ISSN 07479239, Seismic Instruments, 2015, Vol. 51, No. 2, pp. 176–189. © Allerton Press, Inc., 2015. Original Russian Text © V.N. Solovjev, I.N. Tikhonov, A.I. Kozhurin, 2014, published in Voprosy Inzhenernoi Seismologii, 2014, Vol. 41, No. 2, pp. 60–76. Refinement of Background Seismicity on the Survey Plot of Sakhalinskaya GRES2 (Sakhalin Island) V. N. Solovjeva, I. N. Tikhonova, and A. I. Kozhurinb, c a Institute of Marine Geology and Geophysics, Far Eastern Branch, Russian Academy of Sciences, ul. Nauki 1B, YuzhnoSakhalinsk, 693022 Russia b Institute of Volcanology and Seismology, Far Eastern Branch, Russian Academy of Sciences, br Piipa 9, PetropavlovskKamchatskii, 683006 Russia c Geological Institute, Russian Academy of Sciences, Pyzhevskii per. 7, Moscow, 119017 Russia emails: [email protected], [email protected] Abstract—The background seismicity of the territory in the vicinity of the site of the planned construction of the Sakhalinskaya GRES2 power plant, in the region of the Poyasok Isthmus, is refined. The interpretation of aerial photos of the territory under study is performed including the survey plot. Five active faults, previ ously unknown, are discovered, four of which are identified with a high degree of reliability. All of them are located beyond the survey plot limits. Within the survey plot, no indications of active faults are found. Design response spectra are calculated for return periods of 500, 1000, and 5000 years, and accelerogram analogues are selected. Maps of detailed seismic zoning (DSZ) are developed in parameters of the macroseismic inten sity (IMSK) and the peak ground acceleration (PGA, g) with 5% damping for the return periods of 500, 1000, and 5000 years. The IMSK assessments of DSZ maps at the central survey point for mean grounds of category II appeared to be lower in comparison with the similar GSZ97 data by 0.4 (for map A), 1.0 (for map B), and 0.3 (for map C). Keywords: Sakhalin Island, shallow seismicity, active faults, models of seismicity, probabilistic analysis, seis mic hazard assessment, detailed seismic zoning DOI: 10.3103/S0747923915020085 INTRODUCTION be assessed at three levels when constructing objects of different responsibility. Map A is intended for objects The official documents determining a level of of normal (largescale construction) and reduced background seismicity and influence of ground responsibility; maps B and C are destined for objects of conditions for the Russian Federation territory are increased liability (especially dangerous, technically SNIP II781* (2000) and a set of maps of general complicated or unique buildings). seismic zoning (GSZ) GSZ97 (Ulomov and Shumil ina, 1999). The GSZ97 maps reflect the calculated The fact that, according to map A, North Sakhalin intensity of seismic shaking in numbers of the MSK and the western part of Middle Sakhalin are related to 64 scale expected on the given area with the given the 9point zone indicates a high level of seismicity of probability p (%) during the certain time interval t on Sakhalin Island. The survey object (Sakhalinskaya mean grounds corresponding to the grounds of cate GRES2) is situated in Tomari borough of Sakhalin gory II according to SNIP II781*. Oblast, north of Il’inskii village, in the region of the Poyasok Isthmus. For Il’inskii village, a level of seis The set of GSZ97 maps consists of three maps: mic hazard is determined by the GSZ97 data as GSZ97A (map A), GSZ97B (map B), and GSZ 8 (map A), 9 (map B), and 9 (map C) (Ulomov and 97C (map C), which correspond to probabilities of 10, Shumilina, 1999). 5, and 1%, respectively, of possible exceedance (or 90, The GSZ maps are made on a scale from 95, and 99%, respectively, of nonexceedance) of the 1 : 8000000 to 1 : 2500000. When a background level design seismic intensity over 50year intervals of time, of seismic hazard is assessed, calculations are per which is appropriate to the recurrence of the seismic formed taking into account regional (generalized) effect on the Earth’s surface on the average of once seismotectonic models. In this connection, the trans every 500, 1000, and 5000 years. fer from GSZ maps to those of larger scale (1 : Thus, the maps display the numbers that (with cor 500000–1 : 100000 and larger) is an urgent problem. rection for ground conditions) are initial data for These studies are conventionally called the refinement the calculation of a seismic load on a building. The of initial seismicity or refinement of GSZ maps. The GSZ97 map set allows the seismic hazard degree to problem is resolved by the conduction of detailed seis 176 REFINEMENT OF BACKGROUND SEISMICITY ON THE SURVEY PLOT 177 mic zoning (DSZ), taking into account local features of a seismotectonic model of one or other region. In ab1 this case, the separation of zones of earthquake source II 6 occurrence (ESO) is performed using largerscale 2 maps (in comparison to GSZ97 maps) on the basis of 3 data on geological structure, tectonics, geophysical fields, seismic statistics, and other materials available. 