On 25 October 2010 at 14:42 GMT, an estimated magnitude 7.5 earthquake stuck off central West Sumatra. The earthquake produced near vertical motion, and source details and characteristics can be found at www.usgs.gov. Both the Pacific Tsunami Warning Center and the Alaskan Tsunami Warning Center generated warning messages. The first PTWC message came about 7 minutes post event, a time interval indicating considerable progress over earlier messages. The PTWC cancelled the warning about 2 hours later, the message is attached here.
The earthquake consisted of primarily vertical motion and produced what the PTWC referred to as a “significant” tsunami. PMEL calculated the far field evolution of the tsunami as shown in the figure (very preliminary), minutes after the event.
Press reports – see http://www.surf2surf.com/page.php?id=667 – raise the question whether the tsunami was larger than calculated from seismic inversion alone. (The latter is the process through which available worldwide seismic recordings of the specific event are used to estimate the type of dislocation, depth of initiation of the rupture, geometry and size, and then calculate the initial tsunami). Tens have reportedly been drowned or swept away, in stark contrast to the 2005 event, which was greater. Records from the two working tsunameters operated by Australia suggest that the tsunami was larger than estimates from seismic inversions. The tide gauge in Padang registered 1m peak to trough amplitude. The tide gauge record at Cocos Island shows a wave height about 1/5 of that of the 2004 Indonesian tsunami and 1/2 that of the 2005 Sumatran tsunami. Based solely from comparison of the size of these events, one would had expected a tsunami of about 1/2 the size recorded in Cocos. However, harbor resonance and very local amplification may have affected the tide gage measurements, but the casualty reports seem to confirm a deadly tsunami. Could this be another elusive slow earthquake ?
We hope that there are only tens of tsunami fatalities as initially “reported” and that Padang and Bengkulu will consider this event as one more warning for the real BIG one – see the 2006 PNAS paper by Borrero et al below.
Costas Synolakis, 26 October 12:00 GMT.
ITIC Tsunami Bulletin Board
TSUNAMI BULLETIN NUMBER 003
PACIFIC TSUNAMI WARNING CENTER/NOAA/NWS
ISSUED AT 1642Z 25 OCT 2010THIS BULLETIN IS FOR ALL AREAS OF THE INDIAN OCEAN.
… THE TSUNAMI WATCH IS CANCELLED …
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 – 1442Z 25 OCT 2010
COORDINATES – 3.1 SOUTH 100.1 EAST
DEPTH – 33 KM
LOCATION – SOUTHERN SUMATRA INDONESIA
MAGNITUDE – 7.5EVALUATION
A SIGNIFICANT TSUNAMI WAS GENERATED BY THIS EARTHQUAKE.
HOWEVER…SEA LEVEL READINGS NOW INDICATE THAT THE THREAT HAS
DIMINISHED OR IS OVER FOR MOST AREAS. THEREFORE THE TSUNAMI
WATCH ISSUED BY THIS CENTER IS NOW CANCELLED.FOR ANY AFFECTED AREAS – WHEN NO MAJOR WAVES HAVE OCCURRED FOR AT
LEAST TWO HOURS AFTER THE ESTIMATED ARRIVAL TIME 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.THIS WILL BE THE FINAL BULLETIN ISSUED BY THE PACIFIC TSUNAMI
WARNING CENTER FOR THIS EVENT UNLESS ADDITIONAL INFORMATION
BECOMES AVAILABLE.THE JAPAN METEOROLOGICAL AGENCY MAY ISSUE ADDITIONAL INFORMATION
FOR THIS EVENT. IN THE CASE OF CONFLICTING INFORMATION…THE
MORE CONSERVATIVE INFORMATION SHOULD BE USED FOR SAFETY.