Ocean Bottom Seismology Research Articles

Ocean-Bottom Seismology of Glacial Earthquakes: The Concept, Lessons Learned, and Mind the Sediments

Abstract About 70% of Earth’s surface is covered by ocean, for which seismic observations are challenging. Seafloor seismology overcame this fundamental difficulty and radically transformed the earth sciences, as it expanded the coverage of seismic networks and revealed otherwise inaccessible features. At the same time, there has been a recent increase in the number of studies on cryoseismology. These have yielded multiple discoveries but are limited primarily to land and ice-surface receivers. Near ice calving fronts, such surface stations are noisy, primarily due to crevassing and wind, are hazardous to maintain, and can be lost due to iceberg calving. To circumvent these issues, we have applied ocean-bottom seismology to the calving front of a tidewater glacier in northwest Greenland. We present details of this experiment, and describe the technical challenges, noise analysis, and examples of recorded data. This includes tide-modulated seismicity with thousands of icequakes per day and the first near-source (∼200–640 m) underwater record of a major kilometer-scale calving event in Greenland, which generated a glacial earthquake that was detectable ∼420 km away. We also identified a decrease in bottom-water temperature, presumably due to modified water stratification driven by extreme Greenland glacial melting, at the end of July 2019. Importantly, we identify glacial sediments as the key reason for the anomalously long (∼9.7 hr) delay in the sensor release from the fjord seafloor. Our study demonstrates a methodology to undertake innovative, interdisciplinary, near-source studies on glacier basal sliding, calving, and marine-mammal vocalizations.

Three-dimensional magnetic structure and genesis of the oceanic crust of the northern South China Sea

• Magnetic inversion based on model weighting with variable inc and dec is developed. • Inversion method applied in the South China Sea northeast margin. • The distribution of magnetic bodies is north-to-south and east-to-west. • The tectonic patterns are due to the underplating activity in the oceanic crust . • The magnetic structure was disrupted, resulting in the magnetic quiet zone. The north continental margin of the South China Sea is an area conducive to examining the deep structure, tectonic evolution, and dynamic mechanisms of this region. As there is an influence of oblique magnetization in mid-latitude areas on the magnetic data pertaining to this area, this study develops and validates an inversion method for magnetic anomalies that accounts for inclination and declination. The inversion of the total magnetic intensity, ΔT , in the northern South China Sea was then used to reveal the distribution of magnetic bodies in this area. The inversion results show that there is significant tectonic zoning in the area under study, with north-to-south and east-to-west belts showing a pattern consistent with the main tectonic direction in this area. There are also distinct differences between the high magnetic anomaly belt in the continental margin and the magnetic inversion features of the magnetic quiet zone in the south. Combined with ocean-bottom seismology profiles and geothermal data, the observed phenomena may be closely related to magmatic underplating activity in the deep crust caused by plate subduction.

Dynamics of the Continental Margins in South China Sea: Scientific Experiments and Research Progresses

AbstractComprehensive geophysical surveys, including the multi‐channel seismic survey with long streamer and large power source and the refraction/reflection seismic survey of ocean bottom seismology (OBS) have been deployed in the southern margin and the southwest sub‐basin of South China Sea for the first time. Based on the interpretation of these geophysical data, combined with studies on tectonics structure, geochemistry and dynamic simulation, etc, our project worked out the dynamics of the continental margins in South China Sea. Major conclusions are as follows: Two sea‐floor spreading events occurred in Cenozoic. The first episode happened between 33.5~25 Ma, and after a 1.5 Ma tectonic adjustment, the second episode started between 23.5~15.5 Ma. In both episodes, the South China Sea experienced propagation of rifting and spreading from east to west, resulting in a structural framework featured with blocks in north‐south direction and segments in east‐west direction; there is different structural transformation time, from faulting to depression, for Cenozoic sedimentary basins in the southern and northern margins, corresponding to the end time of early and late sea basin spreading respectively. The continental crust of South China Sea might have experienced depth‐dependent stretching. The lower crust and top of upper mantle have been magnetized and no obvious velocity anomalies of the mantle have been found in oceanic basin. Both the southern and northern margins belong to non‐volcanic continental margins, although lower crustal bodies of high‐velocity occur in the east of the northern margin, but without seaward reflector. The origin and evolution of sediment basins on the northern and southern continental margins are very distinct, and show ring‐like belt distribution characteristics with oil outside and gas inside. We believe that the hydrocarbonbearing prospects in southern continental marginal basins are superior to these of northern continental marginal basins. In Mesozoic, the northern and southern margins of South China Sea were parts of South China block, and were controlled by the Palaeo‐Tethys and Paleo‐Pacific tectonic domains.

