Geological Survey of Denmark and Greenland Bulletin 33, 2015, 25-28 25 Acoustic events on a small seismological network – shock waves from thunder and fireballs Peter H. Voss, Trine Dahl-Jensen and Tine B. Larsen Th e Geological Survey of Denmark and Greenland (GEUS) operates a network of seismic stations in Denmark primarily to detect earthquakes. But from time to time other sources than earthquakes generate seismic signals that are detected at the stations. Here we show that both meteoroids and thun- der have generated seismic signals with high signal-to-noise ratios at some of GEUS’ seismic stations (Fig. 1). Th e pur- pose of the seismic stations is to provide data for the earth- quake database of the Kingdom of Denmark, hosted and maintained by GEUS. In order to avoid that the earthquake database is contaminated by other events not related to tec- tonism, these events are given special markers when possible. Meteoroids In their fall through the atmosphere two meteoroids gener- ated sonic signals close to Danish seismic stations with suf- fi cient energy to cause seismic signals (Fig. 2). Fireballs of the meteoroids were observed. Th e sonic signal is interpreted as the sonic shock wave that is generated when the speed of the fi reball exceeds the speed of sound. Th e fi rst observation was made on 17 January 2009 when a meteoroid entered the atmosphere from space above the Baltic Sea and landed Fig. 1. Map of Denmark showing the location of seismic stations and towns mentioned in the text. The arrows show the paths of fireballs. 10°E 55° 57°N 50 km Denmark Sweden Læsø Aalborg Kattegat Germany Stevns Maribo Frederikshavn Klokkerholm Baltic Sea Seismic station Town or village Fig. 2. A: Seismograms from the two seismic sensors that recorded the passing shock wave of the 2009 fireball. Data have been band-pass filtered from 5 to 10 Hz. B: Seismograms from three seismic sensors that recorded the shock wave of the 2014 fireball showing the first motion downward on three stations. Data have not been filtered. C: The arrows mark the pulses of the N-wave of the 2014 fireball. Data were band-pass filtered from 1 to 10 Hz. B C Station 2,Z Station 4,Z Station 5,Z 1 2 s 1 2 s LLD 2,Z Station 2,Z Station 4,Z 1 2 3 4 5 6 7 8 9 s A LLD 1,Z © 2015 GEUS. Geological Survey of Denmark and Greenland Bulletin 33, 25–28. Open access: www.geus.dk/publications/bull 2626 near the town of Maribo, Lolland (Fig. 1), where a meteorite with a weight of 25.8 g was found (Haack 2012; Haack et al. 2012). Th e second meteoroid was recorded on 16 June 2014 on a small network of six seismic stations north of Aalborg, Jylland. Th is meteoroid entered the atmosphere south of Frederikshavn and possibly ended in the Kattegat south of the island of Læsø (Fig. 1). No meteorite was found. Th e seismograms of the shock waves from the 2009 fi re- ball are seen in Fig. 2A. Th ey were recorded on the GEUS seismic station located on Stevns. Th e station is equipped with two vertical sensors installed 208 metres apart, with sensor LLD2 located 150° south-east by south of sensor LLD1. Th e acoustic signal arrived 0.33 s later at LLD2 than at LLD1 which suggests that the source of the signal was lo- cated to the south-east of the stations. Th e duration of the signals diff ers by a factor of four, which might be an eff ect of the diff erent conditions at the sites where the sensors are in- stalled. Th e LLD1 signal is from the original sensor, located in a small vault dug into the fi eld. Th e LLD2 signal is from an experimental installation of a similar sensor next to the wall of a large barn. Th e longer duration of this signal was prob- ably caused by the resonance of the shock wave by the barn. When seismic data from the 2014 fi reball were fi rst ana- lysed, the lack of S-wave energy suggested that the source of the signals was a mine explosion in Kattegat. But closer in- spection of the signals showed that the fi rst motion of the detected P-wave was downward, consistent with dilatation at the source (Fig 2B). Th e expected fi rst motion of the P-wave from an explosion is upward in response to the compression exerted by the source on the surroundings. An analysis of the signals recorded on the six stations, seen in Fig. 3, shows that the shock wave arrived at