МЕХАНІЗМ ВОГНИЩА ЛИТОВСЬКОГО ЗЕМЛЕТРУСУ НА ОСНОВІ ІНВЕРСІЇ ХВИЛЬОВИХ ФОРМ

Modeling of seismic wave propagation near the earth's surface

The aim of this work was to show that refined analyses of background, low magnitude seismicity allow to delineate the main active faults and to accurately estimate the directions of the regional tectonic stress that characterize the Southern Apennines (Italy), a structurally complex area with high seismic potential. Thanks the presence in the area of an integrated dense and wide dynamic network, was possible to analyzed an high quality microearthquake data-set consisting of 1312 events that occurred from August 2005 to April 2011 by integrating the data recorded at 42 seismic stations of various networks. The refined seismicity location and focal mechanisms well delineate a system of NW-SE striking normal faults along the Apenninic chain and an approximately E-W oriented, strike-slip fault, transversely cutting the belt. The seismicity along the chain does not occur on a single fault but in a volume, delimited by the faults activated during the 1980 Irpinia M 6.9 earthquake, on sub-parallel predominant normal faults. Results show that the recent low magnitude earthquakes belongs to the background seismicity and they are likely generated along the major fault segments activated during the most recent earthquakes, suggesting that they are still active today thirty years after the mainshock occurrences. In this sense, this study gives a new perspective to the application of the high quality records of low magnitude background seismicity for the identification and characterization of active fault systems. The analysis of the stress tensor inversion provides two equivalent models to explain the microearthquake generation along both the NW-SE striking normal faults and the E- W oriented fault with a dominant dextral strike-slip motion, but having different geological interpretations. We suggest that the NW-SE-striking Africa-Eurasia convergence acts in the background of all these structures, playing a primary and unifying role in the seismotectonics of the whole region.

Online Material: Additional figures. Focal mechanism and moment tensor determinations based on regional and local waveforms have become routine tasks in seismology. In recent years, considerable effort has been dedicated to extend software capabilities, in particular towards the inclusion of finite‐extent sources and easy, user‐friendly use. Two examples of these efforts are the FMNEAREG and KIWI software packages. Focal Mechanism from NEAr source to REGional distance records (FMNEAREG; Delouis et al. , 2008; Maercklin et al. , 2011) performs a grid search in order to find pure double‐couple sources, based on a 1D (line) finite‐source representation and complete elastodynamic wave field. The KIWI software (KInematic Waveform Inversion; Cesca et al. , 2010) allows the automatic retrieval of point‐source parameters, and for magnitudes above a given threshold kinematic finite‐fault rupture models can also be obtained. KIWI relies on a pre‐calculated database of Green’s functions. KIWI performs the moment tensor inversion in two steps, the first of which is based on the fit of amplitude spectra. The second step is based on time‐domain waveform fits. The ISOLA, ISOLated Asperities, software package (Sokos and Zahradnik, 2008) also performs waveform inversions to find source parameters. ISOLA is based on FORTRAN codes and offers a user‐friendly MATLAB graphical interface (GUI; www.mathworks.com/products/MATLAB). ISOLA allows for both single‐ and multiple‐point‐source iterative deconvolution (Kikuchi and Kanamori, 1991) inversion of complete regional and local waveforms. The moment tensor is retrieved through a least squares inversion, whereas the position and origin time of the point sources are grid searched. The computation options include inversion to retrieve the full moment tensor (MT), deviatoric MT, and pure double‐couple MT. Finite‐extent source inversions may also be performed by prescribing a priori the double‐couple mechanism (which will remain homogeneous over the fault plane). Green’s functions, including near‐field terms, are calculated using the discrete wavenumber method (Bouchon, …

“Modeling of seismic wave propagation is a core component in almost every aspect of exploration seismology, ranging from survey design methods to imaging and inversion algorithms. The last time SEG published a reprint volume on numerical modeling was in 1990. Since then, the last two decades has seen a step change in the application and use of Âfull wave equation modeling methods enabled by the tremendous increase in available computational power. Full waveform inversion, reverse time migration and 3D elastic finitedifference synthetic data generation are examples of modeling applications that are currently having a fundamental impact on our business. In Numerical Modeling of Seismic Wave Propagation: Gridded Two-way Wave-equation Methods, readers will find many of the wellknown and referenced papers from the exploration seismic community as well as some of the key papers that have impacted other fields of seismology. Because the modeling literature is vast, we have limited the scope of the reprint volume to papers over the last two decades on modeling methods based on the full wave equation. The reprint volume will be of particular interest to researchers and practitioners interested in modeling methods and their applications. The searchable CD includes the 114-page book of abstracts and the full papers.”

