Shallow Seismic Profiles Research Articles

Depositional conditions and organic matter distribution in the Bornholm Basin, Baltic Sea

Information on grain-size distribution and total organic carbon (TOC) content of surface sediment and cores from the Bornholm Basin, together with dating of cores using the 210Pb method and shallow seismic chirp profiling, has been analysed to elucidate long-term accumulation patterns. The presence of non-depositional areas with lag sediments and low TOC content below the wave base indicates that inflows of dense bottom water originating in the North Sea and associated near-bottom currents have strong influence on the depositional patterns of bulk sediment and organic matter in this deep basin. The general fining in mean grain size towards the northeast corresponds to the direction of inflow currents and prevailing winds. Recent and previously found 210Pb-based mean accumulation rates vary greatly within the basin, between 129 and 1,144 g m−2 year−1. The accumulation rate may vary by a factor of three even between stations located only 3–4 km apart. Rates recorded close to a seismic profile are consistent with the variation in Holocene sediment thickness. This variation reflects a depositional system controlled by near-bottom inflow currents, consisting of a large-scale channel and a wedge-formed sediment package. The spatial variation in TOC content depends partly on water depth, presumably due to generally poorer degradation in the deepest part of the basin because of less frequent oxygen supply by inflow water. Moreover, there is a tendency of higher TOC contents in the southern part of the basin, which may be due to the input of sediments originating from the Oder River. Compared to values for the central, deep Baltic Sea, TOC contents show lower values of 4–6% and insignificant temporal variations. This may be due to the Bornholm Basin being located much closer to the source of the more oxic inflow water, resulting in more favourable degradation conditions.

Geologic characteristics of the central stretch of the Ticona Channel, north-central Illinois

The Ticona Channel is located in north-central Illinois and occurs in Grundy, LaSalle, and Putnam counties. It is a buried bedrock valley that served as the principal paleodrainage system in north-central Illinois during the Illinoian and pre-Illinoian. This study focused on the part of the Ticona Channel within the Leonore 7.5 Quadrangle. The geometry and stratigraphy of sediments that fill the Ticona Channel were investigated using high-resolution, shallow seismic reflection profiling, traditional field geologic mapping techniques, borehole data, and water-well-log data. The valley is about 2 km (1 mi) wide and approximately 60 m (200 ft) deep. The U-shape channel is straight, trends east-west, and has only one mappable tributary. The valley is carved into the Pennsylvanian Carbondale Formation in the eastern part of the study area; it has incised into the Ordovician Prairie du Chien Group in the west. At its base, the Ticona Channel is filled with the Pearl Formation, which is coarse-grained sand and gravel that was deposited during the Illinoian glaciation. The Pearl Formation is overlain by Illinoian till of the Glasford Formation and is capped by Wedron Group sediments from the Wisconsinan stage.

Sedimentary evolution of the Holocene subaqueous clinoform off the Shandong Peninsula in the Yellow Sea

