Hypocentral Depth Research Articles

Repeatable Source, Path, and Site Effects from the 2019 M 7.1 Ridgecrest Earthquake Sequence

ABSTRACTWe use a large instrumental dataset from the 2019 Ridgecrest earthquake sequence (Rekoske et al., 2019, 2020) to examine repeatable source-, path-, and site-specific ground motions. A mixed-effects analysis is used to partition total residuals relative to the Boore et al. (2014; hereafter, BSSA14) ground-motion model. We calculate the Arias intensity stress drop for the earthquakes and find strong correlation with our event terms, indicating that they are consistent with source processes. We look for physically meaningful trends in the partitioned residuals and test the ability of BSSA14 to capture the behavior we observe in the data.We find that BSSA14 is a good match to the median observations for M>4. However, we find bias for individual events, especially those with small magnitude and hypocentral depth≥7 km, for which peak ground acceleration is underpredicted by a factor of 2.5. Although the site amplification term captures the median site response when all sites are considered together, it does not capture variations at individual stations across a range of site conditions. We find strong basin amplification in the Los Angeles, Ventura, and San Gabriel basins. We find weak amplification in the San Bernardino basin, which is contrary to simulation-based findings showing a channeling effect from an event with a north–south azimuth. This and an additional set of ground motions from earthquakes southwest of Los Angeles suggest that there is an azimuth-dependent southern California basin response related to the orientation of regional structures when ground motion from waves traveling south–north are compared with those in the east–west direction. These findings exhibit the power of large, spatially dense ground-motion datasets and make clear that nonergodic models are a way to reduce bias and uncertainty in ground-motion estimation for applications like the U.S. Geological Survey National Seismic Hazard Model and the ShakeAlert earthquake early warning System.

Stress-Drop and Source Scaling of the 2019 Ridgecrest, California, Earthquake Sequence

ABSTRACTStress drop, while difficult to measure reliably and at scale, is a key source parameter for understanding the earthquake rupture process and its relationship to strong ground motion. Here, we use a P-wave spectral decomposition approach, designed for large and densely sampled datasets, to measure earthquake stress drop in the region surrounding the 2019 Ridgecrest, California, earthquake sequence. With more than 11,000 measurements of earthquake stress drop in the 20-yr time period from 2000 through 2019, this dataset provides an opportunity to understand how coseismic stress changes and how other geophysical factors relate to the distribution of stress drop and its evolution in space and time. We observe a mild but persistent deviation from self-similar scaling, with larger events having systematically higher stress drops, though this trend depends on the assumption of an omega-square source spectral model. Earthquake stress drop increases with hypocentral depth in this study region, and the Ridgecrest aftershocks tend to have higher stress drops than the pre-event seismicity. This is in part due to their deeper hypocenters. Coherent spatial patterns of stress drop in the aftershock sequence correlate with the slip distribution of the M 7.1 mainshock, whose northwest rupture tip terminated in a long-lived zone of enervated stress drop. Although physical interpretation of these results is complicated by the trade-offs between the timing, depth, and location of these earthquakes, the observations provide new insight into the physics of the earthquake source in an area of renewed seismic activity in southern California.

Absolute Location of 2019 Ridgecrest Seismicity Reveals a Shallow Mw 7.1 Hypocenter, Migrating and Pulsing Mw 7.1 Foreshocks, and Duplex Mw 6.4 Ruptures

