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Magnitude change in
UC Berkeley Earthquake Catalog

February 8, 1995

The UC Berkeley Seismological Laboratory recently discovered a bug in the program used to compute earthquake magnitudes in the UCB catalog. Under certain conditions some station corrections were not always applied to the magnitude estimate. This problem existed for events in the UCB catalog from 1984 through 1995.

As of February 8, 1995, we have corrected this bug, and have re-computed the magnitudes for all events in the 1984-1995 catalogs. The average bias in the magnitude estimate due to this bug was 0.1 magnitude units.

However, at the time we corrected this bug, we decided to changed the definition of the magnitudes provided in the catalog in order to more clearly define the type of information used to calculate the magnitude.

For the last 5 years, the Seismological Laboratory has archived "synthetic" Wood-Anderson (WAS) amplitude readings from its broadband seismometers in addition to the traditional Wood-Andersion (WA) amplitude readings taken from photographic records. Prior to February 8, 1995, both WA and WAS readings for the stations ARC, BKS, MHC, and MIN were used to compute local magnitude (Ml).

In revising the definition of magnitudes in our catalog, we are attempting to provide a clear-cut break between the use of the synthetic records and the traditional readings. We are now distinguishing between these two types of amplitude readings.

Ml magnitude calculations (effective Feb 8, 1995):

  1. For events prior to 1/16/93, only WA amplitude readings are used for determining Ml magnitude.
  2. For all events after that 1/16/93, only WAS records are used for determining Ml magnitude. In both cases, the Ml is computed from the four original stations.

"Network Average" Ml calculations:

  1. The magnitude referred to as "network average" is the local magnitude computed from the average of all BDSN stations which provided WAS readings. This definition has not changed.

Because of the effort we have put into calibrating the Wood-Anderson synthetics, we feel that this definition will provide continuity of magnitude in our catalog.

This change in the definition of magnitude means that there is considerable scatter in the comparison of old and revised numbers. The scatter is more pronounced for smaller events (M < 3.0).

If you have further questions about this change in magnitude calculation, please send email to: info@seismo.berkeley.edu