domingo, 5 de julio de 2015


Bulletin of the Seismological Society of America September 1912 2:201-208

Preliminary Shocks.--During the month of May very few seismic disturbances were registered at the national observatory, and those few were of low intensity. On June 1st at 6 h. 26 m. a.m., the first shock of this seismic period was registered, and consisted of two sharp jerks separated by minor vibrations. On the 3rd at 3 h. 50 m. p.m. there was another short shake having an intensity of II that was noticed by many persons. These shocks were all so slight that no notice was received concerning them from other parts of the republic.

The Sarchí earthquake.--On the 6th at 00 h. 40 m. in the morning a long shock was felt having an intensity of VI R.-F. scale. This caused a general panic, and a large number of the people remained awake the rest of the night in fear of heavier shocks. The detailed study of this earthquake must be incomplete because twenty-seven minutes before it took place a small shake disarranged the Ewing seismoscope so that the traces of the three components are blended into a single line. The other seismographs, including those of the Women's College and that of Señor A. Pérez at Mojón, give clear tracings, but they make it possible to reconstruct only a part of the disturbance.

Record of the June 6, 1912 earthquake.

The earthquake began with a series of vibrations which reached their maximum in a few seconds with oscillations violent enough to awaken persons asleep. The amplitude of the movement diminished rapidly until it almost stopped, but within three seconds the movement was renewed with increased violence. This strong shock lasted from ten to eleven seconds and ended with a rocking movement of great amplitude which the seismographs show to have had a dominant direction of northwest-southeast. Electric globes and other hanging objects swung in the same direction as was observed at the time of the Toro Amarillo earthquake. Some clocks were stopped. Twenty minutes later a roar was heard, but it stopped suddenly, and was followed by another lighter one. This earthquake was very similar to the Toro Amarillo shock of August 28th of last year. Only a few notices were sent in by telegraph and many of these were of little value. It was necessary to call for other and more exact data, and even then the information did not come or it was brief and obscure. However, the information received went to show that the shock was felt from Puntarenas to Limon with less violence along the Pacific coast. To the north in Las Cañas, Santa Cruz, Colonia Carmona and Muelle of San Carlos, the movement was said to have been almost imperceptible. Don Theodoro Koschny reports, however, that the intensity at his place was VI R.-F. scale. In the south it was reported as fairly strong at San Marcos and thereabout; but the little information received shows that still further south the intensity was very low.

The following morning it was known that this earthquake had been very serious in Grecia, Sarchí, Naranjo and other neighboring settlements, both on account of the destruction caused by the earthquake itself and on account of the enormous flood that followed it. For the purpose of locating the epicenter, determining the area of the disturbance, and of gathering data regarding the damage done by the earthquake and by the floods, and for determining the origin of the floods,

The authors of the present report went to Grecia on the 10th of June. One of us started on the evening of the 7th and, accompanied by a photographer, Sr. Gómez, for two days travelled over the whole southwestern part of the area affected. On the day of our arrival at Grecia and at our suggestion Sr. Francisco Arias Fernandez called a meeting of some of the important persons who are acquainted with the region damaged. At this meeting were present señores Francisco Arias Fernández, Dr. Santiago Zamora, Dr. Julio Borbón, señores don Hermenegildo Bolaños, don Pedro Quirós, don Alfredo Gómez, and don Pantaleón Serrano. The special purpose of this meeting was to get a general idea of the region, to ascertain the courses of the streams and the general aspect of the entire zone affected. The information furnished by these gentlemen was of the greatest service, and decided us to set out for the headwaters of the river Sarchí on the 11th in order to ascertain the cause of the floods. A trip in the direction of Toro Amarillo offered many serious difficulties.

The damaged zone.--The evidences of the greatest violence were shown on the crest of the range that separates the great collecting basin of the Rio Colorado, an important affluent of Rio Grande de Tárcoles, from the headwaters of the tributaries of the river Toro Amarillo, which is about five kilometers west of the volcano Poás. Numerous brooks and rivers drain the summit, and the high hills and slopes between the streams are highly cultivated.

