Estimation Of Source Parameters Research Articles

地震災害を考える 予測と対策 震源断層‐活断層システムのイメージングの意義と現状

Knowledge of the geometry and connectivity of seismogenic and active fault systems is a key to understanding on-going active tectonic processes and evaluating the risk of destructive earthquakes. Knowing the geometry of the deeper extensions of active faults in the seismogenic zone, contributes to estimating source parameters for scenario earthquakes, like fault geometry, co-seismic displacement, and the location of possible asperities. Together with monitoring seismicity and surface deformation, delineating the geometry of seismogenic faults allows us to construct quantitative models of crustal deformation. Seismic reflection profiling is a powerful tool to discern the deep geometry of faults. Since the Kobe earthquake of 1995, deep seismic profilings have been carried out across some active faults in the Japanese islands, such as the Ou Backbone Range in northern Honshu. Through these seismic experiments, using vibroseis trucks and explosive sources, the geometry of seismogenic and active fault systems is successfully obtained in spite of some difficulties due to rough topography, high attenuation and man-made noise. Deep seismic profiling has a great potential for the direct imaging deep crustal structures, including deep fault geometry.

Distributed source localization using ESPRIT algorithm

A new algorithm based on ESPRIT is proposed for the estimation of the central angle and angular extension of distributed sources. The central angles are estimated using TLS-ESPRIT for both incoherently distributed (ID) and coherently distributed (CD) sources. For CD sources, the extension width is estimated by constructing a one-dimensional (1-D) distributed source parameter estimator (DSPE) spectrum for each source. For ID sources, the extension widths are estimated using the central moments of the distribution. The algorithm can be used for sources with different angular distributions.

Source Depth and Mechanism Inversion at Teleseismic Distances Using a Neighborhood Algorithm

We performed nonlinear waveform inversion for source depth, time function, and mechanism, by modeling direct P and S waves and corresponding surface reflections at teleseismic distances. This technique was applied to moderate size events, and so we make use of short period or broadband records, and utilize SV waveforms in addition to P and SH . For the inversion we used a direct search method called the neighborhood algorithm (NA), which requires just two control parameters to guide the search in a conceptually simple manner, and is based on the rank of a user-defined misfit measure. We use a simple generalized ray scheme to calculate synthetic seismograms for comparison with observations, and show that the use of a derivative-free method such as the NA allows us to easily substitute more complex synthetics if necessary. The source mechanism is represented in two different ways; the superposition of a double-couple component with an isotropic component, and a general moment tensor with six independent components. Good results are obtained with both synthetic input data and real data. We achieve good depth resolution and obtain useful constraints on the source-time function and source mechanism, including an isotropic component estimate. Such estimates provide important discriminants between man-made events and earthquakes. We illustrate inversion with real data using two earthquakes, and in both cases the source parameter estimates compare well with the corresponding centroid moment tensor solutions. We also apply our technique to a known nuclear explosion and obtain a very shallow depth estimate and a large isotropic component.

Source parameters and tectonic interpretation of recent earthquakes (1995–1997) in the Pannonian basin

From January 1995 to December 1997, about 74 earthquakes were located in the Pannonian basin and digitally recorded by a recently established network of seismological stations in Hungary. On reviewing the notable events, about 12 earthquakes were reported as felt with maximum intensity varying between 4 and 6 MSK. The dynamic source parameters of these earthquakes have been derived from P-wave displacement spectra. The displacement source spectra obtained are characterised by relatively small values of corner frequency ( f 0) ranging between 2.5 and 10 Hz. The seismic moments change from 1.48×10 20 to 1.3×10 23 dyne cm, stress drops from 0.25 to 76.75 bar, fault length from 0.42 to 1.7 km and relative displacement from 0.05 to 15.35 cm. The estimated source parameters suggest a good agreement with the scaling law for small earthquakes. The small values of stress drops in the studied earthquakes can be attributed to the low strength of crustal materials in the Pannonian basin. However, the values of stress drops are not different for earthquake with thrust or normal faulting focal mechanism solutions. It can be speculated that an increase of the seismic activity in the Pannonian basin can be predicted in the long run because extensional development ceased and structural inversion is in progress. Seismic hazard assessment is a delicate job due to the inadequate knowledge of the seismo-active faults, particularly in the interior part of the Pannonian basin.

