scholarly journals The May 7 - 11, 2016 Earthquake Sequence at Rivera Fault Zone

2020 ◽  
Author(s):  
Francisco J Nunez-Cornu ◽  
Diego Cordoba ◽  
William L Bandy ◽  
Juan José Dañobeitia ◽  
Carlos Mortera-Gutierrez ◽  
...  
Keyword(s):  
Pressure Sensors ◽  
Velocity Model ◽  
Tectonic Stress ◽  
Seismic Network ◽  
P Wave ◽  
Earthquake Sequence ◽  
Ocean Bottom ◽  
Transform Fault ◽  
Stress Regime ◽  
Short Period

<p>The geodynamic complexity in the interaction between Rivera, Cocos and NOAM plates is mainly reflected in the high and not well located seismicity of the region. In the framework of TsuJal Project, a study of the passive seismic activity was carried out. A temporal seismic network with 25 Obsidian stations with sensor Le-3D MkIII were deploying from the northern part of Nayarit state to the south of Colima state, including the Marias Islands, in addition to the Jalisco telemetric Seismic Network, being a total of 50 seismic stations on land. Offshore, ten Ocean Bottom Seismographs type LCHEAPO 2000 with 4 channels (3 seismic short period and 1 pressure sensors) were deployed and recover by the BO El Puma from UNAM in an array from the Marias Islands to off coast of the border of Colima and Michoacan state, in the period from 19th April to 7th November 2016.</p><p>A seismic sequence started on May 7, 2016 with an earthquake Mw = 5.6 reported by CMT-Harvard, USGS and SSN at the area north of Paleo Rivera Transform fault and west of the Middle America Trench, an area with a very complex tectonics due to the interaction of Rivera, Cocos and NOAM plates.</p><p>An analysis of this earthquake sequence from May 7 to May 11 using data from OBS and adequate P-Wave velocity model for Rivera plate is presented, 87 earthquakes were located. Data from onland stations were integrated after a travel-time residual analysis.</p><p>We observed that the new location is about 50 km southwest direction, from previous one, between the Paleo Rivera Transform fault and the northern tip of the East Pacific Rise – Pacific Cocos Segment.  This area has a different tectonic stress regime.</p>

scholarly journals A first look at the Gargano (southern Italy) seismicity as seen by the local scale OTRIONS seismic network

2014 ◽  
Vol 57 (4) ◽  
Author(s):  
Salvatore de Lorenzo ◽  
Annalisa Romeo ◽  
Luigi Falco ◽  
Maddalena Michele ◽  
Andrea Tallarico
Keyword(s):  
Southern Italy ◽  
Velocity Model ◽  
Local Scale ◽  
Seismic Network ◽  
P Wave ◽  
Travel Times ◽  
Velocity Models ◽  
P Wave Velocity ◽  
Short Period ◽  
Inversion Procedure

<p>On April 2013, a local scale seismic network, named OTRIONS, composed of twelve short period (1 Hz) three component seismometers, has been located in the northern part of the Apulia (southern Italy). In the first two months of data acquisition, the network recorded about one hundred very small (M<span><sub>L</sub></span>&lt;2) magnitude earthquakes. A three-layer 1D V<span><sub>P</sub></span> velocity model was preliminarily computed, using the recordings of earthquakes occurred in the area in the period 2006-2012 and recorded by the national seismic network of INGV (Istituto Nazionale di Geofisica e Vulcanologia). This model was calibrated by means of a multi-scale approach, based on a global search of the minimum misfit between observed and theoretical travel times. At each step of the inversion, a grid-search technique was implemented to infer the elastic properties of the layers, by using HYPO71 to compute the forward models. In a further step, we used P and S travel times of both INGV and OTRIONS events to infer a minimum 1D V<span><sub>P</sub></span> velocity model, using a classical linearized inversion approach. Owing to the relatively small number of data and poor coverage of the area, in the inversion procedure, the V<span><sub>P</sub></span>/V<span><sub>S</sub></span> ratio was fixed to 1.82, as inferred from a modified Wadati diagram. The final 1D velocity model was obtained by averaging the inversion results arising from nine different initial velocity models. The inferred V<span><sub>P</sub></span> velocity model shows a gradual increase of P wave velocity with increasing the depth. The model is well constrained by data until to a depth of about 25-30 km.</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.

scholarly journals The effects of tectonic release on short-period P waves from NTS explosions

