The 2018 Mw 6.8 Zakynthos, Greece, Earthquake: Dominant Strike-Slip Faulting near Subducting Slab

2020 ◽  
Vol 91 (2A) ◽  
pp. 721-732 ◽  
Author(s):  
Efthimios Sokos ◽  
František Gallovič ◽  
Christos P. Evangelidis ◽  
Anna Serpetsidaki ◽  
Vladimír Plicka ◽  
...  
Keyword(s):  
Numerical Models ◽  
Waveform Inversion ◽  
Strong Motion ◽  
Satellite System ◽  
Strike Slip ◽  
Plate Motion ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
Motion Data ◽  
Global Navigation Satellite

Abstract With different styles of faulting, the eastern Ionian Sea is an ideal natural laboratory to investigate interactions between adjacent faults during strong earthquakes. The 2018 Mw 6.8 Zakynthos earthquake, well recorded by broadband and strong-motion networks, provides an opportunity to resolve such faulting complexity. Here, we focus on waveform inversion and backprojection of strong-motion data, partly checked by coseismic Global Navigation Satellite System data. We show that the region is under subhorizontal southwest–northeast compression, enabling mixed thrust faulting and strike-slip (SS) faulting. The 2018 mainshock consisted of two fault segments: a low-dip thrust, and a dominant, moderate-dip, right-lateral SS, both in the crust. Slip vectors, oriented to southwest, are consistent with plate motion. The sequence can be explained in terms of trench-orthogonal fractures in the subducting plate and reactivated faults in the upper plate. The 2018 event, and an Mw 6.6 event of 1997, occurred near three localized swarms of 2016 and 2017. Future numerical models of the slab deformation and ocean-bottom seismometer observations may illuminate possible relations among earthquakes, swarms, and fluid paths in the region.

The 2018 Mw 6.8 Zakynthos, Greece, earthquake – strike-slip and thrust faulting in shallow subduction

2020 ◽  
Author(s):  
Efthimios Sokos ◽  
František Gallovič ◽  
Christos P. Evangelidis ◽  
Anna Serpetsidaki ◽  
Vladimír Plicka ◽  
...  
Keyword(s):  
Numerical Models ◽  
Waveform Inversion ◽  
Broad Band ◽  
Strong Motion ◽  
Aftershock Sequence ◽  
Strike Slip ◽  
Plate Motion ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
Fault Interaction

<p>On October 25, 2018, at 22:54 UTC, an Mw 6.8 earthquake occurred southwest of Zakynthos island in the Ionian Sea. This is an area with different styles of faulting and the locus of strong events thus ideal for fault interaction studies. The 2018 Zakynthos earthquake was recorded by broad-band and strong-motion networks and provides an opportunity to resolve such faulting complexity. We used waveform inversion and backprojection of strong motion data, partly verified by co-seismic GNSS data, too. The aftershock sequence was relocated, and the moment tensors of the strongest events were evaluated. Stress inversion shows that the region is under sub-horizontal southwest-northeast compression, enabling mixed thrust- and strike-slip faulting. Based on detailed waveform inversion studies, we conclude that the 2018 mainshock consisted of two fault segments: a low-dip thrust, and a dominant, moderate-dip, right-lateral strike slip, both in the crust. This model explains the observed large negative CLVD component of the mainshock. Slip vectors of both ruptured segments, oriented to SW, are consistent with plate motion in the area. The sequence can be explained in terms of trench-orthogonal fractures in the subducting plate and reactivated faults in the upper plate. The 2018 event, and an Mw 6.6 event of 1997, occurred near three localized swarms of 2016 and 2017. Future numerical models of the slab deformation and ocean-bottom seismometer observations may illuminate possible relations between earthquakes, swarms and fluid paths in the region.</p>

Multi-parameter model building for the Qiuyue structure using four-component ocean-bottom seismometer data

Geophysics ◽  
2021 ◽  
pp. 1-52
Author(s):  
Yuzhu Liu ◽  
Xinquan Huang ◽  
Jizhong Yang ◽  
Xueyi Liu ◽  
Bin Li ◽  
...  
Keyword(s):  
Wave Velocity ◽  
Waveform Inversion ◽  
Velocity Model ◽  
P Wave ◽  
Velocity Analysis ◽  
Full Waveform Inversion ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
Full Waveform ◽  
P Wave Velocity

