Regional relationships among earthquake magnitude scales

Abstract

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 MS and the mb scales were designed to be universal scales. However, both MS and mb magnitudes are often determined beyond the applicable range of the equations used to define the two scales. Furthermore, the MS magnitudes are not generally available for moderate to small earthquakes in most earthquake catalogs. The mb magnitudes are more generally available than MS values; however, there is also much greater variation in the way mb is determined. In particular, a significant change in the mb scale occurred in the early 1960’s when the World‐Wide Standard Seismograph Network (WWSSN) was established. This change in instrumentation used to determine mb values had a significant effect on estimated magnitudes (post‐1960 values are lower) and the saturation level of the mb scale. The older, longer‐period instruments recorded larger mb magnitudes than can be recorded with the WWSSN instruments. In addition, great care must be taken when selecting the mb 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 mb 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 mb values for the two regions. A regional mb 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 mb and ML values for earthquakes in the western United States. An empirical link between the mb of an eastern U.S. earthquake and the ML of an equivalent western earthquake is given by ML = 0.57 + 0.92(mb)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.

A transportable scale for central Europe and implications for low-magnitude discrimination

“…10(b) they are offset by 0.2–0.25 mu because m b ( P ) and m b ( Pn ) measurements for earthquakes were not corrected for regional bias. This offset is less than the bias estimate for western US obtained by Chung & Bernreuter (1981; −0.33 mu), and highlights additional uncertainty in a magnitude requiring further corrections because it does not transport. Thus considering measurement errors and the greater uncertainty in m b ( Pn ) due to regional bias, the results in Fig.…”

Preparation of earthquake catalogs for the National Seismic-Hazard Maps: Contiguous 48 states

“…During the reformatting step each magnitude value was converted to an equivalent bodywave magnitude (called m^*), and for catalogs with multiple magnitude entries a weighted sum of these was used to compute a single body-wave magnitude value as follows. These conversion rules were generalized from ideas presented by Boore and Joyner (1982), Chung and Bernreuter (1981), Veneziano and Van Dyke (1985), and G. Reagor (personal communication, 1995).…”

Analysis of Online News Coverage on Earthquakes Through Text Mining

“…For instance, many news agencies quote earthquake magnitudes published by USGS, while others report the earthquake magnitude from other seismic monitoring agencies. Different earthquake agencies are likely to report slightly different magnitude values either because of different type of estimate (e.g., local, body wave, surface wave, and moment magnitude) or because using different parameters such as the number of seismic stations (e.g., Chung and Bernreuter, 1981;Kanamori, 1983). In other instances, some reporters tend to give rounded "ceiling" values for the earthquake magnitude reporting a higher than the actual value.…”