We developed an empirical magnitude formula for Indonesia using absolute value acceleration integral computed from accelerograms. We used 68 records from the Indonesia strong motion network operated by the Meteorological Climatological and Geophysical Agency (BMKG) for 13 earthquakes that occurred in the Indonesia region between November, 2008 and March, 2010 with focal depths less than 150 km and Mw ≥ 5.9. We followed Wu and Teng (2004) with slight modification to include a term for focal depth to construct a formula for the new magnitude MewBMKG. We used the least square method under constraints to estimate the coefficients for absolute value acceleration integral, hypocentral distances, and focal depths.
We compared the estimates of MewBMKG to the moment magnitudes from the Global CMT catalog. We found a good agreement between them. The RMS of their differences is 0.27 in the magnitude unit. No saturation is observed up to magnitude 7.7. MewBMKG will be able to be obtained within 3 minutes after earthquakes occurs using data from stations in the epicentral distance less than 300 km. This meets the requirement of the Indonesia Tsunami Early Warning System (InaTEWS). We also determined a magnitude, MBMG, (Iman 2007), for which the maximum displacements obtained from accelerograms are used. Differences between MBMG and the moment magnitudes from the Global CMT catalog are smaller than those between MewBMKG and the moment magnitudes from the Global CMT catalog. We discussed possible problems in calculation of these magnitude scales for their implementation into the InaTEWS.