The Trembling Earth

now at blogs.agu.org/tremblingearth

Category Archives: Anniversaries

Remembering Northridge

Today is a day of significant quake anniversaries for the U.S. and Japan.

The last U.S. quake to kill more than a few people struck the L.A. suburb of Northridge 19 years ago today, in the wee hours of the morning. The San Fernando Valley was hit hard, but the whole L.A. area rattled violently, and seismic waves from The Valley were focused through the Santa Monica Mountains into the populous west side, an effect that’s apparent in the “Did-You-Feel-It” map if you’re familiar with the geography of L.A (click there for info if you aren’t. Or for fun if you are).

DYFINorthridge

Damage was widespread and many of the area’s freeways shut down for months after major collapses when their concrete supports failed during the strong shaking. Despite the huge monetary cost (right between hurricanes Andrew and Katrina… and potentially behind Sandy) and moderate death toll, this was not Los Angeles’ “big one.” In the scheme of possible So Cal earthquakes, this was a relatively small one, and rather than hitting the core of the city, it struck a glancing blow by starting in the suburbs and sending most of its energy northward into the mountains. Nonetheless it is Angelenos’ clearest reminder (although its age must have erased it largely from modern relevance) of what to expect when a significant quake strikes the city.

Exactly a year later a quake of nearly the same magnitude struck a much more densely populated corner of the planet: Kobe, Japan. This quake, almost identical in magnitude to CA’s Northridge earthquake of the year before, is the source of that quintessential earthquake footage most everyone will be familiar with:

The Kobe quake’s death toll was two orders of magnitude higher than Northridge’s, and the damage to the port city was devastating, largely because of widespread liquefaction, an effect that was much less prevalent in the coarse sedimentary basins of Mediterranean L.A.

As we mark this anniversary of those significant earthquakes, as usual you should take advantage of this heightened awareness to double-check that you’d be ready were a similar quake to happen today. Let the commemoration serve to remind you that these were real events that really happened, and could happen again at any moment.

An iconic photo of 90s L.A.

An iconic photo of 90s L.A.

Ten years ago Denali shook

Saturday [November 3, 2012]  marked the ten-year anniversary of the largest quake to hit the U.S. since 1964, and the 1906 SF quake before that. The M7.9 Denali earthquake tore a ~250 mile gash through Alaskan glaciers and pine forests along the Denali Fault, which runs beside the eponymous mountain also known as Mt. McKinley, North America’s highest peak.

A detailed topographic survey of the rupture reveals fresh scarps along the sharp linear trace of the fault where it has been offsetting river valleys for millennia, one earthquake at a time. The fault runs from upper left to lower right across this image, and the large deflections of each canyon represent the cumulative result of dozens of 2002-sized earthquakes. This data set is available to view in Google Earth from http://www.OpenTopography.org

Much like the Haida Gwaii earthquake last week, the Denali quake shook mainly sparsely populated areas, so it was much more probable that anyone feeling it would be on the outer fringes of the shaking from the massive temblor. Indeed footage from a home in Anchorage, 160 miles from the epicenter, shows slow rocking–that telltale sign that some place moderately far from you is really getting hammered. Despite the low frequency of the shaking in this video, its strength is clear, and it lasts a very long time–another sign that you’re on the fringes of a huge earthquake.

One of the many legacies of this earthquake was that it put the trans-Alaska oil pipeline to the test. The pipeline crosses the Denali fault nearly perpendicularly, and was constructed with the knowledge and anticipation of offset along the fault. For ~1,000 feet on either side of the fault, the pipeline’s supports rest in tracks that allow it to shift laterally and bend as the ground beneath it carries the tracks in opposite directions. The structure was designed anticipating an earthquake of magnitude 8.0 with 20 feet of coseismic lateral displacement. At nearly the anticipated size, the Denali quake was a resoundingly successful test of earthquake engineering, which spared us an enormous environmental disaster.

A comparison of the 2002 Denali and 1857 San Andreas fault ruptures. The modern recordings from Alaska help guide our expectations about a contemporary repeat of the southern California quake.

Both its size and its geometry suggest that this rupture may be an excellent modern analogue to the earthquake that ripped along the San Andreas Fault in 1857, before southern California was heavily populated. It thus serves as an excellent source of modern data (seismic recordings, satellite imaging, GPS velocities) to help understand what a repeat of the San Andreas rupture will be like. For example, seismic records from the Denali quake are used in structural engineering tests of seismic safety design, simulating the type of shaking that may be expected around Los Angeles in the next SAF quake.

The USGS hosts a great set of photos from the spectacular Denali fault rupture, including offset glaciers, newly formed waterfalls across the scarp, huge landslides, and my personal favorite: this unfortunate tree that was growing directly atop the fault trace and got sheared in half.

