Earthquake cycles

Earthquakes in Taiwan are caused mainly due to collisions between the Philippine Sea Plate and Eurasian Plate. The impact of these collisions has also caused the emergence of the island of Taiwan and her magnificent geological scenery, and a high frequency of earthquakes as well. As a result of this persistent seismic activity, Taiwan will continue to experience earthquakes with magnitude ranging from small to large. A seismic event occurs when accumulated energy is released after reaching a certain point, after which the seismic energy will start to accumulate again. This repeated energy release is the cause of recurring earthquakes. Such repetitive occurrences form the so-called "earthquake cycle" from a statistical point of view.

According to global statistics based on historical data, the mean cycle of recurrence is somewhere between decades and a century for earthquakes of magnitude 6 to 7, and between one and several centuries for those above 7. But statistical cycles are merely figures for reference. For the purpose of thorough interpretation, it is necessary to obtain information about the sample space and the sample size, along with the statistical numbers. Take an earthquake with cycle of 5 years as an example. It may occur twice in 10 years, twenty times in a century, or even a hundred times in 500 years. Interpretations may vary for a 5-year cycle with different sample space and sample size. The variances may be even greater upon further investigation of the elapsed period between two consecutive earthquakes. Even when a cycle can be scientifically estimated by digging trenches to determine the time interval between two fault displacement, it still can not be inferred as the time to wait before the next earthquake occurrence since the fault displacement could have healed and changed its condition after a long period of time. Apart from this, short-term or minor changes in the environment can alter characteristics related to frequency of occurrence of an earthquake.

Furthermore, in Taiwan, environmental characteristics pertaining to earthquakes are quite different between the east side of the island and the west. Earthquakes on the east side of Taiwan are the direct result of plate collision while earthquakes on the west side are the result of fault displacement that was created by the tectonic environment. With so many characteristic differences, an earthquake cycle estimated by combining the data from both sides yields meaningless results. It is as if trying to figure out the probability of a number that will come up when throwing a 6- and an 8- faced dice in a random manner. The result will be totally different from that of casting a single type of dice.

Of course, however scientifically meaningful the statistics may be, the purpose of academic and expert interpretation of earthquake cycles is to remind people that earthquake occurrence is normal in a seismic zone like Taiwan. The longer the time since the last major earthquake, the more probability the next one will soon occur. It is not the cycle people should focus on as much as it is ordinary disaster preparedness tasks.