Thin sand-mud-coal interbedded layers and multiples caused by shallow water pose great challenges to conventional 3D multi-channel seismic techniques used to detect the deeply buried reservoirs in the Qiuyue field. In 2017, a dense ocean-bottom seismometer (OBS) acquisition program acquired a four-component dataset in East China Sea. To delineate the deep reservoir structures in the Qiuyue field, we applied a full-waveform inversion (FWI) workflow to this dense four-component OBS dataset. After preprocessing, including receiver geometry correction, moveout correction, component rotation, and energy transformation from 3D to 2D, a preconditioned first-arrival traveltime tomography based on an improved scattering integral algorithm is applied to construct an initial P-wave velocity model. To eliminate the influence of the wavelet estimation process, a convolutional-wavefield-based objective function for the preprocessed hydrophone component is used during acoustic FWI. By inverting the waveforms associated with early arrivals, a relatively high-resolution underground P-wave velocity model is obtained, with updates at 2.0 km and 4.7 km depth. Initial S-wave velocity and density models are then constructed based on their prior relationships to the P-wave velocity, accompanied by a reciprocal source-independent elastic full-waveform inversion to refine both velocity models. Compared to a traditional workflow, guided by stacking velocity analysis or migration velocity analysis, and using only the pressure component or other single-component, the workflow presented in this study represents a good approach for inverting the four-component OBS dataset to characterize sub-seafloor velocity structures.

Hybrid long-period volcanic events observed in off Nicobar region, the Andaman Sea from a passive OBS experiment

2020 ◽  
Author(s):  
Karanam Kattil Aswini ◽  
Pawan Dewangan ◽  
Kattoju Achuta Kamesh Raju ◽  
Yatheesh Vadakkeyakath ◽  
Pabitra Singha ◽  
...  
Keyword(s):  
High Frequency ◽  
Source Region ◽  
Strike Slip ◽  
Andaman Sea ◽  
Earthquake Swarms ◽  
Ocean Bottom ◽  
Upward Movement ◽  
Ocean Bottom Seismometer ◽  
Long Period ◽  
Back Arc

<p>The off Nicobar region in the Andaman Sea is witnessing frequent earthquake swarms after December 2004 Tsunamigenic earthquake in January 2005, March and October 2014, November 2015 and April 2019. In this study, we present the geophysical evidence of active volcanism in the Off Nicobar back-arc region on 21<sup>st</sup> and 22<sup>nd</sup> March 2014 based on a passive Ocean Bottom Seismometer (OBS) experiment. We detected a series of hybrid earthquake events characterized by the onset of high–frequency signal (1-10 Hz) which is followed by a long period waveform of up to 600s having a range of 0.1-1 Hz. The waveforms appear to be emergent and lack the onset of a distinct S-phase. We also observed a very high frequency (10-40 Hz) hydro-acoustic phase in the coda of long-period events.  These hybrid events are considered to be volcano-tectonic (VT) events that may trigger magmatic activities in the Off Nicobar region. We have identified and located 141 high-frequency events on 21<sup>st</sup> and 22<sup>nd</sup> March 2014 using hypocent v.3.2 program and they are distributed along NW-SE direction aligning with the submarine volcanoes defining the volcanic arc as observed in the high-resolution bathymetry data. The fault plane solution of the major high-frequency events suggests strike-slip faulting with the strike, dip and rake values of 334<sup>°</sup>, 89<sup>°</sup> and 171<sup>°</sup>, respectively along the direction of the prevalent sliver strike-slip faulting in the Andaman back-arc region. We propose that the upward movement of magma is a plausible mechanism which can explain the frequent occurrence of earthquake swarms in the off Nicobar region. The stress generated from magma movement may initially trigger shallow VT events such as faulting or dike intrusions and later generate long period ringing associated with the resonance of the magma chamber. The shallow nature of the events also generates a hydroacoustic wave which is detected in the OBS experiment as the source region is in the SOFAR channel.</p>

scholarly journals Crustal-scale depth imaging via joint full-waveform inversion of ocean-bottom seismometer data and pre-stack depth migration of multichannel seismic data: a case study from the eastern Nankai Trough

Solid Earth ◽  
2019 ◽  
Vol 10 (3) ◽  
pp. 765-784 ◽  
Author(s):  
Andrzej Górszczyk ◽  
Stéphane Operto ◽  
Laure Schenini ◽  
Yasuhiro Yamada
Keyword(s):  
Seismic Data ◽  
Nankai Trough ◽  
Waveform Inversion ◽  
Velocity Model ◽  
Full Waveform Inversion ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
Depth Migration ◽  
Full Waveform ◽  
Air Gun