Localization method based on the higher-order statistics

In the context of the narrow-band or wideband array processing problem, in this paper we develop a robust algorithm to improve the accuracy of the estimation of the direction of arrival of the sources. It is well known that when the noise covariance matrix is unknown, these estimates may be grossly inaccurate. Using both the higher-order statistics for suppression of the additive Gaussian noise, the transformation matrices for estimating the coherent signal subspace, and the high-resolution algorithm a robust and fast algorithm for the source parameters estimation in the presence of noise with an unknown cross-spectral matrix is developed. We shall show that the performance of bearing estimation algorithms improves substantially when our algorithm is used. Several simulation results are presented with different noise spectral matrices.

On the use of complex attributes and the inferred source parameter estimates in the exploration of archaeological sites

Complex attribute analysis is used to extract parameters of the buried structures that give rise to anomalous total magnetic field. The attributes by themselves aid the interpretation because they can delineate the edges of the concealed targets. Tests on synthetic and real data demonstrate this property. Parameters such as the local depth, dip, dip azimuth, strike and susceptibility contrast are obtained from the complex attributes on the basis of an analytical expression for the effect of a simple subsurface model. The simplest one, the sloping contact model, is used in this study. The local parameters of the concealed targets aid the interpretation further. They can also be used alone to interpret the anomalous field. Interference of the signals caused by nearby buried structures and the noise level limit the extent to which the source parameter estimates can be used. Copyright © 2000 John Wiley & Sons, Ltd.

A detailed analysis of two seismic sequences in Abruzzo, Central Apennines, Italy

A three-component digital seismic network has been installed along central Apennines since the end of 1991. Two seismic sequences having main shocks of magnitudes 3.9 and 3.7 were recorded in August 1992 and June 1994, respectively. A detailed analysis of these sequences, including multiplet relocation, fault-plane solutions and source parameter estimation, is performed in the present paper. A correlation analysis allowed us to recognize a number of correlated events in the two sequences which were used for relative locations using a master event technique. This analysis allowed to obtain a better alignment of epicentral data along two almost orthogonal directions, following an Apenninic and an anti-Apenninic trend. For the two sequences, fault-plane solutions were evaluated by using a first arrival technique, resulting in mechanisms with predominant normal faulting for the 1992 and 1994 swarms. S-wave polarization analysis allowed to check the stability of the previous solutions and to reduce their range of uncertainty. The same technique was also applied to derive the composite fault-plane solutions from the aftershocks, resulting in solutions which are in good agreement with those derived from the main shocks of both sequences. Source parameters were then derived from the three-component records of 28 well-recorded events with seismic moment in the range 8.5 × 1010–1.0 × 1014 Nm. Stress drops ranged in the interval 0.3–52.3 bar and source radii were of the order of 100 m. Their scaling relations are in good agreement with other results derived from the analysis of other Italian earthquakes that occurred in regions of predominantly normal faulting tectonics (Apennines and Calabrian arc).

A simple source inversion scheme for displacement seismograms recorded at short distances

