Is­land gives that sink­ing feel­ing

A Stan­ford grad stu­dent says Catalina’s un­der­wa­ter his­tory could put the main­land at risk.

Los Angeles Times - - CALIFORNIA - RONG-GONG LIN II ron.lin@la­times.com

For decades, re­searchers have de­bated whether Santa Catalina is sink­ing or ris­ing.

Now there is new study that makes the case that the is­land is sink­ing, al­beit very slowly.

A Stan­ford Uni­ver­sity re­searcher says new un­der­wa­ter imag­ing sug­gests Catalina could be com­pletely sub­merged in 3 mil­lion years, though that re­mains in dis­pute. He also con­tends that the move­ment could pose a tsunami risk for Los An­ge­les and Or­ange coun­ties.

The study’s au­thor, Stan­ford grad­u­ate stu­dent Chris Castillo, said the po­ten­tial threat should be in­ves­ti­gated fur­ther. He said his con­clu­sions came from un­der­wa­ter images of Catalina made by Stan­ford Uni­ver­sity last year.

The imag­ing showed ev­i­dence of an­cient beaches that Castillo said have sunk be­low the ocean’s waves. The images also showed a large un­der­wa­ter land­slide that oc­curred off the is­land’s north­east­ern shore about 500,000 years ago.

If a sub­ma­rine land­slide hap­pened again, with tons of rocks and dirt plung­ing to­ward the ocean floor, that could cre­ate a tsunami head­ing to­ward the ports of Los An­ge­les and Long Beach and the Or­ange County coast, he said.

In fact, Castillo said his imag­ing sug­gests that the side of Catalina closer to the main­land is sink­ing faster, so the is­land is tilt­ing at a slight an­gle to­ward Long Beach and Or­ange County. That would make it more likely a land­slide could hap­pen again, Castillo said.

Fur­ther re­search is needed to bet­ter un­der­stand how big the tsunami would be, Castillo said. “It’s still some­thing that could do sig­nif­i­cant prop­erty dam­age, es­pe­cially for the mari­nas.... If you see some­thing that could be danger­ous, you need to find out more about it.”

The Cal­i­for­nia Ge­o­log­i­cal Sur­vey’s tsunami pro­gram manager, Rick Wil­son, agreed that a tsunami could have struck the coast from Catalina when the sub­ma­rine land­slide oc­curred about 500,000 years ago. But he said the agency was more in­ter­ested in sub­ma­rine land­slides in the last 10,000 years; a half-mil­lion-yearold de­posit is con­sid­ered very old.

“If it’s half a mil­lion years, it’s ... much older — some­thing that, maybe, it doesn’t have the po­ten­tial to re­peat right now,” Wil­son said. “We try to re­ally fo­cus on sources that hap­pened ei­ther in the last 10,000 years or have the po­ten­tial of oc­cur­ring again in the very near fu­ture.”

Castillo dis­agreed that there was less of a risk and said there could even be a higher risk be­cause there haven’t been land­slides in a long time. “The whole is­land is tip­ping, so there def­i­nitely is a risk,” he said.

Catalina was once part of the seafloor, Castillo said, but be­gan ris­ing from the ocean as a re­sult of height­ened seis­mic ac­tiv­ity on the nearby Santa Cruz-Santa Catalina Ridge fault zone. But about 1 mil­lion years ago, the fault zone be­came less ac­tive, re­sult­ing in the is­land’s grad­ual sink­ing, he said.

Castillo es­ti­mates Catalina is sink­ing an av­er­age of 1 mil­lime­ter ev­ery five years.

Castillo’s re­search comes amid a cen­tury-old de­bate on Catalina’s fate. Catalina is un­usual in that, un­like other Chan­nel Is­lands off the coast of Cal­i­for­nia, it does not have an­cient flat beaches that have been lifted up by earth­quake ac­tiv­ity. One re­searcher in 1897 also sug­gested that Catalina was sink­ing.

Re­searchers from the U.S. Ge­o­log­i­cal Sur­vey came to a dif­fer­ent con­clu­sion. A study pub­lished in 2012 in the jour­nal Ge­o­mor­phol­ogy re­ported what the au­thors said was cred­i­ble ev­i­dence that the is­land was ris­ing up like many other places in South­ern Cal­i­for­nia.

The lead au­thor, USGS ge­ol­o­gist Randall Schu­mann, said the pres­ence of an­cient flat beaches un­der the ocean wa­ter was not proof that Catalina is sink­ing. The is­land could still be up­lift­ing; it’s just that sea lev­els have risen since the last Ice Age, Schu­mann said.

That’s why there are un­der­wa­ter an­cient flat beaches that sur­round other Chan­nel is­lands that are known to be lift­ing up, Schu­mann said.

On Catalina, the lack of an­cient, raised flat beaches called marine ter­races above sea level can be ex­plained in two ways, he said. One, the is­land is mov­ing up so fast that the ocean never has time to cut a flat beach on the shore­line. The other rea­son: An­cient flat beaches have been washed away by pow­er­ful streams fall­ing off swiftly ris­ing hills.

What would help defini­tively de­ter­mine what’s go­ing on is find­ing dat­able fos­sils de­posited at sea level thou­sands of years ago, he said. Then, sci­en­tists can use that date — and how high that fos­sil is to­day — to de­ter­mine whether the is­land is lift­ing up or sink­ing down.

Schu­mann said, how­ever, that those kinds of fos­sils just haven’t been found on Catalina.

“So far, no one has been able to pro­duce con­clu­sive ev­i­dence of ei­ther up­lift or sub­si­dence on Catalina,” Schu­mann said. “It is en­cour­ag­ing to see that sci­en­tists are con­tin­u­ing to de­ter­mine a de­fin­i­tive an­swer to this com­plex ques­tion.”

Castillo’s study was pre­sented at the Seis­mo­log­i­cal So­ci­ety of Amer­ica meet­ing in Pasadena this week.

Gina Fer­azzi Los An­ge­les Times

“THE WHOLE is­land is tip­ping, so there def­i­nitely is a risk,” says Chris Castillo, the au­thor of a new study. But other sci­en­tists say the is­land is ris­ing.

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