ALBERT

Description: The GEOFON program consists of a global seismic network (GE Network), a seismological data centre (GEOFON DC) and a global earthquake monitoring system (GEOFON EQinfo). These three pillars are part of the MESI research infrastructure of the Helmholtz Centre Potsdam - GFZ German Research Centre for Geosciences aiming at facilitating scientific research. GEOFON provides real-time seismic data, access to its own and third party data from the archive facilities as well as global and rapid earthquake information. The GEOFON Seismological Software can be considered a fourth cross-cutting module of the GEOFON Program. Data, services, products and software openly distributed by GEOFON are used by hundreds of scientists and data centres worldwide. Its earthquake information service is accessed directly by tens of thousands of visitors. The SeisComP software package is the flagship software provided to the community, which is geared for seismic observatory and data centre needs and used extensively to support our internal operations. Like all other MESI (Modular Earth Science Infrastructure) modules GEOFON has the majority of users outside the GFZ as well as an external advisory committee that provides advice to the GFZ Executive Board and to the GEOFON team. This report describes the main activities carried out within the three GEOFON pillars and the software development group.

Description: We analyse data from seismic stations surrounding the Alboran Sea between Spain and North Africa to constrain variations of the lithosphere–asthenosphere boundary (LAB) in the region. The technique used is the receiver function technique, which uses S-to-P converted teleseismic waves at the LAB below the seismic stations. We confirm previous data suggesting a shallow (60–90 km) LAB beneath the Iberian Peninsula and we observe a similarly shallow LAB beneath the Alboran Sea where the lithosphere becomes progressively thinner towards the east. A deeper LAB (90–100 km) is observed beneath the Betics, the south of Portugal and Morocco. The structure of the LAB in the entire region does not seem to show any indication of subduction related features. We also observe good P receiver function signals from the seismic discontinuities at 410 and 660 km depth which do not indicate any upper-mantle anomaly beneath the entire region. This is in agreement with the sparse seismic activity in the mantle transition zone suggesting the presence of only weak and regionally confined anomalies.

Description: The volcanism responsible for creating the chain of the Hawaiian islands and seamounts is believed to mark the passage of the oceanic lithosphere over a mantle plume1,2. In this picture hot material rises from great depth within a fixed narrow conduit to the surface, penetrating the moving lithosphere3. Although a number of models describe possible plume–lithosphere interactions4, seismic imaging techniques have not had sufficient resolution to distinguish between them. Here we apply the S-wave ‘receiver function’ technique to data of three permanent seismic broadband stations on the Hawaiian islands, to map the thickness of the underlying lithosphere. We find that under Big Island the lithosphere is 100–110 km thick, as expected for an oceanic plate 90–100 million years old that is not modified by a plume. But the lithosphere thins gradually along the island chain to about 50–60 km below Kauai. The width of the thinning is about 300 km. In this zone, well within the larger-scale topographic swell, we infer that the rejuvenation model5 (where the plume thins the lithosphere) is operative; however, the largerscale topographic swell is probably supported dynamically.

Description: During the TOR-1 passive seismic experiment in 1996/97, a maximum of 139 temporary seismograph stations were operating over the Sorgenfrei-Tornquist Zone (STZ) in an area extending from northern Germany through Denmark to central Sweden. One of the objectives was to study horizontal anisotropy directions in the subcrustal lithosphere and asthenosphere across the Trans-European Suture Zone. To achieve this goal,broad-band and intermediate-period (5 s) data of the TOR-1 stations and additional stations of permanent networks (GRSN, GEOFON) were analysed for splitting of SKS and SKKS phases. As a result of the relatively dense station spacing, the method offers good lateral resolution of anisotropy.Preliminary results suggest that the directions of the fast horizontal S wave velocity are affected by the STZ. In central Europe and southern Sweden, far away from the STZ, fast S wave directions are approximately E-W while they turn more northerly closer to the STZ where they are approximately parallel to the trend of the STZ. No significant shear wave splitting was observed north of 57 degr. N and east of 14 degr. E. Small delay times between 0.2 and 0.5 s observed at the northernmost TOR-1 station T40S and T60S may be controlled by anisotropy in a thickened crust. The mantle contribution of horizontal anisotropy within the STZ is probably constrained to an approximately 60-km-thick zone in the depth range between 70 and 300 km. The observations are consistent with a model where azimuthally anisotropy is not governed by present-day mantle flow in the asthenosphere, but rather is frozen into the subcrustal lithosphere during the last episode of tectonic activity.

