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Magnitude change in
UC Berkeley Earthquake Catalog

January 23, 1996

The UC Berkeley Seismological Laboratory recently recomputed the station adjustments used to calculate local magnitude. Using a data set of over 1000 events, Dr. Robert Uhrhammer has estimated new station adjustments for the stations of the Berkeley Digital Seismic Network. These adjustments have been applied to all synthesized Wood-Anderson readings and new magnitude estimates generated.

As of January 23, 1996, the magnitudes in the UC Berkeley catalog computed with synthesized Wood-Anderson readings reflect these new station adjustments for events from 1988-present.

Historical Context

Richter (1935, 1958) introduced the concept of an instrumental earthquake magnitude scale and defined "local magnitude" (ML) in the following way:

ML = log A - log A0 + dML

where A is the maximum trace amplitude on the photographical recorded Wood-Anderson (WA) torsion seismograph, A0 is a distance-dependent attenuation factor, and dML is a station-dependent magnitude adjustment. The main use of ML has been to provide a simple and quantitative measure of earthquake size.

Over the years, the Berkeley Seismological Laboratory has operated Wood-Anderson seismographs at a number of sites in northern California. Station adjustments were determined for each site in order to maintain continuity of magnitude. From 1962 until 1992, UC Berkeley operated Wood-Anderson torsion seismometers at ARC (Arcata, California), BKS (Berkeley, California), MHC (Mt. Hamilton, California), and MIN (Mineral, California) and utilized the photographic records to determine earthquake magnitude.

Beginning in 1990, the Berkeley Seismological Laboratory initiated a major upgrade of their instrumentation. The Berkeley Digital Seismic Network is a modern array of broadband seismometers and strong motion instruments with 24-bit digital data recording. In addition to the advantages of digital recording over photographic records, these instruments record ground motion over a broader range of the seismic spectrum than the Wood Anderson seismographs.

The Berkeley Seismological Laboratory collocated BDSN instruments with the Wood-Anderson seismographs in order to calibrate the effect of the new instrumentation on local magnitude. For a period of time, the Berkeley Seismological Laboratory archived "synthetic" Wood-Anderson (WAS) amplitude readings from its broadband seismometers in addition to the traditional Wood-Andersion amplitude readings taken from photographic records. In order to synthesize the Wood Anderson response, we have employed the calibration of Uhrhammer and Collins (1990). The collocation of the digital equipment with the original Wood Anderson instruments has allowed us to compare and calibrate the photographic and synthesized values.

On January 15, 1993, the Wood Anderson torsion seismographs at ARC, BKS, and MHC were turned off, followed by the instruments at MIN one year later.

Computation of New Station Adjustments

Our primary concern is to maintain continuity in the determination of ML despite the evolution from analog to digital instrumentation. A related goal is to retain the simplicity of Richter's original methodology. Here, we briefly describe the proceedures used to determine the revised station adjustments (Uhrhammer et al., 1996).

The first step in the analysis was to verify the historical values of dML that were used in the routine determination of ML at UC Berkeley. Analysis was preformed on readings from the photographic records for earthquakes which were recorded at all four station with a minimum amplitude of 0.3mm. In order to avoid biasing ML, the sum of the revised dML was constrained to be constant (+0.2) - the value of the historical adjustments.

Local Magnitude Station Adjustments
Comparision of historical and revised
Station
Location
dML (Historical)
dML (Revised)
ARC
Arcata, CA
+0.2
+0.209 +/- 0.028
BKS
Berkeley, CA
+0.0
-0.035 +/- 0.017
MHC
Mt. Hamilton, CA
+0.1
+0.128 +/- 0.018
MIN
Mineral, CA
-0.1
-0.107 +/- 0.026

To the nearest tenth of a magnitude unit, the two values of dML are not significantly different, confirming the validity of the historical adjustments.