The small area seriously affected is drained on the Rio Grande side by the following streams, named from west to east: the rivers Rosales, El Vigía, San Juan, Sarchí, and Trojas, all of which occupy deep gorges. The rivers Anonos and E1 Gorrión drain the Toro Amarillo slope, the former draining the volcano Poás. The details of the topography of this region are not known and no data are available concerning the geological structure. Two of the rivers mentioned, el Sarchí and el Anonos, drain the opposite sides of a flat divide, having as their source a small swampy area, deficient in vegetation, called "Laguna Vieja." The others mentioned also head at great elevations, and their smaller tributaries are formed by the run-off from the highest hills.

The area affected.--The area materially affected is relatively small. Alajuela was not injured, destructive action commencing at San Isidro, San Pedro, Grecia and Sarchí. The majority of the houses are of adobe, a few of stone rubble work, and some made of double walls each consisting of posts about which cane is woven, and filled with clay. Those well-built successfully resisted the violent shaking. At Grecia we noted a number of public buildings in good condition or only slightly injured. Others, however, were seriously injured and some do not admit of repair. No disturbance of roof tiles was noted, as occurred at Tres Rios during the earthquake of February 2Ist of this year, nor were there any large ruptures in the walls. We are of the opinion that most of the destruction was due to defective structure or poor materials. As already mentioned, we left Grecia on the 11th with Dr. Zamora, don Alfredo Gómez, and don Pantaleón Serrano, for the headwaters of the river Sarchí.

Map of affected areas.

We observed no material damage to the houses on either side of the road although some were poorly constructed of inferior lumber. At San Juan we noticed an adobe house completely destroyed, as may be seen in the accompanying photograph, which formed an interesting contrast to a second wooden house which was absolutely uninjured. We continued along the road on the crest of the hill between the rivers Sarchí and San Juan. A few narrow and short cracks were noted. Farther up the narrow and tortuous road became a trail, penetrating the woods and not yet cleared of undergrowth. Travel was made difficult on account of the branches of the trees, and especially on account of the cracks which we began to note in great numbers. The horses refused to go further and we were compelled to leave them and to continue on foot.

The crevices increased in number, and in places the earth appeared as if turned up by a great plow. On leaving the woods, suddenly an imposing picture was spread out before us: on the left the deep valley of the river Sarchí, in the center the continuation of the hill, and on the right the depths of the river San Juan. In the background the crests of the range projected boldly, showing almost its entire length. Vertical walls indicated great slips on both sides of our ridge; the ground was ruptured by deep long fractures. The double photograph gives a good idea of the enormous mass of earth and rock which was broken off by the earthquake, and was precipitated chiefly into the valley of the river Sarchí. We continued our journey the length of the ridge. The country appeared more and more disturbed, with large deep cracks which completely separated large areas of loose ground. In other places various slips had been held back by the irregularities of the country and formed a strange mixture of trees, leaves and earth. On both sides, as far as could be seen, it was the same; the earth was loose, turned up and about to slip.

We could not carry out our plan of descending into the deep valley of the river. Any such attempt would have been dangerous. We stopped at one place where the ground seemed more firm in order to observe attentively the great changes that had been wrought. Don Pantaleón Serrano, who was well acquainted with the region, pointed out the areas which suffered greatest changes, all of which confirmed our idea of the origin of the flood, which we will explain later. One of us visited the hill situated between the river Sarchí and El Trojas, known as "the island of Matamoros." As can be seen in the photographs, the cracks are large, and the ground presented the same appearance as the area just described. The description which we received of Toro Amarillo coincided with the above to the smallest details. The flood came down mainly via the river Anonos and on the north of the range. There were also great landslides. We estimate the area that slipped down as about 500 manzanas 2 on each side (A manzana is a square measure of 100 x 100 yards).

View of the Sarchi river flooding.

