Scientific Quarterly Journal of Geosciences

Scientific Quarterly Journal of Geosciences

Source Characteristics of the 28thMay2004 Baladeh-Kojour Destructive Earthquake in Central Alborz, Revealed from far Field Waveform Data

Document Type : Original Research Paper

Author
Mohammad Reza Gheitanchi
Institute of Geophysics, Tehran University, Tehran, Iran
Abstract
Source characteristics of the 2004 Baladeh-Kojur destructive earthquake is obtained by inverting far field waveform data. The information from field investigation and aftershock activity are considered as supplementary data to constrain the source parameters. The source characteristics of main shock is explained in terms of at least three major subevents. Rupture initiated in epicentral area with the first subevent and mainly extended toward west in a unilateral manner. The major slip took place during the first 10 seconds and it is concluded that the directivity played main role for producing extensive intensity in the epicentral region. The source mechanism obtained in this study is predominantly thrust and is in agreement with the mechanism of other earthquakes as well as the orientation of tectonic forces in this region. The total seismic moment is calculated to be M0= 4.1×1018 Nm and the total moment magnitude is Mw = 6.3. The source duration of largest subevent is about 7 seconds and its related rupture extension is about 20 km. The calculated maximum dislocation is about 120 cm and the estimated maximum stress drop is about 25 bar. The distribution of strong ground motion indicates two high acceleration areas in epicentral area and toward the west. This indicates that the main shock possibly has a multiple source nature or the media has amplified the ground motion in western part of the epicentral region. This fact should be clarified by more detailed field studies in future. Macroseismic evidence and recorded accelerograms in Baladeh region indicate that the ground strong motion was intense during the main shock. Although Central Alborz had been seismicly active in historical times, there was no evidence that earthquakes as severe as this one had occurred in the vicinity of Baladeh-Kojour region during at least the past 100 years. From the engineering point of view, Baladeh-Kojour earthquake provided ground-motion characteristics of a strong event in the affected area and should be considered as the controlling event for the design of structures with high safety requirements.
Keywords
Baladeh-Kojur earthquake
Seismicity of Central Alborz
Source parameters
Waveform modeling
Focal mechanism
Seismotectonics
Active faults

Subjects

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Scientific Quarterly Journal of Geosciences
Volume 14, Issue 55
Spring 2005, Vol.14, No. 55
Spring 2005
Pages 154-163