Earthquake Preparation Research Articles

Preseismic Groundwater Ion Content Variations: Observational Data in Flowing Wells of the Kamchatka Peninsula and Conceptual Model

Repeated manifestations of hydrogeochemical anomalies in groundwater ion content variations before local strong earthquakes were recorded in three flowing wells of the Petropavlovsk-Kamchatsky test site. A model of changes in chemical composition of groundwater is considered using observational data and modeling of two waters mixing with contrasting composition in a zone of increased permeability in aquifer. Hydrodynamic parameters of the model—relaxation time of water pressure impulses and the time of mixed water movement in aquifer and wellbore—can vary from days to hundreds of days in individual wells. These parameters determine the duration and morphological features of anomaly in mixed water from a well. Using observational data from self-flowing well M-1, an assessment was made of chemical composition and mineralization of two mixing waters and their ratio in mixed water under background conditions and during the preparation of earthquake on 2 March 1992, Mw = 6.9.

Early Manifestations of Short-Term Precursors in Stress-Strain State Dynamics of Southern California

—The stress-strain state before the М = 7.1 Ridgecrest earthquake in Southern California is analyzed based on spatiotemporal distribution of shear strains calculated in the geomechanical model within local ~100 × 100 km crustal segments at a depth of 3–7 km. In the epicentral zone of the earthquake, starting from three years before the event, a successive series of the time intervals, up to the occurrence of the earthquake, when shear deformations are completely absent and rocks are farthest from ultimate strength—the so-called quiescence zones—are established. The spatial distribution of shear strains in the vicinity of the epicentral zone is analyzed during the quiescence intervals and subsequent bursts of maximum amplitude in the epicentral zone itself. The time intervals of the bursts are called excursions. The successive emergence of maxima in shear strain amplitudes in the epicentral zone and surrounding medium during the excursions corresponds to the situation of a swing when the entire preparation region of a future earthquake is rocking up to the moment of event. Consistency of the obtained results with the existing theoretical models of earthquake preparation is discussed.

RECOGNITION AND INTERPRETATION 
 OF THE SPATIAL IRREGULARITIES IONOSPHERE 
 FOR FEBRUARY – MARCH 2010 OVER THE SEISMIC ZONES 
 OF SOUTH AMERICA BY RADIOPHYSICAL METHODS

The ionospheric disturbances detected during large-scale earthquakes occurred of early 2010 in South America on the base the analysis of data GPS-observations are considered. The complex analysis of the ground-based stations data integrated into the international IGS and UNAVCO networks was carried out the selected region. Extensive spatial-time measurements statistics of more than 10 million phase data were processed. Stable estimates of distributions are obtained for ionospheric plasma perturbation parameters. The relative contribution of acoustic-gravity waves to the ionospheric disturbances formation during the large earthquakes preparation is investigated.

Appraisal of contemporaneous application of polarization ratio and fractal analysis for studying possible seismo-electromagnetic emissions during an intense phase of seismicity in and around Assam Valley and the Eastern Himalayas, India

Mechanical deformations from within the earthquake preparation zones are believed to cause seismo-electromagnetic (SEM) emission in ultra-low frequency (ULF) band, i.e. between 0.001 and 10 Hz, and these are studied as a precursor for short range earthquake forecast. Data from highly noise-resilient, 3-component ULF induction coil magnetometers, installed at the Multi-parametric Geophysical Observatory (MPGO), Tezpur, which is situated at a very close proximity with the northern end of the Kopili Fault and the Bomdila Fault, the Main Boundary Thrust of the Eastern Himalaya, the Naga and Disang Thrust, the Assam Syntaxis Zone as well as the two tectonically active Precambrian shields – Shillong and Mikir, was used for the first time to study SEM emissions employing both polarization ratio analysis and fractal analysis in the background of an intense phase of seismicity during the campaign period of April 20 – September 3, 2019. Fifty-one events were filtered via strain radius and index of seismicity calculations to nominate credible events which were studied vis-à-vis polarization ratio and fractal dimension parameters of 3-h night time data, centered around local midnight contemporaneous with the 18–21 UT window of the KP index (global geomagnetic activity index) in the 0.03–0.1 Hz frequency band. The findings show candidate SEM emissions, in the form of enhancements in SZ/SH, associated with all the seven credible events, even as nine enhancements could not be attributed to immediately adjacent credible events. The fractal dimensions, calculated with the spectral density method, ambiguously depicted a gradual increase prior to the intense phase of seismicity.

