Compensated Linear Vector Dipole Research Articles

The Excitation of Lg Waves by Underground Nuclear Explosions

Near-source scattering of explosion-generated Rg wave into S wave appears to be the primary contribution to the low-frequency Lg wave in nuclear explosions, this mechanism has been supported by analysis of regional data from several Yucca Flats, NTS explosions by Patton and Taylor (1995), who further indicated that the prominent low-frequency spectral null in Lg wave is due to Rg wave from a compensated linear vector dipole (CLVD) source. In this study, the mechanism of the excitation of Lg wave in explosions in layered earth structures representative of the East Kazakh test site is further analyzed by using theoretical seismograms, and the result shows that the Lg spectra indicate prominent nulls showing strong dependence on focal depth and in remarkably good agreement with those expected from Rg wave due to a CLVD source.

Analysis of broad-band regional waveforms of the 1996 September 29 earthquake at Bárdarbunga volcano, central Iceland: investigation of the magma injection hypothesis

SUMMARY Large earthquakes near active volcanoes, that exhibit non-double-couple source properties are usually interpreted as the result of either magma intrusion or geometrical complexity along the fault plane. Such an earthquake occurred in 1996 September 29 at Bardarbunga volcano in central Iceland, to be followed 2 days later by a major volcanic eruption at the area between Bardarbunga and the nearby Grimsvotn volcano. Both of these active volcanic centres lie underneath the Vatnajokull glacier, a permanent ice cap that covers a large area of central Iceland. This event was recorded by a temporary network (HOTSPOT) that consisted of 30 broad-band three-component seismometers covering most of Iceland. The waveforms of this event at all stations show an emergent, low-amplitude, high-frequency onset that is superposed on a longer-period signal. The corresponding amplitude spectra show a low-frequency content (<1 Hz) and prominent peaks around the corner frequency (∼0.25 Hz) and higher frequencies. These regional waveforms were inverted in order to obtain the best-fitting deviatoric and full moment tensor using a linear, time-domain inversion method. The results for the deviatoric moment tensor indicate a large (∼60 per cent) compensated linear vector dipole (CLVD) component, a hypocentral depth of 3.5 km, a moment magnitude of 5.4 and a best double-couple solution showing thrust motion in good agreement with the previously published Harvard CMT solution. The results for the full moment tensor on the other hand, indicate an implosive isotropic component of 8.5 per cent, a reduced CLVD component of 47.2 per cent and a best double-couple solution showing normal faulting. However, a statistical F-test revealed that the full moment tensor does not fit the data significantly better than the deviatoric at a confidence level of not more than 76 per cent. All of these results were found not to change substantially when a different source time function was used or when the data were weighted according to their distance from the source. The data are consistent with an earthquake of this magnitude, caused by the failure of an asperity and the formation of a tensile crack due to increasing fluid pressure. The dimensions of the crack may have been 10 × 3k m 2 and 0.5 m thickness and the volume of the injected fluid was found to be 15 × 10 6 m 3 . The calculated viscosity for the fluid (0.04 Pa s) points to the possibility of water being injected rather than magma, that is also supported by the short source duration of the earthquake (∼5 s). Taking into account the water saturation of the upper crust in Vatnajokull due to the presence of the glacier, this event may have been caused by increased pressure of water that was heated by magma injected through a dyke below the asperity.

Seismic moment tensor for intermediate depth earthquakes at regional distances in Southern Spain

A method has been developed to obtain the seismic moment tensor components by linear inversion of P waves recorded at regional distance for intermediate depth earthquakes. The seismic moment tensor is separated into double couple (DC) and compensated linear vector dipole (CLVD) parts. The method has been applied to four earthquakes (64< h<95 km) which occurred in the Malaga region (southern Spain). Solutions for the 1987 event show a percentage of CLVD of 20% with a short source time function and DC part corresponding to vertical motion. For the 1989, 1990 and 1992 earthquakes, percentages of CLVD between 0% and 6% have been found. Comparison with the results obtained in a previous study [Buforn et al., J. Seismol. 1 (1997) 113] by modelling of P waves using a DC model, shows that the use of a more general representation of the source (seismic moment tensor) gives a fit of data for the 1987 event.

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.

