Linear regression relationships have been derived to assess the relationship between China National Network magnitudes and NEIC magnitudes. They are suitable for comparing most types of magnitudes, with the exception of the relation between MS and strongly saturating mb. For a test set of teleseismic body and surface-wave magnitudes determined at CENC for the China National Network in a wide magnitude, period, and distance range, it was shown that the conditions for calculating linear regressions are reasonably met. Because the most suitable linear regression procedure depends both on the error ratio and the absolute errors of the correlated magnitudes, we have estimated these parameters from a Chinese test data set. With the values determined we proved that orthogonal regression is near to optimal for our data. Exceptions are MS-mb relations, for which SR1 is more suitable than OR.

From the comparison of NEIC and CENC magnitudes conclusions have been drawn with respect to the envisaged new IASPEI standards. Band-limited magnitudes such as MS(NEIC) and mb(NEIC) are the most widely determined and are now available for more than 40 years. Such data thus have some well-documented merits, for example, the combined consideration of band-limited MS and mb is effective in discriminating underground nuclear explosions from natural earthquakes (Marshall and Basham, 1972). The MS/mb is also a suitable hazard-relevant parameter, because it provides information about the relative amount of energy release in the high- and low-frequency ranges, which is relevant for seismic-hazard assessment. Moreover, Kaverina et al. (1996) revealed interesting relations of the so-called creepex parameter c = MS – amb – b to earthquake-source geometry and tectonic origin. Thus, there is good reason to continue the tradition of determining MS and mb as established at NEIC and to also calculate these in the future. This is important for mb, in particular, which for small earthquakes is often the only teleseismically measurable magnitude. However, we feel that especially mb measurements would benefit greatly from a more rigorous standardization of the filter response and measurement time window applied. We have shown the effect of different window lengths by comparing mb(NEIC) with mb(CENC) and mb(IDC), which may become larger than 1 m.u. for strong earthquakes. With respect to mb determinations at distances <20°, we came to similar conclusions as NEIC from its own practice, that is, use of the Gutenberg–Richter Q(,h)—at least for short- period data—may not adequately estimate propagation effects at regional distances.

Accordingly, the current IASPEI WG on Magnitudes has recommended to the IASPEI Commission on Seismic Observation and Interpretation (CoSOI), to adopt short-period (T < 3 sec) body-wave magnitude mb as a future standard within the distance range 21° 100°, provided that the maximum P-wave ground amplitude within the entire P-phase train is measured on short-period WWSSN (or equivalently) filtered records.

When comparing MS(20) with a true velocity broadband MS(BB), MS(20) tends to underestimate the surface-wave magnitude when (A/T)max occurs at periods below 18 sec. Although periods T < 18 sec are most frequently observed for regional earthquakes ( < 30°), we have observed them both at the China and the German (GRSN) seismic networks far into the teleseismic distance range. Comparing MS(20) with Chinese MS and MS7 we found systematic differences of the same order and tendency, because the latter are determined in a much wider range of periods and epicentral distances than MS(20) (see equations 3–5 and related text).

By comparing China broadband mB with mb determined at CENC and NEIC we note that mB saturates much later (at about 8) and scales rather well and linearly with both China and NEIC surface-wave magnitudes. Therefore, we strongly recommend a reintroduction of broadband mB into routine international seismological practice. Its determination should become a must for mb >5–5.5, when mb begins to underestimate the P-wave magnitude (Fig. 5, middle-left). The performance of mB is expected to be even better when the bandwidth of the recording system is enlarged in comparison with the China SK and 763 responses used at CENC for mB determination. In this context, we also refer to a recent publication by Bormann and Wylegalla (2005) that indicates that summing up the velocity BB amplitudes of all major subevents in the P-wave train of very large earthquakes one can calculate a cumulative mB (termed mBc), which scales well without saturation with Mw even for the largest earthquakes.

In agreement with our findings the IASPEI WG on Magnitudes has recommended to CoSOI to adopt a broadband surface-wave and body-wave magnitude, with the nomenclature MS(BB) and mB, respectively, as complementary international standards. The broadband velocity amplitudes (corrected by 2) should be calibrated, as MS(20) and mb, with the standard calibration function () according to Vank et al. (1962) and Q(,h)PZ by Gutenberg and Richter (1956). In contrast to MS(20), however, MS(BB) will also cover the regional distance range, whereas the determination of mB should, for the time being, be restricted to the same distance range as for mb (21° 100°). However, one of us (P.B.) found for several recent regional earthquakes with 6.5 < MW < 8 in the distance range between 16° and 20° excellent agreement between mB and Mw. Therefore, we encourage further investigations into the suitability of mB determinations also at distances below 20°. In this context, we note that CENC also determines mB for earthquakes at 100° < 170° by using vertical-component amplitudes of PP waves and the respective Gutenberg–Richter Q-values. About 10% of the mB values determined by the CENC are based on readings from PP waves and they agree well with those from P waves. Bormann and Khalturin (1975) showed that broadband mB for P and PP waves, calibrated with their respective Q-values, scale ideally with an average difference of 0.05 m.u. and an RMSO of ±0.15 m.u. only. This suggests that in the future PP waves should be considered as an acceptable additional candidate for mB determinations, because they appear compatible with those from P waves and permit mB determinations also in the core shadow of P.

The briefly mentioned recommendations of the IASPEI WG on Magnitudes have all been adopted by the CoSOI at the IASPEI General Assembly 2005 in Santiago de Chile. In addition standards for ML and mb(Lg), the conversion of scalar seismic moment Mo in Mw, information on standards for nomenclature, etc., have been adopted. A detailed publication on these new standards is in preparation. Also, the International Seismological Centre (ISC) will be instrumental in disseminating the approved recommendations among seismological centers and in implementing them (ISC Newsletter July–December 2005). Parallel to this, in 2006, CENC and NEIC began to run parallel measurements of both their old and the newly recommended standards. The same is being done at the observatory CLL with data of the German Regional Seismic Network. Thus, new standard magnitude data will soon be available, which will allow us to better test some of the preliminary findings and conclusions presented in this article.

Selected from:

Peter Bormann, Ruifeng Liu, Xiao Ren, Rudolf Gutdeutsch, Diethelm Kaiser, and Silvia Castellaro, 2007. Chinese National Network Magnitudes, Their Relation to NEIC Magnitudes, and Recommendations for New IASPEI Magnitude Standards. Bulletin of the Seismological Society of America, Vol. 97, No. 2: 114 ~ 127

Corresponding author:

Ruifeng LIU