On the observation of magnetic events on broad-band seismometers

Abstract

The objective of this contribution is to get new insights into the effects of magnetic field variations of natural and anthropogenic origin on broad-band seismic stations. Regarding natural sources of magnetic perturbations, we have investigated if the Sudden Storm Commencements (SSC) cataloged during the 24th solar cycle (2008-2019) can be systematically identified in broad-band seismic stations distributed worldwide. The results show that the 23 SSC events with a mean amplitude above 30 nT and most of those with lower energy but still clearly identified in the magnetometer detection network can be observed at broad-band stations' network using a simple low-pass filter. Although the preliminary impulse of those signals is usually stronger at stations located at high latitudes, major SSC are observed at seismic stations distributed worldwide. Regarding anthropogenic sources, we focus on the short period seismic signals recorded in urban environments which are correlated with the activity of the railway transportation system. We have analyzed collocated measurements of electric field and seismic signals within Barcelona, evidencing that significant changes in the electric field following the activity of the transportation systems can be attributed to leakage currents transmitted to the soil by trains. During space weather events, electric currents in the magnetosphere and ionosphere experience large variations inducing telluric currents near the Earth surface, which in turn generate a secondary magnetic field. In the case of underground trains, leakage currents are transmitted to the soil, which in turn can result in local variations in the magnetic field. The observed signals in modern seismometers can be related to the reaction of the suspension springs to these magnetic field variations or to the effect of the magnetic field variations on the force transducers used to keep the mass fixed.

Achievements and Prospects of Global Broadband Seismographic Networks After 30 Years of Continuous Geophysical Observations

“…The sensitivity of seismographs to magnetic field variations has been noted since the 1970s (Tape et al, 2020), particularly at high latitudes where large magnetic field variations during space weather events are easily resolved across multiple global stations (Díaz et al, 2020;Ringler et al, 2020). This makes it advisable to shield seismometers from magnetic field fluctuations using h µ-metal (a nickle-iron alloy with high magnetic permeability), or other ferromagnetic material, and/or to record the local magnetic field to remove unwanted magnetic signal from the seismic data stream both of which are routinely done at most stations operating Streckeisen STS-1 seismometers (Forbriger, 2007).…”

Monitoring storm evolution using a high-density seismic network

“…This approach has been used to study a large variety of processes, many of them related to hydrology and meteorology sciences, as Antarctica ice cover 5 , glaciers 6 , river floods 7 , mountain snowmelt 8 , groundwater variations 9 , rain 10 , wind 11 and thunders 12 . This range of phenomena can be extended to topics related within astronomy and astrophysics, as the monitoring of auroras, electromagnetic storms or bolides entering the atmosphere [12][13][14] and also to the biological sciences, using seismic data to detect marine mammal vocalizations 15 or to investigate the use of vibrations as a communication tool by the elephants 16,17 .…”