APPLICATION OF THERMAL REMOTE SENSING IN EARTHQUAKE PRECURSOR STUDIES

Abstract

Earthquakes are the most unexpected and devastating natural phenomenon occurring on Earth. Uncertainty involved in time and place of their occurrence has intrigued scientists globally but still years of research have failed to reliably predict / forecast earthquakes in terms of location, time and magnitude. Perhaps knowledge about earthquakes is limited and requires a multidisciplinary approach to widen the window of our understanding. There is however, no doubt that preparatory processes to earthquake rupture exist and huge amounts of energy release might be preceded by some precursory phenomena that could be consistently observed and identified. The ever advancing techniques of remote sensing have the potential to contribute and assist human in evaluating natural disasters. Stresses building up during an earthquake preparation phase lead to enhanced thermal infrared emission from earth's surface prior to earthquakes. In this study efforts have been made to establish correlation between transient temporal TIR anomalies, OLR variability and earthquake events through post-earthquake analysis of NOAA-AVHRR thermal images. In this study, nine earthquakes from different parts of the world; Jabalpur earthquake (21 May 1997), Chamoli earthquake (29 Mar 1999), and Yamnotri earthquake (22 Jul 2007) from India; Dabiran earthquake (10 Jul 2003), Kerman earthquake (21 Aug 2003), Ravar earthquake (14 Oct 2004), Fin earthquake (25 Mar 2006) from Iran; Balochistan earthquake (29 Oct 2008) from Pakistan and Vrancea earthquake (27 Oct 2004) from Romania; were investigated for pre-earthquake TIR anomaly and OLR variability detection. Theories explaining physical phenomena of pre-earthquake TIR anomaly generation are widely debated since the first realization ofthis phenomenon in 1988 by Gorny and his co-workers. According to one of the most accepted theory of Earthdegassing; stresses building up in earthquake preparation zone reduce pore spaces in rocks and gases are squeezed out. Localized greenhouse effect created due to the increased concentration of optically absorbing gases and alteration of hydro-geological regime of the region under stress leads to TIR anomaly. P-hole or positive hole, activation theory is empirical theory that explains not only pre-earthquake TIR anomaly but also other precursory signals. P-hole awakening under high pressure conditions and their subsequent migration, accumulation and recombination at Earth's surface leads to libration of energy. This elevates the LST of the epicenter region and l enhanced TIR emission takes place. Seismo-ionosphere coupling theory takes into account cumulative effect of lithosphere, atmosphere and ionospheric processes. Several Remote Sensing Rock Mechanics (RSRM) experiments have also validated the observations of rise in thermal emission from stressed rock volume. Relevant parameters investigated are land surface temperature (LST) and outgoing longwave radiation (OLR). It has been observed that earthquake with magnitude higher than 5 may be preceded by detectable rise in LST and OLR. The LST was seen to increase by 2° - 11°C about 7-13 days before the main shock. Thermal anomaly attains it peak temperature and return to normal conditions once the main event is over. The transient period may range from 9-17 days. Study of earthquakes with series of aftershocks viz. Dabiran, Iran (10 Jul 2003), Kerman, Iran (21 Aug 2003), Vrancea, Romania (27 Oct 2004) reveals that the occurrence of aftershocks prevents the re-establishment of normal conditions even after the main event is over. It was also noticed that magnitude and focal depth play a vital role in intensity and spatial extent of the thermal anomaly. Higher earthquake magnitude and shallower focal depth are favorable conditions for the appearance of intense thermal anomaly with larger spatial extent and vice versa. A prominent observation regarding the earthquakes of moderate magnitude [Fin, Iran (25 Mar 2006) and Balochistan, Pakistan (29 Oct 2008)] is the appearance of a dual TIR peak instead of the single rise observed previously. This may lead us to infer that perhaps the energy accumulated in the stressed rocks may be released sporadically in the form of apparent temperature increment or any other geophysical earthquake precursor. The surface expression of TIR anomaly is also found to be governed by fault characteristics so it may or may not coincide with the epicenter. Analyses of NOAA-AVHRR derived OLR data also reveal significant pre-earthquake and co-seismic variability. OLR values rose to 20 - 55 W/m2 and 7 -14 days before the earthquake attaining maximum which may or may not be followed by a low before the earthquake. Total OLR variability transient period ranged from 11-23 days. Similarity found in development trend, overlapping transient period, disappearance with the earthquake event implies interrelation of TIR anomaly and OLR variability and their connection with the earthquake preparation process. This study also reports Himalayan Thermal Line (HTL) and impact of increased stress conditions on HTL. HTL phenomenon was studied with respect to Chamoli earthquake, India (29 Mar 1999). Coinciding with the contact zone of frontal fold belt and moisture rich, porous, reworked soils; HTL development is affected the II - lithological, structural and hydrological conditions of the region. It shows varying intensity indicating stress conditions of the region. This research strengthens the concept of TIR anomaly phenomenon as a preearthquake process which can be monitored through satellites equipped with thermal sensors. Outgoing longwave radiation variability in response to impending earthquakes has been studied and it has been correlated with TIR anomaly for the first time. Himalayan Thermal Line has also been reported for the first time. Further HTL sensitivity analysis on regional scale may lead to establishing a reasonable correlation between thermal line, tectonic stress and fault/thrust.