Epicentral Zone Research Articles

Planar internal Lamb problem: Waves in the epicentral zone of a vertical power source

We analyze the field in the epicentral zone for the internal Lamb problem of the action of a concentrated force within an elastic half-plane. We compare the solutions obtained using integral representations, geometric-optical methods, and finite-element approximations.

On unknown strong Holocene earthquakes in the south of the Issyk Kul depression, Tien Shan

An area of paleoseismic deformations of seismogravitation and seismotectonic genesis that trace the epicentral zones of ancient earthquakes, is revealed for the first time in the Tossor river basin, the southern Issyk Kul region. The C14 age is the 9th century AD for the recent earthquake and middle-early Holocene for the earlier seismic event. High seismic activity of the Pre-Terskey Border Fault in the south of the Issyk Kul Lake depression is recorded. An upslope-facing scarp and many significant rockslides in the Tossor river basin mark an epicentral zone of strong Holocene earthquakes, which is not consistent with instrumental seismic data and was not taken into account when compiling the latest seismic zoning map of the Kyrgyz Republic.

Andaman-Sumatra island arc: III. Time evolution of seismogenic activation of the arc since the beginning of the 21st century

The spatiotemporal evolution of the intense burst in seismogenesis within the Andaman-Sumatra island arc in 2000–2010 is analyzed. The onset of seismogenic activation was marked by two strong (MS ∼ 7.9, MS ∼ 7.8) quakes that occurred in the lithosphere of the South Sumatra region on June 4, 2000 and by the strongest (MW = 7.3) earthquake of July 25, 2004 that took place in the lower part of the focal zone. These seismic events were the foreshocks of the main episode of seismogenic activation of the island arc—the catastrophic earthquake of December 26, 2004, with its source near the northern coast of Sumatra. The large shocks (MS ∼ 7.7–8.4) that occurred from March 28, 2005 to October 25, 2010 between the source of the Sumatran earthquake and epicentral zone of the foreshocks of June 4, 2000, are the aftershocks of the Sumatran event. The spatiotemporal evolution of seismogenic activation of the Andaman-Sumatra island arc at the beginning of the 21st century is compared to the seismogenic activation of the Kuril-Kamchatka arc in the middle of the 20th century. The positions, geological conditions, and focal parameters of the strongest Sumatran earthquakes of 2000–2010 are determined. The interpretation of the sources relies on the (1) complex analysis of all the manifestations of the earthquakes of 2000–2010 and (2) the established regularities of the occurrence of the earthquakes in the island arcs. The sources of the earthquakes of 2005–2010 are the steep longitudinal (trending along the arc) reverse faults of a backthrust type, which have a bedding depth of about a few dozen kilometers. The reverse-fault sources of the earthquakes of March 28, 2005, September 12, 2007 (11:10 UT), and October 10, 2010 are located in the zone of the Outer Range, and the earthquakes of September 12, 2007 (23:49 UT) fall in the Mentawai Trough. The strike-slip reverse fault, which cuts the island arc, is likely to be the focal mechanism of the earthquake of June 4, 2000. The sources of the events of March 28, 2005 and September 12, 2007 did not coincide with the sources of the historical devastating earthquakes of 1861 and 1833, respectively. The system of seismic sources in the form of a series of subparallel arc-parallel steep reverse faults off the western coast of Sumatra is similar to the system of seismic sources in the Kuril-Kamchatka island arc.

The Skovorodino, 2011, earthquake

On October 14, 2011, an earthquake with the magnitude Mw = 6.0 occurred 15 km northwest of the town of Skovorodino in East Siberia. This earthquake aroused interest among geophysicists. In this paper, we present some results on the prior seismicity in the region, which are based on the macroseismic and instrumental data. The assessment of the seismic regime immediately before the main shock and the analysis of the aftershock sequence rely on the data provided by the field network of the Schmidt Institute of Physics of the Earth of the Russian Academy of Sciences. The network was in operation from the end of July 2011 to January 2012. The analysis of the entire dataset suggests that the Skovorodino earthquake was outstanding in a vast region: only seismic events with low magnitudes (at most 4) occurred in the epicentral zone of this earthquake and its nearest neighborhood. The positions of the source of the main shock and the hypocenters of the after-shocks (more than 1300 events) are determined. The aftershock sequence is anomalous—it lacks sufficiently strong seismic events (M ≥ 3.5). The spatiotemporal distribution of the aftershocks shows two clusters of the epicenters to the west and east of the epicentral zone. These clusters drastically differ in their parameters, indicating that the source has a complex structure. Generally, the source can be understood as a shear displacement on the subvertical fault plane trending approximately along the latitude. The size of the source is 17 × 17 km; the average slip is 63 cm, which is at least 30% larger than the characteristic values for the earthquakes with M = 6.0.