A subaqueous clinoform wraps around the end of the Shandong Peninsula in the Yellow Sea. The clinoform, previously called the Shandong mud wedge, is up to 40 m thick nearshore and thins seaward. Its origin has been controversial since it was first noted in the 1980s. For our recent investigation offshore of the northeastern Shandong Peninsula, high-resolution shallow seismic profiles covering about 1700 km were obtained, and two drilling cores penetrating the clinoform were recovered. We described the lithofacies characteristics, identified benthic foraminifera and ostracoda, measured grain size, analyzed the mineral components of the cores, and obtained AMS 14C dates on 28 samples, to decipher the sedimentary evolution of the clinoform in response to postglacial sea-level changes. The shallow seismic profiles show that the clinoform comprises three units: a lowermost seismic unit (SU 3, less than 2–3 m thick) showing a retrogradational/aggradational stacking pattern with subhorizontal internal reflectors, a middle seismic unit (SU 2, up to 35 m thick) showing a prograding reflection pattern with mostly seaward-stepping reflectors separated by several erosive surfaces, and an uppermost seismic unit (SU 1, maximum thickness 18 m) exhibiting an aggradational reflection pattern with subparallel reflectors. The three seismic units are bounded on the bottom by distinct erosion surfaces. Analyses of the two cores indicate that the clinoform comprises three depositional units, DU 1, DU 2, and DU 3 in descending order, corresponding respectively to SU 1, SU 2, and SU 3. The depositional units are distinguished by lithofacies characteristics and the downcore distributions of benthic foraminiferal and ostracod assemblages. Mineral components of the cores suggest that the clinoform's provenance is predominantly Yellow River-derived sediments. AMS 14C dates suggest that DU 3 was formed from about 11.6 to 9.6 cal kyr BP, DU 2 from 9.6 to 6.5 cal kyr BP, and DU 1 after ca. 6.5 cal kyr BP. Some major meltwater pulse (MWP) events during the deglacial transgression were registered in the clinoform stratigraphy, with the basal surfaces/sediments of DU 3 and DU 2 corresponding respectively to MWP-1B (ca. 11.6–11.3 cal kyr BP) and MWP-1C (ca. 9.6–9.2 cal kyr BP) events, and with DU 2 being largely formed after the 8.4–8.2 cal kyr meltwater event. Rapid centennial- to multidecadal-scale flooding events were presumably responsible for the erosive surfaces separating the downlapping internal reflectors in SU 2. The change of depositional pattern from progradational in SU 2 to aggradational in SU 1 is attributed to a change from strong, dominantly to-and-fro tidal currents to the modern circulation in the Bohai Sea as well as to the weakening of the tidal-current field during the highstand after about 6500 cal yr BP. As a product of complex interactions among sediment supply, sea-level changes, and hydrodynamic regimes, the clinoform can be regarded as the distal part of the tidal depositional system in the eastern Bohai Sea, which was largely derived from the Yellow River.

Paleocanais na plataforma continental interna do Rio Grande do Sul: evidências de uma drenagem fluvial pretérita

Subsurface structures mapped by the Sparker® acoustic system on the inner continental shelf of Rio Grande do Sul State, in southern Brazil were interpreted as paleochannels which developed over a coastal plain environment during the upper Pleistocene and lower Holocene. Two major reflectors displaying expressive lateral continuity were identified. The first reflector, which is shallower, is buried at depths ranging from 10 to 12m and its surface is relatively homogeneous displaying few irregularities associated that channeled structures. The second reflector mapped on depths ranging from 14 a 32m is more discontinuous and irregular and display erosive structures. The overall interpretation of the shallow seismic reflection profiles indicates that this preterit drainage display an N/NE-SE/SW orientation increasing its width and decreasing its depth towards offshore. The presence of paleochannels in the inner continental shelf of RS state supports the paleogeographic models of the area proposed by early studies.

On the Spectral Analysis of Love Waves

The focus of this study is an analytical and computational platform for shallow seismic profiling via the dispersion and attenuation analysis of Love surface waves. Although the Rayleigh waves are now commonly used as an engineering tool for nonintrusive identification of shallow subsurface stratigraphy, little attention has been paid so far to utilizing their horizontally polarized, Love-wave counterpart. The proposed methodology revolves around a causal viscoelastic model for the wave motion in a layered half-space caused by action of a surficial, torsionally vibrating disc. It is shown that the use of Love waves as a sounding tool reduces the number of material parameters relevant to viscoelastic site characterization by precluding the effect of Poisson’s ratio and of the compressional-wave quality factor of each subterranean layer. For practical purposes, solution to the inverse problem is reduced to the minimization of a spectral misfit between experimental observations and theoretical predictions of the horizontally polarized surface ground motion. To maintain the rigor and computational effectiveness of the inverse solution, sensitivities of the viscoelastodynamic model with respect to relevant layer parameters are determined analytically. The Love-wave testing methodology for which this article establishes a necessary theoretical framework may be used either in a stand-alone fashion or as a complement to the (well-established) spectral analysis of Rayleigh waves. The proposed technique may be especially useful for (1) shallow surveying applications in which material dissipation is important and (2) accurate characterization of saturated layers in which vertically polarized waves find limited use owing to their sensitivity to the pore fluid.

