Weak Seismic Activity Research Articles

Variations in seismic parameters for the earthquakes during loading and unloading periods in the Three Gorges Reservoir area

As the largest water conservancy and hydropower project in China, the Three Gorges Reservoir is a weak seismic activity area before impoundment, but the frequency of earthquakes increases significantly after impoundment. The spatial density scanning method was used to obtain the characteristics of spatio-temporal earthquake distribution in the reservoir area during loading and unloading processes. The results show that the frequencies of earthquakes during the loading and unloading processes were higher than that during the low-water-level operation period, which is well explained by the acoustic emission test results. The seismic b-value, fractal dimension D, and spatial correlation length SCL can be used together to indicate stress criticality. To analyze the impacts of reservoir water loading and unloading on seismicity in the reservoir area, time-scan analyses were performed on the b-value, D-value, and SCL of earthquakes near the Zigui segment and the Badong segment. Previous studies argued that the time-varying characteristics of b-values do not hold predictive significance for earthquakes in the M4.0–6.2 range. However, our study found that the time-varying characteristics of b-values are of predictive significance for earthquakes around M4.0. These seismic parameters decrease significantly before moderate earthquakes but at different rates in different regions.

SEISMICITY of the CRIMEAN-BLACK SEA REGION in 2015

In 2015, 58 earthquakes were registered in Crimea in the range of energy classes КП=5.2–10.8. The bulk of the epicenters refers to the Black Sea. The maximum density of epicenters can be traced in Alushta (№ 3) area and in the Kerch-Anapa area (№ 5). Spectral and dynamic parameters of their foci were obtained for 16 events in the region; and for 1 of them a focal mechanism solution was obtained. Seismic processes in the Crimean-Black Sea region were monitored by seven stationary digital seismic stations. In February, the «Tarkhankut» observation point was closed. Representative registration of earthquakes with Кп=9.0 is still provided for main part of the Crimean-Black Sea region. A peculiarity of spatial distribution of earthquakes is the occurrence of two strongest earthquakes in the year under consideration with КП=10.8 in the Kerch-Anapa (№ 5) and Azovo-Kuban (№ 7) areas. The maximum (Кп=10.8, MLWSG=3.8, Mw=3.8) earthquake occurred on August 16 at 22h38m in the Kerch-Anapa region at the depth of h=7 km. This shock caused tremors in Anapa with the intensity of I=2 according the MSK-64 scale. 2015 is characterized by weak seismic activity. The total number of registered earthquakes, ΣN=58, decreased against ΣN=119 in 2014 and is below than the average Nav=69 for the ten-year observation period (2004–2013). The annual seismic energy is ΣE=1.491011J, that is 5.6 times less than the average value ΣEav=8.41011J for the ten-year period. This suggests that we can expect increased seismicity in the region in subsequent years.

Recent Activity and Kinematics of the Bounding Faults of the Catanzaro Trough (Central Calabria, Italy): New Morphotectonic, Geodetic and Seismological Data

A multidisciplinary work integrating structural, geodetic and seismological data was performed in the Catanzaro Trough (central Calabria, Italy) to define the seismotectonic setting of this area. The Catanzaro Trough is a structural depression transversal to the Calabrian Arc, lying in-between two longitudinal grabens: the Crati Basin to the north and the Mesima Basin to the south. The investigated area experienced some of the strongest historical earthquakes of Italy, whose seismogenic sources are still not well defined. We investigated and mapped the major WSW–ENE to WNW–ESE trending normal-oblique Lamezia-Catanzaro Fault System, bounding to the north the Catanzaro Trough. Morphotectonic data reveal that some fault segments have recently been reactivated since they have displaced upper Pleistocene deposits showing typical geomorphic features associated with active normal fault scarps such as triangular and trapezoidal facets, and displaced alluvial fans. The analysis of instrumental seismicity indicates that some clusters of earthquakes have nucleated on the Lamezia-Catanzaro Fault System. In addition, focal mechanisms indicate the prevalence of left-lateral kinematics on E–W roughly oriented fault plains. GPS data confirm that slow left-lateral motion occurs along this fault system. Minor north-dipping normal faults were also mapped in the southern side of the Catanzaro Trough. They show eroded fault scarps along which weak seismic activity and negligible geodetic motion occur. Our study highlights that the Catanzaro Trough is a poliphased Plio-Quaternary extensional basin developed early as a half-graben in the frame of the tear-faulting occurring at the northern edge of the subducting Ionian slab. In this context, the strike-slip motion contributes to the longitudinal segmentation of the Calabrian Arc. In addition, the high number of seismic events evidenced by the instrumental seismicity, the macroseismic intensity distribution of the historical earthquakes and the scaling laws relating to earthquakes and seismogenic faults support the hypothesis that the Lamezia-Catanzaro Fault System may have been responsible for the historical earthquakes since it is capable of triggering earthquakes with magnitude up to 6.9.