We have implemented and tested a simple inversionscheme to retrieve source parameters of small andmoderate earthquakes recorded at close distances. Theinversion assumes that such events may be approximatedby a point-source shear dislocation, and the mediummay be considered an infinite space. Theoreticalseismograms used in the inversion include near- andintermediate-field contributions. The effect of freesurface is approximately taken into account bymultiplying the infinite-space synthetics by two. Thelocation of the event is assumed to be known. Based onan examination of the data, the user chooses thelength of the record and the weight of each trace tobe used in the inversion. The shape of the source-timefunction is estimated from the observed seismograms.A grid search is performed to determine the focalmechanism and the seismic moment which minimizes theerror between the observed and the syntheticseismograms. For two or more recordings, the inversiongives a solution which is consistent with thefirst-motion data and/or the moment tensor inversionof regional seismic waves. If only a singlethree-component recording is available then theinversion yields more than one equally acceptablefocal mechanisms. In such cases, the first-motionpolarities or a prior knowledge of the tectonics ofthe region is essential to constrain the solution. Thetest of the method on near-source broadband recordingsof small and moderate earthquakes in Mexico and theaftershock sequence of 1997 Umbria-Marche, Italy showsthat it gives fast, and surprisingly robust andreliable estimation of source parameters.

Seismic Moment Estimation of Small Earthquakes by Waveform Fitting.

We developed an X-window program for estimating seismic moments and fault parameters of small earthquakes by waveform fitting. The graphical interface was adopted for operational convenience in routine processing. Manual parameter searching was implemented in the tool as well as waveform inversion. Body wave data recorded at stations within 100 kilometers of hypocenters are compared with synthetic records. Frequency components slightly lower than the corner frequency are used to estimate source parameters. Time for calculating synthetic records was reduced by preparing Green's functions beforehand. Source parameters were estimated by the tool for the earthquakes of Mw from 3.5 to 6.1 in the southern Kanto area.

Crustal and upper mantle structure of Peninsular India and source parameters of the 21 May 1997, Jabalpur earthquake (Mw= 5.8): Results from a new regional broadband network

AbstractIn the past two years, a permanent network of 10 broadband seismographs has become operational in the Indian Peninsular shield region. In this article, we analyze the data recorded by this network with two goals: (1) to lay ground for a quick estimation of source parameters of future earthquakes in the region and (2) to estimate the source parameters of the destructive Jabalpur earthquake of 21 May 1997 (Mw = 5.8), which occurred within the network. Toward these goals, we measure Rayleigh- and Love-wave group velocity dispersion curves in the period range of 10 to 75 sec and invert these curves to estimate the crustal and the upper mantle structure below the Indian shield region. Our best model consists of a two-layer crust; the upper layer is 14-km thick with a shear-wave velocity (Vs) of 3.55 km/sec; the corresponding values for the lower layer are 25 km and 3.85 km/sec. Vs for the upper mantle is 4.65 km/sec. Based on this structure, we perform a moment tensor (MT) inversion of the bandpassed (0.05-0.02 Hz) seismograms of the Jabalpur earthquake. The best fit is obtained for a source located at a depth of 42 km, with a seismic moment, M0, of 7.7 × 1024 dyne cm, and a focal mechanism with strike 61°, dip 64°, and rake 74°. These source parameters are similar to those previously reported in the literature, demonstrating that a routine MT inversion of moderate and large Indian Penisular earthquakes is now possible. A careful examination of the seismograms and modeling of the two closest records reveal that the Jabalpur earthquake nucleated in the lower crust at a depth of ∼36 km. It consisted of two subevents separated in time by 0.65 sec, with the second subevent located ∼2 km above the first one. The total source duration was ∼1.4 sec.

Aftershocks of the 1952 Kern County, California, earthquake sequence

AbstractWe have studied the seismograms recorded at the historic Berkeley (BRK) and Pasadena (PAS) stations for 20 aftershocks of the 21 July 1952 Kern County earthquake sequence. These events, in the magnitude range of MW 4.5 to 5.6, are too small to be studied teleseismically, yet they are important for better understanding the tectonics of the southern Sierra Nevada and the Tehachapi Mountains. On-scale recordings of moderate-sized events from this important earthquake sequence were first scanned, digitized, and then subjected to waveform modeling using a seismic moment tensor inverse procedure. In particular, the long-period, three-component Galitzen instrument at BRK and the 6-sec Wood-Anderson at PAS provided very high quality seismograms that could be analyzed in this manner. These two sites have been continuously operated from 1887 and 1927, respectively, and both are current sites of state-of-the-art broadband, high dynamic range instrumentation. First-motion polarities reported by Bath and Richter (1958) were used as additional constraints in the estimation of source parameters. There is considerable variability in the three-component seismograms of the 1952 aftershocks, which in turn result in a diversity of focal mechanisms. The majority of the solutions are northwest-striking reverse mechanisms that likely occurred on various mapped thrust faults in the hanging block of the mainshock. There are several events with northeast-striking, left-lateral mechanisms that are consistent with the strike of the White Wolf fault, as well as several normal slip events. The results of this study indicate that there are a variety of active fault structures adjacent to the White Wolf, Garlock and San Andreas faults in this region.

