LeRoy M. Dorman

Geodetic and seismic constraints on some seismogenic zone processes in Costa Rica

New seismic and geodetic data from Costa Rica provide insight into seismogenic zone processes in Central America, where the Cocos and Caribbean plates converge. Seismic data are from combined land and ocean bottom deployments in the Nicoya peninsula in northern Costa Rica and near the Osa peninsula in southern Costa Rica. In Nicoya, inversion of GPS data suggests two locked patches centered at 14 ± 2 and 39 ± 6 km depth. Interplate microseismicity is concentrated in the more freely slipping intermediate zone, suggesting that small interseismic earthquakes may not accurately outline the updip limit of the seismogenic zone, the rupture zone for future large earthquakes, at least over the short (∼1 year) observation period. We also estimate northwest motion of a coastal “sliver block” at 8 ± 3 mm/yr, probably related to oblique convergence. In the Osa region to the south, convergence is orthogonal to the trench. Cocos-Caribbean relative motion is partitioned here, with ∼8 cm/yr on the Cocos-Panama block boundary (including a component of permanent shortening across the Fila Costena fold and thrust belt) and ∼1 cm/yr on the Panama block–Caribbean boundary. The GPS data suggest that the Cocos plate–Panama block boundary is completely locked from ∼10–50 km depth. This large locked zone, as well as associated forearc and back-arc deformation, may be related to subduction of the shallow Cocos Ridge and/or younger lithosphere compared to Nicoya, with consequent higher coupling and compressive stress in the direction of plate convergence.

Seismic attenuation tomography of the Tonga‐Fiji region using phase pair methods

The anelastic structure of the region surrounding the Tonga slab and Lau back arc spreading center in the southwest Pacific is studied using data from 12 broadband island stations and 30 ocean bottom seismographs. Two differential attenuation methods determine δt* over the frequency band 0.1 to 3.5 Hz for earthquakes in the Tonga slab. The S-P method measures the difference in spectral decay between P and S waves arriving at the same station. The P-P method measures the difference in spectral decay for P waves with different paths through the upper mantle. Eight hundred sixty phase pairs are used to invert for two-dimensional 1/Qα structure using a nonnegative least squares algorithm. A grid search method determines the Qα/Qβ ratio most compatible with both the S-P and P-P differential measurements. The highest attenuation (Qα = 90) is found within the upper 100 km beneath the active portions of the Lau Basin extending westward to the Lau Ridge. These regions probably delineate the source region for the back arc spreading center magmas, expected to be within the upper 100 km based on petrological considerations. The high attenuation regions also correlate well with zones of low P wave velocity determined by regional velocity tomography. Somewhat lower attenuation is found beneath the Fiji Plateau than beneath the Lau Basin. The entire back arc is characterized by a gradual decrease in attenuation to a depth of 300 to 400 km. The slab is imaged as a region of low attenuation (Qα > 900) material. A Qα/Qβ ratio of 1.75 provides the best fit between the S-P and P-P data sets upon inversion. Spectral stacking shows no frequency dependence within the frequency band analyzed.

A low velocity zone underlying a fast-spreading rise crest

WE present the results of an unreversed seismic refraction profile on the East Pacific Rise near the Siqueiros Fracture Zone recorded using a digital ocean bottom seismograph (OBS)1. An analysis of P wave arrival times and amplitudes indicates a velocity gradient in the top 2 km, with the velocity reaching 6.7 km s−1. This is underlain by a low velocity channel some 1.4 km thick in which the velocity decreases to around 4.8 km s−1. Below this low velocity region there is a velocity gradient from 6.2 to 6.8 km s−1 and mantle velocities of 7.7 km s−1 are reached at a depth of 6 km below the sea bed.

Modeling the Tonga slab: Can travel time data resolve a metastable olivine wedge?

