The Median Tectonic Line is an arc-parallel strike-slip fault that accommodates the Pacific Sea and Eurasian plates' oblique convergence at the Nankai Trough. The slip rate of the Median Tectonic Line is about 5 - 8 mm/yr. Although the Median Tectonic Line has been intensively studied, the displacement associated with recent surface-rupturing earthquakes remains unknown. To investigate the subsurface structure and measure cumulative offset due to surface-rupturing earthquakes, we acquired 2D and pseudo 3D ground penetrating radar (GPR) profiles across the ENE-trending Ikeda fault of the Median Tectonic Line in 2018 and 2020.  We conducted the GPR surveys at the Higashi-Miyoshi site, where the active fault trace is marked by two terrace riser offsets. The 2D lines were about 30 - 65 m long, and the pseudo-3D data are sized 20 m × 30 m with a 0.5-m space. We used a 50 MHz GPR system. We also conducted wide-angle measurements to estimate the electromagnetic wave velocity. Using the final depth-converted GPR profiles, we identified several north-trending paleochannels and correlated them based on their location and facies. A paleochannel at ~1.5 m depth is observed on all inline profiles of the pseudo-3D GPR data. We built a 3D model of this paleochannel and estimated the right-lateral offset about 4 - 8 m. The 1596 Keicho-Kinki earthquake is believed to have ruptured the eastern portion of the Median Tectonic Line, including the Ikeda fault. The paleochannel offset may reflect the displacement during the 1596 earthquake.

Tectonic deformation along western Luzon Island, Philippines, is accommodated by the oblique convergence between the South China Sea tectonic plate, the Philippine Mobile Belt, and on- and offshore upper-plate faults. Proxy records of relative sea level (RSL) along the west coast of Luzon can be used to discern the drivers of local and regional RSL change and, subsequently, to infer vertical deformation and accumulated strain on these faults. Unlike glacial isostatic adjustment that is only expected to contribute to RSL at rates below 1 mm/yr over the past ~1500 yr, tectonic deformation in coastal regions may drive RSL change at significantly higher rates. To ascertain the earthquake potential, we use fossil and living coral microatolls to reconstruct RSL. We identified fields of coral microatolls in the La Union province, Philippines, whose radiocarbon and U-Th dates indicate the presence of at least three generations of corals from 1100 cal yr BP to present. The surface morphology of the coral microatolls reveals a gradual RSL rise over the microatolls’ lifetime punctuated by occasional small diedowns. Further analysis of microatoll cross sections and a comparison of successive generations of microatolls reveal instances of decimeters of RSL rise and fall, consistent with land-level changes inferred to be associated with coseismic subsidence and uplift. Through elastic dislocation modeling, we attempt to attribute RSL changes due to interseismic and coseismic deformation by modelling the vertical deformation associated with specific causative faults. These models infer subsidence events from the Manila Trench and sustained uplift in the region accommodated by upper-plate faults. By comparing RSL histories elsewhere along the west coast of Luzon coast, we are attempting to identify the key drivers of RSL change in La Union.

On February 6th 2018, the Mw 6.4 Hualien earthquake took place near the city of Hualien, eastern Taiwan, accompanied with major surface ruptures and liquefactions along both the Milun and Lingding Faults. While post-earthquake field survey provided detailed surface rupture mapping of the Milun Fault, east of the Hualien city, the surface rupture of the Lingding Fault was not well documented due to the fact that the fault rupture aligned with the active Hualien River, which made the researches and surveys less and difficult. In this study, we use pre- and post-event aerial photos to understand the shallow rupture behavior of the Lingding Fault during the 2018 Hualien earthquake. First, we map the plausible co-seismic fault rupture on the Hualien River’s riverbed by visual comparison of the pre- and post-event aerial photos. Second, we use the software package COSI-Corr to determine the spatial pattern and magnitude of deformation across the Lingding fault. Both approaches reveal clear coseismic surface ruptures within the active riverbed of the Hualien River. Our result suggests that the Lingding Fault’s left-lateral surface slip decreased southwards, from ≥1.0m near the mouth of the Hualien River, to ≤0.4 m south of the Mugua River. The pattern of surface displacements is comparable to fault slip pattern estimated from the seismic data inversion result, and has good correlation with the GNSS RTK result (Wu et al., 2019). Altogether, our analyses confirm that the northernmost Lingding Fault did rupture to the surface during the 2018 Hualien earthquake, forming a series of NE-SW strike left-lateral surface ruptures.

