Vol52,2,2009 17-06-2009 19:02 Pagina 149 ANNALS OF GEOPHYSICS, VOL. 52, N. 2, April 2009 Shallow subsurface geology and Vs characteristics of sedimentary units throughout Rasht City, Iran Latif Samadi and Behzad Mehrabi Department of Geology, Tarbiat Moalem University, Tehran, Iran Abstract The Manjil-Rudbar earthquake of June 1990 caused widespread damage to buildings in the city of Rasht locat- ed 60 km from the epicenter. Seismic surveys, including refraction P-wave, S-wave and downhole tests, were carried out to study subsurface geology and classify materials in the city of Rasht. Rasht is built on Quaternary sediments consisting of old marine (Q1m), deltaic (Q2d), undivided deltaic sediments with gravel (Qdg) and young marine (Q2m) deposits. We used the variations of Vp in different materials to separate sedimentary boundaries. The National Earthquake Hazard Reduction Program (NEHRP) scheme was used for site classifica- tion. Average S-wave velocity to a depth of 30 m was used to develop site categories, based on measured Vs val- ues in 35 refraction seismic profiles and 4 downhole tests. For each geological unit histograms of S-wave veloc- ity were calculated. This study reveals that the Vs]30 of most of the city falls into categories D and C of NEHRP site classification. Average horizontal spectral amplification (AHSA) in Rasht was calculated using Vs]30 . The AHSA map clearly indicates that the amplification factor east and north of the city are higher than those of south and central parts. The results show that the lateral changes and heterogeneities in Q1m sediments are significant and most damaged buildings in 1990 Manjil earthquake were located in this unit. Key words Shear wave velocity – Vs]30 – NEHRP earthquake were strongly related to the local site classification – Rasht – Iran site conditions. Investigations of ground-mo- tion properties indicate that the amplitude, fre- quency content, and shape of seismic waves are 1. Introduction modified by the soils and sediments through which they are transmitted (Joyner and Boore, It is well known that local site conditions 1988; Borcherdt, 1994; Zeng, 1993; Olsen and could give rise to significant local amplification Archuleta, 1996). Shear wave velocity is an im- of ground motion during earthquakes. Historic portant parameter for evaluating the dynamic earthquakes have shown that damage to struc- behavior of soil in the shallow subsurface. tures is often significantly greater on soft sedi- Thus, site characterization in calculating seis- ments than on stiff sediments. Wood (1908) and mic hazard is usually based on the near surface Reid (1910) have shown that the intensity of shear wave velocity values. Seismic wave ve- ground motions during the 1906 San Francisco locity in soil depends on the nature of soils and sediments and factors such as cementation, fab- ric, age, heterogeneity, wet or dry conditions (Gorynov, 1992). The Vs in rocks mainly de- pends on the type, mineralogy, texture, degree Mailing address: Dr. Latif Samadi, Department of Geo- of weathering and alteration. The average shear logy, Tarbiat Moalem University, 49 Mofateh Avenue, Teh- wave velocity in the upper 30 m was globally ran, 15614-Iran; e-mail: [email protected] adopted after the National Earthquake Hazard 149 Vol52,2,2009 17-06-2009 19:02 Pagina 150 L. Samadi and B. Mehrabi Reduction Program (NEHRP) classification in Code (Standard No. 2800, Building and Hous- the USA. ing Research Center – BHRC, 1999), it is clas- Midorikawa (1987), Borcherdt et al. (1991), sified as a region of very high earthquake haz- Borcherdt (1992; 1994), Seed et al. (1992), Do- ard. In 1990, Manjil earthquake caused wide- bry et al. (1992), Boore et al. (1997) have spread destruction in the urban area of Rasht. shown supportive evidence for empirical rela- The cities of Rudbar, Manjil, and Lushan, and tionships between site amplification factor and 700 villages were destroyed. At least three hun- Vs(30). Wills et al. (2000) developed a site- dred more villages were slightly damaged. One conditions map for California based on Vs(30) hundred thousand adobe houses sustained ma- measured in 556 seismic profiles and geologi- jor damage or collapsed resulting in more than cal similarities between units. Shafiee and Aza- twenty five thousand fatalities, and sixty thou- di (2007) completed the classification of geo- sand injuries. About five hundred thousand logical units throughout Tehran. NEHRP uses people were left homeless. In 1992, refraction Vs(30) to rank sites into five broad classes seismic tests and some geoelectical sounding (Building Seismic Safety Council – BSSC, were conducted in the marginal territory of 2001), as shown in table I. Consideration of site Rasht. However, due to the lack of suitable da- conditions is a vital step in analyzing and pre- ta base, a through site classification