Mass estimates of the scalar seismic moments of small earthquake foci on southern Sakhalin

The frequency dependence of the function of the seismic wave attenuation was determined for the first time for southern Sakhalin on the basis of seismic coda of local earthquakes using the model of single scattering. The algorithm of the automated definition of the scalar seismic moments was realized for small earthquake foci. Mass estimates of the scalar seismic moments were obtained as exemplified by the after-shocks of the August 17, 2006, Gornozavodsk earthquake (M_W 5.6) and the May–June 2004 Kostromskoe earthquake swarm events, which occurred in South Sakhalin. The dynamic parameters of the earthquake foci were determined from the SH-wave spectra adjusted for absorption and geometrical spreading. The loglinear relationship determined between the seismic moment and the local magnitude is in good agreement with the estimates obtained for other regions and, in a certain sense, does not contradict the average world dependence.     

Structure and peculiarities of the seismic regime in the source zone of the Takoe earthquake on September 1, 2001 (M W 5.2)

The Takoe earthquake (M _W 5.2) occurred between two en-echelon segments of the active Aprelovskii fault on September 1, 2001, and was accompanied by an earthquake swarm, which was successfully recorded by a local network of digital seismic stations located on the southern part of Sakhalin Island. Modern methods were applied to relocate the parameters of the sources for the earthquake swarm event and significantly specify their spatial distribution and relations to the structural-geological features of the complex system of interacting faults. New data on the correlation between the source mechanism and the modern geodynamic setting in the southern part of Sakhalin were obtained.     

The August 2, 2007, Nevelsk earthquake: Instrumental data analysis

A catalogue of aftershocks of the 2007 Nevelsk earthquake (M _w = 6.2) was prepared on the basis of the data from the local network of digital seismic stations established on the southern part of Sakhalin Island. The parameters of the aftershock hypocenters were determined using the method of the seismic wave travel time inversion. The errors in the determination of the coordinates of the seismic events were analyzed. The particularities of the spatiotemporal distribution of the aftershocks in the source zone of the earthquake were established. It was shown that a strong aftershock was a subsource earthquake with its own source zone. This explains the disagreement between the energetic characteristics and the size of the aftershock zone of the Nevelsk earthquake.     

Seismic anisotropy distribution beneath southern Sakhalin

Shear wave splitting parameters from local deep-focus and crustal earthquakes beneath southern Sakhalin and northern Hokkaido have been measured. The study of the split shear wave amplitude, polarization, and splitting parameter distribution revealed their correlation with the geometry of the subsiding Pacific Plate and horizontal heterogeneity of the rheological properties and viscosity of the medium. Comparison of the observed data with those modeled in anisotropic media allows the mantle flow to be oriented NNW beneath southern Sakhalin and northern Hokkaido. Based on the split shear wave time delays, the degree of mantle anisotropy is estimated to be around 1–2% beneath southern Sakhalin and 1.5–2.5% beneath northern Hokkaido. A relatively high anisotropy (2–15%) from local crustal earthquakes is found beneath the Central Sakhalin Fault.     

The Gornozavodsk earthquake of August 17(18), 2006, in the south of Sakhalin Island

The August 17 (18), 2006, Gornozavodsk earthquake (M_w = 5.6) in the southwestern part of Sakhalin was preceded by a number of anomalous seismological and geophysical phenomena. The extensive data recorded by a network of digital seismic stations make it possible to track the aftershock dynamics of the process within 24 hours after the main event. The paper describes various manifestations of the earthquake.     

The Strong Earthquake in Sakhalin Island in September 13, 2020

Doklady Earth Sciences - The seismotectonic position of the strong earthquake that occurred in the southern part of Sakhalin Island on September 13, 2020, is considered. The maximum shaking...     

The tectonic conditions of the August 2, 2007, M ∼ 6.1 Nevelsk earthquake

This paper reports the results of field observation and the study of coastal tectonic deformations related to the Nevelsk tsunamigenic earthquake (August 2, 2007, M ～ 6.1) obtained in August-September of 2007. The earthquake caused a 0.5- to 1.5-m rise of and partial desiccation of the southern, central, and northern benches and the formation of longitudinal structural ridges seaward of Lovetskaya Bay. In the framework of the new model of the Kamyshovy (West Sakhalin) Anticlinorum as a structure of the Quaternary and Middle Quaternary crustal detachment, the relationships between the earthquake and the slow gravitational creep of the upper crust on its western slope with local squeezing of the Middle Miocene Nevelsk siltstones are discussed.     

