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Monday, January 18, 2010

Neuroanatomy of an Earthquake

Last Wednesday, Jan. 12, 2010, an earthquake of magnitude 7.0 shook the island nation of Haiti at 16:53 local time. The epicenter of the quake was located only about six miles from the nation's largest city and capital Port-au-Prince, where an estimated 2.5 million people live. The temblor collapsed buildings within seconds, burying hundreds of thousands under the rubble. The government was not able to mount any first response of note. No rescue teams showed up on the scene. No heavy equipment was rushed in to help free the trapped from under the rubble. People shoveled with bare hands. The police was not to be seen. A human catastrophe of apocalyptic proportions was going to unfold.

In spite of the ensuing international rescue effort and the medical assistance that has begun to reach the country, hundreds of thousands are believed to have died. Many more are feared to succumb to their injuries in coming days. Many destroyed dwellings seem flimsily erected with cinder blocks and mortar. Reinforced concrete structures with multiple floors were not quake-proof.

The tumbling of concrete walls and ceilings caused countless blunt-force traumas and open fractures among the survivors. People with such injuries need immediate medical attention, including surgery. Only few help organizations were prepared for such assistance in the immediate aftermath of the quake. International medical teams including surgeons from Médecins Sans Frontières and Cuba were on location and are to be commended for their decisive action in the first hours after the quake. Meanwhile, aid is pouring in. The relief effort has become more coordinated. Field hospitals are being set up. Food and water are distributed. Security forces can be seen in action.

Hopefully, the basic needs of the survivors will be met in the coming days. It is clear that the horrific consequences of this catastrophe are not entirely natural. Rather, they seem in good part man-made. That is, the Haitian government has failed colossally in its role as a protector of its citizens. Numerous recent accounts of international volunteers who have been looking after the most profound needs of the country's impoverished population bear witness to the enduring insufficiency of medical care and the endemic shortages of the most basic medicines like antibiotics (e.g. Joani in Haiti). Hospitals had been strained at best before the disaster and have now been rendered completely inadequate. The temblor has laid bare once more the societal ills of this aggrieved nation.

I am ill-prepared to discuss the economical, social, and political reasons for Haiti's failure. The Duvaliers and their somewhat more benign successors certainly did not contribute much to lead the country out of its misery. The international community and the independent organizations providing aid to Haiti may wish to re-assess the ways they can possibly help build a more efficient infrastructure and social services in this country in the future.

Watching the horror take its course, I kept wondering whether we cannot find low-cost indicators to predict an earthquake, if only immediately before the temblor strikes. Even if long-term predictions similar to those advising us of the strength and the path of a hurricane do not seem feasible for earthquakes to date, a few more seconds of warning may make a huge difference.

A day after the quake, CBS News posted a video covering a road lined with a row of condominiums on the far side at the time the event.


The 35-second long footage was captured by a rigidly-mounted camera overlooking an intersection. Twenty seconds into the footage, a series of very strong vertical displacements of the ground brings the first building down, raising a huge plume of dust. Could we have noticed early signs in the video warning us that a strong quake was about to happen?

Though the quality of the video is not high fidelity, examining the pictures closely provides a few insights. We need to focus on the roof line of the condos first and then on the utility pole standing on the near corner of the intersection.

A subtle distortion of the roof line announces the first vertical jolt 12 seconds into the footage. Subsequent, arrhythmic displacements gain strength within the next 3 seconds. At 16 seconds, we see a distinct solitary vertical jolt followed by a pause of a second. Then the ground begins to violently shake up and down at precise periodicity. Note that the utility pole sways in elliptical counter-clockwise rotation. At 20 seconds, after six strong vertical jolts in four seconds, the front condo pancakes. Nothing in the first 12 seconds of movie foretells the impending disaster.

I found information on earthquakes pertinent for the interpretation of our observations in the movie on L. Braile's web page at Purdue University's Earth & Atmospheric Sciences. Three types of seismic ground waves that propagate parallel to the earth's surface appear to come to bear in the movie.

Compressional P waves are known to travel fastest and arrive first. They are longitudinal waves, shaking the soil particles back and forth in the direction of propagation like lined-up billiard balls. P waves cause comparably small vertical ground displacement. The next to arrive are shear or S waves. This type of wave propagates transversely with soil particles jolted up and down at larger amplitude than P waves. At last, Rayleigh or R waves arrive. These waves move soil particles in the direction of and vertical to propagation, causing the ground to rotate elliptically as the utility pole in the movie convincingly demonstrates. R waves brought the condo in the movie down. In addition, a fourth type of wave, Love or L waves, has been recognized. I could not detect any manifestation of L waves in the movie.