Focal mechanisms and moment magnitudes of micro-earthquakes in central Brazil by waveform inversion with quality assessment and inference of the local stress field

abstractThe treatment of the seismic source inverse problem, when diverse forms of waveform data are available, is simple and elegant using a moment tensor formalism. If earth structure is known and its effects predictable in terms of vertically inhomogeneous elastic-layered models, then all types of wave phenomena (e.g., surface waves, body waves, leaky modes, etc.) for a purely deviatoric moment tensor point source may be represented by, at most, a sum of three Green's functions. For an arbitrary symmetric moment tensor point source, one additional Green's function is needed for the P-SV system. However elegant this formalism may be for posing the linear inverse problem, the major difficulties lie in earth structure unknowns and resultant nonlinearities in the Green's functions which can cause significant trade-offs with source parameters. A hybrid inversion procedure is set up to gain insight into the probable unknowns in particular problems by incorporating both a linearized least-squares gradient method for the moment tensor or double couple, and smoothed time function parameters, and a nonlinear systematic trial-and-error search for moment tensor or double couple parameters for several assumptions of Green's function. The inversion technique is applied to near-regional waveform data from a small earthquake associated with the Koyna Reservoir which occurred 13 September 1967, as the second in a group of three events with very similar waveshapes, but differing amplitudes. The magnitudes of the second and third events are smaller by −0.2 and −0.8 units, respectively, compared to the first. The absolute magnitude for the first event is poorly constrained but is estimated to be 4.0 to 4.5 rather than the previously published value of 5.5 to 6.0. From the similarity of waveshapes, all three events are inferred to have the same mechanism and occurred within about 2 km of the same hypocenter. The results from moment tensor and double couple inversions for event 2 data indicate that source depth was 5 km and that left-lateral faulting occurred on a plane with a strike of N20°E ± 5°, dip of 90° ± 15°, and a rake of 0° ± 35°. The inferred far-field time function is approximately 3 sec in duration, unusually long for the seismic moment of 9 × 1022 dyne-cm, yielding a possible stress drop of about 0.05 bars. A fault map was constructed from LANDSAT image interpretation and shows a predominance of NNW to NNE striking faults in the Koyna area which is used to infer the appropriate nodal plane in the inversion results. These faults tend to define a broad en-echelon zone which parallels the Western Ghats in this area.

We report determinations of the focal mechanisms for Sakhalin earthquakes during 2006 through 2015 by two different methods: the polarity of first motions using the FOCMEC program and the inversion of waveforms using the ISOLA program. We show that the resulting moment tensor solutions fairly well fit the tectonic crustal stresses for Sakhalin as determined by previous workers. We have examined the component of the moment tensor that involves a linear dipole. This is at its maximum in areas of active mud volcanism and industrial activities in oil-and-gas fields.

New constraints on the seismotectonics and crustal structure of the Eastern Trans-Mexican Volcanic Belt based on local seismicity, focal mechanisms and a preliminary state of stress evaluation

<p>Geomechanical modelling is widely used to simulate the triggering of induced earthquakes in a gas-producing region, such as in the Groningen gas field. Dynamic simulation can provide information on the process of dynamic rupture during earthquake nucleation and on the generated seismic wavefield. Through geomechanical modelling, one can investigate the effects of the model parameters, e.g., depletion pattern and friction parameters. In the modelling, the dynamic rupture at a finite fault is simulated both in space and time. The resulting seismic wavefield from such a finite source should be more realistic than that from a point source. Previous studies on the inversion of induced-earthquake data in the Groningen area usually assumed a point source. In the present research, we implement the full moment tensor inversion of the synthetic waveforms caused by the dynamic rupture of a geomechanically simulated finite fault. We then link this moment tensor to the moment tensor obtained from the inversion of field-seismic data for an earlier earthquake, using the same inversion approach in both cases. The inverted moment tensor from the field seismic observation serves as a constraint to our geomechanical simulation. This enables us to perform a more realistic simulation of an induced earthquake.</p>

SUMMARY The analysis of aftershocks is more and more recognized by the seismological community as being an essential tool to improve the understanding of the earthquake faulting process, as well as of the various factors contributing to the generation of earthquake damage. In this context, the success or failure of aftershock monitoring programs depends to a large degree on the performance of the mobile seismic network deployed. In comparison with pure location networks, for aftershock monitoring networks the situation is complicated by the fact that they have to perform well for additional tasks such as determination of focal mechanisms, source-parameter estimation, tomographic experiments etc. We have developed a method to design optimum networks for aftershock recordings based on the technique of simulated annealing. If aftershock data are not yet available for the design of a mobile network, they are simulated for the optimization process from the mainshock-source parameters. Different objective functions to quantify network performance for each intended task are used. For the location problem it is based on the average confidence volume of hypocentral locations while for the focal mechanism determination it is derived from the distribution of rays on the focal sphere. For the tomography problem, the resolution matrix of the inverse problem is used. Furthermore, for each station individual noise levels can be assumed. Optimization for multiple tasks is performed in a sequential way with highest priority given to the location problem. The method has been applied to the Roermond earthquake of 1992 April 13. Optimum subsets of the existing ad hoc multinational temporary network for aftershock recordings of this event have been evaluated.