Long Short Term Memory networks (LSTM)-Monte-Carlo simulation of soil ionization using radon

Earthquake events are usually associated with atmospheric processes variations. Some of the important events in seismic periods include the atmospheric electricity/conductivity modification, which mostly depends on the ion population in the vicinity of earthquake preparation area. Radon, together with cosmic radiation are the major ionization sources in the lower troposphere. The radon induced soil ion-pair production rate was estimated for Erzincan, a city along the North Anatolian Fault Zone, Turkey. The Long Short-Term Memory networks (LSTM's) and Monte Carlo method are proposed to account for uncertainty in estimating the ion production rate. The advantage of the LSTM model is taken to study radon anomalies during the M = 5 Girlevik earthquake, Erzincan. Radon concentration is found to increase prior to the 5.0 Girlevik earthquake. According to estimations, 23 × 109 ion-pairs m−3s−1 were generated at 1 m depth from the Earth's surface during this earthquake. The soil-ion production estimation rate due to radon and its progeny in Erzincan is at the order of 109 ion-pairs m−3s−1. When these generated ions and charged aerosols reach the surface and were added to other ionization sources, parameters such as the near surface conductivity and electric field would be modified. In addition, under pronounced conditions could favor ionospheric perturbations.

Statistical and Criticality Analysis of the Lower Ionosphere Prior to the 30 October 2020 Samos (Greece) Earthquake (M6.9), Based on VLF Electromagnetic Propagation Data as Recorded by a New VLF/LF Receiver Installed in Athens (Greece).

In this work we present the statistical and criticality analysis of the very low frequency (VLF) sub-ionospheric propagation data recorded by a VLF/LF radio receiver which has recently been established at the University of West Attica in Athens (Greece). We investigate a very recent, strong (M6.9), and shallow earthquake (EQ) that occurred on 30 October 2020, very close to the northern coast of the island of Samos (Greece). We focus on the reception data from two VLF transmitters, located in Turkey and Israel, on the basis that the EQ’s epicenter was located within or very close to the 5th Fresnel zone, respectively, of the corresponding sub-ionospheric propagation path. Firstly, we employed in our study the conventional analyses known as the nighttime fluctuation method (NFM) and the terminator time method (TTM), aiming to reveal any statistical anomalies prior to the EQ’s occurrence. These analyses revealed statistical anomalies in the studied sub-ionospheric propagation paths within ~2 weeks and a few days before the EQ’s occurrence. Secondly, we performed criticality analysis using two well-established complex systems’ time series analysis methods—the natural time (NT) analysis method, and the method of critical fluctuations (MCF). The NT analysis method was applied to the VLF propagation quantities of the NFM, revealing criticality indications over a period of ~2 weeks prior to the Samos EQ, whereas MCF was applied to the raw receiver amplitude data, uncovering the time excerpts of the analyzed time series that present criticality which were closest before the Samos EQ. Interestingly, power-law indications were also found shortly after the EQ’s occurrence. However, it is shown that these do not correspond to criticality related to EQ preparation processes. Finally, it is noted that no other complex space-sourced or geophysical phenomenon that could disturb the lower ionosphere did occur during the studied time period or close after, corroborating the view that our results prior to the Samos EQ are likely related to this mainshock.

Low-Frequency Seismic Noise Properties in the Japanese Islands.