EEG source localization and imaging using multiple signal classification approaches.

Equivalent current dipoles are a powerful tool for modeling focal sources. The dipole is often sufficient to adequately represent sources of measured scalp potentials, even when the area of activation exceeds 1 cm2 of cortex. Traditional least-squares fitting techniques involve minimization of an error function with respect to the location and orientation of the dipoles. The existence of multiple local minima in this error function can result in gross errors in the computed source locations. The problem is further compounded by the requirement that the model order, i.e. the number of dipoles, be determined before error minimization can be performed. An incorrect model order can produce additional errors in the estimated source parameters. Both of these problems can be avoided using alternative search strategies based on the MUSIC (multiple signal classification) algorithm. Here the authors review the MUSIC approach and demonstrate its application to the localization of multiple current dipoles from EEG data. The authors also show that the number of detectable sources can be determined in a recursive manner from the data. Also, in contrast to least-squares, the method can find dipolar sources in the presence of additional non-dipolar sources. Finally, extensions of the MUSIC approach to allow the modeling of distributed sources are discussed.

Seismological studies at Parkfield VI: Moment release rates and estimates of source parameters for small repeating earthquakes

AbstractWaveform data from a borehole network of broadband seismographic stations have been used to study microearthquakes along the Parkfield segment of the San Andreas fault (SAF). Analysis of almost 10 years of such data demonstrates that much of the seismicity in this region consists of repeating sequences, quasiperiodic sequences of earthquakes that are essentially identical in terms of waveform, size, and location. Scalar seismic moments have been estimated for 53 of these repeating sequences and combined with equivalent estimates from 8 similar but larger event sequences from the Stone Canyon section of the fault and the main Parkfield sequence. These estimates show that seismic moment is being released as a function of time in a very regular manner. Measurements of the moment release rate, combined with an assumed tectonic loading rate, lead to estimates of the seismic parameters source area, slip, and recurrence interval. Such parameters exhibit a systematic dependence upon source size over a range of 1010 in seismic moment that can be described by three simple scaling relationships. Several implications of these scaling relationships are explored, including the repeat time of earthquakes, average stress drop, strength of the fault, and heat generated by earthquakes. What emerges from this analysis of moment release rates is a quantitative description of an earthquake process that is controlled by small strong asperities that occupy less than 1% of the fault area. This means that the fault is highly heterogeneous with respect to stress, strength, and heat generation. Such heterogeneity helps to explain many of the apparent contradictions that are encountered in the study of earthquakes, such as why faults appear weak, why significant heat flow is not observed, how significant high frequencies can be generated by large earthquakes, and how various geologic features such as pseudotachylytes might form.

Glottal source parameter estimation by comparison of measured signals with simulated signals

Oral flow, electroglottographic (EGG), and photoglottographic (PGG) signals were simultaneously digitized from a single subject phonating sustained vowels. Analogous simulated signals—oral flow, vocal fold contact area, and glottal area, respectively—were produced by a driven, kinematic glottal source model coupled to a wave-reflection-type vocal tract. The vocal tract area function used in the simulation was estimated from magnetic resonance imaging (MRI) data previously published for the subject. This essentially removed the vocal tract as a confounding factor in the analysis. The measured and simulated signals were compared and an attempt was made to adjust model parameters so that the least-squared differences between the signals were minimized. The model parameters, which included subglottal pressure, vocal fold adduction, vocal fold convergence, and fundamental frequency were observed to be estimates for the glottal configuration and control parameters used by the subject.

