Site effects in Avcilar, west of Istanbul, Turkey, from strong- and weak-motion data

Approximately 1000 people were killed in the collapse of buildings in Istanbul, Turkey, during the 17 August 1999 Izmit earthquake, whose epicenter was roughly 90 km cast of the city. Most of the fatalities and damage occurred in the suburb of Avcilar that is 20 km further west of the epicenter than the city proper. To investigate this pattern of damage, the U.S. Geological Survey, in cooperation with the Kandilli Observatory & Earthquake Research Institute (KOERI), deployed portable digital seismographs at seven free-field sites in western Istanbul, to record aftershocks during the period from 24 August to 2 September. The primary objective of this deployment was to study the site effects by comparing the aftershock ground motions recorded at sites inside and outside the damaged area, and to correlate site effects with the distribution of the damaged buildings. In addition to using weak-motion data, mainshock and aftershock acceleration records from the KOERI permanent strong-motion array were also used in estimating the site effects. Site effects were estimated using S waves from both types of records. For the weak-motion data set, 22 events were selected according to the criteria of signal-to-noise ratio (S/N ratio) and the number of stations recording the same event. The magnitudes of these events ranged from 3.0 to 5.2. The acceleration data set consisted of 12 events with magnitudes ranging from 4.3 to 5.8 and included two mainshock events. Results show that the amplifying frequency band is, in general, less than 4 Hz, and the physical properties of the geologic materials are capable of amplifying the motions by a factor of 5-10. In this frequency band, there is a good agreement among the spectral ratios obtained from the two mainshocks and their aftershocks. The damage pattern for the 17 August Izmit earthquake is determined by several factors. However, our study suggests that the site effects in Avcilar played an important role in contributing to the damage

Crustal structure of central Japan and its petrological implications

The crustal structure of central Honshu, Japan, has been influenced by several tectonic events: a series of collision processes during the late Tertiary, extensive faulting, highly compressive deformation and Quaternary volcanism, The southern part of the study area is covered by various granitic rocks which intrude into old Palaeozoic formations. Quaternary volcanoes occupy the northern part and cover the basement, which is mainly composed of granitic rocks and Tertiary formations. A traveltime inversion for velocity and interface depth was applied to the seismic data, together with constraints from amplitude forward modelling to produce a seismic velocity model of the crust in central Honshu, In this experiment both refracted P and S waves are clearly recorded along a 110 km profile. These provide constraints on the 2-D P- and S-wave velocity structure down to a depth of about 10 km. Moreover, clear P-wave reflections from intracrustal interfaces provide constraints on deeper parts of the crust down to about 25 km. For the P-wave velocity model, the velocities of near-surface layers show large lateral variations from 3.7 to 4.4 km s(-1), increasing towards the south, Underlying layers have velocities ranging from 4.85 to 5.6 km s(-1). Velocities in the range 5.85-6.2 km s(-1) characterize the upper crust that extends to a depth of about 15 km, For the S-wave velocity model, the near-surface layer velocities change from 1.7 to 2.55 km s(-1), showing remarkable lateral heterogeneity, whilst the upper crust is characterized by velocities of 3.4-3.6 km s(-1). The deeper parts are modelled by two prominent reflectors in the mid-crust. The shallower one, which delineates the bottom of the upper crust, dips southwards from 14 to 16 km. The deeper reflector lies between 20 and 24 km depth and dips northwards. The velocity in this mid-crustal layer is 6.35 km s(-1) at the top and 6.7 km s(-1) at the bottom. The crustal thickness under the profile could not be determined because of a lack of P-n and/or PmP phases