Zuzana Jechumtálová

Amplitude ratios for complete moment tensor retrieval

[1]xa0We generalized the Snokes method of inversion of amplitude ratios and polarities (P, SH, SV) designed to determine double-couple earthquake focal mechanism (Snoke, 2003). We search for a general dipole source described by unconstrained seismic moment tensor, i.e. non-double-couple (non-DC) source components are allowed as well. Based on synthetic tests simulating various configurations of seismic stations from sparse to dense networks and from low to high number of stations, conditions for successful inversion were found. We specified minimum amount of data, i.e. number of stations used and amplitude ratios employed, for reliable retrieval of non-shear source components. We showed that processing of non-DC data by traditional DC-constrained algorithm could provide distorted fault-plane solution. The approach is demonstrated with local data (1st arrival amplitudes) from 1997 West Bohemia, Czechia earthquake swarm and from 2000 injection experiment in KTB, Germany.

Small Scale Earthquake Mechanisms Induced by Fluid Injection at the Enhanced Geothermal System Reservoir Soultz (Alsace) in 2003 using Alternative Source Models

The geothermal reservoir at Soultz-sous-Forêts is a valuable natural laboratory for understanding the mechanisms of microearthquakes generated during stimulations and circulation tests. An ongoing effort currently exists regarding the retrieval of mechanisms aimed to indicate the type of fracturing of the rock massif. As a default, a moment tensor description has been applied. Nevertheless, the retrieval of the mode of fracturing still remains ambiguous. Recent studies indicate a prevailing shear slip but, rarely, a non-shear pattern has also been observed. The moment tensor, used today as a universal tool for descriptions of the mechanism, captures general balanced dipole sources. However, in the case of small-scale earthquakes, the moment tensor need not always be reliably determined. In an effort to fit the data, there may be notable non-shear components caused by the low quality of input data. Constraining the source model to directly determine a simpler one is convenient for describing the physical phenomena expected for a particular focus. An opening of new fractures can be described, to a first approximation, by a tensile crack, optionally combined with a shear slip. Such an alternative model is called a shear-tensile crack (STC) source model. The combination is practical, and can be used to both identify events that reflect purely mode-I (tensile) failure and to determine the dilation angle of the fracture undergoing shear. The latter is particularly important in enhanced geothermal system reservoirs such as Soultz, where shear-related dilation is believed to be the primary mechanism underpinning permeability creation during stimulation injections. We performed a synthetic case study by simulating seismic data as recorded by the actual seismic array installed at Soultz-sous-Forêts. Synthetic P and S amplitudes for several shear-tensile source models were inverted for several types of station coverage. The analysis explored how results were influenced by mislocation, mismodeling, and noise contamination of data. In most cases, the orientation of the mechanism was well resolved. Determination of shear vs. non-shear content within the mechanism was more difficult. From all of the factors influencing resolution that we explored, the quality of the monitoring system (the number of stations and their distribution with respect to the focus) and noise contamination were of the highest impact. The STC source model yielded considerably less spurious non-shear fracture components than the moment tensor. From the bulk of the seismicity recorded during the stimulation in 2003, we concentrated on the first phase of the injection when only a single borehole at the site was stimulated. We processed thirteen small earthquakes with magnitudes larger than 1.4 that were not treated in previous studies. We determined that their source mechanisms were dominantly pure shear slips on pre-existing faults, just as the earthquakes investigated earlier were. The results were also in agreement with the stress pattern from in situ measurements.

Point-source parameters from noisy waveforms: Error estimate by Monte-Carlo simulation

Abstractu200a—u200aWe introduce an approach to estimate the error in the determination of the point-source mechanism and corresponding source-time function by inverting waveform data contaminated by a noise. No a priori assumptions about the statistical characteristics of the noise are needed. The estimate of the confidence region of the retrieved source is constructed as a set of solutions of the inverse problem for individual realizations of noisy data. They are generated from a sample of observed noise by Monte-Carlo simulation: white noise is convolved with the observed noise sample yielding a particular realization of the noise, which is random but retains the spectral characteristics of the observed sample. This approach allows us to consider even the situations where the noise varies from station to station. However, the basic assumption is that the noise is random, thus the method cannot handle systematic errors, e.g., due to mislocation or incorrect model of the medium.¶Synthetic tests with noisy data simulating the configuration of the seismic network in Friuli, NE Italy, show that determination of the source-time function is rather uncertain even with low-noise contamination but that orientation of the mechanism is recovered very well. As an example, local waveforms of a magnitude 3 event from the 1988 Moggio Udinese swarm, Northern Italy, are processed.

Crustal deformations in the epicentral area of the West Bohemia 2008 earthquake swarm in central Europe

[1]xa0In West Bohemia, central Europe, during October 2008 an earthquake swarm of 25,000 shocks with a maximum event of ML ∼ 3.7–3.8 occurred at depths of 7–11 km. In 2007, annual GPS campaigns were launched. During the co-seismic phase, displacements of a few centimeters were detected at GPS sites. Maximum displacement was revealed at the KOPA site, which subsided by 167 mm. The epicentral area is covered by eluvium of 4–10 m thick, and is located in undulating pastures and well-forested valleys where visible surface soil effects could not be observed. To test possible fault manifestations, rough geomorphologic, geoelectric, and geochemical surveys were performed. GPS and seismic data, with geologic materials, were used to build a forward model for surface displacements, crustal deformations, and shear and normal stress fields. The fields enabled us to better determine crustal deformations and stresses that appeared within the seismic cycle, during the pre-, co-, and post-seismic phases. During the co-seismic phase, modeled fault motions along N-S faults located within the epicentral zone reached 0.6–1 mm/day. Possible structural block rotations were comprised of these motions. A dominant role for stress accumulation, release, and relaxation was assigned to the Marianske Lazně fault zone and the Nový Kostel zone. Strain loads slowly, and when local PT conditions with an action of deep magmatic fluids reach instability, the strain is released and stress balancing occurs. The process leads to the reversible motions known for silent earthquakes. A forward crustal deformation model for West Bohemia is also presented within.