The records of seismic noise in Japan for the period of 1997–2020, which includes the Tohoku seismic catastrophe on 11 March 2011, are considered. The following properties of noise are analyzed: The wavelet-based Donoho–Johnston index, the singularity spectrum support width, and the entropy of the wavelet coefficients. The question of whether precursors of strong earthquakes can be formulated on their basis is investigated. Attention is paid to the time interval after the Tohoku mega-earthquake to the trends in the mean properties of low-frequency seismic noise, which reflect the constant simplification of the statistical structure of seismic vibrations. Estimates of two-dimensional probability densities of extreme values are presented, which highlight the places in which extreme values of seismic noise properties are most often realized. The estimates of the probability densities of extreme values coincide with each other and have a maximum in the region: , . The main conclusions of the conducted studies are that the preparation of a strong earthquake is accompanied by a simplification of the structure of seismic noise. It is shown that bursts of coherence between the time series of the day length and the noise properties within annual time window precede bursts of released seismic energy. The value of the lag in the release of seismic energy relative to bursts of coherence is about 1.5 years, which can be used to declare a time interval of high seismic hazard after reaching the peak of coherence.

Ionosphere Sounding for Pre-seismic Anomalies Identification (INSPIRE): Results of the Project and Perspectives for the Short-Term Earthquake Forecast

The INSPIRE project was dedicated to the study of physical processes and their effects in ionosphere which could be determined as earthquake precursors together with detailed description of the methodology of ionospheric pre-seismic anomalies definition. It was initiated by ESA and carried out by an international consortium. The full set of key parameters of the ionospheric plasma was selected based on the retrospective analysis of the ground-based and satellite measurements of pre-seismic anomalies. Using this classification the multi-instrumental database of worldwide relevant ionospheric measurements (ionosonde and GNSS networks, LEO-satellites within situprobes including DEMETER and FORMOSAT/COSMIC ROC missions) was developed for the time intervals related to selected test cases. As statistical processing shows, the main ionospheric precursors appear approximately 5 days before the earthquake within the time interval of 30 days before and 15 days after an earthquake event. The physical mechanisms of the ionospheric pre-seismic anomalies generation from ground to the ionosphere altitudes were formulated within framework of the Lithosphere-Atmosphere-Ionosphere Coupling (LAIC) model. The processes of precursor’s development were analyzed starting from the crustal movements, radon emission and air ionization, thermal and atmospheric anomalies, electric field and electromagnetic emissions generation, variations of the ionospheric plasma parameters, in particular vertical TEC and vertical profiles of the electron concentration. The assessment of the LAIC model performance with definition of performance criteria for earthquake forecasting probability has been done in statistical and numerical simulation domains of the Global Electric Circuit. The numerical simulations of the earthquake preparation process as an open complex system from start of the final stage of earthquake preparation up to the final point–main shock confirms that in the temporal domain the ionospheric precursors are one of the most late in the sequence of precursors. The general algorithm for the identification of the ionospheric precursors was formalized which also takes into account the external Space Weather factors able to generate the false alarms. The importance of the special stable pattern called the “precursor mask” was highlighted which is based on self-similarity of pre-seismic ionospheric variations. The role of expert decision in pre-seismic anomalies interpretation for generation of seismic warning is important as well. The algorithm performance of the LAIC seismo-ionospheric effect detection module has been demonstrated using the L’Aquila 2009 earthquake as a case study. The results of INSPIRE project have demonstrated that the ionospheric anomalies registered before the strong earthquakes could be used as reliable precursors. The detailed classification of the pre-seismic anomalies was presented in different regions of the ionosphere and signatures of the pre-seismic anomalies as detected by ground and satellite based instruments were described what clarified methodology of the precursor’s identification from ionospheric multi-instrumental measurements. Configuration for the dedicated multi-observation experiment and satellite payload was proposed for the future implementation of the INSPIRE project results. In this regard the multi-instrument set can be divided into two groups: space equipment and ground-based support, which could be used for real-time monitoring. Together with scientific and technical tasks the set of political, logistic and administrative problems (including certification of approaches by seismological community, juridical procedures by the governmental authorities) should be resolved for the real earthquake forecast effectuation.