Moment tensors of the January 1997 earthquake swarm in NW Bohemia (Czech Republic): double-couple vs. non-double-couple events

We investigated moment tensors (MTs) of 70 events of the earthquake swarm which occurred in January 1997 in NW Bohemia. A refined location using the master-event procedure shows that all the foci clustered in a volume of less than 0.5 km 3 comprising two compact clusters—the southern and northern ones. The results of single-source, absolute-moment tensor inversion of the P- and SH-peak amplitudes reveal two types of the source mechanisms, A and B in our denotation, which dominated in the swarm. Type A implies an oblique normal faulting with a nearly pure double-couple (DC) source. For the B type, an oblique-thrust faulting and a combined source [double-couple combined with the isotropic (ISO) and compensated linear-vector dipole (CLVD) components] are typical. Magnitudes of the non-double-couple components of MT appear unrelated to the M L magnitude of the event. The proximity of hypocentres of A and B events guarantees the non-double-couple source mechanisms of the B events not to be an artefact of a mismodelling of the medium. To exclude finiteness of the focus or station-site effects as possible causes of spurious non-double-couple components of MTs of the B events, the residuals of the peak amplitudes across the set of the B events were analysed and the jack-knife test was applied. The A and B events separate in time and space. Consequently, three major phases of swarm activity can be distinguished. In the first, only the southern cluster was active and A events prevailed, while B events dominated in the northern cluster in the third phase. Both A and B events occurred (the former in the southern cluster, the latter in the northern one) during the second phase. The initiation of the B events in the northern cluster are reflected in a pronounced increase in the non-double-couple components of the MTs, which points to tensile-source mechanisms as a consequence of a hypothesised fluid injection.

Can unbiased source be retrieved from anisotropic waveforms by using an isotropic model of the medium?

Anisotropy is frequently present in geological structures, but usually neglected when source parameters are determined through waveform inversion. Due to the coupling of propagation and source effects in the seismic waveforms, such neglect of anisotropy will lead to an error in the retrieved source. The distortion of the mechanism of a double-couple point source located in an anisotropic medium is investigated when inverting waveforms using isotropic Green's functions. The anisotropic medium is considered to be transversely isotropic with six levels of anisotropy ranging from a fairly weak to rather strong anisotropy, up to about 24% in P waves and 11% in S waves. Inversions are based on either only direct P waves or both direct P and S waves. Two different algorithms are employed: the direct parametrization (DIRPAR, a nonlinear algorithm) and the indirect parametrization (INPAR, a hybrid scheme including linear and nonlinear steps) of the source. The orientation of the double-couple mechanism appears to be robustly retrieved. The inclination of the resulting nodal planes is very small, within 10° and 20° from the original solution, even for the highest degree of anisotropy. However, the neglect of anisotropy results in the presence of spurious isotropic and compensated linear-vector dipole (CLVD) components in the moment tensor (MT). This questions the reliability of non-double-couple components reported for numerous earthquakes.

Low-Frequency Properties of Intermediate-Focus Earthquakes

Intermediate-focus earthquakes are known to show features such as non-double-couple focal mechanisms that indicate source complexities. To characterize these features, we have systematically studied the low-frequency radiation from 108 intermediate-focus earthquakes recorded by high-performance seismic networks from 1989–1997 whose total moment was > 3 × 10 18 N m. We determined frequency-dependent focal mechanisms and source phase and amplitude spectra for each earthquake, estimating the uncertainties for all parameters. Frequency-dependent focal mechanisms were obtained from vertical-component, free-oscillation data in 1-mHz bands over the range of 1–11 mHz. Source amplitude and phase-delay spectra were determined to 20 mHz from a combination of free-oscillation and surface-wave data. The population of intermediate-focus earthquakes in our catalog is not equally divided between compressional and tensional stress-release mechanisms; instead, 59% are down-dip tensional, 25% are down-dip compressional, and 16% are neither. We have assessed the statistical significance of any non-double-couple component of the source for every earthquake in the catalog. We represent a deviatoric focal mechanism by its principle axes and a scalar ξ that ranges from –1, for a compensated linear vector dipole (CLVD) with a compressional axis of symmetry through 0 for a double couple, to +1, for a CLVD with a tensional axis of symmetry. ξ varies from –0.91 to +0.64 in this dataset. Fifty-six of the 108 events have a significant CLVD component (ξ ≠ 0) at the 95% confidence level; 15 events have an unusually large CLVD component of the focal mechanism (|ξ| ≥ 0.4). We do not observe a correlation between the CLVD component and seismic moment or depth. T -type CLVD mechanisms correlate weakly with slab stress state, but no correlation was similarly found for P -type CLVD events. Seven earthquakes in the catalog are slow or compound, and three show strong frequency dependence of the moment tensor.

