Precursory Phenomena Research Articles

Deep learning forecasting of large induced earthquakes via precursory signals

Precursory phenomena to earthquakes have always attracted researchers’ attention. Among the most investigated precursors, foreshocks play a key role. However, their prompt identification with respect to background seismicity still remains an issue. The task is worsened when dealing with low-magnitude earthquakes. Despite that, seismology and, in particular real-time seismology, can nowadays benefit from the use of Artificial Intelligence (AI) to face the challenge of effective precursory signals discrimination. Here, we propose a deep learning method named PreD-Net (precursor detection network) to address precursory signal identification of induced earthquakes. PreD-Net has been trained on data related to three different induced seismicity areas, namely The Geysers, located in California, USA, Cooper Basin, Australia, Hengill in Iceland. The network shows a suitable model generalization, providing considerable results on samples that were not used during the network training phase of all the sites. Tests on related samples of induced large events, with the addition of data collected from the Basel catalogue, Switzerland, assess the possibility of building a real-time warning strategy to be used to avoid adverse consequences during field operations.

Difference in the Precursory Process of the Intergranular Corrosion of Aged Al-Cu and Al-Cu-Mg Alloys in 0.1 M NaCl

Real-time in situ optical microscopy observations of the initiation behavior of intergranular corrosion on artificially aged Al-4.5Cu and Al-4.5Cu-1.5Mg were performed in naturally aerated 0.1 M NaCl at pH 6.0. For both alloys, the discoloration of intermetallic particles occurred before intergranular corrosion, and a discolored coarse intermetallic particle on the grain boundary acted as the initiation site for intergranular corrosion (Al 2 Cu for Al-4.5Cu and Al 2 CuMg for Al-4.5Cu-1.5Mg). The discoloration of Al 2 Cu particles was localized and occurred only on a small number of particles. However, almost all Al 2 CuMg particles were discolored; the overall surface of the particles was discolored uniformly. The discoloration of Al 2 Cu on Al-4.5Cu led to micropitting. In contrast, the discoloration of Al 2 CuMg on Al-4.5Cu-1.5Mg caused the trenching of particles. The difference in the initiation behavior of intergranular corrosion was discussed in terms of these precursory phenomena.

Incorporating Foreshocks in an Epidemic-like Description of Seismic Occurrence in Italy

The Epidemic Type Aftershock Sequence (ETAS) model is a widely used tool for cluster analysis and forecasting, owing to its ability to accurately predict aftershock occurrences. However, its capacity to explain the increase in seismic activity prior to large earthquakes—known as foreshocks—has been called into question due to inconsistencies between simulated and experimental catalogs. To address this issue, we introduce a generalization of the ETAS model, called the Epidemic Type Aftershock Foreshock Sequence (ETAFS) model. This model has been shown to accurately describe seismicity in Southern California. In this study, we demonstrate that the ETAFS model is also effective in the Italian catalog, providing good agreement with the instrumental Italian catalogue (ISIDE) in terms of not only the number of aftershocks, but also the number of foreshocks—where the ETAS model fails. These findings suggest that foreshocks cannot be solely explained by cascades of triggered events, but can be reasonably considered as precursory phenomena reflecting the nucleation process of the main event.

Precursory predictors of the onset of stick-slip frictional instability

Defining precursory phenomena that forewarn the onset of stick slip events remains an elusive but central goal in earthquake and other failure forecasting. We present a method to define the timing of instability by separately decoupling velocity and inertial effects in a spring-slider system. System state is defined in phase-space by velocity and inertia that in turn control critical stiffness and define the onset of instability while sliding velocity V = VLk/(k + dF/du) remains finite. A general energy analysis defines velocity and inertial effects (coefficients) that act against each other to promote/suppress instability by respectively increasing/decreasing critical stiffness. Repetitive stick-slip experiments define features of stick-slip cycles and identify a precursory trend in accelerating slip that precedes the onset of unstable sliding. We represent this precursory acceleration (dV/dt) immediately preceding instability by a general power-law relation dV/dt = AVα that reduces to V = B(ci + ti-t)-β. This represents the “true” timing of the onset of stick slip, ti relative to that “projected” from the linearization of the precursory data, tf, with tf=ci + ti. ci represents the small offset between the “predicted” and “true” timing of the onset of stick slip resulting from velocity and inertial effects and may be considered as the error in prediction. This error is one-order of magnitude smaller (∼3 ms) than the lead-time that forewarns of the event (∼40 ms) in our experiments and thus is useful as a predictor. Three independent methods using power-law, linear and criticality relationships confirm the fidelity of the timing of the slip instability transition evident in the stick slip data. This theoretical treatment suggests that the underlying physical meaning of the parameter c in the modified Omori law R∼(c + tM-t)-q is the time of the main shock in advance of the timing of the singularity predicted for an ideal response.

