How do Local Earthquake Tomography and inverted dataset affect earthquake locations? The case study of High Agri Valley (Southern Italy) (original) (raw)
Related papers
2016
In a regional seismological network, the estimation of the epicenter is usually robust, especially for events inside or close to the network boundaries. In contrast, the hypocentral depth is very sensitive to the assumed velocity field. In this study, we compare the hypocenter estimates obtained by a classical algorithm in a simple one-dimensional (1D) model with a recently developed full 3D model that is based on shrinking grids. This study is preliminary, as the 3D Earth model is based on limited data from the literature; however, it demonstrates that different patterns show up when a more representative geological model is adopted. This encourages further studies, based on fully integrated 3D models from active surface seismic, well data and other geophysical measurements. Such an integrated approach has been successfully adopted by the oil and gas industries for decades, which has increased the exploration success rate and the production of hydrocarbon reservoirs. 1.
Annals of geophysics = Annali di geofisica
In a regional seismological network, the estimation of the epicenter is usually robust, especially for events inside or close to the network boundaries. In contrast, the hypocentral depth is very sensitive to the assumed velocity field. In this study, we compare the hypocenter estimates obtained by a classical algorithm in a simple one-dimensional (1D) model with a recently developed full 3D model that is based on shrinking grids. This study is preliminary, as the 3D Earth model is based on limited data from the literature; however, it demonstrates that different patterns show up when a more representative geological model is adopted. This encourages further studies, based on fully integrated 3D models from active surface seismic, well data and other geophysical measurements. Such an integrated approach has been successfully adopted by the oil and gas industries for decades, which has increased the exploration success rate and the production of hydrocarbon reservoirs.
2013
Introduction. Delay time tomography based on local earthquakes can provide a detailed three-dimensional image of seismic velocity structure in areas which are expected to be affected by strong earthquakes. Refined earthquake locations in a reliable velocity model, allow to detect and characterize crustal structures, and embedded active faults with a high seismogenic potential (Eberhart-Philipps, 1993). On the other hand, the availability of a velocity model and the seismicity distribution allow to study the relationship between the geometry and mechanical behavior of a fault or faults system and the physical properties of the host environment (Michael and Eberhart-Phillips, 1991). The accuracy of earthquake locations is strongly controlled by several factors, among which the geometry of the network, the number of available phases especially the capability of identifying both P and S phases, the accuracy of arrival time readings and the knowledge of the crustal structure (Pavlis, 1986). In addition, the use of 1D layered velocity models can introduce systematic errors in the estimation of P-and S-travel times due to the presence of large-scale three-dimensional heterogeneities in the propagation medium (Matrullo et al., 2013). In order to reduce the effect of using a 1D velocity model, relative locations and double differences techniques can be used. With these apporaches, the effect of a poor knowledge of the structure can be cancelled out for two events, very close in space, recorded at the same station travel as they travel following nearly the same path except nearby the source zone (Waldhauser and Ellsworth, 2000). However, Michelini and Lomax (2004) emphasized that systematic errors on earthquake location using double-differences may also be caused if the velocity model is not accurate. In this respect, a joint inversion of hypocenter and velocity parameters could allow to overcome the simplistic assumptions of the location methods mentioned above. The methods used today for the joint tomographic inversion have made substantial progress with respect to the basic theory originally developed by Aki and Lee (1990). Recent methods include efficient techniques for 3D ray tracing calculation using the eikonal equation (Vidale, 1990) for firstarrival traveltimes estimation on a given finite-difference grid in which the precision of traveltime calculation can be significantly improved by a successive integration of the slowness along the ray (e.g, Latorre et al., 2004). High-resolution imaging of the sub-surface with local earthquake data requires the use of large and consistent data sets of first arrival times. The quality and resolution of the medium image depends not only on the source/receiver coverage of the target region but also on the accuracy of the travel time measurements. The common procedure of reading the arrival time of a phase (picking) involves the manual measurement of P-and S-arrival times on recordings of a single event at a time. Systematic errors can be introduced due to inadequate working procedures such as the interaction between the process of picking and the result of the location. The inconsistency of the data can remain unnoticed when the events are analyzed independently from each other, but it may clearly appear when performing a joint determination of the hypocentral and velocity model parameters and reducing the inconsistency would require a complete picking revision. The knoweledge of the S-wave add important constraints to the earthquake location problem. The S-phase is important to derive physical parameters of the subsurfaces. The correct reading of the arrival times of these waves can be complicated by various factors, such as the superposition of the tail of the P-wave, the presence of converted waves generated at different interfaces, and the splitting of S-waves caused by seismic velocity anisotropy
Initial reference models in local earthquake tomography
Journal of Geophysical Research, 1994
The inverse problem of three-dimensional (3-D) local earthquake tomography is formulated as a linear approximation to a nonlinear function. Thus the solutions obtained and the reliability estimates depend on the initial reference model. Inappropriate models may result in artifacts of significant amplitude. Here, we advocate the application of the same inversion formalism to determine hypocenters and one-dimensional (l-D) velocity model parameters, including station corrections, as the first step in the 3-D modeling process. We call the resulting velocity model the minimum 1-D model. For test purposes, a synthetic data set based on the velocity structure of the San Andreas fault zone in central California was constructed. Two sets of 3-D tomographic P velocity results were calculated with identical travel time data and identical inversion parameters. One used an initial 1-D model selected from a priori knowledge of average crustal velocities, and the other used the minimum 1-D model. Where the data well resolve the structure, the 3-D image obtained with the minimum 1-D model is much closer to the true model than the one obtained with the a priori reference model. In zones of poor resolution, there are fewer artifacts in the 3-D image based on the minimum 1-D model. Although major characteristics of the 3-D velocity structure are present in both images, proper interpretation of the results obtained with the a priori 1-D model is seriously compromised by artifacts that distort the image and that go undetected by either resolution or covariance diagnostics.
Improving Seismic Event Location: An Alternative to Three-dimensional Structural Models
Monitoring the Comprehensive Nuclear-Test-Ban Treaty: Sourse Location, 2001
Ð We devise and apply a method to account for the eect of the aspherical structure of the Earth in locating earthquakes. This technique relies upon the ability to detect the average structural signal present in the residuals between source and receiver and correct for this signal during location, using a phenomenological description that we call Empirical Heterogeneity Corrections (EHC). EHC are employed in the relocation of a large set of well-constrained teleseismic earthquakes selected among the events reported by the Bulletins of the International Seismological Centre 1964±1995. The rms length of EHC relocation vectors for these events is about 10 km. The method is also tested against a selected set of ground-truth events, both earthquakes and explosions, whose locations are independently known by nonseismic means. The rms length of the mislocation vectors for the test events, compared to their original mislocation in the reference 1-D model SP6, is reduced in the EHC relocation by 17% for explosions and 12% for earthquakes. Our technique provides a successful alternative to the use of 3-D structural models, approximately reaching the same value of eectiveness in improving event location.
Swiss Journal of Geosciences, 2011
One-dimensional (1D) velocity models are still widely used for computing earthquake locations at seismological centers or in regions where three-dimensional (3D) velocity models are not available due to the lack of data of sufficiently high quality. The concept of the minimum 1D model with appropriate station corrections provides a framework to compute initial hypocenter locations and seismic velocities for local earthquake tomography. Since a minimum 1D model represents a solution to the coupled hypocenter-velocity problem it also represents a suitable velocity model for earthquake location and data quality assessment, such as evaluating the consistency in assigning pre-defined weighting classes and average picking error. Nevertheless, the use of a simple 1D velocity structure in combination with station delays raises the question of how appropriate the minimum 1D model concept is when applied to complex tectonic regions with significant three-dimensional (3D) variations in seismic velocities. In this study we compute one regional minimum 1D model and three local minimum 1D models for selected subregions of the Swiss Alpine region, which exhibits a strongly varying Moho topography. We compare the regional and local minimum 1D models in terms of earthquake locations and data quality assessment to measure their performance. Our results show that the local minimum 1D models provide more realistic hypocenter locations and better data fits than a single model for the Alpine region. We attribute this to the fact that in a local minimum 1D model local and regional effects of the velocity structure can be better separated. Consequently, in tectonically complex regions, minimum 1D models should be computed in sub-regions defined by similar structure, if they are used for earthquake location and data quality assessment.
Geophysical Journal International, 2006
A seismic array of 11 stations maintained for 9 years-from 1985 to 1994-provided data that were exploited for the investigation of the 3-D structure of the upper crust in the Gioia Tauro basin, southwestern Calabria, Italy. A data set of 207 local earthquakes that were located with a minimum of seven observations, a traveltime residual of ≤0.3 s rms and an azimuthal gap ≤180 • was used to compute first a minimum 1-D velocity model for the Gioia Tauro region, which served as initial model for the subsequent 3-D inversion. The inversion for 3-D P-velocity crustal structure was performed by iteratively solving the coupled hypocentre-velocity problem in a least-squares sense. Careful analysis of the resolution capability of our data set outlines the well-resolved features for interpretation. The resulting 3-D velocity model of the Gioia Tauro region shows, in the upper crust, generally higher average velocities in the south and lower average velocities in the north. A pronounced northeast-southwest trending zone of low velocities in the upper 12 km was identified in the area of the Cittanova fault zone (CFZ). Because of the limited dimensions of the seismic array the half graben of Gioia Tauro basin could not be recognized in the tomographic images.
Tectonophysics, 2016
The region between the inner zones of the Alps and Corsica juxtaposes an overthickened crust to an oceanic domain, which makes difficult to ascertain the focal depth of seismic events using routine location codes and average 1D velocity models. The aim of this article is to show that, even with a rather lose monitoring network, accurate routine locations can be achieved by using realistic 3D modelling and advanced location techniques. Previous earthquake tomography studies cover the whole region with spatial resolutions of several tens of kilometres on land, but they fail to resolve the marine domain due to the absence of station coverage and sparse seismicity. To overcome these limitations, we first construct a 3D a-priori P and S velocity model integrating known geophysical and geological information. Significant progress has been achieved in the 3D numerical modelling of complex geological structures by the development of dedicated softwares (e.g. 3D GeoModeller), capable at once of elaborating a 3D structural model from geological and geophysical constraints and, possibly, of refining it by inversion processes (Calcagno et al., 2008). Then, we build an arrival-time catalogue of 1500 events recorded from 2000 to 2011. Hypocentres are then located in this model using a numerical code based on the maximum intersection method (Font et al., 2004), updated by Theunissen et al. (2012), as well as another 3D location technique, the NonLinLoc software (Lomax and Curtis, 2001). The reduction of arrival-time residuals and uncertainties (dh, dz) with respect to classical 1D locations demonstrates the improved accuracy allowed by our approach and confirms the coherence of the 3D geological model built and used in this study. Our results are also compared with previous works that benefitted from the installation of dense temporary networks surrounding the studied epicentre area. The resulting 3D location catalogue allows us to improve the regional seismic hazard assessment, more particularly in the south of the Argentera massif and in the Ligurian basin.
Empirical criteria for the accuracy of earthquake locations on the Croatian territory
Geofizika, 2017
This paper presents the empirically based ground truth criteria, or shorter GT criteria, for the estimation of the epicentral location accuracy of the seismic events recorded at network stations within 400 km around the city of Zagreb. The criteria are based only on the network coverage metrics and the GT5 level represents an absolute location error lower than 5 km. They have been developed using a bootstrap resampling method: same earthquakes have been relocated many times but with different, randomly selected seismic stations. We used 330 reference events taken from the pages of ISC (ISC Reference Event Bulletin, 2008) and showed that the location accuracy is most affected by the distance to the farthest station in the seismic network, while not at all influenced by the distance to the nearest. The developed GT criteria for GT5 95% level of accuracy require 10 or more network stations, all within 125 km from the epicentre, and the secondary azimuthal gap (the largest gap when any given station is removed from the network) less than 200°, or the network quality metric (the deviation between the optimal uniformly distributed network and the actual network) less than 0.41. The obtained results revealed that the global criteria are too restrictive and unsuitable for the studied area since they require more regular networks. With our criteria, it is possible to achieve higher accuracy for the networks with a bigger secondary azimuthal gap or greater network quality metric. In addition, our criteria limitations are shown for the areas with simpler geological structure.