Hydrogeochemistry and Precursory Anomalies in Thermal Springs of Fujian (Southeastern China) Associated with Earthquakes in the Taiwan Strait

Analyzing the hydrochemical composition in thermal springs is an advantageous method for studying the coupling mechanism of the deep and shallow fluids in active fault zones. Here we conducted sampling in 30 thermal springs near fault zones in Fujian Province, and the major elements, trace elements, silica, stable isotopes (δD and δ18O) and strontium isotopes were tested in the laboratory. The results show that (1) the thermal springs in the study area can be divided into six types according to the content of the major elements: HCO3-Na, HCO3·SO4-Na, Cl·HCO3-Na, Cl-Na, Cl-Na·Ca and HCO3·SO4-Ca; (2) hydrogen and oxygen isotopes indicate that precipitation is the main source of recharge for thermal springs in the study area, and the recharge height is between 258 m and 1859 m; (3) the content of SiO2 in the thermal spring varies from 18.1 mg/L to 59.3 mg/L. The geothermal reservoir temperature calculated is 90~226 °C, and the circulation depth is 2.9~5.4 km, except for the W10 thermal spring, whose circulation depth is 8.4 km; and (4) the 87Sr/86Sr of the thermal springs in southwestern Fujian and eastern Fujian has obviously different characteristics, indicating the influence of different rock formations on the groundwater cycle process. Additionally, a continuous measurement of the main anions and cations was performed in five thermal springs every three days since January 2020. There were obvious abnormal changes in the hydrochemical compositions, chlorine in four of the five springs, sodium at three springs, and four ions at one spring, which all showed abnormal high-value changes by 15% to 80%, and which occurred 85~168 days prior to the M6.1 earthquake in Hualien, Taiwan. An inspiration could be provided for obtaining effective earthquake precursor anomalies by monitoring the change in ion concentration in thermal springs.

Diurnal variations of outgoing long wave radiation (OLR) vis a vis 4 January, 2016 Manipur earthquake (Mw: 6.7): An earthquake precursor?

Daily and diurnal variations of OLR from INSAT 3D and Kalpana satellites have been studied during 25 December, 2015 to 5 January, 2016 over the epicentral region of the Manipur earthquake of 4 January, 2016 (Mw 6.7) and other regions within the view of these satellites. The surface temperatures recorded at Imphal around 30 km from epicentre of this earthquake were also examined. It is found that OLR remained low prior to the occurrence of Manipur earthquake while much larger rise occurred over several other regions where no significant seismic activity was reported. The results corroborate inferences reported in earlier studies that OLR or temperature changes are related to meteorological causes and its sole identification as earthquake precursor may be misleading due to poor constraints.

Interacting inclined strike-slip faults in a layered medium

Two inclined, interacting, strike-slip faults, both buried, situated in a viscoelastic layer, resting on and in welded contact with a viscoelastic half space, representing the lithosphere-asthenosphere system, is considered. Solutions are obtained for the displacements, stresses and strains, using a technique involving the use of Green’s functions and integral transforms, for three possible cases - the case when no fault is slipping, the case when one fault is slipping and the other is locked and the case when both the faults are slipping. The effect of sudden movement across one fault on the shear stress near the fault itself and near the other faults has been investigated. Some situations are identified where a sudden movement across one fault results in the release of shear stress near the other fault, reducing the possibility of seismic movements across it. Other situations are also identified where a sudden movement across one fault increases the possibility of seismic fault movements. A detail study may lead to an estimation of the time span between two consecutive seismic events near the mid points of the faults. It is expected that such studies may be useful in understanding the mechanism of earthquake processes and may be identified as an earthquake precursor.

Types of strong earthquake precursor behavior obtained from world and regional catalogs of earthquakes

Typical features of seismicity inherent to the period prior to the strong earthquake occurrence are examined using the regional and world-wide earthquake catalogs by the method of construction of the Generalized vicinity of a large earthquake (GVLE). These features are foreshock power-law cascade, the weak long-term background increase in seismic activity, and the definite clustering of the main events. Worth to note, that all these features appear to indicate the loss of a stability of a system, without any reference to the physical mechanism of development of the instability. Thus, these features are expected to occur in connection with the bifurcation points in systems of other physical nature, examples of such behavior are given. Besides, a tendency of a decrease in earthquakes depth in the close GVLE vicinity was found. This precursor feature has a specific character and evidences in favor of a fluid involvement in the mechanism of earthquake occurrence.

