Figure 1. Unusual phenomena, including a seismic event, were observed near Banjawarn Station in western Australia in 1993. Five years worth of PDE listed seismicity indicate that earthquakes are relatively rare in this region. Of those listed in the PDE, only a few events are large enough to be recorded by IRIS GSN station Narrogin (NWAO).
In June 1995, the event raised concern when newspapers reported on the Aum Shinrikyo's attempt to enrich uranium at Banjawarn Station, just north of the estimated epicenter of the seismic event (Figure 1). Suspicion arose that the series of events from 28 May 1993 were related to activities of the Aum Shinrikyo cult.
a) Explosion or Earthquake? The event was recorded by the IRIS GSN station Narrogin (NWAO) at 650 km distance (Figure 1). Our most sensitive Australian GSN station, Tennant Creek (WRAB), was not operational in May of 1993 and the event was too small to be recorded at more distant GSN stations. On the NWAO recordings, the signal appears to have a high frequency content, although we see only a small range of signal frequency from about 2 Hz to 10 Hz, the nyquist frequency of the instrument. At and below 1 Hz, the signal is buried in noise. Above 2 Hz, the event is clear above the noise level with a small P phase and a large and impulsive looking Lg phase (Figure 2).
In many aspects, the event reflects a typical clandestine scenario: a small magnitude event is observed in a region of active mining where earthquakes occur at shallow depths and instrument coverage is poor. While a range of possible sources have to be considered, we can rule out that the event was caused by a legal mining explosion. Mining regulations for western Australia prohibit the use of explosives after sunset and restricts the size of blasts to 30 tons. The largest mining blasts in this region were recorded with magnitudes between 2.0 and 2.8. The seismic event in question occurred at 11:03 p.m. local time and was 170 times larger than the largest mining blast ever recorded in this region.
The high frequency content and the small size of the event make the analysis problematic. We focus here on a comparison study of the event with nearby earthquakes and mining explosions recorded at station NWAO along a similar source-receiver path as the event. To minimize the possibility of including undeclared mining blasts as earthquakes, only those earthquakes were selected that occurred at night. In addition to the earthquake data, NWAO recorded two mining explosions, both magnitude 2.8 events, in the vicinity of the event.
 Figure 2. The z-components of NWAO recordings of an earthquake, the May 28 mystery event, and a mining blast, bandpass filtered at 2-8 Hz. | Figure 2 shows a comparison of the event with a nearby earthquake and mining blast. The location of the events are marked in Figure 1. In the frequency band 2-8 Hz, the earthquake and the mystery event show relatively weak P-wave and strong S-wave energy. Significantly less S-wave energy is observed on the mining blast record. P/Lg ratios are low for all recordings in the frequency bands 2-4 Hz, 4-6 Hz and 6-8 Hz and show a small separation between earthquakes and mining blasts. Only for the 2-4 Hz frequency band were the S/N ratios high enough for a valid comparison. In this frequency band, the P/Lg ratios for the earthquakes are smaller than the P/Lg ratios for the mining explosions (0.140 and 0.141 for the former; 0.156 and 0.166 for the latter). For the event we calculated a P/Lg ratio 0.111 which is smaller than those for the earthquakes. This distribution of P/Lg ratios may indicate that the source for the May 28, 1993 event is more consistent with an earthquake-like source than with an explosion-like source, although many more regional earthquakes would be needed to appropriately calibrate the data. |
b) Meteorite Impact The seismic energy associated with the magnitude 3.6 event in western Australia is 7.6x1015 ergs (using the formula log (E) ergs = 5.8 + 2.8*M (M-magnitude) used by the Mundaring Geophysical Observatory). The seismic energy, however, would constitute only a fraction of the total energy released by a meteorite. For a static explosion on the surface, only about 0.01% of the total energy is converted to seismic energy. For a deeply buried source in hard rock (e.g. a well coupled underground explosion), less than 1% of the total energy may be converted to seismic energy. Assuming these values as the upper and lower bounds, the total energy of the meteorite at the time of explosion would be between 0.02 kT and 2.0 kT.
The estimated maximum energy level has direct implication for the composition of the meteorite. Recent studies on Earth-crossing objects show that asteroids of stony, carbonaceous, or cometary composition with associated kinetic energies below about 2 megatons do not reach the Earth's surface but typically explode at high altitude. Iron meteorites, however, may reach the Earth's surface with energy levels consistent with those derived from the seismic records.
 Figure 3. An iron-meteorite lass than 3 meters in diameter striking the Earth could generate a seismic event of magnitude 3.6, as observed in western Australia. It is estimated that approximately every 6 years a meteorite will impact on land and generate a seismic signal equivalent to an explosive yield of 1 kT or higher. | Figure 3 shows the modeling results for an iron meteorite with density 7.9 g/cm3, entering the Earth's atmosphere at oblique incidence angles (20, 45 and 60 degrees). We assumed the median impact velocity for Earth-crossing asteroids of 15 km/s. At the most probable angle of 45 degrees for an incident body, iron meteorites with radii between 0.5 m and 1.6 m, after ablation and deceleration in the atmosphere, would release total energy levels consistent with those derived from the seismic records, between 0.02 kT and 2.0 kT energy. Iron meteorites in this energy range typically impact Earth's surface, whereas stoney and carbonaceous objects explode at altitudes above 10km. A low altitude explosion is therefore unlikely. |
The meteorite impact scenario is consistent with the eyewitness observations and with the energy levels derived from seismic records for the event. Unfortunately, because of the lack of signal below 2 Hz, modeling of the NWAO recorded seismic data to further evaluate this scenario is inappropriate. Ultimately, the meteorite scenario could be confirmed if a meteorite was known to have entered the atmosphere over western Australia at the time the events occurred, or if an impact crater could be found in the vicinity of the epicenter. The impact of a meteorite with radius 1.6 meters would generate a crater more than 90 meters in diameter. Despite some preliminary searches, no impact crater has yet been found.
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