Cezar-Ioan Trifu

Frequency‐magnitude distribution of earthquakes in Vrancea: Relevance for a discrete model

A high-resolution analysis of a homogeneous and complete data set of intermediate depth microearthquakes (h > 60 km), extending over an interval of 9.3 years emphasizes a significant deviation at low magnitudes (ML < 4) in a linear frequency-magnitude distribution. It appears as a distinct change in the slope of the cumulative distribution and as a seismicity deficit followed by a relative enhancement in the corresponding noncumulative curve. This is in agreement with the presence of two characteristic mechanisms and allows the identification of both the magnitude threshold of asperity-like earthquakes and the transition zone (ML = 3.3 - 3.9) from crack-like earthquakes (background seismicity) to asperity-like events. These features are better pointed out on well individualized active zones. The background seismicity shows a tremendeous decrease in its b slope, from 1.03 to 0.57, during a 6-year interval before the occurrence of a major earthquake in 1986 ( Mw = 7.3), followed by a fast recovery to 1.03 in 2 years after this event. Such a behavior could be correlated to the continuous growth of the shear stress-free surface on the fault, as in a percolation process, followed by its sudden diminution due to the locking of the fault. The above results provide relevance for the discrete character of faulting and enable a coherent modeling of the earthquake generation process from microearthquake to major event scale lengths.

Recent advances in seismic monitoring technology at Canadian mines

We provide an overview of the current status of seismic monitoring instrumentation employed in Canadian underground mines. Based on several case studies, we outline how passive seismic monitoring techniques are being used to evaluate fractures and stress conditions associated with ore extraction at depth. It is shown that induced microseismicity allows for the remote monitoring of active fractures, delineating modes of failure with advancing excavation fronts, and identifying variations in principal stress orientations during sequential stages of mining. Advances into the characterization of excavation zone of influence through deformation state analysis and the use of seismic hazard analysis to evaluate the potential for ground instability are also discussed.

Non‐similar frequency‐magnitude distribution for M < 1 Seismicity

A series of 1503 induced seismic events ranging in magnitude from −2 to 1 as computed based on a locally calibrated seismic moment scale were located within a well-defined volume at depth. The seismicity analysis was carried out over 2 months and included events recorded before, during, and following the after-shock period of a mN 2.6 rockburst. A non-similar pattern was clearly apparent for both cumulative and non-cumulative frequency-magnitude distributions, showing a relative enhancement of seismicity between magnitudes −0.5 and 0. This supports the existence of non-similar scaling at these magnitudes. The time variation of the b-value, as computed for a limited self-similar magnitude domain (−1.1 to −0.4), did not appear to be related to the rockburst. Instead, it is suggested that the rockburst perturbed the local stress field and consequently the distribution of the very small events. No correlations were found between b and space clustering.

Microseismicity derived fault‐Planes and their relationship to focal mechanism, stress inversion, and geologic data

General stress and faulting trends have been retrieved through the analysis of mining induced microseismic events (M < 0) at two sites, related to an mN 2.6 rockburst, and an excavation at depth. A comparison of results obtained through principal component analysis (PCA) of seismicity, focal mechanism, and stress inversion, with in-situ measurements of stress and structural mapping data, show that: under stable stress conditions, the P, B, T, and stress inversion axes are consistent with in-situ measurements of stresses; stress inversion and PCA fault-planes lie within 10 to 20° of the most significant mapped features at the sites; and the PCA technique provides a robust approach for the determination of fault-planes.

