High Tectonic Stress Research Articles

Earthquake frequency-magnitude distribution and fractal dimension in mainland Southeast Asia

The 2004 Sumatra and 2011 Tohoku earthquakes highlighted the need for a more accurate understanding of earthquake characteristics in both regions. In this study, both the a and b values of the frequency-magnitude distribution (FMD) and the fractal dimension (D C ) were investigated simultaneously from 13 seismic source zones recognized in mainland Southeast Asia (MLSEA). By using the completeness earthquake dataset, the calculated values of b and D C were found to imply variations in seismotectonic stress. The relationships of D C -b and D C -(a/b) were investigated to categorize the level of earthquake hazards of individual seismic source zones, where the calibration curves illustrate a negative correlation between the D C and b values (D c = 2.80 - 1.22b) and a positive correlation between the D C and a/b ratios (D c = 0.27(a/b) - 0.01) with similar regression coefficients (R2 = 0.65 to 0.68) for both regressions. According to the obtained relationships, the Hsenwi-Nanting and Red River fault zones revealed low-stress accumulations. Conversely, the Sumatra-Andaman interplate and intraslab, the Andaman Basin, and the Sumatra fault zone were defined as high-tectonic stress regions that may pose risks of generating large earthquakes in the future.

Mine-Tunnel Design & Monitoring Analysis for Major Faults and Complex Geological Conditions

Mining operations at Nanyang-Slope Coal Mine are considerably challenging. The Nanyang- Slope Coal Field has a complex tectonic structure; having major faults, large fissures and fracture- zones, and great changes in coal-seam thickness. Mine tunnel-way 301 traverses multiple complex faults with developed fractures, broken surrounding rock, and high tectonic stresses. The original rectangular-section tunnel design using anchor-bolt-mesh retainment was prone to rib-spalling, and the problem could not be controlled by use of additional encryption anchors on the tunnel sides. The mud-rock above the tunnel-ceiling is soft and argillized, which expands and softens on contact with water. This caused problems with the epoxy bonding agent failing to effectively secure bolt-anchors to the rock, resulting in anchor-bolt-mesh failure. In consideration of the complex geological conditions, a tunnel construction utilizing a curved wall arch-section and anchor-bolt- mesh-concrete spray was proposed. MJ-40 anchor-rod dynamometers were used to measure the forces in the tunnel rock-anchor bolts; and tunnel-rock deformation and surface displacement was monitored over the course of several weeks using a measuring station. Results show that the construction design effectively lowers the effects of the tectonic stresses on any one part of the tunnel, and demonstrates the safety and reliability of the tunnel-retainment and support design. DOI: http://dx.doi.org/10.11591/telkomnika.v11i3.2302

Mining disturbance effect and mining arrangements analysis of near-fault mining in high tectonic stress region

Dynamic disasters can be easily induced by mining disturbance in the tectonic stress concentration region near the fault. Based on the fault Coulomb failure stress criterion, a dynamic Coulomb failure stress increment model is established for quantitative assessment of mining disturbance effect. Furthermore, the disturbance effects of two possible working face arrangements near the same fault are compared by using the proposed dynamic Coulomb failure stress model and numerical method. The results show that Coulomb failure stress increment, as a comprehensive mechanical parameter, can well reflect the mining disturbance to the stress environment of fault. The disturbance effect on the fault in the case of the working face perpendicular to fault is much less than that of the working face parallel to fault, therefore the dynamic disaster risk induced by the former arrangement is much lower.

Study on Section Shape and Support Parameters of Guanjiao Tunnel under Carbonaceous Slate Stratum of the 2nd Line between Xining and Golmud

