Source Parameters of the February 1997 Garmkhan (North-East Bojnourd), Northeast Iran, Earthquake

The Bojnourd earthquake occurred in a mountainous area in North Khorasan provinc. The mainshock produced extensive destruction. Field investigation and aftershocks distribution suggest a NW-SE trend faulting. The distribution of locally recorded aftershocks was extended to a length of about 40-50 km and a depth of about 30 km. Aftershock activity was scattered indicating a complex mode of faulting. The result of waveform inversion indicated that the mainshock followed mainly strike-slip mechanism and the source process included at least two main fault slip. The source time functions indicates that the major amount of seismic energy was released within the first 10 seconds. Considering the field observation, the distribution of aftershocks and the source mechanism, an average source dimension of about 45 km, a NW-SE strike and a SW dipping fault plane could be estimated. The mechanism for the total source is obtained as (strike, dip, rake) = (323, 89, 178). The total seismic moment was calculated to be M0= 6.7×1025 dyne cm. The calculated maximum dislocation was about 50 cm and the obtained moment magnitude was Mw = 6.5. The average stress drop was estimated to be 25 bar and the average dislocation was 25 cm.