A major shallow earthquake hit near Kathmandu in Nepal just before noon on Saturday local time. Between high population densities, intense prolonged shaking, unstable slopes, and inadequate buildings, this has the makings of a very nasty disaster.
Top image: A magnitude 7.8 earthquake causes severe damage in the Kathmandu Valley of Nepal on April 25, 2015. Credit: AP Photo/ Niranjan Shrestha
The earthquake was felt throughout Nepal, with impacts crossing political borders into northern India and southern Tibet. Image credit: United Nations
Impact of the Earthquake
A magnitude 7.8 earthquake is devastating enough, but with the hypocenter a mere 15 kilometers (9.3 miles) below the surface, that shaking is brutally intense in a localized area. Along with damage from intense, local shaking, this earthquake has likely triggered countless landslides and destabilized even more slopes, increasing the risk of more landslides during the upcoming monsoon season.
Shakemap of the M7.8 earthquake at 11:56am local time on Saturday, April 25, 2015 with the epicenter marked by a star, the rupture zone outlined by the rectangle, and felt surface shaking intensity indicated by the colour code. Image credit: USGS
The surface expression of the earthquake’s hypocenter, the epicenter, hit within just 29 kilometers (18 miles) of the Lamjung, a small city with a population of under 200,000 people. As we learn more about the disaster, it appears that the fault rupture zone extended over 100 kilometers to the southeast, encompassing the high-population capital city of Kathmandu 81 kilometers (50 miles) away. The US Geological Survey shakemap indicates even more intense shaking near the capital than at the epicenter as local geology and soils prone to liquefaction amplified shaking. The Crisis Response Center at Google has launched a Person Finder for the crisis to help share information about the estimated 33 million people within the impact area.
While reports on the ground are understandably scant as people are busy dealing with the destruction, damage, and aftershocks, although it’s clear that Kathmandu suffered extensive damage. By 8pm, the casualty estimates were between 700 and 1,000 people killed, and that number is likely to keep climbing. As always in the wake of a major disaster, the areas where no news is coming out is even more troubling as it suggests that even basic communications infrastructure is severely damaged.
If you felt the earthquake, please fill out a Did You Feel It? report to help researchers better understand the impact of local conditions like surficial geology or specific styles of building on how a major earthquake in this region is experienced on the ground.
While aftershocks are still shaking the region, neighbours are already clustering in an attempt to rescue each other. Community cohesiveness is a primary component of resilience after a disaster: most rescues happen by neighbours within the first 24 hours before specialized teams get mobilized to the area. Something as simple as having frequent house parties so nearby residents know where is the most likely location in a pile of rubble to search for trapped victims can have a dramatic impact on survival rate. Unfortunately, even a smaller aftershock can be enough to knock over a previously-damaged building, putting rescuers at-risk.
Rescuers attempt to locate trapped victims under rubble of collapsed buildings. Image credit: AP/ Niranjan Shrestha
Along with the direct damage from shaking, the earthquake has triggered many landslides and avalanches in the region’s over-steepened slopes. An avalanche hit Khumbu Icefall on Everest, trapping climbers in camps 1 and 2 by destroying the routes out, and running out into basecamp directly where it killed at least ten people and injured uncounted more. (Note: This video circulating is from an event in 2009, not today.)
An earthquake-triggered avalanche hit Everest. Image credit: AP/Azim Afif
In the first official situation report from the United Nations on the state of the crisis, the area hospitals were already overcrowded, with supplies running low. Building damage is severe, and understandably the majority of the population is staying outside for fear of delayed building collapse or new building damage from aftershocks.
Earthquake victims receiving treatment outside of Medicare Hospital in Kathmandu, Nepal. Image credit: AP Photo/ Niranjan Shrestha
International aid requests were made overnight for help with search and rescue, additional medical supplies (including body bags), helicopters for interim transportation, and heavy construction equipment to start clearing the colossal amounts of debris.
An aircraft loaded with relief material leaves New Dehli, India. Image credit: AP/Rajesh Kumar
Several aid organizations including the Red Cross are requesting donations to support immediate relief efforts. Longer term, Geohazards International accepts donations to help mitigate disasters before they happen, and have both previous and ongoing projects in the impacted region. If you can’t donate money but wish to contribute your time, try the Humanitarian OpenStreetMap Team’s mapping project.