Inversion for parameters of tensile earthquakes

The tensile source model generalizes the shear source model by assuming that the slip vector can be arbitrarily oriented with respect to the fault and is not constrained to lie within the fault plane. The proposed inversion for the parameters of tensile sources is based on the evaluation of the isotropic (ISO), compensated linear vector dipole (CLVD), and double‐couple (DC) components in seismic moment tensors. The most significant parameters inverted are the λ/μ ratio at the fault (denoted as the κ parameter) and the inclination α of the slip vector from the fault. The κ parameter is significant for discriminating noisy moment tensors of shear earthquakes from those of tensile earthquakes. The inclination α can be accurately determined from the DC component in the moment tensor because the DC component rapidly decreases with increasing α. For example, the inclination of 20° causes DC being ∼50–60% only. The inversion is applied to earthquakes which occurred in January 1997 in West Bohemia, Czech Republic. It is shown that some of these earthquakes display tensile faulting. The κ parameter is ∼0.1. The inclination of the slip from the fault attains values of up to 20°. This inclination is a result of tensile traction and reduced shear traction along the fault and high‐fluid pressure in the region.

Point-source inversion neglecting a nearby free surface: simulation of the Underground Research Laboratory, Canada

SUMMARY The aim of this study was to investigate the effects of the inconsistency between the model used and the actual structure on the inversion of point-source parameters. Specifically, we studied the effects of ignoring a reflecting interface in the model. Such a situation frequently occurs when events with sources close to mine excavations are recorded and subsequently processed using a simplified model describing the interface only approximately or ignoring it completely. In this case, it is important to ask how reliable the retrieved mechanism is. An inversion algorithm with indirect parametrization of the source (INPAR) was applied. A numerical study was performed, in which the mechanism and source time function of a point source were sought in a model inconsistent with the actual structure. The ray synthetic wavefield generated by a source situated close to a free surface of a homogeneous half-space was treated as the observed data. The free surface was then ignored when constructing the Green’s function necessary for the point-source inversion. Thus, the inconsistent response of the medium including only direct phases was used to invert the observed seismograms, which contain both direct and reflected phases. The configuration of the Underground Research Laboratory (URL) of the Atomic Energy Canada Ltd. was simulated in the synthetic experiments. 16 triaxial sensors were situated around the underground tunnel, the face of which was taken to represent the free surface near the hypocentre. High-frequency seismograms were synthetized with a frequency of around 10 kHz. This is close to the prevailing frequency of the actual URL records. It was found that gross features of the source such as orientation of the double couple and the general features of the source time function can be retrieved satisfactorily when the hypocentre is localized correctly. Formal error analysis, however, yields rather large error estimates due to the omission of the free surface, providing us with acceptably constrained solutions at about 70 per cent confidence level only. Mislocation of the hypocentre of the order of seven to 14 wavelengths and/or contamination of the data by noise with an amplitude amounting to 20 per cent of the data amplitude both distort retrieved source parameters and make them rather uncertain. The retrieved orientation deviates by more than 20u and its 70 per cent confidence region extends several tens of degrees. As a consequence of the mismodelling of the medium represented by the neglect of the free surface, spurious non-double couple (DC) components appear in the mechanism. Two URL events from 1991 September 25 separated by 4 s were processed, and a large majority of the compensated linear-vector dipole (CLVD) was found there. The CLVD along the P-axis at the earlier event was replaced by the CLVD along the T-axis at the following one, which suggests an over-relaxation of the stress during the first event and its partial restoration by the subsequent event.

Source Parameters of Regional Earthquakes in Taiwan: January-December, 1997

Establishment of the ”Broadband Array in Taiwan for Seismology (BATS)” has enabled the systematic determination of reliable source parameters for regional earthquakes in Taiwan through waveform inversion technique. Using an improved algorithm by Kao et at (1998) to overcome the generally higher background noise as well as the heterogeneous velocity structure resulted from the complex tectonic interactions near Taiwan, we report source parameters of 46 events that occurred in 1997. The inversion quality is classified by a combination of a letter (A-F) and a digit (1-4) reflecting the waveform misfit and the compensated linear vector dipole (CLVD) component, respectively. To make the results more accessible and useful to the academic community, both the table that summarizes the reported Source parameters and the complete set of inversion results are available electronically from the BATS web site at http://bats.earth.sinica.edu.tw/CMT_Solutions/cmtF1997.html.

