquakes occur within zones of polymorphic phase transformations in the upper mantle at depths where stick-slip instability cannot occur. Their mechanisms tend to be deviatoric (volume conserving), but non-DC, and their source processes are poorly understood. Automatic global moment tensor services routinely report statistically significant non-DC components for large earthquakes, but detailed reexamination of individual events is required to confirm such results.

Reviews of Geophysics↗

USGS1986

Paleoceanographic and tectonic controls on deposition of the Monterey formation and related siliceous rocks in California

The timing of paleoceanographic and tectonic events that shaped the deposition of the Monterey Formation of California and related siliceous rocks has been determined by application of a refined biochronology. The base of the Monterey at 17.5 Ma coincides with rising global sea level and a switch in biogenous silica deposition from the Caribbean and low-latitude North Atlantic to the North Pacific. Major polar cooling, which began at 15 Ma, postdates the base of the Monterey by more than 2 Ma and cannot be invoked to cause the deposition of diatomaceous sediments occurring in the lowermost Monterey. Later polar cooling in the early late Miocene, however, apparently caused increased upwelling and deposition of purer diatomites in the upper Monterey. The top of the Monterey at about 6 Ma coincides with a major sea level drop and is commonly marked by an unconformity. Equivalent unconformities are widespread around the rim of the North Pacific and typically separate more pelagic sediments from overlying sediments with a greater terrigenous component. Above the Monterey, diatoms persist in California sediments to 4.5–4.0 m.y., where their decline coincides with increased deposition of diatoms in the Antarctic. Carbon isotope records in the Pacific and Indian Oceans record storage of 12 C in the Monterey Formation and equivalent organic-rich sediments around the rim of the North Pacific. A +1.0‰ excursion in δ 13 C beginning at 17.5 Ma coincides with rising sea level and probably reflects storage of organic material in Monterey-like marginal reservoirs. A reverse −1.0‰ shift at 6.2 Ma closely approximates the top of the Monterey and may represent erosion of these marginal reservoirs and reintroduction of stored organic carbon into the ocean—atmosphere system. Initiation of transform faulting and extension in the California margin in the latest Oligocene and early Miocene caused the subsidence of basins which later received Monterey sediments. A major tectonic event centered at 10–9 Ma, which included rotation of the Santa Barbara Basin and a change in Pacific plate motion, is recorded by an unconformity and/or interval of compressed sediments in many Monterey sections. Another change in plate motion at about 6 Ma may be expressed by the angular discordance observed in many sections across the unconformity at the top of the Monterey.

California↗

USGS2018

Observations of rotational motions from local earthquakes using two temporary portable sensors in Waynoka, Oklahoma

Characterizing rotational motions from earthquakes at local distances has the potential to improve earthquake engineering and seismic gradiometry by better characterizing the complete seismic wavefield. Applied Technology Associates (ATA) has developed a proto‐seismic magnetohydrodynamic (SMHD) three‐component rotational rate sensor. We deploy two ATA rotational rate sensors at a temporary aftershock station in Waynoka, Oklahoma. From 27 April to 6 June 2017, we recorded the translational and rotational motions of 155 earthquakes of ML≥2.0 within 220 km of the station. Using the recorded events, we compare peak ground rotation rate (⁠ PGω˙ ⁠) with peak ground velocity (PGV) and with peak ground acceleration (PGA). Our results support previously identified potential relationships between the two quantities. We also compare peak ground rotations (⁠ PGω ⁠) as a function of seismic moment and distance. We found that PGω˙ decays with an exponent of approximately −4.0km−1 for both horizontal and vertical components. On the other hand, PGA decays with an exponent of approximately −1.8km−1 for all components. We compute apparent phase velocity directly from the rotational data for both horizontally polarized shear waves (SH; 379m/s with a standard deviation of 114m/s ⁠) and vertically polarized compression and shear waves (P‐SV; 387m/s with a standard deviation of 121m/s ⁠). Finally, by comparing various rotational and translational components, we look at potential implications for estimating local event source parameters. We found that the absolute correlation of nearby earthquakes decays at a rate of approximately 0.39/km for rotational sensors. This decay rate of absolute correlation is faster on translational sensors with a decay rate of 0.44/km. The latter may help in identifying phenomena such as repeating earthquakes by using differences in correlations as a function of distance and how these differences compare with translational correlations.

