This is because these are already vulnerable to future ground movements that would endanger lives and property.
Dr. Art Daag, chief science research specialist of the PHIVOLCS Central Office, on Monday said the magnitude 6.9 earthquake that rocked Cebu and Negros last Feb. 6 had not only caused landslides in Guihulngan and other areas in northern Negros Oriental, but had also weakened the rock and soil slopes.
Daag, who headed a team of PHIVOLCS that surveyed the post-earthquake damages, said that at least four major landslides had been reported and inspected in Solonggon, La Libertad, and in Planas, Zamora and upper Tinayonan Beach in Guihulngan City.
The Solonggon and Planas landslides claimed the lives of several people, with many still missing and believed to have perished under the rubble. Other areas like Jimalalud, Ayungon and Tayasan have also reported landslides following the powerful earthquake but without casualties.
The PHIVOLCS scientist said the landslides in northern Negros Oriental were of a different type, as these as deep-seated rotational and not shallow in movement. Most of the ground material in the northern quake-hit areas of Negros Oriental is basically composed of limestone, he said.
The main shock and almost 2,000 aftershocks as of this week have created stresses in the mountain slopes, making them weak and vulnerable to further collapse, Daag said.
Weight from heavy rains would also further aggravate the threat as tension cracks have also been monitored in the already weakened slopes, he added.
In its initial survey, Daag said the major landslides measured between 10 to 20 hectares wide and with an estimated height of about 30 meters.
The aftershocks continue, but with the strength and intensity lessening, it was not yet known when they would stop, Daag said. He cited the major earthquake in Baguio City several years ago where the aftershocks continued for three months.