The Earth’s history is periodically marked by large-scale intraplate magmatic events that resulted into voluminous magma eruptions in short duration pulse(s). These Large Igneous Provinces (LIPs) were especially recurrent during the Proterozoic and are often linked with rifting/break-up of supercratons. The associated plumbing system of mafic dykes and sills which then acted as conduits, are now exposed after repetitive erosion and thus, bears an imprint of the respective LIP. Mafic dykes integrated with geochronology and geochemistry, act as unparalleled source for several key information about early earth processes: recognizing LIPs, timing and duration of magmatic activities, location of plume centres, mantle sources and processes, tectonics and prior earth configurations etc. Western Dharwar Craton of the Indian peninsula, is one such terrain that is traversed by numerous Proterozoic mafic dykes of different orientations and ages. Precise Pb-Pb baddeleyite ages on five NW to NNW trending dykes have revealed two distinct mafic magmatic events at 2.21 Ga and 2.18 Ga (Yadav, Sarma, Parashuramulu, 2020; French and Heaman, 2010, respectively). The 2.21-2.18 Ga dyke swarms also recorded in the eastern parts, altogether represent a giant radiating pattern enveloping the entire craton (>1,40,000 km²) and converging towards the Deccan Traps, where its plume centre can be located. Their geochemistry suggests basaltic to basaltic-andesitic composition and sub-alkaline tholeiitic nature. They represent multiple pulses of a single LIP event and fractionated from different mantle melts at varying depths. The overall composition of these dykes is consistent with a mantle plume induced heterogeneous source, that was previously modified by some ancient subduction event. The 2.21-2.18 Ga mafic dyke swarms are also recognized in the Superior, Slave and Greenland Cratons, suggesting their proximity in the Sclavia/Superia Supercraton. Hence, a possible reconstruction has been attempted on the basis of their radiating patterns, compiled geochronology and paleopoles.

The inconsistency of seismic P- and S-wave velocity (V_P and V_S) variations is crucial to resolve the non-thermal heterogeneity in the Earth’s mantle. Seismic tomography reveals a slow anomaly in V_S increasing with depth below ~2200 km, which has been used to argue for the existence of large-scale structures of distinct phases and/or compositions. High density (Fe-rich), high bulk modulus, and low shear modulus may explain the seismic observations and provide for geodynamical stability against entrainment by mantle convection. Such dense “chemical piles” or other structures have been attributed to the accumulation of basaltic crust, core-mantle interactions, cumulates of a basal magma ocean, or residues of early differentiation. However, this anomaly can be alternatively explained by purely thermal variations if tomography models fail to resolve V_P in the lower mantle due to the lack of ray path coverage. Geodynamic models have been used to test the resolution of seismic tomography in previous studies, but the relative resolving power remains unclear. We utilize singular value decomposition of two mutually consistent V_P and V_S models to construct “tomographic filters” and apply them to the outcomes of mantle convection models for both purely thermal and thermal-chemical layering scenarios. This is the first time such a procedure has been performed with both a V_P and V_S model. A filtered lower mantle model with only temperature variations does not yield a large low V_S anomaly, and is contradictory with seismic observations. By contrast, the filtered model predicted using both composition and temperature variations results in a slow V_S anomaly and a neutral V_P anomaly, which provides a good correspondence with observations.

