Source model and the macroseismic effect of the 1991 Racha earthquake

Coherency of the source model of the 1991 Racha earthquake in the Greater Caucasus with different data types is analyzed. Authors, when interpreting macroseismic data, accept complex nature of macroseismic effects generation but, nevertheless, consider that its spatial distribution follows certain regularities. First time in the practice, method of evaluation of the mecroseismic material completeness is proposed based on the intensity attenuation along with distance. It is demonstrated the character of macroseismic intensity attenuation can be used for verification of the source model elements constructed based on other seismological data. Dependence of the macroseismic effect distribution on azimuth in near field of the 1991 Racha earthquake is recognized.     

The 23 April 1909 Benavente earthquake (Portugal): macroseismic field revision

The 23 April 1909 earthquake, with epicentre near Benavente (Portugal), was the largest crustal earthquake in the Iberian Peninsula during the twentieth century (M _w = 6.0). Due to its importance, several studies were developed soon after its occurrence, in Portugal and in Spain. A perusal of the different studies on the macroseismic field of this earthquake showed some discrepancies, in particular on the abnormal patterns of the isoseismal curves in Spain. Besides, a complete list of intensity data points for the event is unavailable at present. Seismic moment, focal mechanism and other earthquake parameters obtained from the instrumental records have been recently reviewed and recalculated. Revision of the macroseismic field of this earthquake poses a unique opportunity to study macroseismic propagation and local effects in central Iberian Peninsula. For this reasons, a search to collect new macroseismic data for this earthquake has been carried out, and a re-evaluation of the whole set has been performed and it is presented here. Special attention is paid to the observed low attenuation of the macroseismic effects, heterogeneous propagation and the distortion introduced by local amplifications. Results of this study indicate, in general, an overestimation of the intensity degrees previously assigned to this earthquake in Spain; also it illustrates how difficult it is to assign an intensity degree to a large town, where local effects play an important role, and confirms the low attenuation of seismic propagation inside the Iberian Peninsula from west and southwest to east and northeast.     

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.     

Deep structure of the Racha earthquake source zone from seismic tomography data

The Racha earthquake of 1991 was the strongest of the earthquakes recorded in the Caucasus. It was studied by an international epicentral expedition. Unique data gathered by this expedition included records of aftershocks whose swarm was very intense. A 3-D velocity model is obtained from analysis of these data by the method of local earthquake tomography. The ancient crystalline basement and the sedimentary-volcanic Mesozoic-Cenozoic cover could be identified from cross sections of the P wave field. The eastern and western boundaries of an uplift in the basement of the Dzirulskii Massif are delineated. Linear low velocity heterogeneities correlating with the active Kakheti-Lechkhumi fault zone and two Trans-Caucasian linear fault zones are discovered in the basement. The cloud of aftershock hypocenters is shown to correlate with a reflector coinciding with the cover-basement interface.     

Assessing the intensity of the September 17, 2003, Khoito-Gol earthquake from instrumental and macroseismic data

The present study is devoted to the assessment of shaking intensities due to the September 17, 2003, earthquake occurring in the Khoito-Gol basin (southwestern flank of the Baikal Rift System). The instrumental and macroseismic data used here were acquired during an investigation into the impact of this seismic event. The highest intensity of shaking, VI, was recorded at Khoito-Gol, the human settlement that was the nearest to the instrumental epicenter. A nonuniform distribution of intensity was found for different directions from the epicenter. A scatter of as much as two intensity units was recorded at one settlement.     

Surface disturbances connected with the Kaliningrad earthquake of September 21, 2004, and their correlation with macroseismic scales

Factual data on surface disturbances that emerged in the proximal zone of the Kaliningrad earthquake on September 21, 2004 (M = 4.9, I ₀ = 6.5) on land and in the adjacent basin are presented. These observations are nearly the first of this kind within the eastern European platform. It is shown that, on land, all disturbances were provoked by the shocks; however, they developed partially or completely as delayed consequences and had a superficial character, being conditioned by extremely unfavorable peculiarities of the local soil and relief. Such disturbances are not provided for in the MSK-64 and INQUA macroseismic scales during shocks with I = 6, which presents a number of problems for further studies and the adjustment of scales in the middle gradation zone.     

