Richard Wunderman is managing editor of the Bulletin of the Global Volcanism Network and a geologist in the Division of Mineral Sciences at the Smithsonian’s National Museum of Natural History. Following the earthquake that rumbled across the East Coast of the United States on Aug. 23, Smithsonianscience.org asked Wunderman a few questions about earthquakes and how scientists measure and assess them. He was assisted by interns Claire Hepper and Robert Dennen.

Q. The earthquake that struck the Washington, D.C. area on Tuesday, Aug. 23, was 5.8 magnitude. Why did the earthquake occur? How do scientists assess earthquake size and impact?

A. The Virginia earthquake released energy stored as accumulated stress in the crust along faults along the coastal side of the Appalachian mountains. The resulting shock waves were felt by people into Canada and Georgia. The main areas of damage were strongest near the epicenter, the point on the Earth’s surface above the earthquake, and minor damage in urban areas at distance, such as Washington D.C. located 85 mi. northeast of the epicenter.

Figure right:  Generalized geologic map showing the location of the M 5.8 EQ of 23 August 2011. The areas in white are ancient rocks of the Appalachian region that transmit seismic energy efficiently. Some other smaller epicenters also shown. Map taken from blog by C.M. Bailey 2011 (Click image to enlarge).

The cause of the stress is plausibly attributed to factors such as ancient mountain building along the East side of the Appalachian mountains, more recent shifts and adjustments in this region, or from stress developed in more recent times. Virginia’s Division of Geology and Mineral Resources has excellent resources here.

Q. The energy released during the earthquake was stress that had been building up along a fault line in central Virginia for a very long time. Now that this stress has been released, can we relax and not expect another quake for many, many years?

A. Yes, we would assume that along the fault where the earthquake occurred, this might help alleviate another large earthquake there. Trouble is, some of this stress has probably shifted to another area on this or adjacent faults. A complex web of short faults has been mapped in the region.

That being said, the last large earthquake was around 114 years ago, also a magnitude 5.8 quake, so it is seems unlikely that there will be another in the near future. Small earthquakes, which cause little to no damage, have been an almost yearly occurrence in the Virginia seismic area and will likely continue at that recurrence interval.

Q. On Aug. 23, the shaking in Washington, D.C. seemed to last only 20 seconds. Is duration one of the characteristics of an earthquake that is used to measure its magnitude?

A. The size of the earthquake is a potentially confusing topic so I’ll start at a point that everyone can relate to. That is, a general sense of the shaking levels from an earthquake, an estimate called intensity. This measure does not rely on instruments and much in the way of numerical measurements, but addresses the important question of what the shaking actually did.

Yes, the damage could indeed relate to duration of the shaking. And, yes, prolonged duration could spell disaster. Other factors could include the earthquake’s focal depth, its peak accelerations, the kinds of waves generated and their frequencies. Relating to the site, factors include the thickness and kind of crustal material through which the shock waves passed, the local rock and soil types, the slope, and water saturation of the ground. Other critical factors include local rocks and structures such as buildings, bridges, and dams, and how they respond.

Earthquakes can range from not felt and no impact, all the way to leveling large areas, causing secondary effects like substantial tsunamis, landslides, and liquefaction (shaking wet sand, mud, or soils such that it flows, almost like water). The most common intensity scale is the Modified Mercalli scale.

Akin to the intensity scale, the U.S. Geological Survey also employs a system called the “felt magnitude.” This is estimated from personal accounts, and basically asks “how strong did the earthquake feel?” In this estimate, the way an earthquake is perceived by individuals is the measure of the earthquake magnitude, and one person may base their interpretation of the earthquake magnitude on the duration or any other characteristic. The USGS compiled “felt magnitude” for the Aug. 23 Virginia earthquake can be viewed here.

Magnitude, as in an M 5.8 earthquake, represents a scale based on the readings from seismometers and the wave traces we call seismograms. There are various kinds of these magnitude scales (eg. the Richter scale) and the magnitudes are often based on the size of the largest waves received (rather than the duration of the earthquake’s waves).

