SPOT REPORT ON THE LANDSLIDE INCIDENT IN BRGY. MARAYAG, LUPON, DAVAO ORIENTAL In the first week of January 2014, a Low Pressure Area (LPA) brought torrential rains for several days in different parts of the region particularly in the provinces of Davao del Norte, Compostela Valley and Davao Oriental causing widespread flooding and several landslides. Based on the Situation Update of the Office of Civil Defense XI dated January 14, 2014, 13 were reported as casualties while 7 are still missing from different areas in the region. Among the fatalities/missing, 3 are from Brgy. Marayag, Lupon, Davao Oriental. Brgy. Marayag with geographic coordinates of 126°9'41.789" east longitude and 7°9'21.292" north latitude is located about 33 kilometers (linear distance) northeast from Lupon proper and can be reached via the unpaved Mati-Maragusan road. The assessed site is characterized by flat to undulating terrain flanked by steep slopes and mountainous terrain which is underlain by poorly indurated and highly weathered diorite to the east and metamorphosed sedimentary rocks to the west (ABS & CAA, July 1982) while the flat central portion is underlain by Recent alluvial deposits. The eastern side is part of the Katian Mountain Range with the highest elevation of 1640 masl and to the west is part of the Tagopo Mountains with its peak at 920 meters. The area is mainly drained by Sumlog River and its several tributaries characterized by angular/rectangular drainage system cutting through steeply incised valleys. This type of drainage pattern is a result of the structural control joints or faults in the bedrock. Sumlog river generally trends in a NNW-SSE direction in the upper reaches or headwaters and shifts to a NE-SW direction from Barangay Don Mariano Marcos as it exits towards Lupon and eventually drop is loads to Cuabo Bay. Based on previous reports (NBA and RTV, Sept 2006), the assessed site as well as the adjacent Brgy. Calapagan is believed to be a pull-apart basin developed due to the movement of the left-lateral strike slip fault within the Philippine Fault Zone. The main fault identified as Bitanagan Fault (ABS & CAA, July 1982) generally trends NNW and passes along the break-in-slope defining the gently undulating terrain and the mountainous area. As per interview, the landslide which is a characteristic of a debris flow type of mass wasting occurred on January 12, 2014 at around 4:00 am. Although several debris flows were noted along the road on the way to Marayag, the area that was severely affected is Po. Madasigon (proper) of Brgy. Marayag damaging several infrastructure and houses including the barangay hall and covered court. During the assessment, it was noted that the debris flow shifted westward towards the barangay hall and houses as the original channel was already filled up by debris deposit. An estimate of 1 to 2-meters thick of debris deposit composed of sand to cobble to boulder-size rocks and uprooted trees and logs covered the area. Most of the gullies in the purok and in the vicinities served as the channel way of the debris flow. According to the locals, the occurrence of this type of natural hazard in the area is common. They added that similar debris flow but in a smaller scale happened in 1976 (?), 1986 (?) and in 1993 (?). Based on the information given, a 10-year flood/debris flow affects the site or its vicinities. Previous reports available in MGB-XI also noted the debris flow that affected Brgy. Calapagan in 2006. Moreover, results of the 1:50,000 scale geohazard mapping, Brgys. Marayag and Calapagan were Republic of the Philippines Department of Environment and Natural Resources MINES AND GEOSCIENCES BUREAU Regional Office No. XI 669 Konsuela Bldg., Aurora Quezon Street, Davao City Tel. No. (082) 222-5601 Telefax No. (082) 221-6535 Email: [email protected]