Circular Fault Research Articles

Fluid-driven slow slip and earthquake nucleation on a slip-weakening circular fault

We investigate the propagation of fluid-driven fault slip on a slip-weakening frictional interface separating two identical half-spaces of a three-dimensional elastic solid. Our focus is on axisymmetric circular shear ruptures as they capture the most essential aspects of the dynamics of unbounded ruptures in three dimensions. In our model, fluid-driven aseismic slip occurs in two modes: as an interfacial rupture that is unconditionally stable, or as the quasi-static nucleation phase of an otherwise dynamic rupture. Unconditionally stable ruptures progress through four stages. Initially, ruptures are diffusively self-similar and the interface behaves as if it were governed by a constant friction coefficient equal to the static friction value. Slip then accelerates due to frictional weakening while the cohesive zone develops. Once the latter gets properly localized, a finite amount of fracture energy emerges along the interface. The rupture dynamics is then governed by an energy balance of the Griffith’s type. In this stage, fault slip transitions from a large-toughness to a small-toughness regime due to the diminishing effect of the fracture energy in the near-front energy balance. Ultimately, self-similarity is recovered and the fault behaves again as having a constant friction coefficient, but this time equal to the dynamic friction value. This condition is equivalent to a fault interface operating to leading order with zero fracture energy. When slow slip is the result of a frustrated dynamic instability, slip also initiates self-similarly at a constant peak friction coefficient. The maximum size that aseismic ruptures can reach before becoming unstable can be as small as a critical nucleation radius (shear modulus divided by the slip-weakening rate) and as large as infinity when faults are close to a well-defined limit that separates the two modes of aseismic sliding. In the former case, earthquake nucleation occurs unaffected by the dynamic friction coefficient. In contrast, the latter case exhibits fracture-mechanics behavior, characterized by a finite influx of elastic strain energy being supplied to and dissipated at the rupture front. We provide analytical and numerical solutions for the problem solved over its full dimensionless parameter space, including expressions for relevant length and time scales characterizing the transition between different stages and regimes. Due to its three-dimensional nature, the model enables quantitative comparisons with field observations as well as preliminary engineering design of hydraulic stimulation operations. Existing laboratory and in-situ experiments of fluid injection into simulated and natural faults are briefly discussed in light of our results.

The Crete Isl. (Greece) Mw6.0 Earthquake of 27 September 2021: Expecting the Unexpected

The 27 September 2021 damaging mainshock (Mw6.0) is the first known strong earthquake that ruptured the Arkalochori area, Crete Isl., Greece, during the entire historical period, making it an unexpected event in the long-term sense. The area is characterized by the presence of the normal active Kastelli Fault (KF) striking NNE-SSW and dipping towards ~WNW. The KF, of surface exposure only ~6 km, at its southern tip is truncated by the nearly perpendicular active Nipiditos fault. The main shock was preceded by foreshock activity lasting for ~3.9 months, thus the mainshock turned out to be an expected event in the short-term sense. Maximum ground subsidence of ~20 cm was estimated from InSAR images, but this also incorporates deformation that may have been caused by the largest aftershock (Mw5.1) of 28 September 2021. The fault model produced from the inversion of InSAR observations indicated strike 216°, dip towards ~NW at angle 53°, rake −95°, and is consistent with fault-plane solutions obtained from routine moment tensor analysis. The geodetic seismic moment calculated from the Okada’s formalism is 1.14 × 1018 N·m (Mw6.0), while a maximum slip of 1.03 m was found at depths from 3.5 km to 5 km. The entire aftershock epicenters cloud strikes in a ~SW-NE direction but is distributed in two clusters, the southern and the northern ones. The foreshock cloud, the main slip patch, the deformation area, and the strongest aftershocks all fall within the southern cluster. The foreshocks concentration at the deepest edge of the main slip patch was a foreshadow of the mainshock nucleation area. The northern cluster, which is very likely due to the gradual expansion of aftershocks, is situated in the KF hanging wall block. To interpret the main seismic slip in the southern cluster area we propose the existence of a buried KF segment at the SSW-wards prolongation of the emerged at the surface segment. Assuming a rectangular seismic fault stress drop Δσ~7 bars was found. However, for a circular fault area, which in this case is more realistic, we get Δσ = 55 bars. This is a relatively large value for Greek earthquakes but is explainable by increased fault rigidity as a result of the long repeat time of strong earthquakes in KF.

