Read more: 鈥�Instant Expert: Earthquakes鈥�
Understanding the shaking caused by earthquakes is crucial if we are to prepare for these events 鈥� but the impact of an earthquake on people and cities depends on more than magnitude alone. The Earth鈥檚 crust can amplify or dampen the severity of shaking
Shake, rattle and roll
Seismic waves cause perceptible ground motion if they are strong enough. For seismic hazard assessment, the study of ground motion is where the rubber meets the road. If we understand the shaking, we can design structures and infrastructures to withstand it.
Advertisement
The severity of earthquake shaking is fundamentally controlled by three factors: earthquake magnitude, the attenuation of energy as waves move through the crust and the modification of shaking due to the local geological structure.
Bigger earthquakes generally create stronger shaking, but not all earthquakes of a given magnitude are created equal. Shaking can depend significantly on factors such as the depth of the earthquake, the orientation of a fault, whether or not the fault break reaches the surface and whether the earthquake rupture is relatively faster or slower than average.
Attenuation of seismic waves varies considerably in different regions. In a place like California or Turkey, where the crust is highly fractured and relatively hot, waves dissipate 鈥� or attenuate 鈥� quickly. Following the 1906 San Francisco earthquake, pioneering geologist G. K. Gilbert observed: 鈥淎t a distance of twenty miles [from the fault] only an occasional chimney was overturned鈥nd not all sleepers were wakened.鈥� In regions that are far from active plate boundaries, such as peninsular India or the central and eastern US, waves travel far more efficiently. The three principal mainshocks of the 1811-1812 New Madrid earthquake sequence in the central US damaged chimneys and woke most sleepers in Louisville, Kentucky, some 400 kilometres away. In 2011, the magnitude 5.8 Virginia earthquake was felt in Wisconsin and Minnesota, over 1500 km away.
Local geological structures such as soft sediment layers can amplify wave amplitudes. For example, the M8 earthquake along the west coast of Mexico in 1985 generated a ringing resonance in the lake-bed sediments that underlie Mexico City. And in Port-au-Prince, some of the most dramatic damage in the 2010 Haiti earthquake was associated with amplification by small-scale topographic features such as hills and ridges.
Characterisation of the full range and nature of site response remains a prime target for ground motion studies, in part because of the potential to map out the variability of hazard throughout an urban region, called 鈥渕icrozonation鈥�. This offers the opportunity to identify those parts of urban areas that are relatively more and less hazardous, which can guide land-use planning and appropriate building codes. Rubber, meet road.
Strongest links
Earthquakes are often related to one another 鈥� one can lead to another 鈥� but there are common misconceptions about what drives them and the ways that they are linked.
It is an enduring misperception that a large earthquake is associated with a sudden lurching of an entire tectonic plate. If one corner of the Pacific plate moves, shouldn鈥檛 it be the case that other parts of the plate will follow suit? The idea might be intuitive, but it is wrong. The Earth鈥檚 tectonic plates are always moving, typically about as fast as human fingernails grow. What actually happens is that adjacent plates lock up, causing warping of the crust and storing energy, but only over a narrow zone along the boundary. So when an earthquake happens, this kink is catching up with the rest of the plate.
Earthquake statistics do tell us, however, that the risk of aftershocks can be substantial: on average, the largest aftershock will be about one magnitude unit smaller than the mainshock. Aftershocks cluster around the fault break, but can also occur on close neighbouring faults. As the citizens of Christchurch, New Zealand, learned in 2011, a typical largest aftershock (M6.1) had far worse consequences than the significantly bigger mainshock (M7), because the aftershock occurred closer to a population centre.
In addition to aftershock hazard, there is always a chance that a big earthquake can beget another big earthquake nearby, typically within tens of kilometres, on a timescale of minutes to decades. For example, the 23 April 1992 M6.1 Joshua Tree earthquake in southern California was followed by the 28 June 1992 M7.3 Landers earthquake, approximately 35 kilometres to the north. Such triggering is understood as a consequence of the stress changes caused by the movements of the rocks. Basically, motion on one fault will mechanically nudge adjacent faults, which can push them over the edge, so to speak, following delays ranging from seconds to years.
An additional mechanism is now recognised as giving rise to triggering: the stress changes associated with seismic waves. Remote triggering occurs commonly 鈥� but not exclusively 鈥� in active volcanic and geothermal areas, where underground magmatic fluid systems can be disrupted by passing seismic waves.
Overwhelmingly, remotely triggered earthquakes are expected to be small. Here again, recent advances in earthquake science as well as centuries of experience tell us that earthquakes do not occur in great apocalyptic cascades. However, in recent decades scientists have learned that faults and earthquakes communicate with one another in far more diverse and interesting ways than the classic foreshock-mainshock-aftershock taxonomy suggests.
Tsunami!
Undersea earthquakes can generate a potentially lethal cascade: a fault break can cause movement of the seafloor, which displaces the water above to form a tsunami wave.
Tsunamis can also be generated when earthquakes trigger undersea slumping of sediments, although these waves are generally more modest in size.
Tsunami waves spread out through the ocean in all directions, travelling in the open ocean about as fast as a jet airplane. They have a very long wavelength and low amplitude at sea, but grow to enormous heights as the wave energy piles up against the shore.