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 &lt;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.

The 1996 June 17 Flores Sea and 1994 March 9 Fiji-Tonga earthquakes: source processes and deep earthquake mechanisms

Source parameters of the 1996 Flores Sea and 1994 Fiji–Tonga deep earthquakes are derived from teleseismic body waves recorded by the global seismic network of broad-band seismograph stations. Both events consisted of several subevents. Models to approximate the spatial and temporal extent of the source process include point sources, propagating point sources and a combination of these. For the Flores Sea event, rupture lasted about 23 s and terminated some 70 km east of the nucleation point as inferred from the duration of P-wave pulses, in agreement with the findings reported by other investigators. Our preferred model suggests bilateral rupture propagation. It consists of four point sources that have variable double-couple radiation patterns and source time histories, and explains well the large compensated linear vector dipole (CLVD) component inferred from the Harvard centroid moment tensor (CMT) solution. The main moment release in the Fiji–Tonga event lasted only about 15 s. Our best model consists of two point sources with a total moment release of 2.8 × 1020 N m. Rupture propagated subhorizontally from the nucleation point to the north. The termination of rupture was located about 40 km to the north of rupture initiation. The inferred velocity of moment release in the Flores Sea and Fiji–Tonga events was 3–5 km s-1, a value which is higher than that inferred for the great Bolivian earthquake of June 1994. Other derived source parameters (static stress drop, radiated seismic energy and maximum seismic efficiency) are also significantly different from those inferred for the 1994 Bolivian event, suggesting that deep earthquake processes do not follow an easily detectable common mechanism.

How well constrained are well‐constrained T, B, and P axes in moment tensor catalogs?

The T, B and P axes of earthquake moment tensors (MT) are often used to evaluate regional stress directions and other tectonic parameters; we here undertake three comparisons to assess the uncertainty in the orientations of these axes. These are (1) a direct comparison of common MT in the Harvard, U.S. Geological Survey, or Earthquake Research Institute (ERI) catalogs; (2) a comparison of MT slip vectors and plate motion vectors in several tectonically straightforward regions; and (3) an analysis of the axial variability in the Harvard and ERI catalogs implied by the reported uncertainties in individual MT components. All three comparisons indicate that there is considerable variability within the catalog concerning the axial orientation of MT, but all suggest that axis orientations of the majority of Harvard MT have uncertainties of 15° or less. For compensated linear vector dipole (CLVD) components among the three catalogs, the correlation is very low. For the Harvard catalog, three statistics are especially useful for selecting better constrained MT; these are (1) the relative error Erel, which is the ratio of the scalar moments of the reported error tensor and of the MT itself; (2)ƒCLVD, a measure of the strength of the CLVD component; and (3) nfree, the number of MT elements not fixed at zero in the inversion. For selecting better constrained MT, the appropriate statistical cutoffs chosen depend on the problem of interest, the data available, arid personal preference. However, for analysis of shallow earthquakes we have used Erel ≤ 0.15, ƒCLVD ≤ 0.20, and nfree = 6. While this eliminates 53% of the catalog, our calculations suggest that nearly all the remaining events have T, B, and P axes with azimuth and inclination angle uncertainties of 5°–10° or less.