Geoscience of the 25 April 2015 Nepal Earthquake
Map of the main earthquake and the first 26 aftershocks over M2.5; larger circles indicate larger magnitude earthquakes. Check for live updates here. Image credit: USGS
Nepal is located on the convergent zone in the Himalayas, where the Indian and Eurasian tectonic plates are colliding at just under a half-centimeter per year. When a convergent zone like this one happens with an oceanic tectonic plate (as in Japan, Chile, and the Pacific Northwest), the denser plate subducts, feeding a volcanic system as it melts below the Earth’s crust and building up stress to produce massive earthquakes when it catastrophically slips. Here, with two continental plates, neither subducts and instead they crumple and shorten, creating an area of uplift and mountain-building, and earthquakes that are just as deadly as their watery counterparts. Convergent zones like this are capable of producing the largest earthquakes in the world as sticks and slips of moving tectonic plates build stress to a critical threshold. Even worse, the faults are long and shallow, dipping under almost the entire country.
Potential earthquakes in the region based on stress-buildup, where yellow indicates how much elastic energy could be released. Image credit: Bilham et al 2001
As the faults shift, they release stress, but build up even more along the locked ends. From research on historical earthquakes in the region, it looks like this newest quake might have filled in a seismic gap, a location previously silent of earthquakes where stress has been building up for decades.
This was a reverse thrust earthquake, exactly the type of focal mechanism we expect to see in a compressive zone like this. It was shallow and dipped slightly downward, following the plate junction, and propagated towards Kathmandu as the fault ruptured.
Initial backprojection of the mechanics of how the earthquake rupture propagated. Video credit: Alex Hutko/IRIS
In general shallower earthquakes are more damaging as the shaking is felt more intensely, although over a smaller area than deeper earthquakes. To make it even worse, Kathmandu is in one of the locations where local geology makes earthquakes substantially worse: a solid rock basin reflects seismic waves, focusing them, while the infilled loose sediment liquefies and amplifies shaking. This is evident in the shakemap where the surface intensity of the earthquake was substantially worse in Kathmandu than at the actual epicenter. Complicated relief efforts, the city is surrounded by mountains with only two main roads that are easily blocked by landslides.
The region has already felt dozens of aftershocks, and more are coming. When the fault slipped, releasing the magnitude 7.8 earthquake, it redistributed stress in the region, triggering more, smaller earthquakes. In general, both the number and magnitude of aftershocks drops off exponentially over time: seismologists expect around 35 M5+ earthquakes in the region over the next few weeks. The largest aftershock so far was a magnitude 6.6, striking just a half-hour after the main quake with an epicenter only a few kilometers to the northeast.
Considering that every aftershock brings the risk of collapsing already destabilized buildings, camping out in the street is a popular option as night falls again in Nepal.
About the only lucky breaks with this earthquake are that it’s so far inland, it couldn’t possibly trigger a tsunami, and that it wasn’t an even-more-massive magnitude M9.0 earthquake, which the region is tectonically capable of producing. A magnitude 7.8 was certainly bad enough: the initial analysis is that the fault rupture lasted 80 to 90 seconds, with intense shaking lasting even longer. You can watch the 3D ground motion close to the epicenter here.
Security camera footage of intense shaking in the Kathmandu Valley. Video credit: War News
Nepal is a land of high topographical relief with over-steeped slopes prone to landslides. The earthquake can trigger landslides right now, but it can also further destabilize already weak slopes. With monsoon season only a few months away, it’s extremely likely that the rains will trigger countless landslides, generating even more chaos to a region that will still be recovering from this weekend’s catastrophe.
Landslide susceptibility model, where red indicates a higher likelihood of slope failure. Image credit: Tom Robson/University of Canterbury
Along with being disasters in their own right that can kill people and damage buildings, landslides can block roads, complicating transportation and logistics in the region, take down telecommunications lines, and temporarily dam rivers. Landslide dams are another delayed catastrophe as they can catastrophically burst, causing downstream flooding.
