scholarly journals Basin inversion: reactivated rift structures in the central Ligurian Sea revealed using ocean bottom seismometers

Solid Earth ◽  
2021 ◽  
Vol 12 (11) ◽  
pp. 2553-2571
Author(s):  
Martin Thorwart ◽  
Anke Dannowski ◽  
Ingo Grevemeyer ◽  
Dietrich Lange ◽  
Heidrun Kopp ◽  
...  
Keyword(s):  
Focal Mechanisms ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
Rift Basin ◽  
S Wave ◽  
Uppermost Mantle ◽  
Ligurian Sea ◽  
Northern Margin ◽  
Basin Inversion ◽  
Ligurian Basin

Abstract. The northern margin of the Ligurian Basin shows notable seismicity at the Alpine front, including frequent magnitude 4 events. Seismicity decreases offshore towards the Basin centre and Corsica, revealing a diffuse distribution of low-magnitude earthquakes. We analyse data of the amphibious AlpArray seismic network with focus on the offshore component, the AlpArray ocean bottom seismometer (OBS) network, consisting of 24 broadband OBSs deployed for 8 months, to reveal the seismicity and depth distribution of micro-earthquakes beneath the Ligurian Sea. Two clusters occurred between ∼ 10 km to ∼ 16 km depth below the sea surface, within the lower crust and uppermost mantle. Thrust faulting focal mechanisms indicate compression and an inversion of the Ligurian Basin, which is an abandoned Oligocene–Miocene rift basin. The basin inversion is suggested to be related to the Africa–Europe plate convergence. The locations and focal mechanisms of seismicity suggest reactivation of pre-existing rift-related structures. Slightly different striking directions of presumed rift-related faults in the basin centre compared to faults further east and hence away from the rift basin may reflect the counter-clockwise rotation of the Corsica–Sardinia block. High mantle S-wave velocities and a low Vp/Vs ratio support the hypothesis of strengthening of crust and uppermost mantle during the Oligocene–Miocene rifting-related extension and thinning of continental crust.

scholarly journals Basin inversion: Reactivated rift structures in the Ligurian Sea revealed by OBS

2021 ◽  
Author(s):  
Martin Thorwart ◽  
Anke Dannowski ◽  
Ingo Grevemeyer ◽  
Dietrich Lange ◽  
Heidrun Kopp ◽  
...  
Keyword(s):  
Focal Mechanisms ◽  
Ocean Bottom ◽  
Uppermost Mantle ◽  
Ligurian Sea ◽  
Northern Margin ◽  
Basin Inversion ◽  
Plate Convergence ◽  
Ocean Bottom Seismometers ◽  
Analyse Data ◽  
Ligurian Basin

Abstract. The northern margin of the Ligurian Basin shows notable seismicity at the Alpine front, including frequent magnitude 4 events. Seismicity decreases offshore towards the Basin centre and Corsica, revealing a diffuse distribution of low magnitude earthquakes. We analyse data of the amphibious AlpArray seismic network with focus on the offshore component, the AlpArray OBS network, consisting of 24 broadband ocean bottom seismometers deployed for eight months, to reveal the seismicity and depth distribution of micro-earthquakes beneath the Ligurian Sea. Two clusters occurred between ~10 km to ~16 km depth below sea surface, within the lower crust and uppermost mantle. Thrust faulting focal mechanisms indicate compression and an inversion of the Ligurian Basin, which is an abandoned Oligocene rift basin. The Basin inversion is suggested to be related to the Africa-Europe plate convergence. The locations and focal mechanisms of seismicity suggest reactivation of pre-existing rift structures. Slightly different striking directions of faults in the basin centre compared to faults further east and hence away from the abandoned rift may mimic the counter-clockwise rotation of the Corsica-Sardinia block during ~20–16 Ma. The observed cluster events support the hypothesis of strengthening of crust and uppermost mantle during rifting related extension and thinning of continental crust.