Structural dynamic analysis of the epicentral zone of the Ilin-Tas earthquake (Feb 14, 2013, Ms = 6.9)

The paper presents the results of analysis of tectonics, modern relief morphology, active faults, and types of Cenozoic deformations in the epicentral area of the Ilin-Tas (Ms = 6.9) earthquake, one of the strongest events registered at the boundary of the Eurasian and North American lithospheric plates in northeast Russia. Geological, tectonic, and geophysical characteristics of the Yana-Indigirka segment of the Chersky seismotectonic zone are studied. The methods of investigation were elaborated at the Institute of the Earth’s Physics, RAS (Moscow), and were adapted to conducting seismotectonic work on the territory of northeast Russia. The results of instrumental observations are summarized, and manifestations of strong seismic events are discussed. Description is given of the structural–tectonic setting in which the Andrei-Tas seismic maximum developed. It originated under the effect of the Kolyma-Omolon terrane (indentor) which “intruded” into the Chersky seismotectonic zone on the side of the North American plate, thus leading to the formation of major seismoactive structures in the frontal Ilin-Tas folded zone. The indentor moved in NE-SW direction, which is consistent with the orientation of the major axis of isoseism ellipses (azimuth 50–85°) constructed from observation of macroseismic effects of the Uyandina (Ms = 5.6), Andrei-Tas (Ms = 6.1), and Ilin-Tas (Ms = 6.9) earthquakes.

Stress changes induced by the 2008 [formula omitted] Wenchuan earthquake, China

We invert the background stress fields of the Longmenshan and its adjacent regions before and after the 2008 great Wenchuan earthquake from focal mechanism data. Our results show that the stress orientations after the 2008 great Wenchuan earthquake are more coherent than those before this event at the epicentral zone. This result includes twofold implications: one is that the Wenchuan event altered the local stress field, the other is that the strength of the Wenchuan fault zone is very high and the characteristic of the seismogenic fault is consistent with the asperity model. We also estimate the stress changes caused by the Wenchuan event using an accurate source model and exhaustive receiver faults. The stress changes of >10kPa mainly occur in the nearby areas of the Longriba, east Kunlun, Qingchuan, Lushan faults and the southeastern section of the Xianshuihe fault, indicating that there is high earthquake hazard in these areas. The seismicity before and after the great Wenchuan earthquake in the Longmenshan and its adjacent regions shows that some subsequent earthquakes in the stress shadow area may be delayed and the 2013 Lushan earthquake may be advanced.

Late quaternary dislocations and seismotectonics of the Racha earthquake source, the Greater Caucasus

The results of studying traces of ancient seismic catastrophes in epicentral zone of the 1991 Racha earthquake are set forth. The source of this earthquake is noteworthy, because it reflects the main tendency in the geological evolution of the region, i.e., thrusting of the southern slope of the Greater Caucasus over the basement of the Transcaucasus Median Mass. It has been ascertained that two large-scale seismic events were related to this source in the Holocene. The approximate duration of the middle Holocene event was 2300 yr (7300–5000 yr ago). The late Holocene event, including the Racha earthquake in 1991, lasted ∼1600 yr. The role of the Racha earthquake in the large-scale seismic cycle is discussed. A surprising circumstance was the absence of datings in the time interval of 2400–9500 yr ago. The lack of these datings could have been caused by the change in climate and absence of paleosoils or by deceleration of seismic activity. The results also allowed us to substantiate seismotectonic segmentation of the southern slope of the Greater Caucasus in the area under consideration.