Oklahoma↗

USGS2010

Use of electrical imaging and distributed temperature sensing methods to characterize surface water–groundwater exchange regulating uranium transport at the Hanford 300 Area, Washington

We explored the use of continuous waterborne electrical imaging (CWEI), in conjunction with fiber‐optic distributed temperature sensor (FO‐DTS) monitoring, to improve the conceptual model for uranium transport within the Columbia River corridor at the Hanford 300 Area, Washington. We first inverted resistivity and induced polarization CWEI data sets for distributions of electrical resistivity and polarizability, from which the spatial complexity of the primary hydrogeologic units was reconstructed. Variations in the depth to the interface between the overlying coarse‐grained, high‐permeability Hanford Formation and the underlying finer‐grained, less permeable Ringold Formation, an important contact that limits vertical migration of contaminants, were resolved along ∼3 km of the river corridor centered on the 300 Area. Polarizability images were translated into lithologic images using established relationships between polarizability and surface area normalized to pore volume ( S por ). The FO‐DTS data recorded along 1.5 km of cable with a 1 m spatial resolution and 5 min sampling interval revealed subreaches showing (1) temperature anomalies (relatively warm in winter and cool in summer) and (2) a strong correlation between temperature and river stage (negative in winter and positive in summer), both indicative of reaches of enhanced surface water–groundwater exchange. The FO‐DTS data sets confirm the hydrologic significance of the variability identified in the CWEI and reveal a pattern of highly focused exchange, concentrated at springs where the Hanford Formation is thickest. Our findings illustrate how the combination of CWEI and FO‐DTS technologies can characterize surface water–groundwater exchange in a complex, coupled river‐aquifer system.

Washington↗

USGS1997

Extensional reactivation of the Chocolate Mountains subduction thrust in the Gavilan Hills of southeastern California

The NE vergent Chocolate Mountains fault of south-eastern California has been interpreted as either a subduction thrust responsible for burial and prograde metamorphism of the ensimatic Orocopia Schist or as a normal fault involved in the exhumation of the schist. Our detailed structural analysis in the Gavilan Hills area provides new evidence to confirm the latter view. A zone of deformation is present at the top of the Orocopia Schist in which lineations are parallel to those in the upper plate of the Chocolate Mountains fault but oblique to ones at relatively deep levels in the schist. Both the Orocopia Schist and upper plate contain several generations of shear zones that show a transition from crystalloblastic through mylonitic to cataclastic textures. These structures formed during retrograde metamorphism and are considered to record the exhumation of the Orocopia Schist during early Tertiary time as a result of subduction return flow. The Gatuna fault, which places low-grade, supracrustal metasediments of the Winterhaven Formation above the gneisses of the upper plate, also seems to have been active at this time. Final unroofing of the Orocopia Schist occurred during early to middle Miocene regional extension and may have involved a second phase of movement on the Gatuna fault. Formation of the Chocolate Mountains fault during exhumation indicates that its top-to-the-NE sense of movement provides no constraint on the polarity of the Orocopia Schist subduction zone. This weakens the case for a previous model involving SW dipping subduction, while providing support for the view that the Orocopia Schist is a correlative of the Franciscan Complex.

California↗

USGS2003

Seismic velocity anisotropy and heterogeneity beneath the Mantle Electromagnetic and Tomography Experiment (MELT) region of the East Pacific Rise from analysis of P and S body waves

