Discrepancies Between PIDC, ISC, and USGS Seismic Magnitudes
Abstract
We seek first to document and then to explain the well-known systematic differences between magnitudes (mb) assigned by the PIDC, ISC, and USGS. To do this, we first obtain "classical magnitudes" that as far as possible reproduce the instrumentation and procedures associated with the Veith-Clawson magnitude scale. Though others claim to assign such magnitudes using broadband data, current practice is notably different from the actual Veith-Clawson protocol, and uses measurements made from narrow-band filtered data derived from broadband instruments. We obtain classical magnitudes by making time-domain measurements using WWSSN seismograms simulated from broadband waveforms, thus allowing us to maintain consistency with the original Veith-Clawson magnitude scale. We have obtained Veith-Clawson body-wave magnitudes using simulated WWSSN short-period signals for 21 earthquakes in 1998 and 1999. All of these events have a Veith-Clawson mb that is greater than the PIDC REB mb. The average discrepancy is 0.5 magnitude units. The discrepancy is at least 0.4 mb units for 71% of the station mb observations, with several observations having an offset greater than 1 magnitude unit. Note that the same broadband seismograms underlie these discrepant magnitude values, so this is not an issue of scatter in magnitudes derived from different seismograms. The choice of Gutenberg-Richter, or Veith-Clawson, for the distance correction factor does not strongly affect the resultant event magnitude. The depth assigned to an event by the USGS NEIC in its Preliminary Determination of Epicenters (PDE) is often greater than the depth given by the PIDC REB, especially for shallow events. This is partially due to the fact that the PIDC uses its default depth of 0 km for a significant number of events, rather than solving for an actual event depth. Since an increase in depth will result in a decrease in the magnitude [for a given measurement of log10(A/T) at a given distance, where A is amplitude
Document Details
- Document Type
- Technical Report
- Publication Date
- Oct 01, 2001
- Accession Number
- ADA529620
Entities
People
- John Granville
- Paul G. Richards
- Won-Young Kim
Organizations
- Columbia University