Guanjiao tunnel being reconstructed is the key engineering in the 2nd line between Xingning and Golmud. It is 32.645km long. Carbonaceous slate stratum thereinto is nearly 300m long. The rock mass was suffered tectonic movement and crushed, and its physical and mechanical properties are poor. So large deformation will occur while tunnelling under high horizontal tectonic stress. Numerical simulation along with field experiments were utilized to study the mechnical effect of tunnel section shape. Two kinds of section shapes were considered, that are horseshoe-shaped section and large curvature wall, quasi-circle-shape section. Results show that quasi-circle-shape section could effectively lessen the deformation of tunnel and minish stress concentration of support. Field experiments of different support forms were also conducted. Support forms include Grid steel frame (4×ф22mm)with 0.5m span, I 20a steel frame with 0.67m span and I 20a steel frame with 0.5m span. Pressure of surrounding rock, axial force of anchor, stress of steel frame and shotcrete under different support form were measured and analysed. Because composite lining was adopted in the tunnel, the contact pressure shared by secondary lining and stress of molded concrete under different primary support stiffness were also analysed. Results showed stress of support are rational under support form of I20a. Deformation of tunnel measured in situ under different support forms showed that support form of I20a could control deformation of surrounding rock. Synthetically considering the above and economic benefit, support form of I 20a steel frame with 0.67m span was deemed to be optimal in carbonaceous slate stratum. That could offer engineering experience to tunneling in soft rock under high tectonic stress.

Sequence of rifting in Afar, Manda‐Hararo rift, Ethiopia, 2005–2009: Time‐space evolution and interactions between dikes from interferometric synthetic aperture radar and static stress change modeling

Thirteen dike intrusions in the Manda Hararo rift, Afar (Ethiopia), from September 2005 to June 2009, studied using an extensive interferometric synthetic aperture radar (InSAR) data set, provide insight into the mechanics of a major active rift. Kinematic inversions of InSAR data reveal that dikes opened by 0.8–3.5 m at an average 5 km depth, with volumes of 0.04–0.2 km3 (with up to 12 m opening and a volume greater than 1 km3 for the September 2005 megadike). Dikes have their source in a midsegment magma reservoir, which induces a local shallowing of the brittle‐ductile boundary, presumably due to thermal weakening of the lithosphere. The smaller dikes in 2006–2009 were emplaced in regions of minimum opening of the September 2005 megadike, above the central magma reservoir. In contrast, the most voluminous dike intrusions in 2006–2009 occurred near the locus of the peak of maximum opening of the September 2005 megadike, ∼10 km north of the magma source. This may suggest that tension on the plate boundary was highest there, both prior to 2005 and possibly also after 2005. Evolution and distribution of normal stress on the plate boundary throughout the rifting episode may indicate that tension near to the magma reservoir is lower than toward segment ends. Average relief of normal stresses of tectonic origin coeval to dike intrusions is comparable with shear stress drops for earthquakes, presumably because dikes in the Manda Hararo rift are intruded at low magma pressure and high tectonic stress.

Real-Time Downhole Monitoring and Logging Reduced Mud Loss Drastically for High-Pressure Gas Wells in Tarim Basin, China

Summary This paper describes a real-time case study to prevent mud loss and blowouts while drilling a high-pressure gas well in Tarim basin, China. The complex geological structure, high tectonic stresses, and overpressured- and fractured-reservoir formations in the field present a huge challenge to drilling. Of the seven wells drilled in the field in 2005, two did not reach target depths, four experienced huge mud loss, and the other experienced a blowout, resulting in lost control of the well. In early 2006, PetroChina teamed up with Schlumberger and Petroleum University of China to form a collaborative technical group to develop a better understanding of mud-loss and blowout mechanisms. The key component of the study was to establish a mechanical Earth model (MEM) based on offset well data before drilling, update the model using downhole monitoring and logging data during drilling, and predict a safe mud-weight window in real time. Real-time prediction of a safe mud-weight window with annular-pressure monitoring helped ensure that downhole annular pressure was maintained within the safe mud-weight window during drilling and tripping. The study resulted in a x17 reduction in mud loss and x10 reduction in total nonproductive time (NPT), which is mainly because of mud-loss reduction and elimination of an extra casing. A better understanding of mud-loss/blowout mechanisms was achieved, and guidelines for preventing mud loss/blowouts specific for this gas field were developed.

Mine Water Discharge and Flooding: A Cause of Severe Earthquakes

Severe earthquakes can be triggered by dewatering and flooding of mines, as these activities alter the loading of the Earth’s crust and tectonic stresses in its interior. Worldwide, more than 200 studies have noted sites where human-induced stresses could have reactivated preexisting faults, triggering earthquakes with seismic moment magnitudes of up to M = 7 on the Richter scale. This can only occur where faults are already under high tectonic stresses that have built up over many years. Stable continental regions are seismically less active than unstable regions (e.g. California, Japan, and Turkey). Consequently, faults in stable continental regions can be more earthquake-trigger sensitive, since accumulated stresses have not reached failure conditions. This paper provides an overview of officially recognized mining-triggered earthquakes with magnitudes M ≥ 5.0. The article illuminates that these earthquakes can cause serious socio-economic losses with negative implications for the long-term sustainable development of countries abundant in natural resources and of mining regions, in particular. Historic data suggest that regional geological conditions (e.g. structural geology and tectonic in-situ stress states) are more important in forecasting the potential of earthquake triggering than the scale of the mining activities. Overall, such forecasts should be made to estimate and mitigate potential socio-economic earthquake risks associated with geoengineering operations of extractive industries such as mining.

