Ancient Earthquakes at Lake Lucerne

A recent survey of the sediments beneath a Swiss lake reveals a series of prehistoric temblors

Michael Schnellmann, Flavio S. Anselmetti, Domenico Giardini, Judith A. McKenzie and Steven N. Ward

“On Tuesday the 18th of September, land, who amply documented the cata- standing of the recurrence times of rare 1601, shortly before two o’clock in the strophic events that followed one of the but strong events. morning, a strong and truly frightening strongest known earthquakes in central Up until just recently, the catalogue earthquake hit the region around Europe. That temblor caused consider- of past earthquakes was based exclu- Lucerne.… Nobody could remember a able damage over much of Switzerland sively on seismographic measurements similar event and even chronicles do and was felt in the parts of France, Ger- and historic documents. In Switzer- not document that the city ever experi- many and Italy. Seismologists estimate land, the first seismograph was in- enced a similar occurrence.” So begins that this quake, had it been recorded stalled in 1911, and the written record the eyewitness report of Renward with modern instruments, would have covers just the last millennium. Thus Cysat, a city clerk in Lucerne, Switzer- ranked something like 6.2 on the the two main sources of information, Richter scale, which would put it on while valuable, are insufficient to iden- Michael Schnellmann is currently pursuing a doc- par with many damaging earthquakes tify places where strong earthquakes torate in geology at the Swiss Federal Institute of that have struck near Los Angeles or strike, say, every few thousand years. Technology (ETH) in Zurich. His thesis advisor, San Francisco in recent times. This shortcoming is a real concern, be- Flavio S. Anselmetti received his Ph.D. from ETH Of course, Californians expect the cause such lengthy intervals between in 1994. He then spent four years in postdoctoral ground to shake now and again. But large earthquakes is typical for regions training at the University of Miami. There Ansel- honestly, who imagines earthquakes such as Switzerland, which are located metti gained considerable experience studying ma- threatening Switzerland? Even the away from the edges of tectonic plates, rine sediments before he returned to his native Swiss, who frequently take precautions where most seismic activity takes place. Switzerland and joined the Geological Institute at against avalanches or floods, rarely The only method to document the ETH, where he currently heads the limnogeology consider the possibility of earthquakes past occurrence (and possible recur- laboratory. Domenico Giardini received his Ph.D. at the University of Bologna in 1987. Since 1997 he in their neighborhood. Yet digging rence) of strong earthquakes in such lo- has been a professor of seismology and geodynamics back far enough into the historical cales is to extend the catalogue of at the Institute of Geophysics at ETH and the direc- record, one finds that Switzerland has known events to prehistoric times. Al- tor of the Swiss Seismological Service. Between in fact experienced several strong though our stone-age forebears left no 1992 and 1999, he served as director of the Global earthquakes, ones that brought about description of ancient earthquakes, na- Seismic Hazard Assessment Program for the UN’s considerable damage to property and ture has recorded much of what took International Decade for Natural Disaster Reduc- loss of life. The 1601 Lucerne event is place. One just needs to uncover and tion. Judith A. McKenzie obtained a doctorate in ge- just one example. Another is an earth- ology from ETH in 1976. During her training in quake near Basel in 1356, which de- Figure 1. Although many of the people liv- Zurich and later at the University of Florida in ing and working around Lake Lucerne are stroyed large parts of that city. Gainesville, she investigated the chemical and bio- keenly aware of certain natural hazards, such chemical sedimentation in both modern lakes and The Basel quake remains the as avalanches and landslides, few think very oceans. In 1996 she became a professor of earth sys- strongest one ever observed in central much about the possibility that a large earth- tem sciences at the Geological Institute at ETH. Europe. A similar event today would quake might strike. Yet powerful temblors Steven N. Ward received a Ph.D. in geophysics kill and injure many people, and it have indeed hit this region. One such event from Princeton University in 1978, where he spe- would cause widespread and costly took place in 1601 and caused considerable cialized in seismic wave theory. After post-doctoral destruction to property. It is, however, damage. There are no other historical records stints at Scripps Institution of Oceanography and difficult to decide how much time and of damaging earthquakes in this area, which Harvard University, he joined the research faculty money should be invested in earth- has made it quite difficult to gauge the likeli- of the University of California, Santa Cruz in quake precautions without knowing hood of a recurrence. To better estimate that 1984. Ward creates computer simulations of dy- threat, the authors undertook to study the how large or frequent future quakes namic Earth processes such as earthquakes, sediment that accumulates beneath the lake. tsunamis and tectonic deformations. Address for are likely to be. This is why we are try- Their examination of this natural archive re- Schnellmann: Geological Institute, ETH Zurich, ing to help assess Switzerland’s little- vealed that four other large quakes have dis- Sonneggstrasse 5, CH-8092 Zurich, Switzerland. known seismic hazards, an effort that rupted this scenic locale during the past sev- Internet: [email protected] requires, at minimum, a good under- eral millennia.