Development of medium-term prediction methods: A case study of the August 14, 2016 Onor (Mw = 5.8) earthquake on Sakhalin

The potential of the load-unload response ratio (LURR) method for medium-term earthquake prediction is studied for Sakhalin Island as an example. An approach to the generation of predicted conditions and assessment of their implementation in real time is considered. The results of a retrospective analysis of other large Sakhalin earthquakes are used for generalization. It is shown that deviations of prediction parameters from specified values are satisfactory for this method. It is recommended that this method be used to compile summaries of medium-term predictions for Sakhalin provided that catalogs are filled as soon as possible.     

Recent crustal movements and the Moneron earthquake of 1971

From the results of high-precision levelling conducted in the southern part of Sakhalin Island, the dynamics of the earth's surface have been determined before the Moneron earthquake in 1971, during its numerous aftershocks and in the following periods. Great subsidence, of up to 7–8 cm, preceded the earthquake, then a 3-year period of steady state followed and at the end of this period the earthquake happened. After this event vertical movements show an inherited character related to the very recent geological structures. Horizontal displacements of the triangulation net, 50 km distant from the earthquake epicentre, were also noted; they had a northeast direction and an amplitude of 7 cm.     

A reappraisal of the 1950 (Mw 6.9) Mondy earthquake, Siberia, and its relationship to the strain pattern at the south-western end of the Baikal rift zone

ABSTRACT The precise nature of the transition between the present-day compressional tectonics in central Mongolia and extensional deformation in the central Baikal rift has still to be determined. For that purpose we have built a comprehensive earthquake focal mechanism data base for the Mongolia – southern Siberia area, from which we map the variations of the stress field. We focus our detailed investigations on the largest seismic event in the transition zone, the 1950 (Mw 6.9) Mondy earthquake, for which several discordant focal mechanisms have been proposed. Using a new approach in source inversion, we resolve the focal mechanism (left-lateral strike slip type on a steep south-dipping fault) and depth (14 ± 3 km) of the Mondy earthquake with a satisfactory accuracy. This seismological information, combined with the geological observations, allows us to decipher the connections between the 1950 mainshock, the local stress tensor and the active faults, which strongly suggest a partitioning of the deformation between two faults, namely the Mondy and Ikhe–Ukghun faults.     

Detailed seismic zoning of Sakhalin

Detailed seismic zoning of Sakhalin based on seismological, tectonic, geomorphological, hydrogeological, and other data is discussed. It is shown that strong crustal earthquakes occurred at the boundary between the Eurasian and Okhotsk plates and their recurrence in Central Sakhalin is equal to the duration of the tectonic cycle (75 years). This boundary in North Sakhalin is marked by the Upper-Piltun fault, which was the epicenter of the 1995 Neftegorsk earthquake with an intensity of 9. The analysis of paleosoils in the fault zone showed that such events repeat with an interval of 400 years. The development of large oil and gas reservoirs on the Sakhalin shelf will be accompanied by intensification of the seismicity, which can reach a magnitude of M = 6.0–6.5 in the Lunskoye field.     

Tertiary geodynamics of Sakhalin (NW Pacific) from anisotropy of magnetic susceptibility fabrics and paleomagnetic data

Sakhalin has been affected by several phases of Cretaceous and Tertiary deformation due to the complex interaction of plates in the northwest Pacific region. A detailed understanding of the strain is important because it will provide constraints on plate-scale processes that control the formation and deformation of marginal sedimentary basins. Anisotropy of magnetic susceptibility (AMS) data were obtained from fine-grained mudstones and siltstones from 22 localities in Sakhalin in order to provide information concerning tectonic strain. AMS data reliably record ancient strain tensor orientations before significant deformation of the sediments occurred. Paleomagnetically determined vertical-axis rotations of crustal rocks allow rotation of the fabrics back to their original orientation. Results from southwest Sakhalin indicate a N035°E-directed net tectonic transport from the mid-Paleocene to the early Miocene, which is consistent with the present-day relative motion between the Okhotsk Sea and Eurasian plates. Reconstruction of early–late Miocene AMS fabrics in east Sakhalin indicates a tectonic transport direction of N040°E. In west Sakhalin, the transport direction appears to have remained relatively consistent from the Oligocene to the late Miocene, but it has a different attitude of N080°E. This suggests local deflection of the stress and strain fields, which was probably associated with opening of the northern Tatar Strait. A northward-directed tectonic transport is observed in Miocene sediments in southeast Sakhalin, mid-Eocene sediments in east Sakhalin, and in Late Cretaceous rocks of west and northern Sakhalin, which may be associated with northwestward motion and subduction of the Pacific Plate in the Tertiary period. The boundaries of the separate regions defined by the AMS data are consistent with present-day plate models and, therefore, provide meaningful constraints on the tectonic evolution of Sakhalin.     