1984 ◽  
Vol 74 (3) ◽  
pp. 819-842
Author(s):  
Thorne Lay ◽  
Terry C. Wallace ◽  
Don V. Helmberger
Keyword(s):  
Frequency Component ◽  
Temporal Variations ◽  
P Wave ◽  
High Frequency Component ◽  
Stress Regime ◽  
P Waves ◽  
Underground Nuclear Explosions ◽  
Long Period ◽  
Short Period ◽  
Amplitude Pattern

Abstract The first cycle (ab amplitude) of teleseismic short-period P waves from underground nuclear explosions at Pahute Mesa (NTS) show a systematic azimuthal amplitude pattern that can possibly be explained by tectonic release. The amplitudes vary by a factor of three, with diminished amplitudes being recorded at azimuths around N25°E. This azimuthal pattern has a strong sin(2φ) component and is observed, to varying degrees, for 25 Pahute Mesa events, but not for events at other sites within the NTS. Events that are known to have large tectonic release have more pronounced sin(2φ) amplitude variations. A synthesis of long-period body and surface wave investigations of tectonic release for Pahute Mesa events shows that, in general, the nonisotropic radiation is equivalent to nearly vertical, right-lateral strike-slip faulting trending from N20°W to due north. Long-period P waves at upper mantle distances demonstrate that there is a significant high-frequency component to the tectonic release. Using the long-period constraints on orientation, moment, and frequency content of the tectonic release, the expected short-period P wave effects are predicted. For models in which the downgoing P wave from the explosion triggers tectonic release within a few kilometers below the shot point, a factor of 2.5 amplitude variation with azimuth is predicted for the short-period ab amplitudes, with the lowest amplitudes expected near N25°E. Rather subtle azimuthal variations in the waveforms are expected, particulary for downward propagating ruptures, which is consistent with the absence of strong variations in the data. The occurrence of the azimuthal pattern, albeit with varying strength, for all of the Pahute Mesa events suggests a tectonic release model in which the shatterzone surrounding the explosion cavity is extended preferentially downward by driving a distributed network of faults and joints underlying the Mesa several kilometers beneath the surface. In this model, all events could have a component of tectonic release which would reflect the regional stress regime, although there may be slight spatial and temporal variations in the tectonic release contribution. Some events may trigger slip on larger throughgoing faults as well. While it is shown that tectonic release can affect teleseismic short-period signals significantly, and may contribute to the Pahute Mesa amplitude pattern, other possible explanations are considered.

scholarly journals Updated determination of earthquake magnitudes at the Swiss Seismological Service

2020 ◽  
Author(s):  
Roman Racine ◽  
Carlo Cauzzi ◽  
John Clinton ◽  
Donat Fäh ◽  
Benjamin Edwards ◽  
...  
Keyword(s):  
Seismic Hazard ◽  
Source Region ◽  
Strong Motion ◽  
Velocity Model ◽  
Seismic Network ◽  
Event Processing ◽  
Waveform Data ◽  
Data Archiving ◽  
Short Period ◽  
Magnitude Determination

&lt;p&gt;The Swiss Seismological Service (SED; http://www.seismo.ethz.ch) at ETH Z&amp;#252;rich is the federal agency in charge of monitoring earthquakes in Switzerland and neighboring areas, and for the assessment of seismic hazard and risk for the region. The SED seismic network largely relies on software and databases integrated in the SeisComP3 monitoring suite for waveform acquisition, automatic and manual event processing, event alerting, web infrastructure, data archiving and dissemination. Data from all digital seismic stations acquired by the SED over the last 30 years - broadband (presently ~230), strong-motion (~185), short-period (~65), permanent and temporary - are homogeneously integrated in the seismic network processing tools and products. Waveform data from the Swiss National Seismic Networks are openly available through the SED website and ORFEUS EIDA / Strong-Motion (http://orfeus-eu.org/data/) data gateways. The SED earthquake catalogue is publicly available through FDSN Event web services at&amp;#160; the SED (http://arclink.ethz.ch/fdsnws/event/1/). The Swiss seismic hazard maps are integrated in the EFEHR portal (http://www.efehr.org). The SED is updating its strategy for magnitude determination to make it fully consistent with the state-of-the-art in engineering seismology and seismic hazard studies in Switzerland, and to optimise the use of its dense seismic monitoring infrastructure. Among the planned changes are the: (a) adoption of a new ML relationship applicable in the near-source region at epicentral distances smaller than 15-20 km; (b) inclusion of ML station corrections based on empirically observed (de)amplification with respect to the Swiss reference rock velocity model and associated predictions; (c)&amp;#160; seamless computation of Mw based on spectral fitting of recorded FAS using a Swiss specific model. In this contribution we present and discuss the updated magnitude computations for a playback dataset of thousands of recorded earthquakes, and compare them with the current official estimates. We discuss the expected impacts of the new magnitude determination strategy on the SED event processing chain in SeisComP3, the SED catalogues and other seismological products. We welcome community feedback on our planned transition strategy.&lt;/p&gt;

