S. S. Arefiev

The 2009 Bhutan and Assam felt earthquakes (Mw 6.3 and 5.1) at the Kopili fault in the northeast Himalaya region

Seismotectonics of the two recent earthquakes, one Mw 6.3 in the Bhutan Himalaya on 21 September 2009 and the other Mw 5.1 in the Assam valley on 19 August 2009, are examined here. The recent seismicity and fault plane solutions of these two felt earthquakes suggest that both the events occurred on the Kopili fault zone, a known active fault zone in the Assam valley, about 300 km long and 50 km wide. The fault zone is transverse to the east–west Himalayan trend, and its intense seismicity indicates that it transgresses into the Himalaya. The geologically mapped curvilinear structure of the Main Central Thrust (MCT) in the Himalaya, where the epicentre of the Bhutan earthquake is located, is possibly caused by the transverse Kopili fault beneath the MCT. This intensely active fault zone may be vulnerable to an impending larger earthquake (M > 7.0) in the region.

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.

Earthquake intensity assessment based on environmental effects: principles and case studies

Abstract The comparison of intensity assessments based on macroseismic data and Earthquake Environmental Effects (EEE) is presented. Specific problems faced when assessing intensities using different types of scales are discussed. Two case studies of recent earthquakes with magnitudes MS=7.4 (Altai, 2003, and Neftegorsk, 1995) are used to illustrate the applicability of the INQUA EEE scale. The Altai earthquake was accompanied by surface faulting of c. 70 km length and up to 2 m of horizontal and 70 cm of vertical offset; secondary EEE were observed over 3000 km2. The dominant type of surface faulting during the Neftegorsk earthquake was strike-slip. The length of surface faulting was up to 46 km, maximum horizontal offset was 8.1 m, and average offset coherent with seismic moment was 3.9 m; secondary EEE were observed occasionally at considerable distance from the epicentre on wet seashore sands. Application of the INQUA scale shows the epicentral intensity of the Altai earthquake to be X degrees. Most consistent with all types of data (rupture length, maximum and average offsets) intensity assessment for the Neftegorsk earthquake which is within the X–XI degree range. Taking into account environmental effects in intensity scales is an essential requirement: it follows from the complex nature of an earthquake impact, which spans a very broad frequency range, including static deformations. The case studies illustrate that the intensity assessment of an earthquake, based only on damage to buildings, will be essentially incomplete.

Deep structure and tomographic imaging of strong earthquake source zones

This work generalizes the results of tomographic imaging performed by the authors for epicentral zones. Seismic events in North Africa (the Mw = 5.8 earthquake of 1985 near the town of Constantine), eastern Anatolia (the Erzincan Mw = 6.7 earthquake of 1992), the Lesser and Greater Caucasus (the 1988 Spitak Mw = 6.8 and the 1991 Racha Mw = 7.0 earthquakes), and northern Sakhalin (the 1995 Neftegorsk Mw = 7.1 earthquake) are examined. It is shown how various morphokinematic types of active faults differ in the resulting tomographic images at various depths. A classification of tomographic images of strong earthquake source zones is proposed in accordance with the rank of their generating faults. The sources of the Spitak, Racha, and Erzincan earthquakes are confined to large boundary faults separating tectonic zones. Lower velocity bands are revealed in the tomographic images, and low velocity “pockets” 1–2 km or somewhat more in width penetrating to a depth of up to 15 km are observed near the fault zones. The Constantine and Neftegorsk earthquakes were generated by faults of a lower rank. The source zones of these events are imaged tomographically as narrow gradient zones.

Results of Dipole Electric Sounding in the Area of the Chirkey Water Reservoir after its Filling

The analysis of the data of variations of the apparent electric resistivity of rocks (AR) in the area of the Chirkey HES has been carried out for the period after the water reservoir was filled. For measurements the method of dipole electric sounding has been used with location of potential dipoles around the water reservoir with a distance between them of 5.2–11.3km. The analysis of the data obtained for the period of observations (1976–1988) has shown that the filling of the water reservoir affected the environment for a long time. After it was filled (1975), at different observation points located around the water reservoir a decrease of 1.6–2.4 times was registered in the AR over the period from 1976–1988. This is connected with the process of inundation of the rock mass, which continued for more than 14 years. The process of water filtering into the surrounding rock mass was complicated; that is, it varied in time and space. At the initial stage of the reservoir filling the AR variations of a high amplitude (30–40%) were observed. These variations are considered to be connected with the seismic regime of the area of the water reservoir. A synchronous decrease in the AR, registered at the receiver points was followed by an increase of the number of the earthquakes of energetic class (K = 10). The given anomalous data are connected by an increase in the water filtration into fractured zones, whose filtration features change on being affected by tectonic stresses.

Structural heterogeneity of the media and macroseismic field formation: Racha, April 29, 1991 earthquake

The points with normal, anomalously low, and anomalously high shaking intensities are recognized in the spatial distribution of macroseismic effects from the 1991 Racha earthquake, Greater Caucasus. Distribution of these points in the epicentral area is not random. Comparison between this distribution and the results of local tomography reveals that seismic wave velocities do not increase in the upper layers (from 0 to 3 km) beneath the points with anomalously high intensity, while a sharp increase in velocity is observed in the depth interval from 6 to 9 km. An original method of b-value mapping is suggested. Application of the method demonstrates that anomalously low intensities correlate to high b-values. This likely reflects higher intensity attenuation associated with higher b-value.