Crustal structure of the Gujarat region, India: New constraints from the analysis of teleseismic receiver functions

In this study, receiver function analysis is carried out at 32 broadband stations spread all over the Gujarat region, located in the western part of India to image the sedimentary structure and investigate the crustal composition for the entire region. The powerful Genetic Algorithm technique is applied to the receiver functions to derive S-velocity structure beneath each site. A detail image in terms of basement depths and Moho thickness for the entire Gujarat region is obtained for the first time. Gujarat comprises of three distinct regions: Kachchh, Saurashtra and Mainland. In Kachchh region, depth of the basement varies from around 1.5km in the eastern part to 6km in the western part and around 2–3km in the northern part to 4–5km in the southern part. In the Saurashtra region, there is not much variation in the depth of the basement and is between 3km and 4km. In Gujarat mainland part, the basement depth is 5–8km in the Cambay basin and western edge of Narmada basin. In other parts of the mainland, it is 3–4km. The depth of Moho beneath each site is obtained using stacking algorithm approach. The Moho is at shallower depth (26–30km) in the western part of Kachchh region. In the eastern part and epicentral zone of the 2001 Bhuj earthquake, large variation in the Moho depths is noticed (36–46km). In the Saurashtra region, the crust is more thick in the northern part. It varies from 36–38km in the southern part to 42–44km in the northern part. In the mainland region, the crust is more thick (40–44km) in the northern and southern part and is shallow in Cambay and Narmada basins (32–36km). The large variations of Poisson’s ratio across Gujarat region may be interpreted as heterogeneity in crustal composition. High values of σ (∼0.30) at many sites in Kachchh and few sites in Saurashtra and Mainland regions may be related to the existence of high-velocity lower crust with a mafic/ultramafic composition and, locally, to the presence of partial melt. The existing tectono-sedimentary models proposed by various researchers were also examined.

Crustal velocity structure of the Deccan Volcanic Province, Indian Peninsula, from observed surface wave dispersion

<p>Through inversion of fundamental mode group velocities of Love and Rayleigh waves, we study the crustal and subcrustal structure across the central Deccan Volcanic Province (DVP), which is one of the world’s largest terrestrial flood basalts. Our analysis is based on broadband seismograms recorded at seismological station Bhopal (BHPL) in the central India from earthquakes located near west coast of India, with an average epicentral distance about 768 km. The recording station and epicentral zone are situated respectively on the northern and southern edges of DVP with wave paths across central DVP. The period of group velocity data ranges from 5 to 60 s for Rayleigh waves and 5 to 45 s for Love waves. Using the genetic algorithm, the observed data have been inverted to obtain the crust and subcrustal velocity structure along the wavepaths. Using this procedure, a similar velocity structure was also obtained earlier for the northwestern DVP, which is in the west of the present study region. Comparison of results show that the crustal thickness decreases westward from central DVP (39.6 km) to northwestern DVP (37.8 km) along with the decrease of thickness of upper crust; while the thickness of lower crust remains nearly same. From east to west S-wave velocity in the upper crust decreases by 2 to 3 per cent, while P-wave velocity in the whole crust and subcrust decreases by 3 to 6 per cent. The P- and S-wave velocities are positively correlated with crustal thickness and negatively correlated with earth’s heat flow. It appears that the elevated crustal and subcrustal temperature in the western side is the main factor for low velocities on this side.</p>

Foreshocks of strong earthquakes

The specific enhancement of ultra-low-frequency (ULF) electromagnetic oscillations a few hours prior to the strong earthquakes, which was previously mentioned in the literature, motivated us to search for the distinctive features of the mechanical (foreshock) activity of the Earth’s crust in the epicentral zones of the future earthquakes. Activation of the foreshocks three hours before the main shock is revealed, which is roughly similar to the enhancement of the specific electromagnetic ULF emission. It is hypothesized that the round-the-world seismic echo signals from the earthquakes, which form the peak of energy release 2 h 50 min before the main events, act as the triggers of the main shocks due to the cumulative action of the surface waves converging to the epicenter. It is established that the frequency of the fluctuations in the foreshock activity decreases at the final stages of the preparation of the main shocks, which probably testifies to the so-called mode softening at the approach of the failure point according to the catastrophe theory.