Study on the ULF Magnetic Field Generated by Earth Currents Relating to Large EQs

AbstractULF magnetic anomalies (f < 1 Hz) have been observed prior to multiple large earthquakes (EQs) during the last three decades. A polarization analysis method was produced and applied in many EQ cases in order to discriminate the weak ULF crustal signals from other signals. Although many publications considered the parameter of polarization ratio (amplitude or power ratio of vertical magnetic field component to horizontal components), a key parameter in the polarization analysis method, useful to detect the possible magnetic signals associated with EQs, there were also many doubts on its reliability. The aim of this work is to evaluate the possibilities of the polarization ratio increases prior to large EQs based on the prevail assumption of Earth currents as the source of ULF emissions during the early stage of EQ preparation independent of the mechanism involved in producing the currents, and the parallel 3‐D finite‐difference time‐domain technique is applied to calculate the electromagnetic responses in the whole inhomogeneous Earth‐ionosphere waveguide. The contributions of the lightning discharges on ULF power are also evaluated based on the gridded flash counts included in the Lightning Imaging Sensor climatology data. The results reveal that the surface vertical magnetic field and polarization ratio are possible to increase locally around the epicenters as long as the intensity of the surface seismic magnetic fields are large enough compared with those produced by other sources. Unfortunately, the actual increases are usually difficult to be distinguished obviously mostly due to the weak seismic increases.

Method for Cognitive Identification of Ionospheric Precursors of Earthquakes

A significant proportion of publications related to the ionospheric disturbances that arise during earthquake preparation over the regions of their preparation refer to these disturbances as anomalies. In this case, the identification of the ionospheric precursor is actually based on an estimate of the amplitude of the deviation of the ionospheric parameters from the undisturbed value. We propose a completely different approach based on the physical mechanism of the generation of disturbances created by the interaction of the ionosphere with the lithosphere and atmosphere. At the same time, this interaction gives the observed variations unique properties that are typical only for earthquake precursors, based on which the precursors are identified with an intelligent algorithm. Another advantage of this approach is that the method, which we call “cognitive identification”, does not require large deviations from unperturbed values, since it is based on recognition of the “image” of the precursor. It is created in a way that considers morphological features of the precursors and can be effectively used even at low values of the signal/noise ratio.

Evidence of critical dynamics in various electromagnetic precursors

A wide variety of electromagnetic phenomena possibly related with earthquake (EQ) preparation processes have been reported in the literature during the last few decades. An interesting aspect in their study is the time series analysis of the related observables aiming at the investigation of any embedded dynamics. In this review article we focus on the study of fracto-electromagnetic emissions (fracto-EME) at the MHz band, the ultra-low frequency (ULF) magnetic field variations (<3 Hz) and the subionospheric very low frequency (VLF) propagation anomalies. We present recent analysis results for these electromagnetic signals using two independent methods which are known for their ability to uncover critical dynamics, the recently proposed method of critical fluctuations (MCF) and the natural time (NT) analysis method. Our results show that all three considered electromagnetic signals present critical characteristics from a few weeks up to a few days before the main shock occurrence. On the other hand, signatures for the departure from the critical (highly symmetrical) state towards a low symmetry state, a state during which there is high localization of the EQ preparatory process, have been identified in specific cases for the MHz fracto-EME as well as for the ULF magnetic field variations. Based on a multidisciplinary analysis, a four-stage model of EQ dynamics by means of fracto-EME in the MHz and kHz bands has recently been proposed. The hypothesis that the precursors considered in this article emerge during the spatially extensive phase of EQ preparation, which corresponds to the first stage of the abovementioned four-stage model, as well as their relation with the foreshock seismic activity are discussed.

The Characteristics of the Short-Period S-wave Attenuation Field in the Lithosphere of Turkmenistan and Northeastern Iran and their Relation to Seismicity

The characteristics of the short-period S-wave attenuation field in the lithosphere of Turkmenistan and northeastern Iran are considered. The data obtained by the Karatau station at distances of up to 1750 km were used. The ratios of the maximum amplitudes of Pn and Sn waves (Sn/Pn parameter) were analyzed. The correlation dependence of this parameter on distance has been obtained. These data are compared with data for the regions of Caucasus, Pamir, and the Baikal Rift Zone. The West Turkmenian depression experiences significantly lower attenuation; the Turan Plate shows an intermediate level and the Alborz Range has a higher level. It is shown that attenuation in the source zones decreases considerably with time after large earthquakes. A zone of higher attenuation has been identified in the eastern Alborz Range; it is probably related to preparation of a large earthquake. The connection of heterogeneities in the attenuation field with deep-seated fluids migration is discussed.

