Based on the spatiotemporally weighted two-step method (STW-TSM), the spatiotemporal characteristics of the residual microwave brightness temperature (MBT) with the Mw7.9 Wenchuan earthquake on May 12, 2008 are revealed by satellite data from the Advanced Microwave Scanning Radiometer - Earth Observing System (AMSR-E) sensor. Two significant MBT positive anomalies are found to be exactly in spatial accordance with two geological Quaternary zones, and the detailed geometric information of the MBT positive anomaly is found to be correlated with the microwave frequencies. After eliminating other possible influential factors, including surface temperature, vegetation index, land-surface roughness, and surface soil moisture under the conditions of space, time, and magnitude, and according to the microwave radiative transfer model, the dielectric variation in the ground surface is suggested to be the primary contributor of the MBT positive anomaly. The positive-hole (P-hole) theory is applied to interpret the geological preference of the MBT positive anomalies through a chain process: crustal stress enhancing-P-hole producing and flowing down stress gradients-surface P-hole accumulating-dielectric constant decreasing-and microwave radiation increasing. The stress-resulting effect of the dielectric decrease on MBT the increase provides a novel mechanism for microwave remote-sensing monitoring of crustal stress field alteration, earthquake preparation, and upcoming shocks. This research has a particular significance for searching potential georelations between the tectonic earthquake preparation and the abnormal satellite MBT.

Two catastrophic earthquakes (EQs) with magnitudes of Mw 7.8 and 7.3 occurred in Gorkha and Dolakha, Nepal on April 25 and May 12, 2015, respectively. By employing the spatio-temporally weighted two-step method (STW-TSM), significant positive and negative microwave brightness temperature (MBT) anomalies preceding the two EQs were uncovered with satellite microwave data from FY-3B. Two strip-shaped positive MBT anomalies appeared in the Higher Himalayan 1 day before the Gorkha shock and the Dolakha shock, respectively, which were parallel to the thrust belt and whose intensity distributions exhibited good topographic consistency with the elevation profiles of the central Himalayas. Referring to the P-hole theory and seismogenic mechanism of the Nepal EQs, seismically activated mobile positive charges are presumed to have caused the surface positive MBT anomalies by reducing the superficial dielectric constant and increasing the local temperature of the Himalayan cliffs. A significant negative MBT anomaly occurred plausibly as early as March 2015 in the southern Gangetic Plain and developed into a stripe parallel to the thrust belt on the days of the Gorkha and Dolakha events. The negative MBT anomalies are suggested to result from soil moisture increases due to local synchronous precipitation. This work presents the first case study to use data from the FY-3B microwave radiation imager (MWRI) to investigate seismic anomalies, and discriminate both positive and negative MBT anomalies preceding an EQ for the first time.

Referring to the first law of geography and its extension in time domain, this research makes improvements on the two-step method (TSM) presented to retrieve seismic microwave brightness temperature (MBT) anomaly, and a new method named spatio-temporally weighted TSM (STW-TSM) is developed. Temporally weighted background is calculated using nonseismic data according to respective time intervals with the shocking year, and applied to remove long-term trend so as to retrieve basic MBT residuals. Spatially weighted background is calculated referring to Euclidean geospatial distance away from the epicenter, and applied to eliminate local meteorological noise so as to retrieve cleaned MBT residuals. With May 2008 Wenchuan earthquake sequence being a case study, significant seismic MBT anomalies at H and V polarizations, 10 days before and after the May 12 Mw7.9 main shock, were retrieved using STW-TSM at 6.9, 10.7, 18.7, and 36.5 GHz, respectively. This study shows that the newly developed STW-TSM is much more qualified. More significant seismic MBT anomalies at H polarization was retrieved as compared to V polarization. More detailed and localized MBT anomalies was uncovered from high frequency bands at 18.7 and 36.5 GHz, while much remarkable anomalies (but less details) were obtained from low frequency bands at 6.9 and 10.7 GHz. This research is valuable for satellite MBT observation, seismic anomaly analysis, and earthquake precursor study.

The additional microwave radiation from underground rock mass (lithosphere) that is caused by tectonic activity and alternating crustal stress has been shown to be a detectable electromagnetic signal via satellite remote sensing. However, the emission of microwave radiation produced inside the lithosphere will be affected by its overburden matter, such as sand, soil, water, and vegetation, and this effect is unknown. In this article, we use a C-band microwave radiometer to detect the variations in the microwave brightness temperature (TB) of rock samples in the process of axial loading. For the rock samples, the surface was bare and covered with dry sand or humid sand. The experimental detection illustrates that the dry sand unexpectedly allows more stress-associated additional radiation (Δ TB) to be received by the radiometer, while humid sand shows significant extinction of Δ TB. Both the radiative transfer theory and the stress-activated positive hole hypothesis are applied to interpret the mechanisms of the effect of dry and humid sand layers on Δ TB. The outflow of stress-activated positive holes is from inside the loaded rock, and hence their accumulation beneath the surface of the sand layer is expected to reduce the local dielectric permittivity of sand, thereby making the detected Δ TB by the microwave radiometer to be larger than that extrapolated theoretically in accordance with radiative transfer theory. This article is valuable for understanding the diverse TB anomalies that have been observed prior to tectonic earthquakes, and it facilitates the evaluation of the potential application of Δ TB to seismic monitoring and earthquake prediction.

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.