Ocean Bottom Seismometer Data Research Articles

Earth's crust model of the South-Okhotsk Basin by wide-angle OBS data

Deep seismic studies of the Sea of Okhotsk region started in late 1950s. Since that time, wide-angle reflection and refraction data on more than two dozen profiles were acquired. Only five of those profiles either crossed or entered the deep-water area of the South-Okhotsk Basin (also known as the Kuril Basin or the South-Okhotsk Deep-Water Trough). Only P-waves were used to develop velocity-interface models in all the early research. Thus, all seismic and geodynamic models of the Okhotsk region were based only on the information on compressional waves. Nevertheless, the use of Vp/Vs ratio in addition to P-wave velocity allows discriminating felsic and mafic crustal layers with similar Vp values. In 2007 the Russian seismic service company Sevmorgeo acquired multi-component data with ocean bottom seismometers (OBS) along the 1700-km-long north-south 2-DV-M Profile. Only P-wave information was used previously to develop models for the entire profile. In this study, a multi-wave processing, analysis, and interpretation of the OBS data are presented for the 550-km-long southern segment of this Profile that crosses the deep-water South-Okhotsk Basin. Within this segment 50 seismometers were deployed with nominal OBS station spacing of 10–12km. Shot point spacing was 250m. Not only primary P-waves and S-waves but also multiples and P-S, S-P converted waves were analyzed in this study to constrain velocity-interface models by means of travel time forward modeling. In offshore deep seismic studies, thick water layer hinders an estimation of velocities in the sedimentary cover and in the upper consolidated crust. Primarily, this is due to the fact that refracted waves propagating in low-velocity solid upper layers interfere with high-amplitude direct water wave. However, in multi-component measurements with ocean bottom seismometers, it is possible to use converted and multiple waves for velocity estimations in these layers. Consequently, one can obtain P- and S-waves velocity models of the sedimentary strata and the upper consolidated crust. Velocity values in the upper consolidated crust beneath the South-Okhotsk Basin (Vp=5.50–5.80km/s, Vp/Vs=1.74–1.76) allow interpretation of this 2.5–3.5-km-thick layer to be consistent with a felsic (granodioritic) crust. These results suggest that the Earth's crust in this region can be considered continental in nature, rather than previously accepted oceanic crust. Even though, the crust is thinned and stretched at this location.

Ridge asymmetry and deep aqueous alteration at the trench observed from Rayleigh wave tomography of the Juan de Fuca plate

AbstractUsing Rayleigh wave tomography of noise‐removed ocean bottom seismometer data from the Cascadia Initiative, we illuminate the structure of the upper mantle beneath the Juan de Fuca plate. Beneath the Juan de Fuca ridge, there is strong asymmetry, with a pronounced low‐velocity zone in the 25–65 km depth range. Extending to the west from the spreading axis, this anomaly has velocities low enough to indicate the presence of melt. The asymmetry in velocity structure and the much greater abundance of seamounts on the west flank of the ridge suggest that dynamic, buoyant upwelling is important, perhaps triggered by thermal or compositional anomalies beneath Axial Seamount. In contrast, there is no evidence for asymmetry in the axial zone or lower than expected velocities beneath the Gorda ridge. On the eastern flank of the Juan de Fuca ridge, the shear velocity in the 25–65 depth range is higher than expected; the lithosphere appears to be colder and thicker than predicted by standard plate cooling models, perhaps caused by the downwelling counterpart of the upwelling on the west side of the ridge. Close to the trench, there is a sharp decrease in shear velocity. We interpret this as aqueous alteration caused by hydrothermal circulation through deep normal faults associated with bending of the plate. Beneath the Astoria and Nitinat fans, where abyssal plain sediment is thickest, the velocity decrease is much smaller, which is consistent with a thick sediment cap that prevents hydrothermal alteration of the plate.

Inversion of ocean-bottom seismometer (OBS) waveforms for oceanic crust structure: a synthetic study

The waveform inversion method is applied—using synthetic ocean-bottom seismometer (OBS) data—to study oceanic crust structure. A niching genetic algorithm (NGA) is used to implement the inversion for the thickness and P-wave velocity of each layer, and to update the model by minimizing the objective function, which consists of the misfit and cross-correlation of observed and synthetic waveforms. The influence of specific NGA method parameters is discussed, and suitable values are presented. The NGA method works well for various observation systems, such as those with irregular and sparse distribution of receivers as well as single receiver systems. A strategy is proposed to accelerate the convergence rate by a factor of five with no increase in computational complexity; this is achieved using a first inversion with several generations to impose a restriction on the preset range of each parameter and then conducting a second inversion with the new range. Despite the successes of this method, its usage is limited. A shallow water layer is not favored because the direct wave in water will suppress the useful reflection signals from the crust. A more precise calculation of the air-gun source signal should be considered in order to better simulate waveforms generated in realistic situations; further studies are required to investigate this issue.

Anisotropy from SKS splitting across the Pacific-North America plate boundary offshore southern California

SKS arrivals from ocean bottom seismometer (OBS) data from an offshore southern California deployment are analysed for shear wave splitting. The project involved 34 OBSs deployed for 12 months in a region extending up to 500 km west of the coastline into the oceanic Pacific plate. The measurement process consisted of removing the effects of anisotropy using a range of values for splitting fast directions and delay times to minimize energy along the transverse seismometer axis. Computed splitting parameters are unexpectedly similar to onland parameters, exhibiting WSW–ENE fast polarization directions and delays between 0.8 and 1.8 s, even for oceanic plate sites. This is the first SKS splitting study to extend across the entire boundary between the North America and Pacific plates, into the oceanic part of the Pacific plate. The splitting results show that the fast direction of anisotropy on the Pacific plate does not align with absolute plate motion (APM), and they extend the trend of anisotropy in southern California an additional 500 km west, well onto the oceanic Pacific plate. We model the finite strain and anisotropy within the asthenosphere associated with density–buoyancy driven mantle flow and the effects of APM. In the absence of plate motion effects, such buoyancy driven mantle flow would be NE-directed beneath the Pacific plate observations. The best-fit patterns of mantle flow are inferred from the tomography-based models that show primary influences from foundering higher-density zones associated with the history of subduction beneath North America. The new offshore SKS measurements, when combined with measurements onshore within the plate boundary zone, indicate that dramatic lateral variations in density-driven upper-mantle flow are required from offshore California into the plate boundary zone in California and western Basin and Range.

CRUSTAL STRUCTURE OF XISHA BLOCK AND ITS TECTONIC ATTRIBUTES

AbstractXisha block, as a micro‐continental massif formed during the process of the formation of the South China Sea (SCS), has recorded important information about the evolutionary history of the SCS. The crustal structure, petrological properties and tectonic attributes of Xisha block are critical to the theory of formation and evolution of the SCS. Based on Ocean Bottom Seismometers (OBS) data acquired from Line OBS2013–3, the P‐wave velocity model of Xisha block along this line is established employing the ray‐tracing and forward travel‐time modeling method. The velocity structure shows that the sedimentary layer is various with a velocity of 2.2∼3.2 km·s–1 and a thickness of 0.8∼3.0 km. The basement interface is locally rugged and rough. The velocity values of the upper and lower crusts are 5.0∼6.4 km·s–1 and 6.5∼6.9 km·s–1, and the upper mantle is a layer with an velocity of 8.0 km·s–1. The average crustal thickness of Xisha block is about 23 km. The depths of the upper and lower crusts are 9 km and 14 km. Moho interface lies at the depth of 23∼27 km. Xisha block is a thinned continental crust as compared with the South China continental margin; it has a size about 9.2×105 km3 calculated from two mutually perpendicular seismic profiles crossing the block. Xisha block and Nansha block are distributed on the two sides of the fossil spreading ridge of Northwest Sub‐basin. They are conjugate to each other since their velocity structures are very similar.

ANALYSIS AND PROCESSING OF ABNORMAL OBS DATA IN THE SOUTH CHINA SEA

AbstractProcessing and analysis of ocean bottom seismometer (OBS) data are of great importance to obtain the deep crustal structure. The deep seismic profile OBS2006‐2 was carried out in 2006 along the extinct spreading ridge of the northwest sub‐basin of the South China Sea. This survey is successful to provide high quality data whose seismic signals can be observed at the offset up to 120 km, but 2 OBSs’ recording data (OBS03 and OBS06) cannot be read correctly and were not used in later structure modeling. However, OBS data are very precious due to the high cost and arduous work. The price is more expensive when encountering severe weather during a survey. This paper is focused on reprocessing of these 2 OBSs’ data by use of the methods of checking data format, comparing signals with adjacent OBSs and resampling the data. Finally we acquire these 2 OBSs’ seismic record sections in which abundant seismic phases are clearly seen. We also obtain the seismic record section of OBS03 along the profile OBS973‐3 in the Nansha Island using the same methods above. The instrument OBS06 along the profile OBS2006‐2 and the instrument OBS03 along the profile OBS973‐3 are the same instrument confirmed by checking their logs recorded in 2006 and 2011, respectively. It demonstrates that the processing method for abnormal OBS data is reliable and effective. Then the ray‐tracing and travel‐time simulation were carried out for OBS06 and OBS03 using the interactive trial‐and‐error 2D ray‐tracing method based on the previous P‐wave velocity model of OBS2006‐2. The input of new travel‐time picks should provide stricter constraints for the deep structure beneath the profile OBS2006‐2. This research on abnormal data reprocessing does not only improve the reliability and resolution of the crustal structure, but also provide valuable experiences for OBS processing for other study areas in the future.

Using 4C OBS to reveal the distribution and velocity attributes of gas hydrates at the northern continental slope of South China Sea

To investigate the distribution and velocity attributes of gas hydrates in the northern continental slope of South China Sea, Guangzhou Marine Geological Survey conducted four-component (4C) ocean-bottom seismometer (OBS) surveys. A case study is presented to show the results of acquiring and processing OBS data for detecting gas hydrates. Key processing steps such as repositioning, reorientation, PZ summation, and mirror imaging are discussed. Repositioning and reorientation find the correct location and direction of nodes. PZ summation matches P- and Z-components and sums them to separate upgoing and downgoing waves. Upgoing waves are used in conventional imaging, whereas downgoing waves are used in mirror imaging. Mirror imaging uses the energy of the receiver ghost reflection to improve the illumination of shallow structures, where gas hydrates and the associated bottom-simulating reflections (BSRs) are located. We developed a new method of velocity analysis using mirror imaging. The proposed method is based on velocity scanning and iterative prestack time migration. The final imaging results are promising. When combined with the derived velocity field, we can characterize the BSR and shallow structures; hence, we conclude that using 4C OBS can reveal the distribution and velocity attributes of gas hydrates.

Seismic anisotropy beneath the Juan de Fuca plate system: Evidence for heterogeneous mantle flow

Here we use SKS shear wave splitting observations from ocean-bottom seismometer data to infer patterns of mantle deformation beneath the Juan de Fuca plate and its adjoining boundaries. Our results indicate that the asthenosphere beneath the Juan de Fuca plate responds largely to absolute plate motion with an anisotropic layer developing rapidly near the ridge and persisting into the subduction zone. Geographically restricted deviations from this pattern indicate the presence of secondary processes. At discrete plate boundaries, such as the Blanco transform fault, seismic anisotropy is attributed to relative plate motion within a narrow zone (<50 km). Beneath the deforming southern Gorda plate region—a diffuse plate boundary—splitting observations similarly suggest deformation dominated by relative motion between the rigid Juan de Fuca and Pacific plates but distributed over a broad zone (∼200 km). Our results are inconsistent with toroidal flow around the southern edge of the subducting slab due to rollback, as suggested by onshore studies. Instead, reorganization of upper mantle flow associated with plate fragmentation seems to dominate the anisotropic signature of southern Cascadia.

Velocity structures imaged from long-offset reflection data and four-component OBS data at Jiulong Methane Reef in the northern South China Sea

In this study, P- and S-wave velocity models were built based on two pre-stack depth migration (PSDM) profiles of long-offset reflection data and 25 four-component ocean-bottom seismometers (OBS) data at Jiulong Methane Reef off SW Taiwan in the passive margin of the northern South China Sea (SCS). According to the velocity models, the average P-wave velocity and Vp/Vs ratio of the free gas beneath the bottom-simulating reflector (BSR) are 1.52–1.58 km/s and 4.5–5.10, respectively. The depth of the BSR is found at 80–300 m below the sea floor and the sedimentary thickness of the hydrate and the free gas are about 50–100 m and 70–100 m, respectively. P-wave velocity of about 1.75 km/s in the hydrate-bearing sediment is southeastward increased to about 1.85 km/s above the BSR. Similarly, Vp/Vs ratio of about 3.06 in the hydrate-bearing sediment is increased southeastward to about 3.48 above the BSR. Based on the pseudo-3D map of gas-hydrate saturation estimated from the PSDM and OBS models, the average saturations of hydrate and free gas at Jiulong Methane Reef are about 7% and 0.9–2.4%, respectively. The highest hydrate saturation (11%) is located at 5–20 m above the BSR in the SE portion of the Jiulong Methane Reef. On the other hand, the highest gas saturation of about 2% is observed at 10–70 m below the BSR in the NW portion of the Jiulong Methane Reef. We suggested that several normal faults dipping southeastward beneath the continental slope provided conduits for gas migrating northwestward at Jiulong Methane Reef. Therefore, the highest gas saturation is observed below the anticline in the NW portion of the Jiulong Methane Reef and the highest hydrate saturation (high P-wave velocity and high Vp/Vs ratio) is identified above the BSR in the SE portion of the Jiulong Methane Reef.

Preface to the Focus Section on Cascadia Initiative Preliminary Results

The Cascadia Initiative (CI) is a community‐based onshore and offshore seismic and geodetic experiment designed to study a variety of questions ranging from megathrust earthquakes, episodic tremor and slip, volcanic arc structure, to the formation, deformation, and hydration of the downgoing Juan de Fuca and Gorda plates (Toomey et al. , 2014; Abers et al. , 2015). The four‐year deployment of the CI (from 2011 to 2015) was funded by the National Science Foundation. It includes a plate‐scale deployment of about 60 broadband ocean‐bottom seismometers (OBSs) over the downgoing Juan de Fuca and Gorda plates, 27 inland broadband EarthScope Transportable Array seismic stations, and the upgrade of 232 onshore Plate Boundary Observatory (PBO) geodetic sites to real‐time data transfer (see Abers et al. , 2015). Seismic data recovered from the CI are archived at the Incorporated Research Institutions for Seismology (IRIS) Data Management Center and openly available to the entire scientific community as quickly as technically feasible (http://ds.iris.edu/mda/_CASCADIA; last accessed July 2015). With the availability of the first three‐year data, a variety of research has been conducted to analyze the data and address the targeted scientific questions. This Seismological Research Letters ( SRL ) focus section presents a selected set of six papers that cover various aspects of this topic, including instrument performance, seismic structure and tsunami studies, and earthquake detection. The articles in this SRL focus section provide a snapshot of current research with the use of the CI seismic data. Sumy et al. (2015) summarizes the characteristics and performance of the CI OBS instruments, data recovery, and correction of horizontal orientation for the first three‐year deployment. …

A single-station method for the detection, classification and location of fin whale calls using ocean-bottom seismic stations.

Passive seismic monitoring in the oceans uses long-term deployments of Ocean Bottom Seismometers (OBSs). An OBS usually records the three components of ground motion and pressure, typically at 100 Hz. This makes the OBS an ideal tool to investigate fin and blue whales that vocalize at frequencies below 45 Hz. Previous applications of OBS data to locate whale calls have relied on single channel analyses that disregard the information that is conveyed by the horizontal seismic channels. Recently, Harris, Matias, Thomas, Harwood, and Geissler [J. Acoust. Soc. Am. 134, 3522-3535 (2013)] presented a method that used all four channels recorded by one OBS to derive the range and azimuth of fin whale calls. In this work, the detection, classification, and ranging of calls using this four-channel method were further investigated, focusing on methods to increase the accuracy of range estimates to direct path arrivals. Corrections to account for the influences of the sound speed in the water layer and the velocity structure in the top strata of the seabed were considered. The single station method discussed here is best implemented when OBSs have been deployed in deep water on top of seabed strata with low P-wave velocity. These conditions maximize the ability to detect and estimate ranges to fin whale calls.

Evidence of serpentinized mantle beneath a non-transform discontinuity at an ultra-slow spreading ridge from wide-angle ocean bottom seismometer data

Non-transform discontinuities (NTDs) are frequently found on ultra-slow spreading ridges. They have crustal structures that differ significantly from those of neighboring neo-volcanic ridges (NVRs). For an ultra-slow spreading ridge with weakened lithosphere and thinned crust, NTDs play an important role in understanding the mechanisms of ridge spreading, hydrothermal venting and mineral resource deposition. In this paper, we present an extensive ocean bottom seismometer (OBS) experiment that was carried out in 2010 at the Southwest Indian Ridge (SWIR) during R/V Dayang Yihao cruise DY115-21. The seismic source was a four-air-gun array with a total volume of or 98.32 L (6000 cubic inches) and a seismic trace spacing of ~250 m. A total of 10832 shots were fired and 52 survey lines (about 2650 km in length) were acquired. Thirty-eight OBSs, each containing a three-component geophone and a hydrophone, successfully recovered information from the crust and upper mantle in NVR and NTD settings. In this paper, we use two wide-angle seismic profiles and a ray-tracing method to constrain the velocity structure of the crust and upper mantle of the NTD between segments 28 and 29 of the SWIR. The processing methods include relocations of shot and OBS positions, time drift corrections and filtering. The widely used ray-theoretical travel-time inversion codes of Zelt and Smith (1992) were used for the modeling. The method includes a damped least square inversion of picked travel times, and we computed the statistical misfit, resolution and uncertainty of model parameters to ensure that the final velocity model had the best fit to our data. The main conclusions are as follows: (1) NTD has a thinner crust (3.2-4.5 km), with an oceanic layer 2 that is 2 km thick and an oceanic layer 3 that is very thin (<1 km) or absent. (2) Oceanic layer 2 has significant lateral velocity variation, indicating the presence of various faults. (3) The uppermost mantle has low velocities (7.2 km/s), indicative of large and deep faults or the presence of serpentinization. We suggest that the low velocity zone in the upmost mantle, together with the thinner crust (<5 km) of the NTD and the presence of faults, result from large-scale serpentinization of the uppermost mantle, providing a good example for investigations of the difference between the Moho discontinuity and a serpentinization front.

Lithospheric structure across the California Continental Borderland from receiver functions

AbstractDue to its complex history of deformation, the California Continental Borderland provides an interesting geological setting for studying how the oceanic and continental lithosphere responds to deformation. We map variations in present‐day lithospheric structure across the region using Ps and Sp receiver functions at permanent stations of the Southern California Seismic Network as well as ocean bottom seismometer (OBS) data gathered by the Asthenospheric and Lithospheric Broadband Architecture from the California Offshore Region Experiment (ALBACORE), which enhances coverage of the borderland and provides first direct constraints on the structure of the Pacific plate west of the Patton Escarpment. Noisiness of OBS data makes strict handpicking and bandpass filtering necessary in order to obtain interpretable receiver functions. Using H‐κ and common‐conversion point stacking, we find pronounced lithospheric differences across structural blocks, which we interpret as indicating that the Outer Borderland has been translated with little to no internal deformation, while the Inner Borderland underwent significant lithospheric thinning, most likely related to accommodating the 90° clockwise rotation of the Western Transverse Range block. West of the Patton Escarpment, we find that the transition to typical oceanic crustal thickness takes place over a lateral distance of ∼ 50 km. We detect an oceanic seismic lithosphere‐asthenosphere transition at 58 km depth west of the Patton Escarpment, consistent with only weak age‐dependence of the depth to the seismic lithosphere‐asthenosphere transition. Sp common‐conversion point stacks confirm wholesale lithospheric thinning of the Inner Borderland and suggest the presence of a slab fragment beneath the Outer Borderland.

Structural control on the nucleation of megathrust earthquakes in the Nankai subduction zone

AbstractTo clarify the causal mechanisms of megathrust earthquakes, we studied the detailed three‐dimensional P and S wave velocities (V), attenuation (Q), and Poisson's ratio (σ) structures of the Nankai subduction zone in southwest Japan, using a large number of high‐quality arrival time and t* data measured precisely from seismograms of local earthquakes. The suboceanic earthquakes used are relocated precisely using sP depth phase and ocean bottom seismometer data. Our results show the existence of two prominent high‐V, high‐Q, and low‐σ patches separated by low‐V, low‐Q, and high‐σ anomalies in the Nankai megathrust zone. Megathrust earthquakes during 1900 to 2013 nucleated in or around the high‐V, high‐Q, and low‐σ patches, which may represent strongly coupled areas (i.e., asperities) in the megathrust zone. Our results indicate that structural heterogeneities in the megathrust zone, such as the subducting seafloor topography and compositional variations, control the nucleation of the Nankai megathrust earthquakes.

Correction of Ocean-Bottom Seismometer Instrumental Clock Errors Using Ambient Seismic Noise

Abstract Very accurate timing of seismic recordings is critical for modern processing techniques. Clock synchronization among the instruments constituting an array is, however, difficult without direct communication between them. Synchronization to Global Positioning System (GPS) time is one option for on‐land deployments, but not for underwater surveys as electromagnetic signals do not propagate efficiently in water. If clock drift is linear, time corrections for ocean‐bottom seismometer (OBS) deployments can be estimated through GPS synchronization before and after the deployment, but this is not sufficient for many applications as the nonlinear component of the drift can reach tens to hundreds of milliseconds for long‐duration experiments. We present two techniques to retrieve timing differences between simultaneous recordings at ocean‐bottom instruments after deployment has ended. Both techniques are based on the analysis of the cross correlation of ambient seismic noise and are effective even if clock drift is nonlinear. The first, called time symmetry analysis, is easy to apply but requires a proper illumination so that the noise cross‐correlation functions are symmetric in time. The second is based on the doublet analysis method and does not have this restriction. Advantages and drawbacks of both approaches are discussed. Application to two OBS data sets shows that both can achieve synchronization of recordings down to about five milliseconds (a few percent of the main period used).

Azimuthal anisotropy from OBS observations in Mahanadi offshore, India

We have carried out an ocean bottom seismometer (OBS) survey in a grid along with multichannel seismic survey for gas hydrate exploration in the Mahanadi offshore, India. Here, we report on some interesting observations in seismic waveform data and their interpretations. These include sudden amplitude dimming in the multichannel data that is azimuth- and space-dependent and a clear manifestation of seismic anisotropy in the region. We observe significant patterns of shear wave splitting in the azimuthal gathers in the OBS data, clearly isolating the fast (S1) and slow (S2) axes of propagation in the radial azimuthal gathers. Further, amplitude nulls and amplitude maxima are observed in the transverse azimuthal gathers. These two features are diagnostic of the existence and orientation of anisotropy which is also modeled by generating full waveform synthetic seismograms. We interpret the occurrence of anisotropy to be due to the presence of fractures. The strike of this fracture set is inferred to be [Formula: see text] from the S1 and S2 orientation and variation in the P-wave amplitude with azimuth. The density of fracture network is estimated by full wave modeling of the OBS data. A good match between the synthetic and observed data is noticed for a near vertical fracture (dip angle of about 85°). The seismic image obtained from the 2D high-resolution multichannel profiles correlate well with the OBS results. Based on these analyses, we are able to delineate a fracture zone, which is linked to the near vertical faulting in the gas hydrate layers.

An automatic single station multipath ranging technique for 20 Hz fin whale calls: Applications for ocean bottom seismometer data in the northeast Pacific

Ocean bottom seismometers (OBSs) deployed to monitor earthquakes also record 20 Hz fin whale vocalizations. Because OBSs are often deployed too far apart to record a call on more than one instrument, and call detections are dependent on the acoustic environment, techniques are required to determine call density from a single instrument. One approach is to use point transect distance sampling, which relies on estimates of horizontal range to each call. An automated technique has been developed to estimate range based on the amplitude and relative timing of multipath arrivals assuming a uniform water depth. The method has been evaluated using independently located calls recorded by a seismic network on the Endeavour Segment of the Juan de Fuca Ridge, a region of rough and partially sedimented seafloor. Results suggest that the method works well to ranges of ~5 km but at larger ranges ambiguities arise. Calls at &amp;gt;10 km range are sometimes located between 5 and 10 km because the times are erroneously fit by one too few water column multiples and the amplitudes have too much scatter to be diagnostic of range. The technique is presently being applied in different acoustic settings to determine whether it is more reliable in flat sedimented regions.

New insights on volcanic and tectonic structures of the southern Tyrrhenian (Italy) from marine and land seismic data

We present results from the first crustal seismic tomography for the southern Tyrrhenian area, which includes ocean bottom seismometer (OBS) data and a bathymetry correction. This area comprises Mt. Etna, the Aeolian Islands, and many volcanic seamounts, including the Marsili Seamount. The seismicity distribution in the area depends on the complex interaction between tectonics and volcanism. The 3‐D velocity model presented in this study is obtained by the inversion of P wave arrival times from crustal earthquakes. We integrate travel time data recorded by an OBS network (Tyrrhenian Deep Sea Experiment), the SN‐1 seafloor observatory, and the land network. Our model shows a high correlation between the P wave anomaly distribution and seismic and volcanic structures. Two main low‐velocity anomalies underlie the central Aeolian Islands and Mt. Etna. The two volumes, which are related to the well‐known active volcanism, are separated and located at different depths. This finding, in agreement with structural, petrography, and GPS data from literature, confirms the independence of the two systems. The strongest negative anomaly is found below Mt. Etna at the base of the crust, and we associate it with the deep feeding system of the volcano. We infer that most of the seismicity is generated in brittle rock volumes that are affected by the action of hot fluids under high pressure due to the active volcanism in the area. Lateral changes of velocity are related to a transition from the western to the central Aeolian Islands and to the passage from continental crust to the Tyrrhenian oceanic uppermost mantle.

On a plane-wave based crosscorrelation-type seismic interferometry

We explored a new approach to retrieve virtual seismic responses from crosscorrelating acquired seismic data in the plane-wave domain. Using this method, slant stacking is first performed over shot or receiver locations of observed seismic data to produce plane-wave transformed gathers. Crosscorrelation is then performed by selecting traces with the same ray parameters from different shot or receiver locations of the plane-wave gathers. Unlike traditional crosscorrelation-type time-space domain interferometry, where full range of ray parameters is used for each survey location, this method directly selects common ray parameters to cancel overlapping raypaths. This approach can be used to retrieve reflections in the presence of dispersive waves and to select certain ranges of ray parameters with directional wave paths for retrieval. It can avoid spurious arrivals in supervirtual interferometry when unwanted arrivals such as reflections break the requirement of conventional interferometry. In addition, computation time can be saved with this approach because plane-wave transform usually results in a reduction of the original data volume. We demonstrate this method with synthetic and ocean bottom seismometer data examples.

Crustal accretion in the Manila trench accretionary wedge at the transition from subduction to mountain-building in Taiwan

New marine seismic reflection and coincident wide-angle ocean-bottom seismometer data acquired offshore Taiwan provide high-resolution constraints on the crustal structure of an incipient mountain belt during the earliest stage of arc–continent collision. The new seismic reflection image and travel-time tomography velocity model show evidence for crust of the distal southern Chinese continental margin being subducted eastward beneath the Manila trench and underplated to the accretionary wedge before collision with the southern Chinese continental shelf. The distal margin crust consists of highly extended continental crust interspersed with volcanic bodies and a high-velocity lower crustal layer of likely magmatic intrusions. The distal margin crust is 10–14km thick outboard of the trench, but thins to 6km thick beneath the lower slope of the Manila trench accretionary wedge. Along the lower slope of the accretionary prism, we image westward-verging imbricate thrusts and folded strata up to 10km thick. A sharp decrease in bathymetry marks the transition from lower to upper slope, where we observe a fast (>6.0km/s) seismic velocity anomaly at the base of the wedge that we interpret as structurally underplated crust from the distal continental margin. Our results support a model of arc–continent collision in Taiwan where the accretionary wedge is first thickened by structural underplating of distal margin crust prior to collision with the continental shelf. The crustal rocks exposed throughout the Central Range in Taiwan may be similarly derived from subducted and structurally underplated crust from the highly extended distal continental margin.