On the dynamics of the electromagnetic parameters of the Northern Tien Shan lithosphere

The results of geophysical monitoring of seismically hazardous regions are of undoubted interest for studying the deep structure of the lithosphere, regional seismicity, modern geodynamics, etc. The work used experimental material, including the results of magnetotelluric monitoring and the catalogs of the KNET (Kyrgyzstan Telemetered Network), KRNET (Kyrgyz Republic Digital Network) and ISC (International Seismological Center) networks obtained in the seismically active zones of the Bishkek Geodynamic Proving Ground (Northern Tien Shan). The analysis of electromagnetic monitoring results of the Northern Tien Shan seismic generating zone was carried out in a wide frequency range. The data on the parameters of the electromagnetic field of the Earth’s crust in the Northern Tien Shan are generalized and systematized. Based on the analysis of these data, the dependence of the response of seismic events in electromagnetic parameters on the distance of earthquake epicenters was studied. The most likely reason for the occurrence of anomalous changes in the electromagnetic field is the activation of deformation processes during the preparation of strong earthquakes. The results of the time-frequency analysis are presented and the features of manifestation are considered depending on the location of the hypoand epicenters of seismic events and their magnitude. Regularities have been established in the behavior of tipper variations for remote and regional earthquakes for the first quarter of 2016 with an energy class from 6 to 10.

One Class of Problems of the Theory of Elasticity and Seismology and a Problem of Earthquake Prediction

Spatial quasi-static and dynamic problems of the theory of elasticity, simulating the behavior of lithospheric plates and blocks of Earth’s crust are solved for a layered package. According to the data of seismic stations, GPS, inclinometers, and other measuring instruments, slow (age-related) and transient motions of lithospheric plates are considered. It is shown that by carrying out regular measurements, based on the obtained solutions, it is possible to trace the entire process of preparation and occurrence of earthquakes.

Changes in Tropospheric Ozone Associated With Strong Earthquakes and Possible Mechanism

The index of ozone anomaly (IOA) has been proposed to detect changes in tropospheric ozone associated with strong earthquakes. The tropospheric ozone prior and after the 2008 Wenchuan earthquake has been analyzed using IOA. Atmospheric infrared sounder ozone volume mixing ratio (O3 VMR) at different pressure levels (600, 500, 400, 300, 200 hPa) for an 18-year period 2003–2020 has been considered to identify the unique behavior associated with the strong earthquakes. Our results show distinct enhancement in tropospheric ozone occurred 5 d (7 May 2008) prior to the main event and distributed along the Longmenshan fault zone. An enhancement in IOA has also been observed around the time of the 2013 Lushan and 2017 Jiuzhaigou earthquakes, but with the different emergence time, which indicates that the unusual behavior of tropospheric ozone depends on the tectonic and geological environment, focal mechanism, focal depth, meteorological conditions, and other factors. The location of increased tropospheric ozone indicates the epicenter of earthquakes. The magnitude of earthquake could be one of the important factors affecting the appearance of the anomalous tropospheric ozone. The possible mechanism for the increased tropospheric ozone associated with strong earthquakes is discussed in this article. The quasi-synchronous changes of tropospheric ozone and other parameters in the lithosphere/atmosphere/ionosphere have been found by combining with the other published results related to the Wenchuan earthquake, which show the existence of coupling during the earthquake preparation phase associated with the lithosphere–atmosphere–ionosphere coupling.

