Bottom Seismic Stations Research Articles

The Seismotectonics and Seismicity of the Laptev Sea Region: The Current Situation and a First Experience in a Year-Long Installation of Ocean Bottom Seismometers on the Shelf

This paper is a review of current concepts concerning the seismotectonics and seismicity in the Laptev Sea region. The chief feature of the region is a rift system extending in the shelf from the continental slope with the adjacent Gakkel Ridge to the mainland coast. There are several models to describe the present-day evolution of the region, but no one of these is preferable because of the lack of local instrumental observations of shelf microseismicity, while such microseismicity is characteristic of rift zones. One alternative approach consists in the installation of ocean bottom seismometers on the shelf itself. During the 73rd cruise of the R/V Akademik Mstislav Keldysh, a temporary network was deployed consisting of 7 broadband bottom seismic stations on the Laptev Sea shelf. Comparative analysis of noise spectra based on records of a hydrophone, a bottom seismograph, and a wavegauge, showed that the noise amplitude strongly depends on wind waves, which is in turn dependent on marine ice cover, so that the recording potential of a local network of bottom seismographs is restricted to the winter period of observation.

О ВОЗМОЖНОСТИ ОЦЕНКИ СЕЙСМИЧЕСКОЙ ОПАСНОСТИ ДЛЯ ОФФШОРНЫХ КОНСТРУКЦИЙ НА ШЕЛЬФЕ КАСПИЯ РАСЧЕТНЫМИ МЕТОДАМИ

In recent years the world has been actively developing oil and gas fields on the shelf, including in seismically active areas. On the seabed it is very difficult to carry out qualitative geological and geophysical studies and seismological observations in full, which are a part of seismic microzoning works. Programs for computational methods during seismic microzoning allow taking into account nonlinear soil properties. In the article the experience of studies on seismic microzoning (2012–2015) for the installation area of offshore structures on the shelf of the Middle Caspian Sea is considered. In conditions of absence of observations by bottom seismic stations the possibility of seismic hazard assessment by computational methods taking into account local soil conditions is shown. Thus the obtained values of seismic intensity are lower in comparison with the results of estimations by method of engineering and geological analogies and method of seismic rigidity. Maximal impacts from zones of possible earthquake sources most dangerous for Middle Caspian Sea have been taken into account by peculiarities of spectral composition of vibrations of ground bases in the form of reaction spectra.

Scaled Energy Estimation for Shallow Slow Earthquakes

AbstractDeep and shallow slow earthquakes occur at the edge of the seismogenic zone in the Nankai subduction zone. Deep slow earthquakes occur under high‐temperature and high‐pressure conditions, whereas shallow slow earthquakes occur at lower temperatures and pressures. Although these two types of slow earthquake show qualitatively similar behaviors, such as slow deformation and depleted high‐frequency seismic signals, their similarities have not yet been evaluated in quantitative ways. In this work, we analyze shallow tectonic tremors accompanied by very low frequency earthquakes off Kii Peninsula, Japan. We estimate the seismic energy of the shallow tectonic tremors by correcting for the site amplifications of ocean bottom seismic stations and seismic attenuation in the shallow accretionary prism. The estimated seismic energy rates of these tectonic tremors are compared with seismic moment rates of accompanying very low frequency earthquakes, revealing that the scaled energy of shallow slow earthquakes is roughly constant, ~10−9 to 10−8, which is 0 to 1 order of magnitude larger than the typical scaled energy of deep slow earthquakes.