ABSTRACTThe 2019 Ridgecrest, California, sequence includes an Mw 6.4 earthquake on 4 July and an Mw 7.1 mainshock 34 hr later. We perform absolute location of Mw≥1.0 Ridgecrest events using multiple velocity models, station corrections, and a location algorithm robust to velocity model and arrival-time error. The obtained seismicity is mainly ∼3–12 km deep, with few shallower events. The Mw 6.4 hypocenter is ∼12 km deep, compatible with hypocentral depths of most Mw≥6 earthquakes in southern California. The Mw 7.1 hypocenter, however, is unusually shallow at ∼4 km. The immediate post-Mw 6.4 seismicity defines a deep, ∼12 km long, southeast–northwest structure containing the Mw 6.4 hypocenter and a shallower, orthogonal, ∼18 km long northeast–southwest structure. These structures have little or no intersection, making the Mw 6.4 event a double earthquake, rupturing first the deeper and then the shallower structure. The ensuing, pre-Mw 7.1 seismicity extends the southeast–northwest structure northwestwards to within ∼3 km of the future Mw 7.1 hypocenter and illuminates a new crossing structure, whereas small clusters of events within 2 km of the future Mw 7.1 hypocenter activate 3–4 times in pulses from a few hours after the Mw 6.4 event through Mw 7.1 initiation. This pre-Mw 7.1 seismicity suggests Mw 7.1 rupture initiation activated as an event in the pulsing clusters, and early Mw 7.1 rupture growth was primed by stress changes from the Mw 6.4 rupture and its aftershocks. Moreover, shallow Mw 7.1 nucleation, for which spontaneous rupture growth into a large earthquake is not expected, may have required this incitation by the Mw 6.4 events, a significant complication for hazard estimation. Otherwise, Mw 7.1-like rupture might not have occurred until much later, perhaps with nucleation at greater depth. The Ridgecrest seismicity defines additional structures around and crossing the main Mw 6.4 and 7.1 rupture zones, but some of this seismicity likely shows delayed activity on pre-existing faults due to stress changes from the main events and not rupture complexity during the larger events.

Cascading Partial Rupture of the Flores Thrust during the 2018 Lombok Earthquake Sequence, Indonesia

AbstractA series of four Mw>6 earthquakes struck the northern region of Lombok, eastern Indonesia, in a span of three weeks from late July to mid-August 2018. The series was thought to be associated with the Flores thrust, but the exact mechanism causing the unusual earthquake series has remained elusive. Our Interferometric Synthetic Aperture Radar analysis, combined with insights from seismology, indicates that the events originated at different hypocenter depths with differing fault geometries, which may explain the cascading behavior of the events, and indicates that better imaging of active fault geometry might provide some insight into future rupture behavior on other similar thrust systems. Our static stress change calculations suggest that the earlier events in the sequence played a role in promoting the later events. In addition, the second event brought the most significant impact on a nearby volcano, by causing volumetric expansion at its shallow magma plumbing system and unclamping its magma ascent zone, which may potentially have an impact on its future eruptive activity. However, no volcanic activity has so far occurred after the earthquakes. Finally, our damage proxy maps suggest that the second event caused the greatest damage to buildings.

The Near‐Fault Ground Motion Characteristics of Sustained and Unsustained Free Surface‐Induced Supershear Rupture on Strike‐Slip Faults

AbstractThe free surface is shown to be one of the key factors that may promote supershear rupture propagation on strike‐slip faults even if its initial shear stress is not larger enough as predicted by the Burridge‐Andrews mechanism. However, previous study has shown the free surface‐induced supershear rupture may be unsustained, which turns to sub‐Rayleigh rupture itself as the rupture propagates. We study the near‐field ground motion of sustained and unsustained supershear ruptures using the finite difference method based on the dynamic rupture processes of three vertical strike‐slip faults with the same initial stresses and different hypocenter depths. Both the unsustained supershear ruptures with shallower hypocenter depths show the sub‐Rayleigh characteristics in the peak ground velocity distribution, that is, strong amplitude is noticed beyond the end of the fault without any observed Mach cone. We observe that the arrival time differences between the maximum fault‐perpendicular and fault‐parallel velocity reveal the Mach cone clearly for the sustained supershear rupture. A distinct supershear phase in the high‐frequency seismograms is observed. We also compare the 70° dip strike‐slip case in which unsustained supershear rupture may still present sub‐Rayleigh characteristics in the near‐field ground motion, and the break of symmetry perturbs the normal stress as rupture propagates, which has a key influence in the rupture propagation and ground motion. Our work provides a new insight to understand the supershear rupture in nature earthquakes and the relationship between the ground motion and the rupture process on the fault.

