The 2004 May 28 Firoozabad-Kojour (Iran) Earthquake, Ms6.3, and its Aftershocks Based on Recorded Data at Local Seismological Stations

A moderate earthquake (Ms=6. 3) struck the coastal region of north of Iran and the central Alborz on 28 May, 2004 was responsible of several damages and about 35 casualties. The mainshock was followed by a large number of aftershocks, the largest one reaching Ml=4. 8, based on the analysis of local waveforms. We study the mainshock, first major aftershock, and about 240 aftershocks recorded by Iranian National Seismic Network (INSN), Tehran Telemetry Seismic Network (Institute of Geophysics, Tehran University), and our temporary local seismological stations, which were installed on 30 May, around the epicentral area of this earthquake. Using waveforms of all permanent stations, the coordinates of the mainshock was determined as 36. 30 °N for latitude and 51. 60 °E for longitude. The analysis of aftershocks indicates that the seismic activity migrate from east where the mainshock occurred toward west close to the location of the largest aftershock (36. 36 ° N, 51. 45 ° E). 140 selected aftershocks recorded at a minimum of 6 stations, having rms less than 0. 15 sec and uncertainties less than 2 km, were used to infer a precise geometry of the fault region. The aftershocks distribution has 30 km long and trends NW-SE parallel to the North Alborz and Khazar faults. The focal depths comprised between 10 and 28 km, unusually deep for Iran. Distribution of aftershocks cluster on cross-section defines a fault plane which dips at 40-50 degree south-westward. Its upward continuation can be related to either North Alborz or Khazar faults. Most of the focal mechanisms are consistent with reverse faulting on NW-SE trending faults, parallel to the main active structures of the region. Well constraint focal mechanisms which dip gently at a rate of 25-40ْ indicate the activity of the second mentioned faults during the Firozabad-Kojour earthquake. Existence of focal depths up to 28 km indicates an unusual brittle lower crystalline crust in this part of central Alborz.