Reliability of Seismic Moment Tensor Inversions for Induced Microseismicity at Kidd Mine, Ontario

— A novel seismic moment tensor inversion approach is applied to microseismic events from the Kidd mine with moment magnitudes ranging between −1.2 and 0. Data consist of 35 events recorded on 8 triaxial accelerometers installed underground. Reliable solutions are obtained for 21 events, of which 14 represent pure shear mechanisms, whereas the remaining 7 exhibit a significant positive volumetric component (72–76%), along with some pure shear failure (15–20%). Further analysis indicates that 6 of the events characterized by high volumetric components are located within a sill pillar on the 4,700 level and have subvertical P and subhorizontal T axes. This is in agreement with the presence of tensile cracks close to openings for incipient pillar bursting. The pure shear events are located outside the sill pillar, between the 4,600 and 4,800 levels, and on the 5,600 level within a highly fractured rockmass. For the latter events, the subvertical nodal planes are found to match closely the orientation of subvertical NW-SE fractures aligned parallel to the major faults in the area.

Asperity distribution and percolation as fundamentals of an earthquake cycle

Abstract A seismic cycle model is proposed, based on the existence of an asperity space-strength distribution along the fault plane and the applicability of the percolation theory to earthquakes, as suggested by Vrancea seismotectonic data. Two seismic regions, somewhat decoupled, are emphasized in the lower lithosphere, roughly between 80 and 110 km and between 120 and 170 km depth, and an asperity magnitude quasi-independent dimension of 0.3–0.4 km is determined for small to moderate earthquakes. Accordingly, two concepts are introduced: (1) the active zone—a distinct fault region able to generate a major earthquake by percolation, and characterized by a specific background seismicity, total area, and maximum possible magnitude; (2) the asperity cell—an elementary local stress inhomogeneity of an active zone, generating weak to moderate events. Percolation requires the presence of a critical stress-free surface (44% of the active zone area). The model explains the non-linearity of the frequency-magnitude relationship, and enables the estimation of the major earthquake magnitude domain in each zone. The computed maximum possible magnitudes are Mw = 7.6 and Mw = 7.8, respectively. In agreement with historical data, a complex form of the recurrence period of earthquakes in the major magnitude domain is determined, ranging from about 40 to 100 years, irrespective of the seismic zone. It follows that the seismic activity per unit area is invariant in both zones. This result is considered to be a consequence of an intrinsic material property: the scale invariance of the fragmentation process in the lithospheric material. Different aspects related to the discrete structure of the active zone and the fractal dimension of faulting are also discussed from the point of view of this model.

Fracture coalescence as a mechanism for earthquakes: observations based on mining induced microseismicity

Abstract Microseismic events, with magnitudes less than zero, recorded within a volume at depth in an underground mine are used to evaluate failure conditions associated with a magnitude 2.9 event (rockburst). The approach incorporated several independent methods of analysis, including the principal component analysis (PCA) of seismicity, focal mechanisms, stress inversion, underground in situ stress measurements and structural mapping, and three-dimensional numerical modelling derived stresses. A good correlation was found between stress inversion and PCA derived microseismic failure planes and structural mapping, and between the in situ principal stress orientations and those obtained through stress inversion and numerical modelling. A few days prior to the large event and during the aftershock sequence, a change in microseismic failure plane orientation to a shallower dip angle than mapped underground was found to coincide with the numerical modelling derived plane of maximum slip potential. This orientation, along with measured fracture characteristics (persistence and spacing) and an increase in event clustering into the plane, suggests that the formation of the large event plane of rupture was likely related to the coalescence of mapped fractures. An observed reduction in stress release accompanying the increased tendency towards planarity prior to the large event indicates that the development of a large rupture surface occurred under an increased effective shear stress build up as part of the earthquake generation process, and, the process of coalescence can be considered as the breakage of barriers (new fracture growth) accompanied by an increase in the effective shear stress on the remaining asperities (pre-existing fractures).