A tree, taking advantage of the groundwater source along the Denali fault, suffers the consequences of its opportunism. Photo credit: Peter Haeussler, USGS.

ShakeOut this week! World’s largest ever earthquake drill

This is a big week in U.S. (and world) earthquake history. In the U.S. we mark the anniversaries of several major, important earthquakes that have struck the country.

On October 15, 1979, a M6.9 earthquake struck the Imperial Valley of southern California/northern Baja. In 2006 a M6.7 earthquake rocked the island state of Hawaii, damaging thousands of buildings at a cost of $73 million. On October 16, 1999, people throughout the U.S. southwest were rolled from their slumber by the massive M7.1 Hector Mine earthquake that struck the Mojave desert at 2:46am. And of course… on October 17, 1989, the Battle of the Bay baseball World Series was interrupted in San Francisco by the devastating M6.9 Loma Prieta earthquake. …And let’s not forget the 1935 M6.3 Helena, Montana earthquake of October 19, part of a series that cost the city dearly, and should serve as a reminder that the intermountain west and the Rocky Mountain front are not free from seismic danger.

Capitalizing on that spate of anniversaries, and on the early date in the U.S. school year, a host of quake-dedicated agencies have organized the fifth annual ShakeOut drill.

This year the drill has expanded far beyond California, incorporating participating agencies in countries around the world. The U.S. is divided into official ShakeOut regions, including many individual states and some at-risk regions like the New Madrid seismic zone, each with their own specific issues when it comes to earthquake hazard. Nearly the whole country is covered, so I encourage ALL of you to sign up and Shake Out.

In most of the U.S. the drill takes place on Thursday, October 18, at 10:18am. In the Central U.S. the drill will take place on February 7, to mark the largest of the 1811-12 New Madrid quakes, and Utah will hold theirs on April 17. Participation is voluntary, unless your employer or educator has opted in for you, but already one third of California’s population is registered. Globally, 17 million people are signed up. Of course it behooves everyone to know what to do in an earthquake, and what resources you will have available, so there’s hardly any reason not to join in.

Visit http://www.earthquakecountry.info/ for information on what to do before, during, and after an earthquake, and to obtain resources for your own drill. Every family that faces the threat of earthquakes (that’s all of you) should know how to find each other and cope with the aftermath. You should also brush up on your Drop, Cover, and Hold On. No doorways. No triangle of life. No sprinting down the stairs to the street like a frantic animal. Stay in place and get under something sturdy. The Earthquake Country Alliance has put together an informative set of instructions on how to protect yourself in a wide variety of situations.

Sign up for the drill, and get the word out to your friends and coworkers. This is invaluable practice. We can’t predict earthquakes, but we do now how to deal with them. The best defense we have as individuals is our own awareness and preparation.

Share your ShakeOut plans or experiences in the comments, to help pool preparedness plans.

Landers @ 20

My favorite earthquake photo ever: dextral offset of a desert road surrounded by Joshua trees, taken by geologist Kerry Sieh during a reconnaissance trip five hours after the 1992 Landers earthquake.

Today is another noteworthy anniversary. [Quakiversary, as I’ll start calling them if I’m not careful and don’t restrain my incessant compulsion to merge phrases through obnoxious portmanteau-ing of things.] It marks twenty years since California’s largest earthquake in the last six decades, an earthquake that once again transformed the way we understand fault rupture.

Twenty years ago, on June 28, 1992, Angelenos and others throughout the desert southwest were rolled from their slumber by a massive 5am shock from the Mojave. The M7.3 earthquake ripped erratically through small California desert towns just north of Joshua Tree National Park (then a National Monument). The rupture jumped from one fault to another, linking together fractures in the ground that experts had previously considered discontinuous, unconnected, and thus capable of producing only relatively smaller earthquakes.

Fresh fault rupture passes directly beneath/through a house in Landers, CA after the 1992 earthquake. This scene illustrates the value of setting structures back from active faults as outlined in California’s Alquist-Priolo Zoning Act.

The towns of Yucca Valley and Landers were hit exceptionally hard, with violent shaking and in many cases primary ground rupture ripping through homes and buildings. Other desert towns–Twentynine Palms, Palm Springs, Barstow, Victorville–and the San Bernardino mountains experienced a lengthy period of strong shaking. The vast but more distant populations of Los Angeles and Las Vegas were rocked strongly and slowly, undoubtedly recognizing that something serious was happening somewhere… else. This was not their earthquake.

USGS Community Internet Intensity Map for the June 28, 1992 M7.3 Landers earthquake

Aftershocks rumbled and rattled as people began their days in premature wakefulness, and as details of the disruption in the desert towns occupied the headline spot in the 8am news, a sudden sharper jolt tore outwards through Los Angeles.

