There has recently been much discussion of large-scale interactions of fault zones and the influence of large-scale processes in the preparation and triggering of earthquakes. As a consequence, an official recommendation was issued to set up observational networks at regional scale. In this context, the existing network of standard geomagnetic observatories might play a more important role in future tectonomagnetic studies. The data from standard geomagnetic observatories are basically not appropriate for the detection of small-magnitude and, in most cases, spatially very localized geomagnetic field changes. However, their advantage is a continuity in a long-time period which enables the study of regional tectonomagnetic features and long-term precursory changes. As the first step of a more extensive study aimed at examining the features of observatory data for this purpose, a three-year data set from five European observatories has been analyzed. Some common statistical procedures have been applied along with a simple difference technique and multivariate linear regression to define local geomagnetic field changes. The distribution of M ³ 4.5 earthquakes in Europe, in a corresponding period, was also taken into account. No pronounced field variation, related in time to the M 5.7 Timisoara (Romania) earthquake on July 12, 1991, was found at Grocka observatory at about 80 km from the earthquake epicenter. However, an offset in level of the differences in declination which include Grocka observatory, not seen in the case of differences between other observatories, could be associated with a possible regional effect of the M 4.8 earthquake which occurred in September 1991 at about 70 km SE from Grocka.
A Method for Recognition of Sudden Commencements of Geomagnetic Storms Using Digital Differentiating Filters