Local Seismicity indicates basin inversion in the Ligurian Sea 

2021 ◽  
Author(s):  
Martin Thorwart ◽  
Anke Dannowski ◽  
Ingo Grevemeyer ◽  
Dietrich Lange ◽  
Heidrun Kopp ◽  
...  
Keyword(s):  
Velocity Structure ◽  
Tectonic Setting ◽  
Broad Band ◽  
Ocean Bottom ◽  
Tyrrhenian Sea ◽  
Uppermost Mantle ◽  
Fault Plane Solutions ◽  
Ligurian Sea ◽  
Basin Inversion ◽  
Ligurian Basin

<p>The Alpine orogen and the Apennine system are part of the complex tectonic setting in the Mediterranean Sea caused by the convergence between Africa and Eurasia. Between 30 Ma and 15 Ma the Calabrian subduction retreated in a southeast direction pulling Corsica and Sardinia away from the Eurasian landmass. In this extensional setting, the Ligurian Sea was formed as a back-arc basin. The rifting jumped 15 Ma ago in the Tyrrhenian Sea leaving Corsica and Sardinia in a stable position relative to Eurasia.</p><p>Within the framework of the AlpArray research initiative a long-term seismological experiment was conducted in the Ligurian Sea to investigate the lithospheric structure and the seismicity in the Ligurian basin. The passive seismic network consisted of 29 broad-band ocean bottom stations from Germany and France. It was in operation between June 2017 and February 2018.</p><p>Two seismicity clusters occurred in the centre of the Ligurian Basin. The 18 earthquakes are located in the lower crust and in the upper-most mantle at depths between 10 km and 16 km. Re-location was performed only using picks from the OBS in the centre of the Ligurian Sea to avoid artifacts from the complex 3D velocity structure of the basin. Mantle refractions Pn and Sn have apparent velocities of 8.2 km/s and 4.7 km/s. The low Vp-Vs-ratio of 1.72 indicates a more brittle behaviour of the mantle material.</p><p>Fault plane solutions were determined for four events using also the data of land stations in southern France, Corsica, Sardinia and northern Italy. The focal mechanisms are thrust faulting. Fault planes strike in a NE-SW direction, coinciding with the alignment of the events and the direction of the basin axis.</p><p>We interprete the two earthquake clusters related to the inversion of the Ligurian Basin where the basin’s centre is under compression and stresses are taken up by reactivated faults in the crust and uppermost mantle. The compressional forces could be caused by the convergence of Africa and Europe. In general, observations of earthquakes in continental mantle lithosphere are rare and they reveal on the one hand a strengthening of the crust and uppermost mantle during rifting and on the other hand they support the interpretation that rifting failed in the northern Ligurian Basin.</p><p> </p>

scholarly journals 3D crustal structure of the Ligurian Sea revealed by ambient noise tomography using ocean bottom seismometer data

2021 ◽  
Author(s):  
Felix Noah Wolf ◽  
Dietrich Lange ◽  
Anke Dannowski ◽  
Martin Thorwart ◽  
Wayne Crawford ◽  
...  
Keyword(s):  
Group Velocity ◽  
Ambient Noise ◽  
Cross Correlation ◽  
Time Windows ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
S Wave ◽  
Noise Sources ◽  
Ligurian Sea ◽  
Velocity Models

Abstract. The Liguro-Provençal basin was formed as a back-arc basin of the retreating Calabrian-Apennines subduction zone during the Oligocene and Miocene. The resulting rotation of the Corsica-Sardinia block is associated with rifting, shaping the Ligurian Sea. It is still debated whether oceanic or atypical oceanic crust was formed or if the crust is continental and experienced extreme thinning during the opening of the basin. We invert velocity models using an amphibious network of seismic stations, including 22 broadband Ocean Bottom Seismometers (OBS) to investigate the lithospheric structure of the Ligurian sea. The instruments were installed in the Ligurian Sea for eight months between June 2017 and February 2018 as part of the AlpArray seismic network. Because of additional noise sources in the ocean, OBS data are rarely used for ambient noise studies. However, we attentively pre-process the data, including corrections for instrument tilt and seafloor compliance. We took extra care to exclude higher modes of the ambient-noise Rayleigh waves. We calculate daily cross-correlation functions for the LOBSTER array and surrounding land stations. Additionally, we correlate short time windows that include teleseismic earthquakes that allow us to derive surface wave group velocities for longer periods than using ambient noise only. Group velocity maps are obtained by inverting Green’s functions derived from the cross-correlation of ambient noise and teleseismic events, respectively. We then used the resulting 3D group velocity information to calculate 1D depth inversions for S-wave velocities. The shear-wave velocity results show a deepening of the Moho from 12 km at the southwestern basin centre to 20–25 km at the Ligurian coast in the northeast and over 30 km at the Provençal coast. We find no hint on mantle serpentinisation and no evidence for an Alpine slab, at least down to depths of 25 km. However, we see a separation of the southwestern and northeastern Ligurian Basin that coincides with the promoted prolongation of the Alpine front.