The tectonic evolution of the western Indian ocean is mainly attributed to seafloor spreading along the Central Indian Ridge (CIR) and Carlsberg Ridge (CR). Both these ridges have considerable variations in spreading rates and morpho-tectonic features. The present work is an attempt to analyse the variations in elastic and thermal properties along CIR and CR using satellite-derived geoid and gravity data considering the ridge-plume interaction and influence of diffuse boundary. Analysis of trends of residual geoid vs crustal age reveals significant asymmetry on ridge flanks. A lower slope on either flank suggests off-axis thermal sources, possibly a flow connection from the Reunion plume, causing differential subsidence of the oceanic lithosphere. Geoid - topography ratio (GTR) analysis suggests shallow level of compensation for both ridges. However, the GTR values of CR are found to be distinctly higher (1.3-1.6 m/km) compared to that of CIR (0.9-1.1 m/km). Moreover, the GTR value of northern CIR (1.1m/km) near the diffuse boundary zone is found to be higher than that of the southern part of CIR (0.9m/km). Elastic thickness (Te) of overlapping blocks along both ridges have been estimated using admittance analysis of free-air gravity and topography data. On average, higher Te values (15 km – 16 km) have been observed along CR compared to the CIR (11-14 km). The northern part of CIR has slightly lower Te (11 km) than the southern part (14 km). The obvious distinction in GTR and Te values between the northern and southern parts of CIR indicates the influence of diffuse boundary zone in the thermal structure and elastic properties of the lithosphere. The total analysis reveals that both CR and CIR have been affected by thermal sources other than the mantle upwelling at the spreading centre which reflects in cooling and thermal subsidence of the lithosphere.

A system of Monitors Vibrations and Propagations in the Lithosphere, Atmosphere, and Ionosphere (MVP-LAI) was established in Leshan, Sichuan, China in 2021. Most of observation systems are distributed within a square of 15 x 15 m². Variations of groundwater levels are observed for understanding earthquake-related stress loading on the crust and/or a correction from the precipitation on the other observation. Ground motion and/or vibrations are monitored by using the broadband seismometers and the ground-based GNSS system. Changes of temperature near the surface are monitored by using thermometers and RASS. Air pressure ranged by a few meters in depth and several kilometers above the surface is observed by using the barometers.  Meanwhile, changes of the air pressure could be dominated by winds. A SODAR system is employed to understand a potential source of air-pressure changes. Atmospheric electric field meters and a flux magnetometer are also installed around the station to monitor changes in electromagnetic field before earthquakes that have been widely reported in many studies. Notably, existence of seismo-electromagnetic field caused by changes in ionospheric current about 100 km above the surface that can be observed by using the magnetometer in a relatively-low frequency band. Once acoustic-gravity waves can be generated before earthquakes, clouds associated with earthquakes can be expected. An all-sky camera was installed to prove the existence of earthquake clouds. The VHF coherent scattering radar and the Meteor radar were set about 20 kilometers away from the stations for monitoring changes 80-160 kilometers above the surface.  Pre-earthquake TEC (total electro content) anomalous phenomena can be also detected by the ground-based GNSS system. All the effort is to establish a novel system monitoring changes and/or waves propagations from the lithosphere to the ionosphere for exposing natural signals and physical mechanism behind during the seismogenic processes. 

A precursor mechanism has been presented by earthquake case studies. It is reported that when a major earthquake is forthcoming, the critical change of the crusts in the region of epicenter lead to a great number of rocks being cracked and torn microscopically, possibly some gases passages connected each other, and these passages open to the air through the soil. The special geological movement will significantly release the gases which include micro mount of isotope such as radon. In turn, these radioactive gases undergo a particle decay, then a a particle with MeV energy will nearly ionize million ion pairs, so many positive and negative particles will be departed commonly by the vertically downward normal fair weather atmospheric electrostatic field，gravity and thermal convection before recombination. Finally, these over-dose positive and negative ions will generate a unique reverse vertical atmospheric electric field that could be captured by instruments. The analytical results have been based on research that many earthquake cases have been studied carefully. A very useful method has been proposed in this paper that utilize network monitoring system to get anomalous vertical atmospheric electrostatic field signals under fair weather condition to estimate possible location of the potential epicenter, the magnitude and the shock time of a forthcoming earthquake. 

