Offshore Southern California Research Articles

Motion and rigidity of the Pacific Plate and implications for plate boundary deformation

Using up to 11 years of data from a global network of Global Positioning System (GPS) stations, including 12 stations well distributed across the Pacific Plate, we derive present‐day Euler vectors for the Pacific Plate more precisely than has previously been possible from space geodetic data. After rejecting on statistical grounds the velocity of one station on each of the Pacific and North American plates, we find that the quality of fit of the horizontal velocities of 11 Pacific Plate (PA) stations to the best fitting PA Euler vector is similar to the fit of 11 Australian Plate (AU) velocities to the AU Euler vector and ∼20% better than the fit of nine North American Plate (NA) velocities to the NA Euler vector. The velocities of stations on the Pacific and Australian Plates each fit a rigid plate model with an RMS residual of 0.4 mm/yr, while the North American velocities fit a rigid plate model with an RMS velocity of 0.6 mm/yr. Our best fitting PA/AU relative Euler vector is located ∼170 km southeast of the NUVEL‐1A pole but is not significantly different at the 95% confidence level. It is also close (<70 km in position and <3% in rate) to a pole derived from transform faults identified from satellite altimetry, suggesting that the vector has not changed significantly over the past 3 Myr. Our relative Euler vector is also consistent with all known geological and geodetic evidence concerning the AU/PA plate boundary through New Zealand. The GPS sites offshore of southern California are presently moving 4–5 ± 1 mm/yr relative to predicted Pacific velocity, with their residual velocities in approximately the opposite direction to PA/NA relative motion. Likewise, the easternmost sites in South Island, New Zealand, are moving ∼3 ± 1 mm/yr relative to predicted Pacific velocity, with the residuals in approximately the opposite direction to PA/AU relative motion. These velocity residuals are in the same sense as predicted by elastic strain accumulation on known plate boundary faults but are of a significantly higher magnitude in both southern California and New Zealand, implying that the plate boundary zones in both regions are wider than previously believed.

The last interglacial period on the Pacific Coast of North America: Timing and paleoclimate

New, high-precision U-series ages of solitary corals ( Balanophyllia elegans ) coupled with molluscan faunal data from marine terraces on the Pacific Coast of North America yield information about the timing and warmth of the last interglacial sea-level highstand. Balanophyllia elegans takes up U in isotopic equilibrium with seawater during growth and shortly after death. Corals from the second terrace on San Clemente Island (offshore southern California), the third terrace on Punta Banda (on the Pacific Coast of northern Baja California), and the Discovery Point Formation on Isla de Guadalupe (in the Pacific Ocean offshore Baja California) date to the peak of the last interglacial period and have U-series ages ranging from ca. 123 to 114 ka. The first terrace on Punta Banda has corals with ages ranging from ca. 83 to 80 ka, which corresponds to a sea-level highstand formed in the late last interglacial period. U-series analyses of corals from the Cayucos terrace (central California) and the Nestor terrace at Point Loma (southern California) show that these fossils have evidence of open-system history, similar to what has been reported by other workers for the same localities. Nevertheless, a model of continuous, secondary U and Th uptake shows that two ages of corals are likely present at these localities, representing the ca. 105 and ca. 120 ka sea-level highstands reported elsewhere. U-series ages of last interglacial corals from the Pacific Coast overlap with, but are on average younger than the ages of corals from Barbados, the Bahamas, and Hawaii. This age difference is explained by the nature of the geomorphic response to sea-level change: fringing or barrier reefs on low- latitude coastlines have an accretionary growth style that keeps pace with rising sea level, whether on a tectonically rising or stable coastline. In contrast, midlatitude, high-energy coastlines are sites of platform cutting during the early part of a sea-level high stand and terrace scouring and concomitant sediment and fossil deposition as sea level starts to recede. The youngest ages of corals from the Pacific Coast suggest that sea level was still relatively high at ca. 116 ka, which is not in agreement with other estimates of relatively large global ice volume at that time. Reliably dated, ca. 120 ka marine-terrace deposits on the Pacific Coast have fossil mollusks that indicate water temperatures as warm or warmer than at present. In contrast, ca. 80 ka marine deposits reported here and elsewhere have fossil mollusks indicating cooler-than-modern water temperatures. The presence of both ca. 105 ka and ca. 120 ka corals on the Nestor and Cayucos terraces explains a previously enigmatic mixture of warm-water and cool-water mollusks. At ca. 105 ka, a relatively high sea level with cool waters may have “captured” the terrace formed during the 120 ka sea-level highstand, in areas of low uplift rate. The inference of cooler-than-modern waters off the Pacific Coast of North America at ca. 80 ka and ca. 105 ka, based on marine-terrace faunas, does not agree with estimates of sea-surface temperatures derived from alkenone studies in the Santa Barbara Basin. However, cooler water temperatures at these times are in agreement with paleotemperature estimates from planktonic foraminiferal data for the Santa Barbara Basin. All records, from central California to Baja California, whether from marine terraces or offshore cores, indicate at least seasonably warmer-than-modern waters during the peak of the last interglacial period at ca. 120 ka.

