ALBERT

Description: RV SONNE cruise SO244-2 sailed offshore northern Chile from Nov. 27 to Dec. 13, 2015 to install the seafloor geodetic network GeoSEA (Geodetic Earthquake Observatory on the SEAfloor) on the marine forearc and outer rise of the South American subduction system around 21°S. This segment of the Nazca-South American plate boundary has last ruptured in an earthquake in 1877 and was identified as a seismic gap prior to the 2014 Iquique/Pisagua earthquake (Mw=8.1). The southern portion of the segment remains unbroken by a recent earthquake and is currently in the latest stage of the interseismic phase of the seismic cycle. Seafloor geodetic measurements provide a way to monitor crustal deformation at high resolution comparable to the satellite-based GPS technique upon which terrestrial geodesy is largely based. The GeoSEA Network consists of autonomous seafloor transponders installed on 4 m high tripods, which were lowered to the seabed on the deep-sea cable of RV SONNE. The transponders within an array intercommunicate via acoustic signals for a period of up to 3.5 years. An additional component of the network is GeoSURF, a self-steering autonomous surface vehicle (Wave Glider), which monitors system health and is capable to upload the seafloor data to the sea surface and to transfer it via satellite. We have chosen three areas on the middle and lower slope and the outer rise for the set-up of three sub-arrays. The array in Area 1 on the middle continental slope consists of 8 transponders located in pairs on four topographic ridges, which are surface expressions of faults at depth. Area 2 is located on the outer rise seaward of the trench where 5 stations monitor extension across plate-bending related normal faults. The third area is located at water depth 〉5000 m on the lower continental slope where an array of 10 stations measures diffuse strain build-up. Data from all networks and all stations were successfully uploaded to GeoSURF and/or a HPT modem lowered into the water from RV SONNE. The seabed installation of a total of 23 transponders was completed by December 07, when we proceeded to deploy a total of 14 ocean bottom seismometers (OBS) on the forearc between 19.2°-21.6°S. These instruments will be recovered by RV LANGSETH in Spring/Summer 2016.

Description: 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 homogeneous processing scheme to compute RFs using the time-domain iterative deconvolution method and apply consistent quality control to yield 112 205 high-quality RFs. We then perform time-to-depth migration in a newly implemented 3D spherical coordinate system 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 profiles covering the entire area. We make the RF dataset, the software for the entire 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. How to cite. Michailos, K., Hetényi, G., Scarponi, M., Stipčević, J., Bianchi, I., Bonatto, L., Czuba, W., Di Bona, M., Govoni, A., Hannemann, K., Janik, T., Kalmár, D., Kind, R., Link, F., Lucente, F. P., Monna, S., Montuori, C., Mroczek, S., Paul, A., Piromallo, C., Plomerová, J., Rewers, J., Salimbeni, S., Tilmann, F., Środa, P., Vergne, J., and the AlpArray-PACASE Working Group: Moho depths beneath the European Alps: a homogeneously processed map and receiver functions database, Earth Syst. Sci. Data, 15, 2117–2138, https://doi.org/10.5194/essd-15-2117-2023, 2023.

Description: The network consists of a vertical borehole array equipped with 3C sensors (geophones) for the analysis of swarm earthquakes in the Western Bohemia / Vogtland area located in the German/Czech border region. A surface array is completing the 3D observation of the wave field with 3C sensors (geophones). Waveform data is available from the GEOFON data centre, under network code 6A, and is embargoed until FEB 2035.

Description: The ICDP project "Drilling the Eger Rift" focuses on geodynamic processes in W-Bohemia and Vogtland, such as earthquakes and subsurface fluid flows. Therefore, three boreholes have been drilled and will be instrumented with 3D seismic arrays. The pilot 3D array has 10 borehole geophones with a 10 Hz corner frequency and is surrounded by a surface array with 12 4.5 Hz geophones. The data is recorded by Earth Data Loggers with a sampling rate of 1000 Hz. The goal is to locate seismic noise sources up to a distance of 15 km that may be linked to fluid migration in W-Bohemia. We analyze continuous seismic noise records of more than 20 stations from regional networks and our seismic array. Differential PSDs were calculated in different frequency ranges to find continuous tremor-like seismic sources before the earthquake swarm in April 2021. Our next step is to locate these sources using regional networks and the 3D seismic array. The analysis revealed an increase in seismic noise between 4-8 Hz for a 2-hour period on April 5th, 2021, one day before the earthquake swarm started. Seven stations in the region show a median differential PSD level at least three times higher than the differential PSD level for the entire day. These stations are located up to a distance of 12 km to the Nový Kostel focal zone, where most of the earthquakes occurred. The increase is also compared to Radon concentrations measured at Hartoušov (W-Bohemia) to find possible coincidence.

Description: Superficial geological layers can strongly modify the surface ground motion induced by an earthquake. These so‐called site effects are highly variable from one site to another and still difficult to quantify for complex geological configurations. That is why site‐specific studies can greatly contribute to improve the hazard prediction at a specific site. However, site‐specific studies have historically been considered difficult to carry out in low‐to‐moderate seismicity regions. We present here seismological datasets acquired in the framework of the French–German dense array for seismic site effect estimation project in the heavily industrialized area surrounding the French Tricastin Nuclear Site (TNS). TNS is located above an ancient canyon dug by the Rhône River during the Messinian period. The strong lithological contrast between the sedimentary fill of the canyon and the substratum, as well as its expected confined geometry make this canyon a good candidate for generating site effects that are variable on short spatial scales. To investigate the impact of this geological structure on the seismic motion, we conducted complementary seismic campaigns in the area. The first main campaign consisted of deploying 400 nodes over a 10 × 10 km area for one month and aimed at recording the seismic ambient noise. A second seismic campaign involved the deployment of 49 broadband stations over the same area for more than eight months. This complementary campaign aimed at recording the seismicity (including local, regional, and teleseismic events). These different designs allowed us to target a variety of seismic data at different spatial and temporal scales. Beyond the interest for local operational seismic hazard applications, these datasets may be valuable for studying seismic wave propagation within complex kilometer‐scale sedimentary structures. In this article, we present the deployment designs as well as initial analyses to provide information on the characteristics and the overall quality of the data acquired to future users.