Effects of Rupture Complexity and Stress Regime on Scaling Relations of Induced Microseismic Events

Source parameter scaling relations are examined for microseismic events (−2.4 ≤ M ≤ −0.3) occurring within highly and moderately stressed and fractured rock masses at Strathcona mine, Sudbury, Canada. Insight into scaling is provided by waveform complexities, calculated rupture velocities, and maximum shear stresses based on in situ and numerical modelling data. The importance of normal stress on the failure process is also considered. Our results show that a strong dependence exists between stress release and seismic moment. An observed positive scaling in excess stress release (Δσ/2 − σ a ) is consistent with the concept of overshoot. Rupture velocities ranging from 0.2 to 0.5β and waveform complexities less than 1.5 suggested that overshoot was related to healing behind a slowly advancing rupture front. Scaling in seismic efficiency paralleled that in apparent stress, implying that seismic stress release estimates arc quasi-independent of the maximum shear stress. High levels of normal stress further supported the importance of high resisting stress in the observed overshoot behaviour and its role in the failure process.

Seismic hazard of the Circum-Pannonian region

Construction of a Seismotectonic Model: The Case of Italy.- The Seismotectonic Characteristics of Slovenia.- Characterization of Seismogenic Zones of Romania.- Identification of Future Earthquake Sources in the Carpatho-Balkan Orogenic Belt Using Morphostructural Criteria.- Modelling of Block Structure Dynamics for the Vrancea Region: Source Mechanisms of the Synthetic Earthquakes.- Stress in the Descending Relic Slab beneath the Vrancea Region, Romania.- Upper Crustal Velocity Structure in Slovenia from Rayleigh Wave Dispersion.- Generalised Seismic Hazard Maps for the Pannonian Basin Using Probabilistic Methods.- Seismic Zoning of Slovenia Based on Deterministic Hazard Computations.- A Contribution to Seismic Hazard Assessment in Croatia from Deterministic Modeling.- Synthetic Seismogram Based Deterministic Seismic Zoning for the Hungarian Part of the Pannonian Basin.- Seismic Hazard of Romania: Deterministic Approach.- Estimation of Site Effects in Bucharest Caused by the May 30-31, 1990, Vrancea Seismic Events.- The Dependence of Q with Seismic-induced Strains and Frequencies for Surface Layers from Resonant Columns.

Source parameters of mining-induced seismic events: An evaluation of homogeneous and inhomogeneous faulting models for assessing damage potential

Source parameter estimates based on the homogeneous and inhomogeneous source models have been examined for an anomalous sequence of seven mine-induced events located between 640 and 825 m depth at Strathcona mine, Ontario, and having magnitudes ranging betweenmN 0.8 and 2.7. The derived Brune static stress drops were found to be similar to those observed for natural earthquakes (∼30 bars), whereas dynamic stress drops were found to range up to 250–300 bars. Source radii derived from Madariagas model better fit documented evidence of underground damage. These values of source radii were similar to those observed for the inhomogeneous model. The displacement at the source, based on the observed attenuation relationship, was about 60 mm for three magnitude 2.7 events. This is in agreement with slip values calculated using peak velocities and assuming the asperity as a Brune source within itself (72 mm). By using Madariagas model for the asperity, the slip was over 3 times larger than observed. Peak velocity and acceleration scaling relations with magnitude were investigated by incorporating available South African data, appropriately reduced to Canadian geophysical conditions. The dynamic stress drop scaled as the square root of the seismic moment, similar to reported results in the literature for crustal earthquakes. This behavior suggests that the size of the asperities responsible for the peak ground motion, with respect to the overall source size, follow distributions that may be similar over a wide range of magnitudes. Measurements of source rupture complexity (ranging from 2 to 4) were found to agree with estimates of overall source to asperity radii, suggesting, together with the observed low rupture velocities (0.3 β to 0.6 β), that the sources were somewhat complex. Validation of source model appropriateness was achieved by direct comparison of the predicted ground motion level to observed underground damage in Creighton mine, located within the same regional stress and geological regime as Strathcona mine. Close to the source (<100 m), corresponding to relatively higher damage levels, a good agreement was found between the predicted peak particle velocities for the inhomogeneous model and velocities derived based on established geomechanical relationships. The similarity between asperity radii and the regions of the highest observed damage provided additional support for the use of the inhomogeneous source model in the assessment of damage potential.