Slower than normal rupture propagation is associated with the presence of relatively mechanically weak material in the fault zone . This is particularly the case for some megathrust earthquakes , where the rupture velocity is about 1.0 km per second . These tsunami earthquakes are dangerous because most of the energy release happens at lower frequencies than normal earthquakes and they lack the peaks of seismic wave activity that would alert coastal populations to a possible tsunami risk . Typically the surface wave magnitude for such an event is much smaller than moment magnitude as the former does not capture the longer wavelength energy release . The 1896 Sanriku earthquake went almost unnoticed , but the associated tsunami killed more than 22,000 people
1896 Sanriku earthquake
1896 Sanriku earthquake
Earthquake rupture