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to a truly grand…scale 9。5。 as you will gatherfrom this； measuring earthquakes is not always an exact science； particularly wheninterpreting readings from remote locations。 at all events； both quakes were whopping。 the1960 quake not only caused widespread damage across coastal south america； but also set offa giant tsunami that rolled six thousand miles across the pacific and slapped away much ofdowntown hilo； hawaii； destroying five hundred buildings and killing sixty people。 similarwave surges claimed yet more victims as far away as japan and the philippines。
for pure； focused； devastation； however； probably the most intense earthquake in recordedhistory was one that struck鈥攁nd essentially shook to pieces鈥攍isbon； portugal； on all saintsday （november 1）； 1755。 just before ten in the morning； the city was hit by a suddensideways lurch now estimated at magnitude 9。0 and shaken ferociously for seven full minutes。
the convulsive force was so great that the water rushed out of the city鈥檚 harbor and returnedin a wave fifty feet high； adding to the destruction。 when at last the motion ceased； survivorsenjoyed just three minutes of calm before a second shock came； only slightly less severe thanthe first。 a third and final shock followed two hours later。 at the end of it all； sixty thousandpeople were dead and virtually every building for miles reduced to rubble。 the san franciscoearthquake of 1906； for parison； measured an estimated 7。8 on the richter scale andlasted less than thirty seconds。
earthquakes are fairly mon。 every day on average somewhere in the world there aretwo of magnitude 2。0 or greater鈥攖hat鈥檚 enough to give anyone nearby a pretty good jolt。
although they tend to cluster in certain places鈥攏otably around the rim of the pacific鈥攖heycan occur almost anywhere。 in the united states； only florida； eastern texas； and the uppermidwest seem鈥攕o far鈥攖o be almost entirely immune。 new england has had two quakes ofmagnitude 6。0 or greater in the last two hundred years。 in april 2002； the region experienceda 5。1 magnitude shaking in a quake near lake champlain on the new york鈥搗ermont border；causing extensive local damage and （i can attest） knocking pictures from walls and childrenfrom beds as far away as new hampshire。
the most mon types of earthquakes are those where two plates meet； as in californiaalong the san andreas fault。 as the plates push against each other； pressures build up untilone or the other gives way。 in general； the longer the interval between quakes； the greater thepent…up pressure and thus the greater the scope for a really big jolt。 this is a particular worryfor tokyo； which bill mcguire； a hazards specialist at university college london； describesas 鈥渢he city waiting to die鈥潯。╪ot a motto you will find on many tourism leaflets）。 tokyo standson the boundary of three tectonic plates in a country already well known for its seismicinstability。 in 1995； as you will remember； the city of kobe； three hundred miles to the west；was struck by a magnitude 7。2 quake； which killed 6；394 people。 the damage was estimatedat 99 billion。 but that was as nothing鈥攚ell； as paratively little鈥攑ared with whatmay await tokyo。
tokyo has already suffered one of the most devastating earthquakes in modern times。 onseptember 1； 1923； just before noon； the city was hit by what is known as the great kantoquake鈥攁n event more than ten times more powerful than kobe鈥檚 earthquake。 two hundredthousand people were killed。 since that time； tokyo has been eerily quiet； so the strainbeneath the surface has been building for eighty years。 eventually it is bound to snap。 in 1923；tokyo had a population of about three million。 today it is approaching thirty million。 nobodycares to guess how many people might die； but the potential economic cost has been put ashigh as 7 trillion。
even more unnerving； because they are less well understood and capable of occurringanywhere at any time； are the rarer type of shakings known as intraplate quakes。 thesehappen away from plate boundaries； which makes them wholly unpredictable。 and becausethey e from a much greater depth； they tend to propagate over much wider areas。 themost notorious such quakes ever to hit the united states were a series of three in newmadrid； missouri； in the winter of 1811鈥�12。 the adventure started just after midnight on december 16 when people were awakened first by the noise of panicking farm animals （therestiveness of animals before quakes is not an old wives鈥櫋ale； but is in fact well established；though not at all understood） and then by an almighty rupturing noise from deep within theearth。 emerging from their houses； locals found the land rolling in waves up to three feet highand opening up in fissures several feet deep。 a strong smell of sulfur filled the air。 theshaking lasted for four minutes with the usual devastating effects to property。 among thewitnesses was the artist john james audubon； who happened to be in the area。 the quakeradiated outward with such force that it knocked down chimneys in cincinnati four hundredmiles away and； according to at least one account； 鈥渨recked boats in east coast harbors and 。
。 。 even collapsed scaffolding erected around the capitol building in washington； d。c。鈥潯njanuary 23 and february 4 further quakes of similar magnitude followed。 new madrid hasbeen silent ever since鈥攂ut not surprisingly； since such episodes have never been known tohappen in the same place twice。 as far as we know； they are as random as lightning。 the nextone could be under chicago or paris or kinshasa。 no one can even begin to guess。 and whatcauses these massive intraplate rupturings？ something deep within the earth。 more than thatwe don鈥檛 know。
by the 1960s scientists had grown sufficiently frustrated by how little they understood ofthe earth鈥檚 interior that they decided to try to do something about it。 specifically； they got theidea to drill through the ocean floor （the continental crust was too thick） to the mohodiscontinuity and to extract a piece of the earth鈥檚 mantle for examination at leisure。 thethinking was that if they could understand the nature of the rocks inside the earth； they mightbegin to understand how they interacted； and thus possibly be able to predict earthquakes andother unwele events。
