Combining Geological and Seismological Methods to Re-Estimate the Magnitude of the 1920 Haiyuan Earthquake

Date: 4/26/2019

Time: 06:00 PM

Room: Grand Ballroom

The magnitudes of large earthquakes are important not only because they represent the most devastating events that have occurred but also because of their key role in converting strain rate into earthquake rate for probabilistic seismic hazard assessment. Modern geodetic tools such as InSAR and GPS are generating velocity fields with increasing spatial and temporal resolution, allowing ever more detailed strain rate mapping. The magnitudes of the large historical events, however, are more challenging to determine. Due to their long recurrence periods, many of these events happened before the instrumental period. Their magnitudes were often converted from the intensity of shaking based on historical writings. This method has large uncertainty. The 1920 Haiyuan Earthquake in Gansu, China, is the largest known earthquake in the tectonically active Northeast Tibetan Plateau. It happened 30 years into the instrumental period, but 15 years before the concept of magnitude was coined. Its oft-quoted magnitude, M ~ 8.5, was derived from the intensity of shaking, which we now think is likely overestimated based on geological offset measurements. Early seismological studies were only based on waveforms from one or two stations due to the difficulties of data sharing at that time. However, good preservation and the mass digitization of historical seismograms and the availability of high-resolution optical satellite imagery offered us a golden opportunity to revisit this earthquake and reestimate its magnitude using a combined geological and seismological approach. First, horizontal offsets were measured from orthorectified Pleiades satellite imagery to give a moment magnitude of Mw = 7.64 ± 0.12. Second, historical seismograms were digitised and modelled to give mB = 7.89 ± 0.29, Ms = 8.22 ± 0.21 and Mw = 7.94 ± 0.14. These results confirm that the previously reported magnitude was overestimated. The newly estimated magnitudes will provide an important constraint for future seismic risk assessment in this area.

Presenting Author: Qi Ou

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