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Earthquake could have happened with no physical contact. Just the shock waves hit the earth (think of the airburst of a MOAB, or Hiroshima). More(!) at the link. (And where do I sign up to get a research grant to study this rare event! Seriously - would be interesting and PERHAPS useful!?)

http://www.blackwellpublishing.com/products/journals/aag/AAG_February07/aag_48106.htm

The 1974 Bala earthquake

The article on “The enigmatic Bala earthquake of 1974” (Musson 2006) is an important contribution to the study of a long-neglected geophysical phenomenon, and one that will be essential reading for anyone who is interested in British seismology. However, the author appears to perpetuate some misunderstandings of meteoric processes, which ought to be corrected.

Musson states that “the event is still listed as a possible impact event in Stratford’s (2004) catalogue”, and says that “a magnitude of 3.5 and 4.0 ML would be equivalent to a blast between some hundred tons of TNT and a small nuclear weapon (~4 × 1011 to 4 × 1013 J), and such an impact could not but leave a significant crater”. In fact, the Berwyn Mountains explosion is listed in a table of “Historical fireballs (not impacts) in Britain”. More importantly, the work of Sekanina (1983), Chyba et al. (1993) and Hills and Goda (1993) has shown that large chondritic meteoroids entering the Earth’s atmosphere at interplanetary speeds are subject to aerodynamic pressures that exceed their mechanical strength and cause them to break up or even explode. The meteoroid in effect creates a transient crater in the atmosphere. The kinetic energy of the meteoroid is transferred to the atmosphere, and the longitudinal waves from the explosion can reach the Earth’s surface and shake buildings just as P-waves and S-waves from a seismic focus propagate upwards to the Earth’s surface. There is no need for any large meteorites to reach the ground to cause an observable “earthquake”.

There are many examples of “earth tremors” caused by the atmospheric explosion of large meteoroids. The Tunguska explosion of 30 June 1908 at h~ 8 km was recorded on seismographs in Irkutsk (about 1000 km from the focus of the explosion) and perhaps even as far away as Leningrad (St Petersburg), at r~ 3700 km (Verma 2005). The Chicora (Pennsylvania) fireball of 24 June 1938 was mistaken for the explosion of a magazine; the meteoroid is estimated to have had an initial mass m0~ 500 tonnes, but it yielded only two small meteorites, with a total mass of 303 g (Hey 1966). The great American fireball of 24 March 1933 caused terror across nine states, but the resulting Pasamonte (New Mexico) meteorite fall totalled only 3–4 kg (Hey 1966).

In Britain, the Chilterns fireball of 20 November 1887 was actually mistaken for an earthquake; tremors were felt from Saffron Walden and Cambridge to Wantage (Fordham 1888), and there was slight damage to buildings in Royston and mid-Bedfordshire. It being November, foggy weather prevented visual observations of the fireball along most of its path. The Welsh Borders fireball of 25 January 1894 caused earth tremors at Worcester and at Stokesay (10 km NW of Ludlow), and aroused fears of an explosion at Droitwich (Denning 1894). This fireball attracted the attention of Charles Fort, and as a result it is still occasionally mentioned in books about UFOs, earthlights, etc. Although Musson mentions that the shock wave from the Pontlyfni meteorite of 14 April 1931 was mistaken for an earthquake, he does not explain that Pontlyfni was the smallest meteorite known to have fallen in Britain, with m= 140 g, and that the “earthquake” was due to the atmospheric shock wave rather than the impact of the meteorite, which fell about 10 yards from a Mr Jones and caused no damage at all (King 1931). If Mr Jones had not been on the spot, the meteorite would not have been recovered, and the event might still be regarded as an earthquake or have been entirely forgotten.

Granted that exploding fireballs (“airbursts”) can produce seismic waves in the Earth’s crust, it should be possible to study these waves observationally and theoretically, both to distinguish them from the seismic waves produced by true earthquakes and to relate their properties to those of the airburst that caused them. It might contribute to the study of these “airburst seismic waves” if fireball observing stations were to include a seismograph as one of their instruments.

Fireball statistics suggest that airbursts with E > 1 kton TNT should occur over the British Isles on a timescale of about a century. In view of the arguments presented in this letter, it is worth considering what observable phenomena such an airburst would produce, in particular whether these phenomena could be mistaken for an earthquake. Unfortunately, the rarity of such large fireballs means that none has offered itself for study within the lifetime of advanced seismic arrays in Britain.