Have a look at the event occurring in CNN’s national feed. Skip ahead to 3:00 for the full experience.

The second earthquake, this one of magnitude 6.5, had been unleashed closer to L.A., rather far from the 5am mainshock and on a completely different fault system, centered near the resort town of Big Bear in the San Bernardino Mountains. Now aftershocks were emanating from the mountains above town and the giant tear out in the desert.

Basic map showing location and time of major earthquakes in the 1992 Landers sequence, and their aftershocks. Mapped June 28 fault ruptures are in yellow, other faults in black. The Los Angeles metropolitan area occupies the freeway-filled basin in the lower left.

Field teams from the USGS, Caltech, and other institutions around southern California mobilized immediately to investigate the damage and ground rupture from this earthquake, and thus is it one of the best recorded and documented earthquakes of recent decades. It was the first so have such extensive immediate field coverage.

To mark the current anniversary, the Desert Sun has a slideshow of pictures from the early 90s aftermath, along with some eyewitness reminiscence.

http://www.mydesert.com/article/20120628/NEWS0805/206280319/Landers-quake-still-vivid-minds-survivors

The Landers sequence is my favorite illustration of earthquake behavior because it covered all the basics: There were immediate foreshocks–all <M3–preceding the event by minutes, but a sequence of substantial quakes near the mainshock epicenter had occurred back in April. The aftershocks began immediately, and quickly delineated the extent of fault rupture. Then there’s the triggered earthquake and its own aftershocks. Much research has focused on the stress changes induced by the 7.3 mainshock and how they led to the 6.5 Big Bear quake that happened three hours later, outside of the strict aftershock zone.

We learned a lot about seismicity from this sequence of quakes, but its more lasting impact has been the recognition that nearby faults can link together and rupture in a single earthquake. The Landers earthquake broke open five separate, previously mapped faults, connecting them to each other through a network of unmapped structures that hadn’t been visible in the landscape prior. Since then many more earthquakes have occurred in this manor, including notably its Mojave successor the 1999 M7.1 Hector Mine earthquake, as well as the recent M7.2 El Mayor-Cucapah earthquake just south of the border.

Looking eastward down Linn Road on a 2010 visit to Landers, CA. The 1992 earthquake right-laterally offset the entire street grid several meters, so its effect is still quite visible today, 20 years later.

The well studied rupture from this earthquake is still evident out in the desert as subtle, rain-softened scarps stretching northward from Joshua Tree NP. The entire town of Landers was literally rent in two, with the eastern half sliding meters southward and the western half lurching to the north. The grid of streets still shows this spectacular offset. The whole area is worth a visit if you’re doing some fault-finding tourism, and especially if you love the Mojave.

Yours Truly perched next to the 1992 rupture scarp of the Emerson Fault near Galway Lake Road, site of the maximum slip. The photo is from 2007, 15 years after the quake, and the scarp has deteriorated since then, but will remain a conspicuous step in the landscape for millennia.

40 years of earthquakes on one seismogram

Want to see the pulse of the Earth?

Last year marked the 50th anniversary of the Albuquerque Seismological Laboratory (ASL), a facility in the quiet mountains of New Mexico that is used by the USGS as a hub to maintain both the instruments and data transmission capabilities of the global (GNS) and national (ANSS) seismic networks. The ASL is used to develop new seismograph technology and to test and calibrate instruments in the absence of urban noise and in close proximity to several long-running reference stations.

In honor of the golden anniversary in June 2011 the Incorporated Research Institutions for Seismology put together the [nearly] continuous record of seismograms from these long-running stations in a plot that’s simple, beautiful, and fascinating. I’m going to hang this poster somewhere.

40 years of global earthquakes as recorded at a site outside of Albuquerque, New Mexico. Check out IRIS’s website for the full size version and info about how it was created.

You can read how the plot was generated and details of the more prominent earthquakes at IRIS’s website, but there are some striking things the keen observer will find right off the bat, at least after a brief orientation:

Each of the 465 lines represents a single month of seismic record, squished to fit on the page. The numbers at the bottom represent days of the month, and for months with fewer than 31 days the tail ends of the lines (the final 1-3 days) are filled with zeroes, making them flat lines. Read it from top to bottom and left to right like a book, and you’ll see a rich mixture of small local or regional earthquakes sprinkled among large, long-duration teleseismic earthquakes–ones of substantial magnitude that occurred far around the Earth but were recorded at these stations in Albuquerque thanks to the immense energy they released.

The two largest events are conspicuous in the final years. These of course are the M9.1 Sunda megathrust earthquake that hit Sumatra, Indonesia in 2004, and the M9.0 Tohoku earthquake that hit Japan last year. The bottom right corner is full of other megaquakes, mostly centered in Indonesia, save for the M8.8 Maule, Chile earthquake of 2010.