the stations from an almost easterly direction of approximately 96°, with an apparent surface velocity of 1255.8 m/s. Assuming a sound speed of 342.2 m/s the incidence angle to the seismic stations is 15.9° from vertical. Th ese results fi t well with the fi reball obser- vations published at http://stjerneskud.info. Th e observed fi reball trajectory derived by Sørensen (2014) based on pho- North South East West Fig. 3. Seismic recordings from six seismic stations of the shock wave caused by the fireball on 16 June 2014. Data are the up/down Z compo- nent of the data. Amplitudes are normalised and the data were band-pass filtered from 5 to 10 Hz. The time is UTC. Fig. 4. Photograph showing the meteoroid trajec- tory. The photograph was taken with a specially designed fireball camera on 16 June 2014 at 1:15 p.m. local time, in Klokkerholm, Jylland. The camera was turned upward and equipped with a fisheye objective. The meteor was visible for 3.9 s. The image was created by stacking all video frames showing the fireball. Courtesy of Kim Lang, Klokkerholm and Anton Sørensen, http:// stjerneskud.info. Station 1,Z Station 2,Z Station 3,Z Station 4,Z Station 6,Z Station 5,Z 23:19:00 23:19:1023:19:05 27 tographs taken at two locations in northern Jylland (Fig. 4) corresponds well with our calculation of the azimuth angle. Th e crossing of the azimuth and the trajectory suggest that the source of the shock wave came from an altitude of 46 km in the upper part of the stratosphere over the island of Læsø. But since sound in the atmosphere oft en follows nonlinear ray paths as seen by the low incidence angle, the derived alti- tude is very uncertain. Th e changes in air pressure in the shock wave generate a seismic P-wave in the upper part of the Earth. Since the seis- mic P-wave travels faster than the shock wave, it can some- times be detected just before the arrival of the shock wave as shown by, e.g. Kanamori et al. (1991). But in the data from the two fi reballs that we have observed, we see no indication of P-wave signals arriving before the shock wave. As the sonic boom travels through the atmosphere, the change in pressure has an N-shaped pulse. At the conversion to seismic energy, the N-wave shows as two pulses on the seismogram (Kana- mori et al. 1991; Cates & Sturtevant 2002). Th ree examples of N-waves recorded aft er the 2014 fi reball are shown in Fig. 2C. Th ese should not be confused with P-waves, which arrive through the ground. Thunder Th e six seismic stations north of Aalborg, installed in a net- work with a radius of 5 km, have made it possible to detect signals that are normally regarded as noise, for instance thunder. Th under couples to the ground like the sonic booms from, e.g. meteoroids, but since lightning oft en cov- ers very large areas and occurs in sequences, the seismic signal consists of many peaks and is less impulsive (e.g. Kappus & Vernon 1991). An example of at least seven thunder signals within a short time window is seen in Fig. 5. Th e distance to a thunderstorm that will generate an observable signal depends on the power of the acoustic signal released by the thunder and the composition of the atmosphere at the time of the thunder, since changes in the atmosphere can dampen or amplify acoustic signals in diff erent directions. Most of the thunder we have observed occurred close to our seismic stations, but we have also observed thunder up to 30 km from a seismic station. Th e thunder signals diff er from earth- quake signals and explosions by the absence of body waves and are characterised by an apparent surface velocity around the speed of sound. It is the apparent surface velocity of the thunder signals that discriminates the seismic signals from other noise signals, such as traffi c. Since the distance between seismic stations is usually long (50 km), thunder will most oft en only be recorded by a single seismic station, whereas a minimum of three stations is needed to estimate the appar- ent surface velocity. It is therefore normally not possible to positively identify sonic waves. In order to identify thunder signals we verify the obser- vations with the lightning measurements performed by the Danish Meteorological Institute. Maps of observed light- ning in Denmark are presented on