The Mechanism of Microearthquakes Related to a Gas Storage Using Differently Constrained Source Models: A Case Study of the Háje Location, Czech Republic

The resolution of a source mechanism is investigated in terms of three differently constrained source models: the moment tensor, the shear-tensile crack source model, and the double couple source model. The moment tensor (MT) is an unconstrained description of a general dipole source; the shear-tensile crack (STC) represents a slip along a fault with an off-plane component and the double couple (DC) corresponds to a simple shear slip along a fault. The inversion of body wave amplitudes is applied on microseismic events located in the vicinity of underground gas storage Háje (Czech Republic) where volume changes in the source can be expected. The orientation of the simple shear fracture component is resolved almost always well, independently of the source model used. On the other hand, the non-shear components differ largely among the source models considered, from both the model definition and robustness of the inversion. A comparison of the inversion results for the three alternative source models permits an assessment of the reliability of the non-shear components retrieved. Application of the STC model to all events appears to be the most appropriate. The analysis confirms a shear slip for three events and a tensile fracturing for other three events.

Effects of 1-D versus 3-D velocity models on moment tensor inversion in the Dobrá Voda area in the Little Carpathians region, Slovakia

Retrieving the parameters of a seismic source from seismograms involves deconvolving the response of the medium from seismic records. Thus, in general, source parameters are determined from both seismograms and the Green functions describing the properties of the medium in which the earthquake focus is buried. The quality of each of these two datasets is equally significant for the successful determination of source characteristics. As a rule, both sets are subject to contamination by effects that decrease the resolution of the source parameters. Seismic records are generally contaminated by noise that appears as a spurious signal unrelated to the source. Since an improper model of the medium is quite often employed, due to poor knowledge of the seismic velocity of the area under study, and since the hypocentre may be mislocated, the Green functions are not without fault. Thus, structures not modelled by Green functions are assigned to the source, distorting the source mechanism. To demonstrate these effects, we performed a synthetic case study by simulating seismic observations in the Dobrá Voda area of the Little Carpathians region of Slovakia. Simplified 1-D and 3-D laterally inhomogeneous structural models were constructed, and synthetic data were calculated using the 3-D model. Both models were employed during a moment tensor inversion. The synthetic data were contaminated by random noise up to 10 and 20xa0% of the maximum signal amplitude. We compared the influence of these two effects on retrieving moment tensors, and determined that a poor structural model can be compensated for by high-quality data; and that, in a similar manner, a lack of data can be compensated for by a detailed model of the medium. For examples, five local events from the Dobrá Voda area were processed.

Consistency of stress state, locations and source mechanisms of events induced by hydraulic fracturing: downhole monitoring: Consistency of stress state, locations and source mechanisms

We present results of processed microseismic events induced by hydraulic fracturing and detected using dual downhole monitoring arrays. The results provide valuable insight into hydraulic fracturing. For our study, we detected and located microseismic events and determined their magnitudes, source mechanisms and inverted stress field orientation. Event locations formed a distinct linear trend above the stimulated intervals. Source mechanisms were only computed for high-quality events detected on a sufficient number of receivers. All the detected source mechanisms were dip-slip mechanisms with steep and nearly horizontal nodal planes. The source mechanisms represented shear events and the non-double-couple components were very small. Such small, non-double-couple components are consistent with a noise level in the data and velocity model uncertainties. Strikes of inverted mechanisms corresponding to the nearly vertical fault plane are (within the error of measurements) identical with the strike of the location trend. Ambient principal stress directions were inverted from the source mechanisms. The least principal stress, σ3, was determined perpendicular to the strike of the trend of the locations, indicating that the hydraulic fracture propagated in the direction of maximum horizontal stress. Our analysis indicated that the source mechanisms observed using downhole instruments are consistent with the source mechanisms observed in microseismic monitoring arrays in other locations. Furthermore, the orientation of the inverted principal components of the ambient stress field is in agreement with the orientation of the known regional stress, implying that microseismic events induced by hydraulic fracturing are controlled by the regional stress field.

Smoothing the source time function – a tool to soften inadequate modelling of the medium during inversion of local waveforms?

The inconsistency of records of local earthquakes and synthetic seismograms due to noise contamination of the data and/or to improper Green function because of an inexact structural model often results in high frequency oscillations of the source time function, which prolongs it spuriously. A priori limit on the roughness of the source time function turned out to reduce the spurious oscillations but it keeps its spurious extension unchanged. Thus, it may yield a severe distortion of the seismic moment. However, it reduces the spurious volumetric component of the mechanism resulting from improper modelling of the structure and, thus, may help decide whether the retrieved volumetric component is real or false. The demand for the smoothness is incorporated as a penalty function in the minimization of the least square residuals, which allows us to decide about the degree of the smoothness. The minimization is performed with a simple genetic algorithm, which offers the advantage of a detailed exploration of the model space reducing the danger of being trapped in local minima.