Here, evidently some 30 hours before the shock, earthquake lights were reported in what was soon to be the epicentral region. An hour later a friend told me that the ground all around was a blaze of fire. Some of these later explosions evidently accompanied earthquake aftershocks. The phenomenon of slowly rolling waves, like the waves at sea, was reported from many places in the San Francisco earthquake. Several of these accounts explicitly compare the ground motion observed to that of waves in the ocean.
Similar accounts are also in descriptions of other earthquakes, especially that in Lisbon.
What he presumed was happening was that soft alluvial deposits can be bent and do not fracture as readily as the hard rock beneath. If a great mass of gas suddenly comes from cracks in the rock, it may lift up this carpet and in that case, gravity waves quite similar to the waves in the ocean would be set up. We see that these descriptions make major earthquakes look much like violent eruptions, quite similar to gas eruptions from volcanoes or mud volcanoes. The airborne noises, the flames, the air pollution are all similar, and while most of the intense effects take place at the time of the quake, some of the effects occur as precursors and cannot therefore be ascribed to secondary effects of the mechanical deformation of the ground. It seems very strange that in all the attempts to predict earthquakes, no gas observations are included.