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Earthqukes

Important Earthquakes in 2003

Bam Earthquake of December 26^th

The Bam earthquake of December 26, 2003 (Mw6.5), occurred at 01:56:56 (GMT, 05:26:26 local time) around the city of Bam in the south-east of Iran. This earthquake happened in 5:26:56 am local time when most of the inhabitants were slept, that can be one of the causes of the great life loss.

The Bam earthquake prone area in SE of Iran is an active seismic zone. Towards the northwest of Bam, 4 major earthquakes with the magnitudes greater than 5.6 have stricken the cities and villages since 1981 until 1998. The NW-SE faults (Kuhbanan and Ravar faults) and the north-south faults (Nayband, Chahar-Farsakh, Anduhjerd, Gowk, Sarvestan and Bam faults) have determined the border of the north-south structures in the Lut area with the NW-SE structures. These intersection zones are of the main sources for the disastrous earthquakes. Four great earthquakes have stricken this region during the recent years: Golbaf earthquake of 11 June 1981, Ms6.6, Sirch earthquake of 28 July 1981, Ms7.0, south Golbaf earthquake of 20 November 1989, mb5.6 and the north Golbaf (Fandogha) earthquake of 14 March 1998, Mw6.6.

The epicenter of the bam earthquake has been reported by the United States Geological Survey (USGS) to be located at 29:00N-58.34E, about 185km south east of Kerman .This implies that the epicenter of the earthquake was only a short distance from Bam (29:09N-58.35E) and to the south of the city. The focal depth of the earthquake was reported to be about 7km (BHRC) to 10km (USGS). Studies by Talebian et al based on interferograms derived from coseismic satellite maps suggest that the fault responsible for the Bam earthquake was a blind strike-slip fault located about 5 Km to the west of the visible surface traces of the Bam fault. Accordingly the causative fault extends beneath the Bam city from the south.

 Strong Ground Motions of Bam Earthquake

The Iran Strong Ground Motion Network consists of around 1100 digital instruments dispersed around the country. Out of the 78 instruments within a 300km radius of Bam, the main shock of the Bam earthquake was recorded by 27 instruments. The maximum uncorrected accelerations recorded in Bam station (58.35E, 29.09N) were 0.82g, 1.01g and 0.65g in the longitudinal (east-west), vertical and transverse (north-south) directions, respectively. The recording instrument was located on the ground floor of two-storey Governor’s office building in Bam and importantly has recorded a ground motion very close to the epicenter. Despite severe damages experienced by the building, there was only minor damage in the room where the instrument was located, as reported by BHRC (Fig. 29, 30). Uncorrected ground accelerations for the main shock recorded in Bam station are shown in Figure 28. These records show severe vertical and fault normal accelerations in Bam city with maximum corrected accelerations of 980 and 778 cm/s/s corresponding and peak ground velocities of about 124 and 40 cm/s and a peak ground displacement of 34 and 8 cm, all respectively for horizontal fault-normal and vertical components. The recorded accelerogram of this earthquake is presented in fig 30 along with its Fourier amplitude. The duration of the strong motion based on the evaluation of Arias Intensity was estimated to be about 8 seconds for the fault-normal component

A fore-shock was recorded in Bam station about 53 minutes before the main shock. This fore-shock had a maximum horizontal acceleration of 0.017g and maximum vertical acceleration of 0.08g, with a hypocenter distance of about 10km. The fore-shock was sufficiently strong that a small proportion of the population (according to survivors of the earthquake) took precautionary measures by staying out of their homes.

More than 60 aftershocks were recorded in the 6 days following the main shock. Bam digital instruments functioned for up to 1 hour and 20 minutes after the earthquake, until running out of memory, and managed to record 9 aftershocks. The largest aftershock happened about one hour after the main earthquake with a magnitude of 5.3. Another aftershock with a magnitude of 4.6 was recorded about 12 hours after the main shock at the Mohammad-Abad and Abaragh stations. About 30 hours after the main shock another instrument was installed by BHRC in Bam. This instrument recorded 5 aftershocks with magnitudes ranging between 3 and 4 during the 5 days following installation

Damage from the Bam earthquake was mainly concentrated in Bam City and the surrounding villages. Strong motion attenuation in the east-west (i.e normal to Bam fault) direction was significantly higher than in the north-south direction.