R.N. Hunter 1976 <p>Various periodicities have been suspected in earthquake activity, but none has yet been proved. In his classic text on seismology, K.E Bullen remarked that small but discernable "trigger" forces, such as tidal effects, temperature changes or barometric changes, have been thought to act as "last straw" phenomena when the earthquake would have occurred anyway.&nbsp;</p> application/pdf en U.S Geological Survey Is there a periodicity in the occurrence of earthquakes? article