Long- and Short-term Predictions of Volcanic Eruptions in Santorini

Based on these values and on the hypothesis that these repeat times follow a normal distribution, the probability has been computed of the next strong eruption as a function of the next t years. This probability is 0.1 for the next 10 years, 0.6 for the next 50 years and 0.95 for the next 110 years. It is shown that the total duration, t (in years), of each eruption phase can be used as a measure of the magnitude of the eruption and that t is linearly related to the quiescence period, T, since the time of the previous eruption. This leads to the conclusion that if the 'volcanic energy' which is accumulated under the Santorini caldera today is released, it can produce an eruption equal to that of 1925. The problem of the short-term prediction of volcanic eruptions on Santorini is also discussed and it is concluded that systematic seismological and other observations (temperature measurements, etc) may lead to the prediction of the time of occurrence of the next strong eruption on Santorini.

INTRODUCTION

It is well known that volcanic risk is non-existent everywhere in Greece except in the places along the active volcanic arc of the southern Aegean Sea, Sousaki-Methana-Milos-Santorini-Nisyros (Akylas 1925; Georgalas 1962; Papadopoulos 1985). Along this arc, only three volcanoes are considered as active: Santorini, Nisyros, Methana. The only known eruption of the volcano of Methana (Kameno Vouno) is that which took place in 250 BC which was very weak. The eruptions of the volcano on Nisyros were also very weak (Georgalas 1962). Therefore, according to the available information, the only volcano which can create problems for the inhabitants of this area is that of Santorini. For this reason, in the present paper, an attempt is made to investigate the possibility of long- and short-term prediction of its next strong eruption.

As in the case of earthquake prediction, the prediction of volcanic eruptions can be divided into long-term prediction and short-term prediction. For short-term prediction, the time of the volcanic eruption must be predicted with an accuracy of a few weeks or even of days, while for long-term prediction this time must be known with an accuracy of years. The long-term prediction is usually of probabilistic character and can be considered as a problem of time-dependent volcanic hazard evaluation. Therefore, long-term prediction of a volcanic eruption can be made by deteimining the probability of occurrence of this eruption during the next t years (e.g. t = 20 years).

In the present paper, an attempt is made to contribute to the long-term prediction of the next eruption of the Santorini volcano by determining the probability of occurrence of this eruption as a function of time t (= next t years) and the eruption potential (a measure of available volcanic energy) in the caldera of Santorini. In addition we discuss the possibility of predicting this eruption in the short-term sense.

PROBABILITY OF OCCURENCE OF THE NEXT STRONG VOLCANIC ERUPTION IN SANTORINI

Fig. 1 shows the cumulative number of the known historical volcanic eruptions of the three volcanic centres in the southern Aegean (Methana, Santorini, Nisyros) as a function of time. This plot is based on information given mainly by Akylas (1925) and Georgalas (1962). It is seen that the rate of occurrence of the known eruptions is small up to about AD 1500, and after that this rate becomes much greater. This observation shows that the information available about the volcanic eruptions in the Hellenic arc is complete after AD 1500, and only the data of the last five centuries can be considered complete and therefore suitable for statistical treatment. Since only the volcanic hazard of Santorini is of interest, we will try to use these complete data for probabilistic prediction of the next strong volcanic eruption there.

The largest eruptions of the Santorini volcano during the last five centuries are those which occurred in 1457, 1573, 1650, 1707, 1866 and 1925. The return period, T, that is the time difference between successive volcanic eruptions, is 116, 77, 57, 159 and 60 years. The mean return period is 93.8 years with a standard deviation equal to 43.4. That is:

T = 93.8 yr,  σ = 43.4 yr                           (1)

We can assume that the return period follows the normal distribution with mean and standard deviation given by relations (1) and apply the same method which has been applied for long-term earthquake prediction (Wesnousky et al. 1984; Papazachos et al. 1987) to determine the probability, P, that the next strong eruption will occur within a time t years from present (1989).

To apply this method we also need the time since the last strong eruption (1925) which is t = 64 years (1989-1925).  The results are shown in Fig. 2. It is seen that the probability of the occurrence of a strong volcanic eruption in Santorini is about 0.1 for the next 10 years, 0.6 for the next 50 years and 0.95 for the next 110 years.