On the stress state of the Sakhalin crust according to the data of drilling deep boreholes

The stress state of the sedimentary rocks in the oil-and-gas fields of Sakhalin is estimated and analyzed; data from exploratory boreholes in the eastern Pacific (San-Andreas fault zone) are considered. The vertical and limiting horizontal stresses are calculated for different depths. The maximum sublateral compression can exceed the vertical stress by a factor of 1.2–4 on average in northern and southern Sakhalin. It is shown that the limiting horizontal stress and the maximum shear stress grow as the depth increases.     

Deep structure and seismotectonics of the Southern Sea of Okhotsk region along a profile from southern Sakhalin to the southern Kuril Islands

The results of deep seismic profiling through Southern Sakhalin, the southern Sea of Okhotsk, and the Southern Kuril Islands allowed the identification of deep fault zones, the hypocenter locations, the features of the stress state, and the types of seismic dislocations at the earthquake sources. The east side of the fault was upthrown relative to the west side beneath the southern part of the Tatar Strait and Sakhalin Island and led, as a result of multiple thrusting events along the fault over the geologic history, to the rise and 5-8 km displacement of the seismic boundaries. The true uplift of the Greater Kuril arc block was determined using the focal mechanism solutions. The seismoctectonics and present-day dynamics of the crustal blocks were estimated using a detailed joint analysis of the position of the structural boundaries at the seismic section and the seismotectonic movements according to the earthquake focal mechanisms.     

Seismic risk assessment at the proposed site of Gemsa wind power station,southwestern coast of Gulf of Suez,Egypt

Gemsa has been chosen as the site for one of a new generation of power stations along the south-western margin of the Gulf of Suez. This site has been affected by a number of destructive earthquakes (M_w> 5), in addition to large number of earthquakes with magnitudes of less than 5. In this study seismic activities in the region were collected and re-evaluated, and the main earthquake prone zones were identified. It is indicated that this site is affected by the southern Gulf of Suez, northern Red Sea and Gulf of Aqaba source zones. The southern Gulf of Suez source zone is the nearest to the proposed site. The stochastic simulation method has been applied to estimate the Peak GroundAcceleration at the site of the proposed Gemsa power plant. It was noticed that the pseudo-spectral acceleration (PSA) reaches 175 cm/sec2 resulting from the southern Gulf of Suez seismic source. In addition, the response spectrum was conducted with a damping value of 5% of the critical damping, and the predominant period reached 0.1sec at the site. These results should be taken into consideration by civil engineers and decision-makers for designing earthquake resistant structures.     

The seismic anisotropy beneath Southern Sakhalin: Evidence from the S-wave parameters of deep-focus earthquakes

The measurements of the parameters of split shear (S) waves from local deep-focus earthquakes recorded in 2005–2007 by a network of 12 seismic stations in Southern Sakhalin are presented. The results revealed the heterogeneous distribution of the anisotropic properties beneath Southern Sakhalin. The azimuths of the fast S-wave polarization beneath the stations in the central part of the peninsula are oriented along the NNW and NNE-NE directions normal to and along the Kuril Trench. Beneath the stations located along the western and eastern coasts, the azimuths of the fast S-wave polarization change their direction from NNW in the northern area to E-SE in the southern area. The highest anisotropy degree (up to 0.9–1.5%) is recorded beneath the central part of Southern Sakhalin. The maximum values of the discrepancy in the arrival time of the split S-waves are observed when the azimuth of the fast S-wave is oriented along the NNE beneath the active fault zones. The analysis of the variations of the S-wave lag time shows their weak depth dependence. The highest anisotropy is assumed in the upper layers of the medium (down to a depth of about 250 km). The results obtained for the dominating wave frequency of 1–5 Hz represent mainly the medium-scale anisotropy of the top of the studied region.     

