Popular Science Monthly/Volume 28/April 1886/Earthquakes in Central America
|EARTHQUAKES IN CENTRAL AMERICA.|
OF THE METEOROLOGICO-SEISMIC OBSERVATORY AT SAN SALVADOR.
CENTRAL AMERICA is probably the region of the globe in which the manifestations of volcanic and seismic phenomena are most frequent and continuous. During my residence of four years at San Salvador, 1 have been able to write the detailed history of twenty-three hundred and thirty-two earthquakes, one hundred and thirty-seven volcanic eruptions, twenty-seven ruins of important towns, and the formation of three new volcanoes. Geographically, Central America, founded on the Cordillera of the Andes, forms a connecting link between the two great continental masses through three successive isthmuses, those of Panama or Darien, Izabal, and Tehnantepec. It descends to the Atlantic in two large wedges, ending in Capes Gracias d Dios and Catocha, and rests abruptly on the nearly rectilinear coast of the Pacific. The base of the Cordillera is of primitive formation, and its western flank, with which we are concerned, is formed of Miocene and Pliocene strata, terminating with Quaternary and modern alluvions and more or less recent volcanic flows.
Parallel with this axis runs the remarkable string of volcanoes which, from Chiriqui in the State of Panama, to Soconusco in Mexico, includes not less than one hundred and forty-three volcanic mountains or craters, thirty of which are active, or have been within the three hundred and sixty-three years that separate us from the Spanish Conquest. They do not present themselves, as is generally believed, upon a straight line or along a volcanic fault, nor even on a line broken at two or three points, but in a zone having borders parallel to the Pacific coast, with an average width of about thirty miles. This formation arises from the fact that Central America has had three successive shores, recording as many periods of least movement in the increase of the Cordillera, to each of which corresponds a line of contemporaneous volcanoes. The most ancient shore was of the Miocene period, when a system of trachytic and basaltic eruptions took place; then in the Pliocene rose the chain of the largest number of extinct volcanoes; while in the Quaternary and modern periods appeared the line of existing volcanoes and of others that have since become extinct. It is apparent, then, that the volcanic force has always been near the shore of the ocean, and has moved successively from the east to the west, so as to be at only a short distance from it, as the Cordilleras in their progressive elevation carried the shore farther in that direction. These views, incontestable to me, are plainly read on the strata of the country.
The system of volcanoes is completed by a chain of lakes alternating with them. The principal lakes are those of Managua and of the roads of Fonseca, the latter of which has been put in communication with the ocean by means of some volcanic convulsion. The roads of Nicoya and Chiriqui seem to me to be of the same origin. This part of the system is surely one of the most remarkable aggregations of lakes and volcanoes in the world, and strikingly reminds us, but on a grander scale, of that of the lakes of Limagne, Issoire, and Brassac, with the chain of the puys of Auvergne, which would correspond with the chain of the Marrabios. Starting at the roads of Fonseca, the chain of lakes and volcanoes continues, the former diminishing in importance, to San Salvador and Guatemala. I am not speaking of the numerous picturesque crater-lakes which we meet everywhere in Central America, and which I regard as an accident of no particular importance.
A phenomenon well worthy of attention may be observed at the foot of the chain of volcanoes near Ahuachapan, in San Salvador, in the Ausales, some three or four hundred conical tunnels scattered over a space of about three square leagues, their diameters varying from three or four metres to thirty or thirty-five metres, from which occur, at short intervals, eruptions of vapors, boiling water, and argillaceous mud of many colors. They are grouped by dozens very close together, and poison the plain with their acid and sulphurous emanations. The ground around them resounds under the feet of the traveler, but only along lines which seem to be immediately over the subterranean channels through which the hot water and gases circulate.
From this multiplicity of volcanoes it results that the ground presents a complicated net-work of ancient and modern lava-flows, crossing one another, volcanic alluvions, beds of cinders and tufas, "bad lands," and an extraordinary thermal activity. There also follows a remarkable frequency of earthquakes and subterranean noises, called retumbos. I estimate the average number of shocks felt annually in Central America at two hundred and fifty. Several conclusions may be drawn from the study of the twenty-three hundred and thirty-two earthquakes that have been registered since the conquest. First, contrary to the opinion generally prevailing from Chili to Mexico, the tremors occur about alike through the whole year, and not principally at the transitions between the rainy and dry seasons. But, to perceive this clearly, it is necessary to leave out of the account some series of earthquakes that mask the truth, such as that of December, 1879, at San Salvador, in which more than seven hundred shocks occurred in ten days, and which was the prelude to the appearance of a new volcano in the center of Lake Ilopango. With this precaution, a tendency to equality may be observed between the several months, and I am satisfied that a term of four years will be sufficient to make this equality plain. The same may be said of the retumbos. The maximum of eruptions appears to occur in July. Kluge puts it in August for the whole globe. The coincidence which the same author has predicated between the maxima of auroræ boreales and sun-spots and of volcanic and seismic manifestations has not been historically verified in Central America. The minute study of twenty years of observations at the Institute of Guatemala and my own observations at San Salvador have proved to me that, if the movements of the crust of the earth are connected with those of the barometer, the law of the relation is deeply hidden. I do not deny it, but I have observed nothing analogous to what Scrope believes he has established for Stromboli, and Waltershausen for Etna. Earthquakes and retumbos are apparently more frequent at night than in the daytime. I say apparently, because it may be that manifestations, quite perceptible in the stillness of the night, pass unobserved amid the bustle of the day. From what I have seen, I think I can affirm that the signs of terror given by domestic animals are more marked the longer the shock lasts, and that without reference to its intensity.
