Suspected seismic signals from DFN fireballs

1. Introduction

When a meteoroid enters the atmosphere, it experiences aerodynamic drag and dynamic pressure. Shock waves can be generated by the hypersonic flight in the atmosphere, fragmentation/airburst and impact in the ground. The hypersonic projectile motion in the atmosphere causes the formation of a Mach cone [1-3]. The shock waves generated during this hypersonic entry propagate almost perpendicular to the trajectory. Fragmentation of the meteoroid creates shock waves that propagate omnidirectionally [1,2]. In large impact events, bolides and/or crater events, the first wave to arrive at the seismic station is the P wave generated directly under the terminal point of the trajectory [4]. After the P wave, air-coupled Rayleigh wave arrive. Airwaves generated by the Mach cone will arrive later as they travel at the speed of sound [1]. The airwave that originates from the point of the trajectory having the shortest distance to the seismic station arrives first and they show the strongest seismic signals in time series data [1,4]. In fireball events, airwaves are a dominant seismic signature [1,4].

2. Aim and Methodology

In this study, we searched for seismic signals from fireballs that have been observed by the Desert Fireball Network (DFN), over a 6-year observational period (2014-2019). The DFN is the world’s largest fireball camera network, located in the Australian outback and consisting of 52 observatories, covering an area of 3 million km² aimed to detect fireballs, recover meteorites and to calculate their orbits [5,6]. We used processed trajectory data from the DFN [6], with seismic data acquired from the Australian National Seismograph Network (ANSN).

The criteria that determined if a seismic signal in time series data could be confidently classified as a signal coming from a fireball event were that the amplitude of the signals must be similar or lower than previously confirmed seismic signals from fireballs, the signal must be within the calculated arrival times of the airwave (direct or ground-coupled Rayleigh wave), there must not be any earthquake activity at the same time, and there must not be any clear anthropogenic-related noise.

We checked if a seismic station could encounter the planar wavefront from the Mach cone. If the shortest distance to the seismic station is perpendicular to the bright flight trajectory and arrival times for the airwaves fit, signals are classified as originating from the Mach cone. If the shortest distance is not perpendicular to the bright flight trajectory, any seismic signals (if they fit with calculated arrival times), are assumed to come from an omnidirectional source that could be caused by fragmentation along the trajectory.

3. Results

Weak and short seismic signals were found for 24 fireball events out of 995 surveyed within 200 km of a seismic station (corresponding to 2.4%). The observed seismic signals in our dataset correspond to airwaves (either as direct airwaves or ground-coupled Rayleigh waves). We found 13 fireballs for which we suspect the signals to have originated from the Mach cone traverse and for 11 fireballs we detected signals that might originate from an airburst. No surveyed fireballs were detected by more than one seismic station. The total of 18 out of 24 signals showed the highest peak in vertical component. The shortest distance between the bright flight trajectory to the seismic station is about 50 km. Fireballs for which seismic signals have been detected cover the complete range of impact angles.

4. Discussion and Conclusion

The weak and short signals that we see in our data are likely direct airwaves, or ground-coupled Rayleigh waves generated by fireball events. In many cases it is not possible to distinguish whether the signal originated from the direct airwave or ground-coupled Rayleigh wave due to overlapping arrival time windows and background noise. The reason why we see signals of some fireballs and not others is probably due to distance, directionality, noise, wind and properties of the seismic station.

We report possible detections of seismic signatures originating from 2.4% of surveyed fireballs observed by the DFN. Unlike other studies who used data from images, seismic stations and infrasound to calculate the orbit and energies of meteors, this study uses information about the trajectory and timing of fireballs observed by the DFN to search for seismic signals.

The importance of this work is evident as these impact events occur on a daily basis, yet are rarely reported as seismic events because their impact energy is often not sufficient to cause quakes that are detectable by seismic stations. Furthermore, understanding frequent meteoroid encounters on Earth could help us make better predictions about what may be impacting Earth and other planetary bodies, such as Mars, in terms of small impact events.

References

[1] Edwards W. N. et al. (2008) Rev. Geophys., 46(4).

[2] Tancredi G. et al. (2009) Meteoritics & Planet. Sci., 44, 1967-1984.

[3] Tauzin B. et al. (2013) Geophys. Res., 40(14), 3522-3526.

[4] Brown P. G. et al. (2003) Meteoritics & Planet. Sci., 38, 989-1003.

[5] Devillepoix H. A. R. et al. (2018) Meteoritics & Planet. Sci., 53(10), 2212-2227.

[6]  Devillepoix H. A. R. et al. (2019) MNRAS, 483(4), 5166-5178.