Mechanism of deep cracks in the left bank slope of Jinping first stage hydropower station

The proposed Jinping first stage hydropower station is located on the middle reach of the Yalongjiang River, Sichuan Province, PR China. It consists of a double-curvature arch dam with a maximum height of 305 m, which will be the highest in the world. Cracks are well developed in the left bank slope of the Pusiluogou, the dam site of Jinping first stage hydropower station. Some cracks are even developed at the horizontal distance into rock face (in this paper, the term “horizontal depth” was used to describe the horizontal distance into face of the slope) beyond 200 m, and these cracks are here referred to as deep cracks. The distribution of these deep cracks is far beyond people's experiences in common unloading zones, so the mechanism of the deep cracks is put forward as a new subject. Since these cracks are significant for the stability of the slope, their mechanism becomes one of the key engineering geology problems. A comprehensive study is made in this paper on the distribution characteristics of the cracks as well as their deformation features. The factors that influence their distribution and deformation feature are also analyzed by means of geology, mechanics and numerical simulation. An important conclusion is consequently reached that deep cracks occurred as a result of release of high ground stress including gravitational stress and tectonic stress. The formation of the deep cracks in the slope is a result of unloading process under favorable geology conditions. Fast crust uplift and quick river cutting and high tectonic stress are indispensable regional geological condition, steep and high geometry condition provides favorable geometry condition, hard at the lower part and soft at the upper part of the slope offers suitable rock combination condition, and back facing bending planes and consequent discontinuities supply suitable structure of the slope.

SRC rock mass classification of tunnels under high tectonic stress excavated in weak rocks

This paper describes the application of the SRC rockmass classification system to tunnels under high horizontal tectonic stress excavated in weak rocks. The analysis was performed on 25 tunnels in Spain and Italy, for which it was found that much heavier supports than those estimated by the RMR index were required. SRC and RMR indices and other relevant geomechanical data were obtained during the site investigation and construction stages. Data corresponding to in situ stress measurements, analysis of tectonic structures and instability problems arising during construction were used to asses the state of stress. The relationship between tunnel section convergence and the SRC and RMR indices was also analysed. Support measurements based on SRC and RMR classification were compared with those actually used during construction. These analyses indicate that for most of the tunnels examined, supports estimated using the SRC were much closer to those actually installed than those predicted by the RMR index. Based on the case histories presented, the factors mainly contributing to deformability and consequently to assessing support measurements were: high horizontal tectonic stress, low strength of rocks, overburden thickness and structural anisotropy related to tunnel axis orientation. According to these factors, the tunnels investigated were classified as three types. Tunnels classed as type I were those of low overburden thickness under high horizontal tectonic stress excavated in low strength rocks. The supports installed for these tunnels were much heavier than those predicted by the RMR index, being more in line with those indicated by the SRC index. The type II tunnels had thick overburdens and showed similar stress and strength conditions to the former. The supports installed were practically those foreseen by the SRC index, appreciably differing with respect to the RMR index. Finally, tunnels included in the type III class were those under low to moderate tectonics stress, irrespective of overburden thickness. These tunnels gave rise to RMR and SRC indices that provided acceptable results.

