Cable-Based Geophysical Measurement And Monitoring Systems, New Possibilities For Tsunami Early-Warnings

The disastrous tsunami off Sumatra in December 2004 showed how vulnerable today's industrial infrastructure is, and that almost no risk management for populations living at the coastlines exists. Historically, most governments have continuously ignored the possible destruction caused by earthquakes and their consequences (fire, destruction of infrastructure). Today, only a few mega cities have a seismological network to monitor seismicity on the fault systems they lie on or near. Science has made little progress in the past in understanding the tectonic processes leading to earthquakes and earthquake predictions remain a distant hope. This is a direct consequence of the lack of onshore infrastructure capable of recording the changes in important physical parameters before, during and after an earthquake. The situation is even worse for the offshore parts of large transform fault systems, or even more important for subduction zones. These zones are the tectonically most active regions of the world, and represent an underestimated threat to coastal countries bordering the Pacific, the eastern Indian Ocean and the Mediterranean. Few or no reasonable precautions have been taken to monitor the seismicity of these regions in real time, and thus the risk of them is very poorly understood. Modern marine fiber-optic cables are the best way of networking offshore monitoring arrays. Geophysical sensors can be directly integrated or placed in a special casing. Furthermore, the deployment of such cables even a few decimeters below the seafloor to lower the ambient noise is a standard procedure in industry. For seismicity studies, seismometers, geophones and hydrophones will be the preferred instrumentation. In order to locate the position of an earthquake, such sensors have to be arranged at the end or in extra junctions of the cable in a specific and known manner. Furthermore, miniaturized sensors can be integrated into the cable at certain locations. From recent studies, it is known that the seismicity along offshore tectonic structures is several magnitudes higher than can be detected with onshore instruments. Thus, given present knowledge, the relevance of and risk from marine fault/subduction systems is likely to be underestimated. Similar arguments count for active submarine volcanoes. Here, large landslides can produce local tsunamis. Depending on its layout, a cable system can also serve as a tsunami early warning system along subduction zones, submarine volcanoes or other geological structures with the advantage that all waveforms/sensor information are available to decision makers in real time. To sum up, the impact of such submarine observatories on science would be significant providing the capability to predict problems where -for example- offshore constructions are planned. These observatories might allow a more detailed insight into the dynamics of subduction zones. This insight might provide, after some time, a fresh look at the risk for some coastal areas. From a scientific point of view, it would be desirable, therefore, for marine fiber-optic cables (for example telecommunication cables) to be equipped with regularly spaced geophones/hydrophones. The extra costs for the telecommunication companies would not be significant, but the scientific benefit will be enormous, providing the distribution of the data can be organized.