Методика дешифрирования архивных и современных космических снимков для изучения динамики склоновых процессов в долине р. Гейзерная (Камчатка)

Based on the database of spatial thematic data sources for the Geysernaya River valley (Kamchatka), the work was carried out to identify the areas of the greatest activation of modern exogenous processes and the manifestation of endogenous processes over the maximum possible time interval (more than 50 years). Space images of high spatial resolution (0.5–3 m), received from Key-Hole-4 (1964), GeoEye (2009), Pleiades-1A/1B (2013, 2016), WorldView-2 (2017) satellites, results of field survey and UAV imagery in 2021, literature and archive materials were used as sources. The developed interpretation technique included both sequential interpretation of preliminary geometrically coordinated multi-temporal images and their digital processing: brightness quantization, calculation of vegetation index NDVI, formation of color multi-temporal composite, which allowed to separate vegetation-covered and bare slope areas by formal features and increased the overall reliability of interpretation. Calculations based on the interpretation results determined that the proportion of the total valley area affected by modern slope processes was 10.5 % in 1964, 14.8 % in 2009, and 13 % in 2017. In the process of images interpretation, it was established that slope processes dominated on the left bank of the Geyser River, where more than 75 % of the area of all outcrops (or 10.2 % of the total studied area of the valley) was concentrated, which is apparently caused by confinement of this valley side to the caldera limiting circular fault, which preserves its activity, and also by the presence of the area of modern uplift there. On the right bank of the river, large areas are occupied by ancient large gravitational shifts, possibly associated with collapsing edges of extrusive massifs. Landslides-slides and erosion in the groundwater outlets are predominant among the modern processes here. The right bank accounts for less than 25 % of the area of all outcrops (or 3.3 % of the total study area).

Dynamic fault weakening during earthquakes: Rupture or friction?

An earthquake is an event of dynamic, unstable slip that releases elastic energy stored in the earth's crust. This abrupt energy release requires the weakening of the slipping fault that is manifested by a strength drop from a static level to a dynamic level. The fault weakening occurs in two general zones: The rupture-front at the earthquake tip, and the frictional zone in the wake of the propagating front. Here, we analyze the weakening processes of the rupture-front in experimental stick-slips, and compare these processes to the weakening in the frictional zone. Our experimental analysis focuses on high-speed monitoring (1 MHz) of the strain-fields associated with spontaneous, unstable ruptures propagating bilaterally from a nucleation site along a circular fault. The ruptures propagated at velocities ranging from slow (∼100 m/s) to supershear, and the fault weakening by the rupture-front, defined by WS = [strength drop/peak strength], is proportional to the rupture propagation velocity. WS ranged 0.08-0.58 during tiny slip-displacements of 3.7-43 microns, in agreement with similar weakening intensity in a few sets of dynamic rupture experiments. In contrast, frictional weakening, which is traditionally analyzed in constant-velocity experiments, requires slip-displacements in the range of 0.1-10 m to reach similar weakening intensity. Therefore, in terms of slip-displacements, rupture-front weakening is more efficient than frictional weakening by orders of magnitude. Further, we observed that a rupture-front transported its intense weakening from a nucleation site to the entire experimental fault, and we argue that the equivalent process can occur along natural faults. If the rupture of a tiny earthquake, e.g., M < −5, is of similar scale and intensity as the experimental ruptures, it can locally weaken the host fault, and by doing so could serve as an earthquake nucleus. However, these tiny earthquakes could nucleate large ones only if sufficient elastic energy is available in the surrounding crust for the frictional sliding dissipation.

The relations between the corner frequency, seismic moment and source dynamic parameters derived from the spontaneous rupture of a circular fault

