The global belt of Earth’s subduction zones is the region is where most of the most destructive earthquakes and tsunamis occur. The magnitude (M) of an earthquake is the most important factor that determines its violence and destructiveness, but that is not always the case; a relatively low magnitude earthquake may be devastating in a given zone, since construction criteria in the zone must be accounted for, among other factors. In addition, great tsunamis or large earthquakes (M > 8.0) are usually generated whenever extensive areas of the subduction megathrust face a rupture [1
]. There are examples of earthquake-produced tsunamis in the 2004 Sumatra earthquake (M-9.0) [2
], accounting for 220,000 casualties, or in 2006, with the tsunami that developed from the Java earthquake (M-7.8) [3
]. The intensity and destruction capacity of earthquakes and tsunamis are estimated by considering the area of vertical uplift of the sea bed, and these magnitudes are related to the geometry of the slipping fault when the earthquake rupture is located near the sea floor [4
]. Seismic events produce variations in the magnetic field as well; the study of electromagnetic phenomena precursory of earthquakes and volcanic eruptions has been a very active subject during the last years [5
]. Examples of this being the observation of Ultra Low Frequency (ULF) anomalies (300 Hz–3000 Hz) prior to the development of the Loma Prieta earthquake in 1989 near San Francisco [6
]; later a geomagnetic field variation of 7.2 nT was detected approximately 7 min before the Tohoku earthquake occurred in 2011 and detected by the Iwaki observatory, at 210 km from the epicenter [8
]. Another example is the possible detection of a series of pre-earthquakes magnetic anomalies that occurred on 25 April 2015, with epicentre in Nepal and a 7.8 magnitude. Detection was possible through the use of the Swarm magnetic satellites [11
In addition, after the Fukushima Daiichi nuclear plant disaster and the Great East Japan Earthquake Tsunami, which occurred in 11 March 2011, the requirement of cover human-environment symbiotic points is very important [12
Tsunamis also have a major impact on economic aspects. After a tsunami occurs the flooding of the surrounding lands by seawater is common, damaging those lands and destroying plot borders. This causes damage to property rights and to the land administration system in general [13
]. Therefore, after a Tsunami, it is necessary to re-establish the property lines again, using Global Positioning System (GPS) normally, as was the case in Indonesia [14
]. From a geodesic point of view, it is well known that the definitive displacement of the zones near the epicentre is the results of earthquakes [15
], e.g., the postseismic deformation after 2008 Wencham earthquake in China [16
], or postseismic changes in Thai geodetic network owing to the Sumatra-Andaman mega-thrust earthquake in 2004 [17
] and the Nias earthquake in 2005 [18
Mega-splay faults are caused when very extensive thrust faults rising from the plate frontier megathrust intersects the seafloor along the lower earring of the margin. It has been hypothesized that these mega-splay faults transfer displacement efficiently to the near surface, contributing to the genesis of seismic phenomena. Recently, these facts have been identified as first order characteristics in the Nankai Trough [19
], and are usual in other subduction areas such as Alaska [20
], Sunda [21
], and Colombia [22
To understand the development of tsunamis during the formation of large earthquakes, it is necessary to determine the exact location of the slip of the decomposition system, called splay-frontal [23
]. Instrumental measurements of earthquake and tsunamis largely come from tide seismological and gauge observatories and lately, bottom pressure recorders located in the deep ocean. The first time that the satellites Jason-1 and Topex-Poseidon collected transects of sea-height data using radar altimeter was in the 2004 Indian Ocean tsunami. These data showed a tsunami signal during the across the Indian Ocean [24
]. The number of publications based on earthquake monitoring increases significantly and becomes a topic of increasing interest in the last decade [25
Some authors suggest that a probabilistic approach might be a method for assessing the risk posed by seismic phenomena for a wide range of magnitudes [27
]. Due to the relationship between earthquakes and tsunamis, both the recurrence allocation of events in time and the delivery of earthquake or landslide sizes can be used to calculate the tsunami probability. In coastal locations with a broad register of tsunami is the distribution scope that is similar to the ones from other natural damage such as landslides, forest fires, and earthquakes [1
], which could be described as a power law [28
]. But apart from this approach, methods for early detection are needed.
