Moment Tensor Analysis Research Articles

Seismic Moment from Regional Surface-Wave Amplitudes: Applications to Digital and Analog Seismograms

Accurate, consistent earthquake size estimates are fundamental for seismic hazard evaluation. In central Europe, seismic activity is low and long-term seismicity, available as intensities from written historical records, has to be included for meaningful assessments. We determined seismic moments M 0 of 25 stronger twentieth-century events in Switzerland from surface-wave amplitude measurements. These M 0 can be used to calibrate intensity-moment relations applicable to preinstrumental data. We derived the amplitude-moment relation using digital data from 18 earthquakes in and near Switzerland where independent M 0 estimates exist. The surface-wave amplitudes were measured at empirically determined distance varying reference periods T Δ . For amplitudes measured at T Δ , the distance attenuation term of the surface-wave magnitude relation S (Δ) = log ( A / T ) max + 1.66 log Δ is independent of distance. For log M 0 = M S + C E , we get log M 0 = S (Δ) + 14.90. Uncertainties of ±0.3 for the 14.90-constant correspond to a factor of 2 M 0 uncertainty, which was verified with independent data. Our relation allows fast, direct M 0 determination for current earthquakes, and after recalibration of the constant, the relation can be applied anywhere. We applied our relation to analog seismograms from early-instrumental earthquakes in Switzerland that were collected from several European observatories. Amplitude measurements from scans were performed at large amplifications and corrected for differences between T Δ and actual measurement periods. The resulting magnitudes range from M w = 4.6 to 5.8 for the largest earthquake in Switzerland during the twentieth century. Uncertainties for the early-instrumental events are on the order of 0.4 magnitude units. Online material : Moment-tensor analysis of 14 recent earthquakes.

Acoustic emission for fracture studies using moment tensor analysis

Acoustic emission (AE) has been widely employed for fracture studies because of its high sensitivity to crack initiation and growth. A moment tensor analysis based on the measurement of P-wave amplitudes of detected AE waveforms is especially useful for quantitative evaluation of fractures in terms of crack orientation, direction of crack motions and crack type. A quantitative comparison between prediction and actual cracks has been made to validate the method. This paper deals with AE applications for fracture studies using moment tensor analysis. Effectiveness of the analysis in laboratory tests and field applications is demonstrated.

Observation of Internal Cracking in Concrete by Virtual Reality Modeling of Acoustic Emission Moment Tensor Analysis

Observation of Internal Cracking in Concrete by Virtual Reality Modeling of Acoustic Emission Moment Tensor Analysis

Characterization of Stress-Induced Damage in Rock by the Moment Tensor Analysis of Acoustic Emission

Characterization of Stress-Induced Damage in Rock by the Moment Tensor Analysis of Acoustic Emission

Estimation of cracking and damage mechanisms in rock under triaxial compression by moment tensor analysis of acoustic emission

In highly stressed conditions, the excavation damaged zone induced by stress redistribution and disturbance must be evaluated after tunnel excavation. Therefore, the deformation and fracture characteristics of rock must be investigated. In this study, the fracture and damage mechanisms of rock induced by the accumulation of microcracks were investigated by moment tensor analysis, as well as by the moving point regression technique, both of which were applied to acoustic emission (AE) and strain data obtained from triaxial compression tests. Damage thresholds before the peak strength of rock under triaxial compression were determined by the moving point regression technique using acoustic emission data. The results showed that damage thresholds, except the crack closure stress, increased linearly with confining pressure. The results of the moment tensor analysis showed that shear failure was a major microscopic failure mechanism of rock under triaxial compression. In addition, shear failure became more dominant as the confining pressure increased. In this analysis, the expression of the damage magnitudes in each AE source as relative crack volumes leads to accurate prediction of macroscopic failure mechanisms, as well as major failure planes in the rock. In addition, the orientation of the macroscopic failure plane could be estimated by the orientational distribution of microcracks.