SUMMARYThe stress drop is an important dynamic source parameter for understanding the physics of source processes. The estimation of stress drops for moderate and small earthquakes is based on measurements of the corner frequency ${f_c}$, the seismic moment ${M_0}$ and a specific theoretical model of rupture behaviour. To date, several theoretical rupture models have been used. However, different models cause considerable differences in the estimated stress drop, even in an idealized scenario of circular earthquake rupture. Moreover, most of these models are either kinematic or quasi-dynamic models. Compared with previous models, we use the boundary integral equation method to simulate spontaneous dynamic rupture in a homogeneous elastic full space and then investigate the relations between the corner frequency, seismic moment and source dynamic parameters. Spontaneous ruptures include two states: runaway ruptures, in which the rupture does not stop without a barrier, and self-arresting ruptures, in which the rupture can stop itself after nucleation. The scaling relationships between ${f_c}$, ${M_0}$ and the dynamic parameters for runaway ruptures are different from those for self-arresting ruptures. There are obvious boundaries in those scaling relations that distinguish runaway ruptures from self-arresting ruptures. Because the stress drop varies during the rupture and the rupture shape is not circular, Eshelby's analytical solution may be inaccurate for spontaneous dynamic ruptures. For runaway ruptures, the relations between the corner frequency and dynamic parameters coincide with those in the previous kinematic or quasi-dynamic models. For self-arresting ruptures, the scaling relationships are opposite to those for runaway ruptures. Moreover, the relation between ${f_c}$ and ${M_0}$ for a spontaneous dynamic rupture depends on three factors: the dynamic rupture state, the background stress and the nucleation zone size. The scaling between ${f_c}$ and ${M_0}$ is ${f_c} \propto {M_0^{ - n}}$, where n is larger than 0. Earthquakes with the same dimensionless dynamic parameters but different nucleation zone sizes are self-similar and follow a ${f_c} \propto {M_0^{ - 1/3}}$ scaling law. However, if the nucleation zone size does not change, the relation between ${f_c}$ and ${M_0}$ shows a clear departure from self-similarity due to the rupture state or background stress.

What Is Fling Step? Its Theory, Simulation Method, and Applications to Strong Ground Motion near Surface Fault Ruptures

ABSTRACT We present the theory of the fling step and a theoretical method for simulating accurately the near-fault strong motions, and apply it to reproduce various strong-motion records near surface faults. Theoretically, the fling step is the contribution of the static Green’s function in the representation theorem (Hisada and Bielak, 2003), and we show that this theory holds for any seismic velocity structure. We first demonstrate the validity of this theory using theoretical solutions of a circular fault model in a homogeneous full-space. Next, we apply the theory to layered half-spaces, present a theoretical method based on the wavenumber integration method, and introduce various techniques to simulate the near-fault ground motions including fling steps with high accuracy. Finally, we demonstrate the effectiveness of the method by reproducing various strong-motion records near surface fault ruptures and discuss the characteristics of near-fault strong motions including the fling step and the forward directivity pulse. We made all of the software and data used in this article available on the internet.

Insights into lava dome and spine extrusion using analogue sandbox experiments

Lava dome formation is a common process at stratovolcanoes involving the shallow intrusion or extrusion of viscous lava and may lead to the rise of spines. Spines are protrusions observed to extrude episodically during lava dome growth, yet the structural and mechanical factors controlling their formation are only partially understood. Here, we provide new, detailed insight into lava dome growth and the production of spines using a novel set of analogue experiments extruding sand-plaster mixtures from a fixed-diameter conduit under isothermal conditions. We trace displacement and strain with photogrammetric methods for precise and detailed monitoring of the extrusion process. Results show initial dome growth forming a steep-sided and flat-topped shape through extrusion of new material, leading to slumping of oversteepening slopes, forming a talus. Spines are found to protrude at a later stage through the dome surface along discrete circular faults that originate from the conduit walls, starting a cycle of spine growth and collapse. As our spines only appear after prolonged extrusion, we relate their appearance to the compaction and strengthening of material within the conduit. We find that spine diameter, height and volume are positively correlated with increasing cohesion and therefore material strength. The spine diameter was also observed to be smaller or equal to the diameter of the underlying conduit, as shear extrusion occurs along vertical to outward-dipping fault planes. For natural domes, our findings imply that spine growth may be the consequence of compaction and densification via porosity loss, shearing and/or outgassing of conduit magma during ascent. More efficient compaction will yield wider and taller spines as a result of increasing rock strength. Our study further highlights the relevance of analogue experiments in the study of lava domes and spines, which remain one of the most hazardous and unpredictable features at dome-forming volcanoes worldwide.