Detailed structure of mantle transition zone beneath southeastern China and its implications for thinning of the continental lithosphere

This paper presents a new study on 3-D structure, temperature and water content estimation of the mantle transition zone (MTZ) by P-wave receiver functions common conversion point (CCP) method on broadband array with 267 temporary stations and 203 permanent stations in southeastern China. The results show different structural characteristics of the MTZ to the north and south of the 29°N line. In the northern part, the 660-km discontinuity is generally deeper than globe models, and there are two independent local depressions with characteristics of high velocity, low temperature, and water shortage. We interpret them as two previous subducting plates stagnated in MTZ, possibly related to two separate subduction events at different times. While in the southern part, the 410-km discontinuity is locally depressed, resulting in thinning of the MTZ. Particularly, in the Cathaysian Block south of the 29°N line, based on the MTZ structure and the estimation of temperature and water content, we conside dehydration of the MTZ as one of the important mechanisms of lithospheric thinning through interaction with the asthenosphere. We then propose a dynamic model that combines paleo-Pacific flat-slab subduction and upper mantle convection to explain the lithospheric extension and thinning, magmatic rock distribution, weak seismic activity and basin-and-range geomorphology. This model differs from that north of the 29°N line in the North China Block.

RETRACTED ARTICLE: Fluid-induced high seismicity in Songliao Basin of China

ABSTRACT In recent years, moderate earthquakes with a magnitude of about 5.0 have occurred along the Fuyu/Songyuan-Zhaodong fault frequently. This fault is the most seismic active fault in the Songliao Basin, even in the Northeast Seismic Region of China. Based on previous studies, this fault has obvious segmentation from its activity. Some scholars considered the ground stress changes, which caused by the high pressure water injection method of oil exploration in Chaganhua area, maybe the reasonable interpretation of this phenomenon. This paper tries to synthetically assess and review the activity and segmentation of this fault, combining with previous data and the latest research findings. We also tried to reveal the relationship between the high seismicity and high pressure water injection of oil exploration. This effort will help us to understand the fault active characteristics and preparation tectonic conditions of the moderate earthquake in the weak seismic activity region such as the Songliao Basin or the Northeast Seismic Region of China.

Study of the types and characteristics of geological hazards in reefs of Nansha Waters, South China Sea

Reports of the 19th National Congress of the Communist Party of China identifies that the construction of islands and reefs in South China Sea (SCS) has been actively promoted. It is of great practical significance to study the types and characteristics of geological hazards in Nansha Islands and reefs, and it is the precondition and guarantees to carry out the construction of islands and reefs. At present, the research on marine geological hazards in China focuses on oil and gas enrichment areas such as Bohai bay, southern Yellow Sea and Northern of South China Sea, and there is little research on southern of South China Sea, especially Nansha Reefs. On the basis of collecting and sorting out previous research achievements, combining with existing geological and geophysical data and drilling data, his paper focuses on analysing two single-channel seismic profiles N01 and N02 collected by “Shiyan 2” of South China Sea institute of Chinese academy of sciences in Nansha Islands and reefs in 2013, and studies the types and characteristics of geological hazards in this area. The study shows that the whole Nansha block belongs to areas with weak seismic activity, most of the faults in the island reef area are weak and micro-active, and the volcanic activity in the island reef area is weak, and the coral reef growth rate is fast, which can be suitable for all kinds of engineering construction projects.