38 American Scientist, Volume 92 © Luftbild Schweiz www.americanscientist.org 2004 January–February 39 great mass of soupy mud successively broke several submarine telephone ca- bles lying in its path. Knowing of such events, we rea- soned that the bottom sediments of various Swiss lakes would similarly have recorded past episodes of seismic shaking. It took us a while to test this idea, but after much field and laborato- ry work probing the depths of Lake Lucerne, we were able to discern that four significant prehistoric earthquakes had indeed shaken this locale. Here we would like to recount in some detail how we mounted our investigation.

A History Lesson Our first task in putting together our Figure 2. Although most seismic activity takes place at the boundaries between tectonic plates research program was to consider just (pink lines), earthquakes of substantial size sometimes strike well within plate interiors. The red what sort earthquake signature would dots shows the location of earthquakes with magnitudes of 5.5 or more that have taken place since 1973. (Plate boundaries are from Rice University’s Discovering Plate Boundaries Project; be left in the lake sediments. For that, earthquake epicenters are from the catalog of the U.S. Geological Survey’s National Earth- Cysat’s report of the 1601 quake was quake Information Center.) invaluable. On the morning following the quake, he and his fellow city offi- interpret the hidden geologic archives Land O’Lakes cials rode on horseback across the to glean information about seismic As any geologist will attest, lake sedi- strand while assessing the damage. He events in the distant past. ments provide some of the most sensi- noted the chaotic scene in his chronicle: Paleoseismologists—the scientists tive records of past environmental con- Along the shore of the lake we ob- who specialize in tracking prehistoric ditions, and happily for us, Switzerland served ships, timber, planks, tubes earthquakes—often take advantage of is a country famous for its many majes- and other matters that were not the fact that moderate to strong seismic tic lakes. Such sediments are especially only drifting in the lake, but have shaking leaves characteristic traces at valuable because they accumulate con- been washed ashore and became or immediately below ground level. tinuously, year by year, and thus con- deposited 50 paces [40 to 50 me- Thus many of our colleagues in this tain a complete and often highly de- ters] behind the regular shoreline subfield of geology regularly dig tailed record of past events since the and up to two halberds [three to trenches across the surface of active lake came into existence. In the parts of four meters] above lake level…. faults, a procedure that allows them to Lake Lucerne that we studied, a little Closer towards the city, we saw measure the offsets and timing of an- less than a millimeter of sediment ac- people collecting fishes that were cient quakes. This strategy does not, cumulates each year—and has done so thrown onshore…. In Lucerne the however, work very well in areas that for the several millennia. The composi- ships were torn off the piers and are distant from plate boundaries, tion of this material reflects much about became pushed far out into the where surface ruptures are rare and local conditions at the time of deposi- lake. They were drifting rapidly difficult to identify. In such intra-plate tion. Pollen wafted into the lake, for ex- although they were driven neither regions, one does better to study fea- ample, becomes buried in the muddy by wind, nor rudder or sails…. tures that record earthquake shaking at sediment at the bottom, thus recording Preternaturally, the big river one place regardless of the exact posi- the changing nature of nearby vegeta- Reuss [the normal outflow of tion of the causative fault. tion. And coarse-grained layers docu- Lake Lucerne] flew forth and back Fortunately, there are many sec- ment times when ancient floods swept six times in an hour. ondary effects of ground shaking: Sta- sandy debris into the lake. lactites in caves can break, precarious- Earthquakes, too, can leave perma- Cysat further recorded that several ly poised boulders can topple over, nent traces on the floors of lakes and times the water in the river separating steep slopes can become unstable, and oceans, because they often send sedi- the two parts of the city disappeared al- sandy soils can be forced to flow as a ments tumbling down the submerged most completely, so that “one could liquid. Some of these happenings can, slopes at the margins of these bodies cross the riverbed almost by barely get- however, come about for other rea- of water. The most famous example of ting wet feet, as numerous young peo- sons: A stalactite may break under its this nature is probably the Grand ple did to commemorate this extraordi- own weight, for example, or heavy Banks earthquake of 1929, which had a nary event…. Also the [water-driven] rain may induce a landslide. So the Richter magnitude of 7.2 and triggered mills stopped working.” Cysat also not- main task of paleoseismologists is to a giant submarine slide offshore of ed that “subaqueous mountains and distinguish the triggering mechanism Newfoundland. The sudden flow of hills one could see and reach with bars and, once a seismic event has been sediments down the continental slope during low lake were broken apart and identified, to date the structures it caused a destructive tsunami and cut sucked down into the depth of the leaves in its wake. off transatlantic communication as a lake” and “pieces of meadows were