Physical phenomena observed before strong earthquakes have been reported for centuries. Precursor signals, which include radon anomalies, electrical signals, water level changes and ground lights near the epicenter, can all be used for earthquake prediction. Anomalous negative signals observed by ground-based atmospheric electric field instruments under fair weather conditions constitute a novel earthquake prediction approach. In theory, the abnormal radiation of heat before an earthquake produces fair weather around the epicenter. To determine the near-epicenter weather conditions prior to an earthquake, 99 global earthquake events with magnitudes of 6 or above from 2008 to 2020 were collected. According to Harrison's fair weather criteria, in 76.77% of all statistical cases, the weather was fair 12 hours before the earthquake; in 60.61% of all cases, the weather was fair 24 hours before the event. Moreover, most of these cases without fair weather several hours before the earthquake were near the sea. Among the 34 inland earthquakes, 88.24% were preceded by 12 hours of fair weather, and 70.59% were preceded by fair weather for 24 hours. We conclude that the weather near the epicenter might be fair for several hours before a strong earthquake, especially for inland events.

Oppositely to a previous statistical work using a single time resolution of the total ion density measured onboard the DEMETER satellite, this work deals with statistical seismo-ionospheric influences by comparing different parameters and various time resolutions. The O⁺ density and electron density recorded by the CSES satellite for more than one year and by the DEMETER satellite for about 6.5 years have been utilized to globally search ionospheric perturbations with different time resolutions. A comparison is automatically done by software between the occurrence of these ionospheric perturbations determined by different data sets, and the occurrence of earthquakes under the conditions that these perturbations occur at less than 1500 km and up to 15 days before the earthquakes. Combined with statistical results given by both satellites, it is shown that the detection rate r of earthquakes increases as the data time resolution and the earthquake magnitude increase and as the focal depth decreases. On average, the number of perturbations is higher the day of the earthquake, and then smoothly decreases the days before, which is independent of either ionospheric parameters or time resolutions. The number of right alarms is high near the South Atlantic Magnetic Anomaly area but its relationship with seismic activities is weak. The ion density tends to be more sensitive to seismic activities than the electron density but this needs further investigations. This study shows that the CSES satellite could effectively register ionospheric perturbations due to strong EQs as the DEMETER satellite does.

The North-South Seismic Belt of China is one of the most active seismic areas on the Chinese continent.  More than ten strong earthquakes (Ms > 6) have occurred in this region since 2010.  However, Earthquake-related conductivity anomalies are rarely reported for those earthquakes.  In this study, 3-component geomagnetic data recorded at sixty geomagnetic stations are selected to compute the Parkinson vectors to monitor the changes of conductivity before and after the earthquakes.  Considering most fluxgate magnetometer have only been installed since 2014, we concentrate on earthquakes occurred during 2014–2019.  To mitigate artificial disturbances, low noise data during the 00:00 – 5:00 LT are utilized.  We compute the background distribution and monitoring distribution using the azimuth of the Parkinson vectors at each station within six years and a 15-day moving window, respectively.  The background distribution is subtracted from the monitoring distributions to mitigate the influences of underlying inhomogeneous tectonic structures.  The obtained difference distributions binned by 10° within 400 km from each station are superimposed during 60 days before and after the earthquake to construct integrated maps.  To analyze the potential frequency characteristics, we compute the results form low to high frequency band.  The results show that for four earthquakes, the conductivity anomalies areas appear near the epicenter 10 to 20 days before earthquakes, while the rest two earthquakes have no anomaly.  The conductivity anomalies appear at all study frequency band from 0.0005 Hz to 0.1 Hz, and significantly at 0.001 – 0.005 Hz before earthquakes.  Meanwhile, we find that the lower frequency band corresponds to larger anomalies area.  These results suggest the change of underlying conductivity near the hypocenter is a possible phenomena for strong earthquakes, and the frequency characteristics of the seismo-conductivity anomaly during the earthquake are helpful to understand the pre-earthquake anomalous phenomena.

