A strong earthquake shook northern Taiwan on Sunday. Police said it unleashed rockslides that injured 13 people. They said the injured people were hit by falling rocks at three waterfall tourist sites in suburban Taipei. The quake, measuring 5.5 on the Richter scale, set off rock avalanches in an area covering 1,200 square yards, authorities said. The injured, who were all Taiwanese, were taken to a hospital, police said. One was reported to be in serious condition, and the others suffered only minor injuries. The Central Weather Bureau placed the epicenter of the early-afternoon earthquake in Sanchi, a coastal village about 10 miles north of Taipei. The earthquake was felt in the capital, where residents reported their houses shook violently, the bureau said. The Richter scale measures ground motion as recorded on seismographs. Every increase of one number means a ten-fold increase in intensity. An earthquake of magnitude 5 can cause serious damage in a populated area, and one of magnitude 6 can cause severe damage.