Florian Haslinger

3D crustal structure from local earthquake tomography around the Gulf of Arta (Ionian region, NW Greece)

During summer of 1995 local seismicity was recorded in the area around the Gulf of Arta in northwestern Greece by a dense temporary seismic network. Of the 441 local events observed at 37 stations, 232 well locatable events with a total of 2776 P-phase readings were selected applying the criteria of a minimum of 6 P-observations and an azimuthal gap less than 180°. This data set is used to compute a minimum 1D velocity model for the region. Several tests are conducted to estimate model stability and hypocenter uncertainties, leading to the conclusion that relative hypocenter location accuracy is about 500 m in latitude and longitude and 1 km in depth. The minimum 1D velocity model serves as initial model in the non-linear inversion for three-dimensional P-velocity crustal structure by iteratively solving the coupled hypocenter–velocity problem in a least-squares sense. Careful analysis of the resolution capability of our data set outlines the well resolved features for interpretation. The resulting 3D velocity model shows generally higher average crustal velocities in the east, and the well resolved area of the eastern Gulf of Arta exhibits a homogeneous velocity around 6 km/s for the whole upper crust. A pronounced north–south trending zone of low velocities in the upper 5–10 km is observed in the area of the Katouna fault zone (KFZ). At greater depths (below 10 km) the KFZ is underlain by high-velocity material. E–W profiles suggest a horst–graben structure associated with the KFZ.

Model parametrization in seismic tomography: a choice of consequence for the solution quality

Abstract To better assess quality of three-dimensional (3-D) tomographic images and to better define possible improvements to tomographic inversion procedures, one must consider not only data quality and numerical precision of forward and inverse solvers but also appropriateness of model parametrization and display of results. The quality of the forward solution, in particular, strongly depends on parametrization of the velocity field and is of great importance both for calculation of travel times and partial derivatives that characterize the inverse problem. To achieve a quality in model parametrization appropriate to high-precision forward and inverse algorithms and to high-quality data, we propose a three-grid approach encompassing a seismic, a forward, and an inversion grid. The seismic grid is set up in such a way that it may appropriately account for the highest resolution capability (i.e. optimal data) in the data set and that the 3-D velocity structure is adequately represented to the smallest resolvable detail apriori known to exist in real earth structure. Generally, the seismic grid is of uneven grid spacing and it provides the basis for later display and interpretation. The numerical grid allows a numerically stable computation of travel times and partial derivatives. Its specifications are defined by the individual forward solver and it might vary for different numerical techniques. The inversion grid is based on the seismic grid but must be large enough to guarantee uniform and fair resolution in most areas. For optimal data sets the inversion grid may eventually equal the seismic grid but in reality, the spacing of this grid will depend on the illumination qualities of our data set (ray sampling) and on the maximum matrix size we can invert for. The use of the three-grid approach in seismic tomography allows to adequately and evenly account for characteristics of forward and inverse solution algorithms, apriori knowledge of earth’s structure, and resolution capability of available data set. This results in possibly more accurate and certainly in more reliable tomographic images since the inversion process may be well-tuned to the particular application and since the three-grid approach allows better assessment of solution quality.

Seismic Velocity and Attenuation Structure of the East Rift Zone and South Flank of Kilauea Volcano, Hawaii

It has been proposed that a deep magma body beneath the east rift zone (ERZ) of Kilauea volcano, Hawaii, must be present in order to explain the observed deformation of the south flank. From November 1999 to June 2000, 29 IRIS-PASSCAL three-component seismographs were operated across Kilauea9s ERZ and south flank. Using local earthquakes recorded by these stations, supplemented with data from the Hawaiian Volcano Observatory stations, we investigated the deep structure of the ERZ and south flank through two approaches: seismic velocity tomography and seismic attenuation tomography. Results along two north-northwest-south-southeast profiles are examined. The western profile shows a low-velocity region beneath and south of the Hilina Pali, and earthquakes at depth are sandwiched between a high V p / V s , high Q zone and a low V p / V s , low Q zone. The velocity and attenuation differences are interpreted to reflect the contrast in material properties between the overlying volcanic pile and the underlying fluid-rich ocean sediments and fractured, altered ocean crust beneath the south flank thrust fault. The eastern profile shows an anomalous feature with low V p , low V p / V s , high Q p , and low Q s at about 7 km depth beneath the ERZ. The low V p / V s ratio is inconsistent with the presence of partial melt. Instead, this anomaly is attributed to a trapped reservoir of CO 2 . Our results do not support the existence of a sizable molten or partially molten body at depth beneath the ERZ. If the deep magma body exists, it must be relatively small. Based on synthetic resolution tests, an estimated limit on the maximum allowable cross-sectional size of the deep magma body is about 2 by 4 km.

Tomographic image of P-velocity structure beneath Kilauea's East Rift Zone and South Flank : Seismic evidence for a deep magma body

We present first results from the analysis of P-wave arrival time data recorded from November 11 to December 31, 1999, by a temporary 29-station network installed across Kilauea Volcanos East Rift Zone (ERZ) and South Flank (SF) on Hawaii, augmented by data from the permanent network of the Hawaiian Volcano Observatory. Starting with the inversion result for a minimum 1D velocity model, we use arrival time data from 135 local earthquakes to invert for the 3D P-velocity structure. The resulting tomographic image shows evidence for a deep magma body beneath the ERZ just east of its southward bend, and a smaller magma body at about 5 km depth beneath the WNW-ESE trending segment of the ERZ. We also observe a drastic change in velocities south of the Hilina fault system from velocities around 5.5 km/s in the west to 6.5 km/s to the east.

The European Experience of Educational Seismology

This chapter provides an overview of the last two decades’ European experiences in educational seismology and describes the different contexts in which they have been developed. The basic idea of these educational projects is that seismology may represent an efficient communication vehicle for teaching a wide range of basic earth science topics through laboratory practices and educational activities. Moreover, it is also an effective tool to raise in the young citizens the awareness on the earthquake risk and possible mitigation actions. In this frame, several seismic stations with different technologies were installed in schools across Europe. The scientific support of researchers and the need to establish strong links between teachers and researchers attribute to the school an active role in the knowledge process using the scientific laboratory practice by adopting the ‘learning by doing’ modern approach of science communication (R. Schank C. Cleary, 1995). Some educational activities correlated with seismological projects are presented, following different strategies depending on the country, but all aimed at building a new way to communicate science in the schools. The new vogue is the opening towards social media and blogs. This generalises the concept of an educational Geoscience website making it an e-platform for science communication and multimedia data sharing, where researchers, teachers, students and education operators can interact and constantly be kept informed of ongoing activities and relevant events.