Stratigraphy, age, and correlation of voluminous debris‐avalanche events from an ancestral Egmont Volcano: Implications for coastal plain construction and regional hazard assessment

Two previously unrecognised debris‐avalanche deposits have been identified on the eastern flanks of Egmont Volcano beneath a thick mantle of tephric and andic soil material that has mostly subdued their topographic expression. The Ngaere Formation is a c. 23 14C ka large volume (>5.85 km3) debris‐avalanche deposit that is widely distributed over 320–500 km2 of the north‐east, south‐east, and south portions of the Egmont ring plain. The second deposit, Okawa Formation, is a c. 105 ka large volume (>3.62 km3) debris‐avalanche deposit that has been mapped over a minimum area of 255 km2 in northern and north‐eastern Taranaki. Both debris‐avalanche formations contain axial facies with hummocks composed mainly of block‐supported brecciated andesitic debris. A less conspicuous marginal facies, texturally resembling a mudflow, is more extensive. A third debris‐avalanche deposit (Motunui Formation) is extensively preserved along the north Taranaki coast where it is truncated by a c. 127 ka wave cut surface (NT2) and closely overlies a c. 210 ka wave cut surface (NT3). The source of this debris‐avalanche deposit is unknown. Side‐scan sonar and shallow seismic profiling have been useful in accurately delineating the distribution of combined Okawa and Motunui debris‐avalanche deposits in the offshore environment but cannot distinguish between the two deposits or enable onshore spatial and volumetric estimates for each unit to be revised. However, the widespread occurrence of debris‐avalanche rock material offshore does emphasise the importance of this lag material altering the orientation of the coast influencing both wave climate and rates of coastal erosion. Similarly, the extensive onshore occurrence of debris‐avalanche rock material appears to be a significant factor in widening of the north Taranaki coastal plain and preservation of the NT2 and NT3 uplifted marine terrace surfaces. Initiation of collapse by magmatically‐induced seismicity is apparently common at many stratovolcanoes. Emplacement of Ngaere Formation was immediately preceded by a magmatic fall unit and is directly overlain by a closely spaced sequence of 13 fall units. In contrast, there is no evidence to indicate that an eruptive event triggered or immediately followed the Okawa debris‐avalanche event, but seismically induced gravitational sliding cannot be discounted. Egmont Volcano has repetitively collapsed over its c. 127 ka history and has generated at least five voluminous landscape‐forming debris‐avalanche deposits. Probabilistically‐based return times are calculated at c. 1967 14C yr for volumes ≥0.15 km3 and c. 21 000 14C yr for volumes ≥7.5 km3. Despite lower return times in comparison to tephra emission, Egmont Volcano is an inherently unstable cone because it comprises interbedded lavas and unconsolidated volcaniclastic deposits with a high slope angle overlying a faulted basement of Tertiary sediments. Should eruptive activity recommence and coincide with significant upper cone dilation, then the likelihood of a gravitational cone collapse is expected to increase although critical thresholds remain to be modelled. Fortunately, the Taranaki Regional Volcanic Contingency Plan is based on pre‐emptive evacuation which is intended to minimise loss of life in advance of an eruptive and/or cone collapse event occurring.

The movement age of hidden fault and analysis on width of its effect zone from shallow seismic sounding and drilling data

Field experimental seismic sounding permitted us to obtain optimal shallow seismic reflection sounding parameters. In process of data processing, we obtained a high-qualitative shallow seismic reflection sounding profile by using the techniques such as filtering, edition surgical blanking, prediction deconvolution, fitting static correlation of first arrival time, and velocity analysis. Comprehensive analysis on the information of reflection wave groups along the seismic sounding profile and the stratigraphic and neogeochronological data obtained from many drills near the sounding line reveals that the upper termination of the detected fault zone is located at depth of 75–80 m, in the Middle Pleistocene deposits dated to be about 220 ka BP. The continuity, discontinuity, increasing and decreasing amount of reflection wave groups and change of their configurations, in combination with geological columns of drills, permitted us to know that the width of upper termination of the fault zone is 100 m. It can be inferred from the variation of number of reflection wave groups along the profile that the scarp of hidden fault is 200 m wide and the fault is a synsedimentary active fault in the Early Pleistocene and the early stage of Middle Pleistocene. No tectonic movement, which offset the covering deposits, had occurred since the late stage of Middle Pleistocene.

