A transportable scale for central Europe and implications for low-magnitude discrimination

Abstract

S U M M A R YWe have extended the m b (Lg) method of Nuttli using root-mean-square (rms) amplitudes corrected for noise and a −1 dependence for geometrical spreading. Lg waves recorded on the German Regional Seismic Network (GRSN) for earthquakes in south-central Europe were used to develop an m b (Lg) formula requiring a new calibration constant C rms to keep rms m b (Lg) on the same baseline as Nuttli's traditional formula based on 3rd-peak amplitudes. GRSN stations had to be calibrated for site terms and for Lg attenuation. Lateral variations in Lg Q appear to be significant across the study area, and a regional Q model consisting of constant-Q partitions north, south and in the central Alps was developed using measurements based on interstation and two-event, single-station methods. When plotted against surface wave estimates of M w , rms m b (Lg) measurements in central Europe are found to be consistent with M w -m b (Lg) relationships for north America and southern Asia, thus supporting the transportability of our m b (Lg) formula. Frequency-wavenumber processing of Gräfenberg Array data enabled us to extract Rayleigh waves for small events, and regional M s were measured using the Marshall and Basham formula. Our M s -m b (Lg) relationship extends to M s 2.5 and agrees well with observations in other regions including the western United States. The discrimination potential of M s -m b (Lg) observations was examined under realistic monitoring conditions, where path corrections were inferred from earthquake data and applied uniformly to natural sources and explosions. Under these conditions, m b (Pn, P) are greater than m b (Lg) for large NTS explosions; however, M s -m b scaling slopes are steeper for P waves than they are for Lg, and M s −m b observations for NTS explosions converge near m b 4. Thus, allowing for measurement errors and additional uncertainty in m b (Pn) due to regional bias, there is little difference in the discrimination potential for Pn and Lg waves at small magnitudes. As such, a regional M s −m b discriminant based on Lg might be preferred owing to the better detectability of Lg waves for small earthquakes. These results need to be confirmed for explosions at other test sites. Compared to teleseismic experience, regional M s −m b observations extend the discrimination capability to lower magnitudes by at least one M s unit.Current developments in nuclear test detection are often motivated by the need to improve broad-area monitoring capabilities at low magnitudes. Such is the motivation of this paper, where we follow up recent studies (Patton 2001a; referred to as P01a) investigating the transportability of Nuttli's m b (Lg) and the discrimination potential of M s −m b (Lg) at low magnitudes. Lg waves are well suited for low-magnitude monitoring since they are usually the largest signals on regional seismograms. Establishing the transportability of m b (Lg) would be an important step towards developing M s −m b (Lg) relationships for broad-area discrimination at low magnitudes.The pot...

Lg Body-Wave Magnitude Scaling for the Continental Margin around Korea and Japan

“…The distance-dependent variations of C rms;Nuttli and C rms;Patton may be caused due to discrepancies between actual and theoretical Lg geometrical-spreading effects (e.g., YANG, 2002). Note that this observation is not consistent with PATTON and SCHLITTENHARDT (2005), who proposed to implement a distance-independent calibration constant ðC rms;Patton Þ of 90 lm for central Europe.…”

“…The m b ðLgÞ based on Lg amplitudes on SP-WWSSN was calculated by (NUTTLI, 1986;PATTON and SCHLITTENHARDT, 2005) …”

“…1, the Lg body-wave magnitudes based on rms amplitudes, m b ðLg; rmsÞ; can be calculated similarly (PATTON and SCHLITTENHARDT, 2005):…”

Regional observations of the second North Korean nuclear test on 2009 May 25

“…Traditionally, m b − M s discrimination is used for teleseismic events and measures the amplitudes of high‐frequency P waves and long‐period surface waves. For screening low‐magnitude events, m b − M s discrimination has been extended to use regional observations (Patton & Schlittenhardt 2005; Zhou et al 2006). Therefore, in this study, we measured m b ( Lg ) and M s (VMAX) for the discrimination.…”

“… Pn / Lg spectral ratios are estimated at the same frequency band, whereas m b and M s are measured using high‐frequency body waves and long‐period shear waves, respectively. Based on successful m b − M s discrimination of teleseismic events, the method has been extended to identify regional events at low magnitudes (Patton & Schlittenhardt 2005; Zhou et al 2006). For NK1ST, however, the m b − M s signature showed poor discrimination performance (Patton & Taylor 2008).…”