The causes of the Guinsaugon landslide
On 17 February 2006, a catastrophic landslide buried the village of Guinsaugon, the second largest village of St. Bernard town (Southern Leyte, Philippines) killing more than a thousand residents and displacing approximately 19,000 people. The landslide originated on an approximately 800 m high escarpment produced by the Philippine Fault that bisects Leyte and the major islands of the Philippines. In a recent article which synthesized the papers presented during an international conference in Leyte 2008 and published in the international journal Bulletin of Engineering Geology and the Environment, Guthrie and co-workers (2009) arrived at the following conclusions:"The approximately 15 million m3 landslide was a result of progressive failures and tectonic weakening in a region made especially vulnerable by the inter-reaction of geological/tectonic, climatic and cultural factors. In southern Leyte, geology and tectonics (including historical seismicity, the progressive disintegration of the rock mass, the development of smectite layers and the continuous development and movement of shears within the Philippine Fault Zone) combine in steep rugged terrain to produce a series of massive landslides ([10 million m3) of which the Guinsaugon event is the latest."
"The presence of rice paddies in the valley bottom had a major effect on the mobility of the rock avalanche, which increased the vulnerability of communities established to tend these fields. Having considered the available evidence, it is concluded that the landslide was not triggered by a seismic event that occurred several minutes afterward and that the recorded seismic signature was not a trace of the landslide itself. Rather, it is considered that the earthquake could be a result of tectonic unloading after the landslide occurred, or completely independent of the landslide event."
"The role of climate is, in some respects, similar to that of the seismic event. In terms of the trigger, the storm rainfall that occurred several days prior to the landslide undoubtedly raised pore water pressures in the source rock mass. However, progressive failure relies less and less on pore water pressure as failure becomes imminent. The danger of relying on triggers to ascertain the probability of failure is exemplified by the Guinsaugon event; in the lag time between the end of the period of heavy rainfall and the occurrence of the rockslide-debris avalanche, evacuated residents had returned to their homes. Possible trigger mechanisms can be incidental to the landslide itself; however, the progressive development of a large failure often produces telltale signs that are observable by a community of non-experts."
Our own field investigations have shown two important aspects of the landslide not very well taken up in the report. The first is about the role of the thin layers of mudstone in between thick layers of sandstone/siltstones which could have served as lubricant for the landsliding process. The other is the great possibility that the Guinsaugon village developed on old landslide debris. This was clearly shown by the fact that the lower hills not affected by the recent landslide showed comparable materials as the landslide area. Also, the behaviour of the stream tells us a very important information.
It is very likely that the stream was covered by landslide debris in the past which is the reason why it changed its course and appeared to go around the community. Early settlers may have found the sligthtly elevated part of the area convenient to build their houses since it was elevated (and thus not prone to flooding) but without any idea that it was a landslide debris. The tragic landslide was waiting to happen. It was just a matter of time. Unfortunately, the people were not aware of this.
The role of the paddy fields as claimed by the paper needs more scientific investigation. I am not convinced that it played a major role considering the fact that the debris itself was already saturated with water. The clayey soil material from the hillside probably had more influenced on the movement of the debris than the paddy soil.
"The presence of rice paddies in the valley bottom had a major effect on the mobility of the rock avalanche, which increased the vulnerability of communities established to tend these fields. Having considered the available evidence, it is concluded that the landslide was not triggered by a seismic event that occurred several minutes afterward and that the recorded seismic signature was not a trace of the landslide itself. Rather, it is considered that the earthquake could be a result of tectonic unloading after the landslide occurred, or completely independent of the landslide event."
"The role of climate is, in some respects, similar to that of the seismic event. In terms of the trigger, the storm rainfall that occurred several days prior to the landslide undoubtedly raised pore water pressures in the source rock mass. However, progressive failure relies less and less on pore water pressure as failure becomes imminent. The danger of relying on triggers to ascertain the probability of failure is exemplified by the Guinsaugon event; in the lag time between the end of the period of heavy rainfall and the occurrence of the rockslide-debris avalanche, evacuated residents had returned to their homes. Possible trigger mechanisms can be incidental to the landslide itself; however, the progressive development of a large failure often produces telltale signs that are observable by a community of non-experts."
Our own field investigations have shown two important aspects of the landslide not very well taken up in the report. The first is about the role of the thin layers of mudstone in between thick layers of sandstone/siltstones which could have served as lubricant for the landsliding process. The other is the great possibility that the Guinsaugon village developed on old landslide debris. This was clearly shown by the fact that the lower hills not affected by the recent landslide showed comparable materials as the landslide area. Also, the behaviour of the stream tells us a very important information.
It is very likely that the stream was covered by landslide debris in the past which is the reason why it changed its course and appeared to go around the community. Early settlers may have found the sligthtly elevated part of the area convenient to build their houses since it was elevated (and thus not prone to flooding) but without any idea that it was a landslide debris. The tragic landslide was waiting to happen. It was just a matter of time. Unfortunately, the people were not aware of this.
The role of the paddy fields as claimed by the paper needs more scientific investigation. I am not convinced that it played a major role considering the fact that the debris itself was already saturated with water. The clayey soil material from the hillside probably had more influenced on the movement of the debris than the paddy soil.Reference
R. H. Guthrie, S. G. Evans, S. G. Catane, M. A. H. Zarco, and R. M. Saturay Jr. 2009. The 17 February 2006 rock slide-debris avalanche at Guinsaugon Philippines: a synthesis. Bulletin of Engineering Geology and the Environment 68:201–213
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