Historical accounts of pre-eruption seismicity of Katla, Hekla, Öræfajökull and other volcanoes in Iceland

Detecting unusual activity leading to the outbreak of a volcanic eruption is of vital importance for the short-term warning to the local population of an impending eruption. The varied volcano types of Iceland and range of ambient conditions at which eruptions occur offer an unusually wide spectrum of volcanic phenomena and volcanic hazards during the initial phase of eruptions. A recent study of eruptions in Iceland during the last four decades of instrumental observations has revealed that all the eruptions had a detectable precursory seismic activity, that under favourable conditions can be used to issue short-term warnings to the surrounding communities. Considerable documentary data also exist for pre-instrumental times extending several centuries back in time, that can be compared to the instrumental experience. This is true in particular for two of the most active volcanoes, Katla and Hekla, that are sufficiently close to the populated areas of the country. All seven confirmed eruptions of Katla since 1625 were preceded by felt earthquakes, beginning one to nine hours before the eruption was detected and two to over twelve hours before a jökulhlaup from this partly sub-glacial volcano reached the inhabited areas. The behaviour of Hekla is quite different. Large eruptions from the main edifice of Hekla since 1510 were generally accompanied by rather weak seismic activity. Earthquakes are usually felt only minutes before the first explosion occurs, in the 1947 case even several minutes after the first explosion of the volcano. Eruptions of the Hekla volcanic system outside the main edifice are, on the other hand, accompanied by considerable seismic activity, and the precursor times may be more than three hours, even much longer. The two historical eruptions of Öræfajökull, in 1362 and 1727, were apparently preceded by felt seismicity, sufficient to alarm the local population.

Features of Winter Thunderstorms in Kamchatka

Winter thunderstorms in Kamchatka are a rare meteorological phenomenon. Temporal variations of the quasi-static electric field and meteorological values at the Paratunka observatory of the Institute of Cosmophysical Research and Radio Wave Propagation, Far East Branch, Russian Academy of Sciences (φ = 52.97° N; λ = 158.25° E), as well as data on solar, seismic and cyclonic activities available on INTERNET, are used to study the nature of this phenomenon. It is shown that powerful solar flares accompanied by increased radiation in the visible and infrared spectra, as well as the infrared radiation from the Earth that enters the atmosphere before powerful earthquakes with a magnitude of M > 8 may serve as an additional heat source in the surface atmosphere of Kamchatkan for the formation of thunderstorm activity. The contribution of tropical cyclones to these processes with weak seismic activity is not clearly defined and requires further detailed study.

Winter thunderstorms in Kamchatka

Winter thunderstorms in Kamchatka are rare meteorological phenomena. To investigate the nature of this phenomenon, time variations of quasistatic electric field and meteorological quantities at «Paratunka» observatory, IKIR FEB RAS (φ = 52:97° N; λ = 158:25° E), and the data on solar, seismic and cyclonic activities available in INTERNET were used as a thunderstorm activity indicator. It was shown that powerful solar flares accompanied by radiation increases in visible and infra-red spectra as well as the Earth infra-red radiation entering the atmosphere before powerful earthquakes with magnitude M > 8 may be additional sources of heat in the near-ground atmosphere of Kamchatka. The impact of tropical cyclones on the processes of thunderstorm activity formation in Kamchatka during weak seismic activity has been determined insufficiently and requires further detailed investigation.

Correlations between soil gas and seismic activity in the Generalized Haiyuan Fault Zone, north-central China

Radon and mercury concentrations were measured in 10 fault gas profiles in Generalized Haiyuan Fault. This paper aims to predetermine the potential seismic risk in different segments of the fault zone from the perspective of geochemistry. The background value and anomaly threshold were adopted and synthesized using the maximum value method and average method to calculate concentration intensity values of radon and mercury. Fault soil gas mercury and radon concentrations show a decreasing gradient from NW to SE indicating evident segmentation. Higher values are mostly distributed in the Maomao Mountain–Tiger Mountain fault and Jingtai area. Combined with the seismotectonic background of historical and recent earthquakes and the spatial distribution characteristics of b-values, the fault soil gas concentration intensity shows a close correlation with earthquake activity within the fault zone. Concentrations of fault gas are higher and the b-value lower in areas of strong seismic activity, and regions with weak seismic activity correspond to lower fault gas concentrations and higher b-values. It is thus considered that the Jingtai area may be more dangerous than the other areas. This paper could provide vital background information for earthquake prediction in the Generalized Haiyuan Fault Zone.