Modeling frictional precursory phenomena using a wear-based rate- and state-dependent friction model in the laboratory

We examine numerical models that employ the rate-and-state frictional (RSF) framework to investigate earthquake sequences using laboratory driven descriptions of heterogeneous frictional properties. Using previously obtained experimental measurement of roughness, we observed that wear produced a bimodal Gaussian distribution of surface heights, which we hypothesized produced spatial heterogeneity of the critical slip distance Dc. In this numerical study, the fault surface was binarized into discrete smooth or rough sections, producing a barcode style version of frictional heterogeneity. The fault was predominantly rough except for two dominant asperities (A1 and A2) representative of larger polished sections. We simulated the resistive effect of increasing the fracture energy (toughness) of the rough barriers while maintaining constant properties of the embedded brittle/smooth asperities. Our numerical simulations generated burst-like seismic events and aseismic transients throughout the interseismic phase. At the late interseismic phase, bursts of seismicity (foreshocks) interacted with the accelerating preslip region at the transition to the preseismic (nucleation) phase. At lower levels of toughness heterogeneity, the slip rate increase was roughly inversely proportional to the time-to-failure tf for larger events. As fault toughness was increased, the dominant asperities initiated nucleation and thus force deviations of the fault from the smooth 1/tf acceleration observed for the homogeneous case, producing a rate-dependent cascade response. The calculations were validated by comparing two independently measured metrics from the experiments: (1) The expansion rate of slow ruptures during the interseismic and preslip phase and (2) the scalar seismic moment and source dimensions. While our study does not address the scaling problem, these results help to understand laboratory experiments that investigate transition to the preseismic (nucleation) phase during complex earthquake sequences.

A Practical Method for Short-term Earthquake Prediction Using Multiple High-frequency Tremor Events

The substantial damage induced by major earthquakes requires the preparation of prevention methods before serious shocks occur. For a half-century, researchers have tried to develop an efficient method for earthquake prediction using modern scientific methods without practical results except for two rare successes; as a result, the general evaluation is pessimistic. Among many phenomena, seismic activity has been the approach most often investigated. Particularly, foreshocks seem to offer the most potential. However, foreshocks are found to precede only a small fraction of major earthquakes and provide precursor parameters with too many kinds of diversity.
 We need to find another seismic or similar phenomenon in the nucleation period with characteristics expected for foreshocks, i.e., a stable rate of occurrence and extremely large anomalies immediately before major earthquakes [1,2]. Here we make a special seismic catalog of high frequency tremors deduced anew from continuous seismic data of just before major earthquakes using the extensive network, High-net of Japan. Analyses of catalog of three major and one little bit smaller earthquakes show that there are three successive precursory phenomena, first at some six weeks, second at some four weeks, and finally immediately before the earthquake.
 These results can provide evidence to predict major earthquakes without high rate of diversity among threshold parameters; i.e., the selected threshold values for distinguishing precursor candidates are quite stable. Each precursory activity provides the three items of prediction, namely, occurrence time, epicenter and magnitude, with sufficient accuracy for practical disaster prevention efforts. The positive results will contribute to developing practical prediction methods to be used for the mitigation of serious earthquake disasters.
 The proposed system is now in the level of POC, and expected to start in Japan without large difficulties because of sufficient level of observation network and storage of past data of some twenty years.

Observation of rapid long-range seismic bursts in the Japan Trench subduction leading to the nucleation of the Tohoku earthquake

Earthquake precursory phenomena like foreshocks are common but the physical processes which produce them and lead to rupture are poorly understood. Looking for clues about these processes, we explore here the long-term evolution of seismicity in a broad section of the Japan Trench slab around the 2011 Tohoku earthquake location and report the existence of long-range seismic bursts moving rapidly (∼1-2 days or less) between the deep and the shallow slab. Their presence is evidenced by synchronizations of deep and shallow seismic activities in the slab. Probabilities that these synchronizations could be due to chance are infinitesimal (<10−5). We show that a significant part of the activity in this subduction occurs during these bursts and that they are responsible for the foreshock sequence which preceded the earthquake. These observations support the existence of fluid channels connecting the deep slab, where water is released during dehydration, to the shallow seismogenic zone.