While the great number of fissures observed on the crests of the hills was caused in part by the sudden slipping of the sides, yet the intensity of the shock or shocks, which was very high, contributed to their formation, and especially to those of moderate depth. The special topography of the area also was favorable to rupturing and landslides, but in neighboring places of like topography no such damage occurred. The great disturbance affects the crest between the rivers Sarchí and Anonos. The epicenter includes the higher region, about 1 to 1.5 square kilometers. An earthquake of such violence as this lost intensity rapidly. At a distance of 34 kilometers (at San Jos6) it caused no damage. At 105 kilometers, at Colonia, Carmona, it was scarcely noticeable, and at Las Cañas, 97 kilometers distant, it was also weak. At San Marcos (65 kilometers) it was not severe.

The substance of the above may be summarized as follows: The earthquake of the 6th of June, or the earthquake of Sarchí, had its epicenter five kilometers west of the volcano Poás at the summit of the range, between the sources of the rivers Sarchí and Anonos. Its origin or epicenter was not deep and the area of high seismic intensity was very limited. Various smaller shocks occurred some hours before the great earthquake, almost all proceeded by a seismic rumbling, followed by others that were not strong enough to be registered at San José, The subsequent shocks were very insignificant. On the 9th, at 9:16 a.m., the Ewing seismograph registered a very distinct tracing on two successive shocks thirty-five seconds apart. The second was the stronger and lasted thirty-six seconds. At 1:50 p.m. a microseism was registered. One of us (Biolley) was at the headwaters of the river Sarchí and describes the phenomena as follows: "I arrived at the headwaters of the river Sarchí at 9 a.m. A few minutes thereafter occurred a vibratory temblor, which I estimate of an intensity of VII, and this was followed by thirty-fir e other shocks, intensity VI and V and the weakest at IV, besides more than a hundred between III and II." Such activity was purely local, for only the double shock at 9:16 was felt at San Jos6, thirty-four kilometers distant. This confirmed the superficial origin of the entire series of shocks which comprise the earthquake of Sarchí. Since the epicenter of these earthquakes is so close to the volcano Poás, one immediately queries: Are these disturbances of volcanic or tectonic origin? 

It is a fact that no abnormal phenomena were observed within the crater of Poás. Some eruptions occurred, both large and small, but one cannot consider these ordinary manifestations alone as evidences of a greater activity. At times of great eruptions (1905) no earthquakes were felt whose epicenter was in the neighborhood of the volcano. Is the earthquake of Sarchí one of the many manifestations of the seismic activity which has kept us in constant alarm during the last two years?

Concerning these definite points, we do not hesitate to confess that any opinion is premature, for we lack the numerous observations necessary in order to deduce any opinion as to the cause or causes of the earthquake. On the other hand we make no predictions of any kind, nor discuss theories more or less well founded. The diverse theories and explanations concerning large or small earthquakes are more or less matters of personal opinion, and as long as science offers us no guidance by means of observation and experience, it is not possible to reach conclusions regarding points still under discussion. Let us then leave the discussion of causes of this earthquake, whether volcanic or tectonic,-to await the time when these become more definite in one sense or another, or they terminate with a reduced number of subsequent shocks, as was the case in the former destructive earthquakes of Toro Amarillo and of Guatuso.

The flood.--The report by certain inhabitants appears confirmed, that ten minutes after the strong shock they heard an intense and prolonged noise that was heard over a considerable area. Don Pantaleón Serrano states that the night of the earthquake he was at his farm near the sources of the river Sarchí, and that the noise was heard "a little bit" after the earthquake. This noise, which on account of its strange character terrified all the inhabitants of the region, was produced by a tremendous flood which came down the deep gorge of the river Sarchí. It is estimated that an hour after the earthquake a large volume of water passed Grecia, taking out the bridge on the road to Sarchí. As is known, the flood descended the river Colorado and reached the Rio Grande, a distance approximately of thirty-five kilometers. By 4 a.m. it had subsided to a large extent, but there were, before and after this hour, other floods both small and large. Nearly all of the 7th, the water of the river Sarchl was muddy.

Intersection of Sarchí and Colorado rivers.

