اﻳﺮان ژﺋﻮﻓﻴﺰﻳﻚ ﻛﻨﻔﺮاﻧﺲ ﺳﻴﺰدﻫﻤﻴﻦ137 The ML Scale for the central and west Alborz Rooh Allah Askari, Abdolreza Ghods Institute for Advanced Studies in Basics Sciences ﭼﻜﻴﺪه ﻣﻄﺎﻟﻌ اﻳﻦ در ﻪ رﻛﻮردﻫﺎي داﻣﻨﻪ ﺑﻴﺸﻴﻨﻪ از اﺳﺘﻔﺎده ﺑﺎ ﻟﺮزه ﻣﺼﻨﻮﻋﻲ ووداﻧﺪرﺳﻮن ﻧﮕﺎﺷﺖ ﺷﺪ اﺳﺘﺨﺮاج از ه1290 ﺑﻪ ﻣﺮﺑﻮط رﻛﻮرد59 ﺛﺒﺖ زﻟﺰﻟﻪ ﻟﺮزه ﺗﻮﺳﻂ ﺷﺪه ﻧﮕﺎﺷ ﺖ ﻟﺮزه ﻣﺮﻛﺰ ﻫﺎي ﻛﺸﻮري ﻧﮕﺎري ژﺋﻮﻓﻴﺰﻳﻚ ﻣﻮﺳﺴﻪ ﺑﻪ واﺑﺴﺘﻪ داﻧﺸ ﺗﻬﺮان ﮕﺎه ﺑﺰرﮔﺎي ﺑﺮاي ﺗﺠﺮﺑﻲ ﻛﺎﻫﻨﺪﮔﻲ راﺑﻄﻪ ﻳﻚ، ﻏ و ﻣﺮﻛﺰي اﻟﺒﺮز ﻣﻨﺎﻃﻖ ﺑﺮاي ﻣﺤﻠﻲ ﺷﺪ ﻣﺤﺎﺳﺒﻪ ﺮﺑﻲ: 0.6556 r 0.002304 (r) log 1.057 log 10 0 10 + + × = − A در ﻛﻪ ﻓﻮق، راﺑﻄﻪr اﺳﺖ ﻛﻴﻠﻮﻣﺘﺮ ﺑﺮﺣﺴﺐ ﻛﺎﻧﻮﻧﻲ ﻓﺎﺻﻠﻪ. اﻳﻦ از ﺑﻴﺶ ﻓﻮاﺻﻞ ﺑﺮاي را ﺑﻴﺸﺘﺮي ﻛﺎﻫﻨﺪﮔﻲ ﻣﻴﺰان راﺑﻄﻪ250 ﺑﻪ ﻧﺴﺒﺖ ﻛﻴﻠﻮﻣﺘﺮ ﺑﻮر و ﻫﻮﺗﻮن راﺑﻄﻪ) 1987 ( ﻣﻲ ﭘﻴﺸﻨﻬﺎد ﻛﻨﺪ. Abstract Using the maximum trace amplitude measured on synthetic Wood-Anderson seismograms of 1290 records of 59 events as recorded by short period Iranian Seismic Telemetry Network of Tehran University, we calculated an empirical attenuation curve for the local magnitude scale Ml for the central and west Alborz as, 0.6556 r 0.002304 (r) log 1.057 log 10 0 10 + + × = − A where r is distance in km. The relationship gives larger attenuation for distances above 250 km once compared with Hutton and Boore’s (1987) relationship which is used routinely in some of Iranian seismic networks. Introduction Developed by Richter (1935) for earthquakes in southern California, Local magnitude (ML) was the first magnitude scale to describe the size of an earthquake. It was derived from the amplitudes recorded by the Wood-Anderson torsion seismograph with a natural period of 0.8 sec, a damping constant of h=0.8 and static magnification, V=2800. The local magnitude is considered in most engineering applications because ML is determined within the period range of greatest engineering interest (typically 0.2 to 3 sec); besides this feature, the source distances for ML scale are generally shorter than those used for other magnitude scales including the determination of seismic design criteria for major projects. Since ML is now widely used in regions completely different from southern California, it is necessary to determine ML for any region. According to geological differences, the attenuation relations introduced for other seismic regions (e.g. Hutton and Boore, 1987; Kim, 1998; Savage and Anderson, 1995) do not generally result to unbiased estimates of magnitudes in the central and west Alborz. Availability of a large number of short period seismological data recorded in the study area by the Tehran University’s Iran Seismic Telemetry Network is an incentive motivating us to use the data to calculate ML scale for the region. Data The analysis presented here is based on the data derived from the data bank of the Iranian Seismic Telemetry Network. Founded in 1995, the Iranian Seismic Telemetry Network (http://irsc.ut.ac.ir/) is the first modern seismic network of Iran. The network is equipped with 3-component short period SS-1 seismometers with eigenfrequency of 1 Hz, 24 or 16-bit analog to digital digitizer and with sampling rate equal to 50 samples per second. The data used in this study are 1290 records of 59 earthquakes that happened in the study area during 1996 to 2007 (Fig. 1). The selected events have a magnitude range of 3 to 5.7 as calculated by Hutton and Boore’s (1987) ML relationship. We chose the events with magnitude window of 3 to 5.7 to avoid saturation of ML scale for large earthquakes (ML larger than 6) and to have good signal to noise ratios for measurement of amplitude. Primitive Analysis of Data By removing the instrument response of each record and convolving the signal resulted with the standard Wood- Anderson torsion seismograph response, the synthetic Wood-Anderson is obtained for N-S and E-W seismograms and maximum amplitude are measured on them. Following the method originally used by Richter (1935), ML is calculated using following relationship,