samedi 27 février 2010

Raz de marée ?


(image d'un précédent séisme- celui d'aujourd'hui est parti plus au sud du Chili)

Tremblement de terre au Chili magnitude 8,8. L'onde se propage sur le Pacifique (durée en h)

Rappel: L'échelle de Richter est logarithmique de base 10: elle mesure le logarithme de l'énergie libérée.

Ne pas confondre Magnitude (M), Energie (E), et Intensité (I):


les formules sont, pour l'échelle de Richter:

Energie libérée: log(E) = 11,4 + 1,5 M (où log est le logarithme décimal, comme en chimie pour le pH: c'est-à-dire log(x) = ln(x) / ln10

Intensité I = Io.10^M où Io est l'intensité de référence.

Donc, entre 2 séismes de magnitudes M1 et M2 on a:

Pour l'énergie: log(E2) -log(E1) = 1,5 (M2 - M1) d'où E2 / E1 = 10^[ 1,5( M2 - M1)]

Pour l'intensité: I2 / I1 = 10^(M2-M1)


Ainsi, un tremblement de terre enregistrant 8,8 sur l'échelle de Richter transporte
10^[1,5( 8,8 - 7,8)] = 30 fois plus d'énergie qu'un de 7,8.

Entre celui-ci (mag 8,8) et celui d'Haïti (mag 7), l'énergie libérée est: 10^[ 1, 5 ( 8,8 - 7)] = 500 fois plus importante.

C'est le 5e plus fort tremblement de terre depuis 1900.


Déplacement de l'onde:

Chili-Marquises: 6800 km
Chili-Raiatea: 7500 km donc 700 km de plus, donc environ 1h de décalage par rapport aux Marquises si l'onde voyage à 700 km/h.




Pour plus d'info: site de la NOAA Pacific Tsunami Warning Center

dont voici le bulletin:

TSUNAMI BULLETIN NUMBER 014
PACIFIC TSUNAMI WARNING CENTER/NOAA/NWS
ISSUED AT 1934Z 27 FEB 2010

THIS BULLETIN APPLIES TO AREAS WITHIN AND BORDERING THE PACIFIC
OCEAN AND ADJACENT SEAS...EXCEPT ALASKA...BRITISH COLUMBIA...
WASHINGTON...OREGON AND CALIFORNIA.

... A WIDESPREAD TSUNAMI WARNING IS IN EFFECT ...

A TSUNAMI WARNING IS IN EFFECT FOR

CHILE / PERU / ECUADOR / COLOMBIA / ANTARCTICA / PANAMA /
COSTA RICA / NICARAGUA / PITCAIRN / HONDURAS / EL SALVADOR /
GUATEMALA / FR. POLYNESIA / MEXICO / COOK ISLANDS / KIRIBATI /
KERMADEC IS / NIUE / NEW ZEALAND / TONGA / AMERICAN SAMOA /
SAMOA / JARVIS IS. / WALLIS-FUTUNA / TOKELAU / FIJI /
AUSTRALIA / HAWAII / PALMYRA IS. / TUVALU / VANUATU /
HOWLAND-BAKER / NEW CALEDONIA / JOHNSTON IS. / SOLOMON IS. /
NAURU / MARSHALL IS. / MIDWAY IS. / KOSRAE / PAPUA NEW GUINEA /
POHNPEI / WAKE IS. / CHUUK / RUSSIA / MARCUS IS. / INDONESIA /
N. MARIANAS / GUAM / YAP / BELAU / JAPAN / PHILIPPINES /
CHINESE TAIPEI

THIS BULLETIN IS ISSUED AS ADVICE TO GOVERNMENT AGENCIES. ONLY
NATIONAL AND LOCAL GOVERNMENT AGENCIES HAVE THE AUTHORITY TO MAKE
DECISIONS REGARDING THE OFFICIAL STATE OF ALERT IN THEIR AREA AND
ANY ACTIONS TO BE TAKEN IN RESPONSE.

AN EARTHQUAKE HAS OCCURRED WITH THESE PRELIMINARY PARAMETERS

ORIGIN TIME - 0634Z 27 FEB 2010
COORDINATES - 36.1 SOUTH 72.6 WEST
DEPTH - 55 KM
LOCATION - NEAR COAST OF CENTRAL CHILE
MAGNITUDE - 8.8

