Tag Archives: landslides

Landslide Risk fr/PNG Massive Earthquake

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Massive but deep earthquake below Eastern Papua New Guinea – main concern : landslides

Last update: December 14, 2011 at 10:01 am by By 

Earthquake overview :A strong but very deep earthquake happened at 03:04 PM (15:04) below eastern Papua New Guinea. The depth of the earthquake will have weakened the shaking even just above the epicenter. Our main concern are possible landslides.

Update 14/12 – 09:55 UTC :   TJ Charlton, a person knowing the epicenter area very well, wrote a few minutes ago : Lots of legal and illegal gold mining in the Wau area. I am fearful of avalanches (ER : so are we) and some mine shafts collapsing…I was in Wau just a few months ago. Talk in Wau town (after a phone call) was that “trees were swaying from side to side” during the quake.  (ER : the good news is that the phone was still working)

Update : From James Daniell CATDAT database :
On 26th, February 1963, a M7.3 earthquake, 171km depth struck at -7.5, 146.2 (around 50km west of this epicenter), with only very minor damage reported and intensities of up to 7.
Other earthquakes to have hit the region have occurred in 2001 and 2010, although these are around 80km away, both being M6.2 and only causing minor damage. These were also shallower.
Generally PNG builds of materials that resist earthquakes well and it is unlikely that major damage occurred from this one.

Landscape in the greater area of the epicenter – picture courtesy Grant I. Tebbutt

Update : Mc Adam National park was at a relatively close distance from the epicenter (approx. 40 to 60 km).

Update : The closest settlements are Were Were, Winima, Kaisenik, Kwembu, Kudjeru and Wau

Update : The greater area of the epicenter has almost NO roads but a lot of small settlements . Based on our own experience with PNG earthquakes we do not expect to receive a lot of additional information from the inland settlements. They have NO phone lines (neither fixed or cell) and can mostly be reached by trails or dirt roads only.

Update : A 7.3 earthquake who occurred in 1987 at a distance of 193 km from the current epicenter had NO shaking deaths (focal depth unknown)

Update : USGS is expecting that 506,000 people will have experienced a VI (strong) MMI shaking, 973,000 people a moderate V MMI shaking

Update : WAPMERR, a specialized theoretical damage calculator expects NO fatalities and a max. of 30 injured people as a result of this earthquake.

Update : GDACS, who has great demographic number has calculated that 1,376 people are living within a radius of 10 km. 8,212 within a radius of 20 km and 314,609 within a radius of 100 km from the epicenter.

Update : The exact place of the epicenter is of lesser importance for these deep earthquakes. Even living above the epicenter one can compare it (simplified) with living at 121 km from the epicenter of the earthquake (depth was 121 km)

Update : This was the official NOAA message confirming our own estimate :
BASED ON ALL AVAILABLE DATA A DESTRUCTIVE PACIFIC-WIDE TSUNAMI IS NOT EXPECTED AND THERE IS NO TSUNAMI THREAT TO HAWAII.

for more information and updates, go to:    http://earthquake-report.com/2011/12/14/massive-but-deep-earthquake-below-eastern-papua-new-guinea/

Landslides fr/September Earthquake — Sikkim/Darjeeling

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Earthquake induced landslides in the Sikkim-Darjeeling Himalayas

Last update: November 25, 2011 at 7:37 pm by By 

 

Aftermath of the 18th September 2011 Sikkim earthquake

By : Indranil Chakraborty, Dr. Saibal Ghosh, Debasish Bhattacharya & Anjan Bora, Engineering Geology Division, Geological Survey of India, Eastern Region, Kolkata

Introduction

According to the Indian Meteorological Department (IMD) 1, a strong earthquake of Magnitude M: 6.8 shook Sikkim and Darjeeling areas of India and adjoining Nepal at 18.11 hours IST on 18th September 2011 with its epicenter (27.7oN, 88.2oE) lying near the Sikkim-Nepal border, about 68 km northwest of Gangtok, Sikkim with the depth of its hypocenter at about 10 km. As per the USGS estimates 2, the epicenter of this strong earthquake (6.9 Mw) lies slightly west of the location shown by IMD. This earthquake was widely felt in Sikkim, West Bengal to Rajasthan in the west and the northeastern states of India, and also in Nepal, Bhutan and Bangladesh. This major earthquake triggered several landslides in the region and a large number of which were studied during a recent post-earthquake fieldtrip (22-29 September 2011) to the Darjeeling-Sikkim Himalayas. The fieldwork was planned within four days’ of the event to the worst-affected areas in Sikkim-Darjeeling Himalayas that are accessible by roads. The aim of this study was to rapidly assess the damage caused by these earthquake induced landslides along with collection of the relevant landslide-related attributes for inventory and further study.
It is also pertinent to mention here that moderate to heavy monsoon rainfall prior to the occurrence of 18th September 2011 earthquake, during and post earthquake event is reported from some areas, which perhaps also contributed to some extent in lowering the shearing strengths of already-loosened (due to earthquake shaking) slope forming mass and ultimately triggered some landslides at selected locations just immediately after the earthquake shock. Due to this reason, it was considered wise to name these landslides as earthquake induced landslides instead of the co-seismic landslides in truest sense. Although, present field observation took due care not to mix up with landslides that are triggered by rainfall prior to 18th September earthquake.

