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ed some kind ofbarrier。 from this he deduced that the earth has a core。 three years later a croatianseismologist named andrija mohorovi？i′c was studying graphs from an earthquake in zagrebwhen he noticed a similar odd deflection； but at a shallower level。 he had discovered theboundary between the crust and the layer immediately below； the mantle； this zone has beenknown ever since as the mohorovi？i′c discontinuity； or moho for short。
we were beginning to get a vague idea of the earth’s layered interior—though it really wasonly vague。 not until 1936 did a danish scientist named inge lehmann； studyingseismographs of earthquakes in new zealand； discover that there were two cores—an innerone that we now believe to be solid and an outer one （the one that oldham had detected） thatis thought to be liquid and the seat of magnetism。
at just about the time that lehmann was refining our basic understanding of the earth’sinterior by studying the seismic waves of earthquakes； two geologists at caltech in californiawere devising a way to make parisons between one earthquake and the next。 they werecharles richter and beno gutenberg； though for reasons that have nothing to do with fairnessthe scale became known almost at once as richter’s alone。 （it has nothing to do with richtereither。 a modest fellow； he never referred to the scale by his own name； but always called it“the magnitude scale。”）the richter scale has always been widely misunderstood by nonscientists； though perhapsa little less so now than in its early days when visitors to richter’s office often asked to seehis celebrated scale； thinking it was some kind of machine。 the scale is of course more anidea than an object； an arbitrary measure of the earth’s tremblings based on surfacemeasurements。 it rises exponentially； so that a 7。3 quake is fifty times more powerful than a6。3 earthquake and 2；500 times more powerful than a 5。3 earthquake。
at least theoretically； there is no upper limit for an earthquake—nor； e to that； a lowerlimit。 the scale is a simple measure of force； but says nothing about damage。 a magnitude 7quake happening deep in the mantle—say； four hundred miles down—might cause no surfacedamage at all； while a significantly smaller one happening just four miles under the surfacecould wreak widespread devastation。 much； too； depends on the nature of the subsoil； thequake’s duration； the frequency and severity of aftershocks； and the physical setting of theaffected area。 all this means that the most fearsome quakes are not necessarily the mostforceful； though force obviously counts for a lot。
the largest earthquake since the scale’s invention was （depending on which source youcredit） either one centered on prince william sound in alaska in march 1964； whichmeasured 9。2 on the richter scale； or one in the pacific ocean off the coast of chile in 1960；which was initially logged at 8。6 magnitude but later revised upward by some authorities（including the united states geological survey） 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—and essentially shook to pieces—lisbon； 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’s 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—that’s enough to give anyone nearby a pretty good jolt。
although they tend to cluster in certain places—notably around the rim of the pacific—theycan occur almost anywhere。 in the united states； only florida； eastern texas； and the uppermidwest seem—so far—to 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–vermont 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 “the city waiting to die” （not 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—well； as paratively little—pared with whatmay await tokyo。
tokyo has alread