40 American Scientist, Volume 92 figure 4 bottom figure 4 top 0 depth

two-way traveltime two-way 0 1 2 N kilometers

Figure 3. Structural details in the sediments of Lake Lucerne could be charted using reflection seismic profiling. This method uses an acoustic transducer affixed to the hull of a survey boat to emit a pulse of sound en- ergy, which partially reflects from the lake bottom and from the interfaces between sedimentary layers of distinctive composition (far left). The same transducer picks up these faint echoes, which are recorded elec- tronically. Repeating this sounding procedure at closely spaced intervals and plotting the results side by side (left) produces a facsimile of the structure within the lake-bottom sediments. The authors conducted seis- mic profiling while cruising some 300 kilometers over a zigzag course that allowed them to chart the western portion of the lake (top). moved over more than a stone’s throw Switzerland, Giardini needed to know sel that ETH operates on Lake Lucerne off their original positions and deep where, when and how often big earth- and started to explore the bottom sedi- gaps opened in the ground.” quakes had taken place. When Ansel- ments using reflection seismology. This Reading his account, it was easy for metti reported the findings for Lake method is similar to the sonography us to imagine that these dramatic Lucerne, Giardini quickly appreciated that is used in medicine: Just as doc- events would have left permanent that these slide deposits could be tors are able to peek inside human marks on the floor of the lake. And in- viewed as so many smoking guns, in- bodies with ultrasound, geologists can deed, we were reasonably sure that dicators of hitherto unknown earth- image the internal structure of the sed- they did, because in the early 1980s quakes of Switzerland’s distant past. iments that collect beneath bodies of members of the limnogeology labora- This minor epiphany sparked what re- water by sending sound waves down- tory at the Swiss Federal Institute of mains a close collaboration between ward from the surface and recording Technology in Zurich (which often the Swiss Seismological Survey and the the faint echoes that return. In this case, goes by its German acronym ETH) had ETH limnogeology laboratory. the acoustic transducer (which acts as discovered two large deposits at the Having worked out the goal and both a loudspeaker and microphone) bottom of the lake, the result of sub- strategy of this research project, is mounted on the hull of a ship, where aqueous mudslides, which they be- McKenzie, Anselmetti and Giardini just it transmits an acoustic signal into the lieved the 1601 quake had caused. needed to find a doctoral student, who, water. Some of this sound energy In 1996, soon after taking the reins of course, they hoped would do the bounces back from the floor of the lake to the limnogeology laboratory, one of heavy lifting. So they approached an- and from distinct depositional layers us (McKenzie) eagerly picked up on other one of us (Schnellmann), just after within the lake-bottom muds. The re- this line of research. Together with an- he returned from doing fieldwork in a flected signal, which is picked up with other one of us (Anselmetti), she dis- borax mine in Turkey. On a boat trip on the same acoustic transducer and covered numerous slide deposits, Lake Lucerne, Anselmetti convinced recorded aboard the research vessel, many of them deeper (and thus older) Schnellmann to sign on and help un- thus carries information about the than the ones previously studied. It ravel the secrets hidden below the sur- structure of the sediments. was thus clear that these older features face of the lake. Although Schnellmann Over many days of fieldwork, we must have been deposited in prehis- wondered a bit whether this attempt at gathered reflection-seismic data over a toric times and that if one could distin- tracking ancient earthquakes would distance that measured more than 300 guish slides triggered by earthquakes prove to be an interesting and mean- kilometers in all. Following a zigzag from slides brought about by other ingful topic for the beginning of a sci- course, we collected soundings over a processes, these ancient lake deposits entific career, the prospect of spending dense grid of survey lines, which al- would provide an earthquake history the next few years cruising scenic Lake lowed us to piece together a three- of the area for the past 15 millennia. Lucerne (instead of breathing dust in a dimensional picture of the lake-bottom Coincidently, at this time another distant borax mine) made the decision sediments after we returned to the lab. one of us (Giardini), director of the easy enough. In pouring over our data, we found Swiss Seismological Service, was look- much evidence for subaqueous mass ing for just such an extended catalogue Fishing for Answers movements. These traces were either of earthquakes. Being responsible for In June, 2001, Schnellman and Ansel- failure scars on slopes, marking where the assessment of seismic hazards for metti procured use of the research ves- large chunks of material had fallen www.americanscientist.org 2004 January–February 41 south north