The Occurrence of a Lower Viscosity Layer in the Crust of Old Cratons as a Cause of the Strongly Differentiated Character of Postglacial Uplift

Rapid glacio-isostatic rebound in Fennoscandia and Canada that is nonuniform in time and space indicates that there is a layer with strongly decreased viscosity at shallow crustal depths. The upper boundary of the layer is near the depth of 15 km, which corresponds to the maximum depth of earthquake hypocenters in the Precambrian cratons of the Kola Peninsula and Karelia. The position of the lower boundary is less distinct; however, most likely it is located near the base of the crust. The formation of such a layer in the Pliocene–Quaternary occurred due to infiltration of a large volume of mantle fluids into the crust. In many regions, this has led to retrograde metamorphism with rock expansion and a strong decrease in rocks viscosity.

Practical limitations of earthquake early warning

Earthquake early warning (EEW) entails detection of initial earthquake shaking and rapid estimation and notification to users prior to imminent, stronger shaking. EEW (ShakeAlert Phase 1, version 2.0) went operational in California in October 2019 and is coming to the rest of the U.S. West Coast. But what are the technical and social challenges to delivering actionable information on earthquake shaking before it arrives? Although there will be tangible benefits, there are also limitations. Basic seismological principles, alert communication challenges, and potential response actions, as well as substantial lessons learned from the use of EEW in Japan, point to more limited opportunities to warn and protect than perhaps many expect. This is in part because potential warning times vary by region and are influenced by tectonic environment, hypocentral depth, and the fault’s proximity to the alert user. For the U.S. West Coast, particularly for crustal earthquakes, warning times are shorter—and possible mitigation actions are likely to be less effective—than often maintained. Nevertheless, EEW is an additional arrow in the quiver of earthquake information tools available in the service of earthquake risk reduction. What is called for, then, is transparency and balance in the EEW discussion: along with its potential, the acknowledgment of EEW’s inherent and practical limitations is needed. Recognizing these limitations could, in fact, make EEW implementation more successful as part of a holistic earthquake mitigation strategy, where its role among other earthquake information tools is quite natural.

Watts Bar Nuclear Power Plant Strong-Motion Records of the 12 December 2018 M 4.4 Decatur, Tennessee, Earthquake

Abstract The moment magnitude M 4.4 on 12 December 2018 Decatur, Tennessee, earthquake occurred in the eastern Tennessee seismic zone. Although the causative fault is not known, the earthquake had a predominantly strike-slip mechanism with an estimated hypocentral depth of about 8 km. It was felt over a distance of 500 km stretching from Southern Kentucky to Georgia. Strong shaking, capable of causing slight damage, was reported near the epicenter. The Watts Bar nuclear power plant (NPP) is only 4.9 km from the epicenter of the earthquake and experienced only slight shaking. The earthquake was recorded by the plant’s seismic strong-motion instrumentation installed at four different locations. Near-real-time calculations by the plant operators indicated that the operating basis earthquake (OBE) ground motion was not exceeded during the earthquake. We obtained and processed the recorded motions to calculate corrected accelerations, velocities, and displacements. In addition, we computed the Fourier and 5% damped response spectra to compare them with the plant’s OBE. Comparisons of the ground-motion prediction models with the digital recordings at the plant site indicated that recorded ground motions were significantly below the predicted results calculated using the ground-motion prediction models approved for regulatory use. Availability of high-quality, digital recordings in this case helped make a quick decision about the ground motions not exceeding the OBE and hence prevented unnecessary shutdown of the NPP. Availability of earthquake recordings from the four locations in the NPP also presented an opportunity to analyze the linear response of plant structures.

Using seismic attributes in seismotectonic research: an application to the Norcia <i>M</i><sub>w</sub> = 6.5 earthquake (30 October 2016) in central Italy