We use teleseismic P and S delay times and shear wave splitting measurements to constrain isotropic and anisotropic heterogeneity in the mantle beneath the southern East Pacific Rise (SEPR). The data comprise 462 P and S delay times and 18 shear wave splitting observations recorded during the Mantle Electromagnetic and Tomography (MELT) Experiment. We estimate the mantle melt content (F) and temperature (T) variation from the isotropic velocity variation. Our results indicate that the maximum variation in F beneath our array is between zero and ???1.2%, and maximum variation in T is between zero and ???100 K. We favor an explanation having partial contributions from both T and F. We approximate the seismic anisotropy of the upper mantle with hexagonal symmetry, consistent with the assumption of two dimensionality of mantle flow. Our new tomographic technique uses a nonlinear inversion of P and slow S polarization delay times to simultaneously solve for coupled VP and VS heterogeneity throughout the model and for the magnitude of anisotropy within discrete domains. The domain dimensions and the dip of the anisotropy are fixed for each inversion but are varied in a grid search, obtaining the misfit of the models to the body wave delay data and to split times of vertically propagating S waves. The data misfit and the isotropic heterogeneity are sensitive to domain dimensions and dip of anisotropy. In a region centered beneath the SEPR the best average dip of the hexagonal symmetry axis is horizontal or dipping shallowly (<30??) west. Given the resolution of our data, a subaxial region characterized by vertically aligned symmetry axes may exist but is limited to be <80 km deep. We infer that the mantle flow beneath the SEPR is consistent with shallow asthenospheric return flow from the direction of the South Pacific superswell.

Journal of Geophysical Research B: Solid Earth↗

USGS2000

Global ice-core research: Understanding and applying environmental records of the past

One way to study Earth’s past environmental conditions is to look at ice cores recovered from glaciers. Every year a layer of snow accumulates on glaciers, like a page in a history book, and eventually turns to ice. Like reading the pages of a history book, analyzing the layers in a glacial ice core for specific chemical and physical components is a way of “reading” the environmental changes of the past. Information from ice cores collected from Greenland and Antarctica already has provided important historical clues toward a better understanding of modern global environmental changes (Dansgaard and Oeschger, 1989; Lorius and others, 1989). Environmental changes are of major concern at low- or mid-latitude regions of our Earth simply because this is where 80 to 90 percent of the world’s human population live. Ice cores collected from isolated polar regions are, at best, proxy indicators of low- and mid-latitude environmental changes. Because polar icecore research is limiting in this sense, ice cores from low- and mid-latitude glaciers are being used to study past environmental changes in order to better understand and predict future environmental changes that may affect the populated regions of the world.

Idaho;Wyoming↗

USGS1973

Bottom features and processes related to drifting ice on the Arctic shelf, Alaska

Early investigations of artic shelf regions led to the hypothesis that certain micro-relief forms are related to the action of grounded ice (for example, Rex, 1955). Since the introduction of side-scan sonar as a tool for ocean-floor surveys, a number of workers have described the occurrence of linear bottom features produced by grounded ice. Such features have been found on modern polar shelves (Skinner, 1971; Pelletier and Shearer, 1972; Kovacs, 1972; Reimnitz and Barnes, 1972; Brooks, 1973), and as relict features (Berkson and Clay, 1973; Belderson and Wilson, 1973; Belderson and others, 1973). Studies of core samples, high resolution seismic profiles, and diving observations indicate that the sediments of the Beaufort Sea Shelf are highly distorted by the action of grounding ice, and that the recurrence rate of gouging is very high. It has also become evident that grounding ice is contributing considerably to the sediment transport processes of modern Arctic shelves. Sediments from similar environments exposed on the continents today should also contain the record of ice gouging and related processes. Thus it has become apparent that drifting ice is an important agent influencing the sedimentary structures and the sediment transport regime of Arctic shelves today and has been in the past. The diagrams presented here, with supporting evidence in the form of side-scan sonar records, and ice and bottom photos, demonstrate the most prevalent processes and types of bottom features observed on the continental shelf off northern Alaska. As the map shows, most of the shelf is affected by these processes today.

Alaska↗

USGS2014

Quaternary ostracode and foraminiferal biostratigraphy and paleoceanography in the western Arctic Ocean