Relationship Between Seismic Moment and Source Duration for Seismogenic Earthquakes in Taiwan: Implications for the Product of Static Stress Drop and the Cube of Rupture Velocity

A systematic analysis of the source duration (τ) and seismic moment (M0) for seismogenic earthquakes (MW 5.5–7.1) in the Taiwan region was completed by using a teleseismic P-wave inversion method. Irrespective of the source self-similarity, the M0–τ relationship derived in this study had a power-law form, namely M0 ∝ τ3, under the assumption that ΔσVr3 is constant following a circular fault model (Δσ: static stress drop; Vr: rupture velocity). For Taiwan’s earthquakes, the derived M0–τ relationship not only provides information to predict the source duration of large earthquakes, but also probes the rupture features of seismogenic earthquakes. That is, there are different rupture patterns for earthquakes, but the product ΔσVr3 remains nearly constant.

Micro‐Seismic Monitoring of a Shear Fault Within a Floating Ice Plate

AbstractThe deformation of a circular fault in a thin floating ice plate imposed by a slow rotational displacement is investigated. Temporal changes in shear strength, as a proxy for the resistance of the fault as a whole, are monitored by the torque required to impose a constant displacement rate. Micro‐seismic monitoring is used to study the relationship between fault average resistance (torque) and micro‐ruptures. The size distribution of ruptures follows a power‐law scaling characterized by an unusually high exponent (b≃3), characteristic of a deformation driven by small ruptures. In strong contrast to the typical brittle dynamics of crustal faults, an 'apparently aseismic' deformation regime is observed in which small undetected seismic ruptures, below the detection level of the monitoring system, control the slip budget. Most (≃71%) of the detected ruptures are organized in bursts with highly similar waveforms, suggesting that these ruptures are only a passive by‐product of apparently aseismic slip events. The seismic signature of this deformation regime has strong similarities with crustal faulting in settings characterized by high temperature and with non‐volcanic tremors.

Influence of the Lineament Geological Features on the Hydrologic Flow Direction of Wadi Al Kuf Catchment Area, Cyrenaica, Northeastern Libya

Wadi Al Kuf Catchment Area WKCA is one of the largest watershed basins on Al Jabal al Akhdar Cyrenaica anticlinorium, the area is more than 960Km2, and considers as a semi-wet basin. This basin highly affected with lineaments geological features just like morphometric and tectonics types including fissures, fault systems and joints set systems in the highly karst lime stones of Al Jabal al Akhdar group lithological formations. These lineaments phenomena were measured and extracted from the radar images of digital terrain model of 30 meters space grid, and the hyper spectral Landsat 8 of 15 meters pixel resolution, they were processed and interpreted by several geospatial geomatics and geological software. The direction orientation and the rock density of these fissures, fractures, joints set systems, faults and the morphometric dendritic drainage pattern had been measured and illustrated from the rose diagram analysis and the geological map. The mainstream of this catchment area WKCA is the 6th order and mainly parallel to the main trend direction with the first escarpment circular fault at the major orogeny tectonic fault of Al Jabal al Akhdar uplift, and these lineaments features is averaged 58.3o with the azimuth degree along the mainstream. The drainage density, lineaments density analysis and distribution of the WKCA have been classified as low lineaments rock fractures in the eastern boundary of the basin, moderate lineaments rock fissures in the middle of the basin and high density of rock fracture in the western and northern boundary of the basin, these had reflected the deep percolations and infiltrations to the ground water-bearing aquifer in the WKCA through the secondary and the tertiary porosity of the hydrological karst system.

A Dynamic Explanation for the Ruptures of Repeating Earthquakes on the San Andreas Fault at Parkfield

AbstractAnalyzing the relative source time functions (STFs) of repeating earthquakes (REs) at Parkfield and their spectra indicates that there are two groups of REs. One group of REs (G1) displays simple relative STFs and smooth spectra, whereas the other group (G2) displays complex relative STFs and spectra with obvious holes. We use the boundary integral equation method with different configurations of parameters to simulate the spontaneous dynamic ruptures and fit the observed characteristics. Finally, we find that the differences in the observed characteristics are due to the different configurations of the dynamic parameters. Earthquakes in G1 are self‐arresting ruptures, and earthquakes in G2 are runaway ruptures. Moreover, holes in the spectra of G2 are caused by the dynamic rupture propagation, and the frequency of the holes is a function of the radius of the circular fault, the angle between the station azimuth and the rupture propagation, and the rupture velocity.

