Subtitles section Play video Print subtitles A magnitude 7.8 earthquake struck San Francisco early on the morning of April 18, 1906. The event marked the first time that the consequences of a major US natural disaster were recorded on film. A subsequent report demonstrated that the earthquake occurred on the northern half of a single continuous structure that we now know as the San Andreas Fault. The 1906 earthquake and subsequent fire killed more than 3000 San Francisco residents and left over half the city's population homeless. Today, nearly 40 million people live in California with many of them concentrated in major cities located close to the San Andreas and other fault systems. What can we do to mitigate the potential damages and loss of life from a future dangerous earthquake? To answer that question we need to understand the nature of earthquake hazards. When earthquakes occur the earth shakes for seconds or maybe even minutes. However much of the damage with earthquakes is not a result of the shaking itself but may be driven by other factors such as the design of buildings and the underlying geology. Our objective for this lesson is to familiarize you with the principle hazards associated with earthquake activity and the factors that can magnify their impact. One obvious consequence of shaking associated with an earthquake is the collapse of buildings and other structures. Rapid vertical and horizontal movements can shift homes off their foundations, collapse multi-story office blocks and destroy elevated roadways. The colors on this map indicate the severity of the shaking associated with the San Francisco earthquake. The red color indicates that shaking was most intense along the fault, and the transition from red to yellow to green indicates that shaking decreased as we get farther to the east. This graph illustrates that instruments at some stations near the source of the 1994 Northridge earthquake recorded ground shaking that approached 1g. A vertical acceleration of 1g is sufficient to overcome gravity, throwing objects into the air. Cities in areas of seismic risk typically have strict building regulations that require that new structures withstand accelerations of 0.5g or more with little damage. Even 60 miles or 95 kilometers from the epicenter, there is sufficient shaking to cause damage to older buildings. But shaking isn't just about the distance from the fault. Look again at this map of the 1906 earthquake. Notice this red area here around Santa Rosa. Even though Santa Rosa was more than 30 kilometers from the fault, is suffered some of the worst damage as a result of the earthquake. The underlying geology and building design can combine to exaggerate or dampen the effects of earthquake shaking. or example, the 49-story Transamerica Pyramid shook for more than a minute during a major earthquake but no one was seriously injured and the tower was undamaged. Buildings or structures have a natural resonance or frequency of vibration related to the motion of an earthquake. The greatest damage occurs when ground motion matches the resonance of a building. For example, taller structures will exhibit greater oscillations back and forth with slower ground motions. In contrast, short structures, like two-story homes, will vibrate more violently with faster ground motions. The same earthquake will produce different shaking effects in different earth materials. Bedrock typically produces more frequent, smaller vibrations – the type of shaking that is more likely to damage shorter structures such as two-story homes. Weaker materials such as muddy soils can amplify ground shaking and produce larger, low resonance vibrations that are more dangerous for multi-story structures. Elsewhere the shaking may cause ground failure. Landslides can occur in areas with steep slopes, blocking roads and burying homes under debris. Liquefaction occurs when shaking causes the compaction of sediment increasing water pressure and causing water and water-saturated material to be ejected at the surface. In this example, a block of granite sinks into the underlying saturated sands when the material is shaken vigorously. Local features like these sand boils or sand volcanoes are often produced. Elsewhere, liquefaction results in subsidence of the land surface and causes objects to collapse into the slurry of water and sediment. In the most extreme events this can cause whole apartment buildings collapse as the ground beneath them gives way. Fault movement during earthquakes can result in adjacent pieces of the land surface being displaced by up to several meters. Some blocks may move up or down, while others shift from side to side. This is one of the easier hazards to avoid, the general rule is “Don't build anything on or near an active fault.” We see examples of the effects of ground shaking and ground failure following the magnitude 9.2 Great Alaska earthquake of 1964. This remains the largest US earthquake ever recorded. Liquefaction, subsidence, and landslides swallowed buildings and left behind a chaotic urban landscape. The town of Valdez was moved to a more stable location following the earthquake. We was one major learning objective for this lesson. How confident are you that you could complete this task?