Friday, 20 September 2013

Location and volume of the La Pintada landslide

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Heavy rainfall from hurricane Manuel initiated a large landslide which struck the village of La Pintada (Mexico) this Monday. 58 people have been reported missing in the small village, and at least 20 buildings have been destroyed. Although helicopters have rescued most of the villagers, 45 people remain, and authorities are concerned that "The rest of the hill could fall.". It is almost inevitable that such a large landslide in the narrow valley will cause major difficulties for rescue crews in the coming days, and access into much of the remote coffee growing region is likely to be challenging in the wake of the storm.

An aerial view of the La Pintada landslide
Source: Amador Narcia/Twitter

The above image is one of the first released of the region, and clearly conveys the devastating effect of the landslide. Getting information out to concerned parties can be critically important in such situations, although an accurate sense of scale and location data for early social media images is rarely available. Using the 'Add Photo' dialogue in Google Earth, however, we can use media images to locate, map, and make early decisions regarding ongoing risks and emergency response very soon after disastrous events. Using the Google Earth API we are able to approximately locate the position from which the above photograph was taken, as well as the position of an image which seems to provide an overview of the deposit area prior to the failure. The interactive map below represents an initial interpretation of the spatial extent of the landslide deposit (outlined in brown), as well as an initial guess as to what may be the source of the failure (outlined in grey).

Interactive map indicating the estimated source (grey), and mapped deposit (brown), as well as two photographs covering the area of the damaged village (double click to view). An updated .kmz file (including additional aerial photographs and and revised map of the scarp from 21/09/13) is available as a Google Earth download here

As is evident above, the landslide initiated on a partially vegetated slope to the north of the village, and as reported, appears to have buried much of the eastern side of the community. Survivors report a loud rumbling, and very rapid failure, which is evident from the map as at least one building seems to have been pushed around 30 m from it's foundations (white arrow).  A depression formed at the back of the deposit (adjacent to the hillslope) indicates the energetic landslide mass probably scoured sediment out from the toe of the slope, and did not stop moving until the whole mass reached the valley floor. Calculation of the landslide volume using a combination of the measurement tools in Google Earth and an estimation of the deposit thickness from the aerial photograph suggests approximately 100,000 m^3 of debris remains in the village, although the failure may have been larger as there has been some erosion of the deposit adjacent to the river channel. 

An image taken looking east, across the village prior to the landslide. The mass travelled from left to right, through the buildings in the farground.
Source: gatopelu008\Panoramio

While it is not immediately apparent why this particular slope failed during the storm, there appears to be some evidence for planar failures on similarly oriented slopes elsewhere in the valley. Judging from the weathered appearance of the landslide deposit, it seems likely that bedrock degradation has been ongoing as part of the landscape's natural weathering and erosion cycle. As in many sub-tropical regions, it is possible that either historical, or recent, removal of vegetation led to a local acceleration of these natural processes, decreasing the stability of slope materials, and may have increased the chance of failure during the extreme environmental event.

Map of La Pintada landslide deposit (brown) and assumed source area (grey) derived from the above Twitter photograph

Although there is definitely room to streamline the process of mapping from media photographs, combining Google Earth with social media in this case provides important quantitative insights into the landslide occurrence in a relatively short period of time (~1 hr). Given the limited resources available in many remote or undeveloped regions, these technical aspects are often not addressed, and even more rarely published. Expanding on methods such as those presented here could therefore be a useful means for both local authorities, and the wider research community to investigate and document natural disasters in the future.