ALBERT

Description: This paper presents the main recent results obtained by the seismological and geophysical monitoring arrays in operation in the rift of Corinth, Greece. The Corinth Rift Laboratory (CRL) is set up near the western end of the rift, where instrumental seismicity and strain rate is highest. The seismicity is clustered between 5 and 10 km, defining an active layer, gently dipping north, on which the main normal faults, mostly dipping north, are rooting. It may be interpreted as a detachment zone, possibly related to the Phyllade thrust nappe. Young, active normal faults connecting the Aigion to the Psathopyrgos faults seem to control the spatial distribution of the microseismicity. This seismic activity is interpreted as a seismic creep from GPS measurements, which shows evidence for fast continuous slip on the deepest part on the detachment zone. Offshore, either the shallowest part of the faults is creeping, or the strain is relaxed in the shallow sediments, as inferred from the large NS strain gradient reported by GPS. The predicted subsidence of the central part of the rift is well fitted by the new continuous GPS measurements. The location of shallow earthquakes (between 5 and 3.5 km in depth) recorded on the on-shore Helike and Aigion faults are compatible with 50° and 60° mean dip angles, respectively. The offshore faults also show indirect evidence for high dip angles. This strongly differs from the low dip values reported for active faults more to the east of the rift, suggesting a significant structural or rheological change, possibly related to the hypothetical presence of the Phyllade nappe. Large seismic swarms, lasting weeks to months, seem to activate recent synrift as well as pre-rift faults. Most of the faults of the investigated area are in their latest part of cycle, so that the probability of at least one moderate to large earthquake (M = 6 to 6.7) is very high within a few decades. Furthermore, the region west to Aigion is likely to be in an accelerated state of extension, possibly 2 to 3 times its mean interseismic value. High resolution strain measurement, with a borehole dilatometer and long base hydrostatic tiltmeters, started end of 2002. A transient strain has been recorded by the dilatometer, lasting one hour, coincident with a local magnitude 3.7 earthquake. It is most probably associated with a slow slip event of magnitude around 5 ± 0.5. The pore pressure data from the 1 km deep AIG10 borehole, crossing the Aigion fault at depth, shows a 1 MPa overpressure and a large sensitivity to crustal strain changes.

Description: Published

Description: 7-30

Description: 2T. Deformazione crostale attiva

Description: JCR Journal

Description: 〈span〉〈div〉ABSTRACT〈/div〉The 2017 North Korea test is analyzed together with the previous 2009–2016 tests, and a generalized source model is derived using waveform data. Data are represented by low‐frequency records of 11 broadband near‐regional stations (epicentral distances 140–310 km), bandpassed from 0.03 to 0.09 Hz. The events feature a significant degree of similarity. Therefore, mean records can be calculated by averaging the five tests, using the cross‐correlation shifts and amplitude scaling. The mean records are inverted for the full moment tensor in terms of its posterior probability density function. The mean‐source model reveals significant uncertainties and parameter tradeoffs, due to well‐known resolution problems at shallow depths and long wavelengths. Nevertheless, the moment tensor is undoubtedly dominated by its nonshear parts, that is, the isotropic component, and compensated linear vector dipole (inclined ∼15° to the vertical). The source type is very close to an opening crack, consistent with existing physical models of explosive shallow sources, accompanied by material damage. The generalized source model presented here is new. It can be used as a prior, realistically constrained model, applicable in early discriminations between natural earthquakes and explosions at the test site. Users at any station (not involved in this study) could precompute template synthetics in their preferred frequency ranges and velocity models. If fitting with real data by a single‐constant source scaling, a real‐time indication of an explosion similar to the previous Democratic People’s Republic of Korea (DPRK) tests can be obtained.〈/span〉

