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Remarkable Earthquakes in Central New Mexico in 1906 and 1907
By Harry Fielding Reid
Between July 2, 1906 until well into the year 1907 scarcely a day passed that slight shocks or tremors were not felt at Socorro, in the middle of New Mexico, and in its vicinity; and shocks severe enough to do some damage occurred on July 12th and 16th and on November 15th. The series was inaugurated by smart shocks at 3:15 and 3:30 a.m. on July 2d, felt probably everywhere within fifty miles of Socorro. At Socorro they were strong enough to upset small objects (VI)[*], and at Magdalena, twenty miles west, somewhat less strong(V). Six shocks were reported in Magdalena between 3 and 4 a.m. and ten shocks in Socorro between 3:15 a.m. and noon; tremors continued during the afternoon and night. On July 7th smart shocks were felt at Socorro at 1:30 and 4:10 a.m. and at Magdalena about 3 a.m. (All times are in Mountain Standard Time, seven hours slow of Greenwich mean time.)
On July 12th came the first severe shock at 5:15 a.m. which lasted from fifteen to twenty seconds at Socorro (VIII). The walls of many adobe houses were cracked and some brick chimneys were thrown down. Many boulders were shaken down upon the branch railroad to Magdalena a few miles west of Socorro, breaking one rail and a number of ties. The following places report having felt the shock: Socorro, and small towns nearby (VIII); San Antonio (VII-VIII), Elmendorf, sixteen miles south of Socorro, and Carthage, sixteen miles southeast, (VI-VII), San Marcial (VI), Rosedale (IV?), Alamagordo, Silver City, Lake Valley (III-IV), Albuquerque, El Paso (III). It was also felt on the ranches to the east and southeast of Socorro. A slight shock was reported from Fort Wingate about 5 a.m. on July 11th; it is probable that this is an error and that the shock was felt on the 12th; Socorro reports only very slight shocks on the 11th and no other place reports any. The following towns report
― 11 ―not having felt the shock: San Mateo, Estancia, Las Lunas, Peralta, Cerrillos, Las Vegas, Santa Rosa, Deming, and Roswell; the first three were probably near the limit of the sensible shock. Many light shocks were felt during the rest of the day in the general neighborhood of Socorro.
The second severe earthquake came on July 16th. Slight shocks were felt in Socorro on the previous night and a sharp one at 6 a.m. (VI?), another slight one at 10 a.m., and three strong shocks at 12 noon, 12:05 and 12:08 p.m. Lighter shocks were felt at 12:30, 1, 1:30, 2:40, 3, 4:10, 6, 8:45, 10, and 11:20 p.m., also on the 17th at 1, 3, 4:20 and 6 a.m., and more on the succeeding days.
The midday shocks of July 16th were slightly stronger than the shock of the 12th. They upset more chimneys, cracked more houses, and threw out some brick gables in Socorro, made a few cracks and overthrew a few chimneys in San Antonio and increased the apprehensions of the inhabitants. Many of the people of Socorro left their houses and lived in tents, not in general because the houses were so badly damaged, but from fear of more severe shocks which might bring the houses down upon them. The Socorro Hotel, a brick building in the eastern part of the town was abandoned on account of the injuries it had sustained, and the majority of the houses of the town suffered some damage. The southeast corner of the brick post-office was thrown out, and its eastern wall bulged; the court house and the high school lost several chimneys; but the School of Mines building, about one mile from Socorro, was not injured. A crack appeared in the adobe walk near the station, but this was insignificant, and apparently not due to the earthquakes but to drying during the hot weather. The hot springs near Socorro did not have their temperature increased as reported. Sensational accounts were published in the papers a few days later stating that Socorro was in ruins, that the inhabitants were fleeing and that a drenching rain had increased the misery. These accounts were so exaggerated as to be practically untrue; a few persons left Socorro, and the rain was unimportant.
The shocks of the 16th were felt practically over the same region as the shock of the 12th and were reported as follows: Socorro and neighborhood (VIII), San Antonio (VII-VIII), Sabinal (V-VI), Rosedale (V), Hillsboro (IV-V), Peralta (IV), Datil (IV?), Albuquerque, Rincon, El Paso (III-IV), Alto, Lake Valley, Fort Bayard (III). The shock was reported as felt at Raton, 235 miles from Socorro to the northeast, and at Douglas, Arizona, 250 miles to the southwest,
― 12 ―but it is doubtful if these reports are correct. A train on the Magdalena branch of the railroad about ten miles west of Socorro had stopped to remove some boulders from the track, when the strong shock occurred; it nearly derailed the train and was apparently more severe at this point than at Socorro.