Oceanic lithosphericS-wave velocities from the analysis ofP-wave polarization at the ocean floor

Our knowledge of the absolute S wave velocities of the oceanic lithosphere is mainly based on global surface wave tomography, local active seismic or compliance measurements using oceanic infragravity waves. The results of tomography give a rather smooth picture of the actual S wave velocity structure and local measurements have limitations regarding the range of elastic parameters or the geometry of the measurement. Here, we use the P wave polarization (apparent P wave incidence angle) of teleseismic events to investigate the S wave velocity structure of the oceanic crust and the upper tens of kilometres of the mantle beneath single stations. In this study, we present an up to our knowledge new relation of the apparent P wave incidence angle at the ocean bottom dependent on the half space S wave velocity. We analyse the angle in different period ranges at ocean bottom stations (OBS) to derive apparent S wave velocity profiles. These profiles are dependent on the S wave velocity as well as on the thickness of the layers in the subsurface. Consequently, their interpretation results in a set of equally valid models. We analyse the apparent P wave incidence angles of an OBS data set which was collected in the Eastern Mid Atlantic. We are able to determine reasonable S wave velocity-depth models by a three step quantitative modelling after a manual data quality control, although layer resonance sometimes influences the estimated apparent S wave velocities. The apparent S wave velocity profiles are well explained by an oceanic PREM model in which the upper part is replaced by four layers consisting of a water column, a sediment, a crust and a layer representing the uppermost mantle. The obtained sediment has a thickness between 0.3 km and 0.9 km with S wave velocities between 0.7 km s−1 and 1.4 km s−1. The estimated total crustal thickness varies between 4 km and 10 km with S wave velocities between 3.5 km s−1 and 4.3 km s−1. We find a slight increase of the total crustal thickness from ∼5 km to ∼8 km towards the South in the direction of a major plate boundary, the Gloria Fault. The observed crustal thickening can be related with the known dominant compression in the vicinity of the fault. Furthermore, the resulting mantle S wave velocities decrease from values around 5.5 km s−1 to 4.5 km s−1 towards the fault. This decrease is probably caused by serpentinization and indicates that the oceanic transform fault affects a broad region in the uppermost mantle. Conclusively, the presented method is useful for the estimation of the local S wave velocity structure beneath ocean bottom seismic stations. It is easy to implement and consists of two main steps: (1) measurement of apparent P wave incidence angles in different period ranges for real and synthetic data, and (2) comparison of the determined apparent S wave velocities for real and synthetic data to estimate S wave velocity-depth models.

Deep seismic soundings on the 1-AP profile in the Barents Sea: Methods and results

Profile 1-AP with a length of 1300 km intersects the Barents Sea from The Kola Peninsula to Franz Josef Land. The combined Common Depth Point (CDP) and Deep Seismic Sounding (DSS) seismic studies were carried out on this profile. The DSS measurements were conducted with the standalone bottom seismic stations with an interval of 5–20 km between them. The stations recorded the signals generated by the large air guns with a step of 250 m. Based on these data, the detailed P-velocity section of the Earth’s crust and uppermost mantle have been constructed for the entire profile and the S-velocity section for its southern part. The use of a variety of methods for constructing the velocity sections enabled us to assess the capabilities of each method from the standpoint of the highest reliability and informativity of the models. The ray tracing method yielded the best results. The 1-PR profile crosses two large basins—the South Barents and North Barents ones, with the thickness of the sediments increasing from 8 to 10 km in the south to 12–15 km in the north. The Earth’s crust pertains to the continental type along the entire profile. Its thickness averages 32 to 36 km and only increases to 43 km at the boundary between the two basins. The distinct change in the wave field at this boundary suggests the presence of a large deep fault in this zone. The high-velocity blocks are revealed in the crust of the South Barents basin, whereas the North Barents crust is characterized by relatively low velocities.