Figure left: A simplified illustration of earthquake-triggered waves and the resulting seismogram from a site at distance (at left; tracing time on the x-axis and amplitude on the y-axis). The seismogram contains labels indicating the first arrival of waves; the largest amplitude waves in the case shown are waves traveling along the ground surface called Rayleigh waves. The depiction omits the wave paths moving to the left (as well as some other waves). Taken from Stein and Wysession (2003; their online PowerPoint presentation for Chapter 1 of An Introduction to Earthquakes, Seismology, Earthquakes, and Earth Structure).

The distance from the earthquake and its depth are first calculated, and then the instrument readings at various distances from the epicenter are compensated for, and in fact, there are many corrections and assumptions. These values are often computed from multiple stations using computers. They are often refined somewhat, based on new information and various corrections. Comparisons between intensity and magnitude scales are inexact.

Q. Earthquakes by their very nature are relatively short-lived events, over in a few seconds . . . correct? Or do some earthquakes last a long time?

A. Generally the shaking of the Earth as a result of the fracturing along the fault lasts only a few seconds. That said, in a large earthquake, the propagating waves from the fault rupture can shake for minutes, since for one thing, they come from various parts of the fault and also because the waves reflect (bounce around), actually probing the reaches of the entire planet and providing key clues to the various inner parts of the planet (crust, mantle, and two-part core).

Thick sedimentary valleys and basins such as found in many Western USA settings can behave somewhat like jello, and shake much longer (and stronger, that is, with higher amplitude) than the initial passing waves, essentially amplifying the earthquakes intensity.

Weak vibrations triggered by strong earthquakes may cause the entire planet to ring like a bell at amplitudes and frequencies detected instrumentally but not felt by people. This ringing can go on for weeks in the case of a strong earthquake.

Aftershocks, which are the Earth settling and readjusting, can recur sporadically for weeks to more than a decade after an earthquake has occurred, generally diminishing with time in number and strength.

Q. Much of the structural damage to buildings caused by the Aug. 23 earthquake, it seems, occurred to areas that are high up–cracks at the top of the Washington Monument, the spires of the National Cathedral, and the towers and chimneys of the Smithsonian Castle. Why is that?

A. Hold a stiff fishing pole by the handle and oriented vertically. If you give the handle a shake, the rod’s tip will move as the waves arrive, and excursions there will be comparatively large. Structures made of, or faced with stone, offer special challenges, as the ‘building blocks’ they may not hang together during the deformation and shaking.

Image right and above: The Aug.23 earthquake caused damage to the chimneys and other high-up structures on the east end of the Smithsonian Castle. (Photos by Mark Avino and Eric Long)

Q. Is there a way to keep track of how much compression or tension is building up between rock plates on either side of a fault line?

A. There are multiple things than can be measured to keep track of offset along a fault over time. Strain meters measure the amount of strain (the movement or deformation caused by stress) present in the ground. Methods employing GPS or radar look at ground-surface deformation. One of my favorite methods is a simple line of survey monuments across arranged across a fault, repeatedly re-surveyed over time. After an earthquake, features such as roads, fences, and railroad tracks can record earthquake offset. You can see other famous examples here.

(Image left. A photo of an offset fence seen soon after the M 7.8 Great San Francisco earthquake of 1906. The San Andreas fault’s surface trace is drawn as a colored band. This area is now part of Point Reyes National Seashore. Image from Stein and Wysession 2003; online PowerPoint presentation).

Q. Where were you when the earthquake struck and what were your immediate thoughts?

A. I was at my desk in the Smithsonian’s Museum of Natural History on the Mall. Shaking grew to the point where it upset me as some of my geologic colleagues screamed and I heard glassware breaking. No one was hurt. In this commotion I had walked from my desk and braced myself in the office doorway, but the shaking stopped soon after that. As the event had escalated, my thoughts went to the building’s masonry construction and the well known, water-saturated fill in this part of the low-lying Washington, D.C. region. Neither of those thoughts were comforting, but the building withstood the stress without serious damage.

My wife and daughter were in the family car stopped at a traffic light when they felt weird unexplained vibrations. My wife immediately asked my daughter ‘What did daddy do to the car now?’ She attributed the vibration to engine or suspension trouble caused by you-know-who. The announcements of the earthquake saved me from near-certain allegations of car abuse.