scholarly journals OBS Data Analysis to Quantify Gas Hydrate and Free Gas in the South Shetland Margin (Antarctica)

Energies ◽  
2018 ◽  
Vol 11 (12) ◽  
pp. 3290 ◽  
Author(s):  
Sha Song ◽  
Umberta Tinivella ◽  
Michela Giustiniani ◽  
Sunny Singhroha ◽  
Stefan Bünz ◽  
...  
Keyword(s):  
Velocity Field ◽  
Wave Velocity ◽  
Gas Hydrate ◽  
P Wave ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
S Wave ◽  
Free Gas ◽  
Hydrate Reservoir ◽  
Gas Hydrate Reservoir

The presence of a gas hydrate reservoir and free gas layer along the South Shetland margin (offshore Antarctic Peninsula) has been well documented in recent years. In order to better characterize gas hydrate reservoirs, with a particular focus on the quantification of gas hydrate and free gas and the petrophysical properties of the subsurface, we performed travel time inversion of ocean-bottom seismometer data in order to obtain detailed P- and S-wave velocity estimates of the sediments. The P-wave velocity field is determined by the inversion of P-wave refractions and reflections, while the S-wave velocity field is obtained from converted-wave reflections received on the horizontal components of ocean-bottom seismometer data. The resulting velocity fields are used to estimate gas hydrate and free gas concentrations using a modified Biot‐Geertsma‐Smit theory. The results show that hydrate concentration ranges from 10% to 15% of total volume and free gas concentration is approximately 0.3% to 0.8% of total volume. The comparison of Poisson’s ratio with previous studies in this area indicates that the gas hydrate reservoir shows no significant regional variations.

scholarly journals Crust and upper mantle structure of the Ligurian Sea revealed by ambient noise tomography using ocean bottom seismometer data

2020 ◽  
Author(s):  
Felix Noah Wolf ◽  
Dietrich Lange ◽  
Heidrun Kopp ◽  
Anke Dannowski ◽  
Ingo Grevemeyer ◽  
...  
Keyword(s):  
Phase Velocity ◽  
Ambient Noise ◽  
Cross Correlation ◽  
Velocity Structure ◽  
Signal To Noise Ratio ◽  
Time Windows ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
Ligurian Sea ◽  
Crust And Upper Mantle

<p>The Liguro-Provencal-basin was formed as a back-arc basin of the retreating Calabrian-Apennines subduction zone during the Oligocene and Miocene. The resulting rotation of the Corsica-Sardinia block at roughly 32–24 Ma is associated with rifting, shaping the Ligurian Sea. It is highly debated though, whether oceanic or atypical oceanic crust was formed or if the crust is continental and experienced extreme thinning during the opening of the basin.</p><p>To investigate the velocity structure of the Ligurian Sea a network (LOBSTER) of 29 broadband Ocean Bottom Seismometer (OBS) was installed jointly by GEOMAR (Germany) and ISTerre (France). The LOBSTER array measured continuously for eight months between June 2017 and February 2018 and is part of the AlpArray seismic network. AlpArray is a European initiative to further reveal the geophysical and geological properties of the greater Alpine area.</p><p>We contribute to the debate by surveying the type of crust and lithosphere flooring the Ligurian Sea.<br>Because of additional noise sources in the ocean, causing instrument tilt or seafloor compliance, OBS data are rarely used for ambient noise studies. However, we extensively pre-process the data to improve the signal-to-noise ratio. Then, we calculate daily cross-correlation functions for the LOBSTER array and surrounding land stations. Additionally, we correlate short time windows that include strong events. The cross-correlations of these are dominated by earthquake signals and allow us to derive surface wave group velocities for longer periods than using ambient noise only. Finally, phase velocity maps are obtained by inverting Green’s functions derived from cross-correlation of ambient noise and teleseismic events, respectively. The phase velocity maps show strong heterogeneities for short periods (5-15 s, corresponding to shallow depths). Causes for these include varying sediment thickness, fault zones and magmatism. For longer periods (20-80 s) the velocity structure smoothens and reveals mantle velocities north-northwest of the basin centre. This might hint on an asymmetric opening of the basin. We do not see strong indications for an oceanic spreading centre in the Ligurian basin.</p>

scholarly journals Seismicity cluster below the Moho indicates thrust faulting in the central Ligurian Basin