CRACKING MECHANISMS DUE TO CORROSION IN CONCRETE IDENTIFIED BYAE AND BEM

Due to expansion of corrosive product, various patterns of cracks are nucleated around reinforcement in concrete. Crack kinematics are identified by acoustic emission (AE)-SiGMA (Simplified Green's function for Moment tensor Analysis) procedure. Numerical analysis is conducted by the boundary element method (BEM) based on the concept of the linear elastic fracture mechanics (LEFM). Crack traces due to corrosion are simulated by the two-domain BEM. The behaviors of the dimensionless stress intensity factors (KI/KIC and KII/KIC), and the cracking modes are studied for the different types of crack patterns. It is found that cracking mechanisms due to corrosion of reinforcement are mostly associated with the mode-I failure.

Mechanisms of crack propagation due to corrosion of reinforcement in concrete by AE-SiGMA and BEM

Nucleation of corrosion cracking is analyzed by acoustic emission-simplified Green's functions for moment tensor analysis, by which crack kinematics of locations, types and orientations are quantitatively determined. These kinematical outcomes are obtained as 3D locations and vectors, and thus visualized three-dimensionally by using virtual reality modeling language. The procedure is applied to corrosion cracking in reinforced concrete. To clarify the mechanisms of crack extension due to corrosion, numerical analysis is conducted by the boundary element method. A relation between the stress intensity factors and the increment of crack extension is studied on three crack patterns due to corrosion. The dimensionless stress intensity factors and the cracking modes are investigated. It is confirmed that cracking mechanisms due to corrosion of reinforcement are dominantly of the mode-I crack, along with mixed-mode and mode-II cracks.

Moment Magnitude-Local Magnitude Calibration for Earthquakes off Canada's West Coast

Local magnitude ( M L ) values of earthquakes off Canada9s west coast are known to be underestimated by at least 0.5 magnitude units compared with other magnitude scales. Moment magnitude ( M w ), derived from moment tensor analysis, provides the most robust estimate of the magnitude of earthquakes. Moment tensor analysis of regional seismic data in western Canada is now possible due to the installation of more than 40 three-component broadband stations in western Canada, the U.S. Pacific Northwest, and southeast Alaska. Moment tensor solutions are now possible down to M ∼4.0. More than 230 regional moment tensor solutions have been calculated off Canada9s west coast at the Geological Survey of Canada for 1995–2002. These solutions, along with 14 previous solutions by Oregon State University in 1994–1995 and 13 Harvard solutions for 1984–1993, allow a systematic M w – M L calibration for earthquakes in this region. The study area extends from the Queen Charlotte Islands region in the north to the area off the west coast of southern Vancouver Island. At the northern end of the study area, where there is little oceanic crust in the source-receiver travel path, M w is systematically larger than M L by 0.28 ± 0.08 magnitude units. At the southern end of the study area, where there is a significant amount of ocean crust in the source-receiver travel path, M w is systematically larger than M L by 0.62 ± 0.08 magnitude units. Calibration of M L with M w will allow the western Canadian earthquake database to be used more effectively for tectonic studies and seismic hazard analysis.

Moment Tensor Analysis of Acoustic Emission in Concrete Specimens Failed in Four-Point Bending

Moment Tensor Analysis of Acoustic Emission in Concrete Specimens Failed in Four-Point Bending

Surface wave tomography and seismic source studies at Campi Flegrei (Italy)

Abstract We analyse a set of events, recorded in March 1984 during a period of intense seismicity of the last bradyseismic crisis, to determine shallow velocity models and seismic source moment tensors in the Campi Flegrei region. The velocity models of the crust are obtained by surface wave tomography. Two inland negative anomalies of group velocity resulting in our tomographic maps well agree with previous independent measurements whereas a third anomaly, located in the Pozzuoli Gulf and not evidenced by previous studies, is in correspondence with the caldera position. The inversion of the dispersion curves allows us to extend the already available velocity models within the uppermost 2 km of the crust. The obtained models are employed to study seismic source features by the moment tensor analysis. Preliminary synthetic tests are performed in order to minimise artefacts in the moment tensor solutions possibly due to numerical noise, to station distribution or to the structural modelling approximation. Most of the investigated earthquakes are deviatoric mechanisms and the increase of the isotropic component is observed in connection with the change of the seismicity rate in the investigated temporal window.

