Abstract
A Mw 7.3 earthquake occurred near the Iran-Iraq border on November 12, 2017, as the result of oblique-thrust squeezing of the Eurasian plate and the Arabian plate. By employing the spatio-temporally weighted two-step method (STW-TSM) and microwave brightness temperature (MBT) data from AMSR-2 instrument on board Aqua satellite, this paper investigates carefully the spatiotemporal features of multi-frequency MBT anomalies relating to the earthquake. Soil moisture (SM), satellite cloud image, regional geological map and surface landcover data are utilized to discriminate the potential MBT anomalies revealed from STW-TSM. The low-frequency MBT residual images shows that positive anomalies mainly occurred in the mountainous Urmia lake and the plain region, which were 300 km north and 200 km southwest about to the epicenter, respectively. The north MBT anomaly firstly appeared 51 days before the mainshock and its magnitude increased over time with a maximum of about +40K. Then the anomaly disappeared 3 days before, reappeared 1d after and diminished completely 10 days after the mainshock. Meanwhile, the southwest MBT anomaly firstly occurred 18 days before and peaked 3 days before the mainshock with a maximum of about +20K, and then diminished gradually with aftershocks. It is speculated that the positive MBT anomaly in the Urmia lake was caused by microwave dielectric property change of water body due to gas bubbles leaking from the bottom of the lake disturbed by local crust stress alteration, while the southwest MBT positive anomaly was caused by microwave dielectric constant change of shallow surface due to accumulation of seismically-activated positive charges originated at deep crust. Besides, some accidental abnormal residual stripes existed in line with satellite orbit, which turned out to be periodic data errors of the satellite sensor. High-frequency MBT residual images exhibit some significant negative anomalies, including a narrow stripe pointing to the forthcoming epicenter, which were confirmed to be caused by synchronous altostratus clouds. This study is of guidance meaning for distinguishing non-seismic disturbances and identifying seismic MBT anomaly before, during and after some large earthquakes.
Highlights
Microwave radiative signals are capable of penetrating thick fog and clouds, and do not rely on the Sun as the source for illumination
Soil moisture (SM) data retrieved from 10.65 GHz microwave brightness temperature (MBT) data, satellite cloud images at nighttime derived from Meteosat-8, SRTM-DEM dataset with 30 m resolution, geological map provided by the Geological Survey of Canada (GSC), and land cover data with 30 m spatial resolution provided by the National Geomatics Center of China (NGCC) are used in this research, in order to conduct the discriminating analysis using multi-source auxiliary data
The spatiotemporal evolution of multi-frequency MBT anomalies associated with the 2017 Mw7.3 Sarpol Zahab earthquake is carefully investigated and analyzed
Summary
Microwave radiative signals are capable of penetrating thick fog and clouds, and do not rely on the Sun as the source for illumination. These particular attributes allow microwave radiation monitoring of the Earth’s surface become valid under almost all-weather conditions (Ulaby and Long, 2015). Current studies are based on different methods and various microwave satellite data, they have uncovered valuable seismic-related thermal anomalies before, during and after the earthquakes. This indicates that it is a feasible and promising way to use satellite MBT data to reveal particular phenomena and to explore geophysical mechanism of seismic thermal anomaly
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