A MODEL FOR THE DETERMINATION OF THE ERUPTION POTENTIAL OF THE SANTORINI CALDERA TODAY

Since the activity of the Santorini volcanic centre is limited to the caldera of Santorini during the last three centuries (after 1650), we made an effort to examine the possibility of the determination of the eruption potential of this caldera, that is, of the determination of a measure of the volcanic energy which has been concentrated under the caldera and will be released as volcanic eruption. We made the hypothesis that such a measure of the available volcanic energy is the total duration, t (in years), of the eruption phase which is due to the release of this energy. If our hypothesis is correct, this duration t must: a) be a measure of the magnitude of the volcanic eruption, that is, the larger the volcanic eruption the longer its duration, and b) must increase with the time, T, elapsed since the previous eruption, because it has been observed that the longer the elapsed time, T, the larger the new eruption (Akylas 1925).

Table 1 is based on information given by Georgalas (1962) and shows the dates of the start and end of each eruption as well as the duration, t, of each eruption and the duration, T, of the quiescence period before the corresponding eruption. The duration of the 1573 eruption is not given in this table because it is not known. Also, no information is given for the 1950 eruption because this eruption was rather small in comparison with others listed in this table.

Table 1: Dates of the volcanic eruptions in the caldera of Santorini, duration, t, of each eruption phase, and quiescence time, T, preceding the correpsonding eruption

If we arrange the eruptions which occurred in the caldera of Santorini during the last three centuries in decreasing order of duration t, this order is 1866 (t = 4.7 yr), 1707 (t = 4.3 yr), 1925 (t = 2.6 yr), 1939 (t = 1.7 yr). It is seen that this is also the decreasing order of the magnitude (or intensity) of these eruptions. The eruption of 1866 was the greatest because during this eruption Nea Kameni, which was created during the 1707 eruption, became four times larger, and the 1925 eruption was clearly smaller than the 1707 and larger than the 1939 (Georgalas 1962). Therefore, our first condition for our hypothesis, that is, that the duration, t, of each eruption phase increases with the magnitude of the eruption is valid.

Fig. 3 shows the duration, t, of each phase of the four eruptions (1707, 1866, 1925, 1939) in the caldera of Santorini against the preceding quiescence time, T, of each eruption. The linear relation between these two quantities (t, T) is obvious. Therefore, the second condition for our hypothesis is also valid, that is, the longer the quiescence time since the last eruption, the longer the duration phase of the new eruption.

Therefore, we can conclude that the quiescence time, T, of the volcanic centre of the Santorini caldera can be considered as a measure of the available volcanic energy which will be released during the next eruption phase. In addition, the linear quantitive relation between t and T can be used to determine t since T is known. That is, we can calculate a measure of the magnitude of the volcanic eruption which will occur at any time in the future if the available volcanic energy at that time is released and produces an eruption. By the application of the least squares method to the data of Table 1 we get the relation:

t = 1.61 + 0.02T                                       (2)

Fifty years have elapsed since the last important eruption in the caldera of Santorini (1939). Using this value in relation (2) we get t = 2.6 years. By comparing this value with those shown in Table 1 we can conclude that if the available volcanic energy under the caldera of Santorini were released today it would produce a volcanic eruption of the same magnitude with that of 1925. The relation (2), which expresses a magnitude predictable model, can be used to estimate a measure of the available volcanic energy at any time in the future.

POSSIBILITY OF SHORT-TERM PREDICTION OF VOLCANIC ERUPTIONS IN SANTORINI

Short-term prediction of a volcanic eruption means accurate determination of the place where the eruption will occur, of its magnitude (or intensity) and of the time when this eruptions will occur.  

With respect to the place of the eruption, the problem is simple in the present case, because the available information shows that the probability of a large eruption occurring in other places of the volcanic arc of the southern Aegean is very small. In other words, the first of the three parameters (space, magnitude, time) is known with a high degree of probability.

The problem of the magnitude of the next large eruption in Santorini is also relatively simple, because, as we have already seen, this eruption will be of the same order of magnitude as the largest eruptions which have occurred during the last five hundred years here and the sooner this eruption occurs the smaller will be its magnitude.

The prediction of the accurate time of occurrence of a volcanic eruption is a difficult problem. However, this is rather easier in comparison with the corresponding problem of earthquake prediction, because the precursory phenomena are much clearer in the case of volcanic eruptions than in the case of earthquakes. It is known that before the occurrence of large eruptions precursory phenomena of several kinds have been observed. The most frequent are: swarms of earthquakes, unusual increase of temperature, unusual release of gases; geomorphological changes in the volcanic area and geophysical changes (gravimetric, magnetic). From the description of the volcanic eruptions in Santorini published by several authors it is clear that precursory phenomena of the first category (swarms of earthquakes) have been observed before almost all known large volcanic eruptions in Santorini. Therefore, we can conclude that the installation of a small network of seismometers in the area of Santorini for continuous recording of earthquakes, regular measurements of the temperature and systematic observation for detection of other unusual phenomena can conuibute to the prediction of the next large eruption of the Santorini volcano.

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