Complex sentence: Its preparedness was tested on 14 June 2005 when an earthquake measuring 7.0 on the Richter scale hit offshore; much of the city was evacuated in a matter of 20 minutes when a tsunami warning was issued, but no waves were reported.
Split into simpler sentences: Its preparedness was tested on 14 June 2005 when an earthquake measuring 7.0 on the Richter scale hit offshore. Reportedly, much of the city was evacuated in a matter of 20 minutes when a tsunami warning was issued, but no waves were reported.