Multifrequential periodogram analysis of earthquake occurrence: An alternative approach to the Schuster spectrum, with two examples in central California

Periodic earthquake occurrences may reflect links with semidiurnal to multiyear tides, seasonal hydrological loads, and similar to 14 month pole tide forcing. The Schuster spectrum is a recent extension of Schuster's traditional test for periodicity analysis in seismology. We present an alternative approach: the multifrequential periodogramanalysis (MFPA), performed on time series of monthly earthquake numbers. We explore if seismicity in two central California regions, the Central San Andreas Fault near Parkfield (CSAF-PKD) and the Sierra Nevada-Eastern California Shear Zone (SN-ECSZ), exhibits periodic behavior at periods of 2 months to several years. Original and declustered catalogs spanning up to 26 years were analyzed with both methods. For CSAF-PKD, the MFPA resolves similar to 1 year periodicities, with additional statistically significant periods of similar to 6 and similar to 4 months; for SN-ECSZ, it finds a strong similar to 14 month periodic component. Unlike the Schuster spectrum, the MFPA has an exact modified statistic at non-Fourier frequencies. Informed by the MFPA period estimates, trigonometric models with periods of 12, 6, and 4 months (Model 1) and 14.24 and 12 months (Model 2) were fitted to time series of earthquake numbers. For CSAF-PKD, Model 1 shows a peak annual earthquake occurrence during August-November and a secondary peak in April. Similar peaks, or troughs, are found in annual and semiannual components of pole tide and tide-induced stressmodel time series and fault normal-stress reduction from seasonal hydrological unloading. For SN-ECSZ, the dominant similar to 14 month periodicity prevents regular annual peaking, and Model 2 provides a better fit (Delta(R) over bar (2)(adjusted): 2.4%). This new MFPA application resolves several periodicities in earthquake catalogs that reveal external periodic forcing.