Publications

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2013
Yildiz, S, Sabra K, Dorman LM, Kuperman WA.  2013.  Using hydroacoustic stations as water column seismometers. Geophysical Research Letters. 40:2573-2578.   10.1002/grl.50371   AbstractWebsite

Getting seismic data from the deep oceans usually involves ocean-bottom seismometers, but hydrophone arrays may provide a practical alternative means of obtaining vector data. We here explore this possibility using hydrophone stations of the International Monitoring System, which have been used to study icebergs and T-wave propagation among others. These stations consist of three hydrophones at about the depth of the deep sound channel in a horizontal triangle array with 2km sides. We use data from these stations in the very low-frequency regime (0.01-0.05Hz band) to demonstrate that these stations can also be used as water column seismometers. By differencing the acoustic pressure, we obtain vector quantities analogous to what a seismometer would record. Comparing processed hydrophone station records of the 2004 Great Sumatra-Andaman Earthquake with broadband seismograms from a nearby island station, we find that the differenced hydrophones are indeed a practical surrogate for seismometers.

2006
Dorman, LM, Sauter AW.  2006.  A reusable implosive seismic source for midwater or seafloor use. Geophysics. 71:Q19-Q24.   10.1190/1.2335512   AbstractWebsite

We have developed a new implosive seismic or acoustic source for seafloor or midwater use. The fact that this device does not use pyrotechnics simplifies logistic and permitting problems. It produces relatively little high-frequency output, so it is wildlife friendly. This device enables us to place the source nearer to the image target compared to surface sources, which thus increases resolution. The simple 20-1 version we have constructed must be reset after each use by bringing it to the sea surface. We present measurements of seafloor shear velocity at a depth of about I km in the San Diego Trough. There the surficial shear velocity is 16 m/s, and the gradient is about 10 s(-1).

DeShon, HR, Schwartz SY, Newman AV, Gonzalez V, Protti M, Dorman LRM, Dixon TH, Sampson DE, Flueh ER.  2006.  Seismogenic zone structure beneath the Nicoya Peninsula, Costa Rica, from three-dimensional local earthquake P- and S-wave tomography. Geophysical Journal International. 164:109-124.   10.1111/j.1365-246X.2005.02809.X   AbstractWebsite

The subduction plate interface along the Nicoya Peninsula, Costa Rica, generates damaging large (M-w > 7.5) earthquakes. We present hypocenters and 3-D seismic velocity models (V-P and V-P/V-S) calculated using simultaneous inversion of P- and S-wave arrival time data recorded from small magnitude, local earthquakes to elucidate seismogenic zone structure. In this region, interseismic cycle microseismicity does not uniquely define the potential rupture extent of large earthquakes. Plate interface microseismicity extends from 12 to 26 and from 17 to 28 km below sea level beneath the southern and northern Nicoya Peninsula, respectively. Microseismicity offset across the plate suture of East Pacific Rise-derived and Cocos-Nazca Spreading Center-derived oceanic lithosphere is similar to 5 km, revising earlier estimates suggesting similar to 10 km of offset. Interplate seismicity begins downdip of increased locking along the plate interface imaged using GPS and a region of low V-P along the plate interface. The downdip edge of plate interface microseismicity occurs updip of the oceanic slab and continental Moho intersection, possibly due to the onset of ductile behaviour. Slow forearc mantle wedge P-wave velocities suggest 20-30 per cent serpentinization across the Nicoya Peninsula region while calculated V-P/V-S values suggest 0-10 per cent serpentinization. Interpretation of V-P/V-S resolution at depth is complicated however due to ray path distribution. We posit that the forearc mantle wedge is regionally serpentinized but may still be able to sustain rupture during the largest seismogenic zone earthquakes.

