Myanmar's SuperShear earthquake

We now know that the death toll from the earthquake in Myanmar exceeds 10,000. Any number with four zeros after it is suspicious and it truly an estimate That estimate is reasonable for the size of the earthquake in a setting where building codes may be weak (hard to know) and/or poorly enforced for reasons of institutional oversight and /or corruption. That's expected. Sadly.

From a seismological perspective this is just what you would expect. The area of shaking shown above form a very distinct linear shape that relates to the orientation of the Saigang Fault that rupured to cause the earthquake. Strong shaking occurs very close to the epicenter and along the fault to the south. The former capital Yangon is in a region of light shakng . The new capital, Naypyidaw is in a area of strong shaking. Mandalay is very close to the epicenter and experienced very strong shaking.

Some things about this earthquake are not so expected. Shaking and damage were sever in Bangkok, more than a 1000km distant from the epicenter. Bangkok isn't even in the Light region of shaking intensity. So why was the damage so great in Bangkok? One reason is always construction standards. The very tall building that collapsed was under construction so was just a skeleton with no strengthening exterior walls. That certianly contributed.

But this earthquake itself was unusual. When the Earth breaks in an earthquake it is typical that the rupture begins at a point and propagates away from that point along the fault. Because seismic waves from an earthquake travel very fast, the waves propagate faster than the rupture. In rare cases, as seems to be the case for this earthquake the rupture of the crust travels faster than the seismic waves. This phenomenon is referred to as SuperShear . It can concentrate energy ahead of the seismic waves in a way that cannot be estimated by standard analyses of shaking as in the map shown above.

In Thailand resue and recover was swift and effective. The fatality total looks to be in the 10s.

Pancake collapse in Myanmar

Many of the buildings that collapsed in both Myanmar and Bangkok experienced pancake collapse. Buildings collapse in a near vertical manner with each floor falling onto the floor below. This is often due to the way in which the building was constructed with very heavy solid floors and relatively weal walls. The image at left is an example. The near-horizontal slabs seen in the image are floors of the building that collapsed on top of one another.

Often the lowest story collapses first because it has the most weight to support. Once the lower floor collapses the upper floors progressively fall onto one another.