Sea-level changes and depositional environments in the İzmit Gulf, eastern Marmara Sea, during the late glacial–Holocene period

Offshore and onshore stratigraphic studies, together with high-resolution shallow seismic reflection profiling and multibeam bathymetric mapping, were carried out in the western and central part of the İzmit Gulf. These studies indicate that the İzmit Gulf was a lacustrine environment as part of the Marmara ‘Lake’ during the late glaciation and early deglaciation until ∼12 kyr BP, when the Marmara Basin was inundated by the Mediterranean waters. Correlation of 14C-dated onshore and offshore stratigraphic units in the Western İzmit Gulf indicates that generally coarse late glacial sediments overlie a marked erosional surface formed during the low water level of the Marmara ‘Lake’. These coarse sediments are succeeded by 10.4–7 kyr BP old transgressive, and by late Holocene post-transgression mud units. The bathymetry and sub-bottom Chirp profiles clearly show that the paleoshoreline of the Gulf was located at −85 m, having been controlled by the bedrock sill depth of the Çanakkale Strait. Another paleoshoreline observed at −65 m on the northern margin of the Western İzmit and Karamürsel basins was probably formed during the Younger Dryas sea-level stillstand. The shelf areas during this time were colonized by bioherms, which were subsequently drowned and disappeared after a further rise of the sea level. The presence of a −65 m marine paleoshoreline in the Karamürsel Basin indicates that the sill restricting this basin to the west was much deeper than its present −55 m level and was located further south. The Gölcük Basin, restricted by a −38 m sill to its west, was probably not flooded by marine waters until ∼9 kyr BP.

Variable post-Paleozoic deformation detected by seismic reflection profiling across the northwestern “prong” of New Madrid seismic zone

High-resolution shallow seismic reflection profiles across the northwesternmost part of the New Madrid seismic zone (NMSZ) and northwestern margin of the Reelfoot rift, near the confluence of the Ohio and Mississippi Rivers in the northern Mississippi embayment, reveal intense structural deformation that apparently took place during the late Paleozoic and/or Mesozoic up to near the end of the Cretaceous Period. The seismic profiles were sited on both sides of the northeast-trending Olmsted fault, defined by varying elevations of the top of Mississippian (locally base of Cretaceous) bedrock. The trend of this fault is close to and parallel with an unusually straight segment of the Ohio River and is approximately on trend with the westernmost of two groups of northeast-aligned epicenters (“prongs”) in the NMSZ. Initially suspected on the basis of pre-existing borehole data, the deformation along the fault has been confirmed by four seismic reflection profiles, combined with some new information from drilling. The new data reveal (1) many high-angle normal and reverse faults expressed as narrow grabens and anticlines (suggesting both extensional and compressional regimes) that involved the largest displacements during the late Cretaceous (McNairy); (2) a different style of deformation involving probably more horizontal displacements (i.e., thrusting) that occurred at the end of this phase near the end of McNairy deposition, with some fault offsets of Paleocene and younger units; (3) zones of steeply dipping faults that bound chaotic blocks similar to that observed previously from the nearby Commerce geophysical lineament (CGL); and (4) complex internal deformation stratigraphically restricted to the McNairy, suggestive of major sediment liquefaction or landsliding. Our results thus confirm the prevalence of complex Cretaceous deformations continuing up into Tertiary strata near the northern terminus of the NMSZ.

Characteristics of seismostratigraphy and analysis of structurallithofacies in the southern Yellow Sea Cenozoic basin

In this paper, the upper structural layer of the southern Yellow Sea Basin (SYSB) is further divided into three layers based on the latest obtained high-resolution single-channel seismic profiles. Combined with the borehole data, the characteristics and evolvement of structure and environment since middle-late Tertiary are discussed. The study shows that the acoustic-base indicated by shallow seismic profiles represents the basic shape of the SYSB in late Oligocene-early Miocene. Based on the comprehensive analysis of some typical profiles crossing the basin about seismostratigraphy, gravity and magnetics, the basin edge fault and some basin inner structure are identified. Furthermore, the acoustic-base is classified into four types of basal lithofacies: Eogene basin basement, Paleozoic basement, Mesozoic basement and magma intrusion basement.

Reply: “Characteristic features of the North Anatolian Fault in the Eastern Marmara Region and its tectonic evolution”

Shallow seismic profiles are used to image the tectonic and stratigraphic setting in Izmit Bay, eastern Marmara Sea. Four seismic units were detected in the sediments of Izmit Bay, which carry the effects of the tectonic uplift and global sea-level variations. The area appears to be a negative flower structure controlled by the northern branch of the North Anatolian Fault. In the late Pliocene the North Anatolian Fault reached Marmara Sea as a master fault. Smallscale faults were evolved around the master fault under a regime of dextral shear. The northward bending of the master fault gave rise to en-echelon faults, opening the sub-basins in Izmit Bay as releasing bends. The secondary faults which developed as low-angle oblique to the master fault are also the products of dextral shear. To the south, short dextral ENE^WSW trending faults are interpreted as ‘P’-shears, while to the north, dextral WNW^ESE trending faults represents ‘R’-shears. These faults absorbed the deformation at the western end of the ruptured zone during the

