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

Allen, Trevor I.

doi:10.1007/s10950-021-10004-5

Cited by 4 publications

(2 citation statements)

References 62 publications

(79 reference statements)

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“…For example, the mean rate of M W 5.0 earthquakes per unit area was rounded, and then doubled as a conservative measure to account for uncertainty in earthquake occurrence. In contrast, the NSHA18 first corrected the magnitudes of historical earthquakes to account for the use of Californian local magnitude formulae (Allen, 2021) and then used the mean earthquake rate (with some uncertainty) to estimate the source-rate model. Lam et al (2016) also assumed a b- value that is lower than that estimated through the NSHA18 (Allen et al, 2018b).…”

Section: Existing National Hazard Modelsmentioning

confidence: 99%

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Exploring Australian hazard map exceedance using an Atlas of historical ShakeMaps

2023

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This article explores several area-based tests of long-term seismic hazard forecasts for the Australian continent. Using the observed seismicity, ShakeMaps are calculated for earthquakes that are expected to have generated moderate-to-high levels of ground shaking within continental Australia in the past 50 years (January 1972 through December 2021). A “composite ShakeMap” is generated that extracts the maximum peak ground acceleration “observed” in this 50-year period for any site within the continent. The fractional exceedance area of this composite map is compared with four generations of Australian seismic hazard maps for a 10% probability of exceedance in 50 years (~1/500 annual exceedance probability) developed since 1990. In general, all these models appear to forecast higher seismic hazard relative to the ground motions that are estimated to have occurred in the last 50 years, with the most recent hazard model yielding a fractional exceedance area most similar to the target 1/500 annual exceedance probability. The sensitivity of these results to various modeling assumptions was tested by exploring an alternative ground-motion characterization model that forecasts higher overall ground-shaking intensities. The sensitivity of these results is also tested with the interjection of a rare scenario earthquake with an expected regional recurrence of approximately 8700 years. While these area-based analyses do not provide a robust assessment of the performance of the candidate seismic hazard for any specific location given the limited independent data, they do provide—to the first order—a guide to the performance of the respective maps at a continental scale.

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Section: Existing National Hazard Modelsmentioning

confidence: 99%

“…Finally, there remain uncertainties in catalog magnitudes, both through the correction of local magnitudes due to the use of inappropriate magnitude formulae (Allen, 2021) and through the conversion of local magnitudes to moment magnitudes (Allen et al, 2018c), as required for generating ground-shaking fields in ShakeMap.…”

Section: Assumptions and Limitationsmentioning

confidence: 99%

Exploring Australian hazard map exceedance using an Atlas of historical ShakeMaps

2023

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Standardizing Earthquake Magnitudes for the 2022 Revision of the Aotearoa New Zealand National Seismic Hazard Model

Christophersen,

Bourguignon,

Rhoades

et al. 2023

Bulletin of the Seismological Society of America

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The 2022 revision of the New Zealand National Seismic Hazard Model—Te Tauira Matapae Pūmate Rū i Aotearoa—requires an earthquake catalog that ideally measures earthquake size in moment magnitude. However, regional moment tensor solutions, which allow the calculation of moment magnitude MwNZ, were introduced in New Zealand only in 2007. The most reported magnitude in the national New Zealand earthquake catalog is a variation of local magnitude ML. In New Zealand, ML is systematically larger than MwNZ over a wide magnitude range. Furthermore, the introduction of the earthquake analysis system SeisComP in 2012 caused step changes in the catalog. We address the problems by converting magnitudes using regressions to define a standardized magnitude as a proxy for MwNZ. A new magnitude, MLNZ20, has an attenuation relation and station corrections consistent on average with MwNZ. We have calculated MLNZ20 for nearly 250,000 earthquakes between 2000 and 2020. MLNZ20 is a reasonable proxy for MwNZ for earthquakes with ML<5.5. For earthquakes with ML>4.6, MwNZ is reliably available. We have applied ordinary least squares (OLS) regression for MwNZ and MLNZ20 on ML before and after 2012. We argue that OLS is the most appropriate method to calculate a proxy for MwNZ from individual ML measurements. The slope of the OLS regression compares well to the slope from the method of moments, which accommodates equation error that is present when there is scatter beyond measurement error, as is the case for our magnitude data. We have defined as a proxy for MwNZ a standardized magnitude Mstd, which is Mw when available, MLNZ20 with some restrictions as a second choice, and otherwise the magnitude derived from regression. Standardization of the magnitudes reduces the total number of earthquakes with a magnitude of ≥4.95 by more than half and corrects step changes in the spatial distribution of earthquakes between 2011 and 2012.

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Resolving the Location and Magnitude of the 1918 Queensland (Bundaberg), Australia, Earthquake

Martin,

Cummins,

Griffin

et al. 2024

Bulletin of the Seismological Society of America

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Eastern Queensland (Australia) was struck by a major earthquake at ≈04:14 a.m. local time on 7 June 1918. Most previous studies have suggested that the epicenter of this earthquake lies off the coast of Bundaberg, between the port cities of Gladstone and Rockhampton. This epicentral location was based upon instrumental observations from the Riverview College observatory in Sydney. However, this epicenter lies ≈250 km to the northeast of an inland region that experienced both the strongest shaking effects and numerous felt aftershocks. We revisited available macroseismic data from 224 geographic locations and surviving instrumental observations for the 1918 Queensland earthquake to show that the most likely epicentral location was inland at ≈24.93° S and ≈150.88° E in the Banana Shire and North Burnett region. The re-estimated instrumental magnitude of Mw 6.0 ± 0.3 (1σ) makes it one of the largest onshore earthquakes in eastern Australia in the past century. Our observations also offer support for a viewpoint proposed in 1935 by an eminent Queensland geologist, Walter Heywood Bryan, that the 1918 earthquake was inland. Our study highlights the benefit of the critical evaluation of primary source materials, both archival and seismological, to study historical earthquakes in Australia that are relevant for modern seismic hazard analysis.

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

Cited by 4 publications

References 62 publications

Exploring Australian hazard map exceedance using an Atlas of historical ShakeMaps

Exploring Australian hazard map exceedance using an Atlas of historical ShakeMaps

Standardizing Earthquake Magnitudes for the 2022 Revision of the Aotearoa New Zealand National Seismic Hazard Model

Resolving the Location and Magnitude of the 1918 Queensland (Bundaberg), Australia, Earthquake

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