Thirty-six hours later another flood came down. The sight of the enormous mass of mud along the passage of the river Sarchí gives evidence that the inundation was considerable. The water looked like thick mud of brown or almost black color. This color is characteristic of wet ground and the odor was that of small lakes and swamps. This odor was reported as that of some sulphurous gas, and was assumed to be hydrogen sulphate or sulphur dioxide, but this opinion was not confirmed but disproved by the doctors who were present at the conference mentioned and who visited the river a few minutes after the flood.

Dr. Michaud has given us the results of an analysis made by himself which shows that the muddy water contains 8 mg. of sulphuric acid per liter, and that the residue contained aluminum sulphate and clay. Microscopic study of the sediment showed it to be the result of the decomposition of volcanic rocks. The temperature of the water of the river Sarchí was about 19 °C. (temperature of the air 23 °C.). There were no reports of the temperature of the flood water being raised, nor did those who were at work cleaning the road notice anything particular in this respect. The flood reached its maximum at a place called Cocobola, at an elevation of 1170 meters. The water not being able to pass the curves jumped over the ridges, stripping them bare of trees, branches and mud. At the juncture with the river San Juan the flood back watered a distance up that stream. On the north side of the range another tremendous flood descended simultaneously the river Anonos and carried everything before it, including the dwellings of the inhabitants, who were drowned. Up to the date of our visit the details of this flood were not known, but it was colossal. Some declare that it was greater than that of the Sarchí and calculate its width as 500 meters. It may be mentioned that in both floods a great number of dead fish were noted.

Causes.--It is immediately apparent among all the rivers that rise on both sides of the range only the Sarchí and Anonos were flooded. In all the others that drain from these same mountains there was no sign of a great flood. The waters of the rivers Itiquís, Poás and Prendas were scarcely muddied and the water plants along their shores show no sign of being disturbed. The same may be said even more forcibly of the rivers Trojas, San Juan and Vigía, for there were landslides of no small size in the headwaters of these streams which muddied up their water. The idea that both floods were caused by rains must be discarded for two reasons. First, because there was very little rain on the day of the earthquake as well as the preceding days; and second, because if this were true the same thing should have happened in all the rivers mentioned, which was not the case.

We must, therefore, search in other directions for the cause. There is a flat, known as "Potrerillos" or "Laguna Vieja," situated on the top of the range, between the headwaters of the Sarchí and Anonos. The ground is soft, wet and supports a scanty vegetation. This region is undoubtedly an ancient crater-lake, whose wails, sapped by erosion were thrown from their foundations by the violence of the earthquake, which, as has been seen, has its epicenter at the same place. The great mass of accumulated water burst forth suddenly and encountered on the way the great earth falls, and formed mud mixtures which it swept along its course with enormous velocity. It is not possible to conceive that such a mass of water could have come from any other source. The earthquake then only hastened a phenomenon which sooner or later would have taken place of itself. It is well known that one must cross two ancient craters before reaching the present vent of Poás. The soil and subsoil of these show cracks in which there is always standing water.

Carl V. Seebach, to whom we owe important studies of our volcanoes, says: "I have never heard of a more complicated volcano than Poás." It is certain that the mass known as the volcano Poás consists of a series of very ancient craters, many of which are covered by thick forests and of which we have no knowledge. The commission that undertook the study of the earthquake of Toro Amarillo indicated the existence of an immense extinct crater about 12 kilometers west of the present crater. Detailed study of the entire region will show the existence of other craters in other directions. It is not strange that the crater lake should exist between this ancient extinct crater and the present vent that caused the great flood. With the few topographic details in existence we cannot think of any other cause capable of explaining such a wonderful phenomenon. The profound chasm of the Sarchí may have been formed on account of having been the natural drain of this lake in remote times, as appears to be confirmed by the reports we received in Grecia. We were assured, moreover, that the water of the Sarchí is very like that of the Anonos. The presence of sulphuric acid and the crystals Observed in the mud suggest the volcanic nature of the region. We must add, however, that no earthquake has been registered in the National Observatory since the 9th until the present date, and the volcano Poás has not shown indications of any considerable increase in its activity. Nor have we received reports confirming the existence of new volcanoes. 

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