Pick your most significant historical earthquake from the last 4 decades and try to find it on this plot! Or just marvel at this continuous record of our rocky planet’s pulse.

Trembling from above – first-hand tornado wrath

This past weekend The New Yorker‘s new “Weekend Reading” feature directed me to a fascinating disaster survival narrative that was so good I figure I can take a little departure from solid Earth phenomena to point you all to the enthralling account… and the first-hand videos that accompany it.

The path of the May 22, 2011 EF-5 tornado that tore through Joplin, Missouri. The tornado scored a direct strike on the city’s hospital and high school, and ravaged neighborhoods in between. The New York Times has an interactive feature with before-and-after imagery: http://www.nytimes.com/interactive/2011/05/25/us/joplin-aerial.html

The disaster in question was an EF-5 tornado that scored a direct hit on the modest southwest Missouri town of Joplin on May 22, 2011. In one of those extremely rare and extremely unlucky coincidences, the massive tornado formed right on the western outskirts of town and plowed its 3/4-mile-wide way through the heart of the 175,000-person city, lightening up and lifting off only once it reached the eastern edge of town. The path of the twister could scarcely have been worse: the strongest winds were sustained for a 6-mile path that coincided nearly exactly with the extent of the city, and its trajectory took it directly across the hospital, the high school, a middle school, several large retail complexes, and countless residential neighborhoods, all with wind speeds approaching or exceeding 200 mph.

Power went out throughout the city early in the event, as increasingly frantic newscasters on TV and radio tried to supplement the city’s 3-minute-long siren warnings to convey the dire danger of this particular storm. Naturally, many residents, having been through big midwestern storms before, did not expect what was to come. In the videos that exist of the event (there are surprisingly many), most people do not take shelter until the monstrous tornado is directly upon them, revealing some level of optimism–or at least disbelief–at what was unfolding.

The eye-witness accounts of the tornado are gut-wrenching. In one particularly remarkable video, a group of strangers has gathered in a convenience store on the eastern edge of town. They’re all there for different reasons, although they’ve been driven inside to take shelter from the storm. As they gradually realize the gravity of the situation, they have no clue that this huge looming tornado has already done most of its damage to their southwest, literally ripping their city to shreds. They also don’t–and couldn’t possibly–know what is about to hit them. It is their stories that are compiled in the captivating article in–of all places–Esquire. The article is so well written that you’ll think you’re among the survivors, huddling together and fearing for your life. It also links to that infamous gas station video, which is fairly emotionally strenuous. I’ve never watched anything so tense; Hollywood eat your heart out.

These links should take you through the event, starting with the article:

“Heavenly Father!” “I love you all!” “I love everyone!” “Jesus! Jesus! Jesus! Jesus! Jesus! Jesus! Jesus! Jesus! Jesus! Jesus! Jesus! Jesus! Jesus! Jesus! Jesus!” “I love all of you!”

And here–hold on to your seats–is the video:

The videographer who captured that emotionally draining scene went back the next day to revisit the site that brought them all together in what they thought was their moment of death. Here’s what the place looked like:

Because the destruction is entirely wrought in the deep dark of the tornado, a different video may illuminate the unbelievable winds that defined it:

Joplin tornado recorded by home security camera (youtube)

There are a few additional home videos of the tornado tearing through the neighborhoods, and although the videographers survive in safe interior rooms or basements, these intense clips may be difficult to watch, especially considering the immense loss these people faced upon emerging from their shelters:

Here a family hides as the approaching roar (“That sounds like a train. Is that the tornado??”) and pitch black of the tornado itself descend upon their house. Definitely grab your headphones/good speakers for this one.

http://youtu.be/fo81EQN98Oo

Here’s another family at home… in this one the people’s terror is pretty vicariously horrifying:

http://youtu.be/bZvo39XT8bk

In both of those, the sound of the tornado–that concentrated zone of 200-mph winds tearing its way toward the camera–is quite remarkable.

Then there’s a group of tornado chase tourists who get more than they bargained for. Their panic is evident as they realize the gravity of the situation… and ultimately they can hardly outrun it.

http://youtu.be/CburjPYmSdo

I encourage you to learn more about the humbling event itself, at NOAA’s info page about it:

http://www.crh.noaa.gov/sgf/?n=event_2011may22_survey

…and to keep this experience in mind when the sirens go off in your midwestern town. In the case of tornadoes, we have the technology for advance warning… don’t squander it, and don’t take it for granted!

Seattle’s “Big One”… almost

Instrumentally recorded shaking intensity throughout Washington state, interpolated among the network of local seismometers that recorded this earthquake..