Fault and stress magnitude controls on variations in the orientation of in situ stress

Abstract There has been a great deal of interest in mapping regional trends in stress orientation and their relationships to crustal deformation processes over the last quarter century. Less emphasis has been placed on explaining local variations in the stress field. However, it is variations in the local stress field which are most critical to characterizing hydrocarbon reservoirs. Fracture orientation, well stability, well orientation, and permeability anisotropy are all strongly affected by variations in the local stress field. Using stress orientation data from a number of fields in different tectonic environments, we have tried to determine some of the tectonic and geological controls on variations in in situ stress orientation. We find that nearness to faults, fault structure, and magnitude of tectonic stress play primary roles in determining whether the regional stress field will be perturbed in a given reservoir and whether small-scale variations in the stress field can be expected. We find that high tectonic stress environments (large horizontal differential stress) lead to a much more consistent local stress field than more tectonically quiescent areas. Faults can play a large role in rotating the local stress field. The smaller the difference between the maximum and minimum horizontal stress magnitude (i.e. lower tectonic stress), the larger a fault’s zone of influence is. Large-scale faulting with segmentation of the reservoir into discrete fault blocks can lead to significant stress orientation variations across the reservoir even in areas of large differential stress.

Drillstring Vibration and Wellbore Instability

Wellbore instability can be attributed to several causes. The ones thought to be most important include: chemical interaction with the drilling fluid, high tectonic stresses, and insufficient mud weight. Drillstring vibration, although not traditionally addressed as a potential cause, might influence the stability of wellbores drilled in specific formations. Evidence of the strong correlation between severe vibration and wellbore instability has been reported in the literature. However, a more thorough understanding of the phenomenon is still lacking. This paper describes a study that has been developed by PETROBRAS focusing on how drillstring vibration impacts wellbore instability. Vibration has been monitored in some wells, and events related to borehole enlargement were observed. Four field cases are presented showing a strong correlation between high vibration level and wellbore enlargement in different lithologies. Other sources of wellbore enlargement have also been identified, and they can be clearly separated from vibration.

A model for the 1995 Kozani-Grevena seismic sequence

Accurate locations of aftershocks, fault plane solutions for the main shock and for a large number of aftershocks as well as geographical distribution of macroseismic intensities led to a reliable estimation of the fault parameters and to a better understanding of the rupture process for the 1995 Kozani-Grevena destructive earthquake. The corresponding fault has a length L = 30 km, a width w = 10 km, strikes in an ENE direction (N65 °E), dips to NNW and the mean displacement on the fault during the generation of this earthquake is about 50 cm. Aseismic preshock slip in an aseismic area of the central part of the fault induced high tectonic stress in the rest of the fault. This stress reached the foreshock barrier's strength near the shallow western boundary of the aseismic area, where the two largest foreshocks occurred, and then the ultimate mainshock barrier's strength near the deep eastern boundary of the aseismic area where the mainshock occurred. From the focus of the mainshock, which was located in the deepest and northernmost part of the fault, the rupture propagated both up-dip and bilaterally and terminated vertically at a depth of about 4 km (blind fault) and horizontally at the eastern and western end of the aftershock zone. The vertical termination of the rupture not at the surface but at some depth is attributed to properties of the uppermost part of the crust or to slipping on minor shallow faults, while the horizontal east and west termination to the intersection of this fault with other faults which strike in a NE direction (geometrical barriers).

Unloading and weathering as an indicator of the occurrence of current stresses in the rock foundations of hydraulic structures

The complex structure and incompletely clear processes of formation of the stress fields hinder their evaluation and require the combined used of methods most suitable for particular geologic conditions [1]. At present rather considerable practical experience has been gained in studying the stress--strain state of rock masses, which are the environment and foundation of hydraulic structures. A comparison of the results of comprehensive studies gives grounds to speak about the existence of certain regularities in the manifestation of current stresses through certain fixed or dynamic characteristics both of the natural geologic environment and of the natural--technogenic system. Such characteristics, called indicators, make it possible at any stage of surveys, construction, or operation of the structure to obtain an idea about the general or particular character of existing stresses in the mass as a whole or in its individual structural elements. It is obvious that quantitative measurements of stresses existing in the mass are top-priority for design studies; however, a qualitative characterization of its stress--strain state obtained on the basis of indirect investigations and analysis of indirect indicators also has great practical significance. This is important at the survey and construction stages for optimal selection of the sites of taking expensive quantitative measurements and correcting design decisions, and in the operating stage it is an essential supplement to geodynamic monitoring [7], especially when interpreting detected off-design deformations of the dam or foundation and when developing additional control or protective measures. The detection of indicators can be regarded as a variant of the empirical engineering-geologic approach to the problem of studying the current stress-strain state of rock masses based on the method of evaluating stresses "from the opposite," i.e., from the presence of various characteristics of the properties and state of the rocks and masses as well as from the presence or intensity of occurrence of various processes corresponding to a certain character of the existing stresses. Rock masses can be tentatively divided into two groups on the basis of a combined analysis of the characteristics of the geologic structure and occurrence of current natural stresses carried out for a number of objects. The first group includes masses in which horizontal or almost horizontal stresses predominate over vertical ones near the surface. The second group includes masses with a predominance of vertical over horizontal stresses. Masses of the first group are characterized by: high current horizontal tectonic stresses;