Abstract. In seismotectonic studies, seismic reflection data are a powerful tool to unravel the complex deep architecture of active faults. Such tectonic structures are usually mapped at the surface through traditional geological surveying, whilst seismic reflection data may help to trace their continuation from the near surface down to hypocentral depths. On seismic reflection data, seismic attributes are commonly used by the oil and gas industry to aid exploration. In this study, we propose using seismic attributes in seismotectonic research for the first time. The study area is a geologically complex region of central Italy, struck during 2016–2017 by a long-lasting seismic sequence, including a Mw 6.5 main shock. Three vintage seismic reflection profiles are currently the only ones available at the regional scale across the epicentral zone. These represent a singular opportunity to attempt a seismic attribute analysis by running attributes like the “energy” and the “pseudo-relief”. Our results are critical, as they provide information on the relatively deep structural setting, mapping a prominent, high-amplitude regional reflector interpreted as the top of basement, which is an important rheological boundary. Complex patterns of high-angle discontinuities crossing the reflectors have also been identified by seismic attributes. These steeply dipping fabrics are interpreted as the expression of fault zones belonging to the active normal fault systems responsible for the seismicity of the region. Such peculiar seismic signatures of faulting are consistent with the principal geological and tectonic structures exposed at surface. In addition, we also provide convincing evidence of an important primary tectonic structure currently debated in the literature (the Norcia antithetic fault) as well as several buried secondary fault splays. This work demonstrates that seismic attribute analysis, even if used on low-quality vintage 2D data, may contribute to improving the subsurface geological interpretation in areas characterized by limited and/or low-quality subsurface data but with potentially high seismic hazard.

Seismicity of the Erguna Region (Northeastern China): Evidence for Local Stress Redistribution

ABSTRACTWe consider the seismicity of the Erguna region in northeast China (48°–51° N, 117°–123° E) which is poorly studied from seismological point of view as it is characterized by a low level of seismic activity. We calculate focal parameters (focal mechanisms, scalar seismic moments, moment magnitudes, and hypocentral depths) for seven regional earthquakes with Mw 4.2–4.6 that occurred in 2000–2017 using global seismic data of Rayleigh- and Love-wave amplitude spectra and P-wave first-motion polarities recorded at regional stations. It has been shown that the study earthquakes are of small magnitudes (Mw 4.2–4.6), of various hypocentral depths (3–37 km), and are characterized by different kinematics in their sources (normal and thrust faults, strike slips). The different faulting mechanisms could reflect local stress redistribution in small-scale crustal blocks bordered by local short-length nonconnecting faults. The available geophysical and geological data evidence that the observed features of the seismic process in the Erguna region—low-seismic activity and inhomogeneity of the stress-strain field—are likely to be controlled by the structure of the crust and the upper mantle.

The November 2019 Seismic Sequence in Albania: Geodetic Constraints and Fault Interaction

The seismic sequence of November 2019 in Albania culminating with the Mw = 6.4 event of 26 November 2019 was examined from the geodetic (InSAR and GNSS), structural, and historical viewpoints, with some ideas on possible areas of greater hazard. We present accurate estimates of the coseismic displacements using permanent GNSS stations active before and after the sequence, as well as SAR interferograms with Sentinel-1 in ascending and descending mode. When compared with the displacements predicted by a dislocation model on an elastic half space using the moment tensor information of a reverse fault mechanism, the InSAR and GNSS data fit at the mm level provided the hypocentral depth is set to 8 ± 2 km. Next, we examined the elastic stress generated by the Mw = 7.2 Montenegro earthquake of 1979, with the Albania 2019 event as receiver fault, to conclude that the Coulomb stress transfer, at least for the elastic component, was too small to have influenced the 2019 Albania event. A somewhat different picture emerges from the combined elastic deformation resulting after the two (1979 and 2019) events: we investigated the fault geometries where the Coulomb stress is maximized and concluded that the geometry with highest induced Coulomb stress, of the order of ca. 2–3 bar (0.2–0.3 MPa), is that of a vertical, dextral strike slip fault, striking SW to NE. This optimal receiver fault is located between the faults activated in 1979 and 2019, and very closely resembles the Lezhe fault, which marks the transition between the Dinarides and the Albanides.

