109
Although the NOAA database reports 2224 earthquake in the time-period 1000-1903, only the 40% of them have a magnitude value. Out of these, M ≼ 7 earthquakes are 422, to be compared with the 715 earthquakes of the same size in GHEC. In addition, NOAA reports Mw only for 8 of the M ≼ 7 earthquakes, most of the remaining being Ms (68%) or not specified (27%), (see Table 1.8 for the magnitude types in GHEC). Hough (2013) recently pointed out the scarce reliability of high magnitude values in NOAA, among which the most striking is probably the magnitude 8.5 attributed to the 25 July 1855 Valais (Switzerland), resulting from a confusion with epicentral intensity. An exhaustive comparison of NOAA and GHEC would require a one-to-one check of the earthquakes, which is out of the scope of this report. In the following, two examples of such a comparison are shown for the Himalayas and Indonesia. GHEC lists 20 earthquakes in the Himalayas, mostly derived from Ambraseys and Douglas (2004), Szeliga et al. (2010) and Min Ziqun (1995). In the same area NOAA reports a total of 12 earthquakes (Figure 3.9), out of which only six with magnitude. Even the largest event in the area, the 2 April 1762, Mw 8.8 according to Cummins (2007), has no magnitude in NOAA. All the earthquakes in NOAA are also in GHEC, but the differences in both the epicentral location (Figure 3.10) and magnitude (Figure 3.11) are remarkable. In addition, the 26 August 1833 Nyalam earthquake is duplicated in NOAA, once with the correct date and magnitude (Ms 8), and once dated 30 May with M 7.5 and a significantly different location (more than 500 km to the West).
Figure 3.10 GHEC and NOAA content in the Himalayas