Morphology and activity of the Helgoland Mud Volcano in the Sorokin Trough, northern Black Sea

Abstract Morphological change and activity of submarine mud volcanoes (MVs) are still poorly understood. In this study, we integrated acoustic subbottom profiles, Autonomous Underwater Vehicle (AUV) high-resolution bathymetric and backscatter data, Remotely Operated Vehicle (ROV) seafloor observations, Bottom Water Temperatures (BWT), and compositions of gas samples collected from three research expeditions, to study the morphology and activity of the Helgoland mud volcano (Helgoland MV) in the Sorokin Trough of the Black Sea. Based on the micro bathymetric and backscatter maps, we conducted a geomorphological analysis which differentiates the Helgoland MV into five regions, which are: the active center, the inner crest, the inner moat, the outer crest, and an outer moat. Special seafloor features, such as gas bubble emissions, mud breccia and mud pools, were observed in different regions by ROV observations. They indicate a heterogeneous distribution of materials and activities at the Helgoland MV. Active mud extrusion from the center was inferred from BWT elevations (up to 1.652 °C) that showed the same orientation as a major fault (NW-SE) at the location. Methane stable carbon isotopes (−58.1∼−63.6‰) and molecular compositions of light hydrocarbons (C1/C2+ = 2836, CH4 = 99.545%, C2H6 = 0.033%, C3H8 = 0.002%, CO2 = 0.417%) suggest a predominance of biogenic hydrocarbons with an admixture of thermogenic gas. Circular faults surrounding the Helgoland MV, as observed in the bathymetric and sub-bottom profiles, are speculated to be formed due to subsidence of the MV caldera and gravity driven sediment movement. A conceptual model is proposed to explain the interrelationship between the seafloor morphology and activity of the Helgoland MV. Combining multidisciplinary data, especially from the application of ROVs and AUVs, broaden the scope in which the activity and morphology of submarine mud volcanoes can be investigated.

Shear heat model for gouge free dip-slip listric normal faults

Abstract Shear heating due to brittle faulting is important in petroleum geosciences, tectonics and seismic studies. Temporal variation of shear heat in a listric normal fault, with a circular arc-shaped fault plane is simulated in this work, which was not done so far. The work is expected to have a far reaching implication in tectonics and petroleum geosciences since listric faults can be closely associated with hydrocarbon reserves (e.g., Valdiya and Sanwal, 2017 ). For such a fault plane devoid of gouge and any secondary faulting, shear heat is proportional to the mass of the hanging-wall block, the coefficient of friction acting between the hanging-wall and the footwall block, and radius of the circular arc. Shear heating intensifies temporally as the hanging wall block slides down and reaches progressively gentler fault dip.

Spatial pattern in stress drops of moderate-sized earthquakes on the Pacific Plate off the south-east of Hokkaido, Japan: implications for the heterogeneity of frictional properties

ᅟWe show that the spatial heterogeneity in the coseismic displacement of large earthquakes likely reflects the spatial characteristics of the frictional properties and that it can be inferred from the stress drop of moderate-sized earthquakes. We analyzed stress drops of 686 earthquakes with magnitudes of 4.0 to 5.0 off the south-east of Hokkaido, Japan, and investigated the spatial heterogeneity between the difference of shear strength and dynamic stress level on the Pacific Plate. We deconvolved observed P and S waves with those of collocated small earthquakes and derived the source effect of the earthquakes. We then estimated the corner frequencies of the earthquakes and calculated stress drops using a circular fault model. The values of stress drops showed a spatial pattern consistent with slip distributions of historical large earthquakes. Earthquakes that occurred in the area with a large coseismic slip during the 1968 Tokachi-oki (MW 8.2) and the 2003 Tokachi-oki (MW 8.0) earthquakes had large values of stress drop, whereas earthquakes in the afterslip area of the 2003 Tokachi-oki earthquake showed smaller values. In addition, an area between coseismic ruptures of the 1973 Nemuro-oki (MW 7.8) and the 2003 Tokachi-oki earthquakes had a large value of stress drop. Ruptures occurred in this area during the 1952 Tokachi-oki earthquake (MW 8.1), and the area acted as a barrier during the 2003 Tokachi-oki earthquake. These facts suggest that the frictional properties of the plate interface show little temporal change, and their spatial pattern can be monitored by stress drops of moderate-sized earthquakes. The spatial heterogeneity provides important information for estimating the slip pattern of a future large earthquake and discussing a policy for disaster mitigation, especially for regions in which slip patterns of historical large earthquakes are unclear.Graphical abstract

