Mapping of Eastern North Atlantic Ocean seismicity from Po/So observations at a mid-aperture seismological broad-band deep sea array

Dahm, Torsten; Hannemann, Katrin

doi:10.1093/gji/ggaa054

Cited by 7 publications

(14 citation statements)

References 56 publications

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“…Since several decades, oceanic Pn and Sn waves-often referred to as Po and So waves-have been observed worldwide (Walker and Sutton 1971;Kennett et al 2014;Krüger et al 2020), but their properties and propagation have been seldom investigated and analysed in depth. These waves are reported to travel distances up to thousands of kilometres and show high-frequency (up to 30 Hz) Pand S-wave arrivals with tens to hundreds of seconds long seismic coda.…”

Section: Introductionmentioning

confidence: 99%

Seismic scattering and absorption of oceanic lithosphericSwaves in the Eastern North Atlantic

Hannemann

Eulenfeld

Dahm

2021

Geophysical Journal International

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Summary The scattering and absorption of high frequency seismic waves in the oceanic lithosphere is to date only poorly constrained by observations. Such estimates would not only improve our understanding of the propagation of seismic waves, but also unravel the small scale nature of the lithosphere and its variability. Our study benefits from two exceptional situations: (1) we deployed over 10 months a mid-aperture seismological array in the central part of the Eastern North Atlantic in 5 km water depth, and (2) we could observe in total 340 high frequency (up to 30 Hz) Po and So arrivals with tens of to hundreds of seconds long seismic coda from local and regional earthquakes in a wide range of backazimuths and epicentral distances up to 850 km with a travel path in the oceanic lithosphere. Moreover, the array was located about 100 km North of the Gloria fault, defining the plate boundary between the Eurasian and African plate at this location which also allows an investigation of the influence of an abrupt change in lithospheric age (20 Ma in this case) on seismic waves. The waves travel with velocities indicating upper mantle material. We use So waves and their coda of pre-selected earthquakes to estimate frequency-dependent seismic scattering and intrinsic attenuation parameters. The estimated scattering attenuation coefficients are between 10−4 m−1 and 4 · 10−5 m−1 and are typical for the lithosphere or the upper mantle. Furthermore, the total quality factors for So waves below 5 Hz are between 20-500 and are well below estimates from previous modelling for observations in the Pacific Ocean. This implies that the Atlantic ocean is more attenuative for So waves compared to the Pacific Ocean which is inline with the expected behaviour for the lithospheric structures resulting from the slower spreading rates in the Atlantic Ocean. The results for the analysed events indicate that for frequencies above 3 Hz, intrinsic attenuation is equal or slightly stronger than scattering attenuation and that the So wave coda is weakly influenced by the oceanic crust. Both observations are in agreement with the proposed propagation mechanism of scattering in the oceanic mantle lithosphere. Furthermore, we observe an age dependence which shows that an increase in lithospheric age is associated with a decrease in attenuation. However, we also observe a trade-off of this age dependent effect with either a change in lithospheric thickness or thermal variations e.g. due to small scale upwellings in the upper mantle in the Southeast close to Madeira and the Canaries. Moreover, the influence of the nearby Gloria fault is visible in a reduction of the intrinsic attenuation below 3 Hz for estimates across the fault. This is the first study to estimate seismic scattering and absorption parameters of So waves for an area with several hundreds of kilometres radius centred in the Eastern North Atlantic and using them to characterise the nature of the oceanic lithosphere.

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Section: Introductionmentioning

confidence: 99%

Seismic scattering and absorption of oceanic lithosphericSwaves in the Eastern North Atlantic

Hannemann

Eulenfeld

Dahm

2021

Geophysical Journal International

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“…To the right is a zoom-in of the signal arrival displayed in Fig. 7 were later applied to the detection and classification of ground-coupled airwaves from volcanoes over sparse seismic networks (Fee et al 2016) and to parameter estimation for seismic signals on large Ocean Bottom Seismometer networks (Krüger et al 2020). In each of these studies, the incoherent method is both possible and effective due to the relatively large moveouts of the signal arrivals over the large aperture arrays/networks.…”

Section: Incoherent Processing For High-frequency Signals At Mjarmentioning

confidence: 99%

“…In this section, we demonstrate a strategy for improving incoherent slowness estimates at MJAR using a slightly different approach. Instead of timedomain stacking of the incoherent detection statistic traces, Krüger et al (2020) apply a bandpass filter in a suitably low-frequency band in order that coherent f-k analysis can be performed. This filtering captures those features of the detection statistic traces which are common to all sensors in the network and appears to provide more robust estimates than the smoothing operator proposed by Gibbons (2014).…”

Section: Incoherent Processing For High-frequency Signals At Mjarmentioning

confidence: 99%

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CTBT seismic monitoring using coherent and incoherent array processing

