By Saugat Kandel
April 25: The Gorkha Earthquake
On April 25, a magnitude 7.8 earthquake hit Nepal with Gorkha as the epicenter - killing more than 8,000 people, injuring more than 19,000 people, destroying nearly 300,000 houses, and displacing about 2 million people in its wake. This earthquake literally changed the shape of the earth, raising the landmass around Kathmandu by up to 1.4 meters, while lowering the landmass around the mountainous region. The sudden, violent movements in the earth's crust caused major landslides in parts of Gorkha, Dhading, Nuwakot, Rasuwa, Sindhupalchok, Dolakha, Ramechhap, and Kavre. These movements also significantly loosened up the soil in the affected areas, and consequently escalated the risk of landslides with the imminent monsoon season.
May 12: The Kodari Earthquake - an aftershock?
The April 25 earthquake was followed by a subsequent earthquake on May 12 of magnitude 7.3, with its epicenter 18 km SE of Kodari. While the Kodari quake was clearly a consequence of the first, most likely caused by the redistribution of the stress on the tectonic plates, it was not an aftershock - the epicenter of this quake was 90 miles East of the epicenter of the Gorkha quake, just outside the ruptures caused on April 25. This quake caused its own ruptures, distinctly separate from the ruptures due to the Gorkha quake.
What are aftershocks anyway?
Aftershocks are earthquakes that follow a major earthquake, in the same ruptures in the tectonic plate as caused by the first earthquake. Aftershocks are normal occurrences after major quakes, and are expected to decrease in frequency and magnitude in the days and weeks after the major quake. Even if the earthquake near Kodari is considered an aftershock, it was clearly an outlier in this regard. The probability of this event occurring was less than 1 in 200.
Will there be more aftershocks?
While it is not yet possible to predict the exact timing or location of an earthquake, it is, however, possible to estimate the probable number and magnitude of aftershocks in the time period following an earthquake. The U.S. Geological Survey has provided such an estimate for the week subsequent to May 12. For the week of May 13 to May 19, the USGS estimates:
Mitigating the risks posed by the aftershocks
To mitigate the risks posed by the aftershocks, the One Stop Portal team recommends that everyone in the affected area check their places of work or residence for structural damage before choosing to return there. We also recommend that everyone have an emergency kit ready both at home and at work, just in case an earthquake strikes again.
What about landslides?
In a mountainous country like Nepal, any earthquake can significantly loosen the soil and weaken the slopes of the hills and the mountains, thereby greatly increasing the risk of landslides. The earthquake near Gorkha caused extensive shaking in the central hilly and mountainous regions, weakening their slopes. The May 12 earthquakes in Kodari (magnitude 7.3) and Ramechhap (magnitude 6.3) occurred directly beneath the area most affected by the landslides due to the earlier Gorkha earthquake, possibly triggering multiple immediate landslides. These landslides are expected to have been comprised of rockfalls and debris avalanches, causing significant destruction of lives and properties.
Landslides in the shorter term
With the continuing aftershocks, an important cause for concern is the stability of the Sunkoshi dam, which may have either been weakened or rendered susceptible to rupture by the landslides from the weakened slopes above the dam. A collapse of this dam could flood the downstream region resulting in extensive loss of lives, property and flora & fauna.
Another cause for concern is the stability of the glaciers and glacial lakes in the mountainous region affected by the quakes near Gorkha and Kodari . The collection of debris from landslides can either cause avalanches from the glaciers, or flooding if it breaks the ice walls of the glacial lakes. Either of these events could cause enormous damage and loss of lives in the downstream region.
Landslides in the longer term
The areas affected by the earthquakes consist of areas with high rates of seasonal rainfall-triggered landslides. The slopes in these areas have been significantly weakened by the major earthquakes, with the ongoing aftershocks only compounding the damage. It is logical to expect that the risk of landslides in these areas will escalate further once the monsoon rains begin. This could lead to catastrophic damage - loss of lives, loss of property, and also the blockage of all of Nepal's major trade routes to China - in the longer term.
Mitigating measures:
Given that landslides pose such significant risks, it is imperative that Nepal follow these mitigating measures:
April 25: The Gorkha Earthquake
On April 25, a magnitude 7.8 earthquake hit Nepal with Gorkha as the epicenter - killing more than 8,000 people, injuring more than 19,000 people, destroying nearly 300,000 houses, and displacing about 2 million people in its wake. This earthquake literally changed the shape of the earth, raising the landmass around Kathmandu by up to 1.4 meters, while lowering the landmass around the mountainous region. The sudden, violent movements in the earth's crust caused major landslides in parts of Gorkha, Dhading, Nuwakot, Rasuwa, Sindhupalchok, Dolakha, Ramechhap, and Kavre. These movements also significantly loosened up the soil in the affected areas, and consequently escalated the risk of landslides with the imminent monsoon season.
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| Ground displacement after the April 25 earthquake. Source: NASA Earth Observatory |
May 12: The Kodari Earthquake - an aftershock?
The April 25 earthquake was followed by a subsequent earthquake on May 12 of magnitude 7.3, with its epicenter 18 km SE of Kodari. While the Kodari quake was clearly a consequence of the first, most likely caused by the redistribution of the stress on the tectonic plates, it was not an aftershock - the epicenter of this quake was 90 miles East of the epicenter of the Gorkha quake, just outside the ruptures caused on April 25. This quake caused its own ruptures, distinctly separate from the ruptures due to the Gorkha quake.
