Magnitude relations between the teleseismic mb, the regional mR and Mw for intraplate earthquakes in Brazil

Abstract

Magnitude conversions between different scales are important for comparison of seismic activity between different regions, homogenization of earthquake catalogs for seismic hazard calculations, and use of ground motion relations derived from other regions. Despite the trend of adopting Mw as the standard, reference magnitude scale, other scales are still much used for practical reasons and ease of calculation, such as the local ML for events detected by local networks, and short-period teleseismic mb for events in worldwide catalogues. In Brazil, a regional magnitude (mR), based on the maximum amplitude in the whole P-wave train recorded between 200 and 1500 km distance, has been adopted in our catalogues. The regional magnitude is equivalent to the teleseismic mb from 3.5 to 5.5.Empirical relations between the regional (mR), the teleseismic (mb) and the moment (Mw) magnitudes were determined for Brazilian intraplate earthquakes. Mw can be estimated from m by Mw = 1.10 m - 0.69. In this empirical relation, “m" is mb, mR, or the average of the two. However, despite the average consistency of the regional (mR) and teleseismic (mb) scales, it has been found that difference between mb and mR depend on the type of faulting mechanism: mb > mR for dip-slip earthquakes (reverse and normal faulting), and mb < mR for strike-slip events. This can be explained by the differences in the average radiation patterns of upper crustal earthquakes recorded at regional distances (more horizontally oriented take-off angles) or at teleseismic distances (more vertically oriented take-off angles). This brings the possibility to use the difference between regional and teleseismic magnitudes to indicate the probable type of focal mechanism.