2020 ◽  
Author(s):  
Martin Thorwart ◽  
Anke Dannowski ◽  
Heidrun Kopp ◽  
Dietrich Lange ◽  
Wayne Crawford ◽  
...  
Keyword(s):  
Ocean Bottom ◽  
Tyrrhenian Sea ◽  
Seismic Profiles ◽  
Ligurian Sea ◽  
Thrust Faulting ◽  
Research Initiative ◽  
Passive Seismic ◽  
Long Term Experiment ◽  
Back Arc ◽  
Ligurian Basin

<p><span>The Alpine orogen and the Apennines system </span><span>are</span><span> part of the complex tectonic settings in the Mediterranean Sea caused by the convergence between Africa and Eurasia. Between 30 Ma and 15 Ma, the Calabrian Subduction retreated in southeast direction pulling Corsica and Sardinia away from the Eurasian continent. In this extensional setting, the Ligurian Sea was formed as a back-arc basin. The rifting jumped 15 MA ago </span><span>to</span><span> the Tyrrhenian Sea leaving Corsica and Sardinia in a stable position relative to Eurasia </span><span>as observed by GPS measurements.</span></p><p><span>Within the framework of the AlpArray research initiative and its German component “4D Mountain building” (SPP2017 4D-MB) a long-term experiment was conducted in the Ligurian sea to investigate the lithosphere structure and the seismicity in the Ligurian basin. The passive seismic network </span><span>was operated by France and Germany and </span><span>consisted of 29 br</span><span>oad-band ocean bottom stations. It was in operation between June 2017 and February 2018. At the end of the experiment </span><span>two</span><span> active seismic profiles were conducted additionally.</span></p><p><span>A cluster of 15 events with </span><span>m</span><span>agnitudes lower than 2.5 occurred in the centre of the Ligurian Basin. The earthquakes are located at a </span><span>depth </span><span>of 20 km to 35 km, i.e. </span><span>10 - 25 km below the Moho.</span><span> The cluster was not continuously active but had several active periods </span><span>which</span><span> lasted between 2 and 5 days.</span></p><p><span>A f</span><span>ault plane solution could be determined of the larger events in the cluster. The mechanism is a thrust faulting. </span><span>S</span><span>maller events </span><span>should have a similar mechanism</span> <span>due to the highly</span> <span>coherent </span><span>waveforms. </span><span>A</span> <span>similar </span><span>mechanism </span><span>was </span><span>observed for the Mw=4.9 earthquake on 07.07.2011 which occurred 50 km east of the cluster. Both solutions show a SW-NE striking, northwest dipping fault plane. </span><span>This indicates a convergence in NW-SE direction between Corsica and Eurasia.</span></p>

A new method to estimate ocean-bottom-seismometer orientation using teleseismic receiver functions

2020 ◽  
Vol 221 (2) ◽  
pp. 893-904
Author(s):  
Hong Zheng ◽  
Jianke Fan ◽  
Dapeng Zhao ◽  
Cuilin Li ◽  
Dongdong Dong ◽  
...  
Keyword(s):  
Epicentral Distance ◽  
Synthetic Data ◽  
Receiver Functions ◽  
P Wave ◽  
New Method ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
S Wave ◽  
P Waves ◽  
The Western Pacific

SUMMARY The orientation of an ocean-bottom-seismometer (OBS) is a critical parameter for analysing three-component seismograms, but it is difficult to estimate because of the uncontrollable OBS posture after its deployment. In this study, we develop a new and effective method to estimate the OBS orientation by fitting the amplitude of direct P wave of teleseismic receiver functions. The reliability of this method is verified using synthetic data and observed waveforms recorded at land seismic stations in Shandong Province, China. Our extensive synthetic tests show that our new method is little affected by a thin sedimentary layer that has a low S-wave velocity. The orientations of OBS stations that we deployed in the Yap subduction zone in the Western Pacific Ocean are estimated and corrected using our new method. After the correction, the direct P waves of teleseismic receiver functions show very good consistency. The effects of white and coloured noise in different levels, epicentral distance and backazimuth are also investigated, and the results show that these factors have small effects on the new method. We also examine the effect of sensor tilting on estimation of the OBS orientation, and find that a tilting correction should be made before the misorientation correction. We compare the OBS orientations determined with the new method and other methods and find that they are generally consistent with each other. We also discuss advantages and shortcomings of various methods, and think that our new method is more robust than the existing methods.

Sign in / Sign up

Export Citation Format

Share Document