2 July 2019

The huge September 2018 Naga landslide in the Philippines: a first published analysis

Posted by Dave Petley

Naga City landslide.

Overview of the 2018 Naga landslide. Image via Asian Policy Press.

The huge September 2018 Naga landslide in the Philippines: a first published analysis

The September 2018 Naga landslide was a 27 million cubic metre failure, associated with mining, on Cebu island in the Philippines, which killed 134 people and destroyed almost 60 houses:-

Naga City landslide

The September 2018 Naga landslide in the Philippines. Image via Youtube

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The failure initiated in an area in which there had been extensive mining, and concerned about instability in the area had been raised ahead of the landslide.  In a paper just published in the journal LandslidesCatane et al. (2019) provide a first analysis of this fascinating landslide.  They have mapped the failure using a combination of methods, and have drawn information about activities on the slope prior to failure from a range of sources.  There is some CCTV footage available of the time of failure, which has allowed detailed analysis of the initiation of the slide.

Interestingly, Catane et al. (2019) show that geologists from the Mines and Geosciences Bureau mapped the landslide site in August 2018 – the month before the failure – and logged extensive cracking across the slope, interpreted by the authors as extensional precursory indications of ongoing failure. These cracks are clearly documented on the following video, posted to Youtube:

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These cracks show that the slope was creeping for at least three weeks prior to the failure event.  In response local residents in Sitio Tagaytay were evacuated.  Sadly, as no estimate of the long runout of the landslide was possible (the landslide traveled a distance of 1.34 km), the residents further along the track were not relocated.  It was these people who became the casualties of the landslide.

Catane et al. (2019) interpret the landslide as being a low angled block slide, noting that many of the limestone blocks remained intact and upright.   The landslide moved at a velocity of up to about 8 metres per second, which seems quite rapid, but not exceptional, for such a failure.  In retrospect the runout distance is also not exceptional given total vertical extent of 210 metres and the large volume of the landslide.

An initial analysis of the stability of the slope suggests that prior to failure the slope might have been in a pseudo-unstable state, even under dry conditions.  This should lead to considerable scrutiny of the mining operations.  Interestingly, the authors find no evidence for a direct trigger event for the final failure of the Naga landslide, having analysed both meteorological and seismic data, implying that this was a classic progressive failure event.

The event now seems to be a prime location for a back analysis using INSAR data.  The results are likely to be very revealing – I suspect that it will show that deformation was developing over a much longer time period than was observed at the site.

Overall, this is an excellent first review of this very important landslide.  The tragedy is of course that signs of instability were apparent long before the slope collapsed.  Finding effective mechanisms to identify the potential for such a site to undergo rapid failure must continue to be a priority.

Reference

Catane, S.G., Veracruz, N.A.S., Flora, J.R.R. et al. 2019. Mechanism of a low-angle translational block slide: evidence from the September 2018 Naga landslide, Philippines. Landslides. https://doi.org/10.1007/s10346-019-01212-9