Earthquakes are caused primarily by tectonic stresses building up slowly and suddenly being released along faults in the Earth’s crust. However, evidence has been mounting that earthquakes are also affected by stresses from other sources, including loading by surface water (such as lake levels and snowpack), changes in water pressure at depth, and even tidal stresses caused by the sun and moon’s gravitational pull. The periodic nature of all of these changes causes seismicity to be modulated in time, but many of the processes have similar periods, making it difficult to understand which is most important for affecting earthquakes.
Dutilleul et al.  provide additional evidence that shallow earthquakes along the San Andreas Fault near Parkfield, California, have a significant 6-month periodicity, with earthquake rates from 2006 to 2014 being preferentially higher in June and December. Interestingly, this 6-month periodicity has a very different timing from 1994 to 2002, when peaks occurred in March-April and September, nearly exactly out of phase with the 2006 to 2014 modulation.
Stress changes from water loading (precipitation) were previously thought to be the most likely cause of the periodicity, but attempts to model the seismicity changes were unsuccessful, making it unclear why the periodicity exists. Given the major seismic hazards from large earthquakes on the San Andreas Fault, it is crucial to better understand what physical processes contribute significantly to variability in seismicity.
Citation: Dutilleul, P., Johnson, C. W., & Bürgmann, R. . Periodicity Analysis of Earthquake Occurrence and Hypocenter Depth near Parkfield, California, 1994–2002 versus 2006‐2014. Geophysical Research Letters, 48, e2020GL089673. https://doi.org/10.1029/2020GL089673
―Victor Tsai, Associate Editor, Geophysical Research Letters