We present the results of detailed modeling of the Tonga slab with the goals of determining whether high-resolution travel time data (1) can be fit by simple thermal and petrological slab models and (2) can resolve a metastable olivine wedge at depths greater than the equilibrium olivine-spinel phase boundary. We model arrival times recorded by a 1000 km line of 23 ocean bottom seismometers (OBS) and island broadband seismic stations extending from the Lau backarc basin, across the Tonga trench and onto the Pacific plate. The data consist of 388 local, P wave travel times from 17 deep and 3 intermediate earthquakes recorded during the 3-month OBS deployment in late 1994. We locate the events using both local and teleseismic arrival times, and apply a relocation operator to the theoretical travel times to simulate the biases introduced in the data by locating the events with a reference Earth model. The modeling consists of grid searches using a three-dimensional finite difference algorithm to compute local, first arriving travel times for equilibrium and metastable P wave velocity models constructed from thermal, mineralogical, and morphological constraints. The travel time anomalies are well fit by standard slab thermal models and P velocity temperature derivatives of −0.4 to −0.3 ms−1°C−1. Forward calculations indicate that the presence of a metastable olivine wedge has a subtle effect on the travel times due to the tendency of first arriving waves to avoid the low-velocity region. Wedge velocity models provide a slightly better fit to the data than equilibrium models, but F tests indicate the improvement is not significant at the 95% level. Our results suggest that providing direct seismological evidence of a wedge of metastable olivine in subduction zones will require either waveform modeling or the observation of later arriving phases created by the depressed phase boundary.

Jasper Seamount structure: Seafloor seismic refraction tomography

The velocity structure of Jasper Seamount was modeled using one- and three-dimensional inversions of P wave travel times. The results represent the first detailed seismic images of a submerged, intraplate volcano. Two seismic refraction experiments were completed on Jasper Seamount, incorporating ocean bottom seismometers and navigated seafloor shots. The P wave travel times were first used to compute a one-dimensional velocity profile which served as a starting model for a three-dimensional tomographic inversion. The seamount P velocities are significantly slower than those observed in typical oceanic crust at equivalent subbasement depths. This suggests that Jasper Seamount is constructed predominantly of extrusive lavas with high average porosity. The velocity models confirm morphological predictions: Jasper Seamount is a shield volcano with rift zone development. High seismic velocities were detected beneath the large radial ridges while low velocities characterize the shallow summit and flanks. Comparisons between P velocity models of Jasper Seamount and the island of Hawaii reveal that these two shield volcanoes are not structurally proportional. Jasper Seamount is far smaller than Hawaii, yet both volcanoes exhibit an outer extrusive layer of similar thickness. This suggests that seamount size influences the intrusive/extrusive proportions; density equilibrium between melt and country rock may explain this behavior.

Coherence lengths of seafloor noise: Effect of ocean bottom structure

Results are reported from an experiment, conducted in 1987, in which an ocean bottom seismograph array of 150‐m aperture and minimally redundant design was used to record the ambient noise in deep water off the California coast. The minimum interelement spacing among the nine instruments was 8 m. The coherence lengths observed imply that the noise field in the 0.05‐ to 5‐Hz band are fundamental and higher‐mode Rayleigh/Stoneley/Scholte waves and the relative amplitudes of the modes imply that the excitation occurs within 20 km of the array. These observations imply that the noise energy is scattered into the seafloor waveguide at the boundaries of the sediment pond in which the array was sited. The implications for sub‐bottom sensors are discussed.

Ocean bottom seismometer facilities available

The Office of Naval Research, together with Scripps Institution of Oceanography, University of Washington, Massachusetts Institute of Technology, and Woods Hole Oceanographic Institution, is pleased to announce the formation of two national Ocean Bottom Seismometer (OBS) facilities. Recent advances in marine seismic and acoustic research, including whole Earth tomography, seismic refraction tomography, detailed passive seismology, high-resolution seismic refraction, and marine ambient noise studies, require a suite of identical calibrated seafloor instruments for analysis of array data collected by OBS capable of sustained deployment periods. Such instruments require a recording capability that is substantially improved in terms of bandwidth, recording capability, fidelity, and deployment duration over that possible just a few years ago. Recognizing a deficiency in existing instrumentation, in 1987 ONR embarked on an effort to fund the design and construction of a new generation of OBS. Thirty-one instruments are now available for general use, and we encourage investigators to use the national OBS facilities as an effective means of acquiring state-of-the-art ocean floor seismic data. The two OBS facilities will be managed and operated on a joint institutional basis by WHOI and MIT, and SIO and UW, respectively. While the instruments will be managed and operated by the OBS facilities, ownership of the OBS will be retained by

