Seismometer Data Research Articles

Final breakup of continental block and opening of oceanic lithosphere: insights from deep crustal structure and tectonic evolution of the ocean–continent transition zone in the northern South China Sea

As an important transition domain between the thinnest continental crust and the initial oceanic crust, the ocean–continent transition zone (OCT) preserves a wealth of information on the processes of lithospheric extension and breakup. The northern margin of the South China Sea (SCS) is a non‐typical, magma‐poor passive continental margin with a particular structural pattern and sedimentary formation. The OCT in the northern margin of the SCS was developed by a series of continental rifting and seafloor spreading. Based on several multi‐channel seismic reflection profiles, crustal‐scale structural profiles extracted from gravity data and ocean‐bottom seismometers (OBS) data, this paper studies the deep crustal structures, deformation and geodynamics of the OCT to further understand the structural patterns and tectonic process from the lithospheric extension to its breakup. The OCT of the northern margin of the SCS is characterized by a crustal thinning and high‐velocity layer (HVL) in the lower crust. The OCT has three structural units: rift depression, volcanic zone and tilted fault blocks. The HVL is mainly limited in the eastern portion of the northern SCS. The HVL is distributed essentially within the OCT but does not clearly develop everywhere in the OCT. We propose a hybrid model of rifting to fit characteristics of the northern margin of the SCS. A crustal‐scale largely symmetric process predominated at the initial rifting stage. The basin‐controlling faults subsequently penetrated the entire crust, resulting in asymmetric tectonic evolution. Then, the OCT occupied this asymmetric location. Copyright © 2016 John Wiley & Sons, Ltd.

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.

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.

High-quality lowest-frequency normal mode strain observations at the Black Forest Observatory (SW-Germany) and comparison with horizontal broad-band seismometer data and synthetics

We present spectra concentrating on the lowest-frequency normal modes of the Earth obtained from records of the invar-wire strainmeters and STS-1 broad-band seismometers located in the Black Forest Observatory, Germany after the disastrous earthquakes off the NW coast of Sumatra in 2004 and off the coast near Tohoku, Japan in 2011. We compare the spectra to ones obtained from synthetic seismograms computed using a mode summation technique for an anelastic, elliptical, rotating, spherically symmetric Earth model. The synthetics include strain–strain-coupling effects by using coupling coefficients obtained from comparisons between Earth tide signals recorded by the strainmeters and synthetic tidal records. We show that for the low-frequency toroidal and spheroidal modes up to 1 mHz, the strainmeters produce better signal-to-noise ratios than the broad-band horizontal seismometers. Overall, the comparison with the synthetics is satisfactory but not as good as for vertical accelerations. In particular, we demonstrate the high quality of the strainmeter data by showing the Coriolis splitting of toroidal modes for the first time in individual records, the first clear observation of the singlet 2S0/1 and the detection of the fundamental radial mode 0S0 with good signal-to-noise ratio and with a strain amplitude of 10^−11. We also identify the latter mode in a record of the Isabella strainmeter after the great Chilean quake in 1960, the detection of which was missed by the original studies.

A joint electromagnetic and seismic study of an active pockmark within the hydrate stability field at the Vestnesa Ridge, West Svalbard margin

AbstractWe acquired coincident marine controlled source electromagnetic (CSEM), high‐resolution seismic reflection and ocean‐bottom seismometer (OBS) data over an active pockmark in the crest of the southern part of the Vestnesa Ridge, to estimate fluid composition within an underlying fluid‐migration chimney. Synthetic model studies suggest resistivity obtained from CSEM data can resolve gas or hydrate saturation greater than 5% within the chimney. Acoustic chimneys imaged by seismic reflection data beneath the pockmark and on the ridge flanks were found to be associated with high‐resistivity anomalies (+2–4 Ωm). High‐velocity anomalies (+0.3 km/s), within the gas‐hydrate stability zone (GHSZ) and low‐velocity anomalies (−0.2 km/s) underlying the GHSZ, were also observed. Joint analysis of the resistivity and velocity anomaly indicates pore saturation of up to 52% hydrate with 28% free gas, or up to 73% hydrate with 4% free gas, within the chimney beneath the pockmark assuming a nonuniform and uniform fluid distribution, respectively. Similarly, we estimate up to 30% hydrate with 4% free gas or 30% hydrate with 2% free gas within the pore space of the GHSZ outside the central chimney assuming a nonuniform and uniform fluid distribution, respectively. High levels of free‐gas saturation in the top part of the chimney are consistent with episodic gas venting from the pockmark.