Newly emerging cases of hydraulic fracturing induced seismicity in the Duvernay East Shale Basin

Hydraulic fracturing (HF) in the Duvernay Formation near Fox Creek, Alberta, Canada, has produced some of the most prolific clusters of induced seismicity. In this paper, we describe newly emerging clusters of events occurring in previously quiescent (and undeveloped) areas of the Duvernay Formation. In the Duvernay East Shale Basin, an industry supplemented waveform database is examined using a variety of seismological techniques. Here, strike-slip events as large as 4.18 ML have been felt in nearby cities. Temporal relationships suggest these clusters are almost certainly (>99.7% confidence) caused by nearby HF. Spatially, these earthquakes tend to occur in the strata directly overlying the Duvernay Formation and laterally near a horizontal well. One exceptional cluster clearly delineates a 1.5 km offset between its centroid and the inducing horizontal well, raising questions about the earthquake triggering mechanism. Westward in the Duvernay, two minor clusters within the Willesden Green region appear to be tenuously related to HF completions. This study also places constraints on the structural geology of faulting occurring in the Rocky Mountain House Seismogenic Zone – a region of induced seismicity related to secondary recovery in the Strachan D-3A pool. Here, our moment tensors and double difference relocations describe thrust-slip on a fault underlying the target formation. Last, the largest event (4.27 ML) in our study area was likely natural, due to its deeper hypocentral depth (~15 km) and non-correspondence with industrial operations. Overall, the newly emerging clusters are consistent with reactivation of basement rooted faults.

How do detachment faults form at ultraslow mid-ocean ridges in a thick axial lithosphere?

Slow-spreading mid-ocean ridges develop large offset normal faults, or detachments, that exhume mantle-derived peridotites to the seafloor. Successive detachments that flip polarity (flip-flop detachments) have been documented in corridors of nearly amagmatic spreading at the eastern Southwest Indian Ridge (SWIR). The depth of earthquake hypocenters supports the existence of a very thick axial lithosphere with a brittle lid thicker than 20 km. This is also consistent with our petrological observations on dredged peridotites from the area (plagioclase-free, spinel-bearing dynamically recrystallized assemblages in shear zones) that reveal high stress (80–270 MPa) and high temperature (>800°C) deformation combining brittle and ductile mechanisms. Thermo-mechanical models using strain dependent cohesion weakening to localize deformation do not produce detachments for a brittle lithosphere thicker than 20 km. Here, we explore two other weakening mechanisms observed in situ, and for which we have temperature constraints from rock sampling: serpentinization (at T<350°C) and grain size reduction by dynamic recrystallization (at T∼800–1000°C). Serpentinization and more generally hydrous alteration minerals have been shown to localize strain at detachment-dominated slow-spreading ridge locations. Here we show that grain size reduction due to dynamic recrystallization also occurs in peridotites from the eastern SWIR. Our models explore the effect of implementing these weakening mechanisms separately or together in an amagmatic spreading context, of varying the threshold conditions for their activation, and the thermal conditions on-axis. We show that serpentinization alone does not produce flip-flop detachments, while grain size reduction alone does, albeit with unrealistically high associated topographic relief. Combining the two, we observe the activation of several modes of axial deformation: horst mode, spider mode, and long or short-lived flip-flop detachments. Flip-flop detachment faulting, once initiated, can be activated over long periods of time, with fault topography and offsets that are consistent with geological observations in the eastern SWIR nearly amagmatic study area. The transition between these modes appears to depend on the amount and location of serpentine that has been produced in previous stages of faulting.

Earthquake Source Parameters in Southwestern China and Their Rheological Implications

AbstractEarthquake depth distribution provides key information on rheological behavior of the crust, which usually shows a brittle–ductile transition at a depth of about 10 km. In this study, we use the generalized cut-and-paste method to obtain source parameters of 571 earthquakes in the Sichuan–Yunnan region of China between 2009 and 2017. We were able to successfully determine focal mechanisms, moment magnitudes, and centroid depths of 536 earthquakes with a minimum moment magnitude of 3.2. Our moment magnitudes and centroid depths are systematically smaller than the magnitudes (Ms and mb) and hypocenter depths from the China Earthquake Network Center and International Seismological Centre catalogs for M≥4.0 earthquakes. The earthquake depths in the Sichuan–Yunnan region are mostly in a 5–9 km range, with an average at 7.6 km. About 23% earthquakes have centroid depths &amp;lt;5 km and are concentrated in the southern Sichuan basin. Only very few earthquakes are deeper than 19 km. Compared with the earthquake depth distribution in southern California, the Sichuan–Yunnan region has many shallower earthquakes. The depth distribution suggests that the brittle–ductile transition in the Sichuan–Yunnan region is shallower than the transition beneath southern California, which is probably due to the existence of newborn faults in the Sichuan–Yunnan region.