Sneak Previews

The November-December issue of Geophysics contains two case histories. In “A 3D resistivity investigation of a contaminated site at Lernacken, Sweden,” Dahlin et al. delineate ground contaminated by former sludge ponds. Musil et al. (“Shallow seismic surveying of an Alpine rock glacier”) delineate the internal structure and base of a glacier from shallow seismic profiles and four 70-m deep boreholes.

Shallow seismic surveying of an Alpine rock glacier

To map the internal structure and lower boundary of an alpine rock glacier, we recorded three shallow seismic profiles and drilled four ∼70‐m‐deep holes through to the underlying bedrock. Although analysis of the seismic data using standard reflection processing schemes did not yield conclusive results because of the dominantly low‐frequency returned signals and the presence of strong source‐generated noise, tomographic inversions of first‐arrival times were successful in mapping several critical subsurface features. A thin, low‐velocity layer of loose boulders, air voids, and snow was found to extend across the entire surveyed area. Below this layer, two distinct velocity regimes superimposed on a general increase in velocity with depth were identified. A broad regime of high velocities was interpreted to contain boulders with numerous ice‐filled voids, whereas an adjacent regime of relatively low velocities was explained in terms of boulders with air‐ and water‐filled voids. This latter region of degraded permafrost, which was unexpected, may be an early result of global warming. The transition from rock glacier to gneissic bedrock was delineated approximately by the 4300‐m/s isovelocity line. Although poorly resolved, its depth varied from ∼35 to ∼70 m beneath the surface of the rock glacier. The tomographic inversion results also provided an explanation for the occurrence of strong source‐generated noise in our data. Using the derived velocity distributions as input, synthetic shot gathers were calculated using a finite‐difference viscoelastic scheme that accounted for surface topography. The resultant synthetic data were dominated by the overwhelming effects of back‐ and side‐scattered waves from shallow heterogeneities and high‐amplitude guided phases that originated from the thin, surficial low‐velocity zone. Had the guided phases not been correctly identified and eliminated during processing, they would have appeared as reflections in the stacked sections. Since low‐velocity layers overlying heterogeneous media are common features of the shallow subsurface, our findings may have implications for a broad range of engineering‐scale seismic investigations (e.g., studies of landfills, rockslides, moraines, talus slopes, alluvial fans).

Anatomy of the buried Burdekin River channel across the Great Barrier Reef shelf: how does a major river operate on a tropical mixed siliciclastic/carbonate margin during sea level lowstand?

A large palaeochannel on the northeastern Australian continental shelf has been imaged by a series of shallow seismic reflection profiles. The buried channel forms an important Pleistocene route of the Burdekin River and extends almost continuously for ∼160 km from the present coast to the outermost reef. The channel floor profile steps across the shelf with alternating segments of gentle gradient (flats) and steeper gradient (ramps). Channel sinuosity as interpreted from seismic records varies among segments between 1 and 1.72, with no consistent relationship between sinuosity and gradient. The lower and upper parts of the channel fill have different geometry and reflection character, suggesting channel excavation and initial filling occurred during a different regime than final filling. In one section of the shelf, about the −50 m isobath, the channel is difficult to define and appears to have wandered significantly, either because it has been modified by shoreface erosion ca. 10.5 ka or because the river encountered a change in topography in front of karstified reefs. As the channel passes between the numerous outer shelf reefs, in water depths of 70–80 m, it becomes progressively smaller, conspicuously underfilled, and absent entirely over the outermost 10 km of the shelf. No discrete lowstand river mouth could be recognised on the present shelf edge. The elevations of flat segments on the channel floor profile show considerable similarity to published elevations of stillstands or brief rises in sea level attained during the long-term drawdown associated with the last glacial cycle (125–20 ka) and are interpreted to have formed during this stepwise drop in sea level. Channels were cut and partially filled during the fall and lowstand and then backfilled during the Holocene transgression. The ancestral channel of the Burdekin River therefore preserves a rare insight into the stratigraphic record of falling sea level during the last glacial.