10 meters

200 meters

west east

10 meters

200 meters

Figure 4. Seismic reflection profiles obtained over two cruise tracks reveal distinctive deposits indicative of slumping along the margins of the lake (colored zones). The top panel shows a north-to-south section that cuts through the center of a large deep basin (red line at right in the top panel of Figure 3). The bottom panel shows an east-to-west section near the western margin of the lake (red line at left in Figure 3). The charac- ter of the seismic pattern within the colored zones is somewhat chaotic, like a static-filled television screen, whereas the normal sedimentary layers outside them produce continuous light and dark lines. Guided by these images and many of the other seismic sections they obtained for Lake Lucerne, the authors retrieved sediment cores from key positions, including the center of the large basin (black line in upper panel). This roughly 10-meter-long core penetrated three slump-induced deposits of different ages (pink, purple, green). away, or buried slide deposits, where distinct basins, and we decided not to we found that in the center of two well- the slumped material had settled. Such examine those with adjacent river separated sub-basins, these slide de- accumulations can easily be recognized deltas, which we knew could be the posits are overlain by layers of homo- on the seismic sections because nor- source of slump deposits that didn’t geneous mud that are as much as two mal, undisturbed lake sediments show have anything to do with earthquakes.) meters thick—a result, no doubt, of a clear horizontal layering, whereas the At this point we realized that many of great mass of sediment that was held intensely reworked slide deposits give the newly discovered deposits were at in suspension in the waters of the lake a chaotic signature resembling a static- exactly the same level as the ones the for a short time after the quake before filled TV screen. former ETH lake research group had al- finally settling out. Having dozens of closely spaced ready detected and associated with the Figuring that prehistoric earth- cross-sections in hand made it easy to 1601 earthquake. Indeed, the horizon quakes with magnitudes equal or larg- track prominent layers and to map the corresponding to this event contains at er than the 1601 event would have left extent of individual slumps throughout least 13 widespread slumps, indicating similar deposits, we examined the the fraction of the lake that we had sur- that this quake triggered synchronous seismic cross-sections with great care. veyed. (Lake Lucerne contains several sliding all over the lake. What is more, And we quickly discovered a horizon

