The International Pulsar Timing Array: First data release

International audienceThe highly stable spin of neutron stars can be exploited for a variety of (astro)physical investigations. In particular, arrays of pulsars with rotational periods of the order of milliseconds can be used to detect correlated signals such as those caused by gravitational waves. Three such 'pulsar timing arrays' (PTAs) have been set up around the world over the past decades and collectively form the 'International' PTA (IPTA). In this paper, we describe the first joint analysis of the data from the three regional PTAs, i.e. of the first IPTA data set. We describe the available PTA data, the approach presently followed for its combination and suggest improvements for future PTA research. Particular attention is paid to subtle details (such as underestimation of measurement uncertainty and long-period noise) that have often been ignored but which become important in this unprecedentedly large and inhomogeneous data set. We identify and describe in detail several factors that complicate IPTA research and provide recommendations for future pulsar timing efforts. The first IPTA data release presented here (and available on-line) is used to demonstrate the IPTA's potential of improving upon gravitational-wave limit

Evidence for slab rollback in westernmost Mediterranean from improved upper mantle imaging

This work is funded by the PICASSO project (NSF grant EAR-0808939), The deployment and data processing for Spanish stations was funded by Consolider-Ingenio 2010 project TOPO-IBERIA (CSD2006-00041) as well as ALERT-ES (CGL2010-19803-C03-02).Peer Reviewe

Backarc tectonism, volcanism, and mass wasting shape seafloor morphology in the Santorini-Christiana-Amorgos region of the Hellenic Volcanic Arc

In subduction zone backarcs, extensional deformation and arc volcanism interact and these processes, together with mass wasting, shape the seafloor morphology. We present a new bathymetric map of the Santorini-Christiana-Amorgos backarc region of the Hellenic subduction zone by merging high-resolution multibeam swath data from the R/V Langseth PROTEUS seismic experiment with existing maps. The map together with Knudsen subbottom echosounding profiles reveal that recent tectonism, volcanism, and mass wasting are more prevalent in the Santorini-Amorgos region on the east side of Santorini than in the Christiana Basin on the west side. In the Santorini-Amorgos region, large normal faults form the Anydros and Anafi Basins. Where normal fault segments overlap, two nearby accommodation zones generate a relay ramp and the adjoining Anydros synclinal horst with associated complex faulting and elevated seismicity. The ongoing normal faulting in the Santorini-Amorgos region is accompanied by potentially tsunamigenic submarine landsliding; we identified a large submarine landslide along the Santorini-Amorgos Fault and a smaller landslide with an overlying debris chute along the Amorgos Fault. Volcanic activity is also focused in this eastern region along the Kolumbo lineament within the Anydros Basin. Within the Christiana Basin we discovered the Proteus Knoll and adjacent buried edifice. We suggest that this is an older volcanic edifice formed along the Hellenic Volcanic Arc between Santorini and Milos. Around Santorini itself, features formed during, and immediately after, the Late Bronze Age eruption dominate the seafloor morphology such as the northern strait and wrinkled seafloor pyroclastic flow deposits. This topography is continually reshaped at a smaller scale by ongoing mass wasting. We infer that the earthquake, volcanic, and tsunami activity of the Santorini-Amorgos region is a consequence of focused northwest-southeast extension as the southeastern Aegean moves away from the Attico-Cycladic complex in response to slab breakup and rollback. © 2017 Elsevier B.V

Corrigendum to “Seismic imaging of Santorini: Subsurface constraints on caldera collapse and present-day magma recharge” [Earth Planet. Sci. Lett. 514 (2019) 48–61](S0012821X19301360)(10.1016/j.epsl.2019.02.033)

The authors regret errors in the last two paragraphs of Section 6.3 (p. 57), these should read as follows: 6.3 Multistage, nested caldera collapse: In this scenario, the seismic and geological results from Santorini provide observational evidence for models of multistage, nested caldera formation during progressive caldera subsidence (Acocella, 2006). During stage A, the inner collapse column would be formed along outward-dipping reverse faults with breakup of the roof rock (Fig. 6f). During stage B, a new outer ring of collapse would cause subsidence of the entire topographic caldera and the opening of new vents during phase 4 (Fig. 6c). Accordingly, the geologically distinctive LBA eruptive phases form as a direct result of geological processes occurring during each stage of caldera formation (Fig. 6). It is quite possible that all three of the above scenarios (Fig. 6d-f) play a role in generating the inner cylinder of high porosities at Santorini. Thus, rock breakup by reverse faulting during inner caldera collapse may be accompanied by, or even promoted by, fracturing and reaming of the volcanic vent during violent magma-water interactions. In addition, the upper portions of the high-porosity cylinder are probably formed by the deposition of eruptive volcanic products including tuffs, pyroclasts, and ignimbrites. © 2019 Elsevier B.V

Magma accumulation beneath Santorini volcano, Greece, from P-wave tomography

Despite multidisciplinary evidence for crustal magma accumulation below Santorini volcano, Greece, the structure and melt content of the shallow magmatic system remain poorly constrained. We use three-dimensional (3-D) velocity models from tomographic inversions of active-source seismic P-wave travel times to identify a pronounced low-velocity anomaly (-21%) from 2.8 km to 5 km depth localized below the northern caldera basin. This anomaly is consistent with depth estimates of pre-eruptive storage and a recent inflation episode, supporting the interpretation of a shallow magma body that causes seismic attenuation and ray bending. A suite of synthetic tests shows that the geometry is well recovered while a range of melt contents (4%-13% to fully molten) are allowable. A thin mush region (2%-7% to 3%-10% melt) extends from the main magma body toward the northeast, observed as low velocities confined by tectono-magmatic lineaments. This anomaly terminates northwest of Kolumbo; little to no melt underlies the seamount from 3 to 5 km depth. These structural constraints suggest that crustal extension and edifice loads control the geometry of magma accumulation and emphasize that the shallow crust remains conducive to melt storage shortly after a caldera-forming eruption. © 2019 Geological Society of America. For permission to copy

Tectonism and Its Relation to Magmatism Around Santorini Volcano From Upper Crustal P Wave Velocity

At extensional volcanic arcs, faulting often acts to localize magmatism. Santorini is located on the extended continental crust of the Aegean microplate and is one of the most active volcanoes of the Hellenic arc, but the relationship between tectonism and magmatism remains poorly constrained. As part of the Plumbing Reservoirs Of The Earth Under Santorini experiment, seismic data were acquired across the Santorini caldera and the surrounding region using a dense amphibious array of >14,300 marine sound sources and 156 short-period seismometers, covering an area 120 km by 45 km. Here a P wave velocity model of the shallow, upper-crustal structure (<3-km depth), obtained using travel time tomography, is used to delineate fault zones, sedimentary basins, and tectono-magmatic lineaments. Our interpretation of tectonic boundaries and regional faults are consistent with prior geophysical studies, including the location of basin margins and E-W oriented basement faults within the Christiana Basin west of Santorini. Reduced seismic velocities within the basement east of Santorini, near the Anydros and Anafi Basins, are coincident with a region of extensive NE-SW faulting and active seismicity. The structural differences between the eastern and western sides of Santorini are in agreement with previously proposed models of regional tectonic evolution. Additionally, we find that regional magmatism has been localized in NE-SW trending basin-like structures that connect the Christiana, Santorini, and Kolumbo volcanic centers. At Santorini itself, we find that magmatism has been localized along NE-SW trending lineaments that are subparallel to dikes, active faults, and regional volcanic chains. These results show strong interaction between magmatism and active deformation. ©2019. American Geophysical Union. All Rights Reserved