Aseismic Fault Slip During a Shallow Normal‐Faulting Seismic Swarm Constrained Using a Physically Informed Geodetic Inversion Method

AbstractImproved imaging of the spatio‐temporal growth of fault slip is crucial for understanding the driving mechanisms of earthquakes and faulting. This is especially critical to properly evaluate the evolution of seismic swarms and earthquake precursory phenomena. Fault slip inversion is an ill‐posed problem and hence regularization is required to obtain stable and interpretable solutions. An analysis of compiled finite fault slip models shows that slip distributions can be approximated with a generic elliptical shape, particularly well for M ≤ 7.5 events. Therefore, we introduce a new physically informed regularization to constrain the spatial pattern of slip distribution. Our approach adapts a crack model derived from mechanical laboratory experiments and allows for complex slipping patterns by stacking multiple cracks. The new inversion method successfully recovered different simulated time‐dependent patterns of slip propagation, that is, crack‐like and pulse‐like ruptures, directly using wrapped satellite radar interferometry (InSAR) phase observations. We find that the new method reduces model parameter space, and favors simpler interpretable spatio‐temporal fault slip distributions. We apply the proposed method to the 2011 March–September normal‐faulting seismic swarm at Hawthorne (Nevada, USA), by computing ENVISAT and RADARSAT‐2 interferograms to estimate the spatio‐temporal evolution of fault slip distribution. The results show that (a) aseismic slip might play a significant role during the initial stage and (b) this shallow seismic swarm had slip rates consistent with those of slow earthquake processes. The proposed method will be useful in retrieving time‐dependent fault slip evolution and is expected to be widely applicable to studying fault mechanics, particularly in slow earthquakes.

Worldwide Statistical Correlation of Eight Years of Swarm Satellite Data with M5.5+ Earthquakes: New Hints about the Preseismic Phenomena from Space

Nowadays, the possibility that medium-large earthquakes could produce some electromagnetic ionospheric disturbances during their preparatory phase is controversial in the scientific community. Some previous works using satellite data from DEMETER, Swarm and, recently, CSES provided several pieces of evidence supporting the existence of such precursory phenomena in terms of single case studies and statical analyses. In this work, we applied a Worldwide Statistical Correlation approach to M5.5+ shallow earthquakes using the first 8 years of Swarm (i.e., from November 2013 to November 2021) magnetic field and electron density signals in order to improve the significance of previous statistical studies and provide some new results on how earthquake features could influence ionospheric electromagnetic disturbances. We implemented new methodologies based on the hypothesis that the anticipation time of anomalies of larger earthquakes is usually longer than that of anomalies of smaller magnitude. We also considered the signal’s frequency to introduce a new identification criterion for the anomalies. We find that taking into account the frequency can improve the statistical significance (up to 25% for magnetic data and up to 100% for electron density). Furthermore, we noted that the frequency of the Swarm magnetic field signal of possible precursor anomalies seems to slightly increase as the earthquake is approaching. Finally, we checked a possible relationship between the frequency of the detected anomalies and earthquake features. The earthquake focal mechanism seems to have a low or null influence on the frequency of the detected anomalies, while the epicenter location appears to play an important role. In fact, land earthquakes are more likely to be preceded by slower (lower frequency) magnetic field signals, whereas sea seismic events show a higher probability of being preceded by faster (higher frequency) magnetic field signals.

An Assessment on Earthquake Precursory Signals for Applications of Monitoring and Mitigation Strategies

Forecasting earthquakes entails the existence of time-dependent processes that are dependent on changing conditions deep inside the Earth's crust prior to the occurrence of major seismic activity. Mechanical variables, such as precursory ground deformation and earlier seismic events, have long been the focus of seismology research. The purpose of this review is to acknowledge that there have been numerous reports of a variety of precursory phenomena, including the emission of electromagnetic waves ranging from ultralow frequency (ULF) to visible (VIS) and near-infrared (NIR) light, the emission of radon gas, various electric and magnetic field anomalies, and unusual animal behaviour. Earthquake prediction research has been going on for almost a century with no obvious success.

