ongoing after shocks. >> and what i heard about today, i thought to myself mexico's city's had a histy rof being vulnerable to quakes. that extends some 700 years to the aztecs. mexico city where it's currently located on a dry lake bed. this lake bed right here across this region, there was an island about 700 years ago where the aztecs have populated this region. this lake was a shallow lake. that has all since drained and of course we're left with this island here of one of the most densely populated cities of our planet. and you can feel shaking that can liquefy these sediments. part of the reason you're seeing these structures come down so readily. and these after shocks essentially poses an additional