How strong a quake? Evo­lu­tion of a sci­ence

The Richter scale is gone. It’s now a mat­ter of mag­ni­tude and in­ten­sity.

Los Angeles Times - - THE WORLD - By Rong-Gong Lin II ron.lin@la­

We know an earth­quake is se­vere when it has a high mag­ni­tude num­ber. The 1985 quake that dev­as­tated Mex­ico City was an 8. The quake that shook central Mex­ico this week was a 7.1.

But what ex­actly is mag­ni­tude?

Ba­si­cally, mag­ni­tude is a num­ber rep­re­sent­ing the to­tal en­ergy re­leased in an earth­quake, said seis­mol­o­gist Lucy Jones. The en­ergy re­leased is de­ter­mined by how much rock moves and how far it moves.

Here’s an ex­plainer for what mag­ni­tudes mean, and how that re­lates to the shak­ing we feel.

How does mag­ni­tude re­late to how much en­ergy was pro­duced in an earth­quake?

For each whole-num­ber in­crease in mag­ni­tude, the seis­mic en­ergy re­leased in­creases by about 32 times. That means a mag­ni­tude 7 earth­quake pro­duces 32 times more en­ergy — or is 32 times stronger — than a mag­ni­tude 6.

A mag­ni­tude 8 re­leases 1,000 times more en­ergy than a mag­ni­tude 6, but it re­leases that en­ergy over a larger area and for a longer time, Jones said.

Orig­i­nally, the def­i­ni­tion of mag­ni­tude re­lated to seis­mo­grams, in which ma­chines used an ink sty­lus to record rapid mo­tions on a rolling drum of paper that would mea­sure shak­ing. Mag­ni­tude was about how big the waves were on a seis­mo­gram at a par­tic­u­lar dis­tance from the epi­cen­ter. On the so-called Richter scale, a mag­ni­tude 8 on a seis­mo­gram was 10 times big­ger than a mag­ni­tude 7.

But the Richter scale was even­tu­ally scrapped in fa­vor of what is known as the mo­ment mag­ni­tude scale. The mo­ment mag­ni­tude scale measures the move­ment of rock along the fault, and ac­cu­rately measures larger earth­quakes, which can last for min­utes and af­fect a much larger area; the Richter scale did not ac­cu­rately record such quakes, Jones said.

The U.S. Ge­o­log­i­cal Sur­vey has a cal­cu­la­tor that can help you make th­ese cal­cu­la­tions. So, for in­stance, a mag­ni­tude 8.2 — prob­a­bly the strong­est earth­quake that could hit South­ern Cal­i­for­nia on the San An­dreas fault — would pro­duce an as­ton­ish­ing 178 times more en­ergy than the mag­ni­tude 6.7 Northridge earth­quake in 1994.

What are the earth­quake mag­ni­tudes to worry about?

Mag­ni­tude is im­por­tant, but a key fac­tor is where a quake strikes. The ones most peo­ple care about strike un­der or near heav­ily pop­u­lated ar­eas.

Gen­er­ally, earth­quakes of mag­ni­tude 6 and above are the ones for con­cern. When nearby, they can cause shak­ing in­ten­si­ties that can be­gin to break chim­neys and cause con­sid­er­able dam­age to the most seis­mi­cally vul­ner­a­ble struc­tures, such as non-retro­fit­ted brick build­ings.

Earth­quakes of mag­ni­tude 7 and above can over­turn heavy fur­ni­ture and in­flict con­sid­er­able dam­age in or­di­nary build­ings.

OK, so I un­der­stand that mag­ni­tudes 6s, 7s and 8s can be bad. What does that mean in terms of how bad the shak­ing will be where I’m lo­cated?

Let’s con­sider the 1994 Northridge earth­quake.

That earth­quake was a mag­ni­tude 6.7. But the shak­ing in­ten­sity var­ied by where you were when it hit.

If you were in Northridge, right on top of where the earth­quake fault moved, you faced what’s known as in­ten­sity 9 shak­ing, de­fined by the Mod­i­fied Mer­calli In­ten­sity scale as “vi­o­lent” — enough to shift a struc­ture off its foun­da­tion.

Yet the down­town L.A. re­gion got far less in­tense shak­ing from the same earth­quake — in­ten­sity 6 or 7, where dam­age is neg­li­gi­ble in build­ings of good de­sign and con­struc­tion.

