The deadliest earthquake of the 20th Century occured on July 28, 1976 in the Chinese city of Tangshan at 3:42 AM and lasted for around 15 seconds. Measuring 7.8 on the Richter scale, the Tangshan Earthquake caught many sleeping, and those who survived the intital quake scattered out of their homes to find a city completely wiped out. The citizens immediately began to help others who were trapped beneath the rubble, and 80% of the survivors buried were saved before a 7.1 aftershock ended all rescue attempts. Aid from other parts of China were seriously hampered by the fact that only one road into the city remained in drivable condition...and was then clogged by relief workers.
The Tangshan Earthquake complete caught the Chinese off guard. Tangshan had originally been rated to be only susceptible to moderate earthquakes, following a massive earthquake program, and so the buildings were not designed to withstand the massive shaking that had occured. Even so, in the days prior to the earthquake people reported strange activities by animals, including a goldfish that constantly jumped out of its bowl and even fireballs on the day prior. These incidents helped the Chinese government to save many people from a previous earthquake in 1975, but were some how ignored in the Tangshan Quake.
Medical supplies, food, water were extremely scarce for many days (The city had been reduced to rubble) and attempts were even made to parachute supplies, but left many hogging the supplies. Eventually, after it was all over the profusely embarrassed government released statistics showing the death toll at 242,419 with 93% of all residental and 78% business structures wiped out. Although many believe the statistic is far too low, it still ranks as the deadliest earthquake of the 20th century.
Sources
http://history1900s.about.com/library/weekly/aa061500a.htm