Temporal Variation of b Value with Statistical Test in Wenchuan Area, China Prior to the 2008 Wenchuan Earthquake

The Gutenberg–Richter b value describes the ratio between large and small events. A number of studies have suggested that the b value decreases before large earthquakes. In this study, we investigate the temporal variation of the b value of an area along the main rupture zone of the 2008 Wenchuan earthquake (M8.0) prior to the great event. Before estimating b values, we tested the earthquake catalog to make sure that we use the reliable frequency–magnitude distribution by the calculation of MC (completeness of magnitude). We define parameter P (ΔAIC ≧ 2) values to examine the significance level of b-value changes in the temporal variation by combining a boostrap method with Akaike’s Information Criterion (AIC). The b value in the main rupture zone shows a long-term decrease trend. We then focus on a smaller area where the initial rupture starts. The results show that b values significantly changed about 3 months before the 2008 Wenchuan earthquake in the initial rupture area, indicating that the b value has a potential capability to monitor and detect precursory phenomena of great earthquakes.

Study of Water Relative Permeability in Fractures Using Well Tests and Radon: Gas Bubbles Effect.

Few published data are available for two-phase flow in fractures from field studies. All measurements of relative permeability reported in the literature were done in laboratory-scale. The in situ water saturations are normally not known for multiphase flow in natural fractures; therefore, the direct measurements of relative permeability are difficult in field-scale. With the help of a case study before and after the 2008 Mw 5.4 Antung earthquake, groundwater radon was used as a tracer to determine the gas and water saturations in a small naturally fractured aquifer. Well tests were also conducted to estimate aquifer transmissivity before and after the 2008 Antung earthquake. Anomalous declines in both groundwater radon concentration and transmissivity were observed precursory to the 2008 Antung earthquake. Both declines are two precursory phenomena having a common effect of gas bubbles. Using the data from well tests and radon tracer, one data point of water relative permeability can be obtained for in situ fractures. This data point reveals strong phase interference between water and gas bubbles for multiphase flow in natural fractures. Both the data of well tests and radon tracer are essential to gain an improved understanding of mass transfer behavior of groundwater-dissolved gases between water and gas phases.

Geo-Chemical Techniques for Earthquake Forecasting in Nigeria

Regardless of the doubt caused by some rounds on the impossibility of earthquake forecast, more and more countries, even at the highest governmental levels, realize that doing nothing is the ostrich position of dread before the real difficulties associated with the creation of a real forecasting system. Nigeria in times past was believed to be aseismic. However, the seismic record of Nigeria from 1933-2021 have demonstrated in contrast to the idea, numerous quakes have been recorded in Nigeria throughout the years. With the development of observation techniques and theoretical knowledge of geochemistry, geochemical observation of faults gas has become a hotspot once more in recent years. Rn, Hg, H2, etc., are used for geochemical observations. 222Rn has a half-life of 3.825 days, a magnitude 5.0 earthquake will be detected through precursory phenomena at a distance not greater than 142 km. Mercury and other elements are used as important detectors for earthquake prediction and they play an important role in revealing the relationship between fluid in the fault zone and the occurrence of earthquakes, the range for a magnitude 5.0 earthquake is limited to 200 km. Hydrogen concentrations have been monitored for precursory variations in many fault systems, using either discrete sampling and laboratory analysis or continuous monitoring of ground gas, using hydrogen-sensitive fuel cells. Precursory changes in groundwater chemistry are often attributed to the mixing of fluids from two or more chemically distinct aquifers, the physical mechanism responsible for the mixing of fluids is, however, not well established.

Tukey Control Chart for Radon Monitoring in Relation to the Seismic Activity

Radon is one of the precursory phenomena that exist in connection to the occurrence of earthquakes and may have potential in forecasting these hazards. The data set in this study contains the observations of radon from August 01, 2014, to January 31, 2015, collected in Sobra city, northern Pakistan. Weibull, gamma, log-normal, log-logistic, and Pareto probability distribution were fitted over the radon on its original scale and a log scale. Log-logistic best fits the radon on both scales. The Tukey control charts reveal several anomalies that were compared with earthquake occurrence. There are five earthquakes that occurred during the same period as the radon monitoring program, having magnitude ranges between 4.9 and 5.5, with the ratio between strain radius and distance to the epicenter greater than or equal to 1. The results of this study demonstrate that, for earthquakes, seismic events show a correlation with increasing concentrations of radon gas before their occurrence.