scholarly journals Travel-time tomography imaging the Ecuadorian subduction, north of the Mw 7.8 Pedernales earthquake

2021 ◽  
Author(s):  
Alexandra Skrubej ◽  
Audrey Galve ◽  
Mireille Laigle ◽  
Andreas Rietbrock ◽  
Philippe Charvis ◽  
...  
Keyword(s):  
Oceanic Crust ◽  
A Priori ◽  
Structural Complexity ◽  
Velocity Model ◽  
P Wave ◽  
Ocean Bottom ◽  
Seismic Reflection Profile ◽  
Seismic Structure ◽  
Aseismic Slip ◽  
Velocity Models

&lt;p&gt;The Ecuadorian subduction regularly hosts large earthquakes. Among them, the Mw 8.8 1906 earthquake is the 7th biggest known event. Following the recent 2016 Mw 7.8 Pedernales earthquake, a large deployment of onshore/offshore seismological stations, in addition to the permanent seismological/geodetical network, revealed a complex slip behavior including the presence&amp;#160; of&amp;#160; seismic and aseismic slip.&lt;/p&gt;&lt;p&gt;During the geophysical experiment HIPER, in march 2020, 47 Ocean Bottom Seismometers (OBS), were densely deployed along a 93-km-long trench-perpendicular profile, recording airgun shots (4990 cu.inch.) performed by R/V Atalante to obtain a high-resolution P-wave velocity image. The profile was located north of the 2016 Pedernales rupture zone passing through an area experiencing aseismic slip and a region of contrasted geodetic interseismic coupling. &amp;#160;&amp;#160;&amp;#160;&lt;/p&gt;&lt;p&gt;We used the traveltime tomography code &amp;#171; tomo2d &amp;#187; (Korenaga et al., 2000) to invert first arrivals and reflected phases recorded by our OBS.&amp;#160; A joint 2D-seismic-reflection profile was acquired (abstract by L. Schenini) and provides details on the oceanic basement topography and on Vp velocities in shallow sedimentary layers.&lt;/p&gt;&lt;p&gt;Regarding the structural complexity in the region, we decided to start the inversion&amp;#160; using an a priori 2D velocity model. Several geophysical experiments have already been conducted offshore-onshore Ecuador (SISTEUR, 2000 ; SALIERI, 2001 and ESMERALDAS, 2005). Compilation of velocity models from tomographic images were used to build two a priori 1D Vp velocity models for both the Nazca oceanic crust and the forearc seismic structure. A 2D a priori Vp velocity model was built by merging the results of the two localized inversions using a selection of OBS on each side of the trench.&lt;/p&gt;&lt;p&gt;We obtain the crustal structure of the upper and subducting plates down to 20 km depth. Beneath the trench, a ~30-km-wide low-Vp anomaly is observed at lithospheric scale. This velocity is 10% lower than the typical Vp values observed for hydrated Pacific-type oceanic crust near the trench (Grevemeyer et al., 2018).&amp;#160;Recorded PmP phases will allow us to further constrain the crustal thickness. While we observe PmP phases in areas of low-Vp, the Moho reflectivity weakens and even disappears from the coincident MCS line. This intriguing observation could highlight processes, such as the presence of fluids or serpentinization, that need to be identified and better understood.&lt;/p&gt;

