Journal of the Earth & Space Physics. Vol. 31, No. 1, 2005, P. 13-21 Magnitude scale in the Tabriz seismic network Rezapour, M * . * Institute of Geophysics, University of Tehran, Tehran, Iran (Received: 4 Jan 2005 , Accepted:31 May 2005) Abstract To record local earthquakes, the telemetric-digital seismic network of the Tabriz comprising of eight three component seismic stations was installed in the north-west of Iran. This array started operation at the end of 1995. Investigation of archived data in the Tabriz seismic network between 1999 to September 2004 shows that magnitude values in the database have not been determined using a single formula or a specific method. Comparison of magnitude values in the database with those values that have been published in the ISC/NEIC bulletin shows that magnitude values calculated in the Tabriz network are underestimated for events occurred in greater distances. By using magnitude values in the database and corresponding m b ISC/NEIC (m b ISC or m b NEIC ) values, a scale as M =log (v/4π)+2.6log (Δ)-2.2 is derived for determination of the magnitude in distance range of 170 to 1000 km, where v is peak-to-peak amplitude in micrometer/second and Δ is epicentral distance in kilometers. This formula gives a better estimate for the magnitude of events in comparison with the formula that is presently used in the Tabriz seismic network. Keywords: Seismic network, Magnitude, Database, Tabriz 1 INTRODUCTION Richter (1935) introduced the universally accepted basis of magnitude as proportional to log(amplitude). A large number of magnitude scales have been developed since then (e.g. Båth, 1981, Kanamori, 1983), most of them are similar to Richter’s scale in the sense that they are based on the logarithm of some amplitude measurement. It is well known that earthquake-magnitude scales do not directly represent any physical parameters of the source. However, they represent the relative scale of earthquakes. During the last 50 years, many seismic networks have been installed. Most of the seismic networks or individual stations use their own formula for magnitude determination. Earthquake magnitudes are both convenient and widely available, although the idea is based solely on an empirical relationship. Earthquake magnitude is routinely estimated by large data centers, such as the International Seismological Centre (ISC), the National Earthquake Information Center (NEIC) of the US Geological Survey, and by many national networks and at individual stations. Magnitude scales are normally estimated in two frequency bands: high-frequency body-waves with periods around one sec (for m b and M L ) and low-frequency surface-waves with periods around 20 sec (for M s ). In order to monitor seismic activity, the telemetric-digital seismic network of the Tabriz comprising of eight three-component seismic stations was installed in the central part of northwest Iran. The network stations have been selected to be in remote areas, away from noise and in good situation from a geological viewpoint and possibly to cover major faults in the area. The coordinates of seismic stations are given in Table 1, and in Figure 1 location of stations are shown on a shaded relief map of the region. This network started operation in late 1995. The recorded waveforms in the Tabriz network are analyzed using Data Analysis Program (DAN), which has been produced by Nanometrics Company. It works under OS/2 in 32 bits or SUN workstations (DAN User’s Guide, 1995). The method employed in this software for determining magnitudes is the use of maximum amplitude measurements from short-period stations, with no site correction. In this article, the bias in the magnitude values, which have been determined in this network is investigated, then a revised scale is derived for use in the Tabriz seismic network.