Development and Preliminary Analysis of a VLF-Band Electromagnetic-Wave Observation System for Short-Term Earthquake Precursory Monitoring

Preseismic VLF electromagnetic pulses occasionally increase a few days before large earthquakes, especially inland earthquakes. More than two decades ago, the Tokai University group developed a digital recording system for collecting the preseismic electromagnetic pulse data and showed remarkable results. However, due to the limitations of personal computers’ data storage and CPU power during that time, they discontinued the observation. We relaunched this research using current technology. This paper shows the development of the new observation system and presents preliminary results. In addition, we introduce an electromagnetic-wave arrival discrimination algorithm that combines the autoregressive model and the Akaike information criterion, which are commonly used for automatic waveform reading in seismology, to obtain accurate data on the time of arrival (TOA) of electromagnetic waves. Then, source positioning was performed using TOA of electromagnetic waves. Seven electromagnetic pulses near the epicenter were observed 2 days before the largest inland earthquake (M = 5.6) that occurred near the observation network during the observation period (2016–2020). These VLF pulses may be a seismic precursory phenomenon because they were not electromagnetic pulses originating from lightning. These results encourage future observations.

Rapid gas desorption and its impact on gas-coal outbursts as two-phase flows

Coal and gas outbursts are a violent release of energy in part driven by rapidly desorbing gas from the fragmenting coal. We present a coupled two-phase model of coal and gas outbursts to define the timing, rate and magnitude of gas desorption and its contribution to the resulting energetics. The model involves a fragmenting ejection of the outburst from an overpressurized coal that retreats omnidirectionally from a point and develops a deepening crater. This model is applied to represents both experiments and in situ observations. These results indicate that the outburst is initially driven by free gas before desorbing gas rate exceeds this free gas liberation rate early into the outburst (at ∼17 s in our model). The cumulative mass of desorbed gas only exceeds the free gas later into the event at approximately double this duration (at ∼29 s in our model). During the outburst, ∼55 % of the expansion energy is contributed by desorbing gas. Using the initial gas emission rates for both non-tectonic and tectonic coals defines a power-law relationship between the initial gas desorption rate and the desorption gas contribution (∼14 %–92 %), indicating that the desorbing gas plays a decisive role in outburst development. Furthermore, taking the gas emission model as a boundary conditions for numerical simulations, the gas pressure potential energy (GPPE) released in the first millisecond at the maximum gas emission rate is derived to characterize its effects on the dynamic characteristics of the outburst two-phase flow. The maximum energy release intensity considering gas desorption is ∼5 times that without gas desorption for non-tectonic coal. For tectonic (mylonitized) coals the energy release is a further ∼4 times greater than that of non-tectonic coals. This paper presents a novel quantitative study defining the role of gas desorption in outbursts and contributes to the understanding of causal mechanisms and precursory phenomena preceding catastrophic outbursts.

Rare Occurrences of Non-cascading Foreshock Activity in Southern California.

Earthquakes preceding large events are commonly referred to as foreshocks. They are often considered as precursory phenomena reflecting the nucleation process of the main rupture. Such foreshock sequences may also be explained by cascades of triggered events. Recent advances in earthquake detection motivates a reevaluation of seismicity variations prior to mainshocks. Based on a highly complete earthquake catalog, previous studies suggested that mainshocks in Southern California are often preceded by anomalously elevated seismicity. In this study, we test the same catalog against the Epidemic Type Aftershock Sequence model that accounts for temporal clustering due to earthquake interactions. We find that 10/53 mainshocks are preceded by a significantly elevated seismic activity compared with our model. This shows that anomalous foreshock activity is relatively uncommon when tested against a model of earthquake interactions. Accounting for the recurrence of anomalies over time, only 3/10 mainshocks present a mainshock‐specific anomaly with a high predictive power.

