Observation
Earthquake forecasting is a branch of seismology that deals with specific limits on the timing, location, and magnitude of future earthquakes, specifically "the determination of parameters for the next strong earthquake in an area. Earthquake forecasting is sometimes referred to as an earthquake. forecast, which is the frequency of earthquakes that cause damage to a particular area over years or decades. And it is defined as the probable estimate of the general earthquake hazard, including magnitude. Prediction is called an earthquake warning. Systems that detect real-time warnings for vulnerable neighborhoods within seconds when an earthquake is detected.
In the 1970s, scientists hoped that a practical method for predicting earthquakes would soon be found, but a series of failures in the 1990s raised many questions about whether this was even possible. There have not been successful predictions of major earthquakes and some claims of success are disputed. For example, the most famous claim of a successful prediction was blamed on the 1975 Haicheng earthquake. A later study noted that the true study lacked short-term prediction. Extensive searches have reported several earthquake predecessors, but, until now, such predecessors have not been reliably identified at important spatial and temporal scales. If a part of the scientific community takes into account non-seismic precursors and provides enough evidence to study them in detail, then prediction may be possible, most scientists are pessimistic and some believe that earthquake Prediction is inherently impossible.
Large earthquakes occur suddenly and often cause disasters. In particular, as cities continue to modernize and industrial facilities become more complex and larger, in recent years, the vulnerability of cities to disaster prevention is increasing. Therefore, there is a growing social demand for early detection of earthquakes to reduce earthquake disasters.
It is clear that earthquake prediction, if put into practice, can help save lives and reduce disasters. So, can earthquakes be predicted? There is excessive pessimism and beliefs on this issue, both of which are unacceptable. Some researchers argue that earthquakes are fundamentally unpredictable because they are a phenomenon of crustal destruction. An extinction event is said to be a random event dominated by various uncertain factors. The glass seems to be breaking in the minds of those who say this. Of course, similar materials such as glass can break suddenly, so it is impossible to predict when they will occur. However, with a single fracture event, in dissimilar materials such as rocks and concrete, the fracture does not occur suddenly, but small fractures and slips first occur in vulnerable areas, then progress slowly and finally occur suddenly. This causes great destruction. Therefore, it is impossible to predict the main rupture by observing and monitoring the minor cracks and slips that precede the main rupture and the various changes that accompany it (called precursor events). It is believed that earthquakes occur when the force exerted on the earth's crust gradually increases, reaches a threshold value, and the fault moves rapidly. In that case, the stress distribution over the fault plane and the surrounding layer is assumed to be highly uneven, and the presence of preceding events is expected, so the extreme pessimism described above does not apply. On the other hand, in the early 1970s, an extended model was proposed based on seismic wave velocity changes and reports of unusual uplift of the crust before earthquakes, and overwhelming confidence in earthquake prediction prevailed. However, the significant anomaly of the underlying seismic wave velocity change was not identified in subsequent accurate observations (although it was small), and it continued to occur in and around the Izu Peninsula from 1974 to 1980. Because the way the earthquake occurred was different from what the model predicted, the model's proposed mechanism had to be re-examined, and radical confidence gradually faded.
In China, the devastating February 1975 earthquake near Haicheng, Liaoning Province (Haicheng earthquake, magnitude M 7.3) was successfully predicted, and many lives were saved. However, on the other hand, the same could not be predicted in the case of the 1976 Dongshan earthquake in China. An estimated 2,42,000 people died in the Tangshan earthquake. The practical application of earthquake prediction can be achieved by imaging the preceding events, but the degree of occurrence of the preceding events depends strongly on the mechanical properties of the seismic fault and its surroundings and shows significant regional features. Earthquakes in the Izu region since the late 1970s have always occurred with clear foreskin, but earthquakes in central California often do not show foreshortening. It seems that there are areas where earthquake prediction is easy and where it is difficult.
Three Stages of Earthquake Forecasting
Earthquake prediction must be applied in practice by carefully observing the characteristics of how the foregoing events occur in each region. Neither belief nor disbelief is acceptable. In general, current methods of earthquake forecasting can be summarized as (1) highly long-term forecasting of the location and magnitude of future earthquakes, and (2) the stress accumulation of various long-term preceding events. This is thought to be achieved through three phases: (3) short-term and long-term prediction based on last-minute forecasts, which capture the precursor event before the likely onset of extinction.
