May 29, 2015

Views of strong shaking in Nepal and what they teach us

Posted by Austin Elliott

When the ground shook throughout Nepal in April, it was neither predicted nor surprising–the paradox of inevitable but chaotic large earthquakes within well known seismic zones. Though warnings of Nepal’s catasrophic earthquake risk have been sounded for years, and though this magnitude 7.8 and its energetic 7.3 aftershock wrought plenty of death, destruction, and tragedy, scientists are finding themselves somewhat surprised at the apparently rather limited degree of overall damage in Kathmandu and elsewhere. Videos that captured the shaking offer a glimpse of the violent terror, but also reveal why the quake may have left many smaller structures standing.

People struggle for balance as the ground flails beneath them. https://youtu.be/AJc5J2iDcRM

People struggle for balance as the ground flails beneath them. https://youtu.be/AJc5J2iDcRM

Violent Shaking

The first footage captured during the earthquake itself emerged from a home security camera system. The following two vantage points around the house show violent shaking (strong enough to rock the concrete retaining structure and vigorously jostle plants and trees) that lasts for at least 50 seconds, a terrifying duration of such strong ground motion. In the second video you see that the jostling is quick enough to yank leaves off trees, and hurl potted plants from their perches above.


Security cameras around a narrow shopping street also capture the violence and extreme duration of the shaking, and the terror it inflicts, from three different perspectives:



Traffic cameras also captured the terrifying violence of the shaking, including some unsurprising topples of precarious and unreinforced monuments. In the first one, major swaying of the trees is testament to the force of the ground motion, and the collapse of a massive decorative arch catches passing drivers unaware.

In the second of the three traffic camera videos here we see the collapse of a small monument clearly erected with no regard for seismic hazard. Furthermore overhead cables snap and land dangerously in the intersection. Clearly it is hard to stand/walk/move. In the second clip, existing cracks in the sidewalk become grinding and gaping weaknesses that allow (or result from?) differential motion of roadway fill and the adjacent building foundations:

These last few traffic cam videos hint at one of the surprising and unique aspects of this earthquake:

Strong long-period ground motion

All earthquakes release rattling, jarring high-frequency energy as rock grinds against rock along a rough, lumpy, chattering fault surface. Larger earthquakes also release longer-wavelength, lower-frequency seismic waves because larger volumes of rock are moving coherently. In general, different things on the surface respond to this spectrum of frequencies differently. The bottles, glasses, and plates rattling in your cupboard are responding to high-frequency seismic waves, whereas the swaying office building is responding to low-frequency waves. That long-period swaying is generally only observed having a strong effect in the upper floors of large buildings, or on slowly sloshing swimming pools.

In this particular earthquake, however, unlike any others I’ve seen footage of, STRONG long-period motion is observed at ground level. You can see this effect in the videos above, but the following two really illustrate the power of the phenomenon. In the first, the gathering crowd in a market square stumbles collectively to the left as the ground pulls rightward beneath them. Unlike being tossed or thrown by strong, quick accelerations as we’ve seen in other major earthquakes, these people are losing their footing and leaning as the ground pulls away from them in one direction for a good 4 seconds. (Note also the strong aftershock at about the 4:00 mark that makes the crowd retreat away from the dangerous buildings.)

This next clip shows the phenomenon brilliantly, and is furthermore practically a Where’s-Waldo of earthquake reactions. There’s just so much going on in every corner of the frame, from people in cars to on cars to on bikes, to on foot, carrying children, leaving buildings… You see the effects of shaking on cars, structures, bikes, fences, trees, utility poles, sidewalks… It’s really an amazing earthquake video. As the strong shaking really sets in you see these same strong, long pulses of acceleration in one direction. In this case they make all of the motorcyclists veer to one side of the road and then back simultaneously, as the roadway is pulled to the side beneath them. Really… look all the way down the road and you’ll see the same veer from ALL of them.

What you’re seeing here is the passage, deep underground, of the main slip pulse of the earthquake. You’re witnessing Kathmandu Valley itself lurching 1.5 meters southward as the fault rupture rips by ~10 km below. Seismograms from instrument KATNP within the city show the effect graphically, with large southward displacement accumulating during one long (~5 second) pulse of positive southward acceleration.