Description: The Mw = 9.3 Sumatra earthquake of 26 December 2004 generated a tsunami that affected the entire Indian Ocean region and caused approximately 230 000 fatalities. In the response to this tragedy the German government funded the German Indonesian Tsunami Early Warning System (GITEWS) Project. The task of the GEOFON group of GFZ Potsdam was to develop and implement the seismological component. In this paper we describe the concept of the GITEWS earthquake monitoring system and report on its present status. The major challenge for earthquake monitoring within a tsunami warning system is to deliver rapid information about location, depth, size and possibly other source parameters. This is particularly true for coast lines adjacent to the potential source areas such as the Sunda trench where these parameters are required within a few minutes after the event in order to be able to warn the population before the potential tsunami hits the neighbouring coastal areas. Therefore, the key for a seismic monitoring system with short warning times adequate for Indonesia is a dense real-time seismic network across Indonesia with densifications close to the Sunda trench. A substantial number of supplementary stations in other Indian Ocean rim countries are added to strengthen the teleseismic monitoring capabilities. The installation of the new GITEWS seismic network – consisting of 31 combined broadband and strong motion stations – out of these 21 stations in Indonesia – is almost completed. The real-time data collection is using a private VSAT communication system with hubs in Jakarta and Vienna. In addition, all available seismic real-time data from the other seismic networks in Indonesia and other Indian Ocean rim countries are acquired also directly by VSAT or by Internet at the Indonesian Tsunami Warning Centre in Jakarta and the resulting "virtual" network of more than 230 stations can jointly be used for seismic data processing. The seismological processing software as part of the GITEWS tsunami control centre is an enhanced version of the widely used SeisComP software and the well established GEOFON earthquake information system operated at GFZ in Potsdam (http://geofon.gfz-potsdam.de/db/eqinfo.php). This recently developed software package (SeisComP3) is reliable, fast and can provide fully automatic earthquake location and magnitude estimates. It uses innovative visualization tools, offers the possibility for manual correction and re-calculation, flexible configuration, support for distributed processing and data and parameter exchange with external monitoring systems. SeisComP3 is not only used for tsunami warning in Indonesia but also in most other Tsunami Warning Centres in the Indian Ocean and Euro-Med regions and in many seismic services worldwide.

Description: Combined P and S receiver functions from seismograms of teleseismic events recorded at 65 temporary and permanent stations in the Aegean region are used to map the geometry of the subducted African and the overriding Aegean plates. We image the Moho of the subducting African plate at depths ranging from 40 km beneath southern Crete and the western Peloponnesus to 160 km beneath the volcanic arc and 220 km beneath northern Greece. However, the dip of the Moho of the subducting African plate is shallower beneath the Peloponnesus than beneath Crete and Rhodes and flattens out beneath the northern Aegean. Observed P-to-S conversions at stations located in the forearc indicate a reversed velocity contrast at the Moho boundary of the Aegean plate, whereas this boundary is observed as a normal velocity contrast by the S-to-P conversions. Our modeling suggests that the presence of a large amount of serpentinite (more than 30%) in the forearc mantle wedge, which generally occurs in the subduction zones, may be the reason for the reverse sign of the P-to-S conversion coefficient. Moho depths for the Aegean plate show that the southern part of the Aegean (crustal thickness of 20–22 km) has been strongly influenced by extension, while the northern Aegean Sea, which at present undergoes the highest crustal deformation, shows a relatively thicker crust (25–28 km). This may imply a recent initiation of the present kinematics in the Aegean. Western Greece (crustal thickness of 32–40 km) is unaffected by the recent extension but underwent crustal thickening during the Hellenides Mountains building event. The depths of the Aegean Moho beneath the margin of the Peloponnesus and Crete (25–28 and 25–33 km, respectively) show that these areas are also likely to be affected by the Aegean extension, even though the Cyclades (crustal thickness of 26–30 km) were not significantly involved in this episode. The Aegean lithosphere-asthenosphere boundary (LAB) mapped with S receiver functions is about 150 km deep beneath mainland Greece, whereas the LAB of the subducted African plate dips from 100 km beneath Crete and the southern Aegean Sea to about 225 km under the volcanic arc. This implies a thickness of 60–65 km for the subducted African lithosphere, suggesting that the Aegean lithosphere was not significantly affected by the extensional process associated with the exhumation of metamorphic core complexes in the Cyclades.