Source Parameters of Regional Earthquakes in Taiwan: July 1995-December 1996

Systematic determination of reliable source parameters for regional earthquakes in Taiwan, particularly offshore events with m(subscript b)＜5.5, has been a difficult task because of poor coverage by the local network and the lack of signals at teleseismic distances. Establishment of the ”Broadband Array in Taiwan for Seismology (BATS)” has greatly improved such a task through moment-tensor inversion of regional waveforms. Our inversion algorithm differs from previous studies in two major aspects. First, we evaluate the characteristics of background noise for individual stations. The results are then used to determine the lower corner of the frequency band used in the inversion to maximize the long-period information in waveforms. The higher corner is set at 0.06-0.08 Hz to avoid the effects of strong lateral heterogeneity and possible epicentral mislocation. Second, to further reduce the uncertainty caused by complex structures, a two-step procedure is adapted to select the best velocity models for different stations in calculating the synthetics. The inversion quality is classified by a combination of a letter (A-F) and a digit (1-4) reflecting the waveform misfit and the compensated linear vector dipole (CLVD) component, respectively. In total, source parameters of 36 events that occurred between July 1995 and December 1996 are reported in this study. For the few events that are big enough to be studied teleseismically, most of our solutions are consistent with those reported by other institutions. We intend to make our inversion results available on a routine basis that they will not only be able to provide precise source parameters for smaller regional earthquakes, but will also serve as an alternative to independently examine solutions of large and moderate-sized events reported from other sources.

Source processes of industrially‐induced earthquakes at The Geysers geothermal area, California

Microearthquake activity at The Geysers geothermal area, California, mirrors the steam production rate, suggesting that the earthquakes are industrially induced. A 15-station network of digital, three‐component seismic stations was operated for one month in 1991, and 3,900 earthquakes were recorded. Highly‐accurate moment tensors were derived for 30 of the best recorded earthquakes by tracing rays through tomographically derived 3-D [Formula: see text] and [Formula: see text] structures, and inverting P- and S-wave polarities and amplitude ratios. The orientations of the P- and T-axes are very scattered, suggesting that there is no strong, systematic deviatoric stress field in the reservoir, which could explain why the earthquakes are not large. Most of the events had significant non‐double‐couple (non-DC) components in their source mechanisms with volumetric components up to ∼30% of the total moment. Explosive and implosive sources were observed in approximately equal numbers, and must be caused by cavity creation (or expansion) and collapse. It is likely that there is a causal relationship between these processes and fluid reinjection and steam withdrawal. Compensated linear vector dipole (CLVD) components were up to 100% of the deviatoric component. Combinations of opening cracks and shear faults cannot explain all the observations, and rapid fluid flow may also be involved. The pattern of non-DC failure at The Geysers contrasts with that of the Hengill‐Grensdalur area in Iceland, a largely unexploited water‐dominated field in an extensional stress regime. These differences are poorly understood but may be linked to the contrasting regional stress regimes and the industrial exploitation at The Geysers.

Recent oceanic intraplate earthquake in Balleny Sea was largest ever detected

The largest oceanic intraplate earthquake ever detected and the largest event worldwide since 1994 occurred in a remote oceanic region near the Balleny Islands on March 25, 1998 (03:12:26 UT; Mw 8.2). An intraplate earthquake of this magnitude is extremely rare, especially for the Antarctic plate, which shows low seismicity.The earthquake occurred about 300 km west of the Balleny Transform between the Antarctic passive margin and the Australian‐Antarctic spreading center (Figure 1). The best double‐couple mechanism from the Harvard Centroid Moment Tensor (CMT) solution shows strike‐slip faulting with nodal planes trending north‐south and east‐west (Figure 1). The CMT solution also shows a large, compensated linear vector dipole (CLVD) component that can be interpreted as indicating northwest‐southeast extension.

Low-frequency Lg from NTS and Kazakh nuclear explosions—observations and interpretation

Abstract The generation of Lg from underground nuclear explosions is still not fully understood. Near-source scattering of explosion-generated Rg into S appears to be responsible for the low-frequency Lg from nuclear explosions at both Nevada test site (NTS) and Kazakh test site (KTS) (Gupta et al., 1991a, 1992). This mechanism has recently been supported by the analysis of regional data from several Yucca Flats, NTS explosions by Patton and Taylor (1995), who further indicate that the prominent low-frequency spectral null in Lg is due to Rg from a compensated linear vector dipole (CLVD) source. In this study, the dependence of spectral nulls on shot depths and other source parameters is first investigated by analyzing Lg from a large number of Yucca Flats (NTS) explosions, with known ground truth, recorded at several broadband stations. Analysis of low-frequency Lg from nuclear explosions at KTS also indicates spectral nulls that are in most cases distinct and identifiable, although generally not as strong as for the NTS shots. Methods of analysis include narrow bandpass filtering, spectral ratios, network averaging, and comparison with synthetic seismograms. Most Lg spectra indicate prominent nulls showing strong dependence on shot depth, and the network-averaged spectral nulls are in remarkably good agreement with those expected from Rg due to a CLVD source at about one-third the shot depth. By providing an improved understanding of Lg from explosions and its usefulness for obtaining source information, these results make an important contribution to regional discrimination and seismic-monitoring capabilities.

