RICHTER MAGNITUDES AND SITE CORRECTIONS USING VERTICAL COMPONENT SEISMOGRAMS

The definition of the Richter ML magnitude scale is in terms of seismi c wave horizontal components recorded on Wood-Anderson seismographs. H owever, at many seismograph sites only the vertical component is avail able, and at sedimentary sites horizontal components are usually signi ficantly amplified, causing complications in the assignment of a magni tude to an earthquake. Because each earthquake can be recorded at a di fferent subset of sites, each subset having a different combination of site amplifications, the assignment of a magnitude is dependent upon the seismograph site combination that records a particular earthquake. Although there is some amplification of the vertical component at sed imentary foundation sites, it is shown that a reduced spread of values Of ML magnitude, consistent with low amplification (bedrock) site mag nitudes, can be achieved using the vertical component to compute the m agnitude and adding 0.2 to adjust to the ML magnitude scale (defined i n terms of the horizontal components). This presupposes that the sites used by Richter were on bedrock; however, even if this is incorrect, it appears to be a necessary precondition for the world-wide unificati on of the Richter scale along with defining the true gain of Wood-Ande rson seismographs rather than accepting the design gain of 2800. Site corrections would be smaller than those established using the horizont al components. Taking into account the use of only the vertical compon ent in the calculation Of ML and including the 0.2 adjustment to the e quivalent horizontal component derived magnitude, the expression for t he calculation of magnitudes in the Victoria region becomes: ML = logA (z) - logS(z) + 0.9 + logR + 0.0056Re-0.0013R where A(z) is the equiva lent Wood-Anderson seismograph displacement amplitude, S(z) is the sit e amplification (vertical component) and R is the hypocentral distance .