Regional distance seismic moment tensors of nuclear explosions

Nuclear explosions, because of their known location, depth and theoretical source mechanism, provide a means to explore the resolution of non-double-couple and isotropic seismic moment tensors. We perform seismic moment tensor inversions on long-period (50–20 s), three-component velocity seismograms from the Caltech TERRAscope network and the Berkeley Digital Seismic Network (BDSN) to determine best-fitting double-couple, isotropic plus double-couple, deviatoric and full-moment tensor source mechanisms for the Little Skull Mountain Earthquake and three large ( M L≥5.5, M W≥4.5) Nevada Test Site (NTS) nuclear explosions (JUNCTION, MONTELLO and BEXAR). The significance of solutions with higher degrees of freedom is evaluated using the F-test. The stability of the moment tensor solutions for variations in station configuration is investigated using a cross-validation method. Our results show that strongly non-double-couple seismic moment tensors and shallow source depth characterize the nuclear explosions. The full-moment tensor inversions recover a volume increase. However, our analysis indicates that the improvement in fit afforded by the extra degree of freedom is not statistically significant due to the similarity of the vertical compensated linear vector dipole (CLVD) and isotropic surface wave Green's functions at these periods. An isotropic plus double-couple source model was found to provide the same level of fit as the deviatoric moment tensor inversions. Determination of the nonisotropic source mechanism is not unique and we discuss our results with respect to the proposed source models for NTS. While the results of this study indicate that regional distance seismic moment tensor analysis is not suitable for directly discriminating nuclear explosions from earthquakes, the shallow source depth and non-double-couple seismic moment tensors obtained for these events suggest that it may be useful for identifying suspect events for further screening.

Regional moment tensor determination in the European–Mediterranean area — initial results

The broadband seismic network in the European–Mediterranean area provides high-quality data. We invert these regional three-component data for the source parameters of moderate-to-strong earthquakes in the entire European–Mediterranean area. Regional seismograms have a good signal-to-noise ratio even for moderate-sized events that are too small for teleseismic analysis. The magnitude threshold for source parameter determination can, thus, be significantly lowered. The threshold depends on the average event–station distances. Within dense broadband networks, we analyze M W≈3.0 earthquakes. In areas far from broadband seismic stations, the lower bound is M W≈4.5–4.8, still considerably lower than the teleseismic analysis threshold (about M W≈5.0–5.3). For larger events, we perform rapid moment tensor analysis using near-real-time data; solutions are posted within hours after event occurrence. In a second step, we merge near-real-time and later available data to obtain a regional moment tensor catalog of moderate-to-large earthquakes for the entire European–Mediterranean area. Within less than 1 year, we have analyzed 67 earthquakes ranging in size from M W=2.9 to 7.5. The solutions cover the seismically active areas of the European–Mediterranean area. Particularly important are solutions for slowly deforming regions where large earthquakes, that could be analyzed with teleseismic data, occur infrequently. The solutions are reliable: for events with independent source parameter estimates, the agreement is generally high. The solutions are robust: variations in epicentral parameters, source depth, or exact choice of stations do not affect source parameter estimates strongly. The moment magnitudes provide a unified estimate of earthquake size for the European–Mediterranean area. We perform regression analyses to link our moment magnitudes with local, body, and surface wave magnitudes.

Discriminating Between Large Mine Collapses and Explosions Using Teleseismic P Waves

Some of the most suspicious seismic disturbances under the Comprehensive Nuclear-Test-Ban Treaty (CTBT) are likely to be those associated with mining, as they are shallow, and at least some have an explosion-like m b :M s signature. Previous research highlighted the potential of broadband teleseismic P waves as a way of identifying large mine tremors. Broadband teleseismic P from two suspected large mine collapses, one in Germany (1302 UT, 13 March 1989, 5.4 m b ) and another in Wyoming (1526 UT, 3 February 1995, 5.3 m b ), show differences in character despite the similarity of the reported ground failure and mine types. We apply a full moment-tensor analysis to the teleseismic P waves and show that the data are inconsistent with either a shallow explosion or an earthquake (double-couple) at depth, but this method is unable to distinguish between a shallow dip-slip source and a closing-crack moment tensor. However, three-component surface-wave seismograms recorded at regional distances fit the shallow closing-crack model, but are inconsistent with a shallow earthquake source, because strong Love waves, expected from a double-couple source, are not observed at a number of stations well distributed in azimuth. Here, we restate the equivalence for shallow sources of the closing-crack model and a gravitational collapse model. We use the latter to model the broadband P waves from these mine tremors and show that, while non-unique, the differences in the observed broadband P waves from the two tremors can be attributed to the area, amount of collapse, depth, and rate of collapse. The collapse model predicts negative first-motion for all P waves in contrast to the positive polarity expected from explosions. Thus, the broadband teleseismic P waves have the potential to discriminate between large collapses and explosions.