2005
Brown, KM, Tryon MD, DeShon HR, Dorman LM, Schwartz SY.  2005.  Correlated transient fluid pulsing and seismic tremor in the Costa Rica subduction zone. Earth and Planetary Science Letters. 238:189-203.   10.1016/j.epsl.2005.06.055   AbstractWebsite

Continuous measurements of fluid flow were made over a six month period across the Nicoya Peninsula, Costa Rica (Pacific), convergent margin utilizing osmotically-driven fluid flow meters designed to quantify both inflow and outflow rates on the order of similar to 10(-5) to 3 cm/d. Significant transience in flow was observed through the surface of the forearc. Three periods of correlated flow signals were seen on the subduction forearc among three instruments located in the out-of-sequence thrust (OOST) zone over along-margin strike distances of similar to 30 km. Amplitudes of ground velocity recorded on collocated ocean bottom seismometers (OBS) increase during the three correlated flow events. The seismic signal has frequency characteristics that resemble volcanic and non-volcanic tremor. We hypothesize that repeated plate boundary slow slip events, potentially originating at the up dip limit of the seismogenic zone, generate the observed signals within the toe of the forearc. We propose a model in which the poro-elastic stress/strain field around a series of creep dislocations simultaneously forces flow through fracture networks in the forearc and oceanic basement rocks and induces diffuse flow through the shallow sediments. The former generates the seismic tremor-like noise recorded by the OBSs and the latter generates the flow transients recorded by the fluid flow meters. We suggest that high sensitivity fluid flow meters can be utilized to detect transient tectonic strain events in offshore environments where traditional geodetic techniques lack resolution or are not possible. (c) 2005 Elsevier B.V. All rights reserved.

2004
Norabuena, E, Dixon TH, Schwartz S, DeShon H, Newman A, Protti M, Gonzalez V, Dorman L, Flueh ER, Lundgren P, Pollitz F, Sampson D.  2004.  Geodetic and seismic constraints on some seismogenic zone processes in Costa Rica. Journal of Geophysical Research-Solid Earth. 109   10.1029/2003jb002931   AbstractWebsite

[1] 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 (similar to 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 similar to 8 cm/yr on the Cocos-Panama block boundary ( including a component of permanent shortening across the Fila Costena fold and thrust belt) and similar to 1 cm/yr on the Panama block - Caribbean boundary. The GPS data suggest that the Cocos plate - Panama block boundary is completely locked from similar to 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.

2003
Maurice, SDR, Wiens DA, Shore PJ, Vera E, Dorman LM.  2003.  Seismicity and tectonics of the South Shetland Islands and Bransfield Strait from a regional broadband seismograph deployment. Journal of Geophysical Research-Solid Earth. 108   10.1029/2003jb002416   AbstractWebsite

[1] We investigate the tectonics of the South Shetland Trench and Bransfield Strait by performing a detailed study of local seismicity. During 1997 - 1999 we deployed seven land seismometers and 14 ocean bottom seismometers in the South Shetland Island region. The data we obtained indicate a high level of local seismicity (m(b) 2 - 5), and we accurately located similar to 150 earthquakes. Many of the earthquakes occur at locations and depths indicative of ongoing subduction in the South Shetland trench. A focal mechanism for the largest event in the forearc indicates shallow angle thrusting. The maximum depth of seismicity is similar to 65 km, but the majority of the events are shallower than 30 km. These seismic results are consistent with recent magnetic, GPS, and multichannel seismic reflection data that suggest continued subduction at a very slow rate. The South Shetland trench thus represents an extreme end-member of hot subduction resulting from slow convergence of young lithosphere, and the absence of intermediate depth earthquakes is consistent with thermal assimilation of the slab at shallow depths. We have located many earthquakes associated with volcanism and rifting in Bransfield Strait. A swarm of events near a submarine volcano suggests current magmatic activity. A normal faulting focal mechanism in the northeastern part of the strait gives evidence of extension. Earthquakes associated with rifting in the northeastern portion of the strait are clustered along well-established rifts, but the seismicity is much more diffuse to the southwest. This observation is consistent with other evidence that extension has propagated from northeast to southwest.