Characteristic features of the North Anatolian Fault in the eastern Marmara region and its tectonic evolution

Shallow seismic profiles are used to image the tectonic and stratigraphic setting in Izmit Bay, eastern Marmara Sea. Four seismic units were detected in the sediments of Izmit Bay, which carry the effects of the tectonic uplift and global sea-level variations. The area appears to be a negative flower structure controlled by the northern branch of the North Anatolian Fault. In the late Pliocene the North Anatolian Fault reached Marmara Sea as a master fault. Small-scale faults were evolved around the master fault under a regime of dextral shear. The northward bending of the master fault gave rise to en-echelon faults, opening the sub-basins in Izmit Bay as releasing bends. The secondary faults which developed as low-angle oblique to the master fault are also the products of dextral shear. To the south, short dextral ENE–WSW trending faults are interpreted as ‘P’-shears, while to the north, dextral WNW–ESE trending faults represents ‘R’-shears. These faults absorbed the deformation at the western end of the ruptured zone during the Izmit Earthquake (August 17, 1999), causing the migration of failure stress westward.

Holocene storage of siliciclastic sediment around islands on the middle shelf of the Great Barrier Reef Platform, north‐east Australia

ABSTRACTThick sequences of sediment surround the Whitsunday Islands on the middle shelf of the Great Barrier Reef (GBR) Platform. Much of this sediment is siliciclastic material deposited since the sea‐level highstand at around 6·5 ka. This raises a mass balance dilemma because modern terrigenous discharge to the GBR Platform is restricted to the inner shelf. Shallow seismic profiles and sediment samples were collected over 450 km2 around the Whitsunday Islands to quantify the mass of siliciclastic sediment for a dynamic model of the shelf. The sea floor and pre‐Holocene surfaces were mapped using 4584 stations along the seismic profiles and a graphical computer program. The total volume of sediment between these two surfaces is 3·67 ± 0·45 × 109 m3. This volume is composed of buried reefs (0·13 ± 0·01 × 109 m3), medium‐ (0·70 ± 0·30 × 109 m3) and fine‐grained shoals (2·84 ± 0·35 × 109 m3). The volume estimates combined with measurements of carbonate concentration, density and porosity indicate that 1850 ± 380 Mt of Holocene siliciclastic sediment surround the Whitsunday Islands in medium‐ (510 ± 225 Mt) and fine‐grained shoals (1340 ± 155 Mt). The total mass of siliciclastic material is 1·7–2·6 times that stored in Cleveland Bay, a similar sized repository on the inner shelf. A simple numerical model has been constructed to explain this large quantity of Holocene siliciclastic sediment. The model results in the appropriate siliciclastic mass next to the Whitsunday Islands by integrating regional shelf processes over time. Unlike the present day, rivers discharged sediment to the middle shelf during the early Holocene. This material was subsequently focused by northward transport into the vicinity of the islands, a geomorphologically complex region that serves as a sediment trap. Although direct riverine inputs to the middle shelf have stopped during the present sea‐level highstand, previously deposited siliciclastic sediment is continually being winnowed from the middle shelf and redeposited next to the Whitsunday Islands. The transport and distribution of siliciclastic sediment on the GBR Platform is thus influenced significantly by storage around islands on the middle shelf.

Weighted stack of shallow seismic reflection line acquired in downtown Osaka City, Japan

A 3.5-km-long shallow seismic reflection profile was acquired along Yodo riverbank across the active Uemachi fault, Osaka City, Japan. The study aimed to explore the subsurface geological structures in and around the Uemachi active fault system and to assess the feasibility of using reflection seismology to investigate subsurface structure in urban areas where large amount of cultural noise is expected to interfere with the seismic signal. Field recording parameters were adapted to the accessibility limitation and safety precautions in the study area. The quality of collected data was marginal and the signal to noise (S/N) ratio was considerably low. The data processing involved intensive frequency–wave number (f–k) filtering and summing adjacent common midpoints (CMPs) before stack as well as weighted stacking. The f–k filtering removed most of the ground roll and guided waves while summation of adjacent common midpoints increased the fold and enhanced the apparent signal to noise ratio of the final section. Trace weighting before stack was remarkably effective in increasing the identity and continuity of reflections. The visual and statistical comparison between weighted stack and the basic stack proved that weighted stacking produces significantly more coherent section. The interpretation of the final seismic section and the correlation with the borehole data in the vicinity confirmed the location of the Uemachi fault plane. The fault is shown in the seismic section as a reverse fault that dips to the east with an angle of about 75°. The seismic line also revealed the possibility of the presence of other two shallow faults on the hanging wall of the Uemachi fault. The two faults are dipping to the same dip direction of Uemachi fault with slightly larger dip angle.

