Extract the answer to the question from the following context. Question: How did this happen? Context: Japanese seismologist Yuji Yagi at the University of Tsukuba said that the earthquake occurred in two stages: "The 155-mile Longmenshan Fault tore in two sections, the first one ripping about seven yards, followed by a second one that sheared four yards." His data also showed that the earthquake lasted about two minutes and released 30 times the energy of the Great Hanshin earthquake of 1995 in Japan, which killed over 6,000 people. He pointed out that the shallowness of the epicenter and the density of population greatly increased the severity of the earthquake. Teruyuki Kato, a seismologist at the University of Tokyo, said that the seismic waves of the quake traveled a long distance without losing their power because of the firmness of the terrain in central China. According to reports from Chengdu, the capital of Sichuan province, the earthquake tremors lasted for "about two or three minutes".
The answer to this question is:
the shallowness of the epicenter and the density of population greatly increased the severity of the earthquake