OFR 97–464: Page [4]
During the reformatting step each reported magnitude was converted to an equivalent moment magnitude (called
mb: Mw*=0.67*(mb+1.5) for mb<3.0; =mb otherwise (3.0,4.0,6.8,7.0)
downweight for mb<4.0 (s/n) and mb>6.8 (saturation); downweight for year<1964
Ms: Mw*=0.67*(Ms+2.7) for Ms<5.5; =Ms otherwise (4.0,5.0,8.3,8.5)
downweight for Ms<5.0 (s/n) and Ms>8.3 (saturation)
ML: Mw*=0.67*(ML+1.5) for ML<4.0; =ML otherwise (-,-,6.8,7.0)
downweight for ML>6.8 (saturation)
Mn: Mw*=0.67*(Mn+1.5) for Mn<3.0; =Mn otherwise (-,-,6.8,7.0)
downweight for Mn>6.8 (saturation)
Mw: Mw* =Mw
MD: same as ML, but don't downweight
FeltArea or MaxIntensity: same as mb, but don't downweight
These conversion rules were generalized from ideas presented by Boore and Joyner (1982), Chung and Bernreuter (1981), and G. Reagor (personal communication, 1995).
The weighting function has a flat top (full weight) and one or two exponential skirts. It has the advantage that the weight is always nonzero, so a single reported value is automatically preserved. The general shape is specified by four values: a lower factor-of-10-down value, a lower full-weight value, an upper full-weight value, and an upper factor-of-10-down value. For example, the weighting function for the Ms-to-Mw* magnitude relationship is specified by magnitude values (4.0,5.0,8.3,8.5). The corresponding weight is 10**((m-5.0)/(5.0-4.0)) for m < 5.0,1 for 5.0 <= m <= 8.3, and 10**((8.3-m)/(8.5-8.3)) for m > 8.3.
Because aftershock and foreshock decisions are based on magnitude, and DNAG records contain the largest of all reported magnitude values, early versions of our algorithm tended to preserve too many DNAG records (at the expense of records from higher-preference catalogs). To fix this, we decided to remove any DNAG record found in the aftershock window of an earthquake from a higher-preference catalog, regardless of magnitude. Also, because our version of the CDMG catalog was cut off below magnitude 4, early versions of our algorithm preserved too many records from lower-preference catalogs in California. (For example, if the CDMG magnitude was 3.8, the earthquake wasnt listed in our version of the CDMG catalog, and we didnt want it in our final WUS catalog. If the corresponding PDE magnitude was 4.1, however, this earthquake would appear as a PDE event. This was especially a problem with the...>>NEXT
Last Update: Thursday, February 16, 2006 9:58