Between the intensity of the earthquakes, the uncertainty of fault locations, the weak building construction, and the high population density, Nepal has a has history of incredibly damaging earthquakes. The earthquakes happen every 70 to 80 years on average. The most recent major earthquake was in 1934 when a magnitude 8.4 earthquake killed over 4,500 people, and destroyed 70% of the entire building stock in the Kathmandu Valley. Although for now all the information on Saturday’s earthquake is preliminary, it looks like the rupture zone of this earthquake may overlap with that event.
Quite a few geophysicists and GIS professionals are spending this weekend running the earthquake through their research models. Although none of this is confirmed yet, it looks like the fault rupture went right through Kathmandu, with over 100 kilometers of fault moving 5 meters in just 90 seconds, yet without producing major visible surface ruptures.
In Kathmandu, the shaking was more intense north of the fault line, a tiny mercy as more vulnerable infrastructure is located to the south.
Local topography adds in one more confounding factor to make earthquake damage even worse: the topography is high-relief, so towns are more likely to be high-density with tall buildings, not sprawling out with shorter buildings. Tall buildings are more difficult and expensive to construct well to resist earthquake damage. Buildings of different heights also respond differently to earthquakes, swaying to different amplitudes depending on how off the earthquake is from their resonance frequency, so areas of dramatically mixed building heights run the risk of adjacent buildings colliding and causing even more damage. Check out more details on the earthquake building damage here.
Nepal’s Minister of Foreign Affairs Mahendra Bahadur Pandey addressed the problem of seismic risk in the country just last month at the United Nations Conference on Disaster Risk Reduction, explaining:
“We are seriously concerned that Nepal continues to remain one of the most vulnerable countries in the world to disasters such as earthquakes, flooding, landslides, glacial lake outburst floods. It is also estimated that the human loss in the Kathmandu Valley alone should there be a major seismic event will be catastrophic.”
While some government spokespeople are making the usual statements of how this was an unforeseen disaster, the disaster risk reduction community strenuously disagrees. Seismologist Sue Hough has compared the Did You Feel It? felt intensity reports with that projected by predictions, and this earthquake is almost exactly as bad as we expected it to be.
Even more damning are papers with titles like, “A perfect storm of earthquake and poor governance could cripple Nepal.” The policy advocacy document published in 2013 outlines the exact scenario we’re facing today with not-at-all reassuring paragraphs like:
The valley has only 12 fire engines, mostly vintage models and mostly out-of-service. The soil under most of the city is vulnerable to liquefaction – a nightmare scenario where the shockwave from the earthquake liquefies parts of the sandy, former lake-bed and parts of the city will simply drop. This will be a disaster of epic proportions. And unlike Port au Prince, landlocked Nepal is 400 miles from the nearest coast and its roads are likely to be blocked for weeks by landslides. It’s a planner’s nightmare.
The Nepal Risk Reduction Consortium responsible for addressing the risk from earthquakes and other disasters in the region does its best, but this is a huge problem with a lot of vulnerability and a lot of people at risk. While undeniably tragic, this kind of damage was anticipated and inevitable without a major influx of capital to completely revamp the country’s building inventory to meet seismic codes. As it is, major historical sites are completely devastated.
Disaster Response In The Aftermath
International aid into Nepal has been rapid, with Search and Rescue teams from India, Pakistan, China, and Israel already landed. Eight medical teams from the United States were already in-country before the disaster, while search and rescue support from Fairfax, Virginia is incoming. Additional teams from the United Kingdom, Japan, and Finland are expected to arrive on Monday.
The damage to buildings is extensive, with at least 80% of all housing in Gorkha, yet overall it’s less extensive than anticipated. Even so, many residents are still camping in the streets out of understandable caution over the structural stability of their homes in the face of ever-more aftershocks. Five shelter areas have been set up in areas belonging to the Armed Police Force, while another 16 locations around Kathmandu have been selected and are in the process of being provided with necessary infrastructure like water tanks. Concerns about sanitation and disease are already growing, with the government requesting vaccines as part of their medical supplies.
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