Dear Max,
That "Bang" you heard at the time of the earthquake might have been the audible manifestation of a phenomenon that has been many times reported but never photographed - a flash of electromagnetic light caused by the sudden release of energy by the earthquake. A direct transfer of energy, as Stales suggests- from medium (rock) to medium (air), would seem likely to produce a rumble more so than a bang. A thunderclap produces a similar "Bang" as air rushes in to fill the vacuum left by a lightning strike.