BDSN Surface Wave Magnitude Calculation

Robert A. Uhrhammer

Introduction

A new algorithm, bdsn_ms, has been developed to expedite the calculation of surface wave magnitude M_s from BDSN vertical component broadband recordings. The algorithm was developed to aid the duty seismologist in the rapid calculation of M_s for significant earthquakes.

  

Table 27.1: bdsn_ms command line parameters.

Description

In order to facilitate the timely calculation of M_s a new algorithm, bdsn_ms, was developed which automates the calculation of M_s once the location of seismic source is known. The processing steps in the algorithm are:

• Calculate distance and azimuth to BDSN stations ans also the appropriate time window that spans the required Rayleigh waves.

• Determine when sufficient time has elapsed since the origin time for the Rayleigh wave to propagate to the BDSN stations then extract the BK.LHZ data.

• Bandpass filter (6PBP @ 17-23 sec) each trace and deconvolve to ground displacement.

• For each trace, determine the maximum trace amplitude (A) in microns and the corresponding period (T) in seconds.

• Calculate M_s for each BDSN station using the Prague formula: $M_{s} = 1.66 \times \log ( \Delta ) + \log ( A/T ) + 3.0$.

• Calculate median and interquartile range. With $\sim$20 BDSN broadband stations, the median and interquartile range provide the most robust estimate of M_s and its uncertainty.

Sample Input and Output

The command line arguments for bdsn_ms are given in Table 27.1and the command invoked to calculate M_s for the major earthquake that occurred in Turkey on August 18, 1999 is:

bdsn_ms 1999.229,00:01 40.702 29.987 17.

The bdsn_ms results for the major earthquake that occurred in Turkey on August 18, 1999 are given in Table 27.2. The file name is: 199908170001.Ms. The resulting BDSN M_s estimate of 7.87 $\pm$0.088 is compatable with the NEIC M_s estimate of 7.8.

  

Table 27.2: Sample bdsn_ms output.

Comparison of NEIC M_s and BDSN M_s Estimates

During the past five years, 673 teleseisms occurred which had M_s determined independently by both the NEIC and BDSN. Empirically, we have found that the BDSN estimates are most robust when data from at least 14 BDSN stations are used for determining M_s. The resulting M_s estimates are compared in Figure 27.1. The standard error between the two estimates is $\pm$0.17. The differences between the two estimates is primarily attributed to the fact that the aperture spanned by the BDSN stations is generally much smaller than the aperture spanned by stations used by the NEIC. The median interquartile range for the 14+ station BDSN estimates is $\pm$0.073.

  

Figure 27.1: Comparison of NEIC and BDSN M_s Estimates. The dashed line has a slope of one.

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