The ML Scale in Norway
Abstract
A new local magnitude M sub L scale has been developed for Norway, based on a regression analysis of synthesized Wood-Anderson records. The scale is applicable for distances up to more than 1000 km, and the data used comprise 741 short period recordings at 21 seismic stations from 195 earthquakes in the magnitude range 1 to 5 occurring in and around Norway over the last 20 years. Magnitude corrections for distance have been evaluated in term a and an anelastic attenuation term b, and the significant regional crustal differences in the area under investigation made it desirable to develop these for several subsets of the data base. The results for a a generally found to be around 0. 0008, consistent with the weak, intraplate attenuation expected for Norway. Compared to interplate California, this difference in attenuation represents more than one magnitude unit at a distance of 1000 km. New ML scales are commonly tied to Richter's original definition at the standard reference hypocentral distance of 100 km. The significantly weaker Lg wave attenuation in Norway, however, requires a smaller reference distance. We have chosen a value of 60 km, based on an overall assessment of regional coverage, focal depths and quality of the data. The resulting ML formula for Norway reads ML=log A sub wa + a log (R/60) + b (R-60) + 2.68 + S where A sub wa is synthesized Wood-Anderson amplitude (in mm), R is hypocentral distance (in km), and S is a station correction term that for all 21 stations is found to lie within the range + or - 0.22.
Document Details
- Document Type
- Technical Report
- Publication Date
- May 31, 1990
- Accession Number
- ADA226859
Entities
People
- A. Alsaker
- A. Dahle
- H. Bungum
- L. B. Kvamme
- R. A. Hansen
Organizations
- NORSAR