We determined m^b, the body-wave magnitude calculated from the maximum amplitude of short-period P waves, of 38 large earthquakes in various tectonic provinces. The data are divided into three groups: group 1 (subduction-zone thrust events); group 2 (nonsubduction-zone dip-slip and oblique-slip events); and group 3 (strike-slip events). Groups 2 and 3 include intraplate earthquakes. Comparison of m^b values for these three groups of events suggests that the source spectral amplitudes of intraplate events at a period of about 1.4 sec is 2 to 5 times larger than those of subduction-zone events with the same Mw. We also determined the source spectra of 28 large earthquakes (Mw = 6.5 to 7.7) directly from Global Digital Seismographic Network (GDSN) data, over a period range from 1 to 10 sec. At periods from 1 to 2 sec, the source spectral amplitudes of intraplate earthquakes are 2 to 4 times larger than those of subduction-zone events with the same Mw. The difference decreases as the period increases to 10 sec.