# Impact of Post-Earthquake Seismic Waves on the Terrestrial Environment
## 3 Gas Anomalies in the CCSD-LOS during the Magnitude-9 Earthquake in Sumatra  
In situ fluid analysis was carried out during the CCSD drilling in 2002-2005. The drilling mud was degassed by using the freeze-drying method to remove steam from the gas, then it was extracted from vacuum pump to the OmniStar spectrometer for on-line analysis of gas components, in order to measure real-time concentration data of He, CH\({}_{4}\), CO\({}_{2}\), H\({}_{2}\), Ar, He, N\({}_{2}\) and other components [16,17]. Figure 2 shows the variations of methane, carbon dioxide and hydrogen with depths observed in the main borehole during the scientific drilling [11]. It can be seen from the records that a high concentration of methane gas rose abnormally in the underground fracture rocks from 2400 m to 3700 m. High concentrations of carbon dioxide and hydrogen are abnormal in a deeper formation below 4900 m. Since these high concentration anomalies of gases are located far from the surface, they seem not coming from the atmosphere. The observation records (Figure 2) prove that certain amount of some gases, such as methane, carbon dioxide and hydrogen, may originate from the deep Earth as their contents usually increases with the depth. The questions are, do they come to the near-surface strata and, if so, how? Figure 1: The CCSD long-term crustal movement observation station (CCSD-LOS). (**a**) The location of the station in China; (**b**) the observation borehole of the CCSD-LOS. The picture shows the derrick and drill pipes for the installation of instruments, with a 5158 m-deep hole below; (**c**) some underground probes installed in the boreholes. They have now been installed in 4 layers underground; and (**d**) the supervisory control room. During the CCSD project performance, it is found that the gas contents in the borehole were changed significantly after big earthquakes. Records of gas contents during main earthquakes of magnitude 8 or larger occurred in Asia were obtained by the CCSD project; among them, the magnitude 8.7-9 Sumatra earthquake, occurring on December 26 th of 2004, is outstanding. The earthquake occurred near the northern part of Sumatra, the focal coordinates are 2.30\({}^{\circ}\) N and 93.10\({}^{\circ}\) E with the epicenter depth of 20.0 km. Figure 3a shows a three-component record of the Sumatra earthquake observed on the CCSD-LOS during scientific drilling, the depth of the seismograph detector is at 1559 m. The strong underground vibrations caused by the seismic waves were clear at the drilling site, and the surface wave caused by the earthquake lasted for 4-5 days after the earthquake, then slow down. Figure 2: The gas anomaly records observed during the CCSD drilling: (**a**) methane, (**b**) carbon dioxide, and (**c**) hydrogen. Figure 3b shows the gas content curves of the borehole fluids underground observed during the CCSD drilling from 7 December 2004 to 11 January 2005. The vertical ordinate is the date of observation; columns from the left to right are the seismic magnitude, records of helium, argon, argon/helium, argon/nitrogen, respectively. It shows that, from December 10 to 26, some earthquakes of magnitude 6 occurred in Sumatra. They caused fluctuations in the concentration of helium and argon slightly. After the magnitude 8.7-9 earthquake in Sumatra on December 26 th, helium concentration slightly decreased, and the argon concentration sharply increased. The high argon anomaly lasted for about five days, as well as the ratio of Ar/He or Ar/N\({}_{2}\). Positive anomalies of argon, argon/helium and argon/nitrogen ratios after the strong earthquakes indicate that there is a certain proportion of high Ar and low He fluid upwelling into the rocks below the CCSD-LOS after the earthquake. The distance between the CCSD-LOS and the epicenter of Sumatra earthquake is about 4000 km. This fluid upwelling at the CCSD-LOS after the earthquake could not come directly from the Sumatra seismic zone.