Investigation of long-term and short-term seismicity in Vietnam

In this paper, seismicity of the territory and continental shelf of Vietnam was investigated in depth using two earthquake catalogs representing long-term and short-term observation periods. The first catalog consists of 894 historical, field investigation and instrumental events covering a period of 880 years, from 1137 to 2017, and the second catalog consists of 587 events instrumentally recorded during 13 years, from 2005 to 2017. From these two catalogs, further subcatalogs were derived: Catalog 1a, 1137 through 2017, all events with M ≥ 5.0 Catalog 1b, 1980 through 2017, all events with M ≥ 3.0 Catalog 2a, 2005 through 2017, all events with M ≥ 0.7 Catalog 2b, 2005 through 2017, all events with M ≥ 3.0 The spatial analysis results show that the largest earthquakes with magnitude of 6.8 have occurred in the northwestern part of the country. Earthquakes with magnitudes exceeding 5.0 are distributed along known active faults systems, with high density in the territory of Northern Vietnam. Statistical methods were applied for declustering the 1a and 2a earthquake catalogs, and then the maximum likelihood method was used to estimate the a and b parameters of the Gutenberg–Richter law and the magnitude of completeness M c for the territory of Vietnam and seismotectonic provinces. For the territory of Vietnam, the b -value determined for the long-term seismicity is around unit, while for the short-term seismicity, the b value reaches unit at the threshold M c = 4.0. A time analysis method was carried out to investigate the distribution of waiting time of earthquakes in Vietnam, using Catalog 2b. Finally, a unified scaling law of earthquakes in Vietnam was established for both long-term and short-term seismicity cases, which links together the Gutenberg–Richter law, the Omori law of aftershocks, and the fractal dimensions of the faults.