February 28 marks the anniversary of Washington state’s most recent large quake, the M6.8 Nisqually earthquake of 2001. The epicenter was at the southern end of Puget Sound, and the 10:54am quake heavily rocked Washington’s most populous region, including the capitol Olympia, Tacoma, and Seattle.

This quake was a relatively “small” quake for the region, considering the Pacific Northwest is periodically rocked by massive subduction zone “megaquakes” like the recent ones in Japan, Indonesia, and Chile, but at a respectable 6.8 and located so close to metropolitan areas, it was a violent and destructive quake nonetheless. It represents a different kind of quake risk to the Pacific Northwest from that posed by the much-publicized Cascadia subduction zone. This quake occurred in the crunching and crumpling upper plate as it grinds over the diving Pacific ocean slab.

There are a couple of decent videos of it occurring, including one from an unstable webcam in a Seattle Microsoft office:

(in that video note the distinct P- and S-wave arrivals: the rumbling Primary waves herald the quake’s onset at this location, giving ample warning for the office occupants to head to shelter before the Secondary and Surface waves arrive with significant lateral forces. Also keep an eye out for the continuing long-period motion and two severe vertical jolts that keep rocking the office as the quake subsides. You’ll notice this especially in the the swinging telephone cords, whose oscillation doesn’t simply dampen, but responds to renewed accelerations during continuing low-frequency shaking.)

Microsoft execs including Bill Gates were giving a press conference on new Microsoft products in a Westin Hotel conference room when the quake struck, so two news crews captured it on film. The quality of these is poor, so turn down your speakers:

 

The quake also interrupts a meeting of Seattle’s City Council:

 

In addition, there are a couple of radio recordings of the quake happening, as documented in this article.

 

This was a significant event for Washington state, so lots of money and research has been dedicated to studying it. Much of this research is highlighted through the University of Washington’s Nisqually Earthquake Information Clearinghouse, a great site for more detailed information about the quake and its effects. There’s also an excellent blog dedicated to chronicling the experiences of Washingtonians during and after the earthquake, at http://isquallyquake.wordpress.com.

The Southern California Earthquake Center has also put together an info page comparing the 2001 Nisqually quake to L.A.’s similarly sized 1994 Northridge Earthquake, at http://www.scec.org/news/01news/feature010313.html.

 

As always, mark this anniversary by considering your own preparedness were an earthquake like this to strike your hometown. Every time it happens we learn more, so we should fare better after each new earthquake.

Legendary phenomena in the 1811-1812 New Madrid earthquakes

200 years ago this February 7, on the western frontier of European settlement in North America, the pioneering westward expanders and the natives whose land they were colonizing were thrown from their sleep in the deep wee hours of a winter night by the culminating temblor of a harrowing, months-long sequence of major earthquakes, aftershocks of which continue to this day.

Map of shaking intensity interpolated from historic accounts of the 2:15am mainshock of the New Madrid sequence. Map courtesy Susan Hough, USGS.

The so-called New Madrid earthquakes–named for a small Missouri settlement near the modern-day borders of Kentucky, Tennessee, Illinois, Indiana, and Arkansas that lay nearest the center of this cataclysmic seismic sequence–are the largest to have struck the eastern United States since well before they became the United States. In the recorded history of western settlement of North America, no quakes outside of the mountainous west match them in size and scope, and only a few come close.

Plenty of people have been and will be reporting on these earthquakes as we celebrate their bicentennial, including the organizers of the Great Central U.S. ShakeOut, which took place this morning to commemorate the massive culminating temblor of the sequence that started in December 1811. Even mapping software purveyor ESRI has put together a commemorative compilation of informative and beautiful interactive maps about the quakes (super cool compilation! If you click on one link in this post, let it be that one).  It is worth reading some of these syntheses and reviews because the earthquake series itself makes a captivating narrative. It’s nearly impossible to imagine the terror with which these relentless temblors must have stricken the settlers, who were already braving the “wild” frontier of a foreign continent. Even the mid-continent’s native inhabitants had not experienced such a thing in scores of generations, and in the early 19th century no one would have had any reasonable framework in which to explain the occurrence of massive earthquakes.

Because the New Madrid quakes occurred so early in our country’s recorded and geographic history, piecing together the events with a modern understanding of earthquakes and plate tectonics has required a great deal of sleuthing, and some of the details gleaned about them remain controversial, most notably their magnitudes (were they more like M7 or more like M8?). The uncertainty regarding the exact size of these earthquakes compounds the issue of determining the seismic hazard posed by recurrence of major earthquakes in the New Madrid Seismic Zone. To understand how seismicity may continue in southeastern Missouri we can look for patterns in the prehistoric record of earthquakes, but ideally we would like some idea of what forces caused these earthquakes to happen here. This remains an open question, and one in particular for which the question of the quakes’ magnitudes may be a crucial bit of information. Researchers have tried to use modern seismicity to constrain the behavior of large earthquakes in the New Madrid Seismic Zone, and some have interpreted the ongoing small quakes there as the tail end of an unsurprising aftershock sequence, suggesting that they don’t represent heightened seismic risk, but that in fact New Madrid is as likely as any number of other places in the eastern U.S. to have more major temblors.

