عنوان مقاله [English]
Since Iran is located on the Alpine-Himalayan seismicbelt and it has high seismicity, study of earthquake seismologyis necessary. Part of alpine-Himalayan seismic belt is Iranplateau that demonstrates high seismicity behavior and it has uniquedeformation.
Seismotectonic studies indicate a high density of active fault existencein this plateau. In all of Iran seismotectonic regime, eastof Iran seismotectonic zone due to the presence of strike slipfault system and occurrence of large earthquakes has a greatimportance. Destructive earthquakes such as 6.6Mw Bam (2006)and 6.7 MwRigan (2010) revealed high potential of earthquakeoccurrence. Intended area in this study is Rigan that is located in Kerman province in Iran. This area has important faults such as Kahurak, Bam, Nosratabad, Shahdad, Guk, Golbaf, Sirch and Sabzevaran which have high seismicity.
In this paper, we consideredcloud formation as earthquake precursor for Mohammad Abad-Rigan’s earthquake (2011) that is known less. Thermal precursor was also consideredin this study. According to the existing theories, Rises in stresscan produce initial fracture in the region. Therefore, with the rise intemperature, water evaporation in the pores of stone is created. Whenvapor has appropriate condition (for example; lower temperature and existingenough water), it convert to clouds. One of the fantastic features of this phenomenon is that, these clouds cannot move in the presence of wind, Because of the steady source of their generations. This fact is a distinguishable thing for recognizing thiscloud among other clouds. In the first part, panchromatic images of62days before the event were taken and then theserow images were geo-referenced. Thus, earthquake clouds were digitally extracted andthe results were superposed on the topographic map of the intended region. It should also be mentioned that earthquake clouds were detected 10 days priorto the earthquake (January 17th).A period of 10 days is a suitabletime for making decisions in decision making organizations such as, Governorates, Municipalities, etc. Verdict basedon earthquake clouds is not enough for a good conclusion about earthquake occurring, and it is necessarythat we apply other precursors and pre-indicators, one of which is thermalinfrared that has great results.
In another part of the study, temperature content of thermal bands (bands 31 and 32) ofMODIS is extracted and Land Surface Temperature (LST) time serieswere created. Temperature variations are always considered as animportant and effective factor in earthquake phenomenon studying. Thermal anomalycan be seen within 1-24 days before earthquake and thetemperature increases 5 to12 degrees and then return to the previousmode after the earthquake. Some other researchers presented the increaseof 2 to 10 degrees. The idea that earthquake may be interrelated withtemperature was proved by applying it in Russia, China and Japan. However, notice that thermal anomaly may occur due to otherreasons except earthquake. When it is because of earthquake, actuallyit is because of the stress existing in the underground layersand changes in soil properties. Zuji et al. (1990) provedthat gases such as methane, carbon dioxide and hydrogen are releasedfrom soil cracks before earthquake which lead to intensification ofchlorofluorocarbons (CFC) and magnetic fields of the earth. There aresome other theories about this phenomenon such as piezoelectric andexpansion forces of the elastic strain that increase temperature.
Aftergetting images from NASA website and preprocessing them by deduction ofAir temperature time series from LST time series, atmospheric effectsthat exist because of the weather condition is eliminated. Obtainedsignal was some noisy. In the next step, the waveletas a powerful filter is applied to time series. Forextracting Interpretable results, Statistical test such as standard deviation mustbe perform on filtered time series. Standard deviation (ST) cancreate normal limited area. By using limited area that producedby ST, thermal anomaly is detected 2 days prior tothe earthquake. Also, with colorization of thermal images and then creationof visual time series, strike of fault line is found.
Finally, by Comparingthe earthquake cloud line, focal mechanismandhigh temperature zone, high correlation was found. These results showthe observed cloud related to Rigan’s earthquake and also showthe high temperature zone related to earthquake event.
Resultsof this study can be used in two aspects, oneof which is the application in early warning system and the otheris the application in geology usage. Second usage helps geophysicist andgeologist to detect hidden and caused fault.