Seismic remote monitoring of stress field

We demonstrate that a real-time based moment tensor analysis using broadband seismic waveforms provides us with the information about the spatio-temporal change of stress field. Such a capability is crucial when monitoring the volcanic activity during the eruption if it causes seismic swarm. We estimated the moment tensors of earthquakes occurring during the swarm activity before the eruption of Usu volcano in March 2000 as well as those during the swarm activity of Miyakejima volcano starting in June 2000. In both cases, we detected the stress change due to the movement of magma reservoir.

Acoustic emission moment tensor analysis: development for crack identification in concrete materials

The moment tensor analysis of acoustic emission (AE) provides quantitative information on kinematics of cracks. Simple AE moment tensor analysis has been developed and named SiGMA-AE analysis. In-plane unconfined compression tests of mortar and concrete plates with a slit are carried out. The inclination of slits are varied as 0, 30 and 45° to the loading axis. The sources are located from detected AE signals. Then, orientations of cracks, crack types and crack volumes are determined by the SiGMA-AE analysis. Furthermore, it is clarified that the damage parameters can be estimated from the moment tensor components. The results show close correlation between the calculated crack volumes and the damage parameters.

Moment tensor analysis of near-field broadband waveforms observed at Aso Volcano, Japan

Locations and focal mechanisms of long-period volcanic events observed at Aso volcano, Japan, are determined by waveform inversion. Near-field broadband three-component seismograms of four to seven stations are simultaneously inverted in the time domain in order to find the six-component seismic moment tensor. A linear inversion is performed at each point of a 3D grid located under the volcano in order to constrain the centroid position and the focal mechanism of the source. The complete displacements, including near-field waves, in a homogeneous half-space for a general point source are taken into account. Inversions of 43 long-period tremors and six phreatic eruptions exhibit mainly an isotropic mechanism, with a minor deviatoric component that may originate from a north–south-trending crack. These events are all located in a small region at about 1.3km depth beneath the active crater. The high accuracy, both of locations (a few hundred meters) and of focal mechanisms given by the inversion, is partly attributed to the rapid variation of the amplitude of the static displacement seen in the long-period part of the near-field seismograms.

Role of kinematically induced horizontal forces in Mediterranean tectonics: insights from numerical modeling

Finite element modeling of the central–eastern Mediterranean region has been carried out to show that the recent/present deformation pattern of this zone, inferred from neotectonic observations and seismic strain rates, may be satisfactorily reproduced as effect of the relative motion of Africa and eastern Anatolia with respect to Eurasia. Numerical modeling involved 2D elastic elements in a plane-stress approximation. The model is constituted by a mosaic of poorly deformable blocks separated by much more deformable decoupling zones, representing consuming boundaries, extensional zones and transcurrent discontinuities, whose location and geometry have been deduced by neotectonic, morphological and seismological information. The calculated displacement field obtained with the modeling parametrization which allows to match the observed strain regimes is compatible with geodetic observations in the study area, but for the Hellenic Arc, where geodetic velocities are higher than those predicted by modeling. This discrepancy could be considerably reduced by adopting a higher deformability of the model in the Hellenic trench, but this condition would contrast with the Plio-Quaternary deformation pattern of the southern Aegean zone, which suggest a considerable slowdown of western Crete since the late Pliocene. Furthermore, geodetic velocities are considerably higher than the motion rates derived by moment tensor analysis in the Hellenic trench and in the internal Aegean area and cannot easily account for the low Quaternary deformation observed in the southern Aegean zone. The above discrepancy could be due to a difference between the “instantaneous” kinematic behavior of the Aegean zone, indicated by geodetic measurements, and the average behavior over longer time intervals, inferred from geological and seismological strain indicators.

