Publications

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1997
Shen, Y, Forsyth DW, Conder J, Dorman LM.  1997.  Investigation of microearthquake activity following an intraplate teleseismic swarm on the west flank of the Southern East Pacific Rise. Journal of Geophysical Research-Solid Earth. 102:459-475.   10.1029/96jb02852   AbstractWebsite

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 pew 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.

M, DL.  1997.  Propagation in marine sediments. Handbook of acoustics. 1( Crocker MJ, Ed.).:409-416., New York: John Wiley Abstract
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1996
Wiggins, SM, Dorman LM, Cornuelle BD, Hildebrand JA.  1996.  Hess Deep rift valley structure from seismic tomography. Journal of Geophysical Research-Solid Earth. 101:22335-22353.   10.1029/96jb01230   AbstractWebsite

We present results from a seismic refraction experiment conducted across the Hess Deep rift valley in the equatorial east Pacific. P wave travel times between seafloor explosions and ocean bottom seismographs are analyzed using an iterative stochastic inverse method to produce a velocity model of the subsurface structure. The resulting velocity model differs from typical young, fast spreading, East Pacific Rise crust by approximately +/-1 km/s with slow velocities beneath the valley of the deep and a fast region forming the intrarift ridge. We interpret these velocity contrasts as lithologies originating at different depths and/or alteration of the preexisting rock units. We use our seismic model, along with petrologic and bathymetric data from previous studies, to produce a structural model. The model supports low-angle detachment faulting with serpentinization of peridotite as the preferred mechanism for creating the distribution and exposure of lower crustal and upper mantle rocks within Hess Deep.

Nolet, G, Dorman LM.  1996.  Waveform analysis of Scholte modes in ocean sediment layers. Geophysical Journal International. 125:385-396.   10.1111/j.1365-246X.1996.tb00006.x   AbstractWebsite

In an effort to determine the characteristics of seismic noise on the ocean bottom and its relationship to the structure of the sea-floor, we have adapted the method of nonlinear waveform fitting to accommodate multidimensional models (shear velocity beta and shear damping Q(s)), and have applied it to invert several records of interface waves (Scholte 1958) from the THUMPER experiment off southern California. Waveform fitting is a very powerful tool to determine the S velocity in the top few metres of the sediment. Starting from beta = 30 m s(-1) at the top clay layer, the S velocity increases with a gradient of 2.8 m s(-1) m(-1) over the first 150 m of sediment. A theoretical estimation of the source strength gives coherent estimates of Q(s) as a function of depth for distances between 400 and 1070 m from the source. The Q(s) models are characterized by very low values (10-20) in the top three metres, but by values in excess of 100 below that level. The results confirm the identification of the noise as harmonics of interface waves. In the area of this experiment, the largest noise amplitudes belong to the fundamental mode and penetrate to a depth of about 20 m into the sediment. The overtone energy can be appreciable too, and is noticeable to about 80 m depth. The Q(s) structure confirms the strong influence that the sea-floor structure has on the noise spectrum. The high attenuation at frequencies above 3-4 Hz suppresses noise propagation and produces low noise at higher frequencies. (Similarly, high attenuation in the asthenosphere suppresses noise propagation below 0.1 Hz.)

1995
Bibee, DL, Dorman LM.  1995.  Full Waveform Inversion of Seismic Interface Wave Data. Full Field Inversion Methods in Ocean and Seismo-Acoustics. ( Diachok O, Caiti A, Gerstoft P, Schmidt H, Eds.)., Dordrecht: Klewer Academic Publishers Abstract

A technique for inverting marine seismic interface wave data for shear wave attenuation parameters of ocean sediments is presented. Because of the difficulty in estimating spectral ratios of seismograms in the case when multiple modes are present, a linear perturbation technique that uses the recorded time series seismogram or its envelope as input data is proposed. The technique is stable for tests using synthetic data. Application of the technique to data collected in the Gulf of Mexico yields a model producing a good fit to the data. However, a simple source model is insufficient to correctly predict modal excitations, and the source must be parameterized as part of the inversion process.

1994
Hammer, PTC, Dorman LM, Hildebrand JA, Cornuelle BD.  1994.  Jasper Seamount Structure - Sea-Floor Seismic-Refraction Tomography. Journal of Geophysical Research-Solid Earth. 99:6731-6752.   10.1029/93jb02170   AbstractWebsite

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.

1993
Dorman, L, et al.  1993.  Deep-water sea-floor array observations of seismo-acoustic noise in the eastern Pacific and comparison with wind and swell. Natural Physical Sources of Underwater Sound. ( Kerman B, Ed.).:165-174., Holland: Kluwer Academic Publishers   10.1007/978-94-011-1626-8_14   Abstract

We report results from the analysis of data from an array of Ocean-Bottom Seismographs (OBSs) employed in an array of 150 meter aperture at a depth of 3800 meters off the California coast. The array recorded noise samples four times per day for a month using pressure and three-component inertial sensors.

Comparison of the month-long noise spectrograms with swell spectrograms and wind hind-casts shows marked similarities. In the 0.05–1.0 Hz range the frequency-doubling of swell energy into sea-floor noise predicted by the wave interaction theory is evident. In the 1–10 Hz range the wind-related effects dominate. Lulls in the wind produce deep notches in the noise level. During times of high wind, saturation of the wind wave spectrum causes limiting and reduces the size of the noise maxima.