The effect of tectonic stress release on explosion P-wave signatures

1976 ◽  
Vol 66 (5) ◽  
pp. 1441-1457
Author(s):  
Thomas C. Bache
Keyword(s):  
Computational Models ◽  
Body Wave ◽  
Tectonic Stress ◽  
P Wave ◽  
General Question ◽  
Appreciable Effect ◽  
Stress Release ◽  
Secondary Sources ◽  
Short Period ◽  
Double Couple

abstract The influence of induced tectonic stress release on the short-period teleseismic P-wave signature of underground nuclear explosions is studied. Primary attention is directed to the first few cycles of the record from which body-wave magnitude (mb) is determined. Computational models for both the explosion and the superimposed tectonic release double couple are employed and theoretical seismograms are computed. Interest is mainly in the largest tectonic release component that seems reasonable using surface-wave observations and independent estimates of the controlling parameters as constraints. It is concluded that for most, perhaps all, events, tectonic release has no appreciable effect on the amplitude of the short-period P waves. Even the frequency content of the early arriving P wave is little affected by tectonic release for most likely circumstances. The computations assume tectonic release due to stress relaxation around the fracture zone created by the explosion. However, the results are extended to apply to the alternate mechanism whereby stress is released along a pre-existing fault plane. Since a number of other mechanisms can cause superposition of a double couple on the explosion, the analysis is relevant to the general question of the size these secondary sources must attain before the short-period P-wave recording is significantly altered.

Estimating the Hypocenter and Mechanism of the August 15, 1991 Centre Hall, Pennsylvania Earthquake Using Single-Station Data

1992 ◽  
Vol 63 (4) ◽  
pp. 541-555 ◽  
Author(s):  
Robert H. Clouser
Keyword(s):  
Wave Dispersion ◽  
Velocity Model ◽  
P Wave ◽  
Digital Data ◽  
Source Depth ◽  
Axis Orientation ◽  
Station Data ◽  
Short Period ◽  
Single Station ◽  
Back Azimuth

Abstract On August 15, 1991 a small (mbLg = 3.0) earthquake occurred near the town of Centre Hall, Pennsylvania. Based on early reports of felt effects and earthquake-generated sounds, the epicenter was placed somewhere ENE of State College, Pennsylvania. Three-component short-period digital data from the DWWSSN station SCP were analyzed to determine the hypocenter. Often, for small earthquakes in regions without dense seismic networks, information about an event must be obtained from single-station data. In this case, since no shallow velocity model exists for the area, simple ideas of wave propagation are invoked to estimate the distance and back-azimuth to the event. The horizontal P-wave particle motion constrained the back-azimuth, after calibrating the horizontal components by measuring the back-azimuth of quarry blast P-waves of known location. Distance determination was hampered by lack of a detailed upper crustal velocity model. Using iterative forward waveform modeling, a velocity model was generated that fit the observed S-minus-P and Rg-minus-P times and Rg-wave dispersion, and which was consistent with known upper crustal velocities in the area. A source depth of less than 1 km was inferred from the Rg-to-S ratio, the depth phase sP, and reports of earthquake-generated sounds. Estimates of the focal mechanism were obtained by a grid search procedure using Green’s functions computed with wavenumber integration for shear dislocation sources. Theoretical and observed amplitudes of sP, direct SH and SV (taken as ratios to the direct P), along with P polarity were compared for all possible combinations of strike, dip, and rake. Though fault plane orientation is poorly constrained, E-W to WNW-ESE P-axis orientation is a robust result of the search. Normal-faulting mechanisms are inconsistent with the data. However, the theoretical SV-to-P ratio is up to a factor of two larger than the observed ratio. This is probably related to an inadequate structure model and waveform sensitivity to source depth. Mechanism P-axis trends are consistent with other regional stress field indicators in the area.

Accurate hypocentre locations in the Middle-Durance Fault Zone, South-Eastern France

2009 ◽  
Vol 1 (4) ◽  
Author(s):  
Sebastiano Imposa ◽  
Jean-Pierre Fourno ◽  
Rosario Raffaele ◽  
Antonio Scaltrito ◽  
Luciano Scarfi
Keyword(s):  
Fault Zone ◽  
Velocity Model ◽  
Seismic Network ◽  
P Wave ◽  
Arrival Times ◽  
One Dimensional ◽  
South Eastern ◽  
Earthquake Locations ◽  
Hypocentre Locations ◽  
Wave Arrival

AbstractA one-dimensional velocity model and station corrections for the Middle-Durance fault zone (south-eastern France) were computed by inverting P-wave arrival times recorded on a local seismic network of 8 stations. A total of 93 local events with a minimum of 6 P-phases, RMS 0.4 s and a maximum gap of 220° were selected. Comparison with previous earthquake locations shows an improvement for the relocated earthquakes. Tests were carried out to verify the robustness of inversion results in order to corroborate the conclusions drawn from our findings. The obtained minimum 1-D velocity model can be used to improve routine earthquake locations and represents a further step toward more detailed seismotectonic studies in this area of south-eastern France.