Body wave inversion of the 1970 and 1963 South American large deep‐focus earthquakes

A comprehensive set of teleseismic waveforms from two South American deep‐focus earthquakes of the predigital era, the 1970 Colombia (Mw = 8.1) and 1963 Peru‐Bolivia (Mw = 7.7) events, are inverted for source mechanism, seismic moment, rupture history, and centroid depth. The P and SH wave inversion of the Colombia event confirms previous work, indicating that rupture occurred on a plane that dips steeply west. Rupture direction paralleled the trend of the Wadati‐Benioff zone. We decompose the source into subevents, based on a source time function which shows two major moment release pulses separated by ∼20 s. The first subevent is located near the initiation point at a depth of ∼630 km. The main moment release was located ∼70 km to the southeast and ∼20 km shallower. Rupture subsequently propagated farther southeast. The source time function has an initial subevent accounting for ∼30% of the moment release of the entire event, whereas the long‐period centroid moment tensor (CMT) analysis [Russakoff et al., 1997] has the initial subevent yielding ∼50%. The high‐angle nodal plane rotated ∼15° clockwise during the rupture, explaining the large compensated linear vector dipole (CLVD) component inferred from CMT solutions. Individual subevents have large CLVD and compressive isotropic components. A full moment tensor inversion of the Colombia and 1994 Bolivia events suggests that the initial subevents might contain a large non‐double‐couple (NDC) component. For the 1963 Peru‐Bolivia event, using P waves, rupture propagated NNW for a distance of ∼70 km, parallel to the high‐angle nodal plane and the trend of the Wadati‐Benioff zone. The focal mechanism changed dramatically after the second subevent, causing a very large NDC component. Both events, together with the 1994 Bolivia earthquake, have a precursor separated in space and time from the main rupture and show rupture velocities varying between 3 and 4 km/s between subevents, with <2.0 km/s on average for the entire event. Low seismic efficiencies and rupture velocities support a highly dissipative, temperature‐dependent rupture mechanism for large deep‐focus South American earthquakes, compared with events in cold subducting slabs.

Quantitative analysis of fracture process in RC column foundation by moment tensor analysis of acoustic emission

After the great Hanshin earthquake, numerous studies have been made to evaluate damage levels, toughness and deformation characteristics of reinforced concrete (RC) columns exposed to the earthquake shock. In the present study, RC columns are subjected to simulated seismic lateral loading under different axial loads. Acoustic emission (AE) is monitored to characterize the seismic behavior of column foundation. AE is shown to be very sensitive to detect cracks generated in the foundation. An AE moment tensor analysis is applied to analyze the fracture process quantitatively. Experimental results demonstrate different fracture behavior, depending on the different axial loads. It is also shown that the moment tensor analysis is very useful for quantitative evaluation of the fracture process.

Experimental and numerical crack analysis of mixed-mode failure in concrete by acoustic emission and boundary element method

Crack extension of mixed-mode in concrete is studied by applying the acoustic emission (AE) and the boundary element method (BEM). The theory of AE wave motions is briefly reviewed, including the historical development. AE waveforms are generated as elastic waves in concrete due to crack nucleation, and can be synthesized by the integral formulation, consisting of the spatial derivatives of Green's functions, the moment tensor, and the source-time function. Kinematics of cracks are represented by the moment tensors, which can be determined by a SiGMA (simplified Green's function for moment tensor analysis) code. In bending tests of notched beams, the crack extension of mixed-mode failure in concrete is studied. Crack traces are analyzed by the two-domain BEM, based on the maximum circumferential stress criterion by Erdogan-Sih, and are in remarkable agreement with those of tested beams. Then, locations, types, and orientations of AE sources are analyzed by the SiGMA to investigate crack kinematics in the fracture process zone. Furthermore, it is attempted to estimate the normalized stress intensity factors K I *= K I / K IC and K II *= K II / K IC .