Although thermal infrared imaging has been developed as an important tool of remote sensing rock mechanics (RSRM) since the 1990s, the relationship between infrared emissivity and rock fragments, which is essential for interpreting positive infrared abnormity, has not been investigated. In this letter, the infrared brightness temperature (IBT) ($T_{b}$) of sandstone, marble, and granite, including an intact rock specimen and its fragments of six-level sizes, was experimentally detected with an infrared imaging system indoor. The relationship between the detected$T_{b}$and the measured size of rock fragments is investigated, and the equivalent emissivity$\varepsilon _{\!\!f}$of rock fragments of different sizes is obtained. It was discovered that$\varepsilon _{\!\!f}$of rock fragments rose up to 8.43% compared to that of intact rock and behaved the maximum as the fragment sizes get close to the mineral particle scale (MPS) of the rock.$T_{b}$is hence locally lifted by 6.16 K. This letter revealed that the rise of$\varepsilon _{\!\!f}$is a third remote sensing mechanism, besides the known thermoelastic and friction thermal effects, of local$T_{b}$enhancement in process of rock loaded to fracturing, which provides new experimental supports to infrared imaging detection on rock fracturing and IBT-based stability analysis in rock engineering and geostructures.

Microwave radiations from ground objects are capable of penetrating the shallow surface, vegetation, and atmosphere, but they are also contaminated by many natural factors. Microwave brightness temperature (MBT) data at different channels (frequencies and polarizations) have been used for studying seismic anomalies for decades. However, there lacks an in-depth research on how MBT responds to the natural but nonseismic (NbNS) factors, and the microwave channels used might not be optimal for earthquake study, which might bias the reliability of the retrieved MBT anomalies. In this study, the monthly mean MBT backgrounds (MMMBs) in and around the Qinghai-Tibet Plateau (QTP) were retrieved by using MBT data from the AMSR-2 sensors. The spatiotemporal characteristics of the MMMBs were carefully investigated and found to behave a significant spatiotemporal differentiation. The NbNS factors with profound influences on MMMBs were theoretically analyzed, and the sensitive microwave channels of these factors affecting MBT were revealed. Furthermore, the set of sensitive channels of different NbNS factors was used to determine reversely the optimal channels for searching weak seismic MBT anomalies in the QTP. The distribution map of the optimal channels has been then employed to explore MBT anomalies associated with ten large earthquakes (EQs) in the QTP, since 1997. The case studies demonstrated the good performances of the suggested channels and well verified their applicability. This study is of scientific meanings for understanding the MBT background in and around the QTP and has guiding significances for microwave channel selection of global seismic MBT anomaly study.

The application of compressive stress on minerals and rocks can reduce dielectric constant ($\varepsilon '$) at microwave frequency, which is of considerable significance to geophysics, mineralogy, petrology, and microwave remote sensing. However, the stress near the surface (e.g., Earth surface) is generally on the order of several MPa only, and its increment is supposed to be excessively slight to influence rock$\varepsilon '$. The cubic- and conical-shaped diorite specimens were specially produced in this study to investigate experimentally the variations of rock$\varepsilon '$and tan$\boldsymbol {\delta }$at the stress-free end as the other end is subjected axially to compressive stress by using an open coaxial resonator probe working at 2.0–18.3 GHz. Surprisingly, the rock$\varepsilon '$at the free end decreased by approximately −13% until loading to fracturing, and rock tan$\boldsymbol {\delta }$demonstrated frequency-dependent variations. This particular phenomenon is supposed to be induced by stress-activated positive holes (h•) that are produced inside the loaded rock volume bearing with mineral peroxy defects, flown out to slightly stressed regions, and, finally, accumulated beneath the free rock surface. Consequently, these phenomena lead to the decrease in the electronic and ionic polarizability and the increase in the local field around dipoles, eventually changing the rock$\varepsilon '$and tan$\boldsymbol {\delta }$at the stress-free end. This study implies that the microwave dielectric property of the surface rock might decrease accordingly with deep stress enhancement, which is an important factor to be considered in the application of radar investigation and microwave remote sensing.

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.

Passive microwave remote sensing is a feasible and promising way to reveal seismic thermal anomaly during the seismogenic phase. However, at present, the commonness study and mechanism exploration of MBT anomalies of different earthquake cases are still in urgent need of advancement. Based on the current microwave satellite observations with long temporal coverage, this research exhibited the MBT anomalies associated with some typical earthquakes by using STW-TSM approach. The results were classified as terrestrial anomalies and aquatic anomalies, and the respective principle model was proposed according to the characteristics of seismogenic environment, P-hole theory, and remote sensing physics.

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.

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 (ATB) to be received by the radiometer, while humid sand shows significant extinction of ATB. 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 ATB. 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 ATB 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 ATB to seismic monitoring and earthquake prediction.

As one of the most important electrical properties, the dielectric permittivity of minerals and rocks has been studied with respect to its influencing factors, such as texture, density, moisture, frequency, pressure, etc., and has aroused great interest in geophysics, mineralogy, petrology, and microwave remote sensing. However, the studies on the effect of stress on the dielectric property of rock are limited to lower frequencies, and the related mechanism is not yet clear. Considering the limitations of traditional testing methods and the complexities of compositions and structures of rock specimens, in this article, we choose the minerals of the feldspar group as specimens to investigate the impact of compressive stress on their dielectric property at a higher frequency range from 2 to 18.3 GHz. For this purpose, an open coaxial resonator probe is applied to measuring the alteration of the dielectric permittivity of the feldspar specimen in the process of increasing compressive stress using a specially designed loading device. The dielectric constants of the feldspar specimen at all of the five high-frequency points demonstrate obviously decreasing trends with increasing compressive stress. The particular discovery is that the ionic and electronic polarizations are much sensitive to the increasing compressive stress in the microwave frequency band, where the minerals behave like ionic crystals. This article implies that the variation of the microwave dielectric permittivity of rock mass under altering crustal stress due to tectonic plate movements and engineering disturbances is an important factor to be considered in applying radar investigation and microwave remote sensing for mineral exploration, geological exploration, and geo-hazard perception.