The Emilia Earthquake: Seismic Performance of Precast Reinforced Concrete Buildings

On 20 and 29 May 2012, two earthquakes of MW5.9 and MW5.8 occurred in the Emilia region of northern Italy, one of the most developed industrial centers in the country. A complete photographic report collected in the epicentral zone shows the seismic vulnerability of precast structures, the damage to which is mainly caused by connection systems. Indeed, the main recorded damage is either the loss of support of structural horizontal elements, due to the failure of friction beam-to-column and roof-to-beam connections, or the collapse of the cladding panels, due to the failure of the panel-to-structure connections. The damage can be explained by the intensity of the recorded seismic event and by the exclusion of the epicentral region from the seismic areas recognized by the Italian building code up to 2003. Simple considerations related to the recorded acceleration spectra allow motivating the extensive damage due to the loss of support.

Using Diffuse Field Theory to Interpret the H/V Spectral Ratio from Earthquake Records in Cibeles Seismic Station, Mexico City

Abstract It has been recently demonstrated that averaging the autocorrelations of fields produced by various almost‐vertical incoming elastic body plane waves upon a layered system approximately leads to the imaginary part of the corresponding 1D Green’s functions for deep sources located underneath the receiver (Kawase et al. , 2011). Thus, the ensemble of these waves from deep earthquakes recorded in a station located in the epicentral zone is interpreted as a diffuse field. In this short note, we extend the study to consider earthquakes recorded in a station located at epicentral distances of up to hundreds of kilometers. We consider the horizontal‐to‐vertical spectral ratio (HVSR) of the averaged P , S , and coda waves and full earthquake records at the Cibeles station (Mexico City Accelerometric Network) and compare these with the results obtained with the corresponding HVSR for the 1D (Kawase et al. , 2011) and the 3D (Sanchez‐Sesma, Rodriguez, et al. , 2011) diffuse fields models. Using the signals of 90 earthquakes recorded at Cibeles, we find that the experimental results have distinctive features compatible with the 3D signature of a diffuse field. We interpret this result as a consequence of the multiple paths that seismic waves undergo from the subducting slab to the Mexico City valley and to the multiple scattering in a complex tectonic environment. Our study strongly suggests that we can use strong‐motion records from earthquakes and apply similar techniques to the ones used to analyze the ambient seismic field. Online Material: Earthquake catalog.

A new approach to recognition of the strong earthquake-prone areas in the Caucasus

Clustering the epicenters of Caucasian earthquakes with magnitudes M ≥ 3.0 is carried out, and the epicentral zones of the probable earthquakes with M ≥ 5.0 areas where epicenters of earthquakes with M ≤ 5.0 may occur are recognized by the Fuzzy Clustering and Zoning (FCAZ) algorithmic system developed by the authors at the Geophysical Center of the Russian Academy of Sciences. These zones correspond well to the locations of the epicenters of earthquakes with M ≥ 5.0. The zones recognized in this study are compared with the zones previously recognized by A.D. Gvishiani et al. in 1988 by the Earthquake-Prone Areas Recognition (EPA) technique. The comparison shows that the zones identified by FCAZ are mainly located inside the EPA-zones. The FCAZ-zones are also compared with the zones previously recognized using gravimetric and geological data. The results obtained by different methods closely agree.

Geological and macroseismic manifestations of the earthquake on October 16, 2011 in the Skovorodino region, Amur oblast

The epicentral zone and settlements that suffered from the MS = 6.1 earthquake in the northwest Amur oblast are examined. Only secondary seismic dislocations were revealed and mapped in detail. The inspection of settlements and inhabitansts inquiry allowed estimation of the intensity of the macroseismic effect based on the MSK-64 scale. These forthwith primary factual data give an idea on the shaking intensity at the distant and nearest zones and precise location of the earthquake focus. The map of isoseists of the highest (7–8) intensity is drawn.

Recurrence of strong earthquakes in the active Hovd Fault Zone, Mongolian Altay

A geological, geomorphic, and paleoseismological study of paleoseismodislocation systems has been carried out. Numerous indications of prehistoric strong earthquakes expressed in sediments deposited in the fault-line region due to the formation of transient barrier lakes were revealed in the active Hovd Fault Zone on the eastern slope of the Mongolian Altay during a paleoseismological study of young near-surface sediments in trenches and dug pits. The paleosoils buried by these sediments were radiocarbon-dated. It has been established that the strongest earthquakes with magnitudes of ∼8, which took place here 7200, 6600, 1500, and later than 700 years ago, substantially changed the topography in the epicentral zone. By analogy with the results of paleoseismological investigations performed in the Gorny Altay and the western Mongolian Altay, the data obtained shed light on the recurrence of the strongest seismic events in the Great Altay.

