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Earthquakes occur when massive amounts of energy are released in the form of seismic waves. The period of these waves ranges from ultra-short to long. This wave of oscillation, which is akin to the intervals of a swinging pendulum, is the deciding factor in how a structure will be affected by an earthquake. Today, long-period waves are attracting special attention because they relate to potentially powerful earthquakes in the sedimentary basins of Japan, such as the Kanto plain, a highly populated region that includes Tokyo and Yokohama.
Periods of Seismic Waves*
*Descriptions of the periods of seismic waves using such adjectives as “long” or “short” are treated as relative measures, and usage of such terms may differ according to researchers' backgrounds. The above table is a generalized way of categorizing types of seismic waves by period.
Dr. Yuki Sakai, an associate professor in the graduate school of Systems and Information Engineering at the University of Tsukuba in Ibaraki, Japan, explains that the destructive power of an earthquake depends on the period of its seismic waves. “A period is counted as one back-and-forth swing. Generally, a mixture of slow vibrations, intensive tremors and other waves occur during an earthquake. However, certain periods of waves will become dominant depending on the conditions where quakes are generated and along the propagation path between the area of seismic origin and the earth's surface. The type of wave produced characterizes the nature of the earthquake.”
Periods of waves can range from ultra-short (less than half a second) to long (more than 5 seconds). Moderately short periods — 1 to 2 seconds — cause the most damage. “Seismologists call this the killer pulse,” Dr. Sakai says. “These 1- to 2-second periods are a particular threat to more than 90 percent of ordinary houses (one/two-story houses) and older multi-story buildings found throughout Japan. After the Great Hanshin Earthquake that struck the Kobe area in 1995, research showed that the most destructive periods of seismic waves were, indeed, 1 to 2 seconds. This is why they've been dubbed the killer pulse.”
Every structure has its own specific period; it is likely to collapse if it's struck by a seismic wave with a period matching its own.“A building's natural period,” Dr. Sakai explains, “depends on its design, size and structure. The taller a building is, the longer this period will be.”
What are the practical ramifications of this fact? Put simply, if a quake's seismic wave period matches that of a particular building, that building could be doomed.
Most quakes that strike Japan have seismic waves with periods of less than 1 second, which may not do much damage to most houses. But a killer pulse works differently. When such a wave starts shaking a house, the motion of the building is extended beyond its unique natural period, to between 1 and 2 seconds. This resonates with the killer pulse, and the structure collapses. Here's an analogy to help understand this phenomenon: If you're swinging back and forth on a swing, and you increase the power so you're going higher and higher, the length of each swing increases. The principle is the same.
What about the taller buildings in large cities? Dr. Sakai says long-period waves can “pass through” smaller buildings but be felt more strongly in skyscrapers, which have their own unique long periods. Long-period waves may not destroy a building, but they have been known to knock out elevators, rattle furniture and cause upper floors to undulate.
Longer-period seismic waves also travel greater distances. If the period is longer, an earthquake will have a greater scope. For example, the 2004 Chuetsu Earthquake in Niigata was felt 200km (124.3 mi.) away in Tokyo, where its far-reaching waves caused elevators to stop working in some high-rise buildings, leaving them stranded between floors.
Japan is situated near the boundaries of four major tectonic plates: Two are oceanic — the Pacific and Philippine Plates, and two are continental — the Eurasian and North American Plates. The oceanic plates are descending beneath the Japanese archipelago. This is why Japan is constantly subject to tremors.
Earthquakes Occurring inside Plates
Some quakes occur deep beneath tectonic plates — so deep that short-period waves most often become dominant.
Earthquakes Occurring between Neighboring Plates
A tectonic plate moves under its neighboring plate in a process called subduction. At a certain point, the tip of the descending plate experiences an abrupt kickback that may result in earthquakes, such as the powerful ones that occurred in Tokai (1854) and Tonankai (1944). This type of quake requires the highest level of alertness from the public.
Crustal Earthquakes
The epicenters of such earthquakes are located near the earth's surface, inside the crust. The type of waves generated at the epicenter depends on the extent to which the crust has collapsed. Killer pulses are quite often generated in this area.
