A Brief Note on Tectonic earthquakes Related to the Activity of Santorini from Antiquity to the Present

This is perfectly consistent with the results of other research carried out regarding other volcanoes, especially that of Indonesia. Simple mechanisms are suggested for the explanation of such relationships.

PREFACE

Blot (1976), Casertano (1976), Hédervári (1976, 1977), Nakamura (1971, 1975), Taylor (1955), Van Bemmelen (1970), Van Padang (1976), Vitaliano & Vitaliano (1971) and other workers have suggested that some kind of correlation exists or at least may exist between tectonic earthquakes and volcanic eruptions and vice versa. This suggestion appears to be very reasonable and logical, as both phenomena are the manifestations of the internal energies of the same planet, and are phenomena which can be experienced generally within the same belts of present-day tectonic activity.

As far as Santorini is concerned, Galanopoulos (1971) has also found such correlations. In general the relationships were observed in the case of phreatic and true magmatic eruptions alike and in the case of andesitic, island-arc volcanoes and basaltic shield volcanoes within oceanic (cratonic) areas alike. Many examples are treated in different papers by one of the authors (Hédervári  1976, 1977).

INTRODUCTION

We have investigated the seismological and volcanological data for Santorini as far back into the past as possible (Galanopoulos 1960, 1961; Kárník 1968, 1971; Georgalas 1962), and, comparing the available data, we have come to the conclusion that such a relationship does exist in the activity of Santorini as well.

The data were divided into two groups:

-   In the first one can find old eruptions since the Bronze Age up to 1711 A.D.

-   In the second the recent eruptions are enumerated up to the present.

For this latter period we have rather exact data for the place and time of occurrence of the shocks. Only those quakes were considered which took place within a few hundred kilometers of Santorini and within a few months before or after the commencement or end of the activity. In certain cases earthquakes occurred during the period of an eruption-cycle of the volcano. No correlation was found between the magnitude or intensity of the shocks and the intensity of the eruption. A similar negative result was obtained by one of us (Hédervári 1976) in the case of Indonesian earthquakes and outbursts. In Indonesia, however, more than 50 cases were found in which one or more tectonic earthquakes triggered eruptions or followed them within a few months and within a few hundred of kilometers measures from the volcanoes in question.

PRESENTATION OF DATA

• Table I

Earthquakes that took place within a circle with a radius of some 420 km around Santorini and within about ± 150 days from the start of a single eruption of short duration; also earthquakes preceding, accompanying and/or following longer periods of volcanic activity which lasted for months or a few years.

r : epicentral distance from the volcano, expressed in km;

t : the time-interval in days between the occurence of the shock and the commencement of the outburst; t was not calculated in cases where the volcanic actiity was of long duration. If t has a negative sign, the shock preceded the start of the eruption. If t is positive, the shock took place after the commencement of the outburst.

? : deduced, very approximately Richter - magnitudes on the basis of the epicentral intensity and curve (a) in Fig. 3. For the exact data of the eruptions see Table II. Letter "i" means intermediate shock, the others are shallow.

• Table II

Known eruptions of Santorini from antiquity to present days, after Georgalas (1962).

-   Around 1500, B.C. : Minoan eruption

-   197, B.C.

-   46, A.D.

-   726. No corresponding earhquakes were found.

-   1570 or 1573

-   1650. September 26 - December 6

-   1707. May 23 - 1711, September 11.

-   1866. January 26 - 1870, October 15. Note: According to A. Sieberg this eruption cycle started on February 1, 1866. We accepted this date; however it can be supposed that some weak activity could occur some days prior to February 1.

-   1925. August 11 - 1926, January

-   1928. January 23 - March 17

-   1939. August 20 - 1941, July

-   1950. January 10 - February 2

CONCLUSIONS

We found that almost all of the known eruptions of Santorini were preceded, accompanied and/or followed by at least one tectonic shock in the proximity of the volcano, within a circle with a radius of a few hundred kilometers and within a time-period usually less than one year. We conclude therefore that th same had to happen at the time of the Minoan eruption as well.

The explanation of such correlations between tectonic earthquakes and volcanic eruptions is as follows:

1. In the case of Phreatic eruptions that are preceded by shocks, we accept Van Padang's model (1976). According to this model, surface water can sink along the fault planes to the still very hot magma chamber, and these fault planes owe their origins to the occurrence of tectonic earthquakes in the volcano's environs.
2. In the case of phreatic eruptions that are followed by shocks, we suppose that the subground explosions can help in the release of tectonic stress. This may lead to earthquakes originating near the volcano.
3. In the case of true magmatic eruptions that are preceded, accompanied and/or followed by shocks, we accept Nakamura's model (1971).

According to this model: "As the compressional crustal strain is gradually stored toward the eathquake to occur, the volcano, located near the potential fault, is also deformed and contracts to some degree. Then the magma in the reservoir is squeezed up through the pipe. The rise of the magmatic head above a certain level in the pipe causes an eruption, which, once started, may proceed as a self-moving machine. Later, when the earthquake occurs, the strain that squeezed up the magma is released. And the head of the magma falls of resulting in the end of the eruption, in case it has still continued." Furthermore: the present author suggests that due to the removal of a remarkable mass of material from the magma chamber onto the surface and into the atmosphere, the mechanical equilibrium of the uppermost layers of the Earth in the volcano's environment can change. This can create newer stresses in the proximity of the volcano and this can lead to (further) tectonic earthquakes after the eruption, as happened after the great eruption of Katmai in Alaska, or after the second paroxysm of Agung's eruption in 1963 in Indonesia.

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