Low stress drops observed for aftershocks of the 2011 Mw 5.7 Prague, Oklahoma, earthquake

AbstractIn November 2011, three Mw ≥ 4.8 earthquakes and thousands of aftershocks occurred along the structurally complex Wilzetta fault system near Prague, Oklahoma. Previous studies suggest that wastewater injection induced a Mw 4.8 foreshock, which subsequently triggered a Mw 5.7 mainshock. We examine source properties of aftershocks with a standard Brune‐type spectral model and jointly solve for seismic moment (M0), corner frequency (f0), and kappa (κ) with an iterative Gauss‐Newton global downhill optimization method. We examine 934 earthquakes with initial moment magnitudes (Mw) between 0.33 and 4.99 based on the pseudospectral acceleration and recover reasonable M0, f0, and κ for 87 earthquakes with Mw 1.83–3.51 determined by spectral fit. We use M0 and f0 to estimate the Brune‐type stress drop, assuming a circular fault and shear‐wave velocity at the hypocentral depth of the event. Our observations suggest that stress drops range between 0.005 and 4.8 MPa with a median of 0.2 MPa (0.03–26.4 MPa with a median of 1.1 MPa for Madariaga‐type), which is significantly lower than typical eastern United States intraplate events (&gt;10 MPa). We find that stress drops correlate weakly with hypocentral depth and magnitude. Additionally, we find the stress drops increase with time after the mainshock, although temporal variation in stress drop is difficult to separate from spatial heterogeneity and changing event locations. The overall low median stress drop suggests that the fault segments may have been primed to fail as a result of high pore fluid pressures, likely related to nearby wastewater injection.

SOURCE MECHANISM AND SIZE OF THE 24 APRIL 2002 ML 5.2 GNJILANE (KOSOVO) EARTHQUAKE

A b s t r a c t: The 24 April 2002 ML 5.2 Gnjilane earthquake was studied first through inversion of the Sg – Lg wave group displacement amplitude spectrum and P-nodal planes determination. The seismic moment, source spectrum corner frequency and Brune equivalent circular fault surface for this shock were obtained, respectively, as M0 = 6.48·1016 N·m, f0 = 0.59 Hz and Σs,eq = 15.2 km2. The P-nodal planes for the four strongest aftershocks and the distribution of other aftershocks’ epicentres were determined, too, and used in identifying the actual source mechanism of the main shock by a simple method that included also the vertical projections on the Earth’s surface of the main shock Σs,eq with the two main shock P-nodal planes as possible fault planes. It was found that the main shock was caused by a normal right lateral faulting in a plane which struck with an azimuth of 238° and dipped toward NNW under an angle of 22°. This faulting was associated with the shear stressed fault structure along the Pliocene-Quarternary sinking valley of Binačka Morava, and it led to activations of other ruptures as sources of a significant number of aftershocks.

Gravity and magnetic modelling of layered mafic–ultramafic intrusions in large igneous province plume centre regions: case studies from the 1.27 Ga Mackenzie, 1.38 Ga Kunene–Kibaran, 0.06 Ga Deccan, and 0.13–0.08 Ga High Arctic events

Gravity and magnetic data from the global EGM2008 and EMAG2 datasets are used to identify geophysical anomalies in large igneous province (LIP) plume centre regions with the goal of characterizing mafic–ultramafic intrusions linked to those LIPs. Geophysical anomalies within 18 LIPs distributed globally are investigated. Four of these LIPs are selected for detailed modelling: the 1.27 Ga Mackenzie, 1.38 Ga Kunene–Kibaran, 0.06 Ga Deccan, and 0.13–0.08 Ga High Arctic LIPs. We recognize three spatial distribution types for intrusions in plume centre regions. These are (1) intrusions emplaced along a circular fault system that circumscribes the plume centre, (2) intrusions emplaced along linear rifts that, in some cases, converge towards the plume centre, and (3) single or unclassified intrusions. Modelling supports that the geophysical anomalies associated with these LIPs tend to be produced by large (radius &gt;30 km) and deep-seated crustal intrusions, with densities consistent with mafic–ultramafic rock and magnetic susceptibilities consistent with serpentinized ultramafic rock, except within the Deccan where intrusions are smaller, mainly mafic in composition, and positioned at shallower depths in the crust.

