Imaging Seismic Wave-Fields with AlpArray and 1 Neighboring European Networks

4 The modern-day coverage and availability of broad-band stations in the greater Alpine area offered by AlpArray, 5 Swath-D and the European seismological networks allows for imaging seismic wave-fields at yet unprecedented 6 resolution. In the AlpArray area and in Italy, the distance of any point to the nearest station is less than 30km, 7 resulting in an average inter-station distance of about 45km. With a much denser deployment in a smaller region 8 of the Alps (320km in length and 140km wide), the Swath-D network possesses an average inter-station distance 9 of about 15km. 10 We provide single event seismogram sections, time slices of teleseismic and regional wave-fields, and wave-field 11 animations to reveal both the resolution capabilities of this dense station distribution as well as the enormous 12 spatio-temporal complexity of seismic wave propagation. The time slices and wave-field animations demonstrate 13 the need for dense regional arrays of broad-band stations, such as provided by AlpArray and neighboring networks, 14 to resolve properties of teleseismic wave-fields. Here we present the images of coherent arrivals of direct body and 15 surface waves, multiple body wave reflections, and multi-orbit phases for teleseismic and regional events with 16 moment magnitudes larger than 6 over a time window of at least 2:45 hours. 17 Spatial observations of the wave-fields illustrate e.g. the decrease in horizontal wavelength from P to S to surface 18 waves and the way in which they considerably deviate from plane waves, due to heterogeneous earth structures 19 along the path from the source to the array and beneath the regional array itself. Tomographic imaging techniques 20 for the deep structure beneath the regional array have to take this spatio-temporal variability into account and 21 correct for it. 22 The lateral resolution of the regional broad-band array is however dependent on station density, in this case 23 limited to about 100km. Only even denser station distributions like those provided by Swath-D suffice to recover 24 wave-fields of short period body and surface waves. 25

In the planning years of the AASN, the number of existing permanent stations with accessible data increased by 54 50%. The corresponding plans for the temporary station sites evolved with time, to ultimately leave no point in 55 the Alps and its surroundings (a 250km wide region from the foothills) farther than 30km away from a broad-band 56 station (≥ 30 sec lower corner frequency). 57 In newly covered areas, hexagonal coverage of temporary sites was applied, resulting in an average distance of 58 52km from a site to the neighboring 6 sites, which is tighter and more compact than previous large networks  The aim of the AlpArray experiment is to image the deep structure of the Alps and to understand the effects of 65 collisional mountain building on a larger scale. The Alps have been the focus of geological research for centuries, 66 with concepts like nappe stacking and subduction first being introduced for the Alpine orogeny (Faccenna et al.,67 2001, Piromallo and Faccenna, 2004, Vignaroli et al., 2008, 2009, Handy et al., 2010. In order to understand the 68 driving forces of mountain building, the slab geometry and deep crustal structure have to be revealed. Because of 69 the small lateral and highly curved geometries, this remains a challenge. Furthermore, major ambiguities regarding 70 the presence of slab segments, slab gaps, and slab polarity switches might be resolved using advanced seismological  for Swath-D (cf. Fig. 1). We show single-event seismogram sections, time slices of the wave-fields for specific 77 phase arrivals, and wave-field animations (cf. Supplementary Materials) for long time windows to illustrate the 78 capabilities of dense regional and local broad-band arrays. In the main text each event is discussed in detail, 79 identifying relevant phases, and describing observed spatial properties of the wave-field.   Tab  All traces are detrended, instrument response-corrected, band-pass filtered between 100 -500s, and resampled to 101 1Hz if necessary. They are technically not perfectly aligned as their start times may be offset from one to another 102 by up to half a sample width due to station effects, but that is negligible as for the purpose of these depictions.

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Each station's start time is therefore rounded to the nearest integer second for realignment purposes.

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After R2 a number of body wave arrivals with more than one around-the-globe orbit enter again from the northeast.

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They are visible in the time range between ca. 145 min and 160 min. These include 8S, then 9S, followed by even 136 higher order S reflections and their respective closely related P phases. To the best of our knowledge these are the 137 first direct observations of such phases in single-event datasets.
138 Fig. 2 shows that also small and late arrivals can be detected by a dense array using recordings of just one event.

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Animations 140 The lateral variability of the wave-fields cannot be adequately represented via a seismogram section, hence a more 141 spatial view of the data is needed, as for example in Fig. 3 -6, 8 -12, and 14.

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In order to make use of the full dynamic range of the color scale at any time, we apply the same time-dependent 143 normalization of the seismograms at each station as for the section (Fig. 2). Additionally, the traces are band-pass 144 filtered down to a minimum period of 20 sec to ensure a good signal-to-noise ratio and coherent phase arrivals 145 given the available stations density. The traces are cut to a length of 220 min starting 10 min before source time 146 for local events and to a length of 165 min starting at source time for teleseismic events.

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Note that the only quality control metric that was used, is a basic percentage threshold regarding the number of 148 samples that must be present in the individual traces for them to be used. Data from stations missing more than 149 10% of their samples are discarded.