Investigation of microearthquake activity following an intraplate teleseismic swarm on the west flank of the Southern East Pacific Rise

Between February 1991 and May 1992, 33 intraplate earthquakes having body wave magnitudes between 4.3 and 6.0 were located on the west flank of the Southern East Pacific Rise by the International Seismological Center. Seven months after the last teleseismic event, we deployed four ocean bottom seismometers at the site of the teleseismic swarm. One hundred and ninety-two microearthquakes were located using P and S travel times of events recorded by three or more instruments during the 16-day deployment. Most of the microearthquakes were in a band about 30 km long and 6 km wide between and parallel to seamount chains. In addition, several events were distributed along a line perpendicular to the main seismicity band and parallel to the ridge axis. The focal depths of the microearthquakes range from 1 to 15 km, and most are between 5 and 12 km, similar to the depth range of the teleseismic events [Hung and Forsyth, 1996]. The composite P wave polarities indicate that the microearthquakes had a variety of focal mechanisms. We developed a new grid-search, inversion technique that utilizes the P wave polarities and the empirically corrected ratios of P and S wave amplitudes to find the focal mechanisms of individual events. Within the acceptable travel time and amplitude misfits, focal solutions are fairly stable. Normal faulting is found in the ridge-parallel seismicity line. The thrust and strike-slip faulting in the main seismicity band is distinctly different from the exclusively normal faulting mechanisms of the teleseismic events. There is no apparent depth dependence of fault types. None of the existing models of the sources of stress (ridge push, thermoelastic stresses, loading by local topographic features, caldera collapse, and north-south extension of the Pacific Plate) provides a satisfactory explanation for both the teleseismic swarm and microearthquakes. We propose a new tectonic scenario. In this scenario, the lithosphere is prestressed by the cooling of the plate. Magma rising from the deeper mantle induces normal faulting ahead of the dike tips in the lower lithosphere, which is already under extensional, thermal stress, producing the larger, teleseismically detected events. Once the dikes propagate into the lithosphere, the region surrounding the dikes behind the tips is compressed by the overpressure of magma. Depending on the geometry of the dikes, the local orientations of the minimum principal stress, and the local weaknesses in the lithosphere, thrust or strike-slip faulting (the microearthquakes) may occur.

A proposed super‐thick sedimentary basin, Bay of Bengal

A super-thick (∼22km) sedimentary basin under the northern Bay of Bengal is proposed. The hypothesis is based on data from surface wave dispersion, seismic refraction, Sn attenuation, and geology. We present new high frequency Sn data which indicate a cold upper mantle beneath the Bay of Bengal. We propose that the oceanic crust in this region is in fact nearly normal, and that the sedimentary section is at least 6 km thicker than previously thought, with velocities at the base of the sediments having been increased to near 6.5 km/sec by high pressure metamorphism. Reinterpretation of the refraction data indicate a post India-Asia collision thickness of over 16 km.

An ocean bottom, microprocessor based seismometer

We describe the design and construction of an ocean bottom seismometer configured as a computer, based on an Intersil IM6100 microprocessor plus appropriate peripheral devices. The sensors consist of triaxial 1 Hz seismometers and a hydrophone, each sensor channel being filtered prior to digitizing so that typical noise spectra are whitened. Digital data are recorded serially on magnetic tape. The instrument is placed on the ocean bottom by allowing it to fall freely from just below the surface. An acoustic system allows precise determination of instrument position, acoustic recall, and transmission of operational information to the surface. Release from an expendable anchor is accomplished by redundant pyrotechnic bolts which can be fired by acoustic command or by precision timers.The operational flexibility provided by the micro-computer, which executes the DEC PDP8/EDEC, PDP8/E and OS/8 are registered trademarks of Digital Equipment Corporation, Maynard, Mass., USA instruction set, enables optimum use of the 6-hr recording capacity (at 128 samples/second/channel) in the context of the particular experiment being performed.