Description: We propose a new approach to resolve the isotropic component of the seismic moment tensor and its uncertainty. In linearized inversion problems, where the earthquake or explosive-source location and origin time are fixed (e.g., assumed to be known), the uncertainty of the moment tensor can be studied through the eigenvalues and the eigenvectors of the design matrix, which allows the representation of the theoretical misfit by means of a 6D error ellipsoid. Because the design matrix depends only on the structural model and receiver source geometry, the analysis can be performed using recorded seismic waveforms, or without. In the nonlinear inversion problems, where the free parameters are eight (e.g., the six elements of the moment tensor, depth, and origin time), we propose a waveform-inversion scheme in which the trace of the moment tensor varies systematically and the remaining seven free parameters are optimized for each specific value of the trace. In this way, a 1D experimental probability density function of the moment tensor trace is constructed. To demonstrate the applicability of the method, we apply it to two shallow earthquakes ( M w  4.9 and 4.7) with epicenters close to the Columbo volcano, located 20 km northeast of the island of Santorini, Aegean Sea, Greece. We use 15 near-regional (60–310 km) records at frequencies below 0.1 Hz and two alternative crustal models. We conclude that the main uncertainties are attributed to the crustal model and to the trade-off between the isotropic component and the source depth.

Description: 〈span〉〈div〉Abstract〈/div〉One of the major challenges for the moment tensor determination is associated with the relatively low‐magnitude events (Mw∼4) recorded by few regional stations at relatively large distances (300–600 km) and analyzed with standard velocity models of the region. Difficulties arise from the fact that synthetics in standard models (e.g., those routinely used in the location) cannot properly match real waveforms and favor the appearance of unmodeled time shifts and amplitude discrepancies (e.g., if VMs are constructed to minimize location residuals, they are not sensitive to uppermost shallow layers, which are insufficiently sampled by rays if shallow sources are missing). The situation is even worse when real waveforms can be matched but the retrieved focal mechanism is incorrect. This article investigates an alternative methodology that is more robust with respect to inappropriate velocity models: the inversion of waveform envelopes. The method is built on an empirical basis. It studies the effects of velocity models on synthetic waveforms and finds that the information about focal mechanism is encoded in the variation of the envelope shapes and amplitudes among the seismogram components. Besides synthetic tests, the method has been tested on real data comprising two earthquakes in Brazil: the 2010 Mw 4.3 Mara Rosa (MR) and the 2017 Mw 4.3 Maranhão earthquakes. When compared with solutions from previous studies, based on many polarities and 〈span〉ad hoc〈/span〉 path‐specific velocity models, we obtained in both cases the same mechanism with a single 1D model and a single‐station polarity constraint. The envelope inversion is a promising technique that might be useful in similar sparse networks, such as the one in Brazil, where standard waveform inversion, in general, is not fully efficient.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉One of the major challenges for the moment tensor determination is associated with the relatively low‐magnitude events (Mw∼4) recorded by few regional stations at relatively large distances (300–600 km) and analyzed with standard velocity models of the region. Difficulties arise from the fact that synthetics in standard models (e.g., those routinely used in the location) cannot properly match real waveforms and favor the appearance of unmodeled time shifts and amplitude discrepancies (e.g., if VMs are constructed to minimize location residuals, they are not sensitive to uppermost shallow layers, which are insufficiently sampled by rays if shallow sources are missing). The situation is even worse when real waveforms can be matched but the retrieved focal mechanism is incorrect. This article investigates an alternative methodology that is more robust with respect to inappropriate velocity models: the inversion of waveform envelopes. The method is built on an empirical basis. It studies the effects of velocity models on synthetic waveforms and finds that the information about focal mechanism is encoded in the variation of the envelope shapes and amplitudes among the seismogram components. Besides synthetic tests, the method has been tested on real data comprising two earthquakes in Brazil: the 2010 Mw 4.3 Mara Rosa (MR) and the 2017 Mw 4.3 Maranhão earthquakes. When compared with solutions from previous studies, based on many polarities and 〈span〉ad hoc〈/span〉 path‐specific velocity models, we obtained in both cases the same mechanism with a single 1D model and a single‐station polarity constraint. The envelope inversion is a promising technique that might be useful in similar sparse networks, such as the one in Brazil, where standard waveform inversion, in general, is not fully efficient.〈/span〉

