[sgxin]

These areas are in western Sichuan, less populated, mostly Tibetans

Should be the case from the low casualties.
This side-way shaking quake was so strong that it even violently swayed roadside trees in the video below.
This is something that I have never seen in other quake videos.

Casualties would be very high had this quake happened in a densely populated city (in any part of the world - even no shaking can dislodged a big piece of earth into the Ulu Pandan Canal) as many buildings would have toppled.