ALBERT

Description: An earthquake swarm of magnitudes up to M L  = 3.2 occurred in the region of West Bohemia/Vogtland (border area between Czech Republic and Germany) in autumn 2000. This swarm consisted of nine episodic phases and lasted 4 months. We retrieved source mechanisms of 102 earthquakes with magnitudes between M L  = 1.6 and 3.2 applying inversion of the peak amplitudes of direct P and SH waves, which were determined from ground motion seismograms. The investigated events cover the whole swarm activity in both time and space. We use data from permanent stations of seismic network WEBNET and from temporal stations, which were deployed in the epicentral area during the swarm; the number of stations varied from 7 to 18. The unconstrained moment tensor (MT) expression of the mechanism, which describes a general system of dipoles, that is both double-couple (DC) and non-DC sources, was applied. MTs of each earthquake were estimated by inversion of three different sets of data: P -wave amplitudes only, P - and SH -wave amplitudes and P -wave amplitudes along with the SH -wave amplitudes from a priori selected four ‘base’ WEBNET stations, the respective MT solutions are nearly identical for each event investigated. The resultant mechanisms of all events are dominantly DCs with only insignificant non-DC components mostly not exceeding 10 per cent. We checked reliability of the MTs in jackknife trials eliminating some data; we simulated the mislocation of hypocentre or contaminated the P - and SH -wave amplitudes by accidental errors. These tests proved stable and well constrained MT solutions. The massive dominance of the DC in all investigated events implies that the 2000 swarm consisted of a large number of pure shears along a fault plane. The focal mechanisms indicate both oblique-normal and oblique-thrust faulting, however, the oblique-normal faulting prevails. The predominant strikes and dips of the oblique-normal events fit well the geometry of the main fault plane Novy Kostel (NK) and also match the strike, dip and rake of the largest M L  = 4.6 earthquake of a strong swarm in 1985/86. On the contrary, the 2000 source mechanisms differ substantially from those of the 1997-swarm (which took place in two fault segments at the edge of the main NK fault plane) in both the faulting and the content of non-DC components. Further, we found that the scalar seismic moment M 0 is related to the local magnitude M L used by WEBNET as M 0 10 1.12M L , which differs from the scaling law using moment magnitude M w , that is M 0 10 1.5M w .

Description: SUMMARY Efforts to determine general moment tensors (MTs) for microearthquakes in volcanic areas are often hampered by small seismic networks, which can lead to poorly constrained hypocentres and inadequate modelling of seismic velocity heterogeneity. In addition, noisy seismic signals can make it difficult to identify phase arrivals correctly for small magnitude events. However, small volcanic earthquakes can have source mechanisms that deviate from brittle double-couple shear failure due to magmatic and/or hydrothermal processes. Thus, determining reliable MTs in such conditions is a challenging but potentially rewarding pursuit. We pursued such a goal at Okmok Volcano, Alaska, which erupted recently in 1997 and in 2008. The Alaska Volcano Observatory operates a seismic network of 12 stations at Okmok and routinely catalogues recorded seismicity. Using these data, we have determined general MTs for seven microearthquakes recorded between 2004 and 2007 by inverting peak amplitude measurements of P and S phases. We computed Green's functions using precisely relocated hypocentres and a 3-D velocity model. We thoroughly assessed the quality of the solutions by computing formal uncertainty estimates, conducting a variety of synthetic and sensitivity tests, and by comparing the MTs to solutions obtained using alternative methods. The results show that MTs are sensitive to station distribution and errors in the data, velocity model and hypocentral parameters. Although each of the seven MTs contains a significant non-shear component, we judge several of the solutions to be unreliable. However, several reliable MTs are obtained for a group of previously identified repeating events, and are interpreted as compensated linear-vector dipole events.

Description: The earthquake source mechanism (description of the geometry of the source and its strength in terms of forces equivalent to rupturing of the rock mass) is routinely modeled as a moment tensor (MT) describing the earthquake focus as a point. However, sometimes this approximation is not satisfied, and seismic radiation keeps the directivity due to the finite extent of the source. Inversion into the MT then may yield a biased mechanism. Synthetic study in the Adamová and Síleny (2010) demonstrated the appearance of spurious non-double-couple components in the mechanism even for a pure double-couple (DC) source. Their method was designed to reduce the spurious source components by using the second-degree moments to evaluate their contribution into the records and subtract it from the data. Here we applied the procedure to five moderate to large regional events with large nonshear components. They are mostly located on large tectonic faults where predominantly pure shear slip is expected. We studied one event on the prominent North Anatolian fault, three events in the Pacific area, and one event in Bolivia. In most cases, the non-DC components essentially were reduced, and the geometry of the mechanism remained largely unchanged. This confirms the hypothesis that part of the non-DC components in regional MT solutions may be spurious due to the neglect of the source finiteness in the routine procedure of the MT retrieval. In addition, the geometrical and kinematical characteristics of the foci provided by the second-degree moments (the source ellipsoid and rupture velocity vector) are mostly consistent with the fault geometry, aftershock distribution, and estimates of rupture speed from available previous studies.