2021

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The detection and location capability of the International Monitoring System for small seismic events in the continental and oceanic regions surrounding the Sea of Japan is determined mainly by three primary seismic arrays: USRK, KSRS, and MJAR. Body wave arrivals are coherent on USRK and KSRS up to frequencies of around 4 Hz and classical array processing methods can detect and extract features for most regional signals on these stations. We demonstrate how empirical matched field processing (EMFP), a generalization of frequency-wavenumber or f-k analysis, can contribute to calibrated direction estimates which mitigate bias resulting from near-station geological structure. It does this by comparing the narrowband phase shifts between the signals on different sensors, observed at a given time, with corresponding measurements on signals from historical seismic events. The EMFP detection statistic is usually evaluated as a function of source location rather than slowness space and the size of the geographical footprint valid for EMFP templates is affected by array geometry, the availablesignal bandwidth, and Earth structure over the propagation path. The MJAR arrayhas similar dimensions to KSRS but is sited in far more complex geology which results in poor parameter estimates with classical f-k analysis for all signals lacking energy at 1 Hz or below. EMFP mitigates the signal incoherence to some degree but the geographical footprint valid for a given matched field template on MJAR is very small. Spectrogram beamforming provides a robust detection algorithm for high-frequency signals at MJAR. The array aperture is large enough that f-k analysis performed on continuous AR-AIC functions, calculated from optimally bandpass-filtered signals at the different sites, can provide robust slowness estimates for regional P-waves. Given a significantly higher SNR for regional S-phases on the horizontal components of the 3-component site of MJAR, we would expect incoherent detection and estimation of S-phases to improve with 3-component sensors at all sites. Given the diversity of the IMS stations, and the diversity of the methods which provide optimal results for a given station, we advocate the development of seismic processing pipelines which can process highly heterogeneous inputs to help associate characteristics of the incoming signals with physical events.

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“…Kao and Shan (2007) implemented the source-scanning algorithm using P-wave using local network data. In the following years, these ideas have been shared with migration techniques applying shift-and-stack envelopes and characteristic functions to locate seismic events (Gharti et al, 2010;Grigoli et al, 2013), see also (Krüger et al, 2020) for an application to OBS data. This paper focuses on a new array technique to overcome the problem of dealing with incoherent and noisy waveforms in large aperture arrays, based on the Continuous Wavelet Transform (CWT) (Grossmann et al, 1989;Kumar and Foufoula Georgiou, 1997).…”

Section: Introductionmentioning

confidence: 99%

Slowness vector estimation over large-aperture sparse arrays with the Continuous Wavelet Transform (CWT): application to Ocean Bottom Seismometers

Cabieces

García-Yeguas

Villaseñor

et al. 2020

Geophysical Journal International

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Summary This work presents a new methodology designed to estimate the slowness vector in large-aperture sparse Ocean Bottom Seismometer (OBS) arrays. The Continuous Wavelet Transform (CWT) is used to convert the original incoherent traces that span a large array, into coherent impulse functions adapted to the array aperture. Subsequently, these impulse functions are beamformed in the frequency domain to estimate the slowness vector. We compare the performance of this new method with that of an alternative solution, based on the fast Short-Term Average/Long-Term Average algorithm and with a method based on the trace envelope, with the ability to derive a very fast detection and slowness vector estimation of seismic signal arrivals. The new array methodology has been applied to data from an OBS deployment with an aperture of 80 km and an interstation-distance of about 40 km, in the vicinity of Cape Saint Vincent (SW Iberia). A set of 17 regional earthquakes with magnitudes 2 < mbLg < 5, has been selected to test the capabilities of detecting and locating regional seismic events with the Cape Saint Vincent OBS Array. We have found that there is a good agreement between the epicentral locations obtained previously by direct search methods and those calculated using the slowness vector estimations resulting from application of the CWT technique. We show that the proposed CWT method can detect seismic signals and estimate the slowness vector from regional earthquakes with high accuracy and robustness under low signal-to-noise ratio conditions. Differences in epicentral distances applying direct search methods and the CWT technique are between 1 km and 21 km with an average value of 12 km. The back-azimuth differences range from 1° to 7° with an average of 1.5° for the P-wave and ranging from 1° to 10° with an average of 3° for the S-wave.

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Mapping of Eastern North Atlantic Ocean seismicity from Po/So observations at a mid-aperture seismological broad-band deep sea array

Cited by 7 publications

References 56 publications

Seismic scattering and absorption of oceanic lithosphericSwaves in the Eastern North Atlantic

Seismic scattering and absorption of oceanic lithosphericSwaves in the Eastern North Atlantic

CTBT seismic monitoring using coherent and incoherent array processing

Slowness vector estimation over large-aperture sparse arrays with the Continuous Wavelet Transform (CWT): application to Ocean Bottom Seismometers

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