关键词: 玛多M_S7.4地震, 断层土壤气, 碳同位素, 氦同位素, 巴颜喀拉块体

Abstract:

An M_S7.4 earthquake occurred in Madoi County, Guoluo Tibetan Autonomous Prefecture, Qinghai Province of China at 02:04 (Beijing Time) on May 22, 2021. A total of seven 800~3 000m trans-fault survey lines were targeted laid along different parts of the seismic surface rupture zone(the west, mid-west, mid-east, and the east), one month after the earthquake when the detailed field investigation of the coseismic displacement and the spread of the seismic surface rupture zone had been carried out. The soil gases were collected and the concentrations of Rn, H₂, Hg, and CO₂ were measured in situ.
The results show that the maximum value of Rn, H₂, Hg and CO₂ concentrations in different fracture sections of the surface rupture was 2.10~39.17kBq/m³(mean value: 14.15kBq/m³), 0.4×10^-6~720.4×10^-6(mean value: 24.93×10^-6), 4~169ng/m³(mean value: 30.72ng/m³)and 0.73%~4.04%(mean value: 0.59%), respectively. In general, the concentration of radon is low in the study area, which may be related to the thick overburden and the lithology dominated by sandstone. The concentration characteristics of hydrogen and mercury released from soil have good consistency, and the concentrations are higher at the east and west ends of the surface rupture zones but were lower in the middle of the rupture zone. This is consistent with the field investigation showing that the earthquake-induced surface rupture zone and deformation are more concentrated in the western section, while the eastern section has a large amount of seismic displacement.
The fault strikes at the east and west ends of the Madoi M_S7.4 earthquake surface rupture have deviated from the NW direction to a certain extent, and there also exits two branching faults and rupture complexities at the east end of the main fault of the Madoi earthquake. In the west end of the surface rupture, i.e., the south of Eling Lake, the fault strike turns to EW direction. We laid two survey lines(line 2 and line 3)at the west end of the rupture, the concentration of Rn, H₂ and Hg escaped from line 3 is the lowest one among all lines while the gas concentration of line 2 is significantly higher. In the vicinity of line 3, the field geological survey did not find the cracked and exposed surface rupture, and only a small number of liquefaction points were distributed near the Eling Lake. The soil gas concentrations and morphological characteristics were consistent with the field phenomena. At the east end of the rupture zone, the soil gas morphological characteristics of the south and north fault branches were inconsistent: the soil gas of the south branch showed a single-peak type which was more similar to that at the west end, but the gas concentration pattern of the north fault branch showed a multiple-peaks type. This phenomenon is consistent with the characteristic shown in the surface fracture mapping, that is, the deformation zone of the rupture where is wider.
To find out the source of soil gas and the possible influencing factors of soil gas concentrations in the study area, the carbon isotope and helium isotope of the collected gas samples were analyzed. The value of ³He/⁴He shows that the noble gas in the study area is mainly an atmospheric source, but the results of δ¹³C and CO₂/³He show that the soil gas along the surface rupture of the Madoi earthquake has the mixed characteristics of atmospheric components and crustal components, which to a certain extent reflects the cutting depth of main fault-Jiangcuo fault may be shallow, and it is speculated that the surface rupture caused by Madoi M_S7.4 earthquake may be confined to the shallow crust.

Key words: Madoi M_S7.4 earthquake, fault soil gas, carbon isotope, ³He/⁴He, Bayan Har block