chi-chi earthquake, also known as the 921 earthquake, was an earthquake in central taiwan. the earthquake happened on september 21, 1999 at 1:47 am local time (september 20 17:47 gmt). it measured 7.3 on the richter scale. the epicentre of the earthquake was at in chichi township of nantou county, hovering around the 12.5 km mark west of the sun moon lake. the depth of the epicenter was 7.0 km.