Blue whale call intensity varies with ambient noise level

Blue whales (Balaenoptera musculus) vary the intensity of their sound production level in response to varying ambient noise levels. Blue whales produce tonal, type B calls with a fundamental frequency of 16 Hz, and a strong third harmonic at 48 Hz. These calls, along with other low-frequency baleen whale vocalizations, ambient sea noise, and shipping noise have been monitored and recorded on continuous seafloor acoustic recorders near Cortez and Tanner Banks, offshore Southern California, since August 2000. By separately comparing the intensity in the 16 and 48 Hz spectral level bands with the intensity of ambient noise at and near those frequencies, we observe a correlation between the sound intensity level of blue whale type B calls and ambient noise levels. This is verified by the inspection of individual call intensity in various noise environments. Preliminary analysis indicates a less than unity proportional increase in the call intensity with increasing ambient noise. A trend of increased call intensity with an increased noise level has implications for the effect of shipping, and other natural and artificial noise sources in the marine environment on the behavior of blue whales.

Acoustic and visual monitoring for marine mammals at Cortez and Tanner Banks

We have implemented continuous acoustic monitoring for baleen whales in the region surrounding Cortez and Tanner Banks, offshore of Southern California, complemented by bimonthly ship-based visual and acoustic observations. Our objective is to develop methods for acoustic monitoring of baleen whales using seafloor acoustic recording packages, the results of which will be used to produce abundance estimates. The joint acoustic and visual effort will answer questions regarding the probability for acoustic detections of individual animals, the quantitative association of calls with a particular species, and how well the number of animals in an area can be assessed acoustically. These findings can then be used to generate a detectability curve for each baleen whale species, from which the seafloor recorder data can be translated into absolute abundance estimates. The use of continuous acoustic monitoring with seafloor recording packages offers the potential for efficient and economical monitoring of marine mammals. The continuous acoustic census approach will be discussed, and compared to opportunistic visual and acoustic survey methods.

Using high-resolution multibeam bathymetry to identify seafloor surface rupture along the Palos Verdes fault complex in offshore southern California

Using high-resolution multibeam bathymetry to identify seafloor surface rupture along the Palos Verdes fault complex in offshore southern California

Earthquake Locations in the Inner Continental Borderland, Offshore Southern California

