The prolonged quiescence anomaly and the future trends of strong earthquakes in Yunnan region

From the Lijiang M_S7.0 earthquake in 1996 to August 22, 2023, a significant phenomenon of seismic quiescence has lasted for 27.55 years, which are the longest quiet period since 1900, both for M≥6.7 and M≥7.0 earthquakes in Yunnan Province. We further analyze and summarize the quiescence anomaly phenomenon of Yunnan’s strong earthquake and its relationship to the seismic status of the Qinghai-Xizang Plateau and Sichuan-Yunnan block on a larger spatio-temporal scale. Meanwhile, we discuss the future seismic trends in Yunnan region. Here are the conclusions.

1） The energy released by all M≥5.0 earthquakes per year in Yunnan region is equivalent to an M6.7 earthquake, which means an M6.7 represents the mean released energy per year, so we choose M6.7 as the lower limit of strong earthquake to study the seismicity in Yunnan region. The retrospective analysis based on historical earthquakes indicates that the characteristics of active-quiet alternation in Yunnan region, Sichuan-Yunnan block, and QinghaiXizang Plateau, which are closely related to tectonic settings of M≥6.7 or M≥7.0 earthquakes.

Since 1887, there are four active-quiet alternative periods for M≥6.7 earthquakes in Yunnan region, the active periods are 1913−1925, 1936−1955, 1970−1979, 1988−1996 with the mean duration of （11.96±5.35） years, while the quiet periods are 1887−1913, 1925−1936, 1955−1970, 1979−1988 with the mean duration （15.07±7.57） years. As for M≥7.0 earthquakes, the active periods are 1913−1925, 1941−1950, 1970−1976, and 1988−1996 with the mean duration （8.40±2.12） years, while the quiet are 1887−1913, 1925−1941, 1950−1970, 1976−1988 with the mean duration （18.58±5.87） years. The annual rate of occurrence is 0.42−0.83 times per year for M≥6.7 earthquakes and 0.27−0.63 times per year for the M≥7.0 earthquakes in Yunnan region.

Since 1887, there have been two active-quiet alternative periods for M≥6.7 earthquakes in the Sichuan-Yunnan block, and the active periods are 1887−1925, 1948−1996 with the mean duration （42.47±7.38） years, while the quiet periods are 1925−1948, 1996−2022 with the mean duration （24.89±2.40） years. The annual rate of occurrence is 0.16−0.25 times per year for M≥6.7 earthquakes in Sichuan-Yunnan block.

Since 1897, there have been three active-quiet alternative periods for M≥7.0 earthquakes in Qinghai-Xizang Plateau. The active periods are 1904−1956, 1970−1976, and 1988 to now, while the quiet periods are 1897−1904, 1956−1970, 1976−1988 with the mean duration （10.88±3.26） years. The annual occurrence rate is 0.88−1.36 times per year for M≥7.0 earthquakes in Qinghai-Xizang Plateau.

2） Since 1887, there have been significant differences in seismicity around Yunnan region for its five quiet periods of M≥6.7 earthquakes. The quiescence periods Ⅰ , Ⅱ , and Ⅴ appeared under the background of the seismic activity of M≥7.0 earthquakes in QinghaiXizang Plateau, namely, seismic quiescence appeared in Yunnan region while activity in Qinghai-Xizang Plateau. However the quiescence periods Ⅲ and Ⅳ appeared under the background of the quiescence of M≥7 earthquakes in Qinghai-Xizang Plateau, namely both quiescence in Yunnan region and Qinghai-Xizang Plateau. Currently, the period Ⅴ has lasted for 27.55 years in Yunnan region, and the surrounding seismic activity is similar to that in the quiet periods Ⅰ and Ⅱ . Therefore we deduce the following seismic trend may be in relatively weak active periods, similar to the periods Ⅰ and Ⅱ .

3） The comparison of seismicity between the Big Triangle area and Qinghai-Xizang Plateau shows that the active period （1904−1956） of M≥7.0 earthquakes in Qinghai-Xizang Plateau corresponds to the active period （1879−1957） of M≥8.0 earthquakes in Big Triangle area; from 1956 to 1988 there were two quiet periods in Qinghai-Xizang Plateau which correspond to the quiet period （1954−2001） in Big Triangle area, seismicity of M≥7.0 earthquakes in Qinghai-Xizang Plateau has been in an active period since 1988 while the seismicity of M≥8.0 earthquakes in Big Triangle area has come into a new active period since 2001. These seem to indicate that the active periods are consistent, as well as the quiet periods of M≥7.0 earthquakes in Qinghai-Xizang Plateau and M≥8.0 earthquakes in the Big Triangle area. The analogy of the current and the period （1840−1957） of M≥8.0 earthquakes in the Big Triangle area suggests that the current active period of M≥7.0 earthquakes and M≥8.0 earthquakes in the Big Triangle area of East Asian continent will last for decades in the future.

4） On September 5, 2022, an M_S6.8 earthquake occurred in Luding, Sichuan Province. The earthquake was located on the Moxi fault, the southeast section of the Xianshuihe fault zone. The quiet period of M≥6.7 earthquakes that lasted for 26.59 years in the SichuanYunnan block since 1996 has been terminated by the Luding M_S6.8 earthquake. The following seismicity may be a new active period of M≥6.7 earthquakes in the Sichuan-Yunnan block that analogizes the seismicity of quiet and active periods around 1948.

5） Since 1997, all earthquakes with M≥7.0 in Qinghai-Xizang Plateau have concentrated in the Bayan Har block except for several occurred in the Himalayan belt and Myanmar arc region. The retrospective analysis based on historical earthquakes indicates that the main region of seismic activity is alternative in the Sichuan-Yunnan block and Bayan Har block: Seismic activity appeared in the Bayan Har block while quiescence in the Sichuan-Yunnan block, by contrast, seismic activity appeared in the Sichuan-Yunnan block while less activity in Bayan Har block. Based on the inference, seismicity will be active in the Sichuan-Yunnan block and weak in the Bayan Har block after the Luding M_S6.8 earthquake in 2022.