Attenuation of Ground-Motion Spectral Amplitudes at short hypocentral distances

The empirical attenuation relationship for spectral amplitudes was calculated to study the attenuation of shear waves of shallow events at close hypocentral distances inside Tarom-Rudbar region, western Alborz. 3122 waveforms (170 shallow events) recorded by 40 seismic passive stations of two local temporary seismic networks were included in this analysis. The selected events have moment magnitudes between 1.8 and 4.2 and epicentral distances of 10 km to 70 km. All events have location accuracy better than 2 km in epicenter and less than 5 km in depth. The good location quality of the events allows us to estimate accurately geometrical spreading of shear waves at close hypocentral distances. By selecting the small events, we can safely treat them as point sources and thus use hypocentral distance as our distance metric. Additionally, for smaller events, we automatically avoid the non-linearity of amplitude of seismic waves with magnitude and its possible trade-off with geometrical spreading. Due to a rather low dependence of geometrical spreading on magnitude (v. NGA-WEST2 models), our approach of using weak-motion data may provide a means to reliable assessment of the geometrical spreading coefficient which can then be used to partially regionalize NGA-WEST2 GMPEs for regions with low rate of seismicity or lack of enough strong motion records.
The shape of the attenuation curve at different frequencies was obtained using non-parametric fit to the data with Robust Lowess algorithm showing a mono-linear curve in the associated distance. Assuming mono-linear attenuation model and using regression, the value of geometrical spreading coefficient in the equation derived from the spectral method was obtained as 1.6 in frequencies higher than 2 Hz. Spectral amplitude attenuation curves show an obvious super-spherical geometrical spreading at close hypocentral distances. We show that the geometrical spreading is strongly super-spherical in close agreement with those used in some of the NGA-WEST2 GMPEs. The observed super-spherical geometrical spreading of seismic waves could drastically change the level of seismic hazard in close hypocentral distances by localizing strong motion to short hypocentral distances. The calculation of geometrical spreading coefficient using data from more frequent small events recorded by dense local networks can be used to partially regionalize the geometric term of GMPEs in regions with small rate of seismicity.
The residuals were averaged on a station-by-station basis to determine station corrections. The calculated station corrections for the study area shows sharp contrast between the northern and southern hills of western Alborz. The stations in the northern hill, mostly in Gilan plain, show higher amplification (positive station corrections) relative to those in the southern hill. The strong amplification of seismic waves has a strong implication for preparation of seismic hazard maps of the densely populated Gilan province