Regional relationships among earthquake magnitude scales

Various magnitude scales commonly used and the interrelationships among them are reviewed. It is shown that problems exist with all of the magnitude scales being used in the United States. When using regional catalogs, for example, it is often necessary to determine how the reported magnitudes were determined. Often such information is not available, although the potential errors are quite large. Both the M S and the m b scales were designed to be universal scales. However, both M S and m b magnitudes are often determined beyond the applicable range of the equations used to define the two scales. Furthermore, the M S magnitudes are not generally available for moderate to small earthquakes in most earthquake catalogs. The m b magnitudes are more generally available than M S values; however, there is also much greater variation in the way m b is determined. In particular, a significant change in the m b scale occurred in the early 1960’s when the World‐Wide Standard Seismograph Network (WWSSN) was established. This change in instrumentation used to determine m b values had a significant effect on estimated magnitudes (post‐1960 values are lower) and the saturation level of the m b scale. The older, longer‐period instruments recorded larger m b magnitudes than can be recorded with the WWSSN instruments. In addition, great care must be taken when selecting the m b magnitudes of western U.S. earthquakes, because the values are often in considerable error owing to the fact that they were determined at distances less than 25° and were not properly corrected for attenuation in the upper mantle, or asthenosphere. The seismic body wave magnitude m b of an earthquake is strongly affected by regional variations in the Q structure, composition, and physical state within the earth. Therefore because of differences in attenuation of P waves between the western and eastern United States, a problem arises when comparing m b values for the two regions. A regional m b magnitude bias exists which, depending on where the earthquake occurs and where the P waves are recorded, can lead to magnitude errors as large as ⅓ unit. There is also a significant difference between m b and M L values for earthquakes in the western United States. An empirical link between the m b of an eastern U.S. earthquake and the M L of an equivalent western earthquake is given by M L = 0.57 + 0.92( m b ) east . This result is important when comparing ground motion between the two regions and for choosing a set of real western U.S. earthquake records to represent eastern earthquakes.