The July 2005 Mayana Landslide, Jagna, Bohol
- Possible Mechanisms and Hazards

Sandra G. Catane, Joseph Foronda, Cristituto Tomarong, Jr.

National Institute of Geological Sciences, University of the Philippines,
Diliman, Quezon City

 

Abstract

On the early morning of July 11, 2005, large limestone blocks slide along a steep scarp, which initiated eastward movement of debris up to the present. Earlier, a surface-wave magnitude 4.9 earthquake with epicenter in Sierra Bullones (about 46 km east of Tabilaran City) had occurred on 31 March 2005. The epicenter is roughly only <10 km from the site of the landslide occurrence . The earthquake probably triggered the landslide in Barangay Mayana. The very large landslide originated as rockfalls along a very steep NW-trending scarp in the Sierra Bullones Limestone in Sitio Balikbayan. The debris fell on an area underlain by older limestone landslide debris and thickly weathered soils from the underlying volcaniclastic rocks of the Late Miocene Carmen Formation. The slope of the landslide is only about 13% (7½º). The landslide has a total length of 1.58 km and an area of 52 ha. as of 13 August, 2005 . The debris moves at an average rate of 22 m/day, is still very active till present time, and has already destroyed large agricultural areas and 67 houses. As of 30 September 2005, it already reached a total length of 2.15 km and an area of 69.4 ha. .

The zone of depletion (proximal part) of the landslide debris occurs mainly in Sitio Balikbayan, has a width of about 400 m, and consists dominantly of blocks of limestones, whereas the debris in the zone of accumulation (distal part) in Sitio Ilaud, with a width of 260 m, consists of relatively intact soils derived from the weathering of the volcaniclastic sediments. The large blocks of limestones in Sitio Balikbayan are unlikely to be remobilized during the rainy season, but can have translational motion as they move as a single mass over a slip plane in the Carmen Formation; whereas, the landslide debris blocking the Mayana River is susceptible to remobilization as debris flows that could extend into populated areas on the lower reaches of the Alihauan River.

Subsequent observations indicate that draining of the small Mayana River landslide-dammed lakes would pose a minimal flooding hazard. However, if the landslide debris would reach the Alihauan River, a landslide dam may form and a catastrophic draining of a lake would pose a serious hazard and warrants immediate action. Construction of a spillway across part of the dam could moderate the impact of catastrophic lake draining and the associated flood.