In search of potential earthquake source regions in the Chinese mainland in the light of ambient shear stress field

Earth media are incomplete media. There exist many cracks in it. The achievements of fracture mechanics show that the strength of the incomplete materials will be much lower than that of the complete materials. We consider that earthquake occurrence is the result of unstable propagation of a crack in crust media in proper condition and the earthquake rupture is the phenomenon of a failure by fast fracture under applied low shear stress. It has already been explained by fracture mechanics. The occurrence of failure by fast fracture is necessarily associated with the presence of high level concentration of local stress and strain. The elastic/plastic stress analysis in cracked pieces by Dugdale indicates that the state of stress at the tip of a crack takes a very important role to crack propagation. A plastic zone has necessarily formed in the tip of a crack due to stress concentration. Therefore, the dislocations at the tip of a crack are naturally a plastic displacement, rather than elastic one. The plastic displacementδ=πaτ 0 2 /2µτ y , whereτ 0 is applied shear stress which is equivalent to initial or tectonic shear stress when the quake occurs,a is the half length of a crack,μ is the rigidity,τ y is the yield stresses in shear. The main seismic dislocations take place exactly at the ends of the crack where the plastic zone had been formed. So, a critical assumption is adopted, i. e. we assume the dislocationD(ξ 1,t) as formula (5) in text. The maximum earthquake dislocationD max=πLτ 0 2 /4πτ y , whereL is the fault length. Ifμ is taken the value in the upper crust,μ=33 GPa; andτ y is taken the average value given from laboratories,τ y =30 MPa. Thus, according to observation values ofD max andL, using the formula, one can estimate the initial shear stresses for large earthquakes. Computations show that the initial shear stresses for large earthquakes all over the world are about 5–20 MPa which have some differences between regions. We further research the characteristics of source spectra and have derived the dependent relation of body wave magnitudem b on the shear stressτ 0 and seismic momentM 0 as formula (11) in text. Thus, the formula provides a possibility of computation of large amount of tectonic shear stress values from seismic data. We consider that the tectonic shear stress field is a main factor which controls the earthquake occurrence. The regions with high tectonic shear stress values are considered to be prone to occur great earthquakes (M s⩾6) and called earthquake hazard regions. Based on this criterion,τ 0 values for all earthquakes withm b⩾3.8 all over China since 1987 have been computed, and the great earthquake hazard regions with magnitude ranges have been zoned in the Chinese mainland. During April 1992–January 31, 1994, there were 9M s⩾6 earthquakes which occurred in the Chinese mainland, 8 earthquakes of the 9 had fallen into the regions delineated by us prior to the earthquake occurrence, with only one failure. This new approach as a method for medium-term prediction of strong earthquakes has been proved by practice to be an efficient one. It has good physical bases and bright prospect and worth further research.

Incremental Stress and Earthquakes

SUMMARY We use the Harvard catalogue of seismic moment tensor solutions to investigate the statistical properties of incremental static stress caused by earthquakes. Using Okada’s (1992) program for a point dislocation in a half-space, we compute normal and shear stress on nodal planes for each earthquake in the catalogue, as well as stress invariants in the focal zone of each event. These accumulated stress values are calculated at the location of any future reference earthquake (pre-stress) and then compared with the stress level measured at the same point after the event (post-stress). Comparison of the statistical distributions for pre- and post-stress indicates that the normal stress level has little influence on an earthquake occurrence. On the other hand, the shear stress in the focal zone of an event is significantly higher before an earthquake than after that earthquake (the stress caused by the reference earthquake is not taken into account in this comparison). Earthquakes are more likely to be induced by incremental shear stress if the stress and the moment tensor of an ensuing event are consistent, i.e. the incremental stress has the same sign as the seismic moment tensor of the reference earthquake. Similar results are obtained if we compare the first and second pre- and post-stress invariants: earthquake triggering is not influenced by the average normal stress (first invariant); contrary to that, the average shear stress (second deviatoric invariant) is significantly larger before an earthquake than after it. The distribution of hypocentres from the PDE catalogue confirms the above conclusions: the incremental stress caused by events in the Harvard list and measured at hypocentre locations of PDE events, is higher for the pre-stress shear component, but shows no significant difference for compressional/dilatational stresses. These findings, if interpreted in a typical framework of the Coulomb failure criterion, would suggest that the effective friction coefficient p is close to zero. Since this effect is observed in various seismotectonic regions for shallow and intermediate earthquakes (with a depth of as much as 300 km), the conventional explanation for the low value of p-high pore pressure-is less plausible. We conjecture that these results can be explained by the fractal pattern of earthquake fault geometry which is due to the fault selforganization in conditions of high lithostatic and tectonic shear stress.

