Summary

What is an earthquake? An earthquake is a shaking of the ground that occurs when brittle rocks being stressed by tectonic forces break suddenly along a fault. When they break, the elastic energy built up over years of slow deformation is released rapidly, and some of it is radiated as seismic waves. The focus of an earthquake is the point at which the fault first breaks; the epicenter is the point on Earth’s surface directly above the focus. The foci of most continental earthquakes are shallow. In subduction zones, however, earthquakes can occur at depths as great as 690 km.

What are the three types of seismic waves? Earthquakes generate three types of seismic waves that can be recorded by seismographs. Two types of waves travel through Earth’s interior: P (primary) waves, which are transmitted by all forms of matter and move fastest, and S (secondary) waves, which are transmitted only by solids and move at a little more than half the velocity of P waves. P waves are compressional waves that travel as a succession of compressions and expansions. S waves are shear waves that displace material at right angles to their path of travel. Surface waves are confined to Earth’s surface and outer layers. They travel slightly more slowly than S waves.

What is earthquake magnitude and how is it measured? Earthquake magnitude is a measure of the size of an earthquake. Richter magnitude is proportional to the logarithm of the amplitude of the largest ground movement recorded by seismographs. Seismologists now prefer to use moment magnitude because it is derived from the physical properties of the faulting that causes the earthquake: the area of faulting and the average fault slip.

How frequently do earthquakes occur? About 1,000,000 earthquakes with magnitudes greater than 2 take place each year. This number decreases by a factor of 10 for each magnitude unit. Hence, there are about 100,000 earthquakes with magnitudes greater than 3, about 1000 with magnitudes greater than 5, and about 10 with magnitudes greater than 7. The largest earthquakes, with magnitudes of 9 to 9.5, are rare and are confined to thrust faults in subduction zones.

What governs the type of faulting that occurs in an earthquake? The fault mechanism of an earthquake is determined by the type of plate boundary at which it occurs. Normal faulting, caused by tensional forces, occurs at divergent boundaries. Strike-slip faulting, caused by shearing forces, occurs along transform-fault boundaries. The largest earthquakes, caused by compressive forces, occur on megathrusts at convergent boundaries. A small number of earthquakes occur far from plate boundaries, mostly on continents.

What are the hazards of earthquakes? Faulting and ground shaking during an earthquake can damage or destroy buildings and other infrastructure. They can also trigger secondary hazards, such as landslides and fires. Earthquakes on the seafloor can trigger tsunamis, which may cause widespread destruction when they reach shallow coastal waters.

What can be done to reduce the risk of earthquakes? Land-use regulations can restrict new building near active fault zones, and construction in high-hazard areas can be regulated by building codes so that buildings and other structures will be strong enough to withstand the expected intensity of seismic shaking. Systems using networks of seismographs and other sensors are being developed to provide early warnings of earthquakes and tsunamis. Public authorities can plan ahead, be prepared, and put early warning systems in place. People living in earthquake-prone areas can be informed about how to prepare and what to do when an earthquake occurs.

Can scientists predict earthquakes? Scientists can characterize the level of seismic hazard in a region, but they cannot consistently predict earthquakes with the accuracy that would be needed to alert a population hours to weeks in advance. The best hope of making such predictions in the future may lie in a better understanding of how variations in stress raise or lower the frequency of seismic events in a regional fault system.

379