Characteristic Earthquake Magnitude Frequency Distributions on Faults Calculated From Consensus Data in California

Abstract

An estimate of the expected earthquake rate at all possible magnitudes is needed for seismic hazard forecasts. Regional earthquake magnitude frequency distributions obey a negative exponential law (Gutenberg‐Richter), but it is unclear if individual faults do. We add three new methods to calculate long‐term California earthquake rupture rates to the existing Uniform California Earthquake Rupture Forecast version 3 efforts to assess method and parameter dependence on magnitude frequency results for individual faults. All solutions show strongly characteristic magnitude‐frequency distributions on the San Andreas and other faults, with higher rates of large earthquakes than would be expected from a Gutenberg‐Richter distribution. This is a necessary outcome that results from fitting high fault slip rates under the overall statewide earthquake rate budget. We find that input data choices can affect the nucleation magnitude‐frequency distribution shape for the San Andreas Fault; solutions are closer to a Gutenberg‐Richter distribution if the maximum magnitude allowed for earthquakes that occur away from mapped faults (background events) is raised above the consensus threshold of M = 7.6, if the moment rate for background events is reduced, or if the overall maximum magnitude is reduced from M = 8.5. We also find that participation magnitude‐frequency distribution shapes can be strongly affected by slip rate discontinuities along faults that may be artifacts related to segment boundaries.

Distribution of Earthquakes on a Branching Fault System Using Integer Programming and Greedy‐Sequential Methods

“…Multiple calculations are made to assess the influence of the initial random hypocenter assignments; that influence is mitigated somewhat by using long simulation periods of 100 kyr or more. See Parsons and Geist (2009) and Parsons et al (2018) for more description of the method.…”

“…The Grand Inversion method uses simulated annealing to directly invert for the rupture rates of different earthquake magnitudes given numerous sources of data and constraints. Simplifying the problem to examine participation and nucleation rupture rates on faults given a regional earthquake catalog, Parsons et al (2018) apply several alternative methods and compare the results to those from the UCERF3 Grand Inversion. These methods include a physics-based earthquake simulator developed by Console et al (2015Console et al ( , 2017 and two combinatorial based methods: the greedy-sequential method first introduced by Parsons and Geist (2009) and the integer programming (IP) method introduced by Geist and Parsons (2018).…”

Complex Earthquake Sequences On Simple Faults

“…Seismic hazard models are one of the most societally relevant products of earthquake research, but they often rely on poorly tested assumptions due to the scarcity of data on the recurrence interval of large earthquakes. Whether individual faults are more characteristic than predicted by the Gutenberg‐Richter distribution remains a subject of discussion (Kagan et al, ; Mulargia et al, ; Page, ; Page & Felzer, ; Parsons & Geist, ; Parsons et al, , , Schwartz, ; Stirling & Gerstenberger, ).…”

Effect of fault damage zones on long-term earthquake behavior on mature strike-slip faults

“…Observations in regional and global earthquake catalogues generally show a log-linear decay of magnitude with increasing number of earthquakes, in agreement with the Gutenberg-Richter distribution. However, large earthquakes along individual faults or fault sections deviate from this behavior, showing a relatively elevated number of 'characteristic earthquakes' (Schwartz & Coppersmith, 1984;Wesnousky, 1994;Parsons et al, 2018) that follow a gaussian distribution in addition to smaller earthquakes that follow the Gutenberg-Richter distribution. This characteristic distribution is used as a basis for rupture forecast models, e.g., (Field et al, 2017).…”