Many earthquakes have been reported to be preceded by foreshocks, most at plate interfaces with apparent accelerating seismic activity in months or days before the mainshock. Here we focused on four large earthquakes of surface magnitude larger than 6.5 in the Sichuan-Yunnan region and used the matched filter technique to detect missing microearthquakes 180 days before the mainshocks. The event number of new catalogs have a remarkable increase, about 22 to 42 times more than listed in the standard catalog. We found that the temporal distribution of p values of foreshock sequences was smaller than 0.01, which indicates the abnormal activity above background seismicity. The low p values that lasted a few days or weeks appeared between about five and one months before the four mainshocks and corresponded to relative-high seismicity rates. The foreshock sequences migrated toward the mainshock hypocenter along seismogenic fault striking, which was similar to the observation of many interplate earthquakes, suggesting that the foreshock concentration to the initial rupture point of mainshocks might be a common precursory feature. We also found some pairs of seismic events with a cross-correlation coefficient higher than 0.9, indicating that repeating earthquakes might exist on the seismogenic faults. Our results suggest the nucleation process of intraplate earthquakes might be driven by aseismic fault slip. The analysis of b-value variations implied that the local stress increased around the hypocenter of the mainshocks before the main rupture. The consistency of spatial distribution between foreshocks and aftershocks implied a possible forecast for the areas of following post-seismic ruptures. 

In order to improve our understanding of the seismic activity of the Baribis Fault in the West Java region, which passes through the southern part of Jakarta, we have conducted seismic monitoring using 14 borehole seismometers deployed from Jakarta to Purwakarta, West Java, from August 2019 to January 2021. We identified and picked P-and S-wave arrival-times carefully by eye to find out the micro-earthquakes around the Baribis Fault. Then we applied the Geiger method using Hypoellipse code to determine earthquake hypocenter locations. Our result shows there are three earthquake epicenters located close to the Baribis Fault, one of which was felt by the people in Karawang on II-III MMI scale. The source mechanism solution of selected event indicates that Baribis Fault is a thrust fault. The distribution of the micro earthquake hypocenters close to the Baribis Fault obtained from this study is important information related to activities and the impact of this fault in the future

Rapid response and ease-of-installation are driving the seismic community towards low power, portable and user-friendly seismometers in a broad range of scenarios. Operators are increasingly in favour of low-maintenance, low-logistics installations with minimal surface impact without compromising on performance when responding to seismic events quickly and in unknown terrain. The Güralp Certimus is a digital broadband seismometer which goes beyond these goals, utilising recent advancements in ocean bottom, borehole and digitiser technology to deliver a complete seismic station in a compact and lightweight package deployable as a surface or posthole instrument. User-configurable long-period corners and sensitivity options allow Certimus to be adapted to the installation environment by individual users before each installation. Certimus uses well-proven digital feedback control to operate at any angle without the use of a gimbal, thereby dramatically improving tilt tolerance and allowing automated self-correction throughout deployment if the station is affected by unstable ground or lack of time to build a more permanent seismic station. An integrated high-performance digitiser allows simple interaction operated via remote user-friendly web interface. Additionally, Certimus is available with a touchscreen LCD display or full anodised aluminium for surface or burial versions. When buried, the Certimus can be equipped with a small surface storage module in line with the GNSS receiver containing a removable microSD card and allowing interaction with the seismometer via Bluetooth, minimising disturbance of the instrument. Ultra-low power consumption is a direct benefit from incorporating OBS technology, with ability to disable unnecessary features and for temporary deployments without rechargeable power supplies. The inbuilt flexibility of Certimus benefits instrument pools where the same instruments can be used for different purposes by different operators and simply configured to suit the current installation as well as dedicated networks with permanent installations.