Detection of Possible Short-Term Ionospheric Precursors of Strong Earthquakes Based on Changes in Diurnal Es Characteristics

The changes in two characteristics of the sporadic Е layer are studied for a pair of stations: the probability of the occurrence PEs and the limiting frequency of the ordinary wave of the sporadic E layer of the ionosphere foEs during a 10-day period of the preparation of 19 crustal earthquakes in the Pacific region with М = 6.5–7.4. The stations are located hundreds of kilometers from each other, but they fall within the zone of the preparation of a particular earthquake (the sizes of the earthquake preparation zone are estimated with formulas known in the scientific literature that relate the size of the radius of the earthquake preparation zone and the earthquake magnitude). The measurement data obtained at the ground stations of ionospheric vertical sounding are analyzed. The deviations of diurnal values of PEs (δPEs) from the median over the studied time interval and the integral diurnal values of the total irregular fluctuations in foEs (ΔfEsΣ) are used to identify possible ionospheric earthquake precursors. The coincidence of the time of appearance of the deviation maxima for both parameters before the earthquakes at each of the stations on the same day (from 1 to 4 days before the earthquake day) is recorded in the diurnal changes in the indicated values during the preparation periods of all of the considered earthquakes. The criteria for the identification of a short-term ionospheric earthquake precursor is discussed. Comparison of the analysis results for manual and automatic ionogram processing showed the prospects for the use the proposed parameters obtained according to the data of the distant ionosondes to identify the short-term ionospheric precursors of an earthquake with М = 6.5–7.0.

A Safe Zone SMOTE Oversampling Algorithm Used in Earthquake Prediction Based on Extreme Imbalanced Precursor Data

Earthquake prediction based on extreme imbalanced precursor data is a challenging task for standard algorithms. Since even if an area is in an earthquake-prone zone, the proportion of days with earthquakes per year is still a minority. The general method is to generate more artificial data for the minority class that is the earthquake occurrence data. But the most popular oversampling methods generate synthetic samples along line segments that join minority class instances, which is not suitable for earthquake precursor data. In this paper, we propose a Safe Zone Synthetic Minority Oversampling Technique (SZ-SMOTE) oversampling method as an enhancement of the SMOTE data generation mechanism. SZ-SMOTE generates synthetic samples with a concentration mechanism in the hyper-sphere area around each selected minority instances. The performance of SZ-SMOTE is compared against no oversampling, SMOTE and its popular modifications adaptive synthetic sampling (ADASYN) and borderline SMOTE (B-SMOTE) on six different classifiers. The experiment results show that the quality of earthquake prediction using SZ-SMOTE as oversampling algorithm significantly outperforms that of using the other oversampling algorithms.

A statistical analysis of the relationship between Pc4 and Pc5 ULF waves, solar winds and geomagnetic storms for predicting earthquake precursor signatures in low latitude regions

Short-term earthquake forecasting is impossible due to the seismometer’s limited sensitivity in detecting the generation of micro-fractures prior to an earthquake. Therefore, there is a strong desire for a non-seismological approach, and one of the most established methods is geomagnetic disturbance observation. Previous research shows that disturbances in the ground geomagnetic field serves as a potential precursor for earthquake studies. It was discovered that electromagnetic waves (EM) in the Ultra-Low Frequency (ULF) range are a promising tool for studying the seismomagnetic effect of earthquake precursors. This study used a multiple regression approach to analyse the preliminary study on the relationship between Pc4 (6.7-22 mHz) and Pc5 (1.7-6.7 mHz) ULF magnetic pulsations, solar wind parameters and geomagnetic indices for predicting earthquake precursor signatures in low latitude regions. The ground geomagnetic field was collected from Davao station (7.00° N, 125.40° E), in the Philippines, which experiences nearby earthquake events (Magnitude <5.0, depth <100 km and epicentre distance from magnetometer station <100 km). The Pc5 ULF waves show the highest variance with four solar wind parameters, namely SWS, SWP, IMF-Bz, SIE and geomagnetic indices (SYM/H) prior to an earthquake event based on the regression model value of R2 = 0.1510. Furthermore, the IMF-Bz, SWS, SWP, SWE, and SYM/H were found to be significantly correlated with Pc5 ULF geomagnetic pulsation. This Pc5 ULF magnetic pulsation behaviour in solar winds and geomagnetic storms establishes the possibility of using Pc5 to predict earthquakes.