Strong motion record from KATNP, processing and figure by PEER Berkeley (http://peer.berkeley.edu/news/2015/05/gorkha-nepal-earthquake-peer-strong-motion-records-server-room-video/)

Strong motion record from KATNP, processing and figure by PEER Berkeley. The upper plot shows acceleration, middle shows velocity, and bottom shows displacement of the ground, revealing motion due to elastic seismic waves, but also the finite net southward shift of 1.5 meters. (http://peer.berkeley.edu/news/2015/05/gorkha-nepal-earthquake-peer-strong-motion-records-server-room-video/)

The static plot of North-South motion above was combined with orthogonal components and animated by Anthony Lomax of ALomax Scientific to produce the following video of the seismometer’s (and everything around it’s) motion during the earthquake.

The Pacific Earthquake Engineering Research Center has overlaid that seismogram on a video from a building 2000 ft. away to illustrate the correspondence of instrumental waveforms with the observed movement. The video, however, is from an upper floor in which we may expect this sort of response. I remain profoundly impressed by the phenomenon as recorded at street level out in the open.
http://peer.berkeley.edu/news/2015/05/gorkha-nepal-earthquake-peer-strong-motion-records-server-room-video/

I’ve never seen this phenomenon–at least to this degree–in decades of earthquake footage. It may be a result of the particular setting of Kathmandu, directly above a wide, flat rupture plane. Most such shallow faults are subduction interfaces, with their earthquakes producing most surface displacement well offshore. As seismologist Susan Hough describes in a couple of eloquent, insightful radio interviews on the BBC’s Science in Action and NPR/WNYC’s On the Media, why this happened and what it means for other large earthquakes in the Himalayas will be one of the major salient lessons from this earthquake.

Collapsing Structures

Despite the huge release of low-frequency energy, quite a few structures were shaken to the point of collapse. Lots of chaotic, noisy videos filmed in the touristy parts of town capture this drama:




A frightening CCTV video from Basantapur Durbar Square shows the threatening wobbling and eventual collapse of a large civic building.

This intense video captures the shaking toppling structure after structure, including a horrific mid-rise apartment block collapse in the distance.

Modern, seismically engineered buildings fared well, as this swaying apartment complex:

Meanwhile, in more rural locales people were able to rush out into the open, as these western trekkers whose jam session was interrupted by the horrendous quake, and whose shocked profanity is excusable as you realize along with them what a tremendous roar the earth is making around them:

Coseismic landslides

Elsewhere in the mountains, the earthquake unleashed torrents of boulders as the high peaks literally crumbled. The scene in rural, mountainous Nepal was one of surprising terror as the old adage about being safer out of doors was turned on its head beneath the steep walls of Himalayan valleys.

One trekker captured the cacophony of stone hostels crumbling and the panicked chaos of aftermath before shutting off the camera as he realized the mountainsides were coming down:

Other trekkers’ video picks up where his left off, as the violent shaking of the quake gives way to rolling, tumbling projectiles:

Out on the trail, trekkers up and down the valleys suddenly realized the implications of their talus-slope paths, as this group, who was well within the rockfall deposition zone:

Valley walls throughout the country suddenly turned into cascades of loosened rocks. There’s not much to do in this situation except pray for luck. This guy appreciates it as the “best selfie ever:

This hiker captured the earthquake out in the forest, in what comes across initially as a rather serene environment for it, but bad news arrives fast, and he documents it artfully.

These videos represent the illuminating highlights I’ve found. There are plenty more from shops, businesses, and factories around Kathmandu, which you can find in my hopefully rather comprehensive YouTube playlist. There is also a whole other suite of videos of the second quake on May 12th. If you know of any other important, illuminating videos, please do share them.

Other noteworthy CCTV videos

Earthquake in a bucket factory – ranks among goofiest places to be in a quake:

Earthquake in a glass shop – ranks as among the worst places to be in a quake. Given the strong southward accelerations from this Himalayan thrust event, perhaps this offers a storage strategy going forward: (for shaking start at 3:15)

Another one from which there are lessons to be learned: furniture straps and wheels. (For shaking start at 12:45.)