Hearing transcript Treasure Island Development Authority hearing April 15 1:00 PM San Francisco:
I am James Pepper.
I am looking at the 1994 U.S. geological survey deep instrumentation array at the treasure island naval station. The lateral stress was six times that of — because of the sand. The effect of the earthquake was six times that of other areas in the bay area.
I hope you all address that .It is in the name of the overall report as being the October 17, 1989 earthquake. Ground motion and ground failure report.
The other thing when you were pounding away on the causeway. Did you all file a permission to do that with the fish and wildlife and the Marine Mammal Commission?
That is the area where the seals feed.
>> sure. May I address the question?
>> yes, please.
>>
URI ELIAHU: So, with regard to the first comment: Yes, there’s a USGS seismic array that’s north of this Stage One area and sort of towards the west edge of the island, and it recorded ground motions during Loma Prieta and other less significant earthquakes. And the seismic response – that is, the lateral acceleration that occurred at the surface of Treasure Island — was significantly greater than it was on Yerba Buena Island because of the amplification that occurs through the soil profile and the bedrock. To put it in perspective, the peak ground acceleration that was recorded was, if I recall correctly, on the order of “point one-two gee” (.12g) – that is “point one-two times the acceleration of gravity.” It’s in that range. And, the design that we design to is four times that. It’s about “point four six (.460) feet,” because we are designing for a much higher intensity event. We’re sitting between the Hayward and San Andreas faults, and so we’re way beyond the measured ground motions in the bay area.
As to the second question about the causeway: We said in the beginning that that sand needed to be densified with dynamic energy. There are two ways to do that. One is impact which I think is what the caller is referring to, and the other is vibration. We chose vibration and not impact, and a significant part of the reason was just what was stated. We really didn’t want to have effects that would go beyond the prism of soil that was being improved. And, to put that in perspective, the efficacy of that system is only about 6 feet away from the vibratory columns that we saw in the presentation. So, it dissipates very quickly. It only improves the specific area where it is applied, unlike big impact energy which has waves that propagate much further out.
Thanks to Carol Harvey for typing out the response from URI ELIAHU. I was using the closed captioning file from the City’s website.
[Editors Note: So the answer is no, they did not get permission from the Marine Mammal Commission to do the causeway.]
Treasure Island California 