Abstract
Abstract. We present an extensive dataset of highly accurate absolute travel times and travel-time residuals of teleseismic P waves recorded by the AlpArray Seismic Network and complementary field experiments in the years from 2015 to 2019. The dataset is intended to serve as the basis for teleseismic travel-time tomography of the upper mantle below the greater Alpine region. In addition, the data may be used as constraints in full-waveform inversion of AlpArray recordings. The dataset comprises about 170 000 onsets derived from records filtered to an upper-corner frequency of 0.5 Hz and 214 000 onsets from records filtered to an upper-corner frequency of 0.1 Hz. The high accuracy of absolute and residual travel times was obtained by applying a specially designed combination of automatic picking, waveform cross-correlation and beamforming. Taking travel-time data for individual events, we are able to visualise in detail the wave fronts of teleseismic P waves as they propagate across AlpArray. Variations of distances between isochrons indicate structural perturbations in the mantle below. Travel-time residuals for individual events exhibit spatially coherent patterns that prove to be stable if events of similar epicentral distance and azimuth are considered. When residuals for all available events are stacked, conspicuous areas of negative residuals emerge that indicate the lateral location of subducting slabs beneath the Apennines and the western, central and eastern Alps. Stacking residuals for events from 90∘ wide azimuthal sectors results in lateral distributions of negative and positive residuals that are generally consistent but differ in detail due to the differing direction of illumination of mantle structures by the incident P waves. Uncertainties of travel-time residuals are estimated from the peak width of the cross-correlation function and its maximum value. The median uncertainty is 0.15 s at 0.5 Hz and 0.18 s at 0.1 Hz, which is more than 10 times lower than the typical travel-time residuals of up to ±2 s. Uncertainties display a regional dependence caused by quality differences between temporary and permanent stations as well as site-specific noise conditions.
Highlights
The recently acquired AlpArray dataset provides a fascinating opportunity to extend our knowledge on the structure of the upper mantle below the greater Alpine area, and in particular to answer long-standing questions regarding the orientation and penetration of lithospheric slabs, their connection to the well-studied crustal structure, and their influence on surface processes
The earliest complementary experiment, partly included in our dataset, is the Eastern Alpine Seismic Investigation (EASI) project with 55 stations deployed on a north– south profile at 13.4◦ E, crossing the Alps from the northern Alpine foreland to the Adriatic Sea, which recorded ground motions for more than a year until August 2015
We found that theoretical phase onsets can differ from actual arrivals by up to some tens of seconds, most probably owing to differences of the true physical properties in the global earth from those of the spherically symmetric earth model, uncertainties in origin time, dispersion processes along the travel path and the use of centroid times of the gCMT catalogue as earthquake origin times
Summary
The recently acquired AlpArray dataset provides a fascinating opportunity to extend our knowledge on the structure of the upper mantle below the greater Alpine area, and in particular to answer long-standing questions regarding the orientation and penetration of lithospheric slabs, their connection to the well-studied crustal structure, and their influence on surface processes. Using AlpArray data, we demonstrate that an appropriate combination of automatic picking, correlation measurements and beamforming can attain the required accuracy and provide both reliable travel-time residuals and absolute travel times Applying this technique, we are able to map the propagation of P wave fronts across the AlpArray network and to obtain sufficiently accurate travel-time residuals at all stations of the network. Stacking of eventspecific travel-time residuals yields very stable patterns that already indicate the approximate location of high- and lowvelocity anomalies in the upper mantle prior to any tomographic inversion We shall use these time measurements in a later study for performing a teleseismic tomography and full waveform inversion
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