42 American Scientist, Volume 92 some three meters below the floor of example), and none of those caused the lake that contains 16 individual multiple slope failures around the mar- slumps. Thick, homogenous mud bod- gins of the lake. So we were clearly see- ies sit directly on top of these deposits ing the results of big earthquakes in in three different sub-basins. We thus our seismic records. suspected that we were seeing the ves- When did these quakes happen? tiges of some enormously violent pre- This question has proved easier to an- historic quake. swer—although doing so required us Still, we wondered a bit at the time to do a lot more fieldwork. To attach 1601 A.D. whether all this slumping could have dates to these events, we needed to re- event happened for a more mundane reason. cover sediment samples from the vari- But we made an observation that gave ous slide deposits, which lie deep be- us confidence that an earthquake was low the floor of the lake, which itself is indeed at work here: Remnants of submerged under some 150 meters of these ancient mud slides were not only water. We therefore went back to the found at the foot of the slopes that line lake with a small pontoon boat, really a the margins of the lake, but they also raft, and a specially designed sampling showed up next to two submerged device called a Kullenberg corer—in hills, one that crests about 85 meters this case a 12-meter-long tube of steel below the surface of the lake. More with a 300-kilogram lead weight on commonplace events, such as storm- top. To take sediment cores, we slowly induced waves or pervasive flooding lowered this ungainly probe through could conceivably trigger slides the water toward the bottom using a around the margin of the lake, but steel cable attached to a powerful they would not have affected the sta- winch. When the corer reached 10 me- bility of lake-bottom slopes that are far ters above the lake floor, a triggering from the shore and under 85 meters of mechanism allowed it to fall freely the water. There was no doubt about it: rest of the way. The tube was thus dri- 470 B.C. We had found the traces of an ancient ven deep into the sediments, filling it event earthquake. with mud, which was held in place by a springy device on the business end Sizing Up the Catch of the corer that prevents the sediment Further probing of our seismic records from sliding back out. rapidly turned up evidence for three Guided by our many seismic cross- more prehistoric quakes of significant sections, we retrieved sediment cores magnitude (that is, ones big enough to from various slide deposits as well as cause multiple slides). But just how from undisturbed sediments. After 7820 B.C. powerful were these ancient quakes? seven days of hard and sometimes event That has remained a difficult question messy work, we returned to our lab in to answer. We can’t even rely on what Zurich with eight sediment cores, each would at first blush seem a reasonable 8 to 10 meters in length, from two dif- surmise: that the bigger the earth- ferent sub-basins. quake, the more sediment gets shifted For geologists, the splitting and around. We hesitate to use such logic opening of recovered cores is a much- because the biggest slide deposit we anticipated event. And indeed, it identified in our studies (containing proved thrilling. Having the lake de- some 17,000,000 cubic meters of mud) posits revealed before our eyes was Figure 5. Examination of the retrieved sedi- was probably caused by a very small like traveling back in time, experienc- ment cores allowed the authors to confirm their interpretation of their seismic sections quake—or perhaps by an entirely dif- ing the various swings in climate and and to obtain organic material suitable for ferent process. We believe this to be changes in local vegetation, seeing the age dating. Images of the sediment recovered true because this huge slump repre- tangible evidence of ancient storms from the position shown in the previous fig- sents an isolated occurrence; no other and floods—and, of course, earth- ure (right, here placed in their proper strati- slope failures took place at that time quakes. They showed themselves as graphic position and stretched horizontally elsewhere in the lake. tortuously folded slide deposits with for clarity) show a good correspondence with Although we can’t estimate the overlying beds of homogeneous mud. the results obtained from seismic profiling at magnitude of these quakes in any These distinctive packages stood out this position (left). The pink layer is a slump quantitative way, we can be reasonably clearly from the thin horizontal layers deposit laid down in 1601, during and