5 In this work, the results are described of the inter pretation of aerial photos that was performed for the purpose of discovering active faults on the territory 4 under study, including the survey plot, whose position is shown in Fig. 1. Calculations for the DSZ of the ter ritory in the vicinity of the Sakhalinskaya GRES2 7 survey plot have been carried out. Materials of multi 3 year investigations of the fault tectonics of the earth’s crust of the region, the historical and instrumental data on the seismicity and seismic regime, the existing seismotectonic models of the region, as well as mod 8 ern technology of probabilistic seismichazard analy sis (PSHA) with identification and quantitative esti mation of the arising uncertainty on a logic tree basis I THE SEA were involved during the workexecution phase. OF OKHOTSK Sakhalin I. ACTIVE FAULTS OF SAKHALIN ISLAND III Active faults within Sakhalin Island form two zones of meridional stretch, which correspond to island elongation (see Fig. 1). The first zone (I, see Fig. 1) is located in the southern and central parts of the island, displaced westward from its axis, and separates the 2 uplift of the Western Sakhalin Mountains from the late TP Cenozoic Susunai and TymPoronai depressions situ ated to the east. There has been no confirmation so far that this zone stretches within the limits of North Sakhalin, though it seems to be quite possible. The zone is known as Tsentral’noSakhalinskaya (Central Sakhalin), or TymPoronaiskaya (Rozhdestvenskii, 1982; 1984), and Klyuchevskaya (Kuchai, 1987). The kinematics of active movements along the zone’s faults is of the thrustfault type (Kuchai, 1987; Bulgakov et al., 2002; Tsutsumi et al., 2005); the revealed recur rence periods (return periods) for the zone’s individ 0100 km ual faults reach 3000–5000 years, while the detected values of the oneevent net displacement are up to 4 m (Strel’tsov and Kozhurin, 2002; Tsutsumi et al., 2005). Strikeslip displacements in the relief’s young forms 1 are detected for none of the zone’s faults, though ech S I III Fig. 1. Main active faults and fault zones of Sakhalin Island (based on SRTM30_PLUS (Becker et al., 2009)): (1) (a) active and (b) supposed faults; (2) thrust faults and reverse faults; (3) survey plot Sakhalinskaya GRES2. On the map: Roman numerals denote (I) Central Sakhalin (Tym Poronai), (II) Northern Sakhalin, and (III) Western Sakhaline fault zones; Arabic numerals denote: (1) Apre lovskii, (2) Klyuchevskoi, (3) Goromai, (4) Piltun, elon elements in the plan view created by them allow (5)Longri, (6) Kheiton, (7) Upper Piltun (Neftegorsk), and (8) Dagi faults. Letters S and TP denote Susunai and the presence of a rightlateral shear component to be TymPoronai central depressions, respectively. supposed. SEISMIC INSTRUMENTS Vol. 51 No. 2 2015 178 SOLOVJEV et al. Faults of the second, SeveroSakhalinskaya 650 km; mainly 280–350 km) activities. Deepfocus (Northern Sakhalin), zone (the name is taken from (mantle) earthquakes are connected with the inclined (Rozhdestvenskii, 1982)) (II, see Fig. 1) stretch closer seismofocal zone passing under the island. It appears to the eastern edge of the island, in its northern low that the mode of crustal seismicity does not depend land part and on the Schmidt Peninsula. These are substantially on the regime of mantle seismicity and (from south to north) the Goromai, Piltun, Longri, deep earthquakes do not present a significant seismic and Kheiton faults. The first two faults are sometimes hazard. In this connection, only crustal earthquakes combined into a single fault: PiltunEkhabinskii are considered below (Fig. 2), the majority of foci of (name after (Rozhdestvenskii, 1982)). In addition to which are located in the upper part of the earth’s crust them, this zone, obviously, includes also (i) the Upper at depths to 30 km, while the distribution maximum Piltun fault, the movement along which in 1995 corresponds to the depth ~10 km. caused the Neftegorsk earthquake and the occurrence Strong (M ~ 5.0 and stronger (larger)) earthquakes of the Neftegorsk seismic rupture, (ii) the Dagi fault are associated with the Western Sakhalin, Central (in the axial part of the Dagi uplift–anticline, see Sakhalin, and Northern Sakhalin faults or with their Fig. 1), and (iii) other relatively nonextended faults splays. On the average, a single earthquake with M ≥ 6 discovered during the interpretation of satellite pho and around 10 events with M ≥ 5 are recorded on tos. Sakhalin Island every 10 years. For the period of Apart from the two zones above considered, the instrumental observations, the most significant seis third zone stands out, the ZapadnoSakhalinskaya mic events on Sakhalin Island were the Lesogorsk– (Western Sakhalin), which stretches, as supposed, in Uglegorsk (1924, MS = 6.8), Onori (1909, MS = 6.1), the foot of the western continental slope of the island Moneron (1971, MS = 7.5), Neftegorsk (1995, MS = (III, see Fig.