The wind estimates are from the meteorological model of the U.S. Navy Fleet Numerical Oceanography Center and the swell estimates are from their Global Spectral Ocean Wave Model.

1992
Brune, JN, Curray JR, Dorman LRM, Raitt R.  1992.  A proposed super-thick sedimentary basin, Bay of Bengal. Geophysical Research Letters. 19:565-568., Washington, DC, United States (USA): American Geophysical Union, Washington, DC   10.1029/91gl03134   AbstractWebsite

A super-thick ( approximately 22km) sedimentary basin under the northern Bay of Bengal is proposed. The hypothesis is based on data from surface wave dispersion, seismic refraction, S (sub n) attenuation, and geology. We present new high frequency S (sub n) 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. Copyright 1992 by the American Geophysical Union.

1991
Bibee, DL, Dorman LM, in Sediments SWM.  1991.  Implications of Deep-water Seismometer Array Measurements for Scholte Wave Propagation. Shear Waves in Marine Sediments. ( Hovem JM, Richardson MD, Stoll RD, Eds.)., Dordrecht: Klewer Academic Publishers Abstract

A field exercise was conducted in March 1990 to make measurements of Scholte wave propagation characteristics in a deep ocean environment. Signals from a series of bottom explosive shots were recorded on an array of ocean bottom seismometers. Clear Scholte phases were observed on the vertical seismometers to ranges of 1.25 km, but were attenuated to noise levels by 2 km range. Collocated hydrophones did not detect the Scholte waves even at the closest ranges. The ratio of pressure (µPa) to vertical ground velocity (nm/s) was 68 dB in time windows dominated by body waves but only 20 dB in windows dominated by the Scholte wave. Group velocities were low (30-100 m/s) and showed considerable variability despite the expected uniformity of the seafloor in this abyssal environment.

Jacobson, RS, Dorman LM, Purdy GM, Schultz A, Solomon SC.  1991.  Ocean-Bottom Seismograph Facilities Available. EOS, Transactions, American Geophysical Union. EOS, Transactions, American Geophysical Union:506-515.   10.1029/90EO00366   AbstractWebsite

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.

Dorman, LM, et al.  1991.  The effect of shear velocity structure on sea floor noise. Shera waves in marine sediments. ( Hovem JM, Richardson MD, Stoll RD, Eds.).:239-245., Holland: Kluwer academic publishers
Schreiner, AE, Dorman LM, Bibee LD.  1991.  Shear wave velocity structure from interface at two deep sites in the Pacific Ocean. Shera waves in marine sediments. ( Hovem JM, Richardson MD, Stoll RD, Eds.).:231-238., Holland: Kluwer academic publishers
1990
Schreiner, AE, Dorman LM.  1990.  Coherence Lengths of Sea-Floor Noise - Effect of Ocean Bottom Structure. Journal of the Acoustical Society of America. 88:1503-1514.   10.1121/1.400307   Website
Hildebrand, JA, Dorman LM.  1990.  Scientists use seismic tomography to look at the interior of Jasper Seamount. Earth in Space. 2:3-6., Washington, DC, United States (USA): American Geophysical Union, Washington, DCWebsite
1989
Hildebrand, JA, Dorman LM, Hammer PTC, Schreiner AE, Cornuelle BD.  1989.  Seismic Tomography of Jasper Seamount. Geophysical Research Letters. 16:1355-1358.   10.1029/GL016i012p01355   Website
Trehu, AM, Ballard A, Dorman LM, Gettrust JF, Klitgord KD, Schreiner A.  1989.  Structure of the Lower Crust beneath the Carolina Trough, United-States Atlantic Continental-Margin. Journal of Geophysical Research-Solid Earth and Planets. 94:10585-10600.   10.1029/JB094iB08p10585   Website
Goodman, D, Bibee LD, Dorman LM.  1989.  Crustal Seismic Structure beneath the West Philippine Sea, 17-Degrees-18-Degrees North. Marine Geophysical Researches. 11:155-168.Website
1986
Schiffelbein, P, Dorman L.  1986.  Spectral Effects of Time-Depth Nonlinearities in Deep-Sea Sediment Records - a Demodulation Technique for Realigning Time and Depth Scales. Journal of Geophysical Research-Solid Earth and Planets. 91:3821-3835.   10.1029/JB091iB03p03821   Website
Sauter, AM, Dorman LM.  1986.  Instrument Calibration of Ocean Bottom Seismographs. Marine Geophysical Researches. 8:265-275.   10.1007/bf00305486   Website
Sauter, AW, Dorman LM, Schreiner AE.  1986.  A study of sea floor structure using ocean bottom shots and receivers. ( Akal T, Berkson JM, Eds.)., New York, NY, United States (USA): Plenum Press, New York, NYWebsite
1983
Dorman, LM.  1983.  Modeling and Parameterization Errors in Body Wave Seismology. Geophysical Journal of the Royal Astronomical Society. 72:571-576.   10.1111/j.1365-246X.1983.tb02820.x   Website
1982
Creager, KC, Dorman LM.  1982.  Location of Instruments on the Seafloor by Joint Adjustment of Instrument and Ship Positions. Journal of Geophysical Research. 87:8379-8388.   10.1029/JB087iB10p08379   Website
1981
Jacobson, RS, Shor GG, Dorman LM.  1981.  Linear Inversion of Body Wave Data .2. Attenuation Versus Depth Using Spectral Ratios. Geophysics. 46:152-162.   10.1190/1.1441185   Website