Сейсмоакустические наблюдения с применением молекулярно-электронных гидрофонов на Сахалине и южных Курильских островах (о. Кунашир)

The article presents the main characteristics and studies spectral and recording capabilities of experimental samples of three types of molecular-electronic hydrophones with different sizes of electrochemical converting cells, which were installed in the central part of the south of Sakhalin Island and on Kunashir Island (southern part of the Kuril ridge) at the end of 2018. A hydrophone on a new technological basis (with an increased sensor sensitivity relative to previously conducted studies) was approved on Kunashir Island. Equipment of this type was used for observations on the territory of the Sakhalin region for the first time. As a result of continuous seismoacoustic observations on Kunashir Island, in seven cases out of 35 studied (from May 1, 2019 to February 29, 2020) earthquakes, a low-frequency anticipatory signal (LFAS) was detected, and all seven events had a depth of hypocenter of more than 80 km. In the area of the Central Sakhalin fault, in addition to the possibility of registering the LFAS, spectral features in the recording of waveforms were studied for different conditions of instrument installation. It is shown that the influence of such atmospheric factor as wind load significantly affects hydrophones located in a shallow water body and a shallow open borehole. It is shown that the influence of such atmospheric factor as wind load significantly affects hydrophones located in a shallow water body and a shallow open borehole. However, molecular-electronic hydrophones have demonstrated the ability to fully record seismic events regardless of the installation conditions of the equipment.

Study of seismic activity at Ceboruco Volcano (Nayarit, Mexico) in the period 2012 to 2014

Ceboruco volcano is a stratovolcano located in western central Mexico within the state of Nayarit. Ceboruco volcano is one of the active volcanoes of the Trans-Mexican Volcanic Belt and lies within the Tepic-Zacoalco Rift structure. Eruptions at Ceboruco volcano have been reported over the last 1000 years with eruptions every 126 years on average, with the most recent eruption occurring in 1870. Current activity at Ceboruco volcano is primarily fumarolic emissions. Continued population and socio-economic growth around the volcano over the last 20 years, make urgent the need to study Ceboruco volcano. Previous investigations of seismicity classified the recorded earthquakes into four families using waveform and spectral features. In this paper, we present a seismicity study from March 2012 to July 2014 using four portable seismic stations within the context of the regional stress and structures near Ceboruco volcano. Of the 489 volcanic earthquakes recorded during this time period, only 33 could be located using P- and S-wave arrivals registered at least three stations. The P- and S-phase arrival times were obtained using particle motion. The epicentral distribution of these earthquakes is around the volcanic edifice along three structural lineaments with preferred ENE-WSW orientations, roughly perpendicular to Tepic-Zacoalco Rift and, following the youngest deformation pattern. Hypocentral depths locate within first 10 km, indicating the earthquakes are a result of local tectonic stresses, intrusions into the magma chamber, or both.

The 2013–2018 Matese and Beneventano Seismic Sequences (Central–Southern Apennines): New Constraints on the Hypocentral Depth Determination

