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Seasonal Variations in the Magnitude-frequency Distribution of California Earthquakes

Description: 

Since 1812, California has hosted 45 earthquakes of magnitude M≥6.5. Of those events, 15 occurred during the months of April and December, while only 3 occurred during the months of July, August and September. Though a chi-squared test comparing the complete distribution of events per month to a uniform distribution produces a sampling probability p-value of greater than 10%, it is still interesting to analyze variations in the magnitude-frequency distributions (MFDs) of more recent California earthquakes for months with many M≥6.5 earthquakes compared to months with few M≥6.5 events. To do so, a catalog of M2+ earthquakes that occurred between 1980-2020 is declustered and separated by their month of occurrence into 12 subsets; a window sampling scheme is then used to combine three consecutive years of earthquakes that occurred during a particular month and organize them into a MFD. A Gutenberg-Richter (GR) relation is then fitted to the distribution and its a- and b-parameters are recorded. Time series plots of the a- and b-parameters demonstrate a sinusoidal oscillation through time with a period length of about one year that fluctuates beyond the 95% confidence interval bounds of neighboring points. Spectral analysis of these time series demonstrates strong peaks at integer multiples of one year^-1. Below 35.5°N, a-parameters (b-parameters), on average, increase (decrease) during the spring months and demonstrate the opposite pattern in the late summer. Above 35.5°N, the seasonal pattern is reversed. Roughly equal numbers of M≥6.5 events occurred on either side of 35.5°N, and so the distribution of months that hosted M≥6.5 earthquakes does not appear influenced by these annual variations. Seasonal variations in earthquake occurrence rates in other regions have been documented and attributed to climatic or astronomical processes; this study will further explore how the occurrence rates of earthquakes, represented by the GR a-parameter, as well as the ratio of large to small earthquakes, represented by the GR b-parameter, may be influenced by exogenous climatic or orbital effects.

Session: Building and Decoding High-resolution Earthquake Catalogs With Statistical and Machine-learning Tools [Poster]

Type: Poster

Date: 4/15/2025

Presentation Time: 08:00 AM (local time)

Presenting Author: Erika

Student Presenter: Yes

Invited Presentation: 

Poster Number: 38

Authors