Imaging Seismic Wave-Fields with AlpArray and Neighboring European Networks

Abstract. The modern-day coverage and availability of broad-band stations in the greater Alpine area offered by AlpArray, Swath-D and the European seismological networks allows for imaging seismic wave-fields at yet unprecedented resolution. In the AlpArray area and in Italy, the distance of any point to the nearest station is less than 30 km, resulting in an average inter-station distance of about 45 km. With a much denser deployment in a smaller region of the Alps (320 km in length and 140 km wide), the Swath-D network possesses an average inter-station distance of about 15 km. We provide single event seismogram sections, time slices of teleseismic and regional wave-fields, and wave-field animations to reveal both the resolution capabilities of this dense station distribution as well as the enormous spatio-temporal complexity of seismic wave propagation. The time slices and wave-field animations demonstrate the need for dense regional arrays of broad-band stations, such as provided by AlpArray and neighboring networks, to resolve properties of teleseismic wave-fields. Here we present the images of coherent arrivals of direct body and surface waves, multiple body wave reflections, and multi-orbit phases for teleseismic and regional events with moment magnitudes larger than 6 over a time window of at least 2:45 hours. Spatial observations of the wave-fields illustrate e.g. the decrease in horizontal wavelength from P to S to surface waves and the way in which they considerably deviate from plane waves, due to heterogeneous earth structures along the path from the source to the array and beneath the regional array itself. Tomographic imaging techniques for the deep structure beneath the regional array have to take this spatio-temporal variability into account and correct for it. The lateral resolution of the regional broad-band array is however dependent on station density, in this case limited to about 100 km. Only even denser station distributions like those provided by Swath-D suffice to recover wave-fields of short period body and surface waves.