Seismo-Electromagnetic Investigation of Earthquakes in Costa Rica

Tuesday, 16 December 2014
Laura Dzieran, Wolfgang Rabbel and Martin Thorwart, University of Kiel, Kiel, Germany
In order to investigate possible seismo-electromagnetic signals of local earthquakes seismological and magnetotelluric measurements were conducted on the Nicoya-Pensinsula of Costa Rica between May 2010 and August 2011. During this time 25 local earthquakes were recorded on the magnetotelluric stations. Analysis shows that the seismological and the electromagnetic data are correlated. P- and S-waves could be identified on all components. The frequency content of the B-field is similar to that of the seismic acceleration. The frequency content of the electric field is limited to 2 to 4 Hz. The waveforms and amplitudes of the seismological and electromagnetic signals correlate partially, but envelopes show up to 1 second time shifts between maxima. We estimated the dimension of different effects to find an explanation for the observed waveform of the data. The coseismoelectric effect of the incoming seismic wave produces an electric field which is in the same order of the observed electric field. But the origin for the observed narrow frequency band as well as the occurrence of a seismo-electric S-phase remain unsolved. The change of the magnetic flux through the coil by tilt induces a voltage which is large enough to explain the signal of the magnetic components. A tilt of 10-5° produces a voltage of 5 mV. Other effects such as a lateral movement of the measurement equipment can be neglected. The most surprising observation of our survey which can not be explained by the existing seismo-electromagnetic theory is that the coda appears similarly strong for seismic and seismo-electric field records. An explanation could be seismo-electric converted waves generated at fluid bearing fractures and faults of the upper crust.