Structure of the Earth’s crust in the northern part of the Barents–Kara region along the 4-AR DSS profile

The 1370 km long 4-AR reference profile crosses the North Barents Basin, the northern end of the Novaya Zemlya Rise, and the North Kara Basin. Integrated geophysical studies including common deep point (CDP) survey and deep seismic sounding (DSS) were carried out along the profiles. The DSS was performed using autonomous bottom seismic stations (ABSS) spaced 10–20 km apart and a powerful air gun producing seismic signals with a step size of 250 m. As a result, detailed P- and S-wave velocity structures of the crust and upper mantle were studied. The basic method was ray-tracing modeling. The Earth’s crust along the entire profile is typically continental with compressional wave velocities of 5.8–7.2 km/s in the consolidated part. Crustal thickness increases from 30 km near the islands of Franz Josef Land to 35 km beneath the North Barents Basin, 50 km beneath the Novaya Zemlya Rise, and 40 km beneath the North Kara Basin. The North Barents Basin 15 km deep is characterized by unusually low velocities in the consolidated crust: The upper crust layer with velocities of 5.8–6.4 km/s has a thickness of about 15 km beneath the basin (usually, this layer wedges beneath deep sedimentary basins). Another special property of the crust in the North Barents Basin is the destroyed structure of the Moho.

Imaging in 2D media

Stacking by CDP technique is inapplicable for processing of data from bottom seismic stations or acoustic sonobuoys. In addition, big amount of unknown velocity and structural parameters of the real layered medium do not allow these parameters to be defined by standard processing methods. Local sloped stacking is proposed for simultaneous obtaining the stacked tracks, travel time curve of a chosen wave, and the first derivative of this travel time curve. The additionally defined parameters are second derivative of this travel time curve and integrated average of squared travel time curve. These data are sufficient to reduce the amount of unknown parameters (down to one–two for each boundary) when layer-by-layer top-to-bottom processing. As a result, the stable estimates of velocity parameters of the layered (isotropic or anisotropic) medium can be obtained and stacked tracks obtained by local sloped staking can be transformed into boundaries in the time and depth sections.

A joint Monte Carlo analysis of seafloor compliance, Rayleigh wave dispersion and receiver functions at ocean bottom seismic stations offshore New Zealand

AbstractTeleseismic body‐wave imaging techniques such as receiver function analysis can be notoriously difficult to employ on ocean‐bottom seismic data due largely to multiple reverberations within the water and low‐velocity sediments. In lieu of suppressing this coherently scattered noise in ocean‐bottom receiver functions, these site effects can be modeled in conjunction with shear velocity information from seafloor compliance and surface wave dispersion measurements to discern crustal structure. A novel technique to estimate 1‐D crustal shear‐velocity profiles from these data using Monte Carlo sampling is presented here. We find that seafloor compliance inversions and P‐S conversions observed in the receiver functions provide complimentary constraints on sediment velocity and thickness. Incoherent noise in receiver functions from the MOANA ocean bottom seismic experiment limit the accuracy of the practical analysis at crustal scales, but synthetic recovery tests and comparison with independent unconstrained nonlinear optimization results affirm the utility of this technique in principle.

Integration of onshore and offshore seismic arrays to study the seismicity of the Calabrian Region: a two steps automatic procedure for the identification of the best stations geometry

Abstract. We plan to deploy in the Taranto Gulf some Ocean Bottom broadband Seismometer with Hydrophones. Our aim is to investigate the offshore seismicity of the Sibari Gulf. The seismographic network optimization consists in the identification of the optimal sites for the installation of the offshore stations, which is a crucial factor for the success of the monitoring campaign. In this paper, we propose a two steps automatic procedure for the identification of the best stations geometry. In the first step, based on the application of a set of a priori criteria, the suitable sites to host the ocean bottom seismic stations are identified. In the second step, the network improvement is evaluated for all the possible stations geometries by means of numerical simulation. The application of this procedure allows us to identify the best stations geometry to be achieved in the monitoring campaign.