Let’s say a mag­ni­tude 7.8 earth­quake strikes the San An­dreas fault in South­ern Cal­i­for­nia. What in­ten­sity shak­ing would South­ern Cal­i­for­nia feel?

Vast swaths of South­ern Cal­i­for­nia would feel in­ten­sity 10 shak­ing, or ex­treme shak­ing — an earth­quake that no one alive to­day has ex­pe­ri­enced in this re­gion. I’ve never heard of this in­ten­sity scale. Can you de­scribe it more?

Yes. Here’s the USGS’ ex­pla­na­tion:

In­ten­sity 10: Ex­treme — Some well-built wooden struc­tures de­stroyed; most ma­sonry and frame struc­tures de­stroyed with foun­da­tions. Rails bent.

In­ten­sity 9: Vi­o­lent — Dam­age con­sid­er­able in spe­cially de­signed struc­tures; well-de­signed frame struc­tures thrown out of plumb. Dam­age great in sub­stan­tial build­ings, with par­tial col­lapse. Build­ings shifted off foun­da­tions.

In­ten­sity 8: Se­vere — Dam­age slight in spe­cially de­signed struc­tures; con­sid­er­able dam­age in or­di­nary sub­stan­tial build­ings with par­tial col­lapse. Dam­age great in poorly built struc­tures. Fall of chim­neys, fac­tory stacks, col­umns, mon­u­ments, walls. Heavy fur­ni­ture over­turned.

In­ten­sity 7: Very strong — Dam­age neg­li­gi­ble in build­ings of good de­sign and con­struc­tion; slight to mod­er­ate in well-built or­di­nary struc­tures; con­sid­er­able dam­age in poorly built or badly de­signed struc­tures; some chim­neys bro­ken.

In­ten­sity 6: Strong — Felt by all, many fright­ened. Some heavy fur­ni­ture moved; a few in­stances of fallen plaster. Dam­age slight.

In­ten­sity 5: Mod­er­ate — Felt by nearly ev­ery­one; many awak­ened. Some dishes, win­dows bro­ken. Un­sta­ble ob­jects over­turned. Pen­du­lum clocks may stop.

In­ten­sity 4: Light — Felt indoors by many, out­doors by few dur­ing the day. At night, some awak­ened. Dishes, win­dows, doors dis­turbed; walls make crack­ing sound. Sen­sa­tion like heavy truck strik­ing build­ing. Stand­ing mo­tor cars rocked no­tice­ably.

In­ten­sity 3: Weak — Felt quite no­tice­ably by per­sons indoors, es­pe­cially on up­per floors of build­ings. Many peo­ple do not rec­og­nize it as an earth­quake. Stand­ing mo­tor cars may rock slightly. Vi­bra­tions sim­i­lar to the passing of a truck. Du­ra­tion es­ti­mated.

In­ten­sity 2: Weak — Felt only by a few per­sons at rest, es­pe­cially on up­per floors of build­ings.

In­ten­sity 1: Not felt — Not felt ex­cept by a very few un­der es­pe­cially fa­vor­able con­di­tions.

Re­mem­ber when peo­ple used to say “Richter scale” when de­scrib­ing mag­ni­tude? What hap­pened to the Richter scale?

The USGS switched its def­i­ni­tion of mag­ni­tude from the Richter scale to the mo­ment mag­ni­tude scale in the late 1990s, Jones said.

The Richter scale, de­vel­oped in the 1930s, was named af­ter American seis­mol­o­gist and physi­cist Charles Richter.

Paul Sakuma As­so­ci­ated Press

IN­TER­STATE 880 in Oakland col­lapsed in the 1989 Loma Pri­eta earth­quake, which had a mag­ni­tude of 6.9

John Down­ing Getty Images

THE MEX­ICO CITY earth­quake in 1985 was a mag­ni­tude 8, de­mol­ish­ing many build­ings in the cap­i­tal, above. This week’s quake in central Mex­ico mea­sured 7.1.

U.S. Ge­o­log­i­cal Sur­vey

THE 1964 earth­quake off the Alaskan coast was among the largest ever recorded — a mag­ni­tude 9.2. It buck­led this stretch of Se­ward High­way near An­chor­age.

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