SATELLITE THERMAL SURVEY - A NEW TOOL FOR THE STUDIES OF SEISMOACTIVE REGIONS

At present remote sensing methods are widely used for investigations in seismoactive regions, principally throug the application of satellite imagery of the Earth surface in the visible and near infra-red (IR) part of the spectrum with high spatial resolution. These images capture the spectral response of the Earth's surface and its relief. They are useful for the study of lineaments, morphological structures and neotectonic movements in seismically active regions. Visible and near IR images have been used to study lineaments and modern tectonic movements in seismoactive regions (Veber K., ۱۹۸۴).In comparison with near-IR and visible images, the Earth surface images obtained in thermal IR part of the spectrurn are generated due to surface temperature. The ground temperature, in its turn, depends on solar heating, surface thermal properties and meteorological conditions as well as on processes occurring in the near surface atmosphere and in the upper crust. It is the latter which are relevant for studies in seismoactive regions.It has been reported that pre-seismic activity alters the characteristics of soils, including soil moisture (Sugisaki R. et al. (۱۹۸۰)), gas content and composition (Sugusaki R., ۱۹۸۰, Rikitake T.۱۹۷۶). There are also numerous observations of surface and near surface temperature changes prior to Earth's crust earthquakes. For example, soil temperature anomalies of ۲.۵ °C were measured in the zone of preparation of the Tangshan earthquake (China, ۱۹۷۶ - magnitude more than ۷.۰) (Wang Linying. Zhu Chuanzeng, ۱۹۸۴). Thermal changes due to stress fields also have been determined in laboratory studies of materials.The advanced space-bom equipment for remote sensing in the IR spectrum: HCMR (HCMM), AVHRR۲ (NOAA), (ERS-۱) allows monitoring of the Earth's thermal field with a spatial resolution of ۰.۵ - ۱.۱ km and with a temperature resolution of ۰.۱۲ -۰.۵ °C. Surveys are repeated every ۱۲ hours for the NOAA series satellites. Therefore, variations of outgoing IR radiation during the period of preparation of earthquakes can be closely monitored.Examination of satellite derived night time thermal images taken over Central Asia (fig. ۱) several days before large Gazli earthquakes in ۱۹۷۶ and ۱۹۸۴ (with magnitudes larger than ۷.۰ on the Richter's scale of magnitudes) was revealed the appearance of abnormal IR emissions. The area of anomaly in both cases exceed ۱۰۰ ۰۰۰ km (fig.۲ and ۳). This coincidence provided the incentive to study long series of thermal satellite images along with seismicity of this region.