Description: We perform a detailed synthetic study on the resolution of non-double-couple (non-DC) components in the seismic moment tensors from short-period data observed at regional networks designed typically for monitoring aftershock sequences of large earthquakes. In addition, we test two different inversion approaches—a linear full moment tensor inversion and a non-linear moment tensor inversion constrained to a shear-tensile source model. The inversions are applied to synthetic first-motion P - and S -wave amplitudes, which mimic seismic observations of aftershocks of the 1999 M w  = 7.4 Izmit earthquake in northwestern Turkey adopting a shear-tensile source model. To analyse the resolution capability for the obtained non-DC components inverted, we contaminate synthetic amplitudes with random noise and incorporate realistic uncertainties in the velocity model as well as in the hypocentre locations. We find that the constrained moment tensor inversion yields significantly smaller errors in the non-DC components than the full moment tensor inversion. In particular, the errors in the compensated linear vector dipole (CLVD) component are reduced if the constrained inversion is applied. Furthermore, we show that including the S -wave amplitudes in addition to P -wave amplitudes into the inversion helps to obtain reliable non-DC components. For the studied station configurations, the resolution remains limited due to the lack of stations with epicentral distances less than 15 km. Assuming realistic noise in waveform data and uncertainties in the velocity model, the errors in the non-DC components are as high as ±15 per cent for the isotropic and CLVD components, respectively, thus being non-negligible in most applications. However, the orientation of P - and T -axes is well determined even when errors in the modelling procedure are high.

Description: The earthquake source mechanism (description of the geometry of the source and its strength in terms of forces equivalent to rupturing of the rock mass) is routinely modeled as a moment tensor (MT) describing the earthquake focus as a point. However, sometimes this approximation is not satisfied, and seismic radiation keeps the directivity due to the finite extent of the source. Inversion into the MT then may yield a biased mechanism. Synthetic study in the Adamova and Sileny (2010) demonstrated the appearance of spurious non-double-couple components in the mechanism even for a pure double-couple (DC) source. Their method was designed to reduce the spurious source components by using the second-degree moments to evaluate their contribution into the records and subtract it from the data. Here we applied the procedure to five moderate to large regional events with large nonshear components. They are mostly located on large tectonic faults where predominantly pure shear slip is expected. We studied one event on the prominent North Anatolian fault, three events in the Pacific area, and one event in Bolivia. In most cases, the non-DC components essentially were reduced, and the geometry of the mechanism remained largely unchanged. This confirms the hypothesis that part of the non-DC components in regional MT solutions may be spurious due to the neglect of the source finiteness in the routine procedure of the MT retrieval. In addition, the geometrical and kinematical characteristics of the foci provided by the second-degree moments (the source ellipsoid and rupture velocity vector) are mostly consistent with the fault geometry, aftershock distribution, and estimates of rupture speed from available previous studies.

Description: A sizable amount of moderate and strong earthquakes exhibit a considerable percentage of non-double-couple (non-DC) components in the mechanism, as reported by agencies determining moment tensors on a routine basis from long-period seismograms. As most of them are tectonic events where a simple shear slip is anticipated along a roughly planar fault (at least in the optics of the long periods used), suspicion arises about their source origin. Leaving aside anisotropy in the source region, we assign them to be side effects of the application of the first degree moment tensor approximation to data still containing (after low-pass filtering) information about the source finiteness. We verify the hypothesis in a synthetic experiment simulating a finite-extent source--a unilaterally propagating shear slip--and invert the synthetic data into moments up to degree 2. The first degree moment--traditional moment tensor--exhibits more that 20% of non-DC components. If we restitute the data by subtracting the contribution of the second degree moments, these spurious components are suppressed and the mechanism becomes almost pure double couple. The orientation of the mechanism is, however, not affected discernibly. Spurious non-DC components can be generated also by noise contamination of the observed seismic records and by using an improper Green's function when inverting the data, which happens in cases of mislocation of the hypocenter and/or mismodeling the velocity/attenuation in the area. In additional synthetic experiments, we demonstrate that reasonably estimated effects just listed do not mask the phenomenon of appearance of the non-DC originated by neglecting the source finiteness and correcting for the second degree moments reveals the proper mechanism.