DON’T LET YOUR GUARD DOWN

Largest Hawai­ian earth­quake in a decade re­minds us to be pre­pared

West Hawaii Today - - Home - HAWAI­IAN VOL­CANO OB­SER­VA­TORY

On June 8, many Is­land of Hawaii res­i­dents were awak­ened by a sharp jolt just past 7 a.m. This nat­u­ral wake-up call was caused by a mag­ni­tude-5.3 earth­quake, the largest to strike Hawaii in over a decade.

As seis­mic waves rip­pled across the is­land, peo­ple de­scribed a roar­ing or rum­bling sound as vi­bra­tions passed through their homes, rat­tling items on walls and shelves. Al­most 1,000 peo­ple sub­mit­ted felt re­ports for the earth­quake through the USGS Did You Feel It? web­site. Sub­mis­sions came in from as far away as the lee­ward coast of Oahu some 310 miles dis­tant.

The USGS Hawai­ian Vol­cano Ob­ser­va­tory (HVO) re­ceived sev­eral re­ports of mod­er­ate shak­ing from across Hawaii Is­land. There were also re­ports of mi­nor dam­age from Vol­cano to Hilo, in­clud­ing a small rock­fall on the Chain of Craters Road in Hawaii Vol­ca­noes Na­tional Park.

Ma­halo to ev­ery­one who sub­mits Did You Feel It? (DYFI)

re­ports af­ter an earth­quake. By do­ing so, you con­trib­ute to the USGS mon­i­tor­ing pro­gram, and help us keep you in­formed and safe.

Data from DYFI re­ports are trans­lated into felt in­ten­sity, which de­scribes what peo­ple ex­pe­ri­ence dur­ing an earth­quake. The USGS also records ground shak­ing in­ten­sity with strong mo­tion ac­celerom­e­ters.

The geo­graphic dis­tri­bu­tion of th­ese in­ten­si­ties is por­trayed through a USGS prod­uct called ShakeMap. The maps com­bine both quan­ti­ta­tive data from seis­mome­ters and qual­i­ta­tive data from DYFI felt re­ports to paint a pic­ture of shak­ing for each earth­quake. This in­for­ma­tion aids post­dis­as­ter re­sponse, as well as earth­quake risk mit­i­ga­tion.

The ShakeMap for the June 8 earth­quake shows that the strong­est ground mo­tions cor­re­sponded to a max­i­mum in­ten­sity of VI on the Mod­i­fied Mer­calli In­ten­sity Scale (https://earth­quake.usgs.gov/ learn/top­ics/mer­calli.php). An in­ten­sity value of VI means that strong shak­ing was felt by all and mi­nor dam­age oc­curred in the earth­quake vicin­ity. This was the high­est in­ten­sity recorded for any earth­quake in the State of Hawaii since the Ki­holo Bay and Mahukona earth­quake se­quence in 2006.

In­stru­ments near the epi­cen­ter of the June 8 earth­quake recorded max­i­mum ac­cel­er­a­tions of 0.16g, which is 16 per­cent as strong as Earth’s grav­i­ta­tional force. Ac­cel­er­a­tions vary up and down as the seis­mic waves pass. If the earth­quake had been larger (greater than mag­ni­tude-6.0), the ac­cel­er­a­tion likely would have ex­ceeded 1g, or 100 per­cent times Earth’s grav­ity. When that hap­pens, ob­jects can be lifted off the ground mo­men­tar­ily. Peo­ple have ob­served such “jump­ing rocks” dur­ing past large earth­quakes on the Is­land of Hawaii.

The June 8 earth­quake oc­curred at a depth of 4 miles be­neath Ki­lauea Vol­cano’s south flank, slightly above the fault sep­a­rat­ing the old oceanic crust from younger vol­canic rocks that make up the is­land. Earth­quakes along this fault oc­cur when the mas­sive vol­canic pile shifts rel­a­tive to the oceanic crust. The largest Hawai­ian earth­quakes in recorded his­tory, in­clud­ing the de­struc­tive mag­ni­tude-7.9 and 7.7 events in 1868 and 1975, re­spec­tively, have oc­curred along this in­ter­face.

Thank­fully, the im­pacts of last month’s mag­ni­tude-5.3 earth­quake were rel­a­tively light. But the next big earth­quake could have a dif­fer­ent out­come. Be­fore it hap­pens, cre­ate a quake-safe plan, put to­gether an emer­gency kit, and keep it handy. And the next time you feel strong shak­ing, re­mem­ber th­ese three ac­tions: Drop, Cover, and Hold on!

If you’re near the beach when a strong earth­quake strikes, head to higher ground away from the shore as soon as shak­ing stops. This could save your life in case of a lo­cally gen­er­ated tsunami.

To learn more about how you, your fam­ily, and co-work­ers can pre­pare for the next big earth­quake, we in­vite you to par­tic­i­pate in the Great Hawaii ShakeOut on Oct. 19.

The June 8 earth­quake, our largest in over a decade, served as a wake-up call in more ways than just dis­turb­ing sleep. It re­minded us that Hawaii is earth­quake coun­try, and we all should be pre­pared for the next one. Vol­cano Watch (http://hvo.wr.usgs.gov/ vol­canowatch/) is a weekly ar­ti­cle and ac­tiv­ity up­date writ­ten by U.S. Ge­o­log­i­cal Sur­vey Hawai­ian Vol­cano Ob­ser­va­tory sci­en­tists.

PHOTO COUR­TESY/ USGS EARTH­QUAKE HAZ­ARDS PRO­GRAM

ShakeMap for the June 8 earth­quake shows that strong in­ten­si­ties (yel­low color) oc­curred near the epi­cen­ter, and weak in­ten­si­ties could be felt as far away as Oahu.

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