Earthquake seasonality before the 1906 San Francisco earthquake

Abstract

Since 1855, 40 earthquakes of magnitude ⩾5.5 have occurred on the San Andreas Fault (SAF) system in coastal central California, between latitudes 36°30′ N and 39°30′ N (Table 1)¹. Of the included 27 ‘independent’ earthquakes, which occurred more than 1yr after or 1 geocentric degree distant from a previous ⩾5.5 mag earthquake, 3 occurred in summer, 7 in autumn, 4 in winter and 13 in spring. All of the spring earthquakes occurred during the 25-yr interval that culminated with the great San Francisco earthquake (8.25 mag) in the spring of 1906 (Fig. 1a, b). I report here that the observed frequency of the spring earthquakes before 1906 significantly exceeds (binomial P = 0.002) seasonal frequencies expected from random variation in the earthquake rate, and thus invites serious inquiry into the subject of earthquake seasonality. More importantly, the time-clustering of spring earthquakes before 1906 raises the question of whether accelerated seasonal seismicity may herald the end of the ‘seismic cycle’ suspected¹ to modulate the long-term pattern of seismicity on the northern segment of the SAF system.