Instrumental Seismology of the Eastern part of the Mosha Fault

Mosha is one of the most important and threatening faults in TehranMegacity (Capital of Iran). Instrumental seismology in the region was limited to insufficient data along with location errors especially in depths as well as the small number of available focal mechanisms in bound with the trends in Hedayati et al. (1976) and Ashtari et al. (2005). In the study ahead, by installing 48 local and temporary seismological stations during June to November 2006, micro earthquakes around the eastern part of the southern flank of central Alborz particularly the Mosha fault zone were recorded and processed. The local temporary network consisted of 24 one-vertical-component TAD-2 Hz, 11 3-components MiniTitan- 5 S and 13 3-components Guralp 6TD 0.02-10 S sensors. Sampling rates were 100 samples/sec the for Guralp sensors and 125 samples/sec for the others in continuous and triggering threshold modes. 115 well recorded micro earthquakes with an appropriate azimuthal gap (Gap ≤ 180°), a trivial residual timing and location errors (RMS ≤ 0.3 sec, Erh ≤ 2 km and Erz ≤ 3 km) were selected and applied for the wave velocity ratio (Vp/Vs) calculation based on Wadati (1933) and Chatelain (1978) approaches (1549 P-wave and 1495 S-wave arrival times). A 1-D model of the upper crustal velocity structure was determined as well. SEISAN software (Havskov and Ottemöller, 2005) for phase readings, Hypo71 (Lee and Lahr, 1975) Hypocenter (Barry, 1994.) for seismic event locations, VELEST (Kissling, 1988) for a crustal velocity layers model and FOCMEC program (Snoke, 2003) for focal mechanism solutions were used. Four layers at the depths 3, 7, 16 and 24 km of the crust were determined with P-wave velocities of 5.4, 5.8, 6.1 and 6.25 km/sec, respectively. Accurate locations of 553 micro earthquakes and 15 A and 31 B (excellent for A and good for B groups in red and blue colors in related figures respectively) classes of focal mechanism solutions of the reliable micro earthquakes with a high quality of P-wave first arrival polarities (more than 8 Pg onset’s signs), were provided for the possible analyses of seismicity, the fault geometries-movements and seismotectonic interpretations. We have found that the Eastern part of Mosha fault, longitudinally located from 51.7° to 52.5°, has a northward high dip angle and complex focal mechanisms. The fault mechanisms varied from thrust, strike slip with a small reverse component to reverse with a small normal component from the West to the East. From grouping analysis of the focal mechanism P (or T) axes, the strikes, N 40 (or N 130) were derived for the compression (or tension) stress direction approximately. The focal mechanisms accompanying with the geodynamic analyses from GPS measurements in the studied area reveal a slip partitioning in the local and regional scale compatible with some conclusions from the previous studies (Ritz et al., 2006 and Tatar et al., 2007). Although micro and large earthquakes nonlinearity relation in stress axes orients as true or not proved is important (Mercier et al., 1991 and Hatzfeld et al., 1999), seismotectonics strain analyses of the micro earthquakes in the studied area show the same results of the large earthquakes stress analyses in stress inversion method by Gillard and Wyss (1995). In addition, this study has demonstrated a seismic active trend as mentioned by Jajrood-Pardis-Absard in the South of Mosha fault. Concentrated seismic activities around Mosha fault in the time of data recording have shown that it is a potential hazard for the studied area.