Method of spectral-time analysis for recognition of anomalies in time series with Raleigh- and tsunami-wave disturbances in signals from hydrostatic pressure sensors of ocean bottom seismic stations

The article deals with the problem of recognition of anomalies in time series with Raleighand tsunami-wave disturbances in signals from hydrostatic pressure sensors (HPS) of ocean bottom seismic stations in the framework of tsunami warning problem. The proposed method of spectral-time analysis (STAN) of signals from the sensors was based on computing the evaluations of functions of frequency-time distribution, decision-making procedures and non-linear filtering for the above problem. The developed STAN method was applied to recognize time intervals with Rayleigh and tsunami-wave disturbances in HPS signals. The proposed STAN method is quite universal and can be used to solve problems of recognition of anomalies in time series of geophysical data of different nature. This is the e-book version of the article, published in the Russian Journal of Earth Sciences (doi:10.2205/2012ES000522). It is generated from the original source file using LaTeX’s epub.cls class.

Ocean Seismic Network Pilot Experiment

The primary goal of the Ocean Seismic Network Pilot Experiment (OSNPE) was to learn how to make high quality broadband seismic measurements on the ocean bottom in preparation for a permanent ocean seismic network. The experiment also had implications for the development of a capability for temporary (e.g., 1 year duration) seismic experiments on the ocean floor. Equipment for installing, operating and monitoring borehole observatories in the deep sea was also tested including a lead‐in package, a logging probe, a wire line packer and a control vehicle. The control vehicle was used in three modes during the experiment: for observation of seafloor features and equipment, for equipment launch and recovery, and for power supply and telemetry between ocean bottom units and the ship. The OSNPE which was completed in June 1998 acquired almost four months of continuous data and it demonstrated clearly that a combination of shallow buried and borehole broadband sensors could provide comparable quality data to broadband seismic installations on islands and continents. Burial in soft mud appears to be adequate at frequencies below the microseism peak. Although the borehole sensor was subject to installation noise at low frequencies (0.6 to 50 mHz), analysis of the OSNPE data provides new insights into our understanding of ocean bottom ambient noise. The OSNPE results clearly demonstrate the importance of sediment borne shear modes in ocean bottom ambient noise behavior. Ambient noise drops significantly at high frequencies for a sensor placed just at the sediment basalt interface. At frequencies above the microseism peak, there are two reasons that ocean bottom stations have been generally regarded as noisier than island or land stations: ocean bottom stations are closer to the noise source (the surface gravity waves) and most ocean bottom stations to date have been installed on low rigidity sediments where they are subject to the effects of shear wave resonances. When sensors are placed in boreholes in basement the performance of ocean bottom seismic stations approaches that of continental and island stations. A broadband borehole seismic station should be included in any real‐time ocean bottom observatory.

Ocean bottom seismometers deployed in Tyrrhenian Sea

The Institute for Geophysics (IfG) at Hamburg University and the Research Center for Marine Geoscience (GEOMAR) of Kiel University have developed new, wideband ocean bottom seismic stations (OBS) for long‐term, deep‐sea deployments of up to 1 year. A first long‐term pilot experiment of these stations was conducted in the Tyrrhenian Sea north of Sicily, in‐cooperation with the first long‐term, deep‐sea test of the European Ocean Bottom Observatory GEOSTAR [Beranzoli et al., 2000] by Istituto Nazionale di Geofisica e Vulcanologia (INGV). The seismic data retrieved prove that the new OBSs are useful for seismological studies. A large number of tele‐seismic earthquakes have been recorded in good quality; waves originating from such events pass the mantle and crust below the network, and thus provide important constraints on their structure.

Geology. Interactions between ridges and plumes.

In the last 30 years, geologists have made much progress on understanding slabs, lithosphere, and mid-ocean ridges but mantle plumes remain enigmatic. In their Perspective, [Montagner and Ritsema][1] chart recent progress, largely based on seismic anisotropy, toward understanding how such plumes interact with mid-ocean ridges. Existing data are insufficient, however, to determine whether mantle plumes act independently from plate tectonics. The authors call for ocean bottom seismic stations to settle this question. [1]: http://www.sciencemag.org/cgi/content/full/294/5546/1472