Era of Unpredictable Earthquakes to Predictable: A New Perspective to Predict Earthquakes to Mitigate Loss of Life and Destruction of Property in Japan, California and Mexico

The single-most important purpose of this observational successful earthquake prediction research is to mitigate the loss of life and destruction of property by earthquakes. The basic innovative concept is entirely new and related to the tectonic plate activities of epicenter zones, may not familiar to many and may not be readily found in the existing literature. This manuscript is prepared based on 35 years of earthquake prediction study and over 10 years of observation of onshore observable precursors and the corresponding earthquakes. Past precursory phenomena for earthquakes have been observed, recorded, analyzed and can be used to predict future earthquakes for seismically risky regions, Japan, California and Mexico. It is significant to mention that there are more than 250 epicenter zones and equal number of precursor areas identified, centrifugal force due to the orbital motion of the earth is the major driving force of tectonic plates, each and every epicenter zone has its own creation of atmospheric weather anomalies, due to the change in position of the orbital motion of the earth. Though all form of weather anomalies are preceded by earthquakes, among them rainfall is the best earthquake precursor and rainfall location is best to identify the future earthquake direction, location frame, time frame and magnitude frame can be established 15 days before earthquakes occur. Based on this prediction method each and every individual earthquake can be predicted without false alarm in Japan, California and Mexico.

From Hector Mine M7.1 to Ridgecrest M7.1 Earthquake. A Look from a 20-Year Perspective

The paper provides a comparative analysis of precursory phenomena in the ionosphere and atmosphere for two strong earthquakes of the same magnitude M7.1 that happened in the same region (North-East from Los Angeles) within a time span of 20 years, the Hector Mine and Ridgecrest earthquakes. Regardless of the similarity of their location (South-Eastern California, near 160 km one from another), there was one essential difference: the Hector Mine earthquake happened during geomagnetically disturbed conditions (essential in the sense of ionospheric precursors identification). In contrast, the quiet geomagnetic conditions characterized the period around the time of the Ridgecrest earthquake. The Hector mine earthquake happened in the middle of the rising phase of the 23-rd solar cycle characterized by high solar activity, while the Ridgecrest earthquake happened by the very end of the 24th cycle under very low solar activity conditions. We provide a comprehensive multi-factor analysis, determine the precursory period for both earthquakes and demonstrate the close similarity of ionospheric precursors. Unlike the majority of papers dealing with earthquake precursor identification based on the “abnormality” of observed time-series mainly determined by amplitude difference between “normal” (usually climatic) behavior and “abnormal” behavior with amplitudes exceeding some pre-established threshold, we used the technique of cognitive recognition of the precursors based on the physical mechanisms of their generation and the morphology of their behavior during the precursory period. These permits to uniquely identify precursors even in conditions of disturbed environment as it was around the time of the Hector Mine earthquake. We demonstrate the close similarity of precursors’ development for both events. The leading time of precursor appearance for the same region and similar magnitude was identical. For the Hector Mine it was 11 October 1999—5 days in advance—and for 2019 Ridgecrest it was 28 June—7 days before the mainshock and five days before the strongest foreshock.

Survey of a relationship between precipitation and major earthquakes along the Peru-Chilean trench (2000–2015)

In this study, the possible relationship between anomalous precipitation and five very large earthquakes along the Peruvian- Chilean trench (2001/06/23 M8.4, 2007/08/15 M8.0, 2010/02/27 M8.8, 2014/04/01 M8.2, and 2015/09/16 M8.3) is surveyed. A precipitation archive and earthquake database has been compiled for 2000–2015 for this part of South America. According to the mean daily precipitation data given by the National Centers for Environmental Prediction (NCEP), an anomalous increase in precipitation occurred within two months before these earthquakes over their respective epicentral areas. The mean 60-day total precipitation before these five earthquakes is 98.8 mm, while the mean 60-day total post-event precipitation is 51.5 mm. Other atmospheric parameters show a similar trend. Detailed results using different time intervals and spatial resolutions support the precipitation enhancement before the major earthquakes in different spatial and temporal scales. Therefore, a positive meaningful relation seems to exist between earthquakes and precipitation before the main shocks (R = 0.711). On this basis, atmospheric anomalies and precipitation enhancement are indicated, which include the appearance of cyclones that cause increased precipitation. This precursory phenomenon appears to be consistent with the Freund theory that specifically includes air ionization at the ground-to-air interface as a result of stress-activation of positive holes in the hypocentral rock volume, the spreading of these electronic charge carriers through the rock column, their arrival at the Earth’s surface, and air ionization.