University of Bergen, Institute of Solid Earth Sciences, Bergen Norway

The general Popularity of magnitude as a convenient and robust measure of earthquake size makes it tempting to examine whether this parameter can be reliably estimated in near real time. In this study we dernonstrate that this is indeed the case conditioned on the design of the signal detector being of STA/LTA type where STA is a short term signal power or rms estimate. Using real data we dernonstrate the Random Vibration Theory relation that Amax (21nN) 1/2 Arms , is valid for non stationary seismic signals. Using Rayleigh's theorem we also estabmlaixshed a relation brmetween Arms and the flat portion of the source spectra. These Amax and Arms estimation procedures are used for determining conventional magnitudes and moment magnitudes for 29 events as recorded by the Norwegian Seismograph Network (NSN). We used here a procedure outlined by Sereno et al. (1988) and also their geometrical spreading and attenuation parameters derived from analysis of NORSAR recordings. Our magnitude and moment magnitude estimates for 5 different frequency bands are in good agreement with the ML estimates derived from the conventional magnitude formulas in combination with empirical correction tables. Surprisingly, the Amax and Arms magnitudes produced consistent negative biased by ca. 0.4 units estimates even in the extreme 4 8 Hz band. In view of the good agreement between various types of magnitude estimates, we constructed conventional magnitude correction tables spreading and attenuation parameters from Sereno et al (1988) for a variety of signal frequency bands. Near real time Amax ad/or Arms or correspondingly event magnitudes would be of significance in automatic phase association analysis, bulletin production for local and regional seismic networks and the earthquakes monitoring performances of such networks.

We review historical earthquake research in Northern Europe. 'Historical' is defined as being identical with seismic events occurring in the pre-instrumental and early instrumental periods between 1073 and the mid-1960s. The first seismographs in this region were installed in Uppsala, Sweden and Bergen, Norway in 1904-1905, but these mechanical pendulum instruments were broad band and amplification factors were modest at around 500. Until the 1960s few modern short period electromagnetic seismographs were deployed. Scientific earthquake studies in this region began during the first decades of the 1800s, while the systematic use of macroseismic questionnaires commenced at the end of that century. Basic research efforts have vigorously been pursued from the 1970s onwards because of the mandatory seismic risk studies for commissioning nuclear power plants in Sweden, Finland, NW Russia, Kola and installations of huge oil platforms in the North Sea. The most comprehensive earthquake database currently available for Northern Europe is the FENCAT catalogue covering about six centuries and representing the accumulation of work conducted by many scientists during the last 200 years. This catalogue is given in parametric form, while original macroseismic observations and intensity maps for the largest earthquakes can be found in various national publications, often in local languages. No database giving intensity data points exists in computerized form for the region. The FENCAT catalogue still contains some spurious events of various kinds but more serious are some recent claims that some of the presumed largest historical earthquakes have been assigned too large magnitude values, which would have implications for earthquake hazard levels implemented in national building codes. We discuss future cooperative measures such as establishing macroseismic data archives as a means for promoting further research on historical earthquakes in Northern Europe.