Catastrophic Rockslide-Debris Avalanche at St.
Bernard, Southern Leyte, Philippines
Shortly after the 10:36 a.m. earthquake (Ms 2.6)
on 17 February 2006, an extremely rapid rockslide-debris avalanche
cascaded down the steep slope of Mt. Can-abag, destroying the entire
village of Guinsaugon, St. Bernard in Southern Leyte, Philippines.
The landslide killed 139 with 980 people still missing and presumed
dead, including 248 pupils trapped inside the primary school
building. The landslide was initiated at the ridge line along a
fault plane associated with the active Philippine Fault Zone. It
started as a block slide that transformed into an avalanche.
Initiation to final deposition took only about five minutes. The
landslide left behind a deep, wedge-shaped scarp. Estimated maximum
velocity of the avalanche ranges 120-130 m/sec.
Based on eyewitness accounts, the failed mass
consists of three rigid blocks that descended successively and
progressively disaggregated forming a series of waves of debris.
Explosions occurred prior to the collapse of the first block and
when the coalesced blocks collided with a ridge at the base of the
slope. The collision caused the debris to breakup into two separate
parts. One part followed the course of the stream, Sapa Aliho, while
bulk of the moving mass followed a straight trajectory, inundating
the valley where Guinsaugon village was located. The debris
avalanche destabilized saturated soil-rich landslide deposits at the
base of the ridge, initiating a secondary debris flow. Air blast
generated by the explosions knocked down trees along the margins of
the deposit. The strongest explosion was heard within a 7-km radius.
The failed mass involved fresh volcaniclastic
rocks variably mixed with soil. The landslide has a total area of
3.2 km2, an estimated volume of 20 M m3 and a
runout distance of 4.1 km. The surface of the deposit exhibits the
hummocky topography typical for avalanches. The landslide debris
dammed at least four rivers.
Earthquakes and high precipitation preceding the
landslide are potential triggers. Cumulative rainfall of 751 mm from
February 01 to 16 was recorded by a rain gauge 7 km west of
Guinsaugon, which is 2.5 times higher than the average monthly
rainfall for February. Excessive rainfall led to elevated hydraulic
pressure in gouge-filled discontinuities, making the slope
marginally stable. The earthquake at 10:36 a.m., which immediately
preceded the landslide, may have been the ultimate trigger.