Abstract. Imaging via pre-stack depth migration (PSDM) of reflection towed-streamer multichannel seismic (MCS) data at the scale of the whole crust is inherently difficult. This is because the depth penetration of the seismic wavefield is controlled, firstly, by the acquisition design, such as streamer length and air-gun source configuration, and secondly by the complexity of the crustal structure. Indeed, the limited length of the streamer makes the estimation of velocities from deep targets challenging due to the velocity–depth ambiguity. This problem is even more pronounced when processing 2-D seismic data due to the lack of multi-azimuthal coverage. Therefore, in order to broaden our knowledge about the deep crust using seismic methods, we present the development of specific imaging workflows that integrate different seismic data. Here we propose the combination of velocity model building using (i) first-arrival tomography (FAT) and full-waveform inversion (FWI) of wide-angle, long-offset data collected by stationary ocean-bottom seismometers (OBSs) and (ii) PSDM of short-spread towed-streamer MCS data for reflectivity imaging, with the former velocity model as a background model. We present an application of such a workflow to seismic data collected by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and the Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER) in the eastern Nankai Trough (Tokai area) during the 2000–2001 Seize France Japan (SFJ) experiment. We show that the FWI model, although derived from OBS data, provides an acceptable background velocity field for the PSDM of the MCS data. From the initial PSDM, we refine the FWI background velocity model by minimizing the residual move-outs (RMOs) picked in the pre-stack-migrated volume through slope tomography (ST), from which we generate a better-focused migrated image. Such integration of different seismic datasets and leading-edge imaging techniques led to greatly improved imaging at different scales. That is, large to intermediate crustal units identified in the high-resolution FWI velocity model extensively complement the short-wavelength reflectivity inferred from the MCS data to better constrain the structural factors controlling the geodynamics of the Nankai Trough.

Soil Coupling Of a Strong Motion, Ocean Bottom Seismometer

1979 ◽  
Author(s):  
R.L. Steinmetz ◽  
P.L. Donoho ◽  
J.D. Murff ◽  
G.V. Latham
Keyword(s):  
Strong Motion ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer

scholarly journals Co-Seismic Ionospheric Disturbances Following the 2016 West Sumatra and 2018 Palu Earthquakes from GPS and GLONASS Measurements

2022 ◽  
Vol 14 (2) ◽  
pp. 401
Author(s):  
Mokhamad Nur Cahyadi ◽  
Buldan Muslim ◽  
Danar Guruh Pratomo ◽  
Ira Mutiara Anjasmara ◽  
Deasy Arisa ◽  
...  
Keyword(s):  
Acoustic Waves ◽  
Satellite System ◽  
Strike Slip ◽  
Line Of Sight ◽  
Ionospheric Disturbances ◽  
Sumatra Earthquake ◽  
Satellite Line ◽  
West Sumatra ◽  
Global Navigation Satellite ◽  
Earthquake Moment

The study of ionospheric disturbances associated with the two large strike-slip earthquakes in Indonesia was investigated, which are West Sumatra on 2 March 2016 (Mw = 7.8), and Palu on 28 September 2018 (Mw = 7.5). The anomalies were observed by measuring co-seismic ionospheric disturbances (CIDs) using the Global Navigation Satellite System (GNSS). The results show positive and negative CIDs polarization changes for the 2016 West Sumatra earthquake, depending on the position of the satellite line-of-sight, while the 2018 Palu earthquake shows negative changes only due to differences in co-seismic vertical crustal displacement. The 2016 West Sumatra earthquake caused uplift and subsidence, while the 2018 Palu earthquake was dominated by subsidence. TEC anomalies occurred about 10 to 15 min after the two earthquakes with amplitude of 2.9 TECU and 0.4 TECU, respectively. The TEC anomaly amplitude was also affected by the magnitude of the earthquake moment. The disturbance signal propagated with a velocity of ~1–1.72 km s−1 for the 2016 West Sumatra earthquake and ~0.97–1.08 km s−1 for the 2018 Palu mainshock earthquake, which are consistent with acoustic waves. The wave also caused an oscillation signal of ∼4 mHz, and their azimuthal asymmetry of propagation confirmed the phenomena in the Southern Hemisphere. The CID signal could be identified at a distance of around 400–1500 km from the epicenter in the southwestern direction.

scholarly journals The October 9, 1996 earthquake in Cyprus: seismological, macroseismic and strong motion data

1999 ◽  
Vol 42 (1) ◽  
Author(s):  
I. Kalogeras ◽  
G. Stavrakakis ◽  
K. Solomi
Keyword(s):  
Eastern Mediterranean ◽  
Strong Motion ◽  
Strike Slip ◽  
Reverse Fault ◽  
Aftershock Activity ◽  
Motion Data ◽  
Attenuation Relationships ◽  
Largest Aftershock ◽  
Sea Area ◽  
Development Department

