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On the 31st of March 2021 at 02:47:08 UTC (03:47:08 local time), a magnitude M2.6 earthquake occurred in the Rockall Trough near the Hebrides Terrace Seamount, approximately 210 km north-northwest of Ireland. The epicentre location is indicated with a red circle in the map below, black lines denote major fault zones in and around Ireland. The earthquake occurred at a depth of 10 km, and was preceded by a magnitude M2.3 earthquake at the same location, occurring at 00:05:02 UTC (01:05:02 local time). Although earthquake detection rates in the Rockall Trough are lower than those of the Irish Sea, several events with magnitude M>2.0 have been detected in this region, most notably from 1980, when an earthquake of magnitude M4.0 occurred near the Hebrides Terrace Seamount.
The event was recorded by the stations of the Irish National Seismic Network (INSN) and can be seen in the waveforms plotted below:
Instruments operated by the INSN track ground motions from natural phenomena, such as earthquakes, but also human-made ground vibrations, so-called ‘seismic noise’. After the lockdown started on the 28th March 2020, some INSN stations detected seismic noise levels significantly lower than before the lockdown. See figure below for seismic noise data from our station located in the Dublin mountains (click figure to enlarge).
Frequently Asked Questions:
What are seismometers for?
Seismometers respond to ground motion and are traditionally used to detect and locate earthquakes. The earthquake information in turn is used by seismologists and geologists to study the Earth’s interior.
What is seismic noise?
Seismic noise is a relatively persistent ground vibration that is not caused by earthquakes (Note: it is not a vibration in the air). Seismologist termed these signals ‘noise’ because they are a nuisance when trying to detect small earthquakes. Seismic noise has very low amplitude and cannot be felt by humans.
What can a seismometer not tell?
Seismometers do not distinguish between human made seismic noise, naturally made seismic noise (e.g. by strong wind) or seismic waves caused by an earthquake. Seismometers simply record ground vibrations no matter what caused them.
How do you know that the seismic noise is made by human activity?
Seismic noise can be caused by human activities like industrial works (e.g. large building sites, quarries, tunnelling), road traffic, rail traffic, airports, but it is also caused by natural phenomena like for example wind or water flow in a river. Human induced seismic noise is readily identified by its clear variation between day and night and also lower amplitudes on weekends.
Why does the diagram above stop at Sunday April 26th?
After Sunday 26th April 2020 seismic noise recorded at this station is dominated by more local noise sources (likely related to site machinery) and therefore it no longer gives an average regional picture.
Does human induced seismic noise originate just from traffic?
No. Large building sites, quarries, tunnelling, road traffic, rail traffic and airports are some other examples of sources for human induced seismic noise.
How far away can the vibrations still be detected?
This depends on the type of vibration source, more specifically on the strength and the frequency of the vibration causing the seismic noise waves. It is not possible to determine source distances or locations with a single station, but the spectral content of the data may indicate if the source is near or far. As an approximate guideline here are some numbers: Depending on rock type seismic noise originating from heavy industry can be detected up to distances of (approximately) 25km; from railways up to 15km; from motorways up to 6km; and from smaller roads up to 1km (International Handbook of Earthquake and Engineering Seismology, Part A, 2002, Edited by William H.K. Lee, Hiroo Kanamori, Paul C. Jennings, Carl Kisslinger, ISBN-10: 0124406521, ISBN-13: 978-0124406520).
How do you know its not a disturbance right beside the station?
Disturbances right beside a station show as very large amplitudes and don’t follow the smooth day/night variation that is observed for human induced noise from further afield.
Is this only seen in Ireland?
No. The effect of lockdown measures is observed at seismic stations worldwide, though only stations in or near urbanized areas show the effect, see for example https://www.nationalgeographic.com/science/2020/04/coronavirus-is-quieting-the-world-seismic-data-shows
Can you see this noise change all over Ireland?
No. Most of the seismic stations in Ireland operate at remote locations, away from urban areas, making them less sensitive to human induced seismic noise.
Why are seismologists interested in seismic noise?
In the 1990s methods were developed to derive ground properties from seismic noise data. In addition noise data are now widely used to monitor natural processes, for example ocean wave state, volcano activity and water flow at inaccessible locations like in caves and glaciers.
On the 10th April 2020 Martin Möllhoff gave this interview about seismic ground vibrations on Tipp FM:
On the 29th January 2020 at 07:27:40 GMT a magnitude M1.0 earthquake occurred offshore Co. Antrim, Northern Ireland. The earthquake located close to the epicentres of the M0.9 earthquake, 26th January 2020 and the M1.6 earthquake, 18th of December 2019. In the map below the epicentre of the M1.0 earthquake is indicated with a red marker. Major fault zones in and around Ireland are shown with black lines.