Sensors can collect data to detect seismic phenomena that can lead to major natural disasters by means of a suitable seismic system and a correct treatment of the information. In addition, these systems could help with the early detection of a disaster, supporting the decisions to minimize human and environmental damage [12
]. In the literature there are several types of sensor systems that measure different environmental responses after an earthquake and could determine the material damage caused. An example is the sensor Structural Health Monitoring (SHM), which measures the acceleration of the response movement of a building after an earthquake [29
]; other sensors measure post-seismic deformations in horizontal structures [30
] or in vertical structures [31
Early detection of this type of seismic events can help save many lives. One such method of early detection could be the presence of ELF wave’s observatories, because of the relationship between Very Low Frequency/Extremely Low Frequency (VLF/ELF) waves and seismic phenomena [32
]. During the eruption of the in the Kelud volcano eruption, seismic signals showing peaks at 3.7 mHz, 4.8 mHz, 5.7 mHz, and 6.8 mHz were observed in Indonesia. This fact suggests that an ionosphere-atmosphere coupling phenomenon was present along with the lithosphere-atmosphere coupling [34
]. In addition, mentioning that such electromagnetic phenomena might be used to predict earthquakes, an example of this is shown in the precursors of the Loma-Prieta earthquake [35
], or for the Great East Japan earthquake of 2011 (or Tohoku earthquake) [36
The aim of this paper is to represent the major worldwide earthquakes and tsunamis of twentieth and twenty-first centuries, and to show the location of ELF wave’s observatories existing in the world accounting for the relationship between the two phenomena. This allows us to visualize the zones high probability of seismic phenomena. Taking into consideration the presence of seismic phenomena and their possible detection, different areas of the world will be shown which are suited to this kind of observatories.
3. Results and Discussion
When considering Figure 5
, we have developed several maps that are raised or propose new ELF sensors as stations to provide coverage to potentially vulnerable regions.
The label of “high seismic risk” could be tagged to the zones of Central America, Andes, the most oriental part of Europe, Philippines, Indonesia, and Japan, with the three last regions having a high occurrence of earthquakes and tsunami. There are frequent seismic events of great magnitude that involve high damages: the Sumatra Tsunami in 2010 [18
] or the Tsunami Japan 2011 [51
], and others. This is one of the reasons for which the study and analysis of this type of event is of great importance for the population. Figure 6
shows the current ELF stations along with the ELF stations, as proposed in Table 2
. It can be observed that in this figure the coverage of the areas with seismic risk is greater than the one established in Figure 5
, where there are only represented the existing ELF stations.
proposes with a minimum of 10 new ELF observatories to cover the seismic risk of all these areas of the earth, with a standard range of 1000 km. With this range, all of the zones of interest could have been covered. So, South America can be covered by three observatories that located in Chile and Peru, while Oceania needs five observatories, and the south of Asia needs at least two of them.
If a smaller range is considered, the numbers of suggested stations should be higher, an example shown in Figure 7
. In this figure the data in Table 3
, where possible ELF stations are established to cover the vulnerable areas seismic coverage with a radius of 500 km are represented.
The minimum number of stations to be considered in this case is 19. So, South America can be covered by six observatories located in Chile, Peru, and Ecuador, while Oceania needs 11 observatories, and the south of Asia needs at least two of them.
There is no theoretical model that establishes a specific range for the detection of this type of phenomena, and appointed error in these measures. The ranges described in Table 1
were estimated empirically according to the sensitivity of the phenomena captured, with usual distances of between 500 km and 1000 km. Consistent with these data, possible ELF stations with a range of 1000 km (Table 2
) and 500 km (Table 3
) have been proposed. The technology improvements allow for a greater sensitivity for the capture of these phenomena.
Recently, the largest number of the ELF observatories resides in the region of Japan; they cover the major part of its territory. The rest of observatories are distributed on the North American coast and Europe most commonly in its oriental zone.
In the Figure 5
, it should be considered that the African continent has less seismic activity that can produce tsunami due to the localization of the tectonic plates, which makes it a stable zone. We can observe that there exist numerous zones in southern hemisphere, where the occurrence of seismic events is elevated. Currently, there are no ELF observatories in these zones, which make the prediction of seismic events possible and thus they are considered as vulnerable areas.
