# 6275
The Pacific Tsunami Warning Center has lifted the Tsunami watch for the Sumatran earthquakes, stating that while a significant tsunami was generated, the threat appears to have passed.
First some excerpts from the PTWC alert, then I’ll be back with a little more on why some undersea earthquakes generate large tsunamis and others do not.
PACIFIC TSUNAMI WARNING CENTER/NOAA/NWS
ISSUED AT 1236Z 11 APR 2012THIS BULLETIN IS FOR ALL AREAS OF THE INDIAN OCEAN.
... THE TSUNAMI WATCH IS CANCELLED FOR BOTH MAIN SHOCK
AT 0839Z AND AFTERSHOCK AT 1043Z ...<SNIP>
EVALUATION
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.
As to why some large undersea earthquakes produce tremendous tsunamis – such as the 30+ meter tidal waves that struck Japan last year – and others do not, it depends primarily on the type of fault line where the earthquake occurs.
Strike-Slip faults cause earth movements that are primarily horizontal, and are less likely to produce large tsunamis.
Credit http://quake.abag.ca.gov/
Thrust faults produce an upward movement of the earth, and these are capable of producing very large tsunamis.
Credit http://quake.abag.ca.gov/
The reality is that most quakes involve some degree of both types of earth movement.
Those with significant amount of vertical and horizontal earth movement are called an oblique-slip quakes.
Credit Wikipedia
The amount of upward thrust generated by an undersea quake will have a direct bearing on the size of any tsunami that may be generated.
It would appear, based on the reports so far, that today’s Sumatran earthquake produced relatively little upward thrust, and therefore only minor tsunamis.