DeShon, HR, Schwartz SY, Bilek SL, Dorman LM, Gonzalez V, Protti JM, Flueh ER, Dixon TH.  2003.  Seismogenic zone structure of the southern Middle America Trench, Costa Rica. Journal of Geophysical Research-Solid Earth. 108   10.1029/2002jb002294   AbstractWebsite

[1] The shallow seismogenic portion of subduction zones generates damaging large and great earthquakes. This study provides structural constraints on the seismogenic zone of the Middle America Trench offshore central Costa Rica and insights into the physical and mechanical characteristics controlling seismogenesis. We have located similar to 300 events that occurred following the M-W 6.9, 20 August 1999, Quepos, Costa Rica, underthrusting earthquake using a three-dimensional velocity model and arrival time data recorded by a temporary local network of land and ocean bottom seismometers. We use aftershock locations to define the geometry and characteristics of the seismogenic zone in this region. These events define a plane dipping at 19degrees that marks the interface between the Cocos Plate and the Panama Block. The majority of aftershocks occur below 10 km and above 30 km depth below sea level, corresponding to 30 - 35 km and 95 km from the trench axis, respectively. Relative event relocation produces a seismicity pattern similar to that obtained using absolute locations, increasing confidence in the geometry of the seismogenic zone. The aftershock locations spatially correlate with the downdip extension of the oceanic Quepos Plateau and reflect the structure of the main shock rupture asperity. This strengthens an earlier argument that the 1999 Quepos earthquake ruptured specific bathymetric highs on the downgoing plate. We believe that subduction of this highly disrupted seafloor has established a set of conditions which presently limit the seismogenic zone to be between 10 and 35 km below sea level.

Crawford, WC, Wiens DA, Dorman LM, Webb SC, Wiens DA.  2003.  Tonga Ridge and Lau Basin crustal structure from seismic refraction data. Journal of Geophysical Research-Solid Earth. 108   10.1029/2001jb001435   AbstractWebsite

[1] The crustal structure across the Tonga-Lau arc-back arc system from the Lau Ridge to the Pacific Plate (178degrees-170degreesW, 18degrees19degreesS) is modeled, using data from an 840-km-long air gun refraction line over 19 ocean bottom seismometers and one land station. The data reveal that the Pacific Plate crust is 5.5 km thick, with a velocity structure similar to that found at the present-day East Pacific Rise (EPR). Beneath Tonga Ridge, an intermediate velocity layer (6-7 km/s) is up to 7.5 km thick and has a velocity-depth distribution similar to andesitic rocks found in continental crust. The crust is abnormally thin (4 km) at the boundary between the Tonga Ridge and the Lau Basin. At the east end of Lau Basin, the crust is 5.5-6.5 km thick and resembles crust formed at the EPR except for a thicker sheeted-dike section (2-3 km) and thinner lower crust (2 km). The Lau Basin crust thickens to 7-8 km near the Central Lau Spreading Center (CLSC), mostly through thickening of the lower crust. The crust thickens again to 8.5-9.5 km at 50 km west of the CLSC, mostly through thickening of the midcrust. In the thick westernmost section, the crustal structure is uniform even though one part of this section formed through extension of arc-type crust while the rest was created at an oceanic spreading center. The relative homogeneity of these rocks suggests that their petrology may be dominated by postemplacement magmatic infilling from a mantle source west of the spreading center.

2002
Newman, AV, Schwartz SY, Gonzalez V, DeShon HR, Protti JM, Dorman LM.  2002.  Along-strike variability in the seismogenic zone below Nicoya Peninsula, Costa Rica. Geophysical Research Letters. 29   10.1029/2002gl015409   AbstractWebsite

[1] At the subduction zone in northwestern Costa Rica, the seismogenic zone lies directly beneath the Nicoya Peninsula, allowing for near source seismic studies of earthquake activity. We located 650 earthquakes along the seismogenic plate interface using a dense seismic network in the vicinity of the Nicoya Peninsula. Using these data we constrained the updip limit of the seismogenic zone there and found a transition in depth, 10 km in the south to 20 km in the north, that occurs where the subducting oceanic crust changes from warmer Cocos-Nazca Spreading center (CNS) origin to colder East Pacific Rise (EPR) origin. We argue that the temperature of the incoming oceanic crust controls the seismogenic updip limit beneath Nicoya, Costa Rica; subducting colder oceanic crust deepens the seismogenic updip limit.