scholarly journals The TOMO-ETNA experiment: an imaging active campaign at Mt. Etna volcano. Context, main objectives, working-plans and involved research projects

2016 ◽  
Vol 59 (4) ◽  
Author(s):  
Jesús M. Ibáñez ◽  
Janire Prudencio ◽  
Alejandro Díaz-Moreno ◽  
Domenico Patanè ◽  
Giuseppe Puglisi ◽  
...  
Keyword(s):  
Seismic Refraction ◽  
Seismic Network ◽  
Second Phase ◽  
Ocean Bottom ◽  
Etna Volcano ◽  
Remotely Operated Vehicle ◽  
Reflection Seismic ◽  
Air Gun ◽  
Short Period ◽  
Surrounding Areas

<p>The TOMO-ETNA experiment was devised to image of the crust underlying the volcanic edifice and, possibly, its plumbing system by using passive and active refraction/reflection seismic methods. This experiment included activities both on-land and offshore with the main objective of obtaining a new high-resolution seismic tomography to improve the knowledge of the crustal structures existing beneath the Etna volcano and northeast Sicily up to Aeolian Islands. The TOMO ETNA experiment was divided in two phases. The first phase started on June 15, 2014 and finalized on July 24, 2014, with the withdrawal of two removable seismic networks (a Short Period Network and a Broadband network composed by 80 and 20 stations respectively) deployed at Etna volcano and surrounding areas. During this first phase the oceanographic research vessel “Sarmiento de Gamboa” and the hydro-oceanographic vessel “Galatea” performed the offshore activities, which includes the deployment of ocean bottom seismometers (OBS), air-gun shooting for Wide Angle Seismic refraction (WAS), Multi-Channel Seismic (MCS) reflection surveys, magnetic surveys and ROV (Remotely Operated Vehicle) dives. This phase finished with the recovery of the short period seismic network. In the second phase the Broadband seismic network remained operative until October 28, 2014, and the R/V “Aegaeo” performed additional MCS surveys during November 19-27, 2014. Overall, the information deriving from TOMO-ETNA experiment could provide the answer to many uncertainties that have arisen while exploiting the large amount of data provided by the cutting-edge monitoring systems of Etna volcano and seismogenic area of eastern Sicily.</p>

scholarly journals Using Teleseismic P-Wave Arrivals to Calibrate the Clock Drift of Autonomous Underwater Hydrophones

2020 ◽  
Author(s):  
Alexey Sukhovich ◽  
Julie Perrot ◽  
Jean-Yves Royer
Keyword(s):  
Drift Rate ◽  
Seismic Velocity ◽  
Velocity Model ◽  
Internal Clock ◽  
P Wave ◽  
Ocean Bottom ◽  
P Waves ◽  
Arrival Times ◽  
Clock Drift ◽  
Common Reference

ABSTRACT Networks of autonomous underwater hydrophones (AUHs) are successfully employed for monitoring the low-level seismicity of mid-oceanic ridges by detecting hydroacoustic phases known as T waves. For a precise localization of a seismic event from T-wave arrival times, all AUHs must be synchronized. To this effect, at the beginning of the experiment, all instrument clocks are set to GPS time, which serves as a common reference. However, during the experiment, the instrument clock often deviates from GPS time, and, because the amount of deviation differs from one instrument to another, the synchronization of the AUHs deteriorates, as the experiment progresses in time. Just after the instrument recovery, the time difference (called “skew”) between the instrument and the GPS clocks is measured. Assuming that the skew varies linearly with time, the correction of a time series for the clock drift is a straightforward procedure. When the final skew cannot be determined, correcting for the clock drift is not possible, and any event localization becomes problematic. In this article, we demonstrate that the clock-drift rate (assumed to be time-independent) can be successfully estimated from arrival times of teleseismic P waves, commonly recorded by AUHs. Using a ray-tracing code, and accounting for the uncertainties in event hypocenter locations, origin times, and the Earth seismic-velocity model, confidence intervals of the estimated drift rates are deduced. The validity of the approach is tested on data from two AUHs with known clock drifts. Our results show that a reliable estimation is possible for skews as small as 4 s per two years (corresponding to a drift rate of about 5.5  ms·day−1). This method can also be applied to correct data of other recording instruments subject to internal-clock drift, such as ocean-bottom seismometers, when the skew is unknown.

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