Description: Two passive seismic experiments have been carried out across the Trans European Suture Zone (TESZ) from northern Germany to southern Sweden (TOR) and across the Proterozoic-Archaean suture in Finland (SVEKALAPKO) to improve our understanding of the processes involved in the creation of the European continent. Teleseismic earthquakes recorded by the studies of the crust-mantle, and upper mantle seismic discontinuities with the receiver function method. Along the TOR network the depth to the Moho increases from 30 km at the southern edge of the profile to 40 km at the Elbe Line. Between the Elbe Line and TESZ the Moho branches off and whereas the deeper branch continues at 40 km depth to the TESZ a second branch appears at 30.35 km depth. The upper branch descends north of the TESZ to below 55 km under the northern end of the TOR profile. The crustal thickening north of the TESZ is accompanied by an increase in average Vp/Vs values, appearance of intracrustal conversion zones and north dipping features which we interpret as remnants of the subduction and subsequent collision between Avalonia and Baltica. In southern Finland beneath the SVEKALAPKO network the Moho starts in the south at the depth of 40-45 km, plunges to about 65 km depth south of the Archaean-Proterozoic suture. This deepening of the Moho is coincident with a north dipping intracrustal structure apparently related to the subduction and collision and of the Proterozoic and Archaean provinces in Proterozoic. North of the line of the suture the Moho rises smoothly to 45-50 km depth in the Archaean province. Along the TOR profile, 410 and 660 discontinuities were hard to detect. However, manyfold stacking of receiver functions revealed that the conversions from the two discontinuities arrive more or less accordingto IASP91 predicted time. Across the SVEKALAPKO network 410 and 660 discontinuities arrive markedly earlier than IASP91 theoretical arrival times. In particular north of the Archaean-Proterozoic suture in Finland the 410 and 660 km conversions arrive about 2s earlier, indicating about 5 per cent higher average upper mantle velocities and lower temperatures than what IASP91 global model predicts. Test

Description: The GLATIS project (Greenland Lithosphere Analysed Teleseismically on the Ice Sheet) with collaborators has operated a total of 16 temporary broadband seismographs for periods from 3 months to 2 years distributed over much of Greenland from late 1999 to the present. The very first results are presented in this paper, where receiver-function analysis has been used to map the depth to Moho in a large region where crustal thicknesses were previously completely unknown. The results suggest that the Proterozoic part of central Greenland consists of two distinct blocks with different depths to Moho. North of the Archean core in southern Greenland is a zone of very thick Proterozoic crust with an average depth to Moho close to 48 km. Further to the north the Proterozoic crust thins to 37–42 km. We suggest that the boundary between thick and thin crust forms the boundary between the geologically defined Nagssugtoqidian and Rinkian mobile belts, which thus can be viewed as two blocks, based on the large difference in depth to Moho (over 6 km). Depth to Moho on the Archean crust is around 40 km. Four of the stations are placed in the interior of Greenland on the ice sheet, where we find the data quality excellent, but receiver-function analyses are complicated by strong converted phases generated at the base of the ice sheet, which in some places is more than 3 km thick.

Description: Seismic anisotropy was investigated by measuring shear-wave splitting at 19 broadband stations in Greenland. We examined mostly SKS and SKKS phases, but also some PKS and depth phases of SKS (e.g. pSKS, sSKS) for deep events. Splitting parameters (fast polarization and time delay) were determined for these phases. The fast polarizations at nine sites in southern Greenland are quite uniformly oriented about N–NE. Two sites in central northern Greenland show a similar geometry to southern Greenland. Similar fast polarizations in southern and central northern Greenland suggest continuity of structural fabric beneath large parts of Greenland. This coherent pattern extends across a number of geological provinces of varying age and suggests a common cause of anisotropy not related to the bitwise formation of the Greenland continental block. Four sites in an east–west oriented belt crossing central Greenland show varying fast polarizations and suggest a separate process causing the anisotropy there, which may indicate that these processes are not currently active. The overall pattern of anisotropy in our results, with the exception of variations across central Greenland, is similar to results obtained from Rayleigh waves. The irregular geometry of splitting across central Greenland may be related to the impact of the Iceland plume at ∼ 60 Ma. Reported splitting time delays range from 0.4 to 1.4 s with an average of 0.8 s, which can generally not be explained by crustal anisotropy alone. If confined to a lithosphere of thickness on the order of 100 km, time delays of up to 1.4 s indicate anisotropy of up to about 6%, assuming that the a crystallographic axis of olivine is preferentially contained in the horizontal plane. We suggest that the anisotropy beneath Greenland is located mainly in the upper mantle but some contributions from the crust and lower mantle may be present.