The effect of focal depth error on moment tensor inversion

In the determination of focal mechanism and rupture process of earthquake sources by using moment tensor inversion technique, it is difficult to guarantee the focal depth used in calculating the Green’s functions (theoretical focal depth) is exactly equal to the real focal depth. The difference between the theoretical and real focal depths, i. e., the focal depth error, will affect the moment tensor inversion to some extent. Using synthetic seismograms, the effect of the focal depth error on moment tensor inversion for three basic types of faults is discussed systematically. For the normal and thrust fault, the focal depth error mainly affects the explosive (EP) component and the compensated linear vector dipole (CLVD) component. In the case that the theoretical focal depth is greater than the real focal depth, the focal depth error causes a false positive EP component and a false negative CLVD component for the normal fault. However, it produces a false negative EP component and a false positive CLVD component for the thrust fault. The absolute values of the false EP and CLVD components for both normal fault and thrust fault cases increase with increasing focal depth error. In the case that the theoretical focal depth is smaller than the real focal depth, the focal depth error causes a false negative EP component and a false positive CLVD component for the normal fault. However, it produces a false positive EP component and a false negative CLVD component for the thrust fault. Similarly, the absolute values of the false EP and CLVD components for both normal fault and thrust fault cases increase with increasing focal depth error. For a pure strike-slip fault the focal depth error mainly affects the shape of source the function, unlike for the normal and thrust faults. The source time functions have artificially extended tails when either the theoretical focal depth is greater or smaller than the real focal depth. The numerical experiments show that the focal depth error less than 20 km has no significant effect on the overall focal mechanism of the earthquake. In addition, the effect of the focal depth error on the inversion result is slighter in case that the theoretical focal depth is greater than the real focal depth than in the case that the theoretical focal depth is smaller than the real focal depth.

Seismic monitoring of explosions: a method to extract information on the isotropic component of the seismic source

The design of a monitoring system for detecting explosions is a very topical problem, both for routine data processing at seismological observatories as well as for the monitoring of a Comprehensive Test Ban Treaty. In this framework it is desirable to have the possibility to quantify the presence of the isotropic component in the seismic source. For this purpose a method is presented, which is based on waveform inversion for the full moment tensor retrieval. The method inverts either full waveforms or separate seismic phases and returns the mechanism and time history of a point source. Moreover, it allows to redefine the hypocentral depth of the event and, in a simplistic way, to optimize the structural model as well. In order to model strong laterally heterogeneous structures, different pairs of structural models can be used for each source-receiver path. The source is decomposed into a volumetric part (V), representing an explosive or implosive component, and into a deviatoric part, containing both the double couple (DC) and the compensated linear vector dipole (CLVD) components. The method is applied to an area in central Switzerland and to the network of the Swiss Seismological Service. The events of interest include both earthquakes and explosions. Despite some modelling inadequacies of the source-time function, the explosions can be well identified with the inverted isotropic component in the source, as long as the number of stations used for the inversion is larger than three. The results of the inversion are better for large epicenter-station distances of the order of 40–90 km.

Seismic moment tensor resolution by waveform inversion of a few local noisy records-II. Application to the Phlegraean Fields (Southern Italy) volcanic tremors

Seismic sources with non-double-couple (non-DC) components are frequently observed in volcanic and geothermal areas. This suggests that the non-DC components may be generated by phenomena such as movements of magmatic fluids under high pressure and intrusions of dykes, which characterize volcanic areas and are believed to be responsible for volcanic tremors. Several volcanic tremors from the Phlegraean Fields, Southern Italy, have been processed with the waveform inversion method, which allows us to estimate the error on the source mechanism and on the source time function due to the noise present in the seismograms, the hypocentre mislocation and, in a simplistic way, an improper structure modelling. The events treated are examples of the seismicity accompanying the upward bradyseism of the area in 1984, and the following downward bradyseism in 1986. Significant non-DC components, i.e. a volumetric part and a compensated linear vector dipole (CLVD), are found in the mechanism, which indicates movement of magmatic fluids in the area.