2011 Tōhoku earthquake and tsunami for the event was based on a calculation that requires only about 3 minutes. This calculation is, in turn, based on the maximum amplitude of the seismic wave. The amplitude of the seismic wave is measured using the , which is similar to Richter magnitude scale. However, these scales "saturate" for earthquakes that are above a certain magnitude (magnitude 8 on the JMA scale); that is, in the case of very large earthquakes, the scales' values change little despite large differences in the earthquakes' energy. This resulted in an underestimation of the tsunami's height in initial reports. Problems in issuing
Magnitude (astronomy) by astronomers are: The difference between these concepts can be seen by comparing two stars. Betelgeuse (apparent magnitude 0.5, absolute magnitude −5.8) appears slightly dimmer in the sky than Alpha Centauri (apparent magnitude 0.0, absolute magnitude 4.4) even though it emits thousands of times more light, because Betelgeuse is much farther away. Under the modern logarithmic magnitude scale, two objects, one of which is used as a reference or baseline, whose intensities (brightnesses) measured from Earth in units of power per unit area (such as watts per square metre, W m) are and , will have magnitudes and related by