Waveform inversion and focal mechanisms of two weak earthquakes in Cordillera Principal (Argentina) between 35° and 35.5° S

<p>The determination of seismic moment tensors (MTs) for microseismicity poses challenges because of both the large number of events that are typically recorded, and their low signal to noise ratio. In recent years, automated moment tensor inversion methods have become more and more accurate, but an objective evaluation of their performance is often problematic due to the absence of site-specific, reference databases for comparison. In this study, we build a database of manually inverted MTs for the recent COSEIMIQ project, using the well-tested FociMT/HybridMT inversion method. COSEISMIQ focussed on microseismic monitoring in the Hellisheiði geothermal field, in the Hengill region, southern Iceland, where a dense network of 33 temporary seismic stations was deployed during 2018-2021, offering an ideal case study for microseismic MT inversion.</p><p>As a first step, we test the efficacy and possible pitfalls of the manual MT inversion on both a realistic and a simplified synthetic events waveform database. After careful, repeated manual tests, we observe that the inversion is robust across widely different choices of frequency band, but can be triggered to fail by not including key stations in some rare source-station geometries.</p><p>We then analyse the real data from the COSEISMIQ experiment, using previously located events from a large, recently developed microseismic catalog of the area. By running preliminary inversions of a subset of events in the centre of the deployment, we are able to pinpoint pre-processing steps that have a key effect on the MT inversion.  We find that in strong noise conditions such as in the Hengill region, the order and phase of the used frequency filter are fundamental parameters in correctly processing the P-wave onset used later for inversion.</p><p>After fine-tuning the event preprocessing, we select a larger subset of 197 events with magnitude > 0.8 from the catalog across the whole COSEISMIQ area, including several seismicity clusters at the edge of the deployment. We then pick all 197 events and invert them first with FociMT, then cluster the events based on their location using K-means clustering, and finally re-invert each cluster using HybridMT. The clustered inversion using HybridMT changes some MT solutions significantly, reducing the intra-cluster MT variance for most clusters. Interestingly, some event clusters show increased variance after the HybridMT inversion, suggesting that these include substantially different source mechanisms within a small area.</p><p>This new database of carefully inverted MT solutions can now be used as a test dataset to evaluate the performance of automated inversion tools.</p>

In this paper, the empirical Green's function is calculated based on a method that inverts the radiation patterns of earthquakes. The method utilizes several close-lying earthquakes with known focal mechanisms that are received by a common seismic station. The empirical Green's function between the given source area and the seismic station is then calculated by extracting common features and phases from earthquake waveforms. This is achieved through removing the effect of focal mechanisms from the waveforms and combining the remaining terms based on a damped least-squares approach. The resulting function represents the main characteristics of the propagation path after the waves have left the source area until they are received by a given seismic station. The application of the method is not limited by any requirement to make any separations of seismic phases and the earthquakes used are not required to have the same focal mechanisms. In this research the method is applied to two different data sets, including local Icelandic and regional Iranian earthquakes. In the latter data set the 1990 June 20 Iranian earthquake (Mw = 7.4) was modelled at broad-band seismic station KIV in Russia.

A systematic study towards understanding the correlation between polarization direction of crustal anisotropy with seismogenic stress field at different locations of the Shillong-Mikir Plateau and its vicinity in North East India is attempted. We used data from a 17-station broadband seismic network. In our earlier work , crustal anisotropic parameters were determined using ANISOMAT + for the 17 seismic stations. In this study, we have estimated stress field around the stations using focal mechanism solutions (FMS). Some 215 FMS are obtained by waveform inversion. These solutions are used for stress tensor inversion to estimate stress field around each location. It is observed that polarization direction of crustal anisotropy is consistent with that of the maximum horizontal stress (Ơmax) as well as the minimum horizontal stress (Ơmin). In addition to this, two orthogonal fast polarizations in some locations are also noted. The bivariate nature of correlations helps us to understand that the major mechanisms of seismic crustal anisotropy are not only due to the regional stress, but active faults and other geological conditions play a significant role in contemporary orientation of seismic crustal anisotropy and seismogenic stress field.

Trench-outer rise earthquakes occur by reactivation or creation of normal faults caused as the oceanic lithosphere approaches a subduction zone and bends into the deep-sea trench. These faults may cut deep enough into the mantle to allow sea water to penetrate into the lithosphere, causing serpentinization. The amount of water carried into the mantle is linked to the maximum depth that the tensional faults cut into the lithosphere, which in turn is directly linked to the maximum focal depths of outer rise normal faulting earthquakes. We analysed teleseismic P and S waves of seven earthquakes from the trench-outer rise offshore of Central America using teleseismic waveform inversion of broad-band data. For the computation of Green's functions for waveform inversion, probabilistic earthquake locations were calculated. To study the rupture process, earthquake centroid depths and focal mechanisms for a sequence of subevents were calculated. Both, hypocentral depths from the relocation process and the estimated centroid depths from the waveform inversion show that all events occur at shallow depths (<30 km). Furthermore, the locations of the subevents relative to each other suggest that fault planes for Mw∼ 6 are in the order of 50 km in length and only 5–10 km in width. Rupture generally propagates downdip and the focal mechanisms change for most events from normal faulting to strike-slip or oblique thrusting with time. The depth at which this mechanism change is observed may represent the depth of the nodal plane between tensional and compressional regions in the incoming plate.