Living with Earthquakes - Earthquake Engineering for Structural Design - page 48

( b)
Figure 2.9 1976 China Tangshan earthquake. (a) Location of earthquake;
(b) Air view of the devastation after the earthquake (USGS, nd)
The 1989 Loma Prieta earthquake (Fig. 2.12) (Wikipedia, nd) occurred when
the crustal rocks comprising the Pacific and North American plates abruptly
slipped 2 m along the boundary of the San Andreas fault system in the direction of
San Francisco city. The epicenter of the quake was an unpopulated area in the
Aptos and Cruz Mountains, near the Loma Prieta Peak. The rupture initiated at the
depth of 18 km and extended 35 km along the fault system without breaking the
surface of the Earth. The number of deaths (only 63) was remarkably low due to
the introduction of modern concepts in design. The worst disaster produced by this
earthquake with the most number of fatalities was due to the collapse of the
Cypress Street Viaduct on the Nimitz Freeway. One 15m section of the San
Francisco Oakland Bay Bridge also collapsed.
The 1994 US Northridge earthquake (Fig. 2.13) (Wikipedia, nd) was unusual
because the epicenter was within a very densely populated metropolitan area of
Los Angeles. Despite the area's proximity to the San Andreas Fault, the Northridge
quake did not occur along this fault, but rather on a previously undiscovered blind
fault. The rupture was initiated at 18 km depth and then spread Northwest. Many
commercial buildings and bridges were damaged and, in terms of property damage,
the earthquake is one of the most costly natural disasters in US history. This was
the first time that an earthquake occurred in a very populated zone and the special
effects of near-fault ground motions (very high velocities and accelerations and
important vertical components of ground motion) were noticed. Many reinforced
concrete (RC) buildings and bridges were severely damaged. But, from the
structural engineering point of view, one of the great surprises of this earthquake
was the discovery of widespread damage to welded connections in modern steel
moment frame buildings. After this earthquake, important changes were made in
design and construction practice, also due to the revision of the building codes.