Determination of causative fault parameters for some recent Iranian ...

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H. Hamzehloo / Journal of Asian Earth Sciences 25 (2005) 621–628
Fig. 1. The HRV located epicenters for the 1994 Zanjiran, the 1997 Ardebil, the 1999 Kareh Bas, the 2002 Avaj, and the 2003 Jahrom earthquakes are shown
with Stars. NTF: North Tabriz Fault; KF: Kazeron Fault.
earthquakes of large magnitude (Berberian, 1976; Berberian
and Papastamatiou, 1978; Jackson and Fitch, 1981;
Ambraseys and Melville, 1982,). According to McKenzie
(1972) interpretation of fault plane solutions in Zagros is
hindered by the absence of observed surface ruptures.
The pattern of seicmicity in the Alborz is discontinuous,
with gaps filled in gradually by relatively large events. Most
of the strong earthquakes are in the eastern and central
Alborz regions. Considering macroseismic data, Ambraseys
and Melville (1982) have found that the large scale
seismicity of the region does not follow any pattern
apparent from surface tectonics, nor is it dependent on the
major known Quaternary faults in the area. Recent large
earthquakes occurring in this region suggest that the
seicmicity is connected with relatively major faults of
recent age that cut across the regional Quaternary
lineaments.
According to Berberian (1979), central Iran is not a linear
seismic zone. It is characterized by scattered seismic
activity with large magnitude earthquakes, long recurrence
periods and seismic gaps along several Quaternary faults
(Fig. 1). The shocks in central Iran are generally shallow and
are usually associated with surface faulting. The Azarbaijan
province in northwest Iran is bounded in the southwest by
the north Tabriz Fault that has a northwest–southeast trend
(Fig. 1). The northeast boundary is a major fault that
separates this province from the Caspian shore province.
The seismicity in this province is sporadic and does not
show well-defined trends (Nowroozi, 1976). According to
Ambraseys and Melville (1982), lack of earthquake
information for the Ardebil area suggests that no earthquakes
of any significance have affected this area in the
recent past. A study of historical earthquakes (pre-1900)
shows that only a few earthquakes have occurred in this
region.
2. Method
A fault plane solution yields information about the nature
of an earthquake. In addition, the usefulness of focal
mechanism studies must be emphasized in its application to
research on regional and global tectonics (Kasahara, 1981).
A fault plane solution allows us to determine the mode of
stress change in the earth, at the specific time and locality of
an earthquake. The information yielded by focal mechanism
studies gives an instantaneous picture of tectonic movements.
Several methods are available to estimate focal
parameters such as first P-motion, S-waves, spectral
amplitude of body wave, finite moving source, surface
waves and moment tensor solution.
Strong motion data play an important role in studying the
nature of an earthquake. Strong motion recording close to