The ongoing scientific controversy over ambiguous interpretation of details of these quakes stems from the nature of the data. Researching “pre-instrumental” earthquakes is a pursuit that fuses seismology, history, and social science, in an effort to understand historic written accounts of the earthquakes in the context of their time and cultural setting. A somewhat recent article in Seismological Research Letters describes the endeavor of anecdotal seismology, and through some colorful examples illustrates how historical reports can be translated into seismological data, clarifying the sources of interpretive ambiguity. The marriage of historical and seismological research to inform our model of seismic events in the eastern U.S. could be and has been the subject of many volumes, so I can’t hope to cover it here.

Instead I’ll draw analogy to this incredible sequence of earthquakes through videos and pictures from recent events, hopefully grounding some of the legendary accounts in footage of real and recognizable phenomena.

To the extent that people have learned about the New Madrid earthquakes of 1811-1812, they have often heard of them referred to as the largest quakes to ever strike the U.S.  Ask California [1857 & 1906] and Alaska [too many to name] and you’ll find this claim is far from true. Along with this hyperbolic appraisal comes the legendary confluence of phenomena eyewitnesses allegedly reported: the Mississippi running backwards, giant fountains of water issuing from the Earth, trees being thrown to the ground, and land sinking into the river. The unimaginable chaos of these phenomena all occurring in the midst of violent shaking defies belief, but contemporary earthquakes and modern video recording technology allow us to ground them in reality, and perhaps to understand them as more modest individual events that have been amplified in intensity by their conflation and coincidence in legend. We can see examples of all four in much more modest earthquakes:

1. The Mississippi running backwards

It’s difficult to imagine what possible physical phenomenon could have led to this observation/claim… unless you understand that the New Madrid quakes–just like all other large temblors–resulted from slip along several geologic faults. At the surface, fault slip breaks and displaces the ground, moving one side in a direction opposite the other. In the case of the causative Reelfoot Fault, the surface trace cut right across the Mississippi River channel, dropping an upstream portion of the river relative to the adjacent reach downstream. This warping has been thoroughly investigated and modeled, and thanks to the September 4, 2010 Darfield earthquake–a M7.1 event that ripped across rivers on New Zealand’s flat Canterbury Plain–we have a beautiful modern analog of the occurrence.

Aerial view of the Horata River spilling off of the fault scarp formed by the September 24, 2010 Darfield earthquake in New Zealand. Image courtesy Dr. Mark Quigley, University of Canterbury, Christchurch, NZ.

Where the 2010 NZ rupture fault sliced across the Horata River, it diverted the water into surrounding farmland, effectively changing the course of the flow. This is precisely analogous to the diversion of the Mississippi that led to both the damming and formation of Reelfoot Lake, and the temporary diversion of river flow back upstream.

2. Fountains of water issuing from the Earth

There are a few processes that may combine to produce this effect. In the past year we’ve seen plenty of examples of sand volcanoes, the eruptive results of shaking-induced soil liquefaction. When subjected to seismic waves (as in this New Zealand aftershock, or the Tohoku quake below), these sand blows can be squeezed into fountains of substantial height. The force of a larger and longer earthquake would undoubtedly increase the height these reach.

Extrusion of liquefied sediment by seismic waves isn’t the only coseismic phenomenon that may throw water high into the air: seiching–harmonic oscillation–of small bodies of water may throw water against their banks and up into the air. We’ve seen this dramatically demonstrated in swimming pools during a M7.2 earthquake, but natural ponds don’t necessarily have the splashing power of sharp corners and hard edges in concrete-walled pools. Nonetheless, with these two phenomena operating in tandem, the amount of water being thrown into the air by the quake would certainly be fodder for tales–legendary or not–of high fountains from the Earth.

3. Trees being thrown to the ground

Videos from several modest (M ~6) earthquakes in the past few years have revealed just how much trees can be wrenched around during shaking. Under the accelerations of earthquakes, trees’ own weight can be a more powerful force than high winds. Here a stand of neighborhood trees sways in a mere 4.4 earthquake in Christchurch:

In a M6 we see through the windows the same effect:

Finally, video the USGS captured at practically the epicenter of the M6.0 2004 Parkfield earthquake shows fairly violent lashing of late summer oaks in the California Coast Ranges.