The stratigraphic distributions of ostracodes and selected calcareous benthic and planktic foraminiferal species were studied in sediment cores from ~ 700 to 2700 m water depth on the Northwind, Mendeleev, and Lomonosov Ridges in the western Arctic Ocean. Microfaunal records in most cores cover mid- to late Quaternary sediments deposited in the last ~ 600 ka, with one record covering the last ~ 1.5 Ma. Results show a progressive faunal turnover during the mid-Pleistocene transition (MPT, ~ 1.2 to 0.7 Ma) and around the mid-Brunhes event (MBE, ~ 0.4 Ma) reflecting major changes in Arctic Ocean temperature, circulation and sea-ice cover. The observed MPT shift is characterized by the extinction of species that today inhabit the sea-ice free subpolar North Atlantic and/or seasonally sea-ice free Nordic Seas ( Echinocythereis sp., Rockallia cf. enigmatica , Krithe cf. aquilonia , Pterygocythereis vannieuwenhuisei ). After a very warm interglacial during marine isotope stage (MIS) 11 dominated by the temperate planktic foraminifer Turborotalita egelida , the MBE experienced a shift to polar assemblages characteristic of predominantly perennial Arctic sea-ice cover during the interglacial and interstadial periods of the last 300 ka. These include the planktic foraminifera Neogloboquadrina pachyderma , the sea-ice dwelling ostracode Acetabulastoma arcticum and associated benthic taxa Pseudocythere caudata , Pedicythere neofluitans , and Polycope spp. Several species can be used as biostratigraphic markers of specific intervals such as ostracodes Rabilimis mirabilis — MIS 5 and P. vannieuwenhuisei extinction after MIS 11, and foraminiferal abundance zones Bulimina aculeata — late MIS 5 and Bolivina arctica — MIS 5-11.

Marine Micropaleontology↗

USGS2020

Towards understanding relationships between atmospheric pressure variations and long-period horizontal seismic data: A case study

Variations in atmospheric pressure have long been known to introduce noise in long-period (>10 s) seismic records. This noise can overwhelm signals of interest such as normal modes and surface waves. Generally, this noise is most pronounced on the horizontal components where it arises due to tilting of the seismometer in response to changes in atmospheric pressure. Several studies have suggested methodologies for correcting unwanted pressure-induced noise using collocated microbarograph records. However, how applicable these corrections are to varying geologic settings and installation types (e.g. vault versus post-hole) is unclear. Using coefficients obtained by solving for the residuals of these corrections, we can empirically determine the sensitivity of instruments in a specific location to the influences of pressure. To better understand how long-period, pressure-induced noise changes with time and emplacement, we examine horizontal seismic records along with barometric pressure at five different Global Seismographic Network stations, all with multiple broadband seismometers. We also analyse three Streckeisen STS-2 broadband seismometers, which are collocated with a microbarograph, at the Albuquerque Seismological Laboratory. We observe periods of high magnitude-squared-coherence (γ 2 -coherence; γ 2 > 0.8) between the seismic and pressure signals which fluctuate through time, frequency, and even between seismic instruments in the same vault. These observations suggest that these tilt-generated signals are highly sensitive to very local (<10 m) site effects. However, we find that in cases where instruments are not located at a large depth (<100 m), the pressure-induced noise is polarized in a nearly constant direction that is consistent with local topographic features or the geometry of the vault. We also find that borehole instruments at a large depth (>100 m) appear to be unaffected by pressure-loading mechanisms outlined by Sorrells (1971) but possibly by Newtonian attraction. Correlating the induced-noise polarization direction with times of high coherence, we work to identify sensors that are ultimately limited by pressure-induced horizontal noise as well as period bands that can benefit from pressure corrections. We find that while the situation is complex, each sensor appears to have its own unique response to pressure. Our findings suggest that we can determine empirical relationships between pressure and induced tilt on a case by case basis.

Geophysical Journal International↗

USGS2004

Seismic reflection and refraction data acquired in Canada Basin, Northwind Ridge and Northwind Basin, Arctic Ocean in 1988, 1992 and 1993