MEASUREMENTS OR REPORTS OF TSUNAMI WAVE ACTIVITY

GAUGE LOCATION LAT LON TIME AMPL PER
------------------- ----- ------ ----- --------------- -----
CABO SAN LUCAS MX 22.9N 109.9W 1833Z 0.36M / 1.2FT 12MIN
DART TONGA 51426 23.0S 168.1W 1844Z 0.04M / 0.1FT 30MIN
HIVA OA MARQUESAS 9.8S 139.0W 1741Z 1.79M / 5.9FT 12MIN
PAPEETE TAHITI 17.5S 149.6W 1810Z 0.16M / 0.5FT 10MIN
NUKU HIVA MARQUESAS 8.9S 140.1W 1745Z 0.95M / 3.1FT 04MIN
MANZANILLO MX 19.1N 104.3W 1705Z 0.32M / 1.0FT 24MIN
DART MANZANILLO 434 16.0N 107.0W 1611Z 0.07M / 0.2FT 24MIN
ACAPULCO MX 16.8N 99.9W 1549Z 0.16M / 0.5FT 24MIN
RIKITEA PF 23.1S 134.9W 1559Z 0.15M / 0.5FT 22MIN
DART MARQUESAS 5140 8.5S 125.0W 1531Z 0.18M / 0.6FT 18MIN
BALTRA GALAPAGS EC 0.4S 90.3W 1452Z 0.35M / 1.2FT 14MIN
EASTER CL 27.2S 109.5W 1205Z 0.35M / 1.1FT 52MIN
ANCUD CL 41.9S 73.8W 0838Z 0.62M / 2.0FT 84MIN
CALLAO LA-PUNTA PE 12.1S 77.2W 1029Z 0.36M / 1.2FT 30MIN
ARICA CL 18.5S 70.3W 1008Z 0.94M / 3.1FT 42MIN
IQUIQUE CL 20.2S 70.1W 0907Z 0.28M / 0.9FT 68MIN
ANTOFAGASTA CL 23.2S 70.4W 0941Z 0.49M / 1.6FT 52MIN
DART LIMA 32412 18.0S 86.4W 0941Z 0.24M / 0.8FT 36MIN
CALDERA CL 27.1S 70.8W 0843Z 0.45M / 1.5FT 20MIN
TALCAHUANO CL 36.7S 73.4W 0653Z 2.34M / 7.7FT 88MIN
COQUIMBO CL 30.0S 71.3W 0852Z 1.32M / 4.3FT 30MIN
CORRAL CL 39.9S 73.4W 0739Z 0.90M / 2.9FT 16MIN
SAN FELIX CL 26.3S 80.1W 0815Z 0.53M / 1.7FT 08MIN
VALPARAISO CL 33.0S 71.6W 0708Z 1.29M / 4.2FT 20MIN

LAT - LATITUDE (N-NORTH, S-SOUTH)
LON - LONGITUDE (E-EAST, W-WEST)
TIME - TIME OF THE MEASUREMENT (Z IS UTC IS GREENWICH TIME)
AMPL - TSUNAMI AMPLITUDE MEASURED RELATIVE TO NORMAL SEA LEVEL.
IT IS ...NOT... CREST-TO-TROUGH WAVE HEIGHT.
VALUES ARE GIVEN IN BOTH METERS(M) AND FEET(FT).
PER - PERIOD OF TIME IN MINUTES(MIN) FROM ONE WAVE TO THE NEXT.

EVALUATION

SEA LEVEL READINGS CONFIRM THAT A TSUNAMI HAS BEEN GENERATED
WHICH COULD CAUSE WIDESPREAD DAMAGE. AUTHORITIES SHOULD TAKE
APPROPRIATE ACTION IN RESPONSE TO THIS THREAT. THIS CENTER WILL
CONTINUE TO MONITOR SEA LEVEL DATA TO DETERMINE THE EXTENT AND
SEVERITY OF THE THREAT.

A TSUNAMI IS A SERIES OF WAVES AND THE FIRST WAVE MAY NOT BE THE
LARGEST. TSUNAMI WAVE HEIGHTS CANNOT BE PREDICTED AND CAN VARY
SIGNIFICANTLY ALONG A COAST DUE TO LOCAL EFFECTS. THE TIME FROM
ONE TSUNAMI WAVE TO THE NEXT CAN BE FIVE MINUTES TO AN HOUR, AND
THE THREAT CAN CONTINUE FOR MANY HOURS AS MULTIPLE WAVES ARRIVE.

FOR ALL AREAS - WHEN NO MAJOR WAVES ARE OBSERVED FOR TWO HOURS
AFTER THE ESTIMATED TIME OF ARRIVAL OR DAMAGING WAVES HAVE NOT
OCCURRED FOR AT LEAST TWO HOURS THEN LOCAL AUTHORITIES CAN ASSUME
THE THREAT IS PASSED. DANGER TO BOATS AND COASTAL STRUCTURES CAN
CONTINUE FOR SEVERAL HOURS DUE TO RAPID CURRENTS. AS LOCAL
CONDITIONS CAN CAUSE A WIDE VARIATION IN TSUNAMI WAVE ACTION THE
ALL CLEAR DETERMINATION MUST BE MADE BY LOCAL AUTHORITIES.

BULLETINS WILL BE ISSUED HOURLY OR SOONER IF CONDITIONS WARRANT.
THE TSUNAMI WARNING WILL REMAIN IN EFFECT UNTIL FURTHER NOTICE.

THE WEST COAST/ALASKA TSUNAMI WARNING CENTER WILL ISSUE PRODUCTS
FOR ALASKA...BRITISH COLUMBIA...WASHINGTON...OREGON...CALIFORNIA.

.

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