Braile exemplifies the different wave types in figure 7 on his web page. The figure shows seismograms recorded in Peru during a magnitude-6.5 quake off the Pacific coast in 1998. The trace representing the displacement in the z-direction (vertical) at the bottom of the figure is most applicable to our observations in the movie. The difference in time between the onset of the P and the S waves multiplied by 8 provides a rough estimate of the distance between recording site and the epicenter of the quake in kilometers. In the Peruvian example the seismic recorder was more than 1000 kilometers removed from the quake's epicenter. The S waves lagged the P waves about 7 minutes. By contrast, according to the United Nations Office for the Coordination of Humanitarian Affairs the distance between the epicenter of the quake in Haiti and downtown Port-au-Prince was only 10 kilometers. This leaves just 2 seconds between the arrival of the P waves and the S waves in the movie. If the P waves were strong enough to cause the earliest vertical displacements we noted, the inhabitants had seven seconds to vacate the condo complex before it crumbled. If the P waves were too weak to cause any visible displacement and the first detectable jolts were caused by S waves, we may add another two seconds according to the rule of thumb explained above. We would have had nine seconds to leave the building, after we first felt the ground shake. This may sound short. However, let us not forget that the world's fastest runner, the Jamaican Usain Bolt, covers 109 yards in that time.  

Would we have recognized that the first mild vibrations were the harbingers of a temblor of catastrophic force arriving shortly? Animals may possess a better sense of such danger than humans. In another contribution, CBS News broadcast a segment interviewing geologists who foresaw a major quake near Haiti a couple of years ago. They just could not predict when disaster would strike. In that broadcast, a video from a different quake shows a dog in an office that unequivocally recognizes the imminent catastrophe before any human. The dog takes flight about 5 seconds before the room begins to shake violently.


Animals are known to behave conspicuously in advance of an earthquake, and researchers have tried to associate specific types of behavior with the event (Mott, 2003). To date, the findings of these studies have not yielded any consistent predictors. However, I would not be surprised, if burrowing animals, that are particularly attuned to the processing of low frequency vibrations, would sense earthquakes early, displaying distinct behaviors that could be useful as warning signs. I have written about such animals in my post entitled "The Quest for the Infrasound Fovea" dated Oct. 12, 2009. In fact, the mammal that survived in the pumice zone after the catastrophic eruption of Mt. St. Helens in 1980 was a tiny whiskered burrowing rodent (Force & Resilience).

Addenda
  • According to Madeline Brand's segment with the title "Technology Works to Provide Early Quake Warning" broadcast on National Public Radio's All Things Considered today, sophisticated technology exists that permits us to identify a dangerous earthquake early enough to provide minutes of advance warning. However, when we listen to the podcast, we must keep in mind that the warning time dwindles as we near the quake's center. Even if such equipment had been installed in Haiti, we still would have had only seconds to act in Port-au-Prince (01/29/10).
  • On Saturday, Feb. 27, 2010, 3:34 local time an earthquake of magnitude 8.8 on the Richter scale originated under the Pacific Ocean at a depth of 22 miles 60 miles offshore Chile near Maule. An account of the event is currently being developed on Wikipedia (2010 Chile earthquake).

    The temblor was measured fifth strongest on record, inflicting wide-spread, severe damage along the coast of Chile. Ensuing tsunami waves, which topped ten feet in some locations caused additional devastation. To date, the loss of life stands at about 800. Mostly old, historic structures suffered catastrophic failure. Earthquake-resistant construction methods in modern buildings saved many lives in metropolitan areas. By contrast, in Haiti such building practices constituted a rarity (03/03/10).
  • According to Terry Wade and Fabian Cambero's post entitled "Chile quake-area still shaking, death toll unclear" on Reuters today, the loss of life owing to last Saturday's earthquake off the Chilean coast and the ensuing tsunami is much smaller than believed earlier. The Chilean government now claims that roughly 300 people died as a result of the catastrophe (03/05/10).
  • In today's post on Reuters with the title "Chileans' quake knowledge saved thousands of lives", Daniel Trotta provides a few more details released by the U.S. Geological Survey about the Chilean quake three weeks ago. A 41-foot jump of the tectonic Nazca plate toward the South America plate triggered the 8.8-magnitude quake, 500 times more powerful than the magnitude-7.0 quake that struck Port-au-Prince six weeks earlier. The Nazca plate usually moves roughly 3 inches per year. In addition to strict building codes enforced in Chile, the low loss of lives compared with Haiti may be attributable to the atypically late arrival of the most destructive jolts after the first noticeable tremors. R and L waves arrived between 20 and 30 seconds after the P waves. In Port-au-Prince, this difference in time was only about 5 to 8 seconds. The Chilean coast was hit by three tsunami waves, surging 26 to 33 feet in some locations. Most lives may have been lost because of the flooding (03/19/10).
  • Hough and others (2010) demonstrate in a recently published seismological research study that topographic features around Port-au-Prince, like a steep ridge, must have amplified ground motion by constructive interference during the Haiti earthquake, in addition to the composition of superficial sedimentary layers in low-lying areas. Perhaps the ridge is the best location to detect an impending quake (10/18/10).
  • Not much progress in a year as this Reuters slide show published today with the title "Haiti's somber anniversary" illustrates (01/11/11).
  • Nova aired this informative program entitled "Deadliest Earthquakes" on recent advances in understanding the nature of earthquakes on Jan. 11, 2011 (01/30/11).
Amplification of seismic waves transitioning through geological layers of different density (courtesy Nuclear and Industrial Safety Agency,  Japan).
References
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