A tree along the San Andreas Fault in Wrightwood, CA, had its top snapped off in an 1812 earthquake, from which it grew two new crowns. The tree no longer exists, but others like it can be found along the 1906 rupture near Point Arena in NorCal. Image from "Mixed Matters"

Though the effects shown above do not amount to trees being thrown to the ground, the earthquakes that produced them were much smaller than the ones that struck Missouri. We have clear evidence along the San Andreas Fault of trees whose tops were snapped off during the 1906 earthquake. This is a common effect in the epicentral region of large quakes.

4. Land sinking into the river

This phenomenon is akin to but distinct from the Mississippi being diverted and running backwards. In fact the underlying process is more closely related to the processes that give rise to sand blows. Shaking liquefies water-saturated soils and they lose their shear strength, rendering them unable to support gravitational loads. Thus the land slumps, under its own weight or the weight of trees, houses, or riverboat moorings, downhill towards unencumbered free edges like river banks. This “lateral spreading” is commonly observed along river banks shaken by earthquakes, and results in lowering and inundation of the ground surface. Examples abound from earthquakes as geographically and tectonically various as the 1964 Good Friday event in Alaska, the 1906 San Francisco earthquake, the 2010 Haiti earthquake, and the 2010 El Mayor-Cucapah earthquake. In all of these events vast swaths of land shook loose and slumped ocean- or river-ward, and effectively “sank”.

The video examples compiled above may not match the apparent drama of those recounted from 1811-12 Missouri, but I find it easy to imagine the cumulative results of decades and decades of re-telling on the details of these accounts. In any case, large earthquakes produce remarkable effects, and although many people around the world witness or experience earthquakes, still relatively few witness the truly violent shaking that occurs near an earthquake’s source. Written and oral accounts give us the most thorough picture, even if we have to take them with a grain of salt. Video may gradually be replacing verbal accounts in objectivity (no relying second-hand information!), but it has yet to become as widely distributed and available as individual eye-witnesses.

Next time you strike up a conversation about these earthquakes, consider yourself informed about many of the features that defined them, but by all means gather more information on your own. My two favorite informative links are the following:

ESRI maps of the 1811-1812 New Madrid quakes:
http://mapstories.esri.com/new-madrid/

CERI compendium of New Madrid primary sources:
http://www.ceri.memphis.edu/compendium/eyewitness/index.html

Happy Bicentennial!

Loma Prieta earthquake anniversary

Twenty two years ago this very day the San Francisco Giants were preparing to play their cross-bay rivals, the Oakland Athletics, in the 1989 World Series–the so-called Battle of the Bay. It was just after 5pm on a beautiful October afternoon, and the nation’s eyes were glued on television coverage from Candlestick Park in southeastern San Francisco. Hordes of Bay Area residents had swarmed bars and gathered at home after early departures from their offices in order to watch a game so crucial to everyone’s pride in the rivalry.

Evening news had begun, and classes were still in session at UC Santa Cruz, 70 miles down the peninsula. At 5:04 and 15 seconds, residents of the forested mountains around Santa Cruz were thrown from their places by a giant lurch of the ground. A sizable fault plane adjacent to the San Andreas beneath the Santa Cruz Mountains had slipped, although the rupture didn’t make it to the ground surface. As the sides of the fault ground past each other, seismic waves emanated westward toward the coast, and rocked the city of Santa Cruz.

USGS animation of shaking intensity produced by the Loma Prieta earthquake

Downtown Santa Cruz was devastated, and people fled buildings on the UC campus, all before cities up the peninsula had even an inkling of an idea that this earthquake was underway. As the seismic waves raced northward through San Francisco, BART operators were ordered to halt their trains, and Candlestick Park started bouncing. With scores of miles separating them from the epicenter, the baseball fans had the roaring rumble of P-waves to herald the quake’s violent shaking, but with nowhere to go and not nearly enough time, the stadium erupted into screams as S-waves rippled through and yanked the decks back and forth. As TV transmission cut out from the stadium, the rest of the country stared shocked and agape at the World Series logo silently standing in on their screens.

Meanwhile the seismic waves continued racing outward to the east, high frequencies getting stifled with every additional kilometer, until they began rocking Sacramento and the Central Valley, many minutes after they’d wreaked their havoc in SF.

Rocking from this earthquake was felt as far away as Nevada and Los Angeles, but it was a relatively modest event when compared with the earthquakes northern California’s major faults are capable of.

"Did You Feel It?" survey results for the 1989 quake

This earthquake–named Loma Prieta after the hills nearest its epicenter–was a truly remarkable event made famous by a still rather unbelievable coincidence of conditions: Hitting the urban population of northern California alone would have made it remarkable, but it did so during live nationwide TV coverage of a major sporting event;  that would have been remarkable in itself, but the baseball game in question was between both of the Major League teams from the region, and deeply involved all of their local fans.

