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Deep subsurface research to help understand earthquakes

October 01, 2001

From 8 to 20 October, TU Delft, "Vrije Universiteit Amsterdam" and The University of Utrecht will be conducting an experiment in the south of the Netherlands. The experiment is to find out how the deep subsurface (about 25 to 30 km under the surface) can be profiled with sound waves. It is the first time that Dutch subsurface will be researched at such a depth.

Research on the deeper part of the earth's crust is important to help better understand earthquakes in the Netherlands. Currently, there is relatively little knowledge on the Dutch subsurface beneath 5km. For South-Limburg, there is even very little known about the ground deeper than 1km. The earthquakes in 1992 in Roermond took place at more than 15 km beneath the surface. The KNMI is currently conducting research on the 'swarm' of earthquakes in Voerendaal, where more than 150 earthquakes have taken place since 20 December 2000. Data on the composition of the ground at great depths could contribute greatly to this research.

The study is limited to the earth's crust, which is approximately 25 to 30km thick, because it is in this crust that the structural characteristics such as faults lie. The best way to map these structural characteristics is with the use of sound waves, so-called seismic waves. A strong sonic source consisting of explosives situated at a depth of a few tens of metres creates the sound waves. The waves travel through the ground and are reflected by discontinuities (such as the Mohorovic discontinuity). The waves return to the surface, where they are registered by vibration sensors, also referred to as geophones. From the data obtained through the readings, various conclusions can be made about the crust.

In the experiment in October, various instruments will be tried in various configurations to find which combinations are best suited for this type of measurement, especially in such a densely populated country as the Netherlands. Another specific aspect of this experiment is that the distance over which the measurements are being made is very large, up to 100km. This makes the logistics and the set-up of the sensors very different than relatively shallow readings (up to a depth of about 5km)

This experiment is unique because it has never been carried out on such a scale in the Netherlands, although it has been done in surrounding countries such as Germany, France and Great Britain. Because of the scale of the experiment, various groups and organisations will be involved. The responsibility lies with TU Delft, "Vrije Universiteit Amsterdam" and The University of Utrecht, combined in the research school "Integrated Solid Earth Sciences" (ISES), in which the most prominent Dutch geo-scientists are represented. Organisations such as the KNMI, TNO and various foreign organisations will also be present.


Delft University of Technology