Variability of megathrust earthquakes in the world revealed by the 2011 Tohoku-oki Earthquake

The seismicity of the Pacific coast of Tohoku, Japan, has been investigated in detail and characterized into regional seismic segments. The 2011 megathrust earthquake of Mw 9.0 on 11 March ruptured almost all of the segments in that area, causing devastating tsunamis. The prime factor that had not been recognized before is the double segmentation along the Japan trench: The apparent absence of earthquakes in the trench-ward segments as opposed to the Japan Island-ward segments that have repeated smaller earthquakes. We term this pattern of seismic activity ‘along-dip double segmentation (ADDS)’. The 2011 Tohoku megathrust earthquake is typical of a class of great earthquakes different from that of the 1960 Chile earthquake, in which a young and buoyant plate is subducting rapidly under the continental plate. In the 1960 Chile case, the seismic activity is characterized by ‘along-strike single segmentation (ASSS)’, where there is weak seismic activity before the main event all over the plate interface of the subduction zone. We study the greatest earthquakes around the world and find that there is a variety of megathrust earthquakes characterized by ASSS to ADDS, where the 2004 Sumatra-Andaman, the 1960 Chile, the 1964 Alaska and the 2011 Tohoku-oki earthquakes are typical end-members.

Analysis on rock burst danger when fully-mechanized caving coal face passed fault with deep mining

When fully-mechanized caving face passed fault, rock burst accidence easily occurred. The SOS microseism monitoring system was applied to monitor the microseismic activities all time occurred in the coal and rock mass near the fault area. Variation features of microseismic energy releasing and microseismic frequency were analyzed. Numerical simulation method was used to research the abutment stress distribution when coal face passed fault, which was compared with microseism occurrence rules. When the coal face approached to fault, the abutment stress increases gradually, so the high stress would accumulate near the fault region. When the coal face left fault, the abutment stress decreased. The SOS microseism monitoring results showed that microseismic activity in the fault area had a high instability. When the coal face neared to the fault, total energy value and frequency released by the microseism steadily increased. The maximum energy peak value also had the tendency to rapidly increase. Before the strong shock occurred, there was a period of weak seismic activity. The weak seismic activity showed energy accumulation for strong shock, which can be used to forecast the danger of rock burst.

Some features of fluid dynamics at platforms as illustrated by the example of variations in subsoil radon and seismic noises in the Moscow and Nizhni Novgorod regions

Huge areas of ancient platforms are traditionally ascribed to geodynamically passive structures of the Earth’s crust. This leads to some conservatism in the interpretation of diverse natural phenomena at platforms (landslide, karst‐suffosional processes, rock impacts and gas explosions in mines, breakups of pipelines, and others), which are typically related to the exogenous change in the geological environment of the sedimentary cover [1]. However, following Kropotkin [2], who pointed to a possible relation of global change in the strain-deformational state of the Earth’s crust with variations in the Earth’s rotation, we demonstrated [3] that these processes affect fluid dynamics in both geosynclinal (seismically active) and platformal regions, thus defining the evolution dynamics of exogenous processes in decompressed areas of sedimentary cover that are formed above faults in the crystalline basement. The processes determine the rhythmic evolution of diverse geodynamic phenomena at platforms, including weak seismic activity [4]. Our previous research in seismically active regions showed that changes in the fluid dynamics of faults caused by variations in the strain-deformed state of the rocks is adequately correlated with variations in the radioactive and hydrocarbon emanation [5] and seismic noises [6]. Temporal series of the measured parameters of the aforementioned fields reflects the entire spectrum of variations in the strain-deformational state of the Earth’s crust, from secular (60 to 70 years) to circadian tidals (12 h and less). This allows one to use the emanation and seismic emission methods as highly sensitive noise-proof technologies for monitoring the geological environment, especially suitable for urban agglomerations, where the level of electromagnetic and vibrational‐noise emanation rules out or constrains the application of many methods of geophysical monitoring.

La tectonique active de la région nord-algérienne

Active tectonics in Algeria is located in the northern part of the country, mainly in the Tell system. In this region, located at the Africa–Eurasia boundary, the tectonic activity expresses the convergence between the two main plates. This leads to the closure of the Neogene sedimentary basins and the ongoing Tell building. Offshore, present deformation affects the abyssal plain located close to the continent, by folding of the Plio-Quaternary sediments cover. Along the slope and the continental platform, active structures with a continental extension crosscut this region. The coastal tectonics generates the coastal uplift as it was shown during the last Boumerdes earthquake of 21 May 2003, where an average uplift of 0.50 m was measured. Onshore, the earthquakes occur along the boundaries of the Neogene sedimentary basins that are located along the coast. These basins underwent compressional deformations giving anticlines, reverse faults or thrust faults oriented NE–SW to NW–SE. These later active structures generated the most important earthquakes. In the High Plateau and the Saharan Atlas, a weak seismic activity is recorded. To cite this article: A. Yelles-Chaouche et al., C. R. Geoscience 338 (2006).