Dorman, LM, Gehringer DD.  2002.  Web browser applet allows visualization of three-dimensional models. EOS, Transactions of the AGU. 83(18)   10.1029/2002EO000136   AbstractWebsite

A new, machine-independent Java Applet, a program that runs in a browser, downloads both byte code and three-dimensional models from a remote Web site and displays them on a local computer. The code is a few hundred kilobytes in size and allows the viewer to control a two-dimensional view of a three-dimensional array, which can be represented by files as small as one byte per node. This code allows both the angle of view and the color map to be controlled by the user.

2001
Tryon, M, Brown K, Dorman LR, Sauter A.  2001.  A new benthic aqueous flux meter for very low to moderate discharge rates. Deep-Sea Research Part I-Oceanographic Research Papers. 48:2121-2146.   10.1016/s0967-0637(01)00002-4   AbstractWebsite

Significant quantities of fluids and dissolved geochemical components are expelled through the sediment surface in ocean margin and sedimented ridge environments. Recently, significant interest has been generated in constraining hydrological processes in these environments, but direct measurement of fluid flow in the marine environment has proven to be difficult and many aspects of marine hydrogeology remain poorly understood. To address the need for a means to make a significant number of direct measurements in a wide range of low to moderate flow environments, we have developed a new type of benthic aqueous flux meter that is capable of measuring diffuse fluid flow through the sediment surface on the order of 0.1 mm yr(-1)-15 m yr(-1) when the flow is through sediments with permeabilities of less than 10(-8) cm(2) (typical seafloor sediments). The instrument measures fluid flow by determining the degree of dilution of a chemical tracer that is injected by an osmotic pump at a known rate into the fluids venting into or out of a collection chamber situated on the sea bed. The pump also withdraws a subsample of this tracer/fluid mix into sample coils allowing a serial record of the flow rates to be determined. Both upward and downward flow can be measured and, when flux rates are high enough to effectively flush the collecting chamber, the instruments also act as geochemical samplers. Three years of laboratory testing and field use have constrained the effects of (1) temperature, pressure, and deployment duration on osmotic pump performance, (2) dispersion/diffusion in the sample coils, and (3) deflection of flow under a range of sediment permeabilities. Recent deployments on the Kodiak and Cascadia accretionary prisms document the range and capabilities of the instrument in the field. (C) 2001 Elsevier Science Ltd. All rights reserved.

Gilbert, HJ, Sheehan AF, Wiens DA, Dueker KG, Dorman LM, Hildebrand J, Webb S.  2001.  Upper mantle discontinuity structure in the region of the Tonga Subduction Zone. Geophysical Research Letters. 28:1855-1858.   10.1029/2000gl012192   AbstractWebsite

We study the mantle structure below the southwest Pacific in order to examine the geometry of the Tonga slab at depth and its interaction with the 410- and 660-km discontinuities thereafter called the 410 and the 660). We utilize data from stations of both the Lau Basin Ocean Bottom Seismogram experiment and island stations of the Southwest Pacific Seismic Experiment. The tectonic complexity of this region, containing both the Tonga subduction zone and the associated Lau back are spreading center make it an ideal area to investigate the upper mantle discontinuities using a high resolution technique such as common conversion point slacking of receiver functions. We produce a high-resolution image of the upper mantle near the Tonga subduction zone to show the interaction between the discontinuities and the subducting slab. Our results show the 410 uplifted by 30 km near the Tonga slab and the 660 depressed by 20 to 30 km as expected for thermally controlled olivine phase transitions.

