|Observatories and Research Facilities for EUropean Seismology|
|Volume 4, no 2||September 2002||Orfeus Newsletter|
A Seismological Overview on the Changureh (Avaj, Iran) Earthquake of 22 June 2002, Mw=6.3Mehdi Zare
Figure 1. General topographic map of NW Iran. The epicenter is located near Changureh. (Basemap from: USGS, Digital Data Series; 2001).
Figure 2. The Landsat satellite image of the epicentral area.
Figure 3. The major faults in western Tehran and the epicentral area of the 22 June 2002 Changureh earthquake. Observed surface fissures are shown in red. The fissures generally follow the E-W trending Divandarreh-Buin lineament.
Figure 4. The fault map of the epicentral area is shown on a 1:500,000 topographic map. The fault plane solution is from the website of Harvard University, Seismology Department.
The focal mechanism of the event, representing mostly compressional deformation (Figure 4), agrees with the field observations. Some surface fissures observed in the region around Changureh are uncontinuously along about 3 km (for example in the road beteen Abdarreh and Changureh; Figure 5). Some other surface fissures have been reported between Changureh and Avaj in a direction from Changureh towards Abegarm. These fissures had the general strike of N70-80W. The length along which the surface fissures have been reported was about 20 km. Some tension cracks are observed as well with N5-20W directions perpendicular to the compressional features. The observed fissure orientations are compared with the fault plane solutions reported by NEIC and Harvard university webpages (2002). The principal axis of the moment tensors (N20E) is shown in Figure 6 as well as the axis reported for the Buin-Zahra earthquake of 1 September 1962 (N27E) (Jackson and McKenzie, 1984). The strikes of the nodal planes reported in the NEIC and Harvard solutions and the trend of the surface fissures are traced in Figure 6. According to the direction of the planes and the principal axis of moment tensors, the fault plane with a slope towards the north and a compressional slip with a slight left-lateral strike-slip displacement can be introduced as the causative earthquake fault plane (Figure 6). Based on the observation made by a local network (described later in this text) the aftershocks are mostly concentrated on a plane sloping towards the north. The major compressional and minor left-lateral strike slip mechanism was already experienced in the Buin-Zahra earthquake of 1 September 1962.
Figure 6. Structural analysis. The fault planes, reported by the NEIC and Harvard, are projected on this figure together with the trend of the surface fissures. Also, the P-axes of the moment tensor solutions of the Buin-Zahra earthquake of 1 September 1962 are shown.
The moment tensor solution given by ERI institute of Tokyo University (ERI, website 2002) gives a similar solution, as well as a focal depth of 7 km and the slip duration of 7 seconds (Figure 7). This focal depth of 6-7 km for the mainshock is justified by estimation of aftershock focal depths. The energy magnitude (Me) of 6.2 is reported by NEIC web site (2002) for the event. Based on the areal extent of the surface fissures and the estimated focal depth, and using the empirical relationship between the fault area and seismic moment (Lay and Wallace, 1995), a magnitude of Mw = 6.3 was estimated for the mainshock. Based on the estimated fault rupture area, the stress drop was found to be very high and the rate of the attenuation of the strong motions (based on the observed intensities) was fast (similarly to known fast attenuation of strong motions in the Zagros area). The duration of the mainshock was short. Such strong motion data and source parameter estimates are currently studied in more detail and the results will be published in the future.
Figure 7. The fault plane solution, slip duration and focal depth of the earthquake, reported by ERI, Tokyo, website (2002).
Table 1). The epicenters of the historical (pre-20th century) earthquakes (Figure 8) are more concentrated towards the southern Alborz and southern Tehran area (Ambarseys and Melville, 1982). The earthquakes during the 20th century occurred along the southern Ghazvin plain and west of Tehran (Figure 9). The major earthquakes along the E-W trending Divandarreh-Buin lineament indicate that the zone is active. Relative to the background seismicity of the region, it seems that the Changureh area probably was a seismic gap along this active structure, which was activated on 22 June 2002.
Figure 8. Historical (pre-20th century) seismicity of the area around Changureh (Ambraseys and Melville, 1982).
Figure 9. 20th century seismicity of the area around Changureh (Ambraseys and Melville, 1982, NEIC). BHRC web site, July 2002). The highest peak acceleration has been recorded at Avaj station (Figure 11) at a hypocentral distance of 28 km. Based on the signal to noise ratio of this record (Figure 12), a band-pass filtering between 0.1 and 25 Hz was applied, and the PGA of this record was obtained to be 423 and 417 cm/sec2 for two horizontals and 248 cm/sec2 for the vertical components, respectively. IIEES has installed a local network of 25 temporary stations in the region around the epicenter during the week after the event, in order to record and study the aftershocks.
Figure 10. Mainshock recorded with the IIEES national network stations Naein (3 upper records) and Zahedan (3 lower records).
Figure 11. Accelerogram of the mainshock.
Figure 12. Signal to noise ratio for the accelerogram recorded in the station Avaj. Straight line is the horizontal component, dashed line is the vertical component.
Figure 13. Iso-intensity map of the Changureh earthquake. Based on the observations of IIEES reconnaissance team leaded by Dr. S. Eshghi (June 2002).