SPOT satellite monitoring of the eruption of Nevado Sabancaya volcano (Southern Peru)

The Nevado Sabancaya volcano (Southern Peru) began a phreatomagmatic eruptive process on 28 May 1990 after 4 years of weak seismic activity and fumarolic emission. SPOT images were acquired on 21 July 1986 and 1 July 1989. Differences between the two dates in snow and ice cover and morphology of the crater allowed us to foresee an increase in activity and possible change in the eruptive process. After the beginning of the eruptive stage, five SPOT images were provided by the CNES (France), the last one dated 12 December 1990. They show variations of ash cover, the opening of fractures, an increase of the crater size, and new mudflows. The 10-m ground resolution and stereoscopic capabilities of Panchromatic SPOT imagery permitted us to follow the major changes of geomorphic features during a volcanic eruptive stage.

Geodynamics of the Shanxi Rift system, China

The Pliocene-Quaternary Shanxi Rift system of northern China transsects the domal axis of the Shanxi Highlands and is defined by an S-shaped string of asymmetric basins, extending from Huailai-Yanqing in the north via Taiyuan to Yuncheng in the south. This rift system has a length of about 1200 km and a width of 20–80 km; it is characterized by strong earthquakes. Its central, NNE-trending, transtensional segment forms the main element of the Shanxi Rift system. Basin subsidence commenced during the early Pliocene along NNE oriented master-faults characterized by large strike-slip rates (5.68–7 mm/yr); crustal extension amounts to some 1.4 km; basins are separated by push-up blocks; destructive earthquakes ( M = 7−8) are unevenly distributed. The ENE trending northern and southern branches of the Shanxi Rift system are characterized by orthogonal crustal extension along ENE striking normal faults, amounting to about 8 km; these areas are characterized by a basin-and-range type structural style and weak seismic activity. The Shanxi Rift system developed on a some 40 km thick, thermally cool Precambrian crustal segment that was overprinted during Mesozoic diastrophic events. There is only minor rift-related volcanic activity. The Moho is pulled up by a few kilometers only under the axial rift zone; sub-Moho compressional velocities are in the 8.0–8.1 km/s range. The Shanxi Rift system evolved in response to the build-up of regional stress fields related to the collision of India and Eurasia; its localization involved reactivation of pre-existing fracture systems.

Laboratory investigation into off-fault seismic activity

Abstract The off-fault pattern of weak seismic activity which was observed in the laboratory around fault pairs with already generated tensile cracks led to the comparison with similar patterns observed in nature in some seismoactive regions. On the basis of the results of the laboratory model experiments, the off-fault seismic activity can be attributed to fault segments oriented obliquely or roughly perpendicular to the main fault system; these fault segments may have originated as tensile fractures during one of the foregoing strong earthquakes. To study the off-fault foreshock and aftershock occurrences, a method of laboratory seismology (physical modeling) was employed. The use of this method is based on the assumption that the obtained information on the seismic regime of fault models is relevant to the seismic manifestation of faults in the Earth. The results of laboratory experiments can thus be used to analyze the pattern of seismic energy release in selected seismoactive fault regions.

Study on the sequential activity and consecutive strong earthquakes in mainland China

There are many cracks that are distributed without order with a prominent direction within the crust. Under the action of the tectonic stress field, these cracks are extended with weak seismic activity in the rock. If a weak earthquake may be shown as an extension in a crack, then the development of the major shock is a rupture process of the rock; it seems to be divided into six stages of sequential activity. Given an empirical formula concerning the aftershock volume V and area S to the magnitude M s of its major shock, we studied the law of releasing energy and the characteristics of the sequential activity of about 90 earthquakes ( M s⩾6.0) in mainland China. However, the volume of some of the aftershock regions does not match the magnitude of the major shock and it appears that the strain energy is not fully released. A lot of energy seems to be stored up and this implies that a strong earthquake could occur. So we may use the empirical formula to predict the magnitude of an approaching consecutive strong earthquake. The observation shows that some medium strength earthquakes often occur near the epicentre of approaching consecutive strong earthquakes. But the site of the approaching consecutive strong earthquakes often lies near the line which links the epicentre or a medium strength earthquake with the outer centre in a triangle formed by the epicentre of the three medium strength earthquakes. This implies that we may use a geometric method to find the seismically dangerous region.