The 1991 Racha earthquake, Caucasus: Multiple source model with compensative type of motion

The source of the 1991 Racha earthquake in the Greater Caucasus generally corresponds to thrusting, which is characteristic of the predominant regional compression stress field. A more adequate view of the rupture process is provided by a complex source model composed of three subsources. This model is reconstructed by the body-wave inversion and consistent with the spatial distribution of the aftershocks. In terms of the suggested model, at the last stage of the rupture process, the opposite slip type (normal faulting) is observed in the source, which seems to be objective. It compensates the rapid (probably short) local redistribution of stresses caused by the thrusts in the first two subsources. The surface deformations observed in the epicentral zones of strong earthquakes are probably the analogs of such a compensative mechanism. For example, in the rear parts of the thrusts associated with the surface ruptures, normal faults trending parallel to the strike of the thrust line occur. Another analog of the compensative motion is probably the peculiarities of the aftershock sources. It has long since been noted (Kuznetsova et al., 1976) that some fault plane solutions in the aftershock sequences of strong earthquakes are close to the main shock solution, while others are different. The explanation of this phenomenon is suggested in (Kuznetsova et al., 1976; Kostrov and Das, 1988). In (Kuznetsova et al., 1976), these events are referred to as the aftershocks due to the fracture growth and aftershocks of relaxation, respectively.

Earthquake clustering in the tectonic pattern and volcanism of the Andaman Sea region

Seismicity pattern in the Andaman Sea region has been analysed to interpret recent dynamics of regional submarine volcanic provinces. Hypocentral determinations and focal mechanisms of available global seismological data have been used. Frequent occurrence of earthquake swarms proved to be an important characteristic of seismicity pattern in the investigated region. Epicenters of the earthquake swarms are situated almost exclusively along a narrow belt forming the easternmost limit of the epicentral area of the Andaman Sea region. The southern zone of the belt between 6° and 10.5° N is trench parallel and coincides with the northward prolongation of the Sumatra volcanic arc. The northern zone of the belt between 10.5° and 13° N is deflected by about 45° northeast and precisely follows the complicated zigzag pattern of the rift system in the middle of the Andaman Basin. Earthquake occurrence in these two zones, the northern and the southern, differs by several aspects — by shape of the epicentral zones of individual swarms, by focal mechanisms and by response to the 2004 Sumatra–Andaman great earthquake. These differences, together with available information on composition of magmas found at the seafloor, lead us to the conclusion that the swarms of the southern zone are induced by intrusions of subduction-generated calc-alkaline magmas whereas the swarms in the northern zone by intrusions of basaltic magmas associated with the seafloor spreading. Earthquake swarm occurrence defines a brittle, seismogenic layer at depths between 9 and 35 km, excludes the existence of large magma reservoirs in respective depth interval and puts their hypothetical location to a greater depth. Episodes of magma ascent from deeper reservoir to shallow magma chamber (depth < 9 km) or up to the surface induce earthquake swarms. The region deserves attention of volcano seismologists through its almost continuous activity and represents an attractive area for on-site geophysical monitoring and direct seafloor observations.

On the near-hourly hidden periodicity of earthquakes

The so-called hour-mark effect, which reflects a response of the lithosphere to anthropogenic forcing, was initially detected when processing the earthquake catalogues by the method of synchronous detection. When attempting to reveal this effect by spectral analysis, we encountered an interesting feature of global seismicity. Namely, the spectrum of seismic activity indeed contains a peak at a frequency of 0.277 mHz, and this peak has a clearly anthropogenic origin (the hour-mark effect). At the same time, the spectrum also contains a stronger peak at a frequency of 0.309 mHz, which corresponds to a period of 54 min. We have independently detected this period in the aftershock sequences in the epicentral zones of large earthquakes and in the variations of seismicity in the antipodal zones. The 54-min periodicity coincides with the fundamental mode 0S2 of the free oscillations of the Earth. It is suggested that oscillations of the Earth as a whole result in a weak but detectable modulation of seismic activity.

