NAPA - A wake-up call?


   It was during one of those predawn hours when slumber is being overtaken by wakefulness, when dreams are slipping quietly back into the recesses of the mind. The sun was making ready to march over the hills and mountains to the east to bring a fresh fall day down upon the inhabitants of this quiet little town known more for the slow growth of grapevines that produce fine wines than what was to be in store on this day. Napa, nestled deep in a valley between two coastal ranges of hills and mountains, was world renown for its slow but unceasing acceptance by many  as being one of the premier wine growing regions of the earth  A normal peaceful sunrise was expected at 6:32 AM. The thermometer read 56 degrees F. The moon was in a waning crescent with nearly all of the moon in shadow. An unusual soft offshore wind was blowing toward the south south west, causing small waves and shimmering lights to flicker across the San Francisco Bay Inlets and the Napa River channels that flow slowly past the recently developed and improved downtown area known as  the Riverfront Development. 


   The Napa Earthquake fault had slumbered for only fourteen years had let the residents know of it's restless nature as recently as September 3, 2000 in Napa and nearby Yountville. On this fall day in August, from the bowels of the earth came a message that reminded everyone of the power and force that can be unleashed by the earth as it undergoes its restless and unruly natural progression. The maturations and expectations of mankind were about to be placed once again back in their proper place and made small and supplicant to father time.


  Unable to sleep, due to a recent accident that had caused some damage to his leg, one resident of the city was working in his yard on a motorcycle repair. His girl friend was still sleeping soundly in the house nearby. The ground began to buck and heave sufficiently strong enough to flip an automobile engine block completely upside down.  Plumged into darkness, the resident heard his girlfriend screaming for help from fear of a collapsing house. In his haste to assist her, he tripped over the engine block and further injured his leg. When he was able to get a flashlight, he found his house teetering at an odd angle. The older foundationa and lower support walls were no match for the earthquake forces  and the house moved precariously close to collapse (see braced house picture in right sidebar).


  Another younger inhabitant of Napa was having a sleep over with friends.  Nicholas  Dillon and his friends had decided to sleep in the living room together rather than use the limited bedrooms available. When the earthquake occured, it peeled the brick masonry materials off the front of the fireplace. The heavy materials rained down upon the sleeping kids and resulted in major injuries to Nicholas including a collapsed lung and a shattered pelvis.


  An event, later to become known as the North Napa Valley Earthquake had just made an appearance onto the stage and into the lives of nearly eighty thousand residents in this quiet little wine growing region. 



            As we pass each and every anniversary of the 1906 San Francisco earthquake it would be wise to reflect on what might happen if we experienced a similar earthquake on the Hayward or San Andreas faults. Damage done to homes in Napa that August has revealed some interesting common weaknesses in our older housing stock. The Napa earthquake was just a moderate 6.0 magnitude and the earthquakes currently viewed as probable on the larger faults in the Bay Area are going to be much larger in magnitude and longer in duration. Prediction has always been an enticing and elusive goal. The damage done in Napa allows us to reflect on what might happen if we get another Big One in the Bay Area and to make some predictions on what type of houses will be lost. In Napa, the houses that failed and were Red Tagged, leaving families without housing, had some interesting similarities. A significant number of small one and two story houses with short cripple walls collapsed. The cripple wall is the short wood wall that extends from the foundation up to support the main floor of the home. The configurations that were the most commonly damaged were houses with shorter than usual sections of cripple wall located on a wall that lies in the East West direction. This was due to the seismic waves generated on a North-South trending fault. In many cases the short wall was further weakened by the layout of the house. If there was an inset entry porch that robbed the wall of the needed length to handle the earthquake loads, the cripple wall collapsed. Another major contributor to damage was a porch or deck that protruded from the building and added loads to the already weak cripple wall. Porches and decks are commonly supported on piers and posts located well away from the home. When the earthquake causes shaking, the decks and porches are like tails wagging the dog. They add a lot of load that must then be carried by the cripple walls. Analysis of the damage done in Napa allows us to concentrate on the houses most vulnerable to damage during a major earthquake.



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