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Understanding the Differences between Three Teleseismic m_b Scales

¹ Lamont-Doherty Earth Observatory of Columbia University
61 Route 9W
Palisades, New York 10964
johng{at}ldeo.columbia.edu
richards{at}ldeo.columbia.edu
wykim{at}ldeo.columbia.edu
sykes{at}ldeo.columbia.edu

We investigate differences between three body-wave magnitude (m_b) scales for 2009 earthquakes from 1996 to 1999 listed in the Preliminary Determination of Epicenter (PDE) bulletin having m_b between 5.0 and 5.5 and that also have moment tensor solutions available from the Harvard Centroid Moment Tensor (CMT) catalog. A total of 31,280 broadband seismograms are analyzed, for an average of 15 stations per event. Both the PDE and Reviewed Event Bulletin (REB) procedures for determining an automated m_b are reproduced, thereby eliminating any discrepancies that result from using different networks of stations. We compare the reproduced PDE and REB magnitudes to another magnitude measurement, m_b(P), that is based on the Worldwide Standard Seismographic Network (WWSSN) short-period instrument. We find that differences between m_b(P), m_b(PDE), and m_b(REB) arise from four factors: response function, length of time window, and corrections for event depth and epicentral distance. Reproduced m_b(PDE) and m_b(P) are strongly correlated, and we expect that magnitudes assigned from WWSSN short-period instruments during the 1970s and 1980s are consistent with those assigned by the automated procedure used since 1991 by the U.S. Geological Survey (USGS), providing stability in m_b measurements over several decades. The difference between m_b(REB) and m_b(P) is much greater because of the significantly shorter REB window length of 5.5 sec and the high-frequency passband of the REB displacement response.

Online Material: Filter parameters and color versions of Figures 8 and 9.

View larger version (41K): [in this window] [in a new window]   Figure 8. (a) Reproduced event mb(PDE) as a function of event mb(P) for PDE bulletin mb 5.0 and 5.5 earthquakes. Reproduced PDE measurements are made using a time window length of 10 sec (to approximate the time window used in the PDE bulletin). Measurements of mb(P) are made using a 15-sec time window and the distance and depth corrections for all magnitudes shown are Gutenberg–Richter (to match PDE bulletin). Correlation coefficient value (R) is shown for each plot. (b) Reproduced event mb(PDE) as a function of event mb(P) for PDE bulletin mb 5.0 and 5.5 earthquakes. Both magnitude types are measured using a 15-sec time window. Distance and depth corrections for all magnitudes shown are Gutenberg–Richter. Correlation coefficient value (R) is shown for each plot. (Color versions of Figures 8a and 8b that include results for all six subsets of earthquakes [PDE bulletin mb 5.0 to 5.5] are available in the electronic supplement to this article.)

View larger version (46K): [in this window] [in a new window]   Figure 9. (a) Reproduced event mb(REB) as a function of event mb(P) for PDE bulletin mb 5.0 and 5.5 earthquakes. Reproduced REB measurements are made using a time window length of 5.5 sec (to match the time window used in the REB catalog). Measurements of mb(P) are made using a 15-sec time window and the distance and depth corrections for all magnitudes shown are Veith–Clawson (to match REB catalog). Correlation coefficient value (R) is shown for each plot. (b) Reproduced event mb(REB) as a function of event mb(P) for PDE bulletin mb 5.0 and 5.5 earthquakes. Both magnitude types are measured using a 15-sec time window. Distance and depth corrections for all magnitudes shown are Veith–Clawson. Correlation coefficient value (R) is shown for each plot. (A Color versions of Figures 9a and 9b that include results for all six subsets of earthquakes [PDE bulletin mb 5.0 to 5.5] are available in the electronic supplement to this article.)

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