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=6.3

Mehdi Zare
International Institute of Earthquake Engineering and Seismology (IIEES), P.O.Box: 19395/3913, Tehran, Iran

Introduction - Faulting, Mechanism and Source Characteristics - Background seismicity - Records - Earthquake Intensity - Conclusions - References

Introduction

The Changureh (Avaj) earthquake of 22 June 2002, Mw = 6.3 (7:28:00 a.m. local time, 2:58:27.2 GMT) was strongly felt in most parts of NW Iran. The epicenter, about 250 km west of Tehran and west of the Takestan-Hamedan road (Figure 1), is located within an roughly east-west oriented valley just west of the Abegarm village (see Figure 2). The greatest damages occured in the villages Abdarreh and Changureh (Figures 3 and 4). The epicenter as reported by NEIC is shown in Figure 4. The official report lists 233 fatalities, about 1500 injured and more than 50000 homeless people. The earthquake affected about 50 villages (most of them partially damaged). The macroseismic intensity of VIII+ (EMS-98 scale) could be assigned to Abdarreh and Changureh in the epicentral area. Based on the extent of the surface fissures and an estimated focal depth of 7 km, a magnitude of Mw = 6.3 has been estimated for the event. The preliminary process of source parameter estimates indicates a high stress drop and fast ground motion attenuation.


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.

Faulting, Mechanism and Source Characteristics

The Changureh earthquake area is characterized by the dominant NW-SE and WNW-ESE directed fault systems (Figure 3). The area includes the major NW-SE and east-west structural trends in the southwestern Alborz region and north-central Iran. There are, hence, major intersections of active tectonic structures that generated great earthquakes already in the past (1177 and 1962 Buin-Zahra earthquakes with estimated magnitudes > 7.0). However, the region of northern Avaj has been seismically less active in historic times (according to the historical and 20th century earthquake catalog). The recent 22 June 2002 earthquake is a major event that seismically illuminated the western continuation of the Divandarreh-Buin lineament (Figure 3).


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.

Click for large figure Click for large figure
Figure 5. Surface fissures between Abdarreh and Changureh (N70-80W).


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).

Background Seismicity

The Changureh earthquake area was the location of major earthquakes in historical times (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).

Records

The mainshock of the 22 June 2002 earthquake has been recorded at many different stations around the world including the IIEES national network stations of Naein and Zahedan (Figure 10). The strong motions of the mainshock have been recorded in 42 stations of the national Iranian strong motion network (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.

Earthquake Intensity

The Changureh earthquake caused heavy destruction in the villages Changureh and Abdarreh and unusually high damage towards Avaj. The extension of damages towards Avaj could be caused by source directivity effects. A maximum intensity of VIII-IX is assigned to the macroseismic epicentral region. The isoseismals have a general trend of WNW-ESE (Figure 13). The village of Changureh was damaged most and more than 150 people of the total of 233 reported life losses, are killed just in this village.


Figure 13. Iso-intensity map of the Changureh earthquake. Based on the observations of IIEES reconnaissance team leaded by Dr. S. Eshghi (June 2002).

Conclusions

The Changureh earthquake of 22 June 2002 (Mw = 6.3) in southern Ghazvin and northern Avaj occurred along an active structure. The fault related to the mainshock was a segment in the continuation of the Divandarreh-Buin structural system, on the western side of the Ipak fault. This segment had no major activity during the known historical seismicity of the region. This event seems to have closed a seismic gap along the activated fault system. The shallow focal depth of 7 km, and the poor quality of the village constructions, caused heavy life losses. The event duration, reported to be 7 seconds, was very short and preliminary estimations of stress drop indicate a very high drop for the mainshock. The attenuation rate of the strong motions was very fast.

References

  • Ambraseys N.N., and C.P. Melville, 1982, A History of Persian Earthquakes, Cambridge Earth Sci. Ser.
  • Building and Housing Research Center (BHRC) website, July 2002
  • Earthquake Research Institute (ERI, University of Tokyo), website, June 2002
  • Eshghi S., M. Zare and M.R. Mahdavifar, 23 June 2002, Preliminary Report of IIEES Reconnaissance Team, The Changureh (Avaj) Earthquake of June 22, 2002
  • Harvard University, Seismology Department, website, June 2002
  • Jackson J. and D.P. McKenzie, 1984. Active Tectonics of the Alpine Himalayan Belt between western Turkey and Pakistan, Geoph. J. R. Astron. Soc., 77, 185-264.
  • Lay T. and T.C. Wallace, 1995. Modern Global Seismology, Academic Press, 517p
  • National Earthquake Information Center (USGS), website, June 2002
  • United States Geological Survey, USGS, Digital Data Series DDS-62-C, 2001.
  • Zaré M., P-Y. Bard, M.Ghafory-Ashtiany, 1999, Site Characterizations for the Iranian Strong Motion network, Journal of Soil Dynamics and Earthquake Engineering, 18, no.2, 101-123.


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