The inner Continental Borderland region, offshore southern California, is tectonically active and contains several faults that are potential seismic hazards to nearby cities. However, fault geometries in this complex region are often poorly constrained due to a lack of surface observations and uncertainties in earthquake locations and focal mechanisms. To improve the accuracy of event locations in this area, we apply new location methods to 4312 offshore seismic events that occurred between 1981 and 1997 in seven different regions within the Borderland. The regions are defined by either temporal or spatial clustering of seismic activity in the Southern California Seismic Network (SCSN) catalog. Obtaining accurate locations for these events is difficult, due to the lack of nearby stations, the limited azimuthal coverage, and uncertainties in the velocity structure for this area. Our location procedure is based on the L-1 norm, grid search, waveform cross-correlation method of Shearer (1997), except that we use a nearest neighbor approach (Astiz et al., 2000) to identify suitable event pairs for waveform cross-correlation and we explore the effect of different velocity models on the locations and associated station terms. In general, our relocated events have small estimated relative location errors and the events are more clustered than the SCSN catalog locations. A quarry on the south tip of Catalina Island provides a test of our location accuracy and suggests that, under ideal conditions, offshore events can be located to within 1 to 2 km of their true locations. Our final locations for most clusters are well correlated with known local tectonic features. We relate the 1981 Santa Barbara Island ( M L = 5.3) earthquake with the Santa Cruz fault, the 13 July 1986 Oceanside ( M L = 5.3) sequence with the San Diego Trough fault zone, and events near San Clemente Island with the known trace of the San Clemente fault zone. Over 3000 of the offshore events during this time period are associated with the 1986 Oceanside earthquake and its extended aftershock sequence. Our locations define a northeast-dipping fault plane for the Oceanside sequence, but in cross-section the events are scattered over a broad zone (about 4-km thick). This could either be an expression of fault complexity or location errors due to unaccounted for variations in the velocity structure. Events that occur near Coronado Bank in the SCSN catalog are relocated closer to the San Diego coast and suggest a shallow-angle, northeast-dipping fault plane at 10 to 15 km depth. Manuscript received 12 February 1999.

Using high-resolution multibeam bathymetry to identify seafloor surface rupture along the Palos Verdes fault complex in offshore southern California

Recently acquired high-resolution multibeam bathymetric data reveal several linear traces that are the surficial expressions of seafloor rupture of Holocene faults on the upper continental slope southeast of the Palos Verdes Peninsula. High-resolution multichannel and boomer seismic-reflection profiles show that these linear ruptures are the surficial expressions of Holocene faults with vertical to steep dips. The most prominent fault on the multibeam bathymetry is about 10 km to the west of the mapped trace of the Palos Verdes fault and extends for at least 14 km between the shelf edge and the base of the continental slope. This fault is informally called the Avalon Knoll fault for the nearby geographic feature of that name. Seismic-reflection profiles show that the Avalon Knoll fault is part of a northwest-trending complex of faults and anticlinal uplifts that are evident as scarps and bathymetric highs on the multibeam bathymetry. This fault complex may extend onshore and contribute to the missing balance of Quaternary uplift determined for the Palos Verdes Hills and not accounted for by vertical uplift along the onshore Palos Verdes fault. We investigate the extent of the newly located offshore Avalon Knoll fault and use this mapped fault length to estimate likely minimum magnitudes for events along this fault.

Oceanside and Thirtymile Bank blind thrusts: Implications for earthquake hazards in coastal southern California

We define an active blind thrust system in offshore southern California that extends from Los Angeles south to the United States–Mexico international border. These blind thrusts formed by tectonic inversion of Miocene extensional detachments. We attribute the 1986 Oceanside (ML 5.3) earthquake, local uplift of marine terraces, seafloor fold scarps, and observed geodetic convergence to motion on these faults. Single and multisegment fault rupture scenarios suggest the potential for large (M 7.1–7.6) but infrequent earthquakes that would affect the Los Angeles and San Diego metropolitan areas.

Analysis of earthquake recordings obtained from the Seafloor Earthquake Measurement System (SEMS) instruments deployed off the coast of southern California

AbstractFor more than 20 years, a program has been underway to obtain records of earthquake shaking on the seafloor at sites offshore of southern California, near oil platforms. The primary goal of the program is to obtain data that can help determine if ground motions at offshore sites are significantly different than those at onshore sites; if so, caution may be necessary in using onshore motions as the basis for the seismic design of oil platforms. We analyze data from eight earthquakes recorded at six offshore sites; these are the most important data recorded on these stations to date. Seven of the earthquakes were recorded at only one offshore station; the eighth event was recorded at two sites. The earthquakes range in magnitude from 4.7 to 6.1. Because of the scarcity of multiple recordings from any one event, most of the analysis is based on the ratio of spectra from vertical and horizontal components of motion. The results clearly show that the offshore motions have very low vertical motions compared to those from an average onshore site, particularly at short periods. Theoretical calculations find that the water layer has little effect on the horizontal components of motion but that it produces a strong spectral null on the vertical component at the resonant frequency of P waves in the water layer. The vertical-to-horizontal ratios for a few selected onshore sites underlain by relatively low shear-wave velocities are similar to the ratios from offshore sites for frequencies less than about one-half the water layer P-wave resonant frequency, suggesting that the shear-wave velocities beneath a site are more important than the water layer in determining the character of the ground motions at lower frequencies.