Pautet, L, Kuperman WA, Dorman L.  2001.  Using refracted shear waves for velocity estimation. Geophysical Prospecting. 49:281-286.   10.1046/j.1365-2478.2001.00252.x   AbstractWebsite

The most difficult parr of multicomponent processing is the estimation of the shear-wave velocity map for migration. We used refracted shear waves and a simple iterative method called wavefield continuation (WFC) to evaluate the shallow shear-wave velocity profile on a real data example. The WFC was developed in 1981 by Clayton and McMechan to determine compressional-wave velocity profiles from refracted compressional waves. The application to refracted shear waves is straightforward. The real data example shows that shear structure can be easily determined independently of the compressional structure.

Smith, GP, Wiens DA, Fischer KM, Dorman LM, Webb SC, Hildebrand JA.  2001.  A complex pattern of mantle flow in the Lau backarc. Science. 292:713-716.   10.1126/science.1058763   AbstractWebsite

Shear-wave splitting analysis of Local events recorded on Land and on the ocean floor in the Tonga are acid Lau backarc indicate a complex pattern of azimuthal anisotropy that cannot be explained by mantle flow coupled to the downgoing plate. These observations suggest that the direction of mantle flow rotates from convergence-parallel in the Fiji plateau to north-south beneath the Lau basin and are-parallel beneath the Tonga are. These results correlate with helium isotopes that map mantle flow of the Samoan plume into the Lau basin through an opening tear in the Pacific plate.

Dorman, LM.  2001.  Seismology Sensors. Encyclopedia of Ocean Sciences. ( Steele JH, Turekian KK, Thorpe SA, Eds.)., Amsterdam: Elsevier ScienceDirect   10.1016/B978-012374473-9.00334-9  
1999
Roth, EG, Wiens DA, Dorman LM, Hildebrand J, Webb SC.  1999.  Seismic attenuation tomography of the Tonga-Fiji region using phase pair methods. Journal of Geophysical Research-Solid Earth. 104:4795-4809.   10.1029/1998jb900052   AbstractWebsite

The anelastic structure of the region surrounding the Tonga slab and Lau back are 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 delta t* 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/Q(alpha), structure using a nonnegative least squares algorithm. A grid search method determines the Q(alpha)/Q(beta) ratio most compatible with both the S-P and P-P differential measurements. The highest attenuation (Q(alpha) = 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 are 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 are 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 (Q(alpha) > 900) material. A Q(alpha)/Q(beta) 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.

Koper, KD, Wiens DA, Dorman L, Hildebrand J, Webb S.  1999.  Constraints on the origin of slab and mantle wedge anomalies in Tonga from the ratio of S to P velocities. Journal of Geophysical Research-Solid Earth. 104:15089-15104.   10.1029/1999jb900130   AbstractWebsite

We examine two prominent upper mantle Velocity anomalies in the southwest Pacific, the Tongs slab anomaly and the corresponding overlying mantle wedge anomaly, using data collected during a combined land-sea deployment of temporary seismometers. The linear geometry and small interstation spacing of the instruments yield high-resolution data along a cross section of the Tonga subduction zone, including the actively spreading Lau back are basin. We estimate the relative variation of P and S velocity, often described as v = delta lnV(s)/delta lnV(p), for the slab and mantle wedge anomalies using two distinct methods: a linear regression of the P and S travel time residuals, and detailed modeling of the velocity structure using a three-dimensional finite difference travel time algorithm. The two methods yield similar results, with v of the slab being 1.1-1.5 and v of the mantle wedge being 1.2-1.3. These values are consistent with experimental data concerning the effect of temperature on P and S wave velocities in the upper mantle and are lower than what is expected for velocity anomalies generated by the presence of partial melt. These observations imply that either the theoretical estimates of v for partial melt are too large or very little partial melt is present beneath the Lau basin. In the latter case, melt must be quickly removed from the rock matrix, such that the velocity anomalies are due to increased temperature, and not melt. The bulk of the velocity anomaly in the mantle wedge can be explained by temperature anomalies of 400-600 degrees C because of the amplification of temperature derivatives of seismic velocity by anelastic effects. Such large thermal anomalies, generated by decreased lithospheric thickness and mantle upwelling beneath the fast spreading Lau back are basin, can still leave the mantle near the solidus, even after accounting for the effect of increased volatile content in the mantle wedge. The lower-amplitude velocity reductions in the deeper wedge are likely related to an increased concentration of volatiles from the subducting slab.

