Unexpected non seismic signals recorded by broadband seismic stations during geomagnetic storms

Abstract:

Broadband seismometer is a very complicated and sensitive device designed to record ground motion with resolutions as low as nm/s. While short period geophone is only mass in a coil the broadband sensor is sophisticated combination of feedback circuits. Experience shows that broadband sensors are very sensitive to ground motion and its orientation, especially tilt. On March 17, 2015 an amazing, strong event was simultaneously recorded by all stations at Z component, with very long period - not typical for seismic events. Records at the same time suggested geomagnetic storm. In this study broadband seismometer response to electromagnetic wave caused by geomagnetic storms is shown and analyzed. Main observation was done using network of 13 broadband stations in northern Poland. Same signal was recorded on all stations on area of 120 km in diameter. Time and shape of recording was exactly the same as recording of magnetic anomaly in permanent magnetic station HEL located about 50 km from the nearest seismic station. The first small event was recorded at about 5 am, and following it strong event started at about 4 pm UTC. An increase of signal amplitude is observed with geomagnetic latitude in the range 52.26 - 53.27^o (corresponding geographic 53.69 - 54.54^oN). Seismic and magnetic records are compared for other permanent stations in the area, as well as for other recent storms. In this study circumstances needed for recording magnetic anomalies with seismic stations are discussed based on years of seismic recording by Polish permanent stations and temporary passive experiments “13BB star” and PASSEQ. National Science Centre Poland provided financial support for this work via NCN grant DEC-2011/02/A/ST10/00284.