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In a new study  in the journal Palaeontology, University of Florida and Smithsonian Tropical Research Institute scientists describe a new 20-foot extinct species discovered in the same Colombian coal mine with Titanoboa, the world’s largest snake. The findings help scientists better understand the diversity of animals that occupied the oldest known rainforest ecosystem, which had higher temperatures than today, and could be useful for understanding the impacts of a warmer climate in the future.

Image right: This illustration shows how Acherontisuchus guajiraensis, a 60-million-year-old ancestor of crocodiles, would have looked in its natural setting. Titanoboa, the world’s largest snake, is pictured in the background. (Illustration by Danielle Byerley/click to enlarge)

The 60-million-year-old freshwater relative to modern crocodiles is the first known land animal from the Paleocene New World tropics specialized for eating fish, meaning it competed with Titanoboa for food. But the giant snake could have consumed its competition, too, researchers say.

“The younger individuals were definitely not safe from Titanoboa, but the biggest of these species would have been a bit much for the 42-foot snake to handle,” said lead author Alex Hastings, a graduate student at the Florida Museum of Natural History and UF’s department of geological sciences.

Image left: University of Florida researchers unearth fossils from the 60-million-year-old Cerrejon formation in northeastern Colombia, one of the world’s largest open-pit coal mines. (Photo by Edwin Cadena)

The new species is a dyrosaurid, commonly believed to be primarily ocean-dwelling, coastal reptiles. The new adult specimens challenge previous theories the animals only would have entered freshwater environments as babies before returning to sea.

Fossils of a partial skeleton of the species, Acherontisuchus guajiraensis, show dyrosaurids were key players in northeastern Colombia and that diversity within the family evolved with environmental changes, such as an asteroid impact or the appearance of competitors from other groups, said Christopher Brochu, an associate professor of vertebrate paleontology in the department of geoscience at the University of Iowa, who was not involved in the study.

“We’re facing some serious ecological changes now,” Brochu said. “A lot of them have to do with climate and if we want to understand how living things are going to respond to changes in climate, we need to understand how they responded in the past. This really is a wonderful group for that because they managed to survive some catastrophes, but they seemed not to survive others and their diversity does seem to change along with these ecological signals.”

Image left: This photograph shows the size difference in the jawbones of two 60-million-year-old crocodile ancestors found in northeastern Colombia by University of Florida researchers.The newly described Acherontisuchus guajiraensis, top, and Cerrejonisuchus improcerus, bottom. C. improcerus was the first ancient crocodyliform found in the Cerrejon open-pit coal mine. The new species is the first known land animal from the Paleocene New World tropics specialized for eating fish. (Photo by Kristen Grace)

The species is the second ancient crocodyliform found in the Cerrejon mine of northern Colombia, one of the world’s largest open-pit coal mines. The excavations were led by study co-authors Jonathan Bloch, Florida Museum associate curator of vertebrate paleontology, and paleobotanist Carlos Jaramillo of the Smithsonian Tropical Research Institute.

“This one is related to a group that typically had these long snouts” Hastings said. “It would have had a relatively similar diet to the other (coastal) species, but surprisingly it lived in a more freshwater environment.”

The genus is named for the river Acheron from Greek mythology, “the river of woe,” since the animal lived in a wide river that emptied into the Caribbean. Unlike the first crocodile relative found in the area, which had a more generalized diet, the snout of the new species was long, narrow and full of pointed teeth, showing a specialization for hunting the lungfish and relatives of bonefish that inhabited the water.

“The general common wisdom was that ancestrally all crocodyliforms looked like a modern alligator, that all of these strange forms descended from a more generalized ancestor, but these guys are showing that sometimes one kind of specialized animal evolved from a very different specialized animal, not a generalized one,” Brochu said. “It’s really showing us a level of complexity to the history that 10 years ago was not anticipated.”

During the Paleocene in South America, the environment was dominated by reptiles, including giant snakes, turtles and crocodiles. The dyrosaurid family originated in Africa about 75 million years ago, toward the end of the age of dinosaurs, and arrived in South America by swimming across the Atlantic Ocean.

