Michael Landes

REFRACTION-SEISMIC INVESTIGATIONS OF THE NORTHERN MASSIF CENTRAL (FRANCE)

Abstract New refraction-seismic data have been obtained in the northern French Massif Central. Seven shots were recorded simultaneously on two parallel, NNW-SSE (i.e., perpendicular to the surface structures) oriented profiles allowing for inline and fan interpretations. The results of a 2D-interpretation reveal a large-scale updoming of the Moho of about 2 km, compared to average western Europe, corresponding to the Cenozoic uplift of the Massif Central. Local crustal thinning of up to 20% is restricted to the Cenozoic grabens as seen at the surface. Beneath the volcanic area of Cantal, one of the largest Cenozoic European shield volcanoes, which was active in Miocene to recent times, significant crustal thickening of 2–3 km is observed. Velocities in the upper mantle are generally in the range of 7.9–8.1 km/s beneath the Limagne graben and surrounding undisturbed areas. Underneath the volcanic area of Cezallier, however, velocity is reduced to about 7.7 km/s, corresponding to the area of crustal thickening. Therefore, the low velocity in this zone is explained by the remainders of cooling magma chambers and heating by ascending hot plume material. The Cenozoic tectonic evolution of the Massif Central has affected crust and upper mantle in different ways. Oligocene rifting corresponds well to local crustal thinning, but no correlation is detected to velocities of the upper mantle. In contrast, the volanic event occurring since Miocene times is related to upper mantle velocity reduction and crustal thickening due to underplating.

Seismic experiments target earthquake‐prone region in Romania

Several major earthquakes struck Romania in the last century, and all of them occurred in the Vrancea zone in the southeast Carpathian Mountains (Figure l). These earthquakes claimed many lives and caused extensive damage. In the 1977 earthquake alone, 1,570 people died and more than 11,300 were injured, 32,900 apartments were completely destroyed or severely damaged, and economic loss was estimated at more than

Shear‐wave splitting observations across southwest Ireland

2 billion. To study the crustal and uppermost mantle structure beneath this seismic high-risk area, two major active-source seismic experiments were carried out in 1999 and 2001, nearly 30 years after the last seismic investigations across the eastern Carpathians in the 1970s. The joint seismic refraction and reflection field effort conducted in August and September 2001 (Figure 1), which used almost 800 stand-alone recorders, is one example of the productive interaction forged during the 1999 experiment [Hauser et al., 2001].

Passive teleseismic experiment explores the deep subsurface of southern Ireland

[1] Shear-wave splitting analyses have been carried out on teleseismic data from the southwest of Ireland acquired during the Irish Seismological Lithospheric Experiment (ISLE). The data were gathered over a ten-month period by a temporary network of 23 broadband and short-period stations. The results are compared with data from two permanent broadband seismic stations, which have recorded SKS and SKKS phases for up to 10 years. The purpose of the experiment was to investigate possible anisotropy within the crust and mantle related to Caledonian deformation. Here we report splitting results which show an average delay time of 1.2 s and a variation of fast-polarisation direction with back azimuth that surprisingly suggests a much deeper origin for anisotropy than was anticipated.

Seismic crustal structure between the Transylvanian Basin and the Black Sea, Romania

The Irish Seismological Lithospheric Experiment (ISLE 2002) has been designed to investigate the deep lithospheric and asthenospheric structure across the late-Caledonian Iapetus Suture Zone in southern Ireland. The project is a collaboration between the Dublin Institute for Advanced Studies (DIAS), Ireland, and the Geophysical Institute (GPI) of the University of Karlsruhe, Germany. This is the first passive teleseismic experiment conducted in Ireland, building upon a large body of earlier work on the crustal structure offshore and onshore Ireland, based on controlled source seismics and potential field studies.