The Theoretical and Practical Foundations of Strong Earthquake Predictability

This paper provides theoretical and practical arguments regarding the possibility of predicting strong and major earthquakes worldwide. Many strong and major earthquakes can be predicted at least two to five months in advance, based on identifying stressed areas that begin to behave abnormally before strong events, with the size of these areas corresponding to Dobrovolsky formula. We make predictions by combining knowledge from many different disciplines: physics, geophysics, seismology, geology, and earth science, among others. An integrated approach is used to identify anomalies and make predictions, including satellite remote sensing techniques and data from ground-based instruments. Terabytes of information are currently processed every day with many different multi-parametric prediction systems applied thereto. Alerts are issued if anomalies are confirmed by a few different systems. It has been found that geophysical patterns of earthquake preparation and stress accumulation are similar for all key seismic regions. The same earthquake prediction methodologies and systems have been successfully applied in global practice since 2013, with the technology successfully used to retrospectively test against more than 700 strong and major earthquakes since 1970.

Seismic Microwave Brightness Temperature Anomaly Detection Using Multitemporal Passive Microwave Satellite Images: Ideas and Limits

Thermal anomalies related to large earthquakes are frequently reported, but focus on the inconsistencies and uncertainties in the uncovered results caused by the diverse method used, data used, and case studies is lacking. Taking seismic anomaly detection using passive microwave remote sensing as an example, this study revealed and illustrated the time series of microwave brightness temperature (MBT) anomaly associated with the May 2008 Wenchuan earthquake, the April 2010 Yushu earthquake, the April 2013 Lushan earthquake, and the April and May 2015 Nepal earthquake sequence. Cross comparison showed that the spatial distribution of MBT anomalies behaves differently with different background-removal methods. More regional and detailed MBT anomalies (but with weaker intensities) were obtained with higher frequency data, whereas more pronounced MBT anomalies (but with fewer details) were acquired with lower frequency data. The amplitude and spatial scales of MBT anomalies at H polarization were larger than that at V polarization. And 10.7 GHz at H polarization was recommended as the optimal band. The spatiotemporal evolutions of MBT anomalies associated with the four selected earthquake cases were compared and found to be closely related to respective earthquake preparation mechanisms. The aforementioned characteristics and uncertainties of MBT anomalies under different conditions were then discriminated and discussed based on microwave remote-sensing physics, and the applications and limitations of the present study were illustrated and analyzed. The findings of this study are of great significance for remote sensing data selection, reliability assessment, and result interpretation in seismic-anomaly research.

A global inventory of concentration measurements of free and dissolved in underground waters molecular hydrogen in the Earth’s crust on land

This paper presents a global inventory of concentration measurements of free and dissolved in underground water molecular hydrogen sampling areas in the Earth’s crust on land. The inventory table indicates their regional belonging, the average hydrogen concentration and standard deviation at each point if concentration exceeds 0.01 %. For information analysis, sampling reservoirs are classified in 5 groups depending on the type of degassing structures. They are ophiolite complexes, geothermal systems, underground waters, ultrabasite massifs, and soils. Samples were taken on 5 continents in 32 countries from 97 reservoirs in 318 sampling areas. The 3481 gas analyzes revealed the presence of hydrogen of different concentrations. The 294 values of R/Ra were presented. All sampling reservoirs are plotted on a world map, which also shows the position of wells in the territory of the former USSR, where hydrogen from underground waters was recorded. In general, most of the reservoirs studied contain hydrogen in small quantities, since on-ly 16 (16.5%) of them have concentration of more than 5 %. The highest concentration of hydrogen (average value of 60.34 %, 7 wells) is in the heads of hydrogeological wells immediately after drilling completion. The concentration of hydrogen in soil near wells is 1070—1600 times less than hydrogen in the wells coming from underground water horizons. Zones of the present-day activation accompanied with intensive geothermal processes generate low concentration hydrogen, whose mean concentration value is 1.6 ± 3.97 % (43 analyses). In soil gases, the hydrogen concentration varies from 0.03 to 0.06% with exception of the Moscow region (0.25 %). The concentration of hydrogen in soil of the zones of active faults varies randomly along the faults depending on the composition of the sedimentary rocks filling them. Only in the zones of active faults during several years of preparation of strong earthquakes in the region hydrogen released to the day surface whose concentration exceeds the background value by 200 times reaching 4 %. The presence of hydrogen in gases with the mantle component was mentioned 14 times in publications mainly due to the values of R/Ra. However, the first integrated study of helium isotopes relationships and seismotomographic data suggests a unique case of occurrence of mantle hydrogen in one of geothermal features of the Yellowstone caldera. The present-day water plays a key role in the formation of hydrogen. Its generation occurs exclusively in the Earth’s crust because of the oxidation of ferrous iron by alkaline water in basic and ultrabasic rocks resulted from various chemical reaction schemes and water vapor always occurrence in magmatic gases where hydrogen is registered 24 times. In rare cases (4), the source of hydrogen in negligible quantities is alkaline water radiolysis or radioactive decay.