Seismic reflection and refraction data were collected in generally ice-covered waters of the Canada Basin and the eastern part of the Chukchi Continental Borderland of the Amerasia Basin, Arctic Ocean, during the late summers of 1988, 1992, and 1993. The data were acquired from a Polar class icebreaker, the U.S. Coast Guard Cutter Polar Star, using a seismic reflection system designed by the U.S. Geological Survey (USGS). The northernmost data extend to 78? 48' N latitude. In 1988, 155 km of reflection data were acquired with a prototype system consisting of a single 195 cubic inch air gun seismic source and a two-channel hydrophone streamer with a 150-m active section. In 1992 and 1993, 500 and 1,900 km, respectively, of seismic reflection profile data were acquired with an improved six air gun, 674 to 1303 cubic inch tuned seismic source array and the same two-channel streamer. In 1993, a 12-channel streamer with a 150-m active section was used to record five of the reflection lines and one line was acquired using a three air gun, 3,000 cubic inch source. All data were recorded with a DFS-V digital seismic recorder. Processed sections feature high quality vertical incidence images to more than 6 km of sub-bottom penetration in the Canada Basin. Refraction data were acquired with U.S. Navy sonobuoys recorded simultaneously with the seismic reflection profiles. In 1988 eight refraction profiles were recorded with the single air gun, and in 1992 and 1993 a total of 47 refraction profiles were recorded with the six air gun array. The sonobuoy refraction records, with offsets up to 35 km, provide acoustic velocity information to complement the short-offset reflection data. The report includes trackline maps showing the location of the data, as well as both digital data files (SEG-Y) and images of all of the profiles.

Open-File Report↗

USGS1977

Paleomagnetic investigation of late Quaternary sediments of south San Francisco Bay, California

Paleomagnetic inclinations of the Late Quaternary sediments of South San Francisco Bay were determined from bore hole samples collected near Dumbarton Bridge. The sediments consist of estuarine muds and nonmarine sand deposits, floored by bedrock of the Mesozoic Franciscan Formation. - Beneath Dumbarton Bridge the entire sedimentary fill is normally polarized; therefore, the fill postdates the Brunhes-Matayama polarity reversal (700,000 y. B.P.). Magnetic time lines such as the Mono Lake excursion (24,000 y. B.P.) and the reversed Blake event (110,000 y B.P.) were not found in this bore hole. In addition to Holocene and modern deposits of San Francisco Bay, an older estuarine unit occurs in the stratigraphic section. The older unit was deposited during a period of high sea level, tentatively correlated with the Sangamon interglacial period. Because evidence of the Blake event is not present in the older estuarine unit, the proposed age of this unit could not be confirmed. Although the Holocene estuarine deposits of South San Francisco Bay carry stable remanent magnetization, a reliable record of geomagnetic secular variation could not be recovered because the water-saturated sdiment was deformed by drilling.

California↗

USGS1978

Time-dependence of sea-ice concentration and multiyear ice fraction in the Arctic Basin

The time variation of the sea-ice concentration and multiyear ice fraction within the pack ice in the Arctic Basin is examined, using microwave images of sea ice recently acquired by the Nimbus-5 spacecraft and the NASA CV-990 airborne laboratory. The images used for these studies were constructed from data acquired from the Electrically Scanned Microwave Radiometer (ESMR) which records radiation from earth and its atmosphere at a wavelength of 1.55 cm. Data are analyzed for four seasons during 1973-1975 to illustrate some basic differences in the properties of the sea ice during those times. Spacecraft data are compared with corresponding NASA CV-990 airborne laboratory data obtained over wide areas in the Arctic Basin during the Main Arctic Ice Dynamics Joint Experiment (1975) to illustrate the applicability of passive-microwave remote sensing for monitoring the time dependence of sea-ice concentration (divergence). These observations indicate significant variations in the sea-ice concentration in the spring, late fall and early winter. In addition, deep in the interior of the Arctic polar sea-ice pack, heretofore unobserved large areas, several hundred kilometers in extent, of sea-ice concentrations as low as 50% are indicated. ?? 1978 D. Reidel Publishing Company.

Boundary-Layer Meteorology↗

USGS2012

Earthquakes in the Orozco transform zone: seismicity, source mechanisms, and tectonics