INTERMEDIATE TERM EARTHQUAKE PREDICTION BASED ON INTEREVENT TIMES OF MAINSHOCKS AND ON SEISMIC TRIGGERING

Two models, which contribute to the knowledge on intermediate term earthquake prediction are further examined, improved and applied. The first of these models, called Time and Magnitude Predictable (TIMAP) regional model is based on repeat times of mainshocks generated by tectonic loading on a network of faults which are located in a certain seismic region (faults’ region). The second model, called Decelerating-Accelerating Strain (D-AS) model, is based on triggering of a mainshock by its preshocks. Parameters of the TIMAP model have been specified for the Aegean area and applied by a backward test in 86 circular faults’ regions of this area. The test shows the validity of this time dependent model with 29% false alarms. Data concerning decelerating and accelerating seismic (Benioff) strain, which preceded 46 strong (M≥6.3) recent mainshocks in a variety of global seismotectonic regimes, show that the generation of a mainshock is triggered by quasi-static stress changes due to accelerating preshocks which occur in a broad (critical) region and by static stress changes due to the large number (frequency of occurrence) of small preshocks generated in a narrow (seismogenic) region. Retrospective predictions (postdictions) of these 46 mainshocks by the D–AS model confirms previous results concerning the prediction uncertainties (2σ) of the model in the origin time (± 2.5 years), epicenter location (≤ 150 km) and magnitude (± 0.4) of an ensuing mainshock with a probability ~ 80%. Information is also given on the successful prediction by the D-AS model of: 1) the Cythera strong (M = 6.9) earthquake which occurred on 8 January 2006 in the southwestern part of the Hellenic Arc and 2) of the Rhodes strong (M = 6.4) earthquake which occurred on 15 July 2008 in the Eastern part of this Arc. A backward combined application of both models in the Aegean area shows an uncertainty ≤120km in the epicenter location of an ensuing mainshock.

Information Extraction and Interpretation Analysis of Mineral Potential Targets Based on ETM+ Data and GIS technology: A Case Study of Copper and Gold Mineralization in Burma

Mineralization-alteration and structure information extraction plays important roles in mineral resource prospecting and assessment using remote sensing data and the Geographical Information System (GIS) technology. Choosing copper and gold mines in Burma as example, the authors adopt band ratio, threshold segmentation and principal component analysis (PCA) to extract the hydroxyl alteration information using ETM+ remote sensing images. Digital elevation model (DEM) (30m spatial resolution) and ETM+ data was used to extract linear and circular faults that are associated with copper and gold mineralization. Combining geological data and the above information, the weights of evidence method and the C-A fractal model was used to integrate and identify the ore-forming favourable zones in this area. Research results show that the high grade potential targets are located with the known copper and gold deposits, and the integrated information can be used to the next exploration for the mineral resource decision-making.

Seismic source spectra and estimated stress drop derived from cohesive-zone models of circular subshear rupture

Earthquake stress drops are often estimated from far-field body wave spectra using measurements of seismic moment, corner frequency and a specific theoretical model of rupture behaviour. The most widely used model is from Madariaga in 1976, who performed finite-difference calculations for a singular crack radially expanding at a constant speed and showed that |$\skew4\bar{f}_{\rm c} = k \beta /a$|⁠, where |$\skew4\bar{f}_{\rm c}$| is spherically averaged corner frequency, β is the shear wave speed, a is the radius of the circular source and k = 0.32 and 0.21 for P and S waves, respectively, assuming the rupture speed Vr = 0.9β. Since stress in the Madariaga model is singular at the rupture front, the finite mesh size and smoothing procedures may have affected the resulting corner frequencies. Here, we investigate the behaviour of source spectra derived from dynamic models of a radially expanding rupture on a circular fault with a cohesive zone that prevents a stress singularity at the rupture front. We find that in the small-scale yielding limit where the cohesive-zone size becomes much smaller than the source dimension, P- and S-wave corner frequencies of far-field body wave spectra are systematically larger than those predicted by the Madariaga model. In particular, the model with rupture speed Vr = 0.9β shows that k = 0.38 for P waves and k = 0.26 for S waves, which are 19 and 24 per cent larger, respectively, than those of Madariaga. Thus for these ruptures, the application of the Madariaga model overestimates stress drops by a factor of 1.7. In addition, the large dependence of corner frequency on take-off angle relative to the source suggests that measurements from a small number of seismic stations are unlikely to produce unbiased estimates of spherically averaged corner frequency.