1998
Forsyth, DW, Scheirer DS, Webb SC, Dorman LM, Orcutt JA, Harding AJ, Blackman DK, Morgan JP, Detrick RS, Shen Y, Wolfe CJ, Canales JP, Toomey DR, Sheehan AF, Solomon SC, Wilcock WSD, Team MS.  1998.  Imaging the deep seismic structure beneath a mid-ocean ridge: The MELT experiment. Science. 280:1215-1218. AbstractWebsite

The Mantle Electromagnetic and Tomography (MELT) Experiment was designed to distinguish between competing models of magma generation beneath mid-ocean ridges. Seismological observations demonstrate that basaltic melt is present beneath the East Pacific Rise spreading center in a broad region several hundred kilometers across and extending to depths greater than 100 kilometers, not just in a narrow region of high melt concentration beneath the spreading center, as predicted by some models. The structure of the ridge system is strongly asymmetric: mantle densities and seismic velocities are lower and seismic anisotropy is stronger to the west of the rise axis.

Forsyth, DW, Webb SC, Dorman LM, Shen Y.  1998.  Phase velocities of Rayleigh waves in the MELT experiment on the East Pacific Rise. Science. 280:1235-1238.   10.1126/science.280.5367.1235   AbstractWebsite

The phase velocities of Rayleigh waves increase more rapidly with distance from the East Pacific Rise (EPR) axis than is predicted by models of conductive cooling of the lithosphere. Low velocities near the axis are probably caused by partial melt at depths of 20 to 70 kilometers in a zone several hundred kilometers wide. The lowest velocities are offset to the west of the EPR, Wave propagation is anisotropic; the fast direction is approximately perpendicular to the ridge, parallel to the spreading direction. Anisotropy increases from a minimum near the axis to 3 percent or more on the flanks.

McGowan, JA, Cayan DR, Dorman LM.  1998.  Climate-ocean variability and ecosystem response in the northeast Pacific. Science. 281:210-217.   10.1126/science.281.5374.210   AbstractWebsite

The role of climatic variation in regulating marine populations and communities is not well understood. To improve our knowledge, the sign, amplitude, and frequency of climatic and biotic variations should be compared as a necessary first step. it is shown that there have been large interannual and interdecadal sea-surface temperature changes off the West Coast of North America during the past 80 years. Interannual anomalies appear and disappear rather suddenly and synchronously along the entire coastline. The frequency of warm events has increased since 1977. Although extensive, serial, biological observations are often incomplete, it is clear that climate-ocean variations have disturbed and changed our coastal ecosystems.

Lewis, BTR, Dorman LM.  1998.  Recording teleseisms on the seafloor; an example from the Juan de Fuca plate. Bulletin of the Seismological Society of America. 88:107-116. AbstractWebsite

In 1991, during an experiment to compare low-frequency seismic noise on a basaltic and a sediment covered seafloor (NOBS), we recorded teleseisms on the Juan de Fuca ridge, the Gorda ridge, and the adjacent Cascadia Basin with the SNAG ocean-bottom seismometers (OBS). These data provide an indication of the type of data that may be obtained from future experiments to record teleseisms and may be helpful in designing these experiments and analyzing the results. We found that although seafloor noise is dominated by microseisms in the band 0.1 to 0.3 Hz, there is a well-developed minimum in noise from about 0.03 to 0.1 Hz (the noise notch). In this noise notch, teleseisms can be most easily detected. In the Cascadia area, the overall noise levels are such that only teleseismic events with magnitude greater than 6.5 were usefully recorded. A magnitude 6.6 event in the New Britain area(Delta = 89 degrees) produced usable P- and surface-wave data only in this noise notch. In the band 0.03 to 0.1 Hz, the character of compressional waves is very sensitive to water depth and the type of sensor. We show that pressure sensors are especially sensitive to reverberation in the ocean and that motion sensors (seismometers) are less sensitive to ocean reverberations and will record teleseismic phases with less distortion than pressure sensors. The Cascadia data indicate enhanced P amplitudes at sites on the ridge axes that could be due to focusing caused by a low-velocity lens. These data suggest that amplitude information may be as, or even more, useful than P delay times for determining upper mantle structure.