A combined study of macroseismic data and focal mechanisms applied to the West-Bohemian earthquake,Czechoslovakia

The main shock of the West-Bohemian earthquake swarm, Czechoslovakia, (magnitudem=4.5, depthh=10 km) exhibits an irregular areal distribution of macroseismic intensities 6° to 7° MSK-64. Four lobes of the 6° isoseismal are found and the maximum observed intensity is located at a distance of 8 km from the instrumentally determined epicentre. This distribution can be explained by the energy flux of the directS wave generated by a circular source, the hypocentral location and focal mechanism of which are taken from independent instrumental studies. The theoretical intensity, which is assumed to be logarithmically proportional to the integrated squared ground-motion velocity (i.e.,I=const+log v ² (t)dt), fits the observed intensity with an overall root-mean-square error less than 0.5°. It is important that the present intensity data can also be equally well explained by the isotropic source. The fit was attained by means of a horizontally layered model though large fault zones and an extended sedimentary basin suggest a significant lateral heterogeneity of the epicentral region. The results encourage a broader application of the simple modelling technique used.     

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 M _S = 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.     

Revising macroseismic data in Switzerland: The December 20, 1720 earthquake in the region of Lake Constance

Upgrading the Earthquake Catalog of Switzerland (ECOS) included revising the earthquake of 1720. This change has major importance for history and seismology.Although that quake has been the subject of several publications, none was based on critical methods. This re-evaluation of the event is built upon a new and more reliable database established after investigating archives and libraries. Using data from such historical sources, we assigned new site intensities, adopting the criteria established by the European macroseismic scale EMS 98 (Grünthal, 1998).We discovered that the event had been assigned an overestimated intensity, due to interpretation errors in former earthquake catalogs and compilations. We recommend reducing the intensity from I₀= VIII to I₀= VI (EMS 98). The moment magnitude is given as M_W= 4.6. Since the event had been considered the largest for its respective area, downgrading it now will influence the seismic hazard assessment for this region.     

The Montenegro,Yugoslavia, earthquake of April 15, 1979: source orientation and strength

Long-period teleseismic P, S and Rayleigh waves and geologic considerations indicate that the Montenegro earthquake involved thrust faulting on a plane striking nearly parallel to the Adriatic coast and dipping ca. 15° toward the Yugoslav mainland. There is some support from modeling of body waves recorded on long-period WWSSN instruments for a focal depth of 22 km, but the possibility of a multiple source and the difficulty of matching some of the detailed characteristics of the P- and S-wave forms reduce our confidence in the determination of the depth. Fortunately, the source orientation and moment of the event are not sensitive functions of the depth. The long-period (256 s) moment was 4.6 × 10¹⁹ Nm (4.6 × 10²⁶ dyne-cm). The moment obtained by fitting the first cycle of P and S waves recorded on WWSSN long-period instruments is about four times smaller. This increase of moment with period is consistent with spectral estimates of the moment from SH waves recorded at SRO and ASRO stations.     

阳江市洋边海2001年4月22日ML4.6级地震影响场

2001年4月22日在阳江市洋边海发生ML4.6级强有感地震,部分房屋受到轻微破坏。震后对地震现场进行了调查,得出这次地震的等烈度线。     

Evaluation of the seismic moment of the April 20, 2013 Lushan earthquake

The April 20, 2013 Lushan earthquake which occurred in Sichuan, China had only moderate thrust. However, the computed seismic moments (M ₀) for the Lushan earthquake calculated by several institutions differ significantly from 0.4 × 10¹⁹ to 1.69 × 10¹⁹ Nm, up to four times difference. We evaluate ten computed M ₀s by using normal mode observations from superconducting gravimeters in Mainland China. We compute synthetic normal modes on the basis of moment tensor solutions and fit them to the observed normal modes. Comparison of our results indicates that M ₀ is the main cause for some large differences between observations and synthetics. We suggest that a moment magnitude of M _w6.6, corresponding to a M ₀ of 0.97–1.08 × 10¹⁹ Nm, characterizes the size and strength of the seismic source of the Lushan earthquake.     