Coping with stress gives Karawanken tunnellers a headache : Martin, D Tunn TunnlgV20, N12, Dec 1988, P19–27

This article describes the Karawanken Tunnel between Austria and Yugoslavia. Extensive deformations caused by high tectonic stress, swelling anhydrite, water and methane gas have given the tunnellers driving the 7.8km Karawanken road tunnel many headaches. A redesigned ventilation system, part longitudinal and part transverse, has cut costs by saving the need for two deep ventilation shafts.

Evaluation of rock mass condition in the vicinity of large-scale caverns in mountain slopes as affected by high tectonic stresses

Evaluation of rock mass condition in the vicinity of large-scale caverns in mountain slopes as affected by high tectonic stresses

Selection and substantiation of effective drainage protection of the gate works of the diversion tunnels of the Rogun hydroelectric station

1. The low permeability and high tectonic compressive stresses in the rock mass made it possible to locate the chamber-type underground structures in the upper pool of the dam under the reservoir bed. In connection with the poor effectiveness of plugging the rocks by grouting under such conditions, a well drainage curtain located on the upstream side of the grouting zone was used as the main measure on protecting the underground chambers from seepage. 2. The high effectiveness of investigating three-dimensional flow in three-dimensional electrolytic EHDA models with simplication of complex drainage devices in them by using the analytic solutions of appropriate fragments was shown. 3. As a result of investigating seepage in the foundation of the Rogun hydroelectric station the following main comparative characteristics were obtained: In the absence of drainage the linings of the gate hoisting chambers of the diversion tunnels will experience a pressure up to 2.50 MPa; With operation of just the main drainage system of the PHC this pressure decreases to 1.80 MPa; With the combined operation of the drainage systems of the PHC and gate works of the diversion tunnels the seepage pressure on the linings of the latter decreases to 0.08–0.32 MPa; With the operation also of the local borehole drainage of the hoisting chambers the pressure in the stretch of the boreholes decreases to 0.01 MPa; With the combined operation of the main drainage systems of the PHC and gate works of the PHC receives 12,300 m3/day of water and the drainage system of the gate works 7650 m3/day. Operation of the borehole drainage increases the seepage flow to the chambers negligibly, by about 300 m3/day.

Source dynamics of two great earthquakes of the Indian subcontinent: The Bihar-Nepal earthquake of January 15, 1934 and the Quetta earthquake of May 30, 1935

abstractSource mechanisms of two major destructive earthquakes which occurred at the Bihar-Nepal border and in the Quetta region on January 15, 1934 and May 30, 1935, respectively, are determined using the P-wave first motions, S-wave polarization angles, and surface-wave spectral data. The high stress drop and apparent stress associated with these events suggest that high tectonic stresses are prevailing in these regions. A major part of the stresses accumulated before the occurrence of the two earthquakes had been released through the main shock. An investigation of temporal and spatial variation of regional seismicity reveals possible existence of seismic gaps before the occurrence of these two major events.

Earthquakes associated with the Clark Hill reservoir, South Carolina — A case of induced seismicity

Following the McCormick County, South Carolina, local magnitude 4.3 earthquake on August 2, 1974, continuous seismic activity has been observed in the area. The epicenters are located within 3 km from the Clark Hill reservoir. The shallow seismic activity appears to be related to water level fluctuations in the reservoir, and follows them by about two days. The frequency of earthquakes is related to the rate of change of water level, while the energy release is seen to depend on the water level itself. Pore pressure fluctuations at focal depths in a regime of high tectonic stresses is the suggested cause for the triggering of earthquakes.