Failure of the Bay Bridge during a 6.9 earthquake 70 miles away... Image courtesy USGS

Gradually (surely aided by the wealth of attention already focused on the Bay Area for the World Series game) information started to spread about the extent of this quake’s effects. The waves had bounced around, slowed down, sloshed, and churned in the poorly consolidated sediments and made-land beneath San Francisco’s Marina District and the industrial coastal stretches of Oakland and the east bay, collapsing houses and double-decker roadways, and igniting fires that for a while brought back visions of 1906.

The eastern span of the Bay Bridge had wriggled and stretched, and during a particularly tensile yank, a segment of the upper deck was pried away and flopped down onto the eastbound deck below.

These details may describe familiar sights to many people. Certainly some of the footage from the Loma Prieta earthquake is the quintessential earthquake footage for many Americans far from significant seismic hazard and exposed to earthquakes only through their interest in baseball.

As you re-live that moment through some of this footage, the most important thing to note is that this was not San Francisco’s earthquake. It was largely Santa Cruz’s. The quake was far south of the peninsula, and had a relatively modest magnitude of 6.9, about an order of magnitude smaller than the real Big One in 1906. The havoc wreaked in 1989 doesn’t come close to illustrating the real effects of a large rupture on one of the faults bounding the Bay Area. Fortunately we can use modern instruments and computing power to compare the Loma Prieta earthquake with simulations of potential earthquakes on both the Hayward and San Andreas faults. We have maps showing the shaking hazard as well as other associated hazards, including landsliding and liquefaction. The USGS has a neat zoomable map of liquefaction susceptibility in the Bay Area–check out where your house stands:

http://geomaps.wr.usgs.gov/sfgeo/liquefaction/susceptibility.html

The 1989 failure of the Bay Bridge spurred the construction of an entirely new bridge span that is soon to be completed, and implements some excellent (and super cool) earthquake-savvy devices. (Watch the video at the link below)

http://baybridgeinfo.org/projects/corridor-overview/seismic-innovations#.TpyuNZyppWo

As is the nature of earthquake science, earthquakes that occur are our primary way about learning what to expect and what to do in the future. San Francisco is likely to have bigger and closer earthquakes than this one, so it’s best to be prepared and know what you’ll be dealing with. Try ShakeOut!

105th Anniversary of SF’s 1906 quake

5:12 a.m. PDT this morning marked the 105-year anniversary of “the San Francisco earthquake,” the >400 km rupture of the northern San Andreas fault that began just offshore and propagated along the fault in both directions for several minutes, all the while churning and twisting the blossoming city above it.

Maximum shaking intensity as a result of the 1906 earthquake: the results of a large concerted effort by the USGS and several California universities to simulate major earthquake ruptures throughout the state

It’s helpful to take a moment and reflect upon the reality of the event. I get the impression that such high-profile events lose their meaning as we talk so commonly about them. “The 1906 quake” is surely a part of the California lexicon, if not the country or world, however it still stuns me to realize that this event did happen. Just a century ago the gleaming city of San Francisco was shaken to its core and burned even further. We have come a long way (basically the whole way) in understanding earthquakes since then, and we now construct buildings with mitigation of earthquake risk in mind, but in America we have yet to be tested by another such earthquake so near a huge population center.

To help transcribe the impact of that turn-of-the-last-century event into modern consciousness, my friend John McDaris among a group at Carleton College’s Science Education Resource Center compiled an awesome array of informative and illustrative resources for the quake’s centennial 5 years ago. These include photos, videos, maps, and–most importantly–animations. I encourage eye-opening perusal of all of them, but especially the animations of modern structures subjected to the shaking of the 1906 earthquake.

An evocative screenshot from a simulation of the Golden Gate Bridge as it would have been shaken by the actual 1906 earthquake. Fortunately the display is exaggerated 100 times for illustration

Have at it! Happy anniversary.

[Update: 4/18/11 3:33pm] In an amusing little fluke of nature (or is it? –it is.) San Francisco had a nice little 3.4 jolt today, centered squarely within the San Andreas fault zone just south of the city. Mother Nature’s tip-o-the-hat to all our commemorations, if you want to think of it that way. To my utter disappointment it didn’t ripple the ground quite hard enough to make it rattle Davis, but plenty o’ San Franciscans got a mid-day jostling.

USGS "shakemap" showing instrumentally recorded shaking amplitude during this afternoon

Don’t make too much of this; M3-4 earthquakes are par for the course in the Bay Area: just as likely to occur any other day. Plenty more have happened not on major anniversaries.

%d bloggers like this: