SEISMICITY of the KOPETDAG REGION in 2015

In 2015, the seismicity of the Kopetdag region was monitored by the network of 32 seismic stations, including 28 digital and 4 analogue stations. The re-equipment of stationary analogue stations of Turkmenistan with digital GEOSIG equipment, which began in 2013, was continued in 2015 – 6 GEOSIG stations were added to 9 stations of this type, and the analogue equipment at the re-equipped stations was stopped. In 2015, the seismic activity A10 in the Kopetdag region was close to the background level for the period 1992–2014, while the number of weak events significantly exceeded the level of the previous year. The seismic activation along the boundaries of the crustal blocks in the north of the Iranian plate, which began in 2012, continued by the October 12, 2015 earthquake with KR=12.7, Mw=5.2. This strongest earthquake in the territory of Turkmenistan in 2015, named the Kenekesir earthquake by the name of the nearest settlement, accompanied by numerous aftershocks – more than 35000 events with KR=3–11 were located during 80 days after the mainshock within 30 km radius. The aftershock series lasted 186 days and ended in 2016. According to the complex of instrumental seismological and tectonic data, oblique-slip with equal normal and strike-slip components occurred in the source of the mainshock. The rupture plane had a southwestern strike and dipped to the northwest. The maximum ("Bath’s") aftershock occurred on November 16 with KR=11.1. Judging by its remoteness from the mainshock in space and time, and the difference in the type of movement in the source (upthrust), it was caused by stress relaxation in the environment.

Precise Locating of the Great 1897 Shillong Plateau Earthquake Using Teleseismic and Regional Seismic Phase Data

Pinched between the Eastern Himalaya and the Indo-Burman ranges, the Shillong Plateau represents a zone of distributed deformation with numerous visible and buried active faults. In 1897, a great (magnitude 8+) earthquake occurred in the area, and although a subsurface rupture plane has been proposed geodetically, its epicenter remained uncertain. We gathered original arrival time data of seismic waves from this early-instrumental era and combined them with modern, 3D velocity models to constrain the origin time and epicenter of this event, including uncertainties. Our results show that the earthquake has taken place in the northwest part of the plateau, at the junction of the short, surface-rupturing Chedrang fault and the buried Oldham fault (26.0°N, 90.7°E). This latter fault has been proposed earlier based on geodetic data and is long enough to host a great earthquake. Rupture has most likely propagated eastward. Stress change from the 1897 earthquake may have ultimately triggered the 1930 M 7.1 Dhubri earthquake, along a fault connecting the Shillong Plateau with the Himalaya.

SMALL-AMPLITUDE DISCONTINUOUS DISTURBANCES IN PLEISTOCENE DEPOSITS OF THE VOLYN-PODILSKA UPLAND

The paper presents the results of the study of the small-amplitude discon¬tinuous disturbances of the possibly cryogenic (thermokarst) origin. The dislocations were found in the outcrops of Middle and Upper Pleistocene sediments of the Volyn-Podilska Upland, accumulated in periglacial or sub-periglacial conditions. The distur¬bances are represented mostly by the micro-normal faults and also by sheared fractures and are very similar to tectonic (seismogenic) discontinuities. The tectonotypic fractures in the near-surface deposits of the Pleistocene terraces of Western Bug and Styr (five sections within Volhynian Upland, four of them – in the valley of Bug), as well as in the cover of the Late Pleistocene sediments on the slope of the valley of Dniester (Galician Prydnisterya) are subjected to consideration, analysis and interpretation. In the last location the ruptures are represented mostly by the dis¬turbances identified as sheared fractures. In all others there are small-amplitude normal faults. One reverse fault, timed to an ice-wedge cast, was also revealed. Typical micro-normal faults of all sections are steep and have a number of other common features, which testifies to the same or almost identical mechanism of their formation. These features, in particular, are as follows: 1) insignificant (usually up to 2–2.5 m) length in cross-section and small (several centimeters) amplitude of displacement along the rupture plane; 2) gradual attenuation of the fractures up and down the section. All micro-normal faults are confined to sediments (thicknesses) that are partially or completely composed of sand. The formation of the micro-normal faults and other examined ruptures can be ex¬plained by the uneven compaction and the gravitational subsidence of the rocks, and in the section on the slope of the Dniester valley – also by their displacement down along the slope. It is probable that these processes occurred due to: 1) the degradation of the permafrost; 2) the dehydration of the sand deposits during a significant decrease in the groundwater levels; 3) the melting of the buried layers and lenses of snow, which were accumulated during the winter season in the thickness of sandy the niveo-aeolian deposits. In the outcrops of this terrace, they occur no less frequently than the confidently identified ice wedge pseudomorphs. Key words: small-amplitude disturbances; microfaults; thermokarst; Volyn-Podilska Upland.

Domino-style earthquakes along blind normal faults in Northern Thessaly (Greece): kinematic evidence from field observations, seismology, SAR interferometry and GNSS

Here we present a joint analysis of the geodetic, seismological and geological data of the March 2021 Northern Thessaly seismic sequence, that were gathered and processed as of April 30, 2021. First, we relocated seismicity data from regional and local networks and inferred the dip-direction (NE) and dip-angle (38°) of the March 3, 2021 rupture plane. Furthermore, we used ascending and descending SAR images acquired by the Sentinel-1 satellites to map the co-seismic displacement field. Our results indicate that the March 3, 2021 Mw=6.3 rupture occurred on a NE-dipping, 39° normal fault located between the villages Zarko (Trikala) and Damasi (Larissa). The event of March 4, 2021 occurred northwest of Damasi, along a fault oriented WNW-ESE and produced less deformation than the event of the previous day. The third event occurred on March 12, 2021 along a south-dipping normal fault. We computed 22 focal mechanisms of aftershocks with M≥4.0 using P-wave first motion polarities. Nearly all focal mechanisms exhibit normal kinematics or have a dominant normal dip-slip component. The use of InSAR was crucial to differentiate the ground deformation between the ruptures. The majority of deformation occurs in the vertical component, with a maximum of 0.39 m of subsidence over the Mw=6.3 rupture plane, south and west of Damasi. A total amount of 0.3 m horizontal displacement (E-W) was measured. We also used GNSS data (at 30-s sampling interval) from twelve permanent stations near the epicentres to obtain 3D seismic offsets of station positions. Only the first event produces significant displacement at the GNSS stations (as predicted by the fault models, themselves very well constrained by InSAR). We calculated several post-seismic interferograms, yet we have observed that there is almost no post-seismic deformation, except in the footwall area (Zarkos mountain). This post-seismic deformation is below the 7 mm level (quarter of a fringe) in the near field and below the 1 mm level at the GNSS sites. The cascading activation of the three events in a SE to NW direction points to a pattern of domino-style earthquakes, along neighbouring fault segments. The kinematics of the ruptures point to a counter-clockwise change in the extension direction of the upper crust (from NE-SW near Damasi to N-S towards northwest, near Verdikoussa).

Combined Geodetic and Seismological Study of the December 2020 Mw = 4.6 Thiva (Central Greece) Shallow Earthquake

On 2 December 2020, a moderate and shallow Mw = 4.6 earthquake occurred in Boeotia (Central Greece) near the city of Thiva. Despite its magnitude, the co-seismic ground deformation field was detectable and measurable by Sentinel-1, ascending and descending, synthetic aperture interferometry radar (InSAR) acquisitions. The closest available GNSS station to the epicenter, located 11 km west, measured no deformation, as expected. We proceeded to the inversion of the deformation source. Moreover, we reassessed seismological data to identify the activated zone, associated with the mainshock and the aftershock sequence. Additionally, we used the rupture plane information from InSAR to better determine the focal mechanism and the centroid location of the mainshock. We observed that the mainshock occurred at a shallower depth and the rupture then expanded downdip, as revealed by the aftershock distribution. Our geodetic inversion modelling indicated the activation of a normal fault with a small left-lateral component, length of 2.0 km, width of 1.7 km, average slip of 0.2 m, a low dip angle of 33°, and a SW dip-direction. The inferred fault top was buried at a depth of ~0.5 km, rooted at a depth of ~1.4 km, with its geodetic centroid buried at 1.0 km. It was aligned with the Kallithea fault. In addition, the dip-up projection of the modeled fault to the surface was located very close (~0.4 km SW) to the mapped (by existing geological observations) trace of the Kallithea fault. The ruptured area was settled in a transition zone. We suggest the installation of at least one GNSS and seismological station near Kallithea; as the activated zone (inferred by the aftershock sequence and InSAR results) could yield events with M≥5.0, according to empirical laws relating to rupture zone dimensions and earthquake magnitude.

A transient in surface motions dominated by deep afterslip subsequent to a shallow supershear earthquake: the 2018 Mw 7.5 Palu case.