Light shocks and tremors continued to be felt almost daily in the vicinity of Socorro, San Antonio and Magdalena with somewhat sharper ones on July 25th at 11:50 a.m., and 30th at 3 p.m., August 5th at 11:20 p.m., and 21st, at 3:30 a.m., October 12th at 1:45 p.m., and 23d, at 11:30 p.m., November 4th at 8 p.m. and December 19th at 5 a.m.
The severest shock of the year occurred on November 15th at 5:15 a.m. and was felt generally throughout central New Mexico. It increased the damage already done in Socorro, but was not much more severe than those of July 12th and 16th. Peralta reported a lighter shock at 2:10 a.m. and about thirty light shocks were felt in Socorro in the course of the day.
The intensity at various places was as follows: Socorro (VIII), San Antonio (VII+), Magdalena, Sabinal (VI-VII), Rosedale, Peralta, Laguna (V), Datil (V?), Hillsboro, Lake Valley, Alamagordo, Willard, Carpenter (IV), Albuquerque (III-IV), Estancia, Torrance (III+), Santa Fe, Cerrilos, Rosewell, El Paso (III), Las Vegas (III?). These have been entered on the map, Fig. 1, and the isoseismal lines drawn. The data are far too meagre to yield more than rough approximations to the positions of the lines. If a circle of 180 miles radius be drawn with Socorro as its center, it will include all places that felt the shock; it encloses an area of about one hundred thousand square miles, and it seems not improbable that the shock could have been felt over all this region.
It has been impossible to draw the isoseismals higher than V on account of insufficient information, for the region is not thickly settled. The isoseismal III of the shocks of July 12th and 16th probably did not differ much in position from isoseismal IV of November 15th and enclosed an area of about 40,000 square miles.
The smaller shocks usually gave the impression of a single vibration forward and back again; the stronger ones consisted of several vibrations, and the very strong ones apparently had vibrations in different directions, as is usual with strong earthquakes. Sounds, sometimes sharp, sometimes dull, and sometimes like a distant rumbling, occurred at the time of many of the shocks, though some shocks seem not to have been accompanied by sound.
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In seeking the origin of this series of shocks we naturally examine the geology of the region. The Rio Grande flows in a broad flat valley on old river deposits hundreds, and in places perhaps more than a thousand, feet thick. The valley is limited a few miles from the river on both sides, by a series of mountain ranges. The whole region is cut by many faults. Flows of eruptive rock have taken place since the Tertiary Period; the Socorro Mountains, immediately west of Socorro, are eruptive; and very recent flows have occurred near Socorro, near San Marcial and elsewhere. The Magdalena Mountains consist of stratified rock resting on early igneous or metamorphic
― 14 ―rock, through which more recent eruptives have forced their way; faults are very numerous.
The formation of a new fault or the movement on an old one, and volcanic explosions, are the two recognized causes of strong earthquakes. The lack of any definite evidence of volcanic activity, the sudden beginning and long continuance of the shocks, their general character and the considerable distance to which the stronger ones were felt, incline us to refer their origins to breaks or slips of the rocks; and the distribution of the intensity suggests that the slips occurred near Socorro--to the west, northwest or southwest. This suggestion is supported by some further evidence. The overthrown chimneys and gables at Socorro fell to the east and the southeastern corner of the postoffice fell outward. The chimneys and gables had their greatest lengths north and south, which would oppose their falling along that line; the fact that they fell to the east and not to the west indicates an origin in general to the west and not to the east as might be supposed; for in any sudden shock the return movement is apt to be stronger than the first forward movement; and in this case the return movement of the earth would have caused the chimneys to fall away from the origin, which would therefore be to the west. We have an example of this principle when window glass is broken by a very loud noise, such as the firing of cannon. The glass falls outward; the first forward movement of the air wave presses the glass in, and the stronger return movement draws it out and breaks it.
About the middle of July Mr. C. B. Allaire at San Antonio and Mr. J. J. Leeson at Socorro, hung up heavy pendulums to record the movement of future shocks. Mr. Allaire's pendulum marked with a pencil on paper, and Mr. Leeson's with a point in a saucer of flour. Before the end of the month Mr. Allaire had recorded two shocks strong enough to yield some information regarding their character. The first indicated a sudden movement of the earth to the southeast; during the return movement there was added a much smaller vibration in a northeast and southwest direction. The second record was like the first but the movement was east instead of southeast. Mr. Leeson reports that his pendulum indicated movements from the northwest. Dr. Bagg set up a similar pendulum in the School of Mines in 1904 and recorded a shock on the night of March 8th of that year in which the first earth movement was toward the east (see below). Unfortunately this pendulum was not kept in operation. From the little information obtainable of the region southeast of Socorro, the intensity
― 15 ―of the shock there seems to have been less than near Socorro or a little west of it.
About three and a half miles