As part of the Rivera Ocean Seismic Experiment, a network of ocean bottom seismometers and hydrophones was deployed in order to determine the seismic characteristics of the Orozco transform fault in the central eastern Pacific. We present hypocentral locations and source mechanisms for 70 earthquakes recorded by this network. All epicenters are within the transform region of the Orozco Fracture Zone and clearly delineate the active plate boundary. About half of the epicenters define a narrow line of activity parallel to the spreading direction and situated along a deep topographic trough that forms the northern boundary of the transform zone (region 1). Most focal depths for these events are very shallow, within 4 km of the seafloor; several well-determined focal depths, however, are as great as 7 km. No shallowing of seismic activity is observed as the rise-transform intersection is approached; to the contrary, the deepest events are within 10 km of the intersection. First motion polarities for most of the earthquakes in region 1 are compatible with right-lateral strike slip faulting along a nearly vertical plane, striking parallel to the spreading direction. Another zone of activity is observed in the central part of the transform (region 2). The apparent horizontal and vertical distribution of activity in this region is more scattered than in the first, and the first motion radiation patterns of these events do not appear to be compatible with any known fault mechanism. Pronounced lateral variations in crustal velocity structure are indicated for the transform region from refraction data and measurements of wave propagation directions. The effect of this lateral heterogeneity on hypocenters and fault plane solutions is evaluated by tracing rays through a three-dimensional velocity grid. While findings for events in region 1 are not significantly affected, in region 2, epicentral mislocations of up to 10 km and azimuthal deflections of up to 45° may result from assuming a laterally homogeneous velocity structure. When corrected for the effects of lateral heterogeneity, the epicenters and fault plane solutions for earthquakes in region 2 are compatible with predominantly normal faulting along a topographic trough trending NW–SE; the focal depths, however, are poorly constrained. These results suggest an en echelon spreading center or leaky transform regime in the central transform region.

Journal of Geophysical Research B: Solid Earth↗

USGS2010

Age and origin of Cretaceous planktonic foraminifers from limestone of the Franciscan Complex near Laytonville, California

Detailed planktonic foraminiferal biostratigraphy from eight measured sections of Cretaceous limestone near Laytonville, California, indicates a composite sequence that extends in age from late Albian to early Turonian. The sequence contains seven biozones and two subzones based on the first and last appearance datums of planktonic foraminifers examined in thin section. Unequivocal biostratigraphic facing directions show four sections are right side up and four are reversed, and confirm the stratigraphic polarity employed in the paleomagnetic studies of Alvarez et al. (1980) and Tarduno et al. (1986). Temporal changes in the microfauna deviate from the global trend. Early Albian through early Cenomanian planktonic foraminiferal assemblages dominated by opportunistic, eurytopic species of Hedbergella and Globigerinelloides give way in the late Cenomanian to assemblages containing more abundant larger, heavily ornamented stenotopic species of Rotalipora and Praeglobotruncana typical of stratified Tethyan oceans. Radiolarian replacement chert increases slightly in the late Albian to early Cenomanian part of the Laytonville sequence and then decreases in the late Cenomanian and early Turonian. Benthic foraminifers show a similar decrease in abundance and diversity. We propose that this complex of biogenic and lithogenic patterns records transit via oceanic plate motion from a depositional site in the southern part of the paleoequatorial zone of high productivity, characterized by pronounced upwelling and habitat destabilization, perhaps augmented by topographic upwelling, to the central part of the equatorial zone dominated by biogenic calcite deposition. Our model of northward transit from below the equator supports the paleomagnetic determinations of Alvarez et al. (1980) and Tarduno et al. (1986). Arrival at the paleoequator coincided with the onset of stratification in the world ocean during the middle Cenomanian, indicated by the diversification of the more complex rotaliporids, and the subsequent intensification of oxygen-depleted intermediate waters in the latest Cenomanian to early Turonian. Two pulses in heterohelicid abundance signify the onset or intensification of an oxygen-minimum layer if Cretaceous heterohelicids occupied habitats analagous to Tertiary biserial heterohelicids. The first pulse beginning in the middle Cenomanian Rotalipora reicheli Zone may indicate a minor expansion of the oxygen-minimum zone or regional upwelling due to transit of the site beneath the equatorial divergence. The second pulse in the late Cenomanian Dicarinella algeriana Subzone signals the onset of upwelling of deeper oceanic water masses that characterized the succeeding Whiteinella archaeocretacea Zone. Organic-rich black shales typical of the Whiteinella archaeocretacea Zone are missing at Laytonville, although samples do contain the low-diversity, partially dissolved, planktonic assemblages that characterize this zone. The lack of black shales suggests that deposition occurred at depths greater than the oxygen-depleted intermediate water depths or alternately indicates paleoceanographic conditions unique to the Pacific.

California↗