Evaluation of the seismic moment of the April 20, 2013 Lushan earthquake

The April 20, 2013 Lushan earthquake which occurred in Sichuan, China had only moderate thrust. However, the computed seismic moments （M0） for the Lushan earthquake calculated by several institutions differ significantly from 0.4 × 1019 to 1.69 ×019 Nm, up to four times difference. We evaluate ten computed Mos by using normal mode observations from superconducting gravimeters in Mainland China. We compute synthetic normal modes on the basis of moment tensor solutions and fit them to the observed normal modes. Comparison of our results indicates that Mo is the main cause for some large differences between observations and synthetics. We sug- gest that a moment magnitude of Mw6.6, corresponding to a Mo of 0.97-1.08 × 1019 Nm, characterizes the size and strength of the seismic source of the Lushan earthquake.     

On the criteria of macroseismic information reliability: A case study of the 1542 earthquake in the Russian North

Historical seismology suggests certain recommendations to assess the reliability of macroseismic information. These recommendations are quite general and have proved their efficiency in many cases. But they should be very carefully applied, because the formal approach used in them is not always reasonable. Based on the analysis of sources relevant to the 1542 earthquake in the north of Russia, it is shown that though the distribution of priorities for sources relying on how close they are to the event works well, it can not be considered a universal rule. Many other details, such as characteristic of the person compiling the document, one’s interests, how free one could access the non-public achirves, and the goal of writing the document, have to be taken into account.     

Rupture process of the M_w7.9 Nepal earthquake April 25, 2015

<正>On April 25,2015,a magnitude Mw7.9 earthquake occurred in the southern Himalaya,Nepal,at 14:11 local time(UTC2015-04-25 06:11).Its epicenter was at 28.147°N,84.708°E with a source depth of 15 km,as determined by the United States Geological Survey(USGS).The earthquake hazard and secondary disasters,including landslides and avalanches,resulted in serious damage to Nepal and surroundings(including Kathmandu and the northern Himalaya of China)and caused huge loss of life and considerable destruction of property.     

The April 24, 2013 Changning M_s4.8 earthquake: a felt earthquake that occurred in Paleozoic sediment

The dense broadband seismic network provides more high-quality waveform that is helpful to improve constraint focal depth of shallow earthquake. Many shallow earthquakes occurring in sediment were regarded as induced events. In Sichuan basin, gas industry and salt mining are dependent on fluid injection technique that triggers microseismicity. We adopted waveform inversion method with regional records to obtain focal mechanism of an Ms 4.8 earthquake at Changning. The result suggested that the Changning earthquake occurred at a ESE thrust fault, and its focal depth was about 3 km. The depth phases including teleseismic pP phase and regional sPL phase shows that the focal depth is about 2 km. The strong, shortperiod surface wave suggests that this event is a very shallow earthquake. The amplitude ratio between Rayleigh wave and direct S wave was also used to estimate the source depth of the mainshock. The focal depth(2–4 km) is far less than the depth of the sedimentary layer thickness(6–8 km) in epicentral region. It is close to the depth of fluid injection of salt mining, which may imply that this event was triggered by the industrial activity.     

The catastrophic earthquake of April 25, 2015, in Nepal: Analysis of seismological data

The paper analyzes available seismic data of the Geophysical Survey of the Russian Academy of Sciences on the catastrophic earthquake with Ms = 7.9 occurred in Nepal on April 25, 2015. It is shown that this earthquake (also called Gorkha) in its coseismic stage reflected the dynamic situation in the collision zone between the Indian and Eurasian plates, and occurred in the area of the Main Frontal Thrust in the Himalayas. In the last 15 years, the seismicity of this area has demonstrated the features of strong earthquake preparation. The study results are presented for the early postseismic stage (in the first month after the mainshock). It is found that the pattern of a decrease in aftershock activity is similar to that obtained by Tatevossian and Aptekman (2008) for the world’s earthquakes with M > 8. It is regular in the first 11–16 days and can be described by the Omori law, whereas on 17th day after the mainshock, the exponent characterizing the rate of change in the flow of events becomes to irregular. The spatial and temporal distribution of aftershocks of the 2015 Gorkha earthquake qualitatively and quantitatively indicates the heterogeneity of a seismogenic interface of the Himalayan arc collision zone between the Indian and Eurasian plates.