The 2015 M 7.8 Nepal earthquake

Amaya Fuenzalida, Stephen Hicks Tom Garth, Lidong Bie

When continents collide: seismic hazard along the Himalayas

Largest earthquake in Nepal for over 80 years; a region of high seismic hazard Strongest shaking in Kathmandu region – 75 km east of epicentre Current estimates suggest around 8,000 fatalities due to shaking and devastating secondary effects (landslides & avalanches) Earthquake caused by the ongoing continental collision between the Indian & Eurasian tectonic plates Unprecedented view of Himalayan earthquake, but remaining questions on regional seismic hazard

Summary

“ Since 1900, 35 earthquakes have killed at least 10,000 people. 26 of these quakes were along the Alpine-Himalayan mountain belt ”

What happened on Saturday 25th April? First alerts Detected by global networks of

seismometers and through social media reports (e.g. Twitter)

~20 minutes after the earthquake:

•  Earthquake occurred at 06:11:26 UTC (11:56:26 in Nepal)

•  Epicentre located 80 km west of Kathmandu

•  Occurred 15 km below the surface

•  Magnitude 7.8

http://earthquake.usgs.gov

Detecting the earthquake

Recordings from the UK

Plot similar seismograms yourself at: http://ds.iris.edu/wilber3/find_stations/5111620

These seismometers are located some 7000 km (4500 miles) from the earthquake’s epicentre.

These recordings allow us to make the first alerts of the earthquake.

Detecting the earthquake Recordings from the US

www.iris.edu

Comparison with other recent earthquakes

High shaking intensity

Modified Mercalli Intensity Scale

I. Instrumental

Not felt by many people unless in favourable conditions.

II. Weak Felt only by a few people at best, especially on the upper floors of buildings. Delicately suspended objects may swing.

III. Slight Felt quite noticeably by people indoors, especially on the upper floors of buildings. Many to do not recognise it as an earthquake. Standing motor cars may rock slightly. Vibration similar to the passing of a truck. Duration estimated.

IV. Moderate Felt indoors by many people, outdoors by a few people during the day. At night, some awakened.

V. Rather Strong

Felt outside by most, may not be felt by some people in non-favourable conditions. Dishes and windows may break and large bells will ring. Vibrations like train passing close to house.

VI. Strong Felt by all; many frightened and run outdoors, walk unsteadily. Windows, dishes, glassware broken; books fall off shelves; some heavy furniture moved or overturned; a few instances of fallen plaster. Damage slight.

VII. Very Strong

Difficult to stand; furniture broken; damage negligible in building of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken. Noticed by people driving motor cars.

VIII. Destructive

Damage slight in specially designed structures; considerable in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture moved.

IX. Violent General panic; damage considerable in poorly designed structures, well designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.

X. Intense Some well build wooden structures destroyed; most masonry and frame structures destroyed with foundation. Rails bent.

XI. Extreme Few, if any masonry structures remain standing. Bridges destroyed. Rails bent greatly.

XII. Cataclysmic

Total destruction – everything is destroyed. Lines of sight and level distorted. Objects thrown into the air. The ground moves in waves or ripples. Large amounts of rock move position. Landscape altered, or leveled by several meters. In some cases, even the routes of rivers are changed.

Predicted ground shaking intensity (image from USGS)

Shaking intensity depends on many factors (depth, magnitude, local geology) Very strong (VII) to violent (IX) shaking was likely felt within ~125 km of the earthquake’s epicentre Nearly 10,000 people were estimated to have been exposed to Destructive (VIII) or Violent (IX) shaking.

Why was the damage so great?

Kathmandu valley is

built on ancient lake (amplifies

ground shaking)

Kathmandu

Steep snow-covered mountains: avalanches

Lack of earthquake-

resistant buildings and

poor education

Villages on steep Himalayan foothills: landslides

Shallow depth and large magnitude of earthquake

Secondary effects: Landslides

Langtang

As with any significant earthquake, aftershocks are expected. Nearly 70 aftershocks have been recorded by the USGS.

The largest recorded aftershocks have been M6.6 and M6.7 events, 40 minutes and 1 day after the mainshock, respectively.

Most aftershocks are located to the southeast of the mainshock hypocentre

Can we expect large (M6.5–7.5) aftershocks in the near future?

M6.6 M6.7

Aftershock hazard

Himalayan collision Collision between of India & Eurasia continents since 40 million years Indian moving rapidly northward at 46 mm/yr (fingernail growth) Two plates can become locked together for 10s – 100s years Stress released during large earthquakes

Collision also causes damaging earthquakes in Tibet / China

Past great earthquakes

http://web.gps.caltech.edu/~avouac/

India-Eurasia collision causes large thrusting earthquakes across the Himalayas

Last large Nepal earthquake in 1934 (M 8)

Past history of earthquakes poorly known

More frequent large ruptures than previously thought?Seismic gaps remain along the Himalayas?

Past great earthquakes

http://web.gps.caltech.edu/~avouac/

2015

India-Eurasia collision causes large thrusting earthquakes across the Himalayas

Last large Nepal earthquake in 1934 (M 8)

Past history of earthquakes poorly known

More frequent large ruptures than previously thought?Seismic gaps remain along the Himalayas?

In which direction did the fault rupture?

www.iris.edu

Slip distribution shows that the maximum slip occurred to the south-east of the epicentre.

Kathmandu is located in the zone of maximum slip

Was all the built-up tectonic stress in this region released by the Nepal earthquake?

How much did the fault move during the earthquake?

The deformation is obtained combining INSAR images from before and after the earthquake

This images confirm the rupture area obtained trough the seismograms by an independent method.

Dense datasets needed for hazard assessment

Summary

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1 Very high earthquake hazard along the Himalayas

Need to communicate seismic hazard and its mitigation with local communities and policy makers http://ewf.nerc.ac.uk

Unprecedented observation of large Himalayan earthquake

Dense ground and satellite observations needed to assess future hazard

Supercomputer models developed to assess shaking and secondary hazards for different earthquake scenarios

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