Attenuation of Ground-Motion Spectral Amplitudes and Its Variations across the Central Alborz Mountains by Khalil Motaghi * and Abdolreza Ghods Abstract We selected 136 earthquakes recorded at stations of the Institute of Geo- physics of the University of Tehran (IGUT) during 1996–2007, in order to estimate the average attenuation parameters and variations of station corrections across the central Alborz mountains, the northern extension of the Alpine-Himalayan orogeny in wes- tern Asia. The selected events provide 1260 high signal-to-noise ratio (SNR) records with relatively good spatial coverage. We graphically find the distances at which the nature of geometrical spreading attenuation (R b ) changes significantly using a locally weighted scatterplot smoothing (LOWESS, local regression smoothing method) called robust LOWESS. A trilinear function with hinges at distances of about 80 and 160 km describes the geometric spreading attenuation with distance. By regressing to the hinged trilinear function, we found that b 1  -1:15  0:21, b 2  0:09  0:31, and a fixed b 3  -0:5 minimize the average absolute value of the Fourier spectrum amplitude residuals. Using an anelastic attenuation coefficient at different frequencies, the direct quality factor Q in the central Alborz region is obtained as Q  109  2f 0:640:04 . The geographical distribution of station corrections at 1 Hz can be clearly separated along a dividing line connecting the North Tehran, Mosha, and Attari faults into two regions: the northern low-attenuating and the southern high attenuating regions. The stations with strong positive station corrections clearly align along the dividing line. This suggests the existence of a higher attenuation zone south of the dividing line. The presence of thick Tertiary–Quaternary sedimentary rocks with thick layers of salt in the southern region may explain the observed greater attenuation of seismic waves. Introduction The central Alborz Mountains of northern Iran (Fig. 1) is a zone of intensely active continental deformation, and it is the most heavily populated region of Iran, including the megacity of Tehran with a population of over 10 million. It is a part of the Arabia–Eurasia collision zone where defor- mation is characterized by partitioning of strain into range- parallel left-lateral strike-slip and thrust faults (Jackson et al., 2002; Allen et al., 2003). The Alborz region is affected by numerous active faults, some of which have great seismic potential, and documented historical seismicity (Ambraseys and Melville, 1982; Berberian and Yeats, 1999). As such, the central Alborz Mountains are a prime candidate for seismic hazard evaluation studies The choice of ground-motion model has a significant impact on hazard estimates for the distance range of 50– 200 km within an active seismic zone such as the central Al- borz. Estimation of ground shaking for a typical frequency range of 0.2–10 Hz is required for the proper design of earth- quake resistant structures and facilities (Raoof et al., 1999) and is considered as input for engineering stochastic ground- motion relationships (Atkinson, 2004). The lack of an appro- priate ground-motion prediction model may result in unde- sirable outcomes, such as unrealistically high or low loading standards in the design and construction of critical infrastruc- ture such as large dams, power stations, and hospitals. For seismological purposes, appropriate attenuation models make it possible to calculate more accurately source para- meters such as magnitude and seismic moment (Atkinson and Mereu, 1992). The Alborz region has experienced very few large events during the operation of the accelerometer network of the Building and Housing Research Center (BHRC). The BHRC network has been operating since 1973 but has recorded ground acceleration for only 22 events in the study area, be- cause of the low seismicity rate (Ghods and Sobouti, 2005). Zare (1999) and Ghasemi et al. (2009) produced ground-motion relationships for Iran using BHRC acceler- *Now at the International Institute of Earthquake Engineering and Seis- mology (IIEES), Tehran, Iran. 1 1 Bulletin of the Seismological Society of America, Vol. 102, No. 4, pp. –, August 2012, doi: 10.1785/0120100325