No abstract

S U M M A R YWe establish reliable and conservative estimates for epicentre location accuracy using data that are readily available in published seismic bulletins. A large variety of seismic studies rely on catalogues of event locations, making proper assessment of location uncertainty critical. Event location and uncertainty parameters in most global, regional and national earthquake catalogues are obtained from traditional linearized inversion methods using a 1-D Earth model to predict traveltimes. Reported catalogue uncertainties are based on the assumption that error processes are Gaussian, zero mean and uncorrelated. Unfortunately, these assumptions are commonly violated, leading to the underestimation of true location uncertainty, especially at high confidence levels. We find that catalogue location accuracy is most reliably estimated by station geometry. We make use of two explosions with exactly known epicentres to develop local network location (0 • -2.5 • ) accuracy criteria. Using Monte Carlo simulations of network geometry, we find that local network locations are accurate to within 5 km with a 95 per cent confidence level when the network meets the following criteria: (1) there are 10 or more stations, all within 250 km, (2) an azimuthal gap of less than 110 • , (3) a secondary azimuthal gap of less than 160 • and (4) at least one station within 30 km. To derive location accuracy criteria for near-regional (2.5 • -10 • ), regional (2.5 • -20 • ) and teleseismic (28 • -91 • ) networks, we use a large data set of exceptionally well-located earthquakes and nuclear explosions. Beyond local distances, we find that the secondary azimuthal gap is sufficient to constrain epicentre accuracy, and location error increases when the secondary azimuthal gap exceeds 120 • . When station coverage meets the criterion of a secondary azimuth gap of less than 120 • , near-regional networks provide 20 km accuracy at the 90 per cent confidence level, while regional and teleseismic networks provide 25 km accuracy at the 90 per cent confidence level.

S U M M A R YThe International Seismological Centre (ISC) is a non-governmental, non-profit organization with the primary mission of producing the definitive account of the Earth's seismicity. The ISC Bulletin covers some 50 yr of seismicity. The recent years have seen a dramatic increase both in the number of reported events and especially in the number of reported phases, owing to the ever-increasing number of stations worldwide. Similar ray paths will produce correlated traveltime prediction errors due to unmodelled heterogeneities in the Earth, resulting in underestimated location uncertainties, and for unfavourable network geometries, location bias. Hence, the denser and more unbalanced the global seismic station coverage becomes, the less defensible is the assumption (that is the observations are independent), which is made by most location algorithms.To address this challenge we have developed a new location algorithm for the ISC that accounts for correlated error structure, and uses all IASPEI standard phases with a valid ak135 traveltime prediction to obtain more accurate event locations. In this paper we describe the new ISC locator, and present validation tests by relocating the ground truth events in the IASPEI Reference Event List, as well as by relocating the entire ISC Bulletin.We show that the new ISC location algorithm provides small, but consistent location improvements, considerable improvements in depth determination and significantly more accurate formal uncertainty estimates. We demonstrate that the new algorithm, through the use of later phases and testing for depth resolution, considerably clusters event locations more tightly, thus providing an improved view of the seismicity of the Earth.

The AlpArray programme is a multinational, European consortium to advance our understanding of orogenesis and its relationship to mantle dynamics, plate reorganizations, surface processes and seismic hazard in the Alps-Apennines-Carpathians-Dinarides orogenic system. The AlpArray Seismic Network has been deployed with contributions from 36 institutions from 11 countries to map physical properties of the lithosphere and asthenosphere in 3D and thus to obtain new, high-resolution geophysical images of structures from the surface down to the base of the mantle transition zone. With over 600 broadband stations Electronic supplementary material The online version of this article (https ://doi.org/10.1007/s1071 2-018-9472-4) contains supplementary material, which is available to authorized users. operated for 2 years, this seismic experiment is one of the largest simultaneously operated seismological networks in the academic domain, employing hexagonal coverage with station spacing at less than 52 km. This dense and regularly spaced experiment is made possible by the coordinated coeval deployment of temporary stations from numerous national pools, including ocean-bottom seismometers, which were funded by different national agencies. They combine with permanent networks, which also required the cooperation of many different operators. Together these stations ultimately fill coverage gaps. Following a short overview of previous large-scale seismological experiments in the Alpine region, we here present the goals, construction, deployment, characteristics and data management of the AlpArray Seismic Network, which will provide data that is expected to be unprecedented in quality to image the complex Alpine mountains at depth.

No abstract

No abstract

SUMMARYRecently developed three-dimensional global seismic-velocity models have demonstrated location improvements through independent regional and teleseismic travel-time calibration. Concurrently, a large set of high quality ground truth (GT) events with location accuracies 10 km or better (GT0-GT10) has been collected for Europe, the Medite rranean, North Africa, the Middle East, and Western Eurasia. In this study, we validate event location improvements using this new data set by applying the regional and teleseismic modelbased travel-time calibrations (independently and jointly) to demons trate that significant improvements can be achieved using 3D global models for locating small events with sparse network data. Besides relocating events using all station arrivals, a subset of the GT events was also relocated using controlled station geom etries generated from a "constrained bootstrapping" technique. The advantages of this approach include: (1) generating simulated sparse networks (Simulated Sparse Network Bulletin or SSNB), (2) increasing the statistical power of the tests, (3) reducing the effect of correlated errors to ensure valid 90% error ellipse coverage statistics, and (4) measuring location bias due to un-modeled three-dim ensional (3D) Earth structures.With respect to the GT events, we compared event relocations, with and without t raveltime calibrations, considering statistics of mislocation, error ellipse area, 90% coverage, origin time bias, origin time errors, and misfit. Relocations of more than 1000 GT0-GT10 reference events show significant reductions in location bias and uncertainty. Pn and/or P calibration reduces mislocation for between 60% and 70% of the events. Joint regional Pn and teleseismic P travel-time calibration provided the largest location improvements, and approximately achieved GT5 accuracy levels. Due to correlated 3 errors, calibrated event locations using large numbers of stations have deficient 90% error ellipse coverage. However, the coverages derived from the model errors are appropriate for sparse regional and teleseismic networks. This validation effort demonstrates that the global model-based travel-time calibrations of Pn and teleseismic P travel-time reduce both location bias and uncertainty over wide areas.

No abstract