In the next step, we compared the maximum trace amplitudes of the Wood Anderson photographic records with the synthesized recordings from the BDSN. No significant or systematic variations were observed, and the values of dML in the table above were then adopted for the BDSN stations at these sites.

The dML for the rest of the BDSN stations was then determined using approximately 1000 earthquakes which occurred between 1992 and 1995.

Local Magnitude Station Adjustments
Newly adopted values
Station
Location
dML
ARC
Arcata, CA
+0.209 +/- 0.028
BKS
Berkeley, CA
-0.035 +/- 0.017
CMB
Colombia College, CA
+0.240 +/- 0.009
HOPS
Hopland Field Station, CA
+0.324 +/- 0.032
JRSC
Jasper Ridge, CA
+0.139 +/- 0.016
MHC
Mt. Hamilton, CA
+0.128 +/- 0.018
MIN
Mineral, CA
-0.107 +/- 0.026
ORV
Oroville, CA
+0.428 +/- 0.011
PKD1
Parkfield, CA
-0.198 +/- 0.013
SAO
San Andreas Geophysical Observatory, CA
+0.314 +/- 0.009
STAN
Stanford, CA
-0.233 +/- 0.017
WDC
Whiskeytown Dam, CA
+0.484 +/- 0.012
YBH
Yreka Blue Hill, CA
+0.499 +/- 0.015

These adjustments vary significantly from one station to another and generally reflect the local geology. For example, the stations in the Sierra generally have very large, positive adjustments.

As new stations are added to the BDSN, they are utilized in the ML estimation without a station adjustment. After a period of 6 months to 1 year, we estimate a station adjustment and recomputed the magnitudes. At the present, KCC is the only station without an adjustment.

Change in UC Berkeley Catalog Magnitudes

As of January 23, 1996, the new station adjustments have been used to recompute all of the "network average" estimates of local magnitude reported in the UC Berkeley catalog, and to recompute the "traditional" local magnitudes based on synthetic Wood Anderson records. These new station adjustments will also be used in all future local magnitude calculations.

The following magnitudes are reported in the UC Berkeley catalog:

"Traditional" ML magnitudes:

  1. For events prior to January 17, 1993, only WA amplitude readings are used for determining "traditional" ML magnitude. There has been no change to these magnitude values.
  2. For events from January 17, 1993 to the present, WAS records from the four original Wood Anderson stations are used for determining "traditional" ML magnitude using the new station adjustment values.

"Network Average" ML magnitudes:

  1. For all events from May 13, 1988 to the present, the "network average" magnitude for all events has been recomputed from the average of all BDSN stations which provided WAS readings using the new station adjustment values.

In general, these adjustments have not significantly changed the reported magnitudes, although those events with a limited number of observations are the most sensitive. To demonstrate the effect of the recalculation, this figure compares the new and old "network average" magnitude for all events affected by the change in station adjustments. The top figure shows the ML difference (new - old) is plotted against the new ML and the bottom figure is a histogram of the ML differences. The horizontal lineations in the top figure are primarily due to the inclusion of small earthquakes where the ML was determined by a single station and the location of the line reflects the difference in the new and old station ML adjustments for that station. The histogram is skewed towards positive differences between the new and old station ML adjustments and the individual spikes are also due to the inclusion of small earthquakes which were recorded by only one station.

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.

References

Richter, C. F., An Instrumental Earthquake Magnitude Scale, Bull. Seis. Soc. Am., 25,1-32, 1935.

Richter, C. F., Elementary Seismology, Freeman and Co., New York, 768 pp., 1958.

Uhrhammer, R. A., and E. R. Collins, Synthesis of Wood-Anderson Seismograms from Broadband Digital Records, Bull. Seis. Soc. Am., 80, 702-716, 1990.

Uhrhammer, R. A., S. J. Loper, and B. Romanowicz, Determination of Local Magnitude using BDSN Broadband Records, Bull. Seis. Soc. Am., submitted, 1996.

Additional Information

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