the institute’s webpage (http://www.dmi.dk/vejr/maalinger/lyn/). Discussion Th e signals caused by sonic booms only constitute a small fraction of the observed seismic events in the GEUS earth- quake database. But when they occur they must be identifi ed so that they do not contaminate the seismic data. Th e sonic boom from the fi reball on 16 June 2014 was not reported by any persons. Th e seismic recordings only con- tain signals u p to 50 Hz which is close to the lower limit of the human hearing range (on average 31 Hz). Th e frequency spectra in Fig. 6 show that the recorded seismic signal of the meteoroid shock wave on average contained less energy above 31 Hz than for instance a thunder signal recorded on 1 No- vember 2014. At around 15 Hz the two signals have similar amplitudes, but again at lower frequencies the thunder signal is on average higher. With an energy content lower than a thunder signal, the sonic boom was probably not audible to humans. Figure 6 also shows the spectra of the P-wave of a local magnitude 1.2 earthquake recorded on 4 August 2014, approximately 39 km from the seismic station. Th e spectra of the P-wave are Station 3,Z Station 5,Z 11:25 11:26 11:27 11:28 Station 1,Z Fig. 5. Seismograms showing 4 minutes of data from the up/down sensor recorded at three stations as a thunderstorm passed by on 1 July, 2014. At least seven signals caused by thunder are seen. Time is UTC, data were band-pass filtered from 10 to 30 Hz. www.dmi.dk/vejr/maalinger/lyn/ 2828 comparable to the thunder signal, at frequencies lower than 40 Hz. Hearing such a signal would require a coupling of the energy from the ground to the air, which would introduce an energy loss. When people send GEUS reports on earth- quakes sounds, they are oft en related to movements in build- ings. GEUS did not receive any reports from this earthquake being felt or heard. It is oft en the ground impact of meteors that is associated with danger, but the meteor that hit Chelyabinsk, Russia, on 15 February 2013 was a clear reminder that the shock wave may pose a signifi cant risk. More than 1600 people were hurt from falling debris and over 7300 buildings were damaged. Seismic waves were observed at distances of more than 4000 km. Th is corresponds to an earthquake magnitude of 3.6 (Heimann et al. 2013). Th e challenge of marking seismic events of non-tectonic nature in the earthquake database remains and we are still not able to identify and mark them all. Although we have recorded explosions, glacial earthquakes, thunder and fi re- balls we still have not detected any footquakes in Denmark. Footquakes are seismic signals from sport events like foot- ball, an example comes from the 2006 African Cup. When Cameroon scored goals during the games, people in Cam- eroon who watched the games on TV jumped a lot and gen- erated simultaneous signals on 20 seismic stations across the country (Euler 2007). References Cates, J.E. & Sturtevant, B. 2002: Seismic detection of sonic booms. Jour- nal of the Acoustical Society of America 111, 614–628. Euler, G.G., Wiens, D.A. & Loft on, K.M. 2007: Footquakes. IRIS News- letter 1, p. 13. Haack, H. 2012: Meteoritter – tidskapsler fra solsystemets oprindelse, 189 pp. Copenhagen: Gyldendal. Haack, H. et al. 2012: Maribo – a new CM fall from Denmark. Meteorit- ics and Planetary Science 47, 30–50. Heimann, S., González, Á., Wang, R., Cesca, S. & Dahm, T. 2013: Seismic characterization of the Chelyabinsk meteor’s terminal explosion. Seis- mological Research Letters 84, 1021–1025. Kanamori, H., Mori, J., Anderson, D.L. & Heaton, T.H. 1991: Seismic excitation by the space shuttle Columbia. Nature 349, 781–782. Kappus, M.E. & Vernon, F.L. 1991: Acoustic signature of thunder from seismic records. Journal of Geophysical Research-Atmospheres 96, 10989–11006. Sørensen, A.N. 2014: http://www.stjerneskud.info/fi reball/ event2014-06-16-01-15/ Authors’ address Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: pv@geus.dk 0 1 2 3 L o g a m p li tu d e 1 2 5 10 20 50 Frequency (Hz) Meteroid Thunder Earthquake Fig. 6. Frequency spectra of meteoroid, thunder and earthquake waves. A time window of 2 s around the signal on the up/down Z-sensor from Sta- tion1 was used. The amplitude is uncorrected. The Nyquist frequency is 50 Hz.