A comprehensive quantification of error location uncertainties for the French earthquake catalog

Locating earthquakes has been a longterm problem in seismology that depends on multiple parameters like station density and spacing, azimuthal gap, velocity models, and phase pick precision. Here, we analyze the current state of the earthquake French catalog for the time period between 2010 until 2018, which we divide into different regions: the Alps, Massif Central, the West, the Pyrenees, the Grand-East and the North. We perform multiple location synthetic tests using as benchmark the earthquake catalog and the evolution of the French seismic network to quantify the improvements in 1) earthquake location through time and 2) the error locations and their uncertainties. For such endeavors, we use NonLinLoc to perform the synthetic tests varying, as input, the stations, the number of stations and phase picks, 1D velocity models and 3D velocity models, and to understand the changes in 1) earthquake hypocenters, 2) ellipsoidal errors and 3) posterior density functions. Then, we relocate the entire catalog using NonLinLoc including 3D velocity models (where available) and compare the hypocentral location differences when we relocate the catalog with 1D velocity models. Additionally, we estimate a quality factor for each of the located earthquakes and report the changes on the quality factor with the temporal evolution of the national seismic network. The resulting catalog and its associated error location will help future seismic hazard estimations in the Metropolitan French area.

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Earthquake relocation in the wester termination of the North Aegean Trough

Bulletin of the Geological Society of Greece ◽

10.12681/bgsg.16828 ◽

2018 ◽

Vol 40 (3) ◽

pp. 1091

Author(s):

Ch. K. Karamanos ◽

G. V. Karakostas ◽

E. E. Papadimitriou ◽

M. Sachpazi

Keyword(s):

Time Delays ◽

Seismic Waves ◽

S Wave ◽

Data Set ◽

Arrival Times ◽

Velocity Models ◽

The North ◽

North Aegean ◽

Focal Parameters ◽

Hypocentral Location

The area of North Aegean Trough exhibits complex tectonic characteristics as a consequence of the presence of complicated active structures. Exploitation of accurately determined earthquake data considerably contributes in the investigation of these structures and such accuracy is seeking through certain procedures. The determination of focal parameters of earthquakes that occurred in this area during 1964-2003 was performed by collecting all the available data for Ρ and S arrivals. After selecting the best solutions from an initial hypocentral location, 739 earthquakes were found that fulfilled certain criteria for the accuracy and used for further processing. The study area was divided in 16 sub regions and by the use of the HYPOINVERSE computer program, the travel time curves were constructed, and were used to define the velocity models for each one of them. For each sub region the time delays were calculated and used as time corrections in the arrival times of the seismic waves. The Vp/Vs ratio, necessary for S—wave velocity models, was calculated with two different methods and was found equal to 1.76. The velocity models and the time delays were used to relocate the events of the whole data set. The relocation resulted in significant improvement of the accuracy in the focal parameters determination.

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Rayleigh wave ellipticity measurements in the North Tanzanian Divergence (Eastern African Rift)

10.5194/egusphere-egu2020-7976 ◽

2020 ◽

Author(s):

Laura Parisi ◽

Andrea Berbellini ◽

P. Martin Mai

Keyword(s):

Rayleigh Wave ◽

Seismic Station ◽

Seismic Network ◽

Body Waves ◽

Earth Structure ◽

Velocity Models ◽

The North ◽

The Earth ◽

Single Station ◽

African Rift

Rayleigh wave ellipticity depends, in theory, only on the Earth structure below a seismic station, offering the advantage of a &#8220;single-station&#8221; method to infer crustal properties. Therefore, ellipticity measurements can be used to construct pseudo 3-D shear velocity models of the earth structure using even seismic stations that did not record simultaneously.&#160;&#160;Based on that, we carried-out ellipticity measurements by using teleseismic waveforms recorded by the OPS seismic network we deployed at the western flank of the North Tanzanian Divergence between June 2016 and May 2018, covering 17 sites. We then expanded our measurements on the waveforms recorded by the adjacent CRAFTI seismic network from January 2013 and December 2014, available on IRIS, which comprised more than 30 sites.&#160;While the OPS network covers the transition between the Tanzania Craton and North Tanzanian Divergence, the CRAFTI network is entirely contained in the North Tanzanian Divergence. Therefore, the imaging that can be obtained by integrating the two asynchronous passive seismology experiments will help to better understand the dynamics of this segment of the eastern branch of the Eastern African Rift.Preliminary results show heterogeneity structure that are in agreement with previous tomographic studies based on ambient noise cross-correlation and body-waves arrival-times. In regions where previous seismological studies are not available, results match the known geological structure of the transition between the Tanzanian Craton and the North Tanzanian Divergence. This demonstrates that measurements of ellipticity can be a useful and integrative tool for earth structure imaging, especially at the edges of the active rifts where the seismicity is scarce.

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New insights into structure and seismicity of the Central Alps from 3D Pg and Sg tomography and improved hypocenter relocations

10.5194/egusphere-egu21-2902 ◽

2021 ◽

Author(s):

Tobias Diehl ◽

Edi Kissling ◽

Marco Herwegh ◽

Stefan Schmid

Keyword(s):

Velocity Structure ◽

Focal Depth ◽

Small Scale ◽

Earthquake Catalog ◽

Depth Range ◽

Central Alps ◽

Quartz Content ◽

Phase Selection ◽

Velocity Inversion ◽

Velocity Models

Accuracy of hypocenter location, in particular focal depth, is a precondition for high-resolution seismotectonic analysis of natural and induced seismicity. For instance, linking seismicity with mapped fault segments requires hypocenter accuracy at the sub-kilometer scale. In this study, we demonstrate that inaccurate velocity models and improper phase selection can bias absolute hypocenter locations and location uncertainties, resulting in errors larger than the targeted accuracy. To avoid such bias in densely instrumented seismic networks, we propose a coupled hypocenter-velocity inversion restricted to direct, upper-crustal Pg and Sg phases. The derived three-dimensional velocity models, combined with dynamic phase selection and non-linear location algorithms result in a highly accurate earthquake catalog, including consistent hypocenter uncertainties. We apply this procedure to about 60&#8217;000 Pg and 30&#8217;000 Sg quality-checked phases of local earthquakes in the Central Alps region. The derived tomographic models image the Vp and Vs velocity structure of the Central Alps&#8217; upper crust at unprecedented resolution, including small-scale anomalies such as those caused by a Permo-Carboniferous trough in the northern foreland, Subalpine Molasse below the Alpine front or crystalline basement units within the Penninic nappes. The external Aar Massif is characterized by low Vp/Vs ratios of about 1.625-1.675 in the depth range of 2-6.5 km, which we relate to a felsic composition of the uplifted crustal block, possibly with increased quartz content. Finally, we discuss along-strike variations imaged by relocated seismicity in the Central Alps and demonstrate how joint interpretation of velocity structure and hypocenters provides additional constraints on lithologies of upper-crustal seismicity.

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Seismic Body-Wave Tomography in Fennoscandia

10.5194/egusphere-egu21-15798 ◽

2021 ◽

Author(s):

Nevra Bulut ◽

Valerie Maupin ◽

Hans Thybo

Keyword(s):

Velocity Structure ◽

Body Wave ◽

Low Frequency ◽

General Purpose ◽

Depth Range ◽

Quality Of Data ◽

S Waves ◽

Velocity Models ◽

The North ◽

Wave Tomography

We present a seismic tomographic image of Fennoscandia based on data from the ScanArray project in Norway, Sweden, and Finland, which operated during 2012-2017, together with data from earlier projects and stationary stations. We use relative traveltime residuals of P- and S- waves in high- and low-frequency bands and apply the frequency-dependent crustal correction. We use seismic signals from earthquakes at epicentral distances between 30&#176; and 104&#176; and magnitudes larger than 5.5. The general purpose of this study is to understand the possible causes of the high topography in Scandinavia along the passive continental margins in the North Atlantic as well as the interrelation between structure at the surface and in the lithospheric mantle.We present an upper-mantle velocity structure for most Fennoscandia derived for the depth range 50-800 km with a 3D multiscale parameterization for an inversion mesh-grid with dimensions dx=dy=17.38 km and dz=23.44 km. In all body-wave tomography methods, smearing of anomalies is expected. Therefore resolution tests are critical for assessing the resolution of the parameters determined in the velocity models. The resolution of the models depends on several factors, including the noise level and general quality of data, the density of observations, the distance and back-azimuthal distribution of sources, the damping applied, and the model parameterization. We use checkerboard and model-driven (block and cylindrical) tests for assessing the resolution of our models.Seismic models derived in this study are compared to existing and past topography to contribute to understanding mechanisms responsible for the topographic changes in the Fennoscandian region. The models also provide a basis for deriving high-resolution models of temperature and compositional anomalies that may contribute to understanding the observed, enigmatic topography.

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2019 Ridgecrest Earthquake Reveals Areas of Los Angeles That Amplify Shaking of High-Rises

Seismological Research Letters ◽

10.1785/0220200170 ◽

2020 ◽

Vol 91 (6) ◽

pp. 3370-3380

Author(s):

Monica D. Kohler ◽

Filippos Filippitzis ◽

Thomas Heaton ◽

Robert W. Clayton ◽

Richard Guy ◽

...

Keyword(s):

Los Angeles ◽

Strong Motion ◽

Ground Level ◽

Site Amplification ◽

Seismic Network ◽

Los Angeles Basin ◽

High Rise ◽

The North ◽

South Central ◽

San Fernando

Abstract The populace of Los Angeles, California, was startled by shaking from the M 7.1 earthquake that struck the city of Ridgecrest located 200 km to the north on 6 July 2019. Although the earthquake did not cause damage in Los Angeles, the experience in high-rise buildings was frightening in contrast to the shaking felt in short buildings. Observations from 560 ground-level accelerometers reveal large variations in shaking in the Los Angeles basin that occurred for more than 2 min. The observations come from the spatially dense Community Seismic Network (CSN), combined with the sparser Southern California Seismic Network and California Strong Motion Instrumentation Program networks. Site amplification factors for periods of 1, 3, 6, and 8 s are computed as the ratio of each station’s response spectral values combined for the two horizontal directions, relative to the average of three bedrock sites. Spatially coherent behavior in site amplification emerges for periods ≥3 s, and the maximum calculated site amplifications are the largest, by factors of 7, 10, and 8, respectively, for 3, 6, and 8 s periods. The dense CSN observations show that the long-period amplification is clearly, but only partially, correlated with the depth to basement. Sites with the largest amplifications for the long periods (≥3 s) are not close to the deepest portion of the basin. At 6 and 8 s periods, the maximum amplifications occur in the western part of the Los Angeles basin and in the south-central San Fernando Valley sedimentary basin. The observations suggest that the excitation of a hypothetical high-rise located in an area characterized by the largest site amplifications could be four times larger than in a downtown Los Angeles location.

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Natural Seismicity in and around the Rome Trough, Eastern Kentucky, from a Temporary Seismic Network

Seismological Research Letters ◽

10.1785/0220190015 ◽

2020 ◽

Vol 91 (3) ◽

pp. 1831-1845 ◽

Cited By ~ 1

Author(s):

N. Seth Carpenter ◽

Andrew S. Holcomb ◽

Edward W. Woolery ◽

Zhenming Wang ◽

John B. Hickman ◽

...

Keyword(s):

New York ◽

Oil And Gas ◽

Seismic Network ◽

Earthquake Activity ◽

Seismic Zone ◽

Injection Wells ◽

Eastern Kentucky ◽

The North ◽

Horizontal Compressive Stress ◽

Natural Seismicity

Abstract The Rome trough is a northeast-trending graben system extending from eastern Kentucky northeastward across West Virginia and Pennsylvania into southern New York. The oil and gas potential of a formation deep in the trough, the Rogersville shale, which is ∼1 km above Precambrian basement, is being tested in eastern Kentucky. Because induced seismicity can occur from fracking formations in close proximity to basement, a temporary seismic network was deployed along the trend of the Rome trough from June 2015 through May 2019 to characterize natural seismicity. Using empirical noise models and theoretical Brune sources, minimum detectable magnitudes, Mmin, were estimated in the study area. The temporary stations reduced Mmin by an estimated 0.3–0.8 magnitude units in the vicinity of wastewater-injection wells and deep oil and gas wells testing the Rogersville shale. The first 3 yr of seismicity detected and located in the study area has been compiled. Consistent with the long-term seismicity patterns in the Advanced National Seismic System Comprehensive Catalog, very few earthquakes occurred in the crust beneath the Rome trough—only three events were recorded—where the temporary network was most sensitive. None of these events appear to have been associated with Rogersville shale oil and gas test wells. Outside of the trough boundary faults, earthquakes are diffusely distributed in zones extending into southern Ohio to the north, and into the eastern Tennessee seismic zone to the south. The orientations of P axes from the seven first-motion focal mechanisms determined in this study are nearly parallel with both the trend of the Rome trough and with the orientation of maximum horizontal compressive stress in the region. This apparent alignment between the regional stress field and the strikes of faults in the trough at seismogenic depths may explain the relative lack of earthquake activity in the trough compared with the surrounding crust to the north and south.

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The Mesozoic-Tertiary evolution of the Aquitaine Basin

Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences ◽

10.1098/rsta.1982.0026 ◽

1982 ◽

Vol 305 (1489) ◽

pp. 63-84 ◽

Cited By ~ 28

Keyword(s):

Upper Cretaceous ◽

Oil Fields ◽

Upper Eocene ◽

The South ◽

Gas Field ◽

Massif Central ◽

Tectonic Plates ◽

The North ◽

Basic Magmatism ◽

Aquitaine Basin

The Aquitaine Basin, situated in southwest France, with an area of about 60 000 km 2 , has the form of a triangle which opens towards the Atlantic (Bay of Biscay) and is limited to the north by the Hercynian basement of Brittany and the Massif Central, and to the south by the Pyrenean Tertiary orogenic belt. Beneath the Tertiary sequence (2 km thick, and which outcrops over much of the basin) a Mesozoic series, up to 10 km thick, rests generally on a tectonized Hercynian basement but locally it covers narrow (NW-SE-trending) post-orogenic trenches of Stephano-Permian age. The Mesozoic history can be subdivided into four major structural-sedimentary episodes: (1) during a Triassic taphrogenic phase a continental-evaporitic complex developed with associated basic magmatism; (2) throughout the Jurassic, a vast lagoonal platform developed, initially (Lower Lias) as a thick evaporitic sequence followed by a uniform shale-carbonate unit, indicating a relative structural stability; (3) the end of the Jurassic and the Lower Cretaceous saw a fragmentation of this platform, due to an interplay between the Iberian and European tectonic plates, resulting in an ensemble of strongly subsident sub-basins; (4) during the Upper Cretaceous and until the end of the Neogene, the evolution of the Aquitaine Basin was influenced by the Pyrenean orogenic phase, with the development, towards the south, of a trench infilled by flysch which, from the Upper Eocene, is succeeded by a thick post-orogenic molasse complex. The main hydrocarbon objectives in the basin are situated in the Jurassic platform (e.g. the Lacq giant gas field) and the Cretaceous sub-basins (e.g. the Cazaux and Parentis oil fields). To date, production has been about 4 x 10 7 m 3 of oil, and about 15 x 10 10 m 3 of gas since the first gas discovery (St Marcet) in 1939.

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Effect of Combining Catalogs with Different Completeness

10.5194/egusphere-egu2020-1749 ◽

2020 ◽

Author(s):

Myunghyun Noh

Keyword(s):

Korean Peninsula ◽

North Korea ◽

Seismic Network ◽

The Other ◽

Earthquake Catalog ◽

Detection Capability ◽

Chi Square ◽

Earthquake Detection ◽

Seismic Networks ◽

Target Monitoring

In most seismic studies, we prefer the earthquake catalog that covers a larger region and/or a longer period. We usually combine two or more catalogs to achieve this goal. When combining catalogs, however, care must be taken because their completeness is not identical so that unexpected flaws may be caused.We tested the effect of combining inhomogeneous catalogs using the catalog of Korea Meteorological Administration (KMA). In fact, KMA provides a single catalog containing the earthquakes occurred in and around the whole Korean Peninsula. Like the other seismic networks, however, the configuration of the KMA seismic network is not uniform over its target monitoring region, so is the earthquake detection capability. The network is denser in the land than in the off-shore. Moreover, there are no seismic information available from North Korea. Based on these, we divided the KMA catalog into three sub-catalogs; SL, NL, and AO catalogs. The SL catalog contains the earthquakes occurred in the land of South Korea while the NL catalog contains those in the land of North Korea. The AO catalog contains all earthquakes occurred in the off-shore surrounding the peninsula.The completeness of a catalog is expressed in terms of mc, the minimum magnitude above which no earthquakes are missing. We used the Chi-square algorithm by Noh (2017) to estimate the mc. It turned out, as expected, that the mc of the SL is the smallest among the three. Those of NL and AO are comparable. The mc of the catalog combining the SL and AO is larger than those of individual catalogs before combining. The mc is largest when combining all the three. If one needs more complete catalog, he or she had better divide the catalog into smaller ones based on the spatiotemporal detectability of the seismic network. Or, one may combine several catalogs to cover a larger region or a longer period at the expense of catalog completeness.

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Bringing slow slip processes into focus

10.5194/egusphere-egu2020-18869 ◽

2020 ◽

Author(s):

Rebecca Bell

Keyword(s):

High Resolution ◽

Subduction Zones ◽

Seismic Velocity ◽

Numerical Models ◽

Fluid Pressure ◽

Geophysical Methods ◽

Rock Properties ◽

Slow Slip ◽

Velocity Models ◽

The North

The discovery of slow slip events (SSEs) at subduction margins in the last two decades has changed our understanding of how stress is released at subduction zones. Fault slip is now viewed as a continuum of different slip modes between regular earthquakes and aseismic creep, and an appreciation of seismic hazard can only be realised by understanding the full spectrum of slip. SSEs may have the potential to trigger destructive earthquakes and tsunami on faults nearby, but whether this is possible and why SSEs occur at all are two of the most important questions in earthquake seismology today. Laboratory and numerical models suggest that slow slip can be spontaneously generated under conditions of very low effective stresses, facilitated by high pore fluid pressure, but it has also been suggested that variations in frictional behaviour, potentially caused by very heterogeneous fault zone lithology, may be required to promote slow slip.Testing these hypotheses is difficult as it requires resolving rock properties at a high resolution many km below the seabed sometimes in km&#8217;s of water, where drilling is technically challenging and expensive. Traditional geophysical methods like travel-time tomography cannot provide fine-scale enough velocity models to probe the rock properties in fault zones specifically. In the last decade, however, computational power has improved to the point where 3D full-waveform inversion (FWI) methods make it possible to use the full wavefield rather than just travel times to produce seismic velocity models with a resolution an order of magnitude better than conventional models. Although the hydrocarbon industry have demonstrated many successful examples of 3D FWI the method requires extremely high density arrays of instruments, very different to the 2D transect data collection style which is still commonly employed at subduction zones.&#160;The north Hikurangi subduction zone, New Zealand is special, as it hosts the world&#8217;s most well characterised shallow SSEs (<2 km to 15 km below the seabed).&#160; This makes it an ideal location to collect 3D data optimally for FWI to resolve rock properties in the slow slip zone. In 2017-2018 an unprecedentedly large 3D experiment including 3D multi-channel seismic reflection, 99 ocean bottom seismometers and 194 onshore seismometers was conducted along the north Hikurangi margin in an 100 km x 15 km area, with an average 2 km instrument spacing. In addition, IODP Expeditions 372 and 375 collected logging-while drilling and core data, and deployed two bore-hole observatories to target slow slip in the same area. In this presentation I will introduce you to this world class 3D dataset and preliminary results, which will enable high resolution 3D models of physical properties to be made to bring slow slip processes into focus. &#160;

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Geodynamic study of the north of the Andes block (Colombia, Panama, Ecuador and Venezuela) through gnss-gps: models of displacements, models of deformation and definition of local and regional geodynamic structures.

10.5194/egusphere-egu2020-21362 ◽

2020 ◽

Author(s):

Berrocoso Manuel ◽

Del Valle Arroyo Pablo Emilio ◽

Colorado Jaramillo David Julián ◽

Gárate Jorge ◽

Fernández-Ros Alberto ◽

...

Keyword(s):

South America ◽

Seismic Activity ◽

Great Part ◽

Velocity Model ◽

South American ◽

The South ◽

South American Plate ◽

Northern Andes ◽

Velocity Models ◽

The North

The northwest of South America is conformed by the territories of Ecuador, Colombia and Venezuela. Great part of these territories make up the Northern Andes Block (BAN). The tectonic and volcanic activity in the northwest of South America is directly related to the interaction of the South American plate, and the Nazca and Caribbean plates, with the Maracaibo and Panama-Choc&#243; micro plates. The high seismic activity and the high magnitude of the recorded earthquakes make any study necessary to define this complex geodynamic region more precisely. This work presents the velocity models obtained through GNSS-GPS observations obtained in public continuous monitoring stations in the region. The observations of the Magna-eco network (Agust&#237;n Codazzi Geographic Institute) are integrated with models already obtained by other authors from the observations of the GEORED network (Colombian Geological Service). The observations have been processed using Bernese software v.52 using the PPP technique; obtaining topocentric time series. To obtain the speeds, a process of filtering and adjustment of the topocentric series has been carried out. Based on this velocity model, regional structures have been defined within the Northern Andes Block through a differentiation process based on the corresponding speeds of the South American, Nazca and Caribbean tectonic plates. Local geodynamic structures within the BAN itself have been established through cluster analysis based on both the direction and the magnitude of each of the vectors obtained. Finally, these structures have been correlated with the most significant geodynamic elements (fractures, faults, subduction processes, etc.) and with the associated seismic activity.

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