What are aftershocks anyway?
Aftershocks are earthquakes that follow a major earthquake, in the same ruptures in the tectonic plate as caused by the first earthquake. Aftershocks are normal occurrences after major quakes, and are expected to decrease in frequency and magnitude in the days and weeks after the major quake. Even if the earthquake near Kodari is considered an aftershock, it was clearly an outlier in this regard. The probability of this event occurring was less than 1 in 200.
Will there be more aftershocks?
While it is not yet possible to predict the exact timing or location of an earthquake, it is, however, possible to estimate the probable number and magnitude of aftershocks in the time period following an earthquake. The U.S. Geological Survey has provided such an estimate for the week subsequent to May 12. For the week of May 13 to May 19, the USGS estimates:
- The chances of a 5-6 magnitude earthquake is 80%. 0 to 4 such earthquakes may occur.
- The chances of a 6-7 magnitude earthquake is 15%, or 1 in 6. While there has already been a 6.3 magnitude earthquake, it does not mean that another such earthquake will not occur.
- The chances of a 7-7.8 earthquake is about 1%, or 1 in 100. Again, this low probability does not exclude the possibility of such an earthquake.
- The chances of an earthquake larger than 7.8 is less than 1 in 300.
- While the larger earthquakes are less probable, it is essential that any decisions taken keep in mind that the larger earthquakes are still possible, and can cause catastrophic damage if not prepared for.
Mitigating the risks posed by the aftershocks
To mitigate the risks posed by the aftershocks, the One Stop Portal team recommends that everyone in the affected area check their places of work or residence for structural damage before choosing to return there. We also recommend that everyone have an emergency kit ready both at home and at work, just in case an earthquake strikes again.
What about landslides?
In a mountainous country like Nepal, any earthquake can significantly loosen the soil and weaken the slopes of the hills and the mountains, thereby greatly increasing the risk of landslides. The earthquake near Gorkha caused extensive shaking in the central hilly and mountainous regions, weakening their slopes. The May 12 earthquakes in Kodari (magnitude 7.3) and Ramechhap (magnitude 6.3) occurred directly beneath the area most affected by the landslides due to the earlier Gorkha earthquake, possibly triggering multiple immediate landslides. These landslides are expected to have been comprised of rockfalls and debris avalanches, causing significant destruction of lives and properties.
 |
| Landslide epicenters due to the Gorkha earthquake and its aftershocks (Until May 11). Source: Earthquakes without Frontiers |
Landslides in the shorter term
With the continuing aftershocks, an important cause for concern is the stability of the Sunkoshi dam, which may have either been weakened or rendered susceptible to rupture by the landslides from the weakened slopes above the dam. A collapse of this dam could flood the downstream region resulting in extensive loss of lives, property and flora & fauna.
Another cause for concern is the stability of the glaciers and glacial lakes in the mountainous region affected by the quakes near Gorkha and Kodari . The collection of debris from landslides can either cause avalanches from the glaciers, or flooding if it breaks the ice walls of the glacial lakes. Either of these events could cause enormous damage and loss of lives in the downstream region.
Landslides in the longer term
The areas affected by the earthquakes consist of areas with high rates of seasonal rainfall-triggered landslides. The slopes in these areas have been significantly weakened by the major earthquakes, with the ongoing aftershocks only compounding the damage. It is logical to expect that the risk of landslides in these areas will escalate further once the monsoon rains begin. This could lead to catastrophic damage - loss of lives, loss of property, and also the blockage of all of Nepal's major trade routes to China - in the longer term.
Mitigating measures:
Given that landslides pose such significant risks, it is imperative that Nepal follow these mitigating measures:
- Immediate inspection of the Sunkoshi dam and the surrounding hillsides for structural integrity. Monitoring of the lake waters for seven to ten days to ensure there are no leakages or further damages from subsequent aftershocks. If any instability is found, the nearby Araniko highway should be closed while repairs are conducted.
- Immediate inspection of the glaciers and glacial lakes. Evacuations of the downstream populace, if necessary.
- Survey of the major transportation corridors in the affected areas to identify where problems are likely to arise, to prepare alternate routes, and "to prepare for the possibility of both immediate action to clear rockfall debris and continued clearance and maintenance of major roads after future landsliding".
- Collection of more data, on the ground or satellite, to identify where any landslides have occurred so far, and where they are likely to occur in the future. The affected populace should be warned and prepared in accordance with the data collected.
Sources:
- USGS Earthquake Hazards Program
- USGS Updated Aftershock Advisory
- "Sensing How Much the Earth Moved in Nepal", Earth Observatory, NASA
- "Segment By Segment", Seismo Blog, UC Berekely Seismological Laboratory
- "Nepal Earthquake: update on Landslide Hazard (8 May)", Earthquakes Without Frontiers
- "Nepal: update on landslide hazard following 12 May 2015 earthquake", Earthquakes Without Frontiers
- "Quick Structural Assessment of Buildings", One Stop Portal
- "Emergency Kits - What Every Home Needs to Have", One Stop Portal
- "Perspectives on the Nepal Earthquake", University of Cambridge
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