of Rasht dicting earthquake ground motion particularly was not accomplished. In 2006, as part of na- for major cities situated in earthquake-prone ar- tional geotechnical and seismological survey eas. Rasht, the capital of Gilan province is lo- program, investigation for strengthening the cated in a tectonically active region, between water supply system of Rasht was conducted the Caspian Sea and the Alborz Mountain (fig. which lasted for a year. In this project, 12 bore- 1). According to the Iranian National Building holes, 26 P and 26 S refraction seismic tests Fig. 1. Major faults, and the position of boreholes in Rasht area. 150 Vol52,2,2009 17-06-2009 19:02 Pagina 151 Shallow subsurface geology and Vs characteristics of sedimentary units throughout Rasht City, Iran Fig. 2. Location map of destroyed or damaged buildings in Rasht city during 1990 Manjil earthquake (Natural Disaster Prevention Center, 1994). were performed. Data of the recent study were south. The southern part of the Caspian shores is compiled together with the results from previ- a narrow coastal plain with an average width of ous studies for creating a suitable seismic data- about 50 km, created by the «regional regres- base. Considering characteristics of geologic sion» of the sea, which probably once extended units of Rasht, this paper describes the effort to as far as the foot of the Alborz Mountains (Ge- define the shear-wave velocity classification for ological survey of Iran, 2004). The depth of Rasht, based on NEHRP site classes. bedrock in the city of Rasht is not known. Based on a geoelectric survey performed down to 200 m some 40 years ago, at 150-160 m depth the 2. Seismicity and geological setting electric resistance drops drastically which might of Rasht area be either bedrock or a thick layer of over-con- solidated clay. At least two water wells have Rasht is located between the Caspian coastal been drilled down to a depth of 150-160 m in the zone in the north and Alborz Mountains in the urban area of Rasht. Therefore the depth of qua- 151 Vol52,2,2009 17-06-2009 19:02 Pagina 152 L. Samadi and B. Mehrabi Fig. 3. Distribution of seismic profiles throughout the Rasht city. ternary deposits in Rasht city is greater than 160 25,000 lives. Three towns of Manjil, Rudbar and m (Haeri and Kholafaie, 1994). The tectonics of Loshan located in the epicentral area were al- the Alborz Mountain Belt is controlled by most demolished showing the highest casualties boundary conditions, due to convergence of the in these areas. The reported magnitude by US- Arabian and Eurasian plates probably com- GS is Ms=7.7 and Mb=6.4 (Niazi and Bozorg- menced in the Cretaceous. Alborz and its asso- nia, 1992; Berberian et al. 1992; Haeri and Kho- ciated mountain range form a continuous wall lafaie, 1994). Figure 1 shows the location of the along the north of Iran. From east to the west, epicenter and major towns and cities that were the width of the coastline of Caspian Sea in- affected by Manjil earthquake. From a micro- creases where the vast Delta of Sefidrud River seismal point of view Manjil was reported as the expands. In the Sefidrud River Delta and Rasht epicenter. However the aftershocks recorded in area on the west, the coastal plain widens to 33 the epicentral region indicated that the epicenter km. The important coastal features along coast- was a few kilometers north of Rudbar (Haeri lines in Rasht area is Sefidrud Delta. Intensive and Kholafaie, 1994). The surface faulting was geological and tectonic investigations reveal the mapped for a length of more than 80 km in seg- vulnerability of the region to earthquake dam- ments (Berberian and Qorashi, 1991). The June age. Historically a destructive Manjil earth- 1990 earthquake caused widespread damage in quake on 20 June 1990 in this region caused ca- areas within a one hundred kilometer radius of sualties which were reported to be more than the epicenter, near the city of Rasht and about 152 Vol52,2,2009 17-06-2009 19:02 Pagina 153 Shallow subsurface geology and Vs characteristics of sedimentary units throughout Rasht City, Iran Fig. 4. Schematic of seismic Profiles: I-refraction P-wave, II-refraction S-wave and III- downhole. two hundred kilometers northwest of Tehran. appeared to cause the partial or total collapse of The study area is the cover territory of Rasht many mid-rise buildings. In Rasht most of the city, bounded between longitudes of 49°32′- four- to eight-story buildings in center and 49°39′E and latitudes of 37°14′-37°19′N (fig. south of the city were damaged and at least 1). Geologically, the city of Rasht is located in three eight-story buildings were completely de- the intercept of 4 different types of Pleistocene molished. A seven-story steel frame building (Q1m, Qdg) and Holocene (Q2m, Q2d) sedi- was deviated from vertical position and a five- ments (fig.