“The same thing that snuffed out the dinosaurs killed off most of the crocodiles alive at the time,” Hastings said. “The dyrosaurids are one of the few groups to survive the extinction and later become more successful.”

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Images right and below: Rough diamonds

“It’s very rare to know where some rough diamonds came from because typically, once they come out of the mine, they go to market and are sold,” says Jeffry Post, curator in the Department of Mineral Sciences. “In most cases, diamonds lose any documentary links to their source by the time they reach the market.” This donation will be a great asset to researchers, allowing them to study specimens and knowing where they originated in the Earth.

The larger diamonds in the Jewelers Mutual donation will be added to the diamond exhibition in the Natural History Museum’s Gem and Mineral Hall. The others will be made available for scientific study. Jewelers Mutual originally acquired the diamonds to display in the company’s onsite gallery of gems and minerals in Neenah.

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Two feet of heavy rain inundated the Panama Canal watershed between Dec. 7 and 10, 2010. Landslides tore down steep slopes, choking rivers with sediment and overwhelming Panama City’s water-treatment plant. Flooding closed the Panama Canal for the first time since 1935. Despite the deluge, the influx of sediments in the water forced authorities to shut down the plant, leaving a million residents of central Panama without clean drinking water for nearly a month.

Image left: Storms trigger landslides that release sediment into rivers and streams (Photo by Robert Stallard)

LightHawk, a conservation organization based in the U.S., donates flights for research and conservation efforts. Retired United Airlines captain David Cole flew the Cessna 206 aircraft, and the four flights yielded images of 191 square miles (495 square kilometers) of watershed. Stallard observed numerous new landslide scars left behind by the December storm, supporting his prediction that landslides supplied much of the suspended sediment that disrupted Panama’s water supply.

The new watershed erosion map will allow Stallard and collaborators from the Panama Canal Authority to calculate the landslide risk of future storms and direct strategies to minimize the effect on Panama’s water supply.

Tropical hydrologists agree that river-borne sediment originates from surface erosion or from deep erosion from landslides. In 1985, Stallard predicted that “deep erosion, not shallow surface erosion, is the primary process controlling the chemistry and sediment levels in many tropical rivers that pass through mountainous areas.” Few studies have been conducted to test this prediction.

Image right: Robert Stallard, STRI staff scientist and USGS hydrologist. (Photo by Marcos Guerra)

Deforestation of steep slopes is the primary factor determining the number of landslides. Six decades of aerial photographs analyzed by USGS researchers in similar landscapes in Puerto Rico showed that landslide frequency doubles outside protected nature preserves, and that roads and infrastructure make landslides eight times more likely. Although landslides happen in natural forests, the objective is to limit their impact through appropriate land-use practices.

“With development, landslide intensity increases dramatically,” said Stallard. “In its history, the Panama Canal watershed has experienced huge floods. It’s still hard to say whether future floods will be accompanied by disastrous landslides like those produced by Hurricane Mitch in Central America.” In 1998, Hurricane Mitch swept across Honduras, Guatemala, Nicaragua and El Salvador causing more than 10,000 deaths and incalculable economic damage. Panama’s proximity to the equator puts the country outside the usual hurricane zone, but prolonged tropical storms may occur.

Erosion control is possible. Partnering with the Panama Canal Authority and Panama’s Environmental Authority, the Smithsonian is conducting a 700 hectare experiment in the canal watershed funded by the HSBC Climate Partnership to compare the effects of land-use choices, such as cattle ranching or reforestation with native tree species on water supply, carbon storage and biodiversity.  Stallard hopes that this research will provide new information about which land uses provide a steady supply of clean water for the Canal.

With the first rains in May, the eight-month wet season begins anew in central Panama. Drinking water flows freely, the rivers are clear and the Panama Canal is open for business. But bare slopes of past landslides continue to create secondary erosion, which will dislodge sediments from the steep, rainy and rugged Panama Canal watershed in 2011. The long-term effects of the 2010 storm may continue as renewed interruptions in the water supply in 2011.–Beth King

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