Background and precursory seismicities along and surrounding the Kunlun fault before the Ms8.1, 2001, Kokoxili earthquake, China

The Ms8.1 Kokoxili, China, earthquake on 14 November, 2001, occurred on the western portion of the Kunlun fault. By analyzing data from geological surveys and records of historical and modern earthquakes, we know that at least 8 Ms6.9-8.1 earthquakes occurred along the Kunlun fault from 1879 AD to 2000 AD. The spatial arrangement of these ruptures indicates the existence of two rupture gaps, the Kokoxili and the Maqu, on the fault before 2001. The Kokoxili gap and surrounding area experienced a long-term seismicity quiescence of Ms ⩾ 5 earthquakes and developed the corresponding background seismic gap at least from 1966 to 2001. The 2001 Kokoxili earthquake occurred on the Kokoxili gap. The Maqu gap on the eastern portion of the Kunlun fault was found to be a site where the seismicity rate of smaller earthquakes increased anomalously about 3.4 years before the Kokoxili earthquake. We analyze this anomalous increase in seismicity rate and conclude that it was a significant precursor of the Kokoxili earthquake. The cases in this study suggest that rupture gaps on a great inland strike-slip fault zone could be sites of potentially major or great earthquakes, or sites of precursory seismicities, emerging prior to major or great earthquakes on the same fault zone.