The Matese and Beneventano areas coincide with the transition from the central to the southern Apennines and are characterized by both SW- and NE-dipping normal faulting seismogenic structures, responsible for the large historical earthquakes. We studied the Matese and Beneventano seismicity by means of high-precision locations of earthquakes spanning from 29 December 2013 to 4 September 2018. Events were located by using all of the available data from temporary and permanent stations in the area and a 1D computed velocity model, inverting the dataset with the Velest code. For events M &gt; 2.8 we used P- and S-waves arrival times of the strong motion stations located in the study area. A constant value of 1.83 for Vp/Vs was computed with a modified Wadati method. The dataset consists of 2378 earthquakes, 18,715 P- and 12,295 S-wave arrival times. We computed 55 new fault plane solutions. The mechanisms show predominantly normal fault movements, with T-axis trends oriented NE–SW. Only relatively small E–W trending clusters in the eastern peripheral zones of the Apenninic belt show right-lateral strike-slip kinematics similar to that observed in the Potenza (1990–1991) and Molise (2002 and 2018) sequences. These belong to transfer zones associated with differential slab retreat of the Adriatic plate subduction beneath the Apennines. The Matese sequence (December 2013–February 2014; main shock Mw 5.0) is the most relevant part of our dataset. Hypocentral depths along the axis of the Apenninic belt are in agreement with previous seismological studies that place most of the earthquakes in the brittle upper crust. We confirm a general deepening of seismicity moving from west to the east along the Apennines. Seismicity depth is controlled by heat-flow, which is lower in the eastern side, thus causing a deeper brittle–ductile transition.

Satellite monitoring of clouds over deep fault zones, short-term earthquake forecast and seismic activity in Ukraine

The article substantiates the possibilities of satellite monitoring of deep fault zones during the formation of a source of strong and catastrophic earthquakes and the manifestations of precursors of strong and catastrophic earthquakes - foreshocks, which manifest themselves in the form of cloud anomalies over a period of time from two weeks to several days. In recent years, cloud cover anomalies have been recorded before relatively weak earthquakes that occurred in southern Ukraine. It is very important that earthquakes occur in the Dnieper-Donetsk Depression (DDZ). An analysis of the tectonic map of the remote sensing data with the position of the epicenters of earthquakes that occurred in 2010 and 2015 showed that these earthquakes are confined to zones of deep faults or their intersections. The article proposed an earthquake prediction technology, which includes two stages. At the first stage, based on the results of spatial-temporal analysis, maps of the parameters of earthquakes (dates, coordinates of epicenters, depths of hypocenters) are built, which occur over a certain period of time, which allows you to determine the area of ​​increased seismic activity. At the second stage, research is being carried out in this area on satellite monitoring of cloud anomalies in order to short-term forecast strong earthquakes.

A Reappraisal of Seismicity Recorded During the 1996 Gjálp Eruption, Iceland, in Light of the 2014–2015 Bárðarbunga–Holuhraun Lateral Dike Intrusion

This study performs a reanalysis of the seismicity recorded during the 1996 Gjalp eruption that occurred at NW Vatnajokull, Iceland. The seismicity was recorded by the temporary HOTSPOT network consisting of 30 three-component broadband stations. In total 301 events were identified between 29 September and 12 October and their phases were manually picked. A velocity model was estimated from P-phase travel times by using VELEST. Events were first located using the algorithm NONLINLOC in order to obtain absolute locations. Precise relative locations were obtained with HYPODD by utilizing catalog and cross-correlation differential travel times. Results show that events clustered first along the SW rim of the Barðarbunga caldera and later along the Gjalp fissure, with most hypocentral depths located between 3 and 8 km. Waveforms of the 10 largest events that followed the Barðarbunga earthquake were inverted in order to obtain moment tensors. For all events we found that the deviatoric moment tensor fits the data better than pure double-couple or full moment tensor solutions. Events along the Barðarbunga caldera exhibited reverse focal mechanisms, while those at the Gjalp fissure exhibited mostly strike-slip faulting. Seismic velocity variations calculated using ambient noise interferometry, point to the possibility that a small subglacial eruption occurred at Barðarbunga before the main earthquake. This removed melt from the magma chamber causing its roof to collapse, and also resulted in the lateral migration of magma towards the Gjalp fissure. The 2014–2015 Barðarbunga–Holuhraun eruption shares common characteristics with the 1996 Gjalp eruption, although the size of the latter was much smaller.

Курило-Охотский регион

The review of the annual seismicity of the Kurilo-Okhotsk region is submitted. Parameters of 863 earthquakes are determined by records of four Kuril stations. Their distribution of hypocenter depth is given. The maximal depth has made h=620 km in 2013. The distribution of earthquakes on the magnitude and their summarized energy on areas of the region is given. The map of epicenters, together with focal mechanisms for 23 earth-quakes is shown. For each area, seismic conditions are described.