shortly sure that they must have been fairly found elsewhere in the sediment. after a historic earthquake. The purple layer shows a thinner slump-induced deposit that large ones. After all, this area of To find the age of each slide deposit, formed earlier, in 470 B.C. according to radio- Switzerland has experienced many we extracted leaves and small pieces of carbon dating. The green layer represents a small earthquakes over the past centu- wood from the undisturbed sediment yet-older event, one that produced only a sin- ry (some five events ranking at magni- directly overlying it. The age of this or- gle slump deposit and was thus probably not tude-5 or more have been recorded, for ganic material could then be deter- an earthquake. www.americanscientist.org 2004 January–February 43 mined using radiocarbon dating. Fur- ilar water movements in lakes have ther clues about the antiquity of these barely been studied. deposits came from two layers of vol- To better comprehend how earth- canic ash that we found in the sedi- quake-triggered mass movements in ments. We were able to tie these ash Lake Lucerne can generate dangerous- layers to prehistoric volcanic eruptions ly large waves, we modeled the tsuna- in eastern France and western Ger- mi-like effect of the subaqueous slide of many. Combining all the dates, for the 470 B.C. We chose to study this particu- organic material and for the two hori- lar event because we had mapped in zons of volcanic ash, we calculated good detail one of the places where the ages for the slide deposits and the four bottom had given way, the pathway of earthquakes that caused them: They sediment movement and the geometry happened in about 470 B.C., 7,820 B.C., of the resultant deposit, which, we pre- 11,960 B.C. and 12,610 B.C. We had suc- sumed, would allow for an accurate re- cessfully produced a timeline for pre- construction of this ancient disruption. historic seismic events in central Our reflection-seismic data showed Switzerland. that the slide broke loose leaving a 9- meter-high scar behind on the margin Kowabunga! of the lake, that it transported a volume Our study of the sediments of Lake of sediment equivalent to a giant cube Lucerne revealed a long history of seis- with edges 100 meters long and that mic shaking in the area, but it did not some of this mud moved as much as answer an important question raised 1,500 meters laterally. by Cysat’s account of the 1601 earth- The numerical model showed waves Figure 6. In 1687, this house, built on the quake: Why did the water in the lake higher than three meters striking the shore of Lake Lucerne, was damaged by a 4- shift as it did? Can underwater mud- shore opposite the site of failure within meter-high wave, which smashed through slides of the size we observed displace a minute after initiation of the slide. the windows on the first floor and flooded enough water to generate 4-meter-high The modeled waves had wavelengths the interior, turning over a table and knock- waves? And do such waves, which greater than a kilometer, which is en- ing down the landlord. The huge wave also might be considered tsunamis of sorts, tirely different from the situation with damaged a nearby village and several har- pose significant hazards to lakeshore ordinary wind-induced surface waves. bors. It arose during calm, clear weather, the communities? In this respect, the computer-simulated result of a spontaneous slumping of sedi- ments on the opposite shore, where a large To estimate the type and amplitude waves indeed resemble mountains of portion of a river delta suddenly disap- of waves one would expect, Schnell- water rising in the center of the sub- peared from view. This historic example man and Anselmetti approached an- basins, just as eyewitnesses to the shows that sediment slumps and their asso- other one of us (Ward), an expert in the events of 1601 long ago described. Try ciated waves do not require ground shaking numerical modeling of tsunamis. to imagine the tempestuous state of the to trigger. Hence for their paleoseismic in- These destructive water waves gener- lake at that time, with large chunks of vestigations, the authors concentrated on a ally result from large displacements of bottom sediment giving way at vari- portion of the lake far from major deltas and sediments at the sea floor. Whereas the ous points around the lake and the re- ascribed to earthquakes only those events occurrence of destructive tsunamis in sulting tsunamis superimposing. The that simultaneously left multiple slump de- the ocean has long been investigated water movements must truly have posits in their wake. and is relatively well understood, sim- been frightening.

