Case study for Natural Hazards - Earthquakes - San Francisco 1989
EARTHQUAKE CASE STUDY; SAN FRANCISCO EARTHQUAKE 1989
Event;On the 17th October 1989, at 17.04, an earthquake hit San Francisco and Oakland. It was centred on the Santa Cruz range, some 60 miles south of the city. It was measured between 6.5 and 7.5 on the Richter scale. Aftershocks continued to hit over the next 36 hours. The major factor was the fact that it was rush hour in San Francisco. The fault ruptured bilaterally, that is in two directions, from the centre of the fault line, causing a relatively short shock. If the fault had ruptured unilaterally, along the entire fault line, the shock would have lasted for almost double the time.
The San Francisco Area.The underlying geology of California causes most of the problems. Much of San Francisco is based on clay at depth. Some areas are built on reclaimed soft soil, e.g. the Marina district was built on reclaimed marshland for an exhibition in 1915. During the past years, the Santa Cruz mountains have moved 6 ft northwards along a 25 mile section of the fault.
The problem comes when the quake occurs. The hard rock of the area provides resistance to the shock waves, and they are impeded and reduced in intensity. The clays and soft soils magnify the waves power, and can cause them to behave lick quicksand, an effect known as liquefaction. Under this, buildings can simply sink, intact, under their own weight. The basal acceleration of the 89 quake was 4%g, but it was propagated to 16%g at the surface by the clay soils. This propagation can cause resonance in mid rise buildings (10-15 storeys), which is why the newer areas had a larger death toll than expected.
The other problem is the location of people on the fault line, through poor education and no planning control exercised. For example, the Haywood fault runs on a similar axis to the San Andreas fault, but is further inland. Much of the buildings located directly above it are seriously at risk, including;
· The Memorial Stadium; an old concrete structure with a 70 000 - 80 000 capacity. It has been offset by 10 in 60 years.
· Berkeley University; which has rigid concrete buildings. 75 000 students are at risk. A report lists 27 buildings as unsafe. Only 3 have been made safe in recent times.
· Office of Emergency Services/Fairmont Hospital - very near the fault, and major nerve centre for response co-ordination in the event of a quake.
If a similar quake hit the Haywood fault, then 8000 would die, 11000 in San Francisco. Up to 300 000 would be injured, and an estimate $100 billion worth of damages would be caused.
35; Prediction, prevention and management.