<div> <div> <div> <p>The 2018 <em>M<sub>w</sub></em> 7.5 Palu earthquake is a remarkable strike-slip event due to its nature as a shallow supershear fault rupture across several segments and a destructive tsunami that followed co-seismic deformation. GPS offsets in the wake of the 2018 earthquake display a transient in the surface motions of northwest Sulawesi. A Bayesian approach identifies (predominantly a-seismic) deep afterslip on and below the co-seismic rupture plane as the dominant physical mechanism causing the cumulative, post-seismic, surface displacements whereas viscous relaxation of the lower crust and poro-elastic rebound contribute negligibly. We confirm a correlation between shallow supershear rupture and post-seismic surface transients with afterslip activity in the zone below an inter-seismically locked fault plane where the slip rate tapers from zero to creeping.</p> </div> </div> </div>

The March 25, 2020 MW 7.5 Paramushir earthquake

The strong earthquake with moment magnitude Mw = 7.5 occurred on March 25, 2020, in the North Kurils to the southeast of the Paramushir Island. The hypocenter of the earthquake was located under the oceanic rise of deep-sea trench in the subducting Pacific lithospheric plate. This earthquake has been the strongest seismic event since 1900 for an area about 800 km long of the outer rise of the trench. It also was the strongest earthquake for the 300-kilometer long area of the Kuril-Kamchatka subduction zone adjacent to the epicenter. The article summarizes the data on the Paramushir earthquake. Tectonic position of the earthquake, source parameters, features of the aftershock process development, as well as coseismic displacement of the nearest continuous GNSS station are considered. The performed analysis did not allow us to clearly determine the rupture plane in the source. Nevertheless, the study of the features of the outer-rise earthquake is a matter of scientific interest, since the stress state of the bending area of the subducting Pacific lithospheric plate reflects the interplate interaction in the subduction zone.

Deep learning of the aftershock hysteresis effect based on the elastic dislocation theory

This paper selects fault source models of typical earthquakes across the globe and uses a volume extending 100 km horizontally from each mainshock rupture plane and 50 km vertically as the primary area of earthquake influence for calculation and analysis. A deep neural network is constructed to model the relationship between elastic stress tensor components and aftershock state at multiple timescales, and the model is evaluated. Finally, based on the aftershock hysteresis model, the aftershock hysteresis effect of the Wenchuan earthquake in 2008 and Tohoku earthquake in 2011 is analyzed, and the aftershock hysteresis effect at different depths is compared and analyzed. The correlation between the aftershock hysteresis effect and the Omori formula is also discussed and analyzed. The constructed aftershock hysteresis model has a good fit to the data and can predict the aftershock pattern at multiple timescales after a large earthquake. Compared with the traditional aftershock spatial analysis method, the model is more effective and fully considers the distribution of actual faults, instead of treating the earthquake as a point source. The expansion rate of the aftershock pattern is negatively correlated with time, and the aftershock patterns at all timescales are roughly similar and anisotropic.

Pseudo-probabilistic identification of fracture network in seismic clouds driven by source parameters

SUMMARY Fracture networks in underground reservoirs are important pathways for fluid flow and can therefore be a deciding factor in the development of such reservoirs for geothermal energy, oil and gas production or underground storage. Yet, they are difficult to characterize since they usually cannot be directly accessed. We propose a new method to compute the likelihood of having a fracture at a given location from induced seismic events and their source parameters. The result takes the form of a so-called pseudo-probabilistic fracture network (PPFN). In addition to the hypocentres of the seismic events used to image the fracture network, their magnitudes and focal mechanisms are also taken into account, thus keeping a closer link with the geophysical properties of the rupture and therefore the geology of the reservoir. The basic principle of the PPFN is to estimate the connectivity between any spatial position in the cloud and the seismic events. This is done by applying weighting functions depending on the distance between a seismic event and any location, the minimum size of the rupture plane derived from the event magnitude, and the orientation of the rupture plane provided by the focal mechanism. The PPFN is first tested on a set of synthetic data sets to validate the approach. Then, it is applied to the seismic cloud induced by the deep hydraulic stimulation of the well GPK2 of the enhanced geothermal site of Soultz-sous-Forêts (France). The application on the synthetic data sets shows that the PPFN is able to reproduce fault planes placed in a cloud of randomly distributed events but is sensitive to the free parameters that define the shape of the weighting functions. When these parameters are chosen in accordance with the scale of investigation, that is, the typical size of the structures of interest, the PPFN is able to determine the position, size and orientation of the structure quite precisely. The application of the PPFN to the GPK2 seismic cloud reveals a large prominent fault in the deep-northern part of the seismic cloud, supporting conclusions from previous work, and a minor structure in the southern upper part, which could also be a branch of the main fault.