Interference of high-latitude geomagnetic pulsations on signals from glacial earthquakes recorded by broadband force-balanced seismic sensors

Seismic broad-band sensors with electromagnetic feedback are sensitive to variations of surrounding magnetic field, including variations of geomagnetic field. Usually, the influence of the geomagnetic field on recordings of such seismometers is ignored. It might be justified for seismic observations at middle and low latitudes. The problem is of high importance, however, for observations in polar regions (above 60 deg. magnetic latitude), where magnitudes of natural magnetic disturbances may be two or even three orders larger. In our study we investigate the effect of ultra-low frequency (ULF) magnetic disturbances, known as geomagnetic pulsations, on signals from glacial earthquakes from Greenland recorded by the STS-2 seismic broadband sensors. The pulsations have their sources and, respectively, maximal amplitudes in the region of the auroral ovals, which surround the magnetic poles in both hemispheres at geomagnetic latitude (MLAT) between 60 deg. and 80 deg. To investigate sensitivity of the STS-2 seismometer to geomagnetic pulsations, we compared the recordings of permanent seismic stations in northern Finland to the data of the magnetometers of the IMAGE network located in the same area. Our results show that temporary variations of magnetic field with periods of 40-150s corresponding to regular Pc4 and irregular Pi2 pulsations can create problem for proper interpretation of seismic observations (in particular, glacial earthquakes) in the auroral region. Moreover, the shape of Pi2 magnetic disturbances and their periods resemble the waveforms of glacial seismic events reported originally by Ekström [2003]. Generally, the influence of geomagnetic pulsations on broadband seismometers deployed in polar regions depends on the position of the station with respect to the geomagnetic latitude and can vary significantly within several hundreds of kilometres. The problem may be treated, however, if combined analysis of recordings of collocated seismic and magnetic instruments is used.