Estimation of source parameters and Q from acceleration recorded in the Vatnafjöll earthquake in south Iceland

Abstract The earthquake in Vatnafjöll, 25 May 1987 (M = 5.8), was the largest earthquake in south Iceland since 1912 (M = 7), and it was one of the first earthquakes recorded on a local network of strong-motion acceleration recorders. In this study, the strong-motion records are used to obtain an estimate of the source parameters and the attenuation of high-frequency waves. The source parameters are obtained by fitting the data by a dislocation source model combined with an exponential model for the attenuation. The root-mean-squared acceleration is also estimated and compared to predicted values using the source parameters. The source parameters estimated here (M0 = 8.2 × 1024 dyn-cm, r = 4.6 km, Δσ = 38 bars, Δu = 36 cm, Td = 3 sec) and estimated moment magnitude (Mw = 5.9) agree well with values obtained previously from teleseismic waves and strain-meter data. Estimates of path-averaged crustal shear-wave quality factors give results in the range Q = 128 to 425, and k = 0.05 sec for the spectral decay parameter, confirming recent measurements of higher crustal Q for this region than in earlier studies.

Seismic moment assessment of earthquakes in stable continental regions-III. New Madrid 1811-1812, Charleston 1886 and Lisbon 1755

The sizes of three major or great historical earthquakes are reassessed using the isoseismal-area-regression tools developed in Parts I and II of this study of stable continental region (SCR) seismicity. The earthquakes are 1811 New Madrid, central United States, and its following sequence; 1886 Charleston, coastal South Carolina; and 1755 Lisbon, oceanic intraplate off the continental shelf of Portugal. The analysis confirms the large size of these events and for the first time places constraints on the uncertainty of their seismic moment release. Because of the exceptionally low seismic-wave attenuation of eastern North America (ENA), a separate North American regression of seismic moment on isoseismal area was developed. Additionally, the unknown western extents of the New Madrid isoseismal areas were calibrated with the patterns of the M 6.3-6.6 1843 and 1895 earthquakes. Application of Part II analysis procedures with these corrections yields New Madrid size estimates, expressed as moment magnitude, of M 8.1±0.31 for the 1811 December 16, M 7.8±0.33 for the 1812 January 23, and M 8.0±0.33 for the 1812 February 7 principal events. The Charleston earthquake's magnitude decreases from M≳7.4 to M 7.3±0.26 after compensation for the effect of coastal plain sediments on its inner isoseismals. Intensity regressions for Lisbon are calibrated against the isoseismal pattern of the nearly co-located M 7.8 1969 St Vincent earthquake, which in this case increases the predicted size of Lisbon from M 8.4 to M 8.7±0.39. These size estimates are supported by data from independent phenomena: extent and severity of liquefaction, the maximum distance of induced landslides, and for Lisbon, tsunami wave amplitudes. Estimated source parameters are controlled by crustal or lithospheric temperature, which governs the depth extent of brittle faulting. Using estimated continental and oceanic geotherms, viable fault lengths are 30–80 km for Charleston, 120–180 km for 1811 New Madrid, and 180–280 km for Lisbon for average displacements of 2–4 m, 8–11 m, and 10–14 m, respectively, and for average static stress and strain drops. At the estimated seismic moments of this study, the 1811 New Madrid and the 1755 Lisbon events are, respectively, the largest known SCR and oceanic lithosphere earthquakes.

Comments on ‘‘Effects of bandwidth on glottal airflow waveforms estimated by inverse filtering’’ [J. Acoust. Soc. Am. 98, 763–767 (1995)

In the subject paper [Alku and Vilkman, J. Acoust. Soc. Am. 98, 763–767 (1995); hence AV], AV describe the results of their research on the effect of bandwidth on estimated voice source parameters. They found that reducing the bandwidth (by low-pass filtering the glottal flow signals) leads to a distortion of the estimated parameters. Although I do agree that low-pass filtering influences the estimate of the voice source parameters, I do not agree with some of their conclusions, explanations, and recommendations. Furthermore, the method they used does not seem to be optimal for the purpose of their research. These matters are discussed in this Letter.