(1) Extremely long-term prediction (a) Since large earthquakes occur repeatedly in the same place, the probability of earthquakes in that area can be estimated by examining how many major earthquakes have occurred in the past. It is noteworthy that there have been frequent major earthquakes in the past, but not in recent years. For example, the Tkai region, including Suruga Bay, experienced repeated major earthquakes in 1707 and 1854, and 130 years have passed since then, so it is estimated that there will be no major earthquakes in the region yet. . Especially if, after a long period of time, other parts of the seismic zone are followed by major earthquakes, but some parts remain intact, the next. Since the probability of a large earthquake is high, the location of a large earthquake can be estimated, the size of the earthquake can be estimated by the size of the area that has not been destroyed, and the timing can be predicted. . up to a limit. The 1973 Nemuro Peninsula-Oki earthquake was a prediction of an earthquake based on this idea. (b) Earthquakes occur when stress accumulation in the crust reaches its limit, so the average critical stress of the crust is 0.5 × 104 to 1 × 104. The magnitude of the seismic acceleration can be estimated somewhat by knowing how much stress has accumulated in comparison. The amount of stress accumulation is determined by geological surveys such as plane and lateral length surveys. However, the critical stress of the crust due to earthquakes shows a large <variation>, and prediction from the value of crustal strain should be very difficult. (c) By knowing the presence of active fault earthquakes and the history of their activity, the probability and urgency of earthquakes in the area can be estimated. In particular, recently, research (the trench method) has been conducted to detect active faults, to find out when faults have moved in the past, and to elucidate the history of seismic activity over long periods. which is useful in the long term. term prediction. Be expected. However, the accuracy of temporal prediction is more general and long-term than (a) and (b).
(2) Long-term projection should occur when the stress level of the crust increases and the accumulation of stress reaches a certain level. A small amount of deformation or slip occurs in low-strength regions in the crust, including the ones above, and displacement due to viscous flows proceeds along the seismic fault. Therefore, various long-term preceding events occur. The most important long-term precursor event is a change in seismic activity. First, the measures before a major earthquake often work over a wide area. Also, closer to the time of the earthquake, the epicenter area becomes very quiet. This is the core of the seismic difference. In such a case, the activity around zero becomes more active and forms a donut shape. The larger the earthquake, the longer the time from the origin of the void to the occurrence of the earthquake, but the <variation> is too large to predict the time of occurrence quantitatively. There are some reports of unusual uplift of the crust based on leveling several years before the earthquake, but in many cases this cannot be called conclusive when examining the accuracy of the leveling. water pipe fittings, etc. According to continuous observations of crustal motion, there is some information that this anomaly appeared long before the earthquake. Groundwater levels and radon concentrations in groundwater can change. Additionally, there are reports that the geomagnetism and geopotential differences have changed. There have been many examples of animals behaving abnormally before the earthquake in China, but this is unlikely because some animals have the ability to exhibit strange reactions that are not understood by modern science. It might not be so. However, this is difficult to expect in a densely populated area like Japan.
(3) Short-term Prediction As noted in the beginning, it is assumed that the core destruction of the crust does not occur suddenly or progress, but leads to rapid core destruction after a possible initial uplift process. It is estimated that during this initial process, small deformations occur one after the other and the steady slippage along the fault plane gradually increases in speed. A variety of immediately preceding events are observed that seem to correspond to such changes.
Namely, foreskin, abnormal crustal movement, changes in geomagnetism and electrical resistivity, the formation of radio waves, changes in groundwater levels, changes in chemical elements such as radon, and changes in temperature in self-springs. A practical earthquake forecast should be obtained by immediately capturing the various antecedent changes prior to these earthquakes and making detailed decisions. A general pattern of how long these preceding events appear before an earthquake is unknown. However, foreshadows are visible from several hours to one to two days in advance, and there are many reports that changes in groundwater levels have already been visible. Since the preceding events tend to have habits by type and region, and their reproducibility appears to be relatively good, the timing of the event can be estimated by careful study of earthquakes in each region.
To summarize the above, if a major earthquake is likely or (ultra) determined to occur relatively soon by long-term forecasting, a 24-hour monitoring network is set up in the area and occurs immediately before Is. It is a way of capturing the event and continuing by making a final prediction.
Japan Earthquake Prediction System
At present, long term forecasting is being considered in Japan. This is done by the Earthquake Prediction Liaison Group. The Coordinating Committee for Earthquake Prediction consists of 30 seismologists from relevant organizations and holds nationwide discussions in regular meetings four times a year. There are two sub-committees to deal with urgent issues. Currently, only the Tokai region is under short-term forecast. The 1978 Large-scale Anti-Earthquake Act designated the Dokai area, including Suruga Bay, as an area to strengthen earthquake disaster prevention measures, and various observatories such as seismic monitoring points, crustal stress monitoring points and groundwater monitoring points. were established. The points are kept and the data is sent to the Japan Meteorological Center in Tokyo, which is under a 24-hour monitoring system. If an accident is detected, the Prime Minister will issue a warning notice after convening the Regional Adjudication Committee to strengthen earthquake disaster prevention measures and confirm the likelihood of an earthquake. The network is one of the most complete in the world in terms of density and quality. Whether a precessing event has been observed for the Tokai earthquake is promising because significant precessional crustal motion accelerated 2 to 3 days before the 1944 Tongai earthquake, which can be said to be a sibling of this earthquake. , There is thus, earthquake prediction has certainly taken a step towards practical application in Japan as well. However, such a system for short-term forecasting is limited to the expected 8-magnitude earthquake in the Dokai region and other earthquakes (probably) occurring in different parts of the country. Richter magnitude 7) for very small earthquakes it is necessary to take one positive measure after another. Systematic observation and surveys are encouraged by relevant national organizations based on the Annual Plan for Earthquake Prediction proposed by the Geological Council, but in addition, local governments and local governments measure well water levels by amateur observers.
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