Abstract :Making Proprietary Data Public to Help Stimulate Industry Interest and Academic Research: An Example from Offshore Southern California

Abstract :Making Proprietary Data Public to Help Stimulate Industry Interest and Academic Research: An Example from Offshore Southern California

Abstract: Making Proprietary Data Public to Help Stimulate Industry Interest and Academic Research: An Example from Offshore Southern California 

Abstract: Making Proprietary Data Public to Help Stimulate Industry Interest and Academic Research: An Example from Offshore Southern California 

Wave Climate Variability in Southern California

The incidence of major wave events is investigated for the Southern California offshore region for the period from 1984 to 1995. The significant influence of the world climate event popularly known as El Niño, which had been shown in an earlier study of the first 83 yr of the century, is seen to be a major influence in the recent decade. Although the overall wave intensity (as measured by the median wave height for all observations during a year) has decreased for the most part in the last 20 yr, the number of large wave events has been very much increased. An averaged sea surface temperature anomaly in the tropics is shown to be well correlated with the increases in large wave events.

Chemically diverse, sporadic volcanism at seamounts offshore southern and Baja California

Compositions of lavas from seven small to medium-sized seamounts, between lat 34.0N25&N, are all highly differentiated trachyte and trachyandesite. Based on 40 Ar/ 39 Ar laser fusion techniques, MORB-like lava from one of the northernedificesisasoldastheunderlying oceanic crust (>20 Ma), indicating that it originatedataspreadingcenter.Otherseamountlavaagesaremuchyoungerthanthe oceanic crust on which they reside, ranging from 16.8 6 0.3 to <7 Ma for some of the northern seamounts to 270 616 ka for the trachyte from Rocas Alijos. Similar highly evolved lavas cap fossil spreading centers like Guadalupe and Socorro Islands, but Rocas Alijos, based on magnetic anomalies, is not an abandoned spreading center but may instead have formed on a leaky transform fault. Some of the seamounts with transitional and alkalic lavas may have formed as part of a short, age-progressive chain formed by a short-lived mantle plume. Many others, aligned along abandoned spreading centers orfaultsandfracturezoneswhichareabundant in the tectonically complex region offshore southern and peninsular California, may have resulted from upwelling mantle diapirs in response to localized extension. Someoftheepisodesofvolcanismappearto havebeencontemporaneouswithvolcanism in the continental borderland and coastal southern California, suggesting linkage between extension along the continental margin and the seamount province farther offshore. The data available for the abundant volcanic edifices of varying sizes, shapes, and orientations in this region suggest that the seamounts formed from multiple episodes of chemically diverse volcanism, tappingvariablyenriched,heterogeneousmantle, which occurred sporadically from early Miocene to late Pleistocene.

Timing and Cause of Middle Tertiary Magmatism in Onshore and Offshore Southern California: ABSTRACT

Timing and Cause of Middle Tertiary Magmatism in Onshore and Offshore Southern California: ABSTRACT

Prediction of Oil Gravity Prior to Drill-Stem Testing in Monterey Formation Reservoirs, Offshore California

This paper discusses empirical geochemical correlations used to predict API oil gravities prior to drill-stem testing in Monterey Formation reservoirs, offshore southern California. The primary objective was to eliminate expensive well testing by identifying intervals that contain low-gravity, nonproducible oil (usually <14° API). However, the correlations proved very successful in accurately predicting (within 4° API) oil gravities that range from 5 to 35° API throughout the offshore Santa Barbara and Santa Maria areas. The primary data are weight-percent sulfur and Rock-Eval pyrolysis of bitumen chemically extracted from reservoir rock samples. In general, reservoirs that contain higher gravity, producible oil have bitumen organic sulfur contents of less than 5 wt. %, Rock-Eval bitumen S1 values greater than 450 mg HC/g bitumen, and Rock-Eval bitumen S1/S2 ratios greater than 1.0. These data are usually supplemented with Rock-Eval pyrolysis of the reservoir rock, where whole-rock S1/S2 ratios greater than 0.30 usually indicate associated oil gravities greater than 14° API. This analytical mix gives a multiple approach for estimating reservoir oil gravities within proposed drill-stem test (DST) intervals. Using this approach, oil gravities of more than 50 DSTs have been accurately predicted in the offshore southern California area. The technique is also useful for reevaluating API gravities in older wells where Monterey reservoirs were not the primary target. Moreover, the technique should have application elsewhere, provided the range of oil gravities are not the result of obvious biodegradation, and sufficient rock and oil samples are available to establish pertinent correlations.