Numerical analysis of interseismic, coseismic and post-seismic phases for normal and reverse faulting earthquakes in Italy

SUMMARYThe preparation, initiation and occurrence dynamics of earthquakes in Italy are governed by several frequently unknown physical mechanisms and parameters. Understanding these mechanisms is crucial for developing new techniques and approaches for earthquake monitoring and hazard assessments. Here, we develop a first-order numerical model simulating quasi-static crustal interseismic loading, coseismic brittle episodic dislocation and post-seismic relaxation for extensional and compressional earthquakes in Italy based on a common framework of lithostatic and tectonic forces. Our model includes an upper crust, where the fault is locked, and a deep crust, where the fault experiences steady shear.The results indicate that during the interseismic phase, the contrasting behaviour between the upper locked fault segment and lower creeping fault segment generates a stretched volume at depth in the hangingwall via extensional tectonics while a contracted volume forms via compressional tectonics. The interseismic stress and strain gradients invert at the coseismic stage, with the interseismic dilated volume contracting during the coseismic stage, and vice versa. Moreover, interseismic stress gradients promote coseismic gravitational subsidence of the hangingwall for normal fault earthquakes and elastic uplift for reverse fault earthquakes. Finally, the post-seismic relaxation is characterized by further ground subsidence and uplift for normal and reverse faulting earthquakes, respectively, which is consistent with the faulting style. The fault is the passive feature, with slipping generating the seismic waves, whereas the energy activating the movement is stored mostly in the hangingwall volume. The main source of energy for normal faulting and thrust is provided by the lithostatic load and elastic load, respectively.

Three-dimensional numerical simulation of the interseismic and coseismic phases associated with the 6 April 2009, Mw 6.3 L'Aquila earthquake (Central Italy)

Although several observations have been reported in the literature before a strong earthquake, their relation with the forthcoming event is often controversial. Since many physical processes and parameters govern the dynamics of preparation, initiation, and occurrence of earthquakes, their understanding is essential for explaining anomalous seismological, geophysical, hydrological and geodetic signals before a strong earthquake that may be considered for seismic monitoring and hazard assessment.In this work, the interseismic and coseismic stress and strain fields associated with the 6 April 2009, Mw 6.3 L'Aquila earthquake are calculated via a 3D numerical model designed to simulate the crustal interseismic loading and the coseismic brittle episodic dislocation along the fault. The model adopts a framework of gravitational and tectonic forces that are compatible with the geodynamics of the Central Apennines region of the Italian territory. The model assumes a brittle upper crust, where the fault has stick-slip behaviour, and a plastic deeper crust, where the fault is in stationary creep.The results indicate that the concurrent action of gravitational and tectonic forces determines steep interseismic stress gradients at the transition between the creeping and locked fault planes that promote the coseismic subsidence of the hanging wall. The interseismic strain above the transition between that locked upper fault and its unlocked lower shear zone develops a dilated volume in the hanging wall and a contracted volume in the footwall. These stress and strain variations are compatible with seismological, geophysical and geodetic anomalies observed before the earthquake, i.e., Vp/Vs anomalies and location of foreshocks. Interseismic stress and strain patterns invert during the coseismic stage. The dilated volume, formed during the interseismic phase, will be contracted at the coseismic stage and, conversely, the footwall volume previously contracted will be expanded.