ARTIFICIAL AND NATURAL CHANGES IN THE RATES OF SEISMIC ACTIVITY - A CASE-STUDY OF THE GARM REGION, TAJIKISTAN (CIS)

Seismicity rate variations observed in earthquake catalogues can be na tural, i.e. associated with temporal variations in the seismic process , or they can be artificial, i.e. related to changes in the networks o f seismic stations used to create these catalogues. Such artificial ch anges must be identified and corrected before real variations can be r eliably examined. This paper demonstrates techniques for modelling of the observed changes in seismicity rates in order to determine their o rigin. The main technique used here has been previously applied to a n umber of catalogues to identify times of rate changes. It can be used to demonstrate the magnitude dependence of these changes and the exist ence of two major types of possible artificial rate variations, detect ion changes and magnitude shifts. In addition, two new techniques are applied. One of them, the variable window technique, makes use of wind ows of different sizes to sample the data, thus addressing some of the shortcomings of the original approach. The other new technique, catal ogue randomization, helps analyse the cause of some of the rate change s; it is most helpful in the cases of pure magnitude shifts. The use o f these techniques is demonstrated examining data from the Garm region , Tadjikistan (former Soviet Union). The Garm earthquake catalogue inc ludes more than 85 000 events for the period 1955 January-1989 April a nd covers an 80 km x 100 km area in the collisional zone between the I ndian and Eurasian plates. Known changes in the Garm network, recently documented in detail, were compared with the rate changes identified with the above techniques. Of the 18 main rate variations identified, at least two-thirds were found to be artificial. Most of them were rea dily associated with reported changes in the network, such as closure and opening of stations, and/or changes in instrumentation. Of the rat e changes determined to be natural, most prominent were the ones assoc iated with a possible quiescence preceding a M6.3 event and its afters hock sequence. Finally, other authors' reports of linear trends and an nual periodicity in the seismic activity at Garm are critically examin ed.