Koper, KD, Wiens DA, Dorman LM, Hildebrand JA, Webb SC.  1998.  Modeling the Tonga slab: Can travel time data resolve a metastable olivine wedge? Journal of Geophysical Research-Solid Earth. 103:30079-30100.   10.1029/98jb01517   AbstractWebsite

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)degrees 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.

1997
Zhao, DP, Xu YB, Wiens DA, Dorman L, Hildebrand J, Webb S.  1997.  Depth extent of the Lau back-arc spreading center and its relation to subduction processes. Science. 278:254-257.   10.1126/science.278.5336.254   AbstractWebsite

Seismic tomography and wave form inversion revealed that very slow velocity anomalies (5 to 7 percent) beneath the active Lau spreading center extend to 100-kilometer depth and are connected to moderately slow anomalies (2 to 4 percent) in the mantle wedge to 400-kilometer depth, These results indicate that geodynamic systems associated with back-are spreading are related to deep processes, such as the convective circulation in the mantle wedge and deep dehydration reactions in the subducting slab, The slow regions associated with the Tonga are and the Lau back are are separated at shallow levels but merge at depths greater than 100 kilometers, suggesting that slab components of back-are magmas occur through mixing at these depths.

Wiggins, SM, Dorman LM, Cornuelle BD.  1997.  Topography can affect linearization in tomographic inversions. Geophysics. 62:1797-1803.   10.1190/1.1444280   AbstractWebsite

Linearized inverse techniques commonly are used to solve for velocity models from traveltime data. The amount that a model may change without producing large, nonlinear changes in the predicted traveltime data is dependent on the surface topography and parameterization. Simple, one-layer, laterally homogeneous, constant-gradient models are used to study analytically and empirically the effect of topography and parameterization on the linearity of the model-data relationship. If, in a weak-velocity-gradient model, rays turn beneath a valley with topography similar to the radius of curvature of the raypaths, then large nonlinearities will result from small model perturbations. Hills, conversely, create environments in which the data are more nearly linearly related to models with the same model perturbations.

Bradley, CR, Stephen RA, Dorman LM, Orcutt JA.  1997.  Very low frequency (0.2-10.0 Hz) seismoacoustic noise below the seafloor. Journal of Geophysical Research-Solid Earth. 102:11703-11718.   10.1029/96jb03183   AbstractWebsite

The sources and propagation of VLF (0.2 --> 10 Hz) ambient noise on and within the deep ocean crust at Deep Sea Drilling Project (DSDP) Hole 534B in the Blake Bahama Basin are shown to be related to the surface sea state and local lithology. This study represents the first experiment where ambient noise is measured simultaneously at several depths below the seafloor. The low-frequency microseism power spectral density (PSD) peak at 0.3 Hz is nearly invariant with depth between 0 and 100 m below the seafloor. PSD levels of the peak are 65 and 75 dB (rel 1 (nm/s(2))(2)/Hz) for the vertical and horizontal components, respectively, and both horizontal and vertical components of the ocean bottom seismometer and borehole array compare favorably. Above 0.5 Hz the noise levels decrease with depth and increasing frequency. At 1.0 Hz, 100 m below the seafloor the noise level is 10 and 20 dB below the levels observed at the seafloor for vertical and horizontal components, respectively. There is evidence that amplification in some frequency bands may make deeper sites noisier than shallower sites in the same well. Temporal variation of the noise shows nonlinear interaction of local water-borne gravity waves to be the dominant source mechanism and that the passing of a local storm generates interface waves and increases the noise level (similar to 10 dB) from 0.3 to 1.5 Hz and 5 to 64 Hz. Between 1.5 and 5 Hz the spectrum is not strongly affected by the passing storm, indicating that the ocean wave spectrum may be saturated.