Focal mechanism solution of the Himalayan earthquake of February 14, 1977

Through a closely spaced local network of seismic stations in Himachal Pradesh, India, supplemented by worldwide P-wave first-motion data, the source mechanism of the February 14, 1977 earthquake which occurred very close to the Rawalpindi area in Pakistan has been determined. The fault-plane solution as reported earlier for this event by Seeber and Armbruster (1979) showed thrust faulting. The reliability of their solution has been tested using more P-wave first-motion data from near Indian stations within the epicentral distance of 4°–7°, as well as from distant stations. The inclusion of data from these stations completely changed the type of faulting from thrust to normal type. The new solution parameters have been briefly discussed in relation to the local geological faults/thrusts.     

Seismicity and volcanism of Jan Mayen Island

The small, arctic Jan Mayen Island, site of the World's northernmost active volcano, Beerenberg, is part of the mid-Atlantic ridge system and located along the Jan Mayen Fracture Zone (JMFZ). Recent data from the local seismic network, and fault plane solutions from the global network, indicate that the island is located at a ridge-crest intersection, which might explain the origin of the island and the associated volcanism. Moreover, the new data suggest a series of offset segments of the Mohn's Ridge, overlapping in an en echelon pattern. In January 1985, a flank eruption was for the first time observed with the local seismic network. Volcanic tremors and low-frequency events were observed on 5 January at 2230 h and 10 hours later the first large earthquake occurred. No visual confirmation of the eruption was made until 6 January at 1630 h. The seismic observations rule out the possibility that the large earthquake caused the eruption; it seems more likely that the changes in local stress conditions triggered the earthquake and that the eruption started before the first large earthquake. Recent observations show that the local network provides an efficient tool for monitoring and warning of volcanic activity. However, since there was no change in the local seismicity in the days or months before the 1985 eruption, it seems to be difficult to make long-term predictions of Beerenberg flank eruptions without using other techniques such as observations of tilt.     

Coseismic deformations of the Earth’s surface in Sakhalin related to the August 2, 2007, M w = 6.2 Nevelsk earthquake

The satellite radiointerferometry data revealed deformations of the coastal part of Sakhalin Island caused by the earthquake with M _w = 6.2 that occurred in the Tatar Strait near Nevelsk. Based on the joint analysis of the satellite and seismological data, dislocation models were contrived for the main shock and its strong aftershocks with the western dip of the fault planes. This made it possible to determine the source mechanisms and the geometrical parameters of the seismic ruptures and to calculate the coseismic vertical and horizontal displacements. In contrast to the one-dimensional model of the insular land displacements determined from the satellite radiointerferometry measurements, this provided a three-dimensional model of the surface deformations for the epicentral zone.     

The velocity structure of the Cariaco sedimentary basin,northeastern Venezuela,from refraction seismic data and possible relationship to earthquake hazard

The main damage from the July 9, 1997, Cariaco earthquake (Ms=6.8) was concentrated in the town of Cariaco and surrounding villages, which are located in the western part of the Cariaco sedimentary basin, close to the Gulf of Cariaco. Casanay, located at the eastern end of the sedimentary basin, suffered considerably less damage. The El Pilar fault, a right-lateral strike-slip fault that generated the earthquake, runs parallel to the southern border of the valley and crosses both towns. The determination of the velocity structure of the basin is the main objective of this study. Seismic refraction data were recorded along three lines, one of them along-strike and two perpendicular to the valley axis in the northern and southern bedrocks. Beneath Cariaco, approximately 1 km thick Quaternary sediments with seismic velocities of 1.9–2.1 km/s and bedrock velocities of more than 4 km/s were observed. The thickness of the Quaternary sediments varies within the basin, and Pleistocene sediments outcrop beneath Casanay. The increased thickness of the unconsolidated, water-saturated Quaternary sediments, together with the difference in the quality of buildings prior to the earthquake, probably is responsible for the damage pattern of the Cariaco earthquake.