the project became known； all but inevitably； as the mohole and it was pretty welldisastrous。 the hope was to lower a drill through 14；000 feet of pacific ocean water off thecoast of mexico and drill some 17；000 feet through relatively thin crustal rock。 drilling froma ship in open waters is； in the words of one oceanographer； 鈥渓ike trying to drill a hole in thesidewalks of new york from atop the empire state building using a strand of spaghetti。鈥�
every attempt ended in failure。 the deepest they penetrated was only about 600 feet。 themohole became known as the no hole。 in 1966； exasperated with ever…rising costs and noresults； congress killed the project。
four years later； soviet scientists decided to try their luck on dry land。 they chose a spot onrussia鈥檚 kola peninsula； near the finnish border； and set to work with the hope of drilling toa depth of fifteen kilometers。 the work proved harder than expected； but the soviets weremendably persistent。 when at last they gave up； nineteen years later； they had drilled to adepth of 12；262 meters； or about 7。6 miles。 bearing in mind that the crust of the earthrepresents only about 0。3 percent of the planet鈥檚 volume and that the kola hole had not cuteven one…third of the way through the crust； we can hardly claim to have conquered theinterior。
interestingly； even though the hole was modest； nearly everything about it was surprising。
seismic wave studies had led the scientists to predict； and pretty confidently； that they wouldencounter sedimentary rock to a depth of 4；700 meters； followed by granite for the next 2；300meters and basalt from there on down。 in the event； the sedimentary layer was 50 percentdeeper than expected and the basaltic layer was never found at all。 moreover； the world downthere was far warmer than anyone had expected； with a temperature at 10；000 meters of 180 degrees centigrade； nearly twice the forecasted level。 most surprising of all was that the rockat that depth was saturated with water鈥攕omething that had not been thought possible。
because we can鈥檛 see into the earth； we have to use other techniques； which mostly involvereading waves as they travel through the interior。 we also know a little bit about the mantlefrom what are known as kimberlite pipes； where diamonds are formed。 what happens is thatdeep in the earth there is an explosion that fires； in effect； a cannonball of magma to thesurface at supersonic speeds。 it is a totally random event。 a kimberlite pipe could explode inyour backyard as you read this。 because they e up from such depths鈥攗p to 120 milesdown鈥攌imberlite pipes bring up all kinds of things not normally found on or near thesurface： a rock called peridotite； crystals of olivine； and鈥攋ust occasionally； in about one pipein a hundred鈥攄iamonds。 lots of carbon es up with kimberlite ejecta； but most isvaporized or turns to graphite。 only occasionally does a hunk of it shoot up at just the rightspeed and cool down with the necessary swiftness to bee a diamond。 it was such a pipethat made johannesburg the most productive diamond mining city in the world； but there maybe others even bigger that we don鈥檛 know about。 geologists know that somewhere in thevicinity of northeastern indiana there is evidence of a pipe or group of pipes that may be trulycolossal。 diamonds up to twenty carats or more have been found at scattered sites throughoutthe region。 but no one has ever found the source。 as john mcphee notes； it may be buriedunder glacially deposited soil； like the manson crater in iowa； or under the great lakes。
so how much do we know about what鈥檚 inside the earth？ very little。 scientists aregenerally agreed that the world beneath us is posed of four layers鈥攔ocky outer crust； amantle of hot； viscous rock； a liquid outer core； and a solid inner core。
1we know that thesurface is dominated by silicates； which are relatively light and not heavy enough to accountfor the planet鈥檚 overall density。 therefore there must be heavier stuff inside。 we know that togenerate our magnetic field somewhere in the interior there must be a concentrated belt ofmetallic elements in a liquid state。 that much is universally agreed upon。 almost everythingbeyond that鈥攈ow the layers interact； what causes them to behave in the way they do； whatthey will do at any time in the future鈥攊s a matter of at least some uncertainty； and generallyquite a lot of uncertainty。
even the one part of it we can see； the crust； is a matter of some fairly strident debate。
nearly all geology texts tell you that continental crust is three to six miles thick under theoceans； about twenty…five miles thick under the continents； and forty to sixty miles thickunder big mountain chains； but there are many puzzling variabilities within thesegeneralizations。 the crust beneath the sierra nevada mountains； for instance； is only aboutnineteen to twenty…five miles thick； and no one knows why。 by all the laws of geophysics thesierra nevadas should be sinking； as if into quicksand。 （some people think they may be。）1for those who crave a more detailed picture of the earths interior； here are the dimensions of the variouslayers； using average figures： from 0 to 40 km （25 mi） is the crust。 from 40 to 400 km （25 to 250 mi） is theupper mantle。 from 400 to 650 km （250 to 400 mi） is a transition zone between the upper and lower mantle。
from 650 to 2；700 km （400 to 1；700 mi） is the lower mantle。 from 2；700 to 2；890 km （1；700 to 1；900 mi） is the〃d〃 layer。 from 2；890 to 5；150 km （1；900 to 3；200 mi） is the outer core； and from 5；150 to 6；378 km （3；200 to3；967 mi） is the inner core。
how and when the earth got its crust are questions that divide geologists into two broadcamps鈥攖hose who think it happened abruptly early in the earth鈥檚 history and those who thinkit happened gradually and rather later。 strength of feeling runs deep on such matters。 richardarmstrong of yale proposed an early…burst theory in the 1960s； then spent the rest of hiscareer fighting