A pragmatic approach to adjusting early instrumental local magnitudes for seismic hazard assessments in Australia

Trevor I Allen

doi:10.1007/s10950-021-10004-5

Abstract

Prior to the development of Australian-specific magnitude formulae, the 1935 magnitude correction factors by Charles Richter—originally developed for southern California—were almost exclusively used to calculate earthquake magnitudes throughout Australia prior to the 1990s. Due to the difference in ground-motion attenuation between southern California and much of the Australian continent, many earthquake magnitudes from the early instrumental era are likely to have been overestimated in the Australian earthquake catalogue. A method is developed that adjusts local magnitudes (ML) using the difference between the original (inappropriate) magnitude formulae (or look-up tables) and the Australian-specific formulae at a distance determined by the nearest recording station likely to have recorded the earthquake. Nationally, these adjustments have reduced the number of earthquakes of ML ≥ 4.5 in the early instrumental catalogue by approximately 25% since 1900, while the number of ML ≥ 5.0 earthquakes has reduced by approximately 32% over the same time period. The reduction in the number of moderate-to-large-magnitude earthquakes over the instrumental period yields long-term earthquake rates that are more consistent with present-day rates, since the development of Australian-specific magnitude formulae (approximately 1990). The adjustment of early instrumental magnitudes to obtain consistently derived earthquake catalogue is important for seismic hazard assessments.

Highlights

Earthquake catalogues are fundamental to probabilistic seismic hazard assessments (PSHAs) in low-seismicity regions where the seismic hazard is commonly characterised by distributed seismicity source models based upon the statistical analysis of catalogues (e.g., Bommer et al 2015; Burkhard and Grünthal 2009; Danciu et al 2018; Gerstenberger et al 2020; Grünthal et al 2018; Onur et al 2020)
For the 2018 National Seismic Hazard Assessment (NSHA18) of Australia (Allen et al 2020), a pragmatic method was developed that adjusts magnitudes using the difference between the original magnitude formula and the Australianspecific corrections at a distance determined by the nearest recording station likely to have recorded the earthquake (Allen et al 2018b)
The method first introduced by Allen (2010) and updated here corrects magnitudes using the difference between the original magnitude formula (e.g., Bakun and Joyner 1984; Richter 1935) and the Australian-specific relationships (e.g., Gaull and Gregson 1991; Greenhalgh and Singh 1986; MichaelLeiba and Malafant 1992) at a distance determined by the nearest recording station likely to have recorded the earthquake

Summary

Introduction

Earthquake catalogues are fundamental to probabilistic seismic hazard assessments (PSHAs) in low-seismicity regions where the seismic hazard is commonly characterised by distributed seismicity source models based upon the statistical analysis of catalogues (e.g., Bommer et al 2015; Burkhard and Grünthal 2009; Danciu et al 2018; Gerstenberger et al 2020; Grünthal et al 2018; Onur et al 2020). These assessments can be highly dependent on relationships between small-tolarge earthquakes (e.g., Gutenberg and Richter 1944) from which earthquake rupture forecasts are generated. Unless these conversions are time-dependent, they commonly assume that the estimation of ML has been consistent for the observation period

Methods

Findings

Discussion

Conclusion