Rescuers dug with picks and shovels trying to reach dozens of people trapped under houses collapsed by a strong earthquake Monday that shook a central Philippine island and set off landslides. At least 13 people were killed and 40 are believed missing, most of them along the shore near the epicenter of the 6.9-magnitude quake that struck in a narrow strait just off Negros Island. In the mountain village of Planas, 9 miles (15 kilometers) from coastal Guihulngan town in Negros Oriental province, as many as 30 houses were buried with at least 40 residents believed trapped, said Gov. Roel Degamo.