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(1) Taiwan (C201802061550A) 164 We start the discussion of the animations with an event located to the east of the network. It took place in Taiwan in the Alps.

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Note that the small circles indicated in Fig. 3 -7 seem to suggest the event being located southwest of the 170 observed region due to their curvature. This is not the case, it is merely an artifact of the projection that they 171 appear to curve away from the source. The source itself is furthermore located at a latitude slightly lower then At about 130 min the returning surface wave R2 emerges over the array from the southwest. It again lacks the 204 high frequencies of R1 (cf. Fig. 7) and exhibits strong dispersion over the course of its arrival, which lasts until 205 ca. 145 min.

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Note that time slices of this kind can also be useful to spot either polarity or timing errors in individual stations.

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Both the figures shown here and the animations clearly point out a few out-of-phase traces, particularly during 208 long wavelength arrivals such as P. Fig. 3 shows a few cases near Venice (red), over the Dinarides (blue), and in 209 southern Germany (blue), for example.

222
For the second event discussed here, the source is located in the South Atlantic, southwest off the coast of Africa, 223 ca. 98 • away from the center of the Alps, with a moment magnitude of M w 6.5 (cf. Tab. 1).

224
The animation starts with ca. 14 min of ambient background signal to be observed before any phases arrive.

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It is mostly comprised of random noise and appears incoherent, though some small-scale coherent wave-fields do phases with about 6 • -7 • . Interestingly, the expected transition from P to P diff occurs at about the reference 231 station at the center of the displayed region (cf. Fig. 8 -12). The wave-front seems largely aligned with the 232 theoretical wave-front (cf. thin gray lines in Fig. 8). It is closely followed by PP and PPP at ca. 18 min and wave. These phases are notably more sensitive to 3D structure, sometimes reaching the array at an oblique angle 247 compared to the theoretical great circle path, which is particularly obvious within the Swath-D network.

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The surface wave R1 reaches the array at ca. 48 min (cf.

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Unlike the body waves, its horizontal wavelength and velocity remain unaffected by the close proximity to the 295 source, because its ray path is bound to the surface, thus leading to a constant angle of incidence regardless of the 296 distance. The coda is very short and at about 5 min the Rayleigh wave is no longer visible as well.

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After R1 the data quickly returns to noise, as there are no phases reaching the array until the arrival of waves that 298 have completed a full global orbit. Noise remains prevalent for about 1 hour. The first returning phases are seen 299 at ca. 62 min, though they do not appear as distinct wave-fronts but rather as highly scattered and deformed 300 wavelets. This is expected as the superposition of arrivals collapsing back into the source from practically every 301 direction after circling the entire earth is bound to be strongly affected by global 3D structure and the earth's 302 ellipticity. This effect would probably be even more pronounced for larger magnitude earthquakes, though none 303 occured during operation of the AlpArray network.

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The arrival of scattered returning higher order reflections continues for over 100 min. At some points the wave-305 field does coalesce into a semi-coherent ring-like wave-front (e.g. at 90 min, likely 10P2S and higher), but most of 306 the time it is rather fragmented, sometimes even seemingly forming standing waves between simultaneous arrivals 307 from opposite directions (e.g. at 110 min, likely 6S and related P reflections).

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The returning Rayleigh wave R2 is seen from ca. 168 min onwards. It first enters from the north as an almost 309 planar, surprisingly intact wave-front, slowly shifting to a south-easterly direction of propagation over the next  The standard deviation (Fig. 21) shows several similar features in Central Italy and Sicily as the previous events.

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R2 is slightly less prominent as a result of its longer travel path and the subsequently more pronounced attenuation.

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It therefore lacks the clear indicators of dispersion that can be observed in the other cases.

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The standard deviation (Fig. 22)  dense arrays are necessary to understand these wave-fields and consequently sparse spatial sampling may lead to 358 severe aliasing that can not be overcome by any imaging techniques. That means large and dense regional arrays 359 represent a prerequisite to measure properties of teleseismic wave-fields.

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It remains however a challenge to extract information on the local structure, because the wave-fields are strongly  Because of the lateral extent of the regional array, it also covers the source region for some events. Therefore, 371 wave-fields in source regions as well as wave propagation over regional distances may be directly observed without 372 spatial gaps. Also, smaller events can be detected and source parameters can be determined using a large number 373 of recordings at close distances.

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In addition to large regional arrays, very dense deployments of broad-band stations like the Swath-D can help to 375 reduce the shortest resolvable wavelength considerably further, however only in smaller regions. Wave-fields of 376 short period and scattered waves can be adequately measured by dense deployments as provided by the Swath-D 377 network. For example, coherent wave-fields in the surface wave coda wave train can only be detected by Swath-D 378 but not the rest of the regional array.

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Finally, it is worth mentioning that the deployment of large, dense regional arrays points also to the importance of 380 a consequent quality control. In particular, timing errors as well as false information on the sensors' properties have 381 to be detected and corrected in order to use the full potential of the arrays, including amplitude information. The 382 obtained data set will be the basis for an improved understanding of seismic wave-fields in a strongly heterogeneous 383 region as well as of the deep structure of the Alpine orogen.