Seismological Report on the 2006 Typhoon Shanshan that Lit up Seismic Stations along Its Way

Typhoons generate ocean waves, which induce ground motions on the seafloor. Such processes can generate microseismic signals observed at far distances (Longuet-Higgins 1950; Tabulevich 1971; Sutton 1996; Bromirski et al. 1999; Tolman 1999; Grevemeyer et al. 2000; Kedar and Webb 2005; Webb 2007; Gerstoft et al. 2008; Kedar et al. 2008; Wilson and Makris 2008; Koper and de Foy 2008). However, direct observations on the deep seafloor under a typhoon were not available until now, due to the difficulty of field operations and the lack of proper instruments installed under the path of a typhoon. Recently we have set up a portable wideband oceanbottom seismology network in Taiwan. We have deployed broadband ocean-bottom seismographs (OBSs) to collect direct observations. For the engineering parameters of the OBS, see Table 1. Such datasets were then combined with data from many permanent land-based seismic stations for analyses. View this table: TABLE 1 Technical Specifications of the Ocean Bottom Seismographs Used for This Study The combined OBSs and land seismic stations have captured a special “seismic event” associated with Typhoon Shanshan— …

Crustal architecture and deep structure of the Ninetyeast Ridge hotspot trail from active-source ocean bottom seismology

A 550-km-long transect across the Ninetyeast Ridge, a major Indian ocean hotspot trail, provided seismic refraction and wide-angle reflection data recorded on 60 ocean bottom instruments. About 24 000 crustal and 15 000 upper mantle arrivals have been picked and used to derive an image of the hotspot track. Two approaches have been chosen: (i) a first-arrival tomographic inversion yielding crustal properties; and (ii) forward modelling of mantle phases revealing the structure at the crust–mantle boundary region and of the uppermost mantle. Away from the volcanic edifice, seismic recordings show the typical phases from oceanic crust, that is, two crustal refraction branches (Pg), a wide-angle reflection from the crust–mantle boundary (PmP) and a wave group turning within the upper mantle (Pn). Approaching the edifice, three additional phases have been detected. We interpret these arrivals as a wide-angle reflection from the base of material trapped under the pre-hotspot crust (Pm2P) and as a wide-angle reflection (PnP) and its associated refraction branch (PN) from a layered upper mantle. The resulting models indicate normal oceanic crust to the west and east of the edifice. Crustal thickness averages 6.5–7 km. Wide-angle reflections from both the pre-hotspot and the post-hotspot crust–mantle boundary suggest that the crust under the ridge has been bent downwards by loading the lithosphere, and hotspot volcanism has underplated the pre-existing crust with material characterized by seismic velocities intermediate between those of mafic lower crustal and ultramafic upper mantle rocks (7.5–7.6 km s−1). In total, the crust is up to ≈ 24 km thick. The ratio between the volume of subcrustal plutonism forming the underplate and extrusive and intrusive volcanism forming the edifice is about 0.7. An important observation is that underplating continued to the east under the Wharton Basin. During the shield-building phase, however, Ninetyeast Ridge was located adjacent to the Broken Ridge and was subsequently pulled apart along a transform fault boundary. Therefore, underplating eastwards of the fracture zone separating the edifice from the Wharton Basin suggests that prolonged crustal growth by subcrustal plutonism occurred over millions of years after the major shield-building stage. This fact, however, requires mantle flow along the fossil hotspot trail. The occurrence of PnP and PN arrivals is probably associated with a layered and anisotropic upper mantle due to the preferential alignment of olivine crystals and may have formed by rising plume material which spread away under the base of the lithosphere.

An extensive Ocean Bottom Seismograph survey in the Vøring basin, Norway

Ocean Bottom Seismographs (OBS) are relatively commonly used in scientific lithospheric investigations in many parts of the world. On the Norwegian continental shelf several investigations of this kind have been performed during the last eight years as a result of the fruitful cooperation between the Institute of Solid Earth Physics, University of Bergen, the Laboratory for Ocean Bottom Seismology, Hokkaido University, the Laboratory for Earthquake Chemistry, Tokyo University, and Statoil.

Linear predictive coding of marine seismic data

This paper examines the utility of linear predictive coding in reducing the amount of data storage required for signals gathered in ocean bottom seismology. In this study, a set of 12 typical signals were repeatedly encoded with the storage allocated decreasing from an initial 12 bits per datum to 2. The error introduced was then compared to the performance achieved by simply rounding off the lowest bits of the data, and to estimates of the rate distortion limit. It was found that this scheme consistently introduced about 15 times (4 bits) less distortion both in terms of the root-mean-square (rms) error and in terms of the maximum error than rounding the data. Moreover, the rms distortion of the data were within a factor of 4 (2 bits) of the rate distortion bound on optimal encoding. Thus, the scheme was seen to be an effective approach to the problem of data compression in the marine environment.