historic (1601 A.D.) prehistoric (470 B.C.) earthquake earthquake

0 1 2 N kilometers

Figure 7. Historic earthquake of 1601 (left) resulted in many slump deposits, which range from less than 5 meters in thickness (yellow) to more than 15 meters (red). (Orange indicates where such deposits are between 5 and 15 meters thick.) In the deepest parts of the lake, these deposits are overlain by a layer of thick, homogeneous mud (hachures), the result of all the stirred-up material that was temporarily held in suspension within the waters of the lake. Discovery of a similar but older set of deposits (right) led the authors to conclude that a prehistoric earthquake must have triggered slumping at various points around the lake (arrows). One key observation was that some of these slumps were shed from the sides of submerged hills, which would not have been affected by more mundane triggering mechanisms, such as widespread flooding.

44 American Scientist, Volume 92 N

0 1 2 kilometers

–3 0 +3

meters

Figure 8. Numerical modeling illuminates how a single failure at the lake margin (hachured area) and its resulting slump deposit (yellow out- line), one of several known to have been caused by the 470 B.C. earthquake, would give rise to a tsunami-like disturbance on the surface. Unlike normal, wind-driven waves, the undulation in water level has an enormous wavelength, nearly a kilometer. It also has an enormous size: One minute after the margin of the lake gives way in the simulation, the peak-to-trough amplitude of the wave is almost 6 meters (left). The wave propagates rapidly away from the site of initiation, traveling about 2 kilometers into two of the arms of the lake during the next minute (cen- ter)—that is, with the speed of highway traffic. Three minutes after the simulated initiation, most of the disturbance is limited to the north- western limb of the lake (right). In actuality, the multiple slope failures at different points around the lake generated several waves of this type, making for what must have been a highly complex pattern on the surface as the various waves interfered.

In his 1601 report, Cysat indicated ty yet to discover. For example, if we Bibliography that the normal outflow of the Lake are ever to estimate earthquake epi- Heezen, B. C., and M. Ewing 1952. Turbidity underwent reversals, moving back centers and magnitudes, one lake will currents and submarine slumps, and the and forth six times in an hour. That is, not be enough; several paleoseismo- 1929 Grand Banks earthquake. American Journal of Science 250:849–873. the period of the water movement graphs will surely be required. Fortu- Korgen, B. J. 1995. Seiches. American Scientist was approximately 10 minutes. Curi- nately, in central Switzerland a net- 83:330–342. ously, this is more than 10 times work of prehistoric seismographs is Schnellmann, M., F. S. Anselmetti, D. Giardini, longer than the period of the virtual available, with each of the many lakes J. A. McKenzie and S. N. Ward. 2002. Pre- tsunamis in the numerical model. We there acting as an independent historic earthquake history revealed suspect that the 10-minute oscillations recorder. Because each lake responds by lacustrine slump deposits. Geology of lake level in 1601 arose only after slightly differently to shaking, the ef- 30:1131–1134. Siegenthaler, C., W. Finger, K. Kelts and W. some delay, the result of a resonance, fect of earthquakes on a specific lake Wang,. 1987. Earthquake and seiche de- as water sloshed back and forth across has to be calibrated using historic posits in Lake Lucerne, Switzerland. Eclogae the lake. The period of such resonant events. In collaboration with the Swiss Geologicae Helvetiae 80:241–260. movements of a large body of water, Seismological Survey, members of the Ward, S. N. 2001. Landslide tsunami. Journal of called seiche, depends on the geome- ETH limnogeology group are now fo- Geophysical Research 106:11201–11216. try of the basin (see “Seiches,” cusing their efforts on four smaller Ward, S. N., and S. Day. 2002. Suboceanic land- July–August 1995). Wind and changes lakes in the vicinity of Lake Lucerne, slides. In 2002 Yearbook of Science and Tech- nology. New York: McGraw-Hill. of atmospheric pressure are known to looking for the fingerprints of historic cause similar oscillations (with lower and prehistoric earthquakes in an ef- amplitudes). Such meteorologically fort that we hope will allow them to induced undulations in Lake Lucerne estimate epicenters and magnitudes. were first studied at the end of the Renward Cysat probably didn’t 19th century—revealing characteristic think that, four centuries after the ink 10-minute shifts, in addition to two had dried, his report would be the ba- longer periods of oscillation. So it sis of a seismological investigation, For relevant Web links, consult this is- makes good sense that earthquake-in- one that not only probed the event he sue of American Scientist Online: duced movements showed these peri- witnessed but also revealed more-an- ods too. cient earthquakes in central Switzer- http://www.americanscientist.org/ In all, we felt we were quite success- land. But maybe he wouldn’t have IssueTOC/issue/406 ful with our investigation of Lake been so surprised. After all he saw in Lucerne, both in understanding the the aftermath, perhaps he sensed that 1601 event and, most importantly, in a shaken lake could serve as a seis- using the lake sediments as prehistoric mometer of sorts, the best one avail- seismographs. But there remains plen- able for his era. www.americanscientist.org 2004 January–February 45