No abstract

In this study, we analyse the 410-km and 660-km upper-mantle transition zone discontinuities as seen from seismic P-to-s wave conversions beneath the Eurasian-African Plate boundary at south Spain and north Morocco. For this purpose we use teleseismic events recorded at 43 broad-band seismic stations deployed mainly by the TopoIberia project. The conversions from the upper-mantle discontinuities arrive in the P-wave coda together with other signals and are usually identified on stacked receiver functions. We build a new processing approach which is leaned on receiver functions and which is based on cross-correlation and stacking techniques to efficiently detect and extract signals by means of their coherence, slowness, traveltime and polarity. In order to add consistency and robustness to the detections, our final results are based on a joint analysis of two different cross-correlation functionals and receiver functions. This permits to assess errors and to bridge observation gaps due to the breakdown of any of the techniques inherent to signal and noise characteristics. Finally, discontinuity depths are determined using time corrections obtained from a 3-D velocity model. We present topography maps for the 410-km and 660-km discontinuities which show a thickening of the transition zone beneath the plate boundary towards Morocco. The transition zone thickness is about global average beneath south Spain (240-250 km) and is thicker beneath east Morocco (250-275 km). This is mainly due to a deeper 660-km discontinuity, while the topography of the 410-km discontinuity is smaller. In the Alboran Sea we find an up to 25 km deflection of the 660-km discontinuity which suggests that the Alboran Sea heterogeneity or slab is still sufficiently cold to depress the post-spinel phase transition. We finally discuss the results in order to add new constraints on temperature and composition to seismic velocity anomalies observed in the transition zone beneath south Spain and north Morocco.

Abstract. Within the framework of the European collaborative research initiative AlpArray (http://www.alparray.ethz. ch), the Istituto Nazionale di Geofisica e Vulcanolgia (INGV) deployed overall 20 broad-band seismic stations in Northern Italy and on two islands in the Tyrrhenian Sea (Capraia and Montecristo) during Fall-Winter 2015.The temporary deployment (16 stations) will run for two to three years and 4 INGV National Seismic Network accelerometric sites are now equipped with additional permanent broad-band sensors. The 16 temporary stations are equipped with REF TEK 130 digitizers and Nanometrics Trillium Compact 120 s sensors, a couple have Nanometrics Trillium 120P sensors and one a Streckeisen STS2.For each site we describe the settings and discuss the noise levels, the site effects and the preliminary sensitivity analysis.

We investigate the mantle transition zone beneath the Chile‐Argentina flat subduction region by means of P‐to‐S conversions at mantle discontinuities from teleseismic events recorded at 103 seismic stations. From the analysis of receiver functions, we obtain clear converted phases from the 410 and 660 discontinuities, and we identify a robust precursory signal to P660s, of negative amplitude, that we name P590s. We observe little frequency dependence in the amplitude of the P410s converted phase, while the P660s is less visible toward higher frequencies. The 410 is on average deeper than 410 km by 10 ± 1 km in the higher‐frequency bands, and it is relatively sharp, being consistent with a 10% velocity jump over less than 20 km. The observed 660 depth varies with frequency; it is deeper by up to 18 ± 2 km for lower frequencies and close to reference at higher frequencies, being consistent with a 13% broad velocity gradient over 30–40 km, probably caused by a composite of multiple phase transitions. The transition zone thickness is controlled by the frequency‐dependent depth variability of the 660. Our findings of relative depth, width, and velocity jump of the detected discontinuities, combined with tomographic images of the mantle transition zone, cannot be explained by thermal variations alone. Compositional constraints from mineral physics show that a near pyrolitic mantle is consistent with the ratio of the estimated velocity jumps. However, the negative P590s phase in this region could be signal from the velocity reduction due to basalt accumulation at the base of the transition zone.

Abstract. Since the 1980s a number of active and passive seismic experiments have revealed significant information about the Earth’s crust in the broader European Alpine region. In this paper, we use seismic waveform data from the AlpArray Seismic Network and three other temporary seismic networks, to perform receiver function (RF) calculations and time−to−depth migration to update the knowledge of the Moho discontinuity beneath the broader European Alps. In particular, we set up a 5 homogeneous processing scheme to compute RFs using the time-domain iterative deconvolution method, and apply consistent quality control to yield 107,633 high-quality RFs. We then perform time−to−depth migration in a newly implemented 3D spherical coordinate system and using a European-scale reference P and S wave velocity model. This approach, together with the dense data coverage, provide us with a 3D migrated volume, from which we present migrated profiles that reflect the first-order crustal thickness structure. We create a detailed Moho map by manually picking the discontinuity in a set of orthogonal 10 profiles covering the entire area. We make the RF dataset, the software for the full processing workflow, as well as the Moho map, openly available; these open-access datasets and results will allow other researchers to build on the current study.