ALBERT

Description: The Changning MS6.0 earthquake and its moderate to strong aftershocks, which occurred in the axial area of Changning anticline, exhibit different features with the moderate to strong earthquakes that occurred in the southern syncline area nearby, although these two areas are only a dozen kilometers away. Whether this difference is only caused by different fault structures, or caused by local change is still a matter of debate. In order to answer this question, basing on the correction of small earthquake location and seismic velocity structure through double-difference tomography inversion, this paper determined the fine structure of the crustal stress field by using comprehensive focal mechanism solutions, and analyzed its mechanical consistency with the focal mechanism solutions of moderate to strong earthquakes. It is found that for the different subareas of Changning area, the maximum principal stress remains horizontal, and its azimuth remains almost East-West oriented although it has a small clockwise rotation from north to south; meanwhile, the stress regime exhibit a significant local change, which is thrust stress regime for the axial area of Changning anticline, against strike-slip stress regime for the southern syncline area. The stress fields in these two subareas highly accord with the focal mechanism solutions of moderate to strong earthquakes in the corresponding area, but poorly accord with, or even contradict that in the non-corresponding area, which indicates that the local stress change of stress field is the necessary mechanical basis for the complex seismicity behavior in Changning area. Additional rock mechanics analysis points out that lateral difference of Poisson's ratio of rock is probably the main cause of this local change of stress field. This paper also discusses the seismogenic fault structure of Changning MS6.0 earthquake sequence, and suggests the activation of the basement fault(s) in the depth range of 6 to 9 km in the axial area of Changning anticline driven by regional stress field is the potential cause of the occurrence of Changning MS6.0 earthquake sequence.

Description: For a better understanding of the sustainability of geothermal resources, we want to quantify subsurface masschanges caused by production and injection of fluids at the Theistareykir geothermal field in Northeast Iceland.For this purpose, we installed three superconducting gravity meters (iGrav006, iGrav015 and iGrav032) and twospring gravity meters (gPhone061 and gPhone128) in vicinity to the new geothermal power plant that started op-eration in October 2017.Prior to the Iceland installation, all gravity meters were setup at the gravimetric observatory J9 in Strasbourg forsimultaneous side-by-side measurements. The obtained data were used for instrumental calibration, comparisonof noise levels and tidal analysis. In Theistareykir, three of our measuring sites are set up close to the geother-mal production and injection wells. The fourth site is located outside the geothermal field, to provide referencemeasurements that are unaffected by the activities of the power plant. At each site additional physical parameters,which influence the local gravity signal, are measured. This includes the continuous monitoring of GPS-positions,rainfall, soil moisture and snow thickness. Moreover, snow weight and snow water equivalent are measured at thesite close to the production wells.Here, we present the results of the unique intercomparison of three superconducting gravity meters and twogPhones at Strasbourg and the initial time series obtained at the geothermal site in Iceland. A preliminary in-terpretation of the gravity variations with regard to the geothermal activities and the hydro-metrological dynamicsis given.