Apples and Oranges: Developing a Consistent Catalog of Local Magnitudes for the National Seismic Hazard Assessment of Australia

Date: 4/26/2019

Time: 06:00 PM

Room: Grand Ballroom

Modern probabilistic seismic hazard assessments rely on earthquake catalogs consistently expressed in terms of moment magnitude, M_W. However, M_W is still not commonly calculated for small local events by many national networks. The preferred magnitude type calculated for local earthquakes by Australia’s National Earthquake Alerts Centre is local magnitude, M_L. For use in seismic hazard forecasts, magnitude conversion equations are often applied to convert M_L to M_W. Unless these conversions are time-dependent, they commonly assume that M_L estimation has been consistent for the observation period. While Australian-specific local magnitude algorithms were developed from the late 1980s and early 1990s, regional, state and university networks did not universally adopt these algorithms, with some authorities continuing to use Californian magnitude algorithms. Californian algorithms are now well-known to overestimate earthquake magnitudes for Australia. Consequently, the national catalogue contains a melange of contributing authorities with their own methods of magnitude estimation.

The challenge for the 2018 National Seismic Hazard Assessment of Australia was to develop a catalog of earthquakes with consistent local magnitudes, which could then be converted to M_W. A method was developed that corrects magnitudes using the difference between the original (inappropriate) magnitude formula and the Australian-specific corrections at a distance determined by the nearest recording station likely to have recorded the earthquake. These corrections have roughly halved the rates of M_L 4.5 earthquakes in the Australian catalogue.

To address ongoing challenges for catalog improvement, Geoscience Australia is digitising printed and hand-written observations preserved on earthquake data sheets. Once complete, this information will provide a valuable resource that will allow for further interrogation of pre-digital data and enable refinement of historical catalogs.

Presenting Author: Trevor Allen

Authors