The first estimations of soil-radon activity near faults in Central Mongolia

Estimation of soil-radon activity, Q was first carried out for faults in Central Mongolia. Eight study sites were located in epicentral zones of Mogod (M = 7.8; 05 January 1967) and Avdar (M = 3.8; 22 March 2009) earthquakes, and in the vicinity of Ulaanbaatar, where small seismic events (M = 1.0–2.5) occurred in the past few years. Profile radon surveys were conducted at fifteen faults that differ in size and geodynamic activity, yet clearly topographically manifested as scarps or straightened segments of valleys of ephemeral streams. By applying the formalized method of processing of the survey results, it was possible to reveal radon anomalies and to establish that their shape, intensity and contrast are mainly determined by the structure of the fault zone. Due to heterogeneous permeability of fault zones, shapes and quantitative parameters of radon anomalies are variable at different faults and in individual cross-sections of one and the same fracture. Radon anomalies in Central Mongolia are diverse, yet the most frequent are the cases where (1) a radon anomaly is discontinuous in shape due to the presence of small domains with minimum values of Q; (2) a major part of the anomaly is located in one fault wall; and (3) a fault scarp is marked by a minimum value of Q. In Central Mongolia, intensities of radon anomalies, Qmax near neotectonic faults differ by more than an order of magnitude. The most intense anomaly (20,200 Bq/m3) is registered at Hustai fault in the vicinity of Ulaanbaatar, which indicates the importance of assessment of radon hazard for the capital city of Mongolia, where almost half the population of the country reside. The contrast of radon anomaly, KQ is determined as a ratio of a maximum value of Qmax to a minimum value of Qmin outside the fault zone; it varies from 1.4 to 17.3 for faults of Central Mongolia. Faults characterized by ultra-high (KQ > 10), high (10 ≥ KQ > 5), increased (5 ≥ KQ > 3), medium (3 ≥ KQ > 2) and low (KQ ≤ 2) radon activity are distinguished. A relative index, KQ can be effectively applied for assessment of geodynamic activity of faults in Central Mongolia. On the one hand, it correlates with sizes and seismic potential of the studied faults; on the other hand, it significantly reduces the complicating influence of regional factors, such as radioactivity of rocks, sediment thickness, meteorological conditions of measurements, etc. The application of KQ, the formalized method of detection of radon anomalies, long-distance base cross-sections, and reduction of the measurement interval near faults – these key features of the profile survey are recommendable for further more accurate estimations on the basis of the first measurements of soil-radon activity in Central Mongolia.

Crustal deformations in the epicentral area of the West Bohemia 2008 earthquake swarm in central Europe

In West Bohemia, central Europe, during October 2008 an earthquake swarm of 25,000 shocks with a maximum event of ML ∼ 3.7–3.8 occurred at depths of 7–11 km. In 2007, annual GPS campaigns were launched. During the co‐seismic phase, displacements of a few centimeters were detected at GPS sites. Maximum displacement was revealed at the KOPA site, which subsided by 167 mm. The epicentral area is covered by eluvium of 4–10 m thick, and is located in undulating pastures and well‐forested valleys where visible surface soil effects could not be observed. To test possible fault manifestations, rough geomorphologic, geoelectric, and geochemical surveys were performed. GPS and seismic data, with geologic materials, were used to build a forward model for surface displacements, crustal deformations, and shear and normal stress fields. The fields enabled us to better determine crustal deformations and stresses that appeared within the seismic cycle, during the pre‐, co‐, and post‐seismic phases. During the co‐seismic phase, modeled fault motions along N‐S faults located within the epicentral zone reached 0.6–1 mm/day. Possible structural block rotations were comprised of these motions. A dominant role for stress accumulation, release, and relaxation was assigned to the Mariánské Lázně fault zone and the Nový Kostel zone. Strain loads slowly, and when local PT conditions with an action of deep magmatic fluids reach instability, the strain is released and stress balancing occurs. The process leads to the reversible motions known for silent earthquakes. A forward crustal deformation model for West Bohemia is also presented within.