Living along our beautiful California coastline provides us all with wonderful scenic views of the Pacific Ocean and Channel Islands. But a violent earth movement along that island chain could produce large waves, whose arrival on local beaches would be in a matter of a few minutes inundating our shores, causing destruction and most likely loss of life. Even remote earthquakes 100’s to 1000’s of miles away can have significant detrimental effects upon our shore lines. Tonight’s speaker will bring clarity to those possibilities and give us some idea as to the frequency of such events both from a geological historical perspective and those more recent events shown in recorded history. The hazard of a major tsunami hitting our coasts is real, knowing more about those possibilities is the subject of tonight’s talk.
Sept 16th A Coastal Tsunami Talk
Mark Legg - Tsunami Expert
Tsunami Observations in Southern California Mark R. Legg – Legg Geophysical, Inc.
Tsunamis are one of the most deadly natural disasters known to mankind. More than 300,000 people have been killed by tsunami attack (inundation) from large earthquakes and volcanic eruptions with wave-trains that cross entire ocean basins to strike vulnerable coasts. Fortunately, here in southern California located far from active subduction zones and somewhat sheltered by our Continental Borderland, historic tsunami wave heights have been modest, generally <2-3 m. However, tsunami currents in harbor areas (and rip currents along beaches) are severe and have caused substantial damage in past events. The major tsunamis of the past decade have provided excellent data to test the numerical models of tsunami generation, progagation, and coastal inundation which are used for tsunami warning and preparedness planning. Abundant photographic and video coverage of several large tsunamis are available to document and study more carefully the character of tsunami attack in our coastal zone. We find that the Santa Ana River between Newport and Huntington Beach provides an excellent location to observe tsunami wave activity where the modest river outflow is significantly affected by each tsunami wavetrain – from drawdown to inflow over multiple cycles. Waves as small as several cm (a few inches) were observed during the night of the Samoa tsunami (September 2008) with several separate wavetrains arriving over a two-hour observation period. The larger tsunamis from Chile (February 2010) and Japan (March 2011) persisted for at least four days, so that we could observe and measure wave heights and tsunami currents over significant tidal cycles. Strong tidal currents during outgoing and incoming cycles enhance the current activity, and the most severe tsunami currents may occur many hours after the initial arrivals. Currents up to 4 knots were observed during the 2011 Tohoku event. Currents up to 14 knots were reported from King Harbor, and strong currents damaged boats and docks in San Diego, Catalina Harbor, and Santa Cruz for the 2010 and 2011 events. We also observed when the tsunami wave activity eroded the sandy river banks and when material was deposited – severe scour around pilings at docks in San Diego Bay was observed after the 2011 Tohoku event. Even with the relatively simple geometry of the river mouth, relatively complex flow patterns are created which resemble observations in major harbors where strong eddies (whirlpools) form that may be destructive to large ships and docks. In addition, we have observed small bores from other long-period surface gravity waves (non-tsunami) at the Santa Ana River- these are most noticeable during the slack tide.