Description: On 23 October 2011 an M w  7.1 earthquake occurred in eastern Turkey, close to the towns of Van and Ercis, causing more than 600 casualties and widespread structural damage. The earthquake ruptured a 60–70 km long northeast–southwest fault with a thrust mechanism, in agreement with regional tectonic stress regime. We studied the fault process of the event and the recorded ground motions using different sets of data. Regional records (0.005–0.010 Hz) are used to constrain the centroid moment tensor solution. Near-regional data, 100–200 km from the fault, are used for relocation of the hypocenter and, in the frequency range 0.05–0.15 Hz, for inversion of the rupture propagation by two methods: multiple point-source model (ISOLA) and multiple finite-extent (MuFEx) source model. MuFEx also provides an estimate of the model uncertainty, which is quite large due to unfavorable station distribution. We arrive at several plausible scenarios (equally well fitting the observed data including Global Positioning System coseismic displacements) with different styles of the rupture propagation. A few alternative source models are used for broadband (0.1–10 Hz) ground-motion simulations by means of the hybrid integral-composite source model. Only models comprising source complexities, such as a delayed rupture of shallow asperities, enable explanation of the acceleration record at the only available near-fault station, which exhibits a long duration and two prominent wave groups. These complex rupture models are used to simulate the ground motion in the near-fault area, specifically, at Van and Ercis, where records of the mainshock were missing, providing reasonable agreement with the observed spatial distribution of damage.

Description: For routine practice, we need simple tools to reliably identify earthquakes with large isotropic (ISO) components. This study aims to highlight a possible indicator. Non-double-couple (non-DC) components of moment tensors (MTs) play a key role in our understanding of faulting earthquake processes and/or in identifying explosions. As opposed to DC components of the calculated seismic source model, the non-DC components (compensated linear vector dipole and ISO) are more vulnerable to errors in location, inaccurate velocity modeling, and noise. Methods for analyzing resolvability of ISO are relatively complicated. We propose a simple procedure to identify an earthquake with a strong ISO component. Recent MT determinations include space and time grid search of the centroid position, mainly the depth and time. The centroid is identified with a trial source position that maximizes correlation between real and synthetic waveforms. In synthetic tests with varying ISO percentage, we compare the correlation-depth dependence for two types of MT inversion: full and deviatoric. We show that in the inversion of data with a significant ISO component under the deviatoric assumption (i.e., when ISO is neglected), we might obtain an inaccurate centroid depth. However, when we make the grid search twice, under the deviatoric-MT and full-MT assumptions, and compare the results, we can obtain an indication of the significant ISO and avoid depth bias. This straightforward method is applied to two shallow earthquakes in Greece (the 27 January 2012 M w  5.3 Cretan Sea earthquake and the 26 June 2009 M w  4.9 Santorini earthquake).

Description: The source complexity of the M w 7.1 (USGS) Van, Eastern Turkey, earthquake of 2011 October 23 is studied using full waveform inversions of seismic records at near-regional distances (120–220 km) and relatively low frequencies (0.05–0.15 Hz). The study relies on iterative deconvolution and on a new method in which pairs of point sources on the fault plane are systematically grid searched, and the moment-rate time functions of the two-point sources are simultaneously calculated by non-negative least-squares inversion. It is demonstrated on synthetic and real data that the wavefield in these ranges is sensitive enough to distinguish two main subevents of the Van earthquake, separated from each other by ~10–15 km and ~4 s. The double-event character of the Van earthquake is indicated even by a simplified single-point source model, optimally when the trial-point source is near the earthquake centroid. The simple indicators of source complexity developed in this paper are useful in practice in the first hours after an earthquake, when the source position is known only approximately and finite-fault models of slip evolution are not yet available.