While I do not think that it is possible in the present state of knowledge to predict earthquakes, I believe that the phenomena are frequently connected with an indefinable aggregation of atmospheric conditions which, subjected to many years of study, might lead to the discovery of some law. This is so true that persons who have lived long in the country often say when they meet, without knowing why, "There will be an earthquake to-day"; and they are seldom mistaken. Towns in Central America, situated near active volcanoes, have much less to fear than those which, being in the dangerous zone, are more distant from them. This may be proved by the local history. Guatemala was destroyed seven times, between 1541 and 1773, while it was near the extinct volcano of Agua; but it has not suffered since 1775, when it was removed to its present position near the active volcano of Fuego, of which forty-four eruptions have taken place. Izalco, built on the flanks of Izalco, a volcano which has had since its formation in 1770 an eruption about every twenty minutes and twenty-one considerable ones, has never been destroyed, nor have Santa Anna, San Miguel, and Masaya, on the slopes of the volcanoes of the same names, which have had respectively seven, ten, and six great eruptions. San Salvador, which is built on the slopes of Quetzaltepec, has been wholly destroyed fourteen times, the last time on the 19th of March, 1873. This volcano may be regarded as extinct, for it has had only one eruption since the conquest, that of the 30th of September, 1659, when the cinders flew as far as Conmyagua, the capital of Honduras, and the lavas formed the immense "bad land" (cheyre) of Quetzaltepec and buried the Indian city of Nejapa. The principal of the eight craters of Quetzaltepec (or San Salvador as it is otherwise called) is remarkable for its perfect regularity and its size, six hundred metres in diameter and depth. The bottom is occupied by an almost inaccessible lake. The appearance of the volcano of Lake Ilopango, in 1879-'80, probably saved San Salvador from a fifteenth destruction. Omoa and Jucuapa, built on the slopes of the extinct volcanoes of the same names, were destroyed on the 4th of August, 1856, and the 2d of October, 1 878.
In a work published by the Government of San Salvador on "Earthquakes and Volcanic Eruptions in Central America," in which I have given a detailed history of the phenomena, I have been able to show, from original documents, that the destruction of Guatemala, on the night of the 10th and 11th of September, 1541, was due, not to an eruption of mud from the extinct volcano of Agua, as some authors suppose, but to the rupture under the weight of the water, assisted by an earthquake, of the walls of its crater, which had been filled by the extraordinary rains of the preceding days. The eruption of Pacaya, on the 18th of February, 1651, and the ruin of Guatemala, which it occasioned, were accompanied by the spectacle of frightened wild animals seeming to seek the protection of man, as they did also during the eruption of Coseguina on the 20th of January, 1835. The year 1770 witnessed the rise of Izalco—"the Lighthouse of the Pacific"—a magnificent volcano, whose eruptions have since followed one another uninterruptedly about every quarter of an hour, with explosions that are frequently heard for ten leagues around. The great eruption of Coseguina, on the 20th, 21st, 22d, and 23d of January, 1835, perhaps one of the most formidable eruptions mentioned in history, the cinders from which flew as far as to Vera Cruz, Havana, Carácas, and Bogotá, was heard over the same circle of seventeen hundred miles in diameter. The well-proved coincidence that these eruptions began on the same day with those of the Chilian volcanoes of Aconcagua and Corcovado, all three situated in the chain of the Andes, is too remarkable not to attract attention. The environs of the active volcano of Momotombo from the 1st to the 20th of April, 1850, witnessed the emergence of the new volcano of Las Pilas, now extinct.
A fact remarked by Humboldt as accompanying the earthquake of the 4th of November, 171)9, at Cumana, was also observed at Guatemala on the 8th of December, 1859. I refer to a sudden and considerable deviation of the magnetic needle, which still continues. To account for it, I propose the theory of a change by the shock in the disposition of the neighboring strata.
A series of more than seven hundred shocks between the 20th and 31st of December, 1879, two of which were disastrous, and which caused much alarm at San Salvador, was the prelude to the appearance, in the neighboring Lake of Ilopango, of a new but ephemeral volcano, whose mass caused the lake to overflow its banks and to produce a terrible inundation in the valley of the Rio Jiboa. The event has been made the subject of a detailed and very interesting study by Messrs. Goodyear and Rockstroh. I will only observe respecting it that two hundred and thirty-seven explosions took place on the 4th of March, 1880, between twenty-five minutes past nine and twenty minutes past ten in the morning, and eight hundred and ninety-seven explosions between eighteen minutes past seven in the evening of the following day and seventeen minutes past three on the next morning.
The retumbos heard at San Salvador and in Colombia on the 27th of August, 1883, were doubtless the echo of the eruption of Krakatoa. I am satisfied that if such a work as I have performed for the small fraction of Central America were done for the whole system of the Cordilleras, from Cape Horn to Behring Strait, and if the different governments would establish meteorologico-seismic observatories, like the one I have directed for four years at San Salvador, it would be possible, in this home of volcanic activity, to form some sound theory of these interesting and terrible phenomena, and perhaps to find some means of announcing them beforehand, as we predict storms on the Atlantic.—Translated for the Popular Science Monthly from the Revue Scientifique.