Study area & Seismicity

The study area falls within the Lesser and Higher Himalayan regions with an elevation ranging from 65 m (around Dudhia, Darjeeling District) through 800-1200 m around Ranipul, Mangan, 1400 m around Darjeeling, 1800-2200 m around Gangtok, Pelling, Ravangla to 2800–3200 m around Kyangsla (near Nathula) with elevations increasing gradually from south to north. In the Lesser Himalayas towards south, the slope is gentler and in the undulating stretches, numerous streams and rivulets flow along the depression and finally join with the trunk streams. In the Higher Himalayan terrain, the topography is highly rugged, characterized by steep slopes with prominent gully erosion. The Tista-Rangit water divide is the main north-south water divide within the Sikkim Himalayas. Another north-south water divide is between Lachen Chu & Lachung Chu which starts at about 3000 m and slowly increases up to 6700 m. North of Dikchu, steep ridges on the eastern side are there but of smaller extent as the Tista valley is sub-divided by the water divide between Lachen Chu and Lachung Chu. On the western part long ridges are present – these are Talang Chu and Zemu Chu water divide. Geologically, the mountainous regions of the Darjeeling and Sikkim Himalayas are part of the active Himalayan Fold-Thrust Belt (FTB), which is geologically and structurally complex exposing a number of overturned (towards south) and thrusted sequence of variably metamorphosed pelitic and psammitic rocks over Mesozoic (Gondwanas) and foreland rocks composed of Tertiary (Siwaliks) sediments in the south3.
As per the Seismic Zonation Map of India4 the state of Sikkim comes under Seismic Zone IV – a zone of considerable vulnerability. The nearby region in the further north comes under Zone V and is highly susceptible to moderate to major earthquakes. In this area, most of the previous earthquakes were of shallow focus (< 40 km) and are commonly of 4.5 to 5.5 Magnitudes on the Richter scale. The regional distribution of earthquakes and lineaments/faults in Sikkim and adjoining Darjeeling areas indicates that a number of past moderate (M 5.0 to 5.9) and slight (M 3 to 5) earthquakes5 are clustered around the Tista Lineament trending NW-SE direction. Northern extension of the Tista lineament cuts across the NE-SW trending Kanchanjungha lineament. Another NW-SE trending lineament marked as a fault along the Tista River between Lachen and Chungthang (and further southeast) appears to have been the cause of some high Magnitude earthquakes in the region6.

Fig. 1: Map showing locations of 18 September 2011 Sikkim earthquake induced landslides observed along important road-corridors in Sikkim-Darjeeling Himalayas (in background multi-spectral IRS P-IV Satellite’s LISS III image of 2005 was used). The locations are plotted using Geographic Coordinates in Latitudes/Longitudes using WGS 84 global datum and UTM projection parameters.

Earthquake induced landslides

Due to the 18th September 2011 Sikkim earthquake (M: 6.8), several “new” and a few “reactivated” landslides have occurred right from the Himalayan foot-hill region (e.g., Dudhia in Kurseong Sub-division, Darjeeling District, West Bengal) up to the higher Himalayan range in the higher reaches of Sikkim-Darjeeling Himalayas. The new landslides that occurred in the lower elevations are mostly concentrated within the terrace deposits of trunk streams such as Tista, Rangit and Balason etc. and within the old colluvial deposits on the lower reaches of slope adjacent to trunk streams (e.g., Jorethang-Rishi-Legship section along the right bank of Rangit river). In the slopes having steep relief, the frequencies of rock/debris fall are more than to slides, whereas field observation clearly indicated that proximal to epicentral region, frequencies of both rock fall and rock slides are much higher than the debris/soil slides and the same are more concentrated towards the crest of steep ridges. Recent field observation of the landslide-related damages further indicated that lithology and geomorphology also played a significant role in causing these earthquake induced landslides. The debris-laden slope which is made of loose unconsolidated material and the slope covered by thin unconsolidated scree deposits have been more prone to failure by this earthquake. Frequency of rock fall and rockslides are more in areas which are generally vulnerable due to steep slope, weathered and fractured lithology and unfavourably jointed and kinematically-unstable slopes.

for more, go to:    http://earthquake-report.com/2011/11/25/earthquake-induced-landslides-in-the-sikkim-darjeeling-himalayas/