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.

Systematic monitoring of instrumentation health in high-density broadband seismic networks

Broadband seismometer data are essential for the development of seismological studies such as those investigating earthquake sources and the Earth’s structure. However, previous studies have revealed that the metadata describing these data can possibly be contaminated by instrumentation response errors that are often difficult to recognize from visual waveform checks. Herein, we report on the development of a systematic method of assessing seismometer conditions when recording ground motions at a period range of 50 to 200 s in observation networks whose station intervals are smaller than 200 km. The method is based on comparisons between teleseismic surface wave records at a target station and those at multiple surrounding reference stations, from which we calculate three index parameters and evaluate in situ instrumentation conditions, including amplitude and phase responses against input ground motions. In our experiments, we applied the proposed method to F-net broadband seismometers covering the Japanese Islands, where station intervals are approximately 100 km. This allows us, through calculations of the index parameters, to evaluate instrumentation health at each station at least once every 60 days. Using our proposed method, we found that approximately 75% of the evaluated index parameters distributed well around the standard values, and for most examined broadband seismometers, response anomalies are not detected at the period range of 50 to 200 s. However, instrumentation errors, such as gain decrease over the evaluated periods and gradual changes in amplitude and phase frequency responses (sometimes covering several years) were identified at a few stations. Additionally, overdamping errors at the STS-1 seismometers, which experience significant amplitude and phase frequency response variations around the 360-s corner, appear to have been common at several stations. In contrast, STS-2 seismometers appear to have functioned more reliably than STS-1 seismometers. We developed a method to evaluate broadband seismometer instrument conditions by comparing teleseismic surface waves observed at a target station with those at multiple surrounding stations. It is believed that the systematic evaluation of instrumentation health using our method will enhance the operation of seismic networks, and allow researchers to eliminate contaminated data before conducting various data analyses.

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.

Measuring of clock drift rates and static time offsets of ocean bottom stations by means of ambient noise

Marine seismology usually relies on temporary deployments of stand alone seismic ocean bottom stations (OBS), which are initialized and synchronized on ship before deployment and re-synchronized and stopped on ship after recovery several months later. In between, the recorder clocks may drift and float at unknown rates. If the clock drifts are large or not linear and cannot be corrected for, seismological applications will be limited to methods not requiring precise common timing. Therefore, for example, array seismological methods, which need very accurate timing between individual stations, would not be applicable for such deployments. We use an OBS test-array of 12 stations and 75 km aperture, deployed for 10 months in the deep sea (4.5–5.5 km) of the mid-eastern Atlantic. The experiment was designed to analyse the potential of broad-band array seismology at the seafloor. After recovery, we identified some stations which either show unusual large clock drifts and/or static time offsets by having a large difference between the internal clock and the GPS-signal (skew). We test the approach of ambient noise cross-correlation to synchronize clocks of a deep water OBS array with km-scale interstation distances. We show that small drift rates and static time offsets can be resolved on vertical components with a standard technique. Larger clock drifts (several seconds per day) can only be accurately recovered if time windows of one input trace are shifted according to the expected drift between a station pair before the cross-correlation. We validate that the drifts extracted from the seismometer data are linear to first order. The same is valid for most of the hydrophones. Moreover, we were able to determine the clock drift at a station where no skew could be measured. Furthermore, we find that instable apparent drift rates at some hydrophones, which are uncorrelated to the seismometer drift recorded at the same digitizer, indicate a malfunction of the hydrophone.

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.

Seismic velocities on the Nova Scotian margin to estimate gas hydrate and free gas concentrations

This article provides new constraints on gas hydrate and free gas concentrations in the sediments at the margin off Nova Scotia. Two-dimensional (2-D) velocity models were constructed through simultaneous travel-time inversion of ocean-bottom seismometer (OBS) data and 2-D single-channel seismic (SCS) data acquired in two surveys, in 2004 and 2006. The surveys, separated by ∼5 km, were carried out in regions where the bottom-simulating reflection (BSR) was identified in seismic reflection datasets from earlier studies and address the question of whether the BSR is a good indicator of significant gas hydrate on the Scotian margin. For both datasets, velocity increases by 200–300 m/s at a depth of approximately 220 m below seafloor (mbsf), but the results of the 2006 survey show a smaller velocity decrease (50–80 m/s) at the base of this high-velocity layer (310–330 mbsf) than the results of the 2004 survey (130 m/s). When converted to gas hydrate concentrations using effective medium theory, the 2-D velocity models for both datasets show a gas hydrate layer of ∼100 m thickness above the identified BSR. Gas hydrate concentrations are estimated at approximately 2–10% for the 2006 data and 8–18% for the 2004 survey. The reduction in gas hydrate concentration relative to the distance from the Mohican Channel structure is most likely related to the low porosity within the mud-dominant sediment at the depth of the BSR. Free gas concentrations were calculated to be 1–2% of the sediment pore space for both datasets.

