EXTRACURRICULAR GEOPHYSICS: OR WHEN INSTRUMENTS PICK UP WHAT THEY WERE NOT DESIGNED TO RECORD

In the wake of the 2004 Sumatra and a few other recent tsunamis, a number of fascinating observations were made on instruments not designed for that specific purpose: in most cases, they express subtle coupling between media of extremely different properties, such as the oceanic column, the solid Earth, or the atmosphere. They include recording of tsunamis by seismometers at land stations and on the ocean bottom, by hydrophones of the CTBTO, the definitive observation and explanation of tsunami shadows, tsunami signatures in the geomagnetic field, the generation of deep infrasound, and the perturbation of the ionosphere detected on GPS receiver arrays. In most cases, these phenomena are readily explained by the continuation (in a mathematical sense) of the tsunami eigenfunction outside of the water column; we will show that in many instances, the order of magnitude of the effect is well predicted by simple arguments derived under the normal mode approach.