On October 9, 1996, an earthquake of magnitude 6.8 occurred in the sea area SW of Cyprus, Eastern Mediterranean. This earthquake, which caused damage mostly in the area of Paphos and Limassol, triggered an accelerograph installed at Yermasoyia dam, north of Limassol. The Geodynamic Institute of the National Observatory of Athens in cooperation with the Geological Survey of Cyprus deployed five digital accelerographs in order to record large aftershocks. Although the aftershock activity lasted over four months and included a large number of earthquakes with magnitudes 4.5 and greater, only the largest aftershock of January 13, 1997, having a magnitude of 5.9, triggered two of these five accelerographs. Moreover another digital accelerograph, operated by the Water Development Department of Cyprus, was triggered and this record was also taken into account in this study. The first Cyprean strong motion records obtained to date, gave us the opportunity to compare the results from their analysis to the already proposed attenuation relationships from other areas of the world with a similar seismotectonic regime. Although a general fitting to the attenuation curves for subduction events and strike-slip/reverse fault events was found, the calculated peak ground accelerations were found to be lower than others. Unfortunately, due to the lack of data from previous Cyprean earthquakes, it was not possible to conclude to precise attenuation

scholarly journals Isla del Coco, on Cocos Plate, converges with Isla de San Andrés, on the Caribbean Plate, at 78mm/yr

2017 ◽  
pp. 33-41
Author(s):  
Mariano Protti ◽  
Víctor González ◽  
Jeffrey Freymueller ◽  
Sarah Doelger
Keyword(s):  
Steady Motion ◽  
Satellite System ◽  
Plate Motion ◽  
Time Interval ◽  
Caribbean Plate ◽  
Cocos Plate ◽  
Position Time Series ◽  
The Caribbean ◽  
Global Navigation Satellite ◽  
San Andres

Isla del Coco is the only land mass of the Cocos Plate that emerges above sea level. This makes it the only place where Cocos Plate motion can be measured using Global Navigation Satellite System (GNSS) monitoring. Global Positioning System (GPS) observations have been carried out sporadically over more than two decades on Isla del Coco, allowing precise measurement of the motion of the Cocos Plate. Recently, in May 2011, a continuous GPS station was built and instrumented at Isla del Coco, in Wafer Bay, by OVSICORI-UNA and UNAVCO, as part of the COCONet regional GNSS network. Position time series from this CGPS station (ISCO: Isla del Coco) show a steady motion of Isla del Coco at a speed of 90.9±1.5mm/yr in the N35oE direction in ITRF2008 and convergence with the Caribbean Plate at 78±1mm/yr. This result is consistent with the findings of the earliest GPS studies, and agrees within uncertainty with the estimated convergence rate of 76.4±x mm/ yr of the MORVEL plate motion model. MORVEL is based on an average over the last 780,000 years, and our result suggests that Cocos-Caribbean plate motions have been constant over that time interval. Citation: Protti, M., V. González, J. Freymueller & S. Doelger. 2012. Isla del Coco, on Cocos Plate, converges with San Andres Island, on the Caribbean Plate, at 78mm/yr. Rev. Biol. Trop. 60 (Suppl. 3): 33-41. Epub 2012 Dec 01.

scholarly journals Looking beyond kinematics: 3D thermo-mechanical modelling reveals the dynamics of transform margins

Solid Earth ◽  
2021 ◽  
Vol 12 (5) ◽  
pp. 1211-1232
Author(s):  
Anthony Jourdon ◽  
Charlie Kergaravat ◽  
Guillaume Duclaux ◽  
Caroline Huguen
Keyword(s):  
Boundary Conditions ◽  
Strain Localization ◽  
Numerical Models ◽  
Motion Vector ◽  
Shear Zones ◽  
Strike Slip ◽  
Plate Motion ◽  
Plate Boundaries ◽  
Mechanical Modelling ◽  
Extension Direction

Abstract. Transform margins represent ∼ 30 % of non-convergent margins worldwide. Their formation and evolution have traditionally been addressed through kinematic models that do not account for the mechanical behaviour of the lithosphere. In this study, we use high-resolution 3D numerical thermo-mechanical modelling to simulate and investigate the evolution of intra-continental strain localization under oblique extension. The obliquity is set through velocity boundary conditions that range from 15∘ (high obliquity) to 75∘ (low obliquity) every 15∘ for rheologies of strong and weak lower continental crust. Numerical models show that the formation of localized strike-slip shear zones leading to transform continental margins always follows a thinning phase during which the lithosphere is thermally and mechanically weakened. For low- (75∘) to intermediate-obliquity (45∘) cases, the strike-slip faults are not parallel to the extension direction but form an angle of 20∘ to 40∘ with the plate motion vector, while for higher obliquities (30∘ to 15∘) the strike-slip faults develop parallel to the extension direction. Numerical models also show that during the thinning of the lithosphere, the stress and strain re-orient while boundary conditions are kept constant. This evolution, due to the weakening of the lithosphere, leads to a strain localization process in three major phases: (1) initiation of strain in a rigid plate where structures are sub-perpendicular to the extension direction; (2) distributed deformation with local stress field variations and formation of transtensional and strike-slip structures; (3) formation of highly localized plate boundaries stopping the intra-continental deformation. Our results call for a thorough re-evaluation of the kinematic approach to studying transform margins.

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