Source characteristics of the Sinaia (Romania) sequence of May–June 1993

A particular sequence of crustal earthquakes ( h < 10 km, M L < 5.1) occurred in the southwestern extremity of the Eastern Carpathian arc bend (Sinaia region) starting on May 4, 1993. The main shock ( M L 5.1) occurred on May 23 after significant 20-day foreshock activity. The first locations delineate an epicentral area of approximately 225 km 2, elongated in a NW-SE direction, which agrees with the fault plane solution for the main shock. The digital recordings of the Romanian telemetered network are analyzed in order to estimate source parameters and to investigate scaling relations. An empirical deconvolution technique and the spectral ratio method are used for this purpose. The main results, such as (1) the departure from a M 1 3 o scaling of the source radius as a function of the seismic moment, (2) the scaling of the peak velocity parameter as a function of local magnitude and (3) the scaling of the dynamic stress drop with earthquake size, are interpreted in terms of an inhomogeneous source model.

Microearthquake analysis for hydraulic fracturing process

The hydraulic fracture technique is widely applied in the enhancement of petroleum and natural gas productions and in the development of geothermal energy. This technique is also used to create an underground fracture zone system for disposal of solid and liquid wastes. This is the most recent development in the application of industrial techniques to environmental protection scientific problems. Knowledge of mechanical properties and geometrical parameters of a hydraulic fracture zone is important for both energy resource development and safe disposal of waste. Hydraulic fracturing often induces many microearthquakes. Analysis of the spatial-temporal distribution of the induced seismicity yields the geometry of a hydraulic fracture zone, and kinetic and dynamic parameters associated with the fracture growth process. Applying a waveform correlation analysis and a space-time grid search method, we precisely determined hypocentral locations for 157 microearthquakes induced by hydraulic fracturing. Spatial distribution of the induced seismicity clearly shows the dimension and orientation of a hydraulic fracture zone. Variation of the seismicity distribution in time and space was used to infer the growth rate and direction of the fracture zone. An empirical Green’s function (EGF) method was applied to earthquake doublets to retrieve relative source time functions (RSTF’s) and to estimate source parameters, such as seismic moment, source radius, and stress drop, for larger microearthquakes. Azimuthal variation of the RSTF of a master event indicates that the source ruptured to the northwest, which aggrees with the fracture zone growth direction. Large variation of stress drops for these induced earthquakes reflects the significant heterogeneity of mechanical properties in the hydraulic fracture zone.

Robust nonadaptive matched‐field beamforming in an uncertain ocean environment.

Matched‐field processing techniques can be both ambiguity‐prone and sensitive to errors in the assumed environmental conditions. Although greater robustness to environmental variability can be obtained by the use of data adaptive methods, such as the MV‐EPC beamformer [J. L. Krolik, J. Acoust. Soc. Am. 92, 1408–1419 (1992)] or joint estimation of source and channel parameters [D. F. Gingras and P. Gersoft, J. Acoust. Soc. Am. 97, 3589–3598 (1995)], these strategies require higher signal‐to‐noise ratios. The methods presented here concern the design of robust nonadaptive beamformer weights with sidelobe levels which are less signal‐to‐noise ratio dependent. As with Chebyshev filters, the weights are designed to minimize the maximum magnitude‐squared sidelobe level. Robustness to environmental mismatch is achieved by defining the sidelobe level as the magnitude‐squared response averaged over an ensemble of environmental conditions. The two algorithms proposed are a fast iterative constrained minimum variance method and the use of a minimax quadratic programming technique. For an uncertain Mediterranean environment, an improvement in the maximum average sidelobe level of as much as 3 dB over the Bartlett beamformer is achieved. [Work supported by NRaD/ONR.]