Re:M7.9 Earthquake in Nepal
by MPSimmons on Saturday April 25, 2015 @07:43AM
(#9438042)
To be fair, even modern cities would have trouble with a 7.9. Maybe not tens of thousands of lives, but the scale is logarithmic, and in this quake, the amplitude was literally 10 times as strong as the 6.9 Loma Prieta quake in San Francisco during the 1989 World Series.
[ Parent
| on HN ]
Closer to 30 times actually. Moment magnitude isn't base 10.
Re:M7.9 Earthquake in Nepal
by will_work4tears on Saturday April 25, 2015 @11:30AM
(#9438561)
To be fair though, he did say the amplitude (shaking apmlitude) was 10 times that of a 6.9, which is true, the energy released was 30+ times though, that is correct. (42+ PetaJoules vs 1.4 PJ in the case of the 6.9)
Actually, there's not a simple relationship between magnitude and intensity. Surface wave amplitude depends on a lot more than the size of the earthquake. (e.g. rupture depth, surficial geology, etc)
Moment magnitude is broadly calibrated to be similar to the old Richter scale, which was based (more or less) on log10 of the surface wave amplitude. However, the surface wave relationship only holds for mid-size earthquakes in southern California. It's different for larger earthquakes and different regions.
Either way, it is fair to say that the peak ground acceleration was not 30 times greater, and was significantly less (probably close to 10x, but could be a lot less or slightly more). It's not a simple relationship, at any rate.