So, in Africa, there is no evidence of significant seismic events recorded and therefore installing ELF observatories would not be a priority, although it would be recommended. However, there are other areas of the world with high earthquake intensity and risk to human life as consequence to the high population density [86
The low atmospheric attenuation with some latitude dependence of ELF signals (with an average of 0.64 dB/km at 40 Hz) allows for wave propagations at global scale [87
]. Despite the global character of these signals, the signals anomalies related to the occurrence of seismic events cannot be detected whenever the magnitude is less than 6 (M < 6). It should be considered that seismic phenomena of such magnitudes can also cause disasters and should be taken into account for their early detection [25
]. This is one of the reasons behind the necessity of establishing new ELF stations in the potentially active regions, such as the Asian zone or the south of America.
The development of new ELF stations in areas of frequent seismic activity could be considered as a very useful tool for the possible detection of seismic phenomena. There are researchers with great experience in the correlation of seismograms with previous anomalies of the natural patterns of ELF signals. The author Hayakawa M. is one of the main advocates of this theory, as indicated by the large number of articles he has developed in this topic, although the list of researchers is rather extensive. Table 4
shows some of the precursor phenomena of seismic events, which have been observed in different earthquakes in recent years.
The table above shows that the number of earthquake precursor electromagnetic phenomena is high, and that the majority are due to anomalies in the signals that were captured in different ELF stations. Such anomalies comprise from the increase or decrease of the usual intensity of the signal, a shift in the frequency of some of its modes of resonance or the existence of signals considered as ELF noise, signals not usually be in this band.
It should be also considered that there is no exact methodology for the temporary detection of these precursory phenomena. Some have been observed for several minutes before an earthquake, others a few days before or even weeks. Therefore, it is an empirical methodology that depends on several factors, such as the magnitude of the seismic event, the proximity to the ELF station, its instrumentation, and even the existing environment. In spite of this, there is evidence that a continuous study of the patterns of ELF band signals (such as RS signal) can be a powerful methodology that allows for the possible early detection of important seismic phenomena.
A seismograph is capable of collecting seismograms from any earthquake (M ≈ 7) at any point on the earth. The filtering effect of the high seismic frequencies allows us to determine the remoteness of the earthquake and to center our study of correlation with the earthquakes located within a radius of between 500 km and 1000 km, if it is located within range of a the ELF station. These are the estimated distances for the detection and correlation of the previous phenomena, as well as a possible explanation for the ELF stations performance ranges that are proposed in this paper.
Correlation studies of the ELF signals captured at the Calar Alto station with data from the Alboran earthquake (M = 6.3) [92
], which occurred in January 2016, are being carried out. The results of this study may be adequate to extract quantitative conclusions of this methodology and to further strengthen its potential for the early detection of possible seismic events.
This paper shows the existing ELF stations and their experimental range of coverage, for the detection of seismic events. On the other hand, the earthquakes with related tsunamis that occurred throughout history were located. When considering the relationship between the detection of precursory electromagnetic phenomena and seismic events, this work highlights the lack of coverage for almost the entire southern hemisphere, warning the community about the seismic risk for the area of South America and South Asia for their high seismic activity. The location of ELF observatories is proposed to provide a minimum coverage of seismic risk alert worldwide, although this methodology is largely experimental and widely studied today.
The ELF stations proposed in this paper are set in the areas of the greatest potential risk in the development of high magnitude seismic events or damage to populated areas, depending on the experimental distances for detecting this kind of phenomena (500 km and 1000 km). The proposed increase in the number of stations allows for greater potential comparative study and better behavior of these events. With this procedure, it should have carried out a greater number of works and the distribution scope is similar to the ones from other natural damage such as landslides, forest fires, and earthquakes studies to confirm and continue with the work that reinforces the use of these observing systems as a tool for early detection of seismic events of interest. So, the goal is to avoid possible global catastrophes with the related study of ELF signals of the observatories.
Despite that the relation between the detection of these seismic events and the ELF signals measurements (highlighting the Schumann resonance signal) is not totally verified; many studies are being contributed into investigation in the last few years. Works that showed anomalies or variations in these ELF signals a few weeks or days before the occurrence of large seismic events occur. These investigations are of interest due the major implications that can soon be developed through them soon. Therefore, this work opens new perspectives in the early detection of earthquakes and tsunamis in the world.