The flow of the undercurrent over the continental borderland off southern California

Complex bathymetric features offshore of southern California have a pronounced effect upon the flow of the California Undercurrent Geostrophic dynamics and water mass characteristics are used to reveal the various paths of flow and spread of undercurrent waters for a July 1985 survey. Undercurrent waters are identified by relatively high spiciness and low dissolved oxygen at densities between σt = 26.4 and 26.9 (∼200 to 400 m). A jetlike core of flow follows the continental slope within the California Bight and exits the bight through a gap in the Santa Rosa‐Cortes Ridge. Mixtures that include undercurrent waters exit the bight through other gaps in the bathymetric ridge and about the southern end of the ridge. As evidenced by patterns in the fields of geostrophic currents and water mass characteristics, baroclinic instabilities develop about the ridge which result in the diversion of undercurrent waters offshore beneath the California Current and into the offshore mesoscale eddy field. The offshore subsurface diversion of undercurrent waters and their inclusion in the offshore mesoscale eddy field are also found in the long‐term mean distributions for July.

Platform-Well Tieback Procedures at Island Esther

Summary On March 2, 1983, Island Esther, a man-made producing island located 1 xh miles [2.4 km] offshore southern California, was destroyed by a severe winter storm. The 90 completed wells survived the storm, but some were badly damaged. In 1985, a platform was erected over these wells, and work to repair the wells began. This paper discusses the methods and equipment used for that operation.

Offshore Production of Heavy Oil

This paper presents methods developed to enable the lifting and producing of low-gravity, high-viscosity crude from offshore fields. Relative advantages and disadvantages of several lifting, treating, and transportation methods are discussed. The paper also reviews actual experiences with heavy-oil production offshore southern California, as well as plans for development of newly discovered heavy-oil fields in the Santa Maria basin offshore central California and offshore Sicily in the Mediterranean Sea.

Exploration Potential in Sale 95: Central and Southern California Offshore: ABSTRACT

Exploration Potential in Sale 95: Central and Southern California Offshore: ABSTRACT

Sword Field, Offshore California: Challenges in Making This Giant Oil Field Commercial: ABSTRACT

The major obstacles and challenges involved in exploration and development of a giant deep-water low-gravity oil field are exemplified in the undeveloped Sword field of offshore southern California. In 1979, Conoco Exploration identified a northeast-southwest-trending basement high in 800-2000 ft deep federal waters 12 mi southwest of Pt. Conception at the western end of the Santa Barbara Channel. The intended reservoir was fractured Miocene Monterey chert, siliceous shales or siltstones, and dolomites that are draped over the axially faulted anticlinal structure. Drilling of the initial well in OCS P-0322 in 1982 resulted in discovering the giant Sword field. A confirmation well drilled in OCS P-0320 indicates in-place reserves of well over 1 billion bbl. Although the discovered potential is significant, the low gravity (8.5/sup 0/-10.5/sup 0/ API) of the oils discovered to date, along with water depths in excess of 1500 ft, currently pose economic challenges to successful field development. Conoco and its partners are addressing the current economic barriers on several fronts. A three-dimensional seismic survey has been conducted to delineate reservoir geometry and to define probable variations in pay thickness and fracturing. A market feasibility study will be undertaken to assess the demand for low gravity crude from more » offshore California. Finally, Conoco has developed proprietary technology called OCHOS (Offshore California Heavy Oil System), which uses an innovative oil and/or water emulsion technique to allow for more economic recovery of high-velocity or low-gravity crudes. « less