The Shandong mud wedge and post-glacial sediment accumulation in the Yellow Sea

Two well-defined deltaic sequences in the Bohai Sea and in the South Yellow Sea represent post-glacial accumulation of Yellow River-derived sediments. Another prominent depocenter on this epicontinental shelf, a pronounced clinoform in the North Yellow Sea, wraps around the northeastern and southeastern end of the Shandong Peninsula, extending into the South Yellow Sea. This Shandong mud wedge is 20 to 40 m thick and contains an estimated 300 km3 of sediment. Radiocarbon dating, shallow seismic profiles, and regional sea-level history suggest that the mud wedge formed when the rate of post-glacial sea-level rise slackened and the summer monsoon intensified, at about 11 ka. Geomorphic configuration and mineralogical data indicate that present-day sediment deposited on the Shandong mud wedge comes not only from the Yellow River but also from coastal erosion and local rivers. Basin-wide circulation in the North Yellow Sea may transport and redistribute fine sediments into and out of the mud wedge.

Shallow Seismic Profiling of the Exhumed Punchbowl Fault Zone, Southern California

The relationship between seismic velocity and internal fault structure was investigated through a shallow seismic refraction experiment across the Punchbowl fault, Devil9s Punchbowl Los Angeles County Park, California. The Punchbowl fault is a northwest-striking, large-displacement fault of the San Andreas system that is exhumed to several kilometers depth and places crystalline basement against arkosic sandstone of the Punchbowl Formation. Seismic refraction profiles using hammer and impulsive shear-wave sources along a 300-m-long line reveal the velocity structure of the fault zone beneath a thin deposit of alluvium. We determine a velocity model assuming the alluvial layer is fairly uniform in velocity and thickness consistent with geologic observations and P -wave travel times. Raytracing with damped least-squares inversion of travel times of P and S waves indicate that the Punchbowl fault is best modeled as a zone several tens of meters wide with velocities reduced by 10%–25% from wall-rock velocities ( V p = 3.2 km/sec for granitic basement and V p = 2.9 km/sec for Punchbowl Formation). Thickness of the low-velocity zone and the variation in seismic velocity across the zone are qualitatively consistent with expectations based on the observed distribution of fault-related fracturing and alteration. Apparent crack densities calculated from measured seismic velocities using O9Connell and Budiansky (1974) formulation for a cracked medium range from about 0.4 in the core to a background crack density of 0.1 in the host rock. The variation in calculated crack density across the fault is similar to observed variations in microfracture density in the Punchbowl Formation sandstone along traverses across the fault. An estimate of the Poisson9s ratio near the fault is about 0.25, suggesting that open cracks in the shallow part of the Punchbowl fault zone are dry, consistent with the geologically inferred location of the groundwater table. Although the seismic data do not completely constrain the velocity structure, the seismic velocity model determined by raytracing and inversion of travel times is admissible on the basis of structural data.

Early Holocene drowned lagoonal deposits from the Kattegat, southern Scandinavia

Shallow seismic profiling indicated the presence of a drowned lagoon–barrier system formed during the transgression of the southern Kattegat, and investigations of core material have confirmed this. Studies of plant and animal macrofossils show that the lagoonal sediments contain a mixture of marine, brackish, lacustrine, telmatic and terrestrial taxa, and analyses of foraminifers indicate brackish‐water conditions. Low oxygen isotope values obtained on shells of marine molluscs also point to lowered salinity. The lagoonal sediments are situated at depths between 24 and 35 m below present sea level. They are dated to between c. 10.5 cal. ka BP and c. 9.5 cal. ka BP, and reflect a period characterized by a moderate relative sea level rise. The lagoonal sediments are underlain by lateglacial glaciomarine clay and silt, which are separated from the Holocene deposits by an unconformity. The earliest Holocene sediments consist of littoral sand with gravel, stones and shells; these sediments were formed during the transgression of the area before the barrier island–lagoon system was developed. The lagoonal sediments are overlain by mud, which contains animal remains that indicate increasing water depths.