A case study: imaging OBS multiples of South China Sea

The subseafloor structure offshore South China Sea was imaged using first-order water-layer multiples from ocean-bottom seismometer data and the results were compared to conventional imaging using primary reflections. The mirror-imaging method employs a primaries-only reverse time pre-stack depth migration algorithm to image the receiver ghosts. The additional travel path of the multiples through the water layer is accounted for by a simple manipulation of the velocity model and processing datum: the receivers lie not on the sea floor but on a sea surface twice as high as the true water column. Migration results show that the multiple-migrated image provides a much broader illumination of the subsurface than the conventional image using the primaries, especially for the very shallow reflections. The resulting image from mirror imaging has illumination comparable to the vertical incidence surface streamer (single-channel) reflection data.

Waveform tomography imaging of a megasplay fault system in the seismogenic Nankai subduction zone

We apply Frequency-domain Waveform Tomography to form quantitative, high-resolution P-wave velocity images of a megasplay fault system within the central Nankai subduction zone offshore of southwest Japan, using controlled source Ocean Bottom Seismometer (OBS) data originally acquired in 2004. The Waveform Tomography method exploits recorded seismic waveforms beyond their first arrivals, and thus achieves a much higher resolution (of the order of a wavelength) than that of conventional Traveltime Tomography methods. Frequency-domain Waveform Tomography facilitates a multi-scale approach to stabilize the inversion, in which initial Traveltime Tomography results are sequentially improved on by first fitting low frequency components of the seismic records (starting at 2.25Hz); higher frequency components (up to 8.5Hz) are then introduced progressively.Our final Waveform Tomography image allows velocity anomalies as small as 700m (horizontally) and 350m (vertically) to be discerned and interpreted, as confirmed by checkerboard modeling tests. The improved explanatory power of the final images is verified by observing that synthetic waveforms calculated from the final results yield much better fit to the observed waveforms than those estimated from the original Traveltime Tomography image. Apparent lithological boundaries from Waveform Tomography agree well with corresponding reflections on seismic migration images, providing further confidence in the validity of the results.The megasplay fault is evident on the final Waveform Tomography image as a sharp velocity discontinuity, delineating the upper surface of a velocity reduction of nearly 1km/s with respect to the regional 1D velocity trend. The megasplay fault can be traced continuously throughout the entire image, from a nearly horizontal section at the landward extent, moving seaward through to a steeper angle penetrating the old accretionary prisms, with several additional splays appearing to branch in the shallow subsurface. The location of the low-velocity zones imaged by our waveform tomography method coincides with two-previously-identified low velocity zones. The image reveals a low velocity zone that is continuous from deeper to shallower portions of the subsurface, suggesting that pore-fluids may be transported from the inner wedge, to the transition zone, and to the surface, through fluid conduits associated with the megasplay fault system.

Response of the Ross Ice Shelf, Antarctica, to ocean gravity-wave forcing

AbstractComparison of the Ross Ice Shelf (RIS, Antarctica) response at near-front seismic station RIS2 with seismometer data collected on tabular iceberg B15A and with land-based seismic stations at Scott Base on Ross Island (SBA) and near Lake Vanda in the Dry Valleys (VNDA) allows identification of RIS-specific signals resulting from gravity-wave forcing that includes meteorologically driven wind waves and swell, infragravity (IG) waves and tsunami waves. The vibration response of the RIS varies with season and with the frequency and amplitude of the gravity-wave forcing. The response of the RIS to IG wave and swell impacts is much greater than that observed at SBA and VNDA. A spectral peak at near-ice-front seismic station RIS2 centered near 0.5 Hz, which persists during April when swell is damped by sea ice, may be a dominant resonance or eigenfrequency of the RIS. High-amplitude swell events excite relatively broadband signals that are likely fracture events (icequakes). Changes in coherence between the vertical and horizontal sensors in the 8–12 Hz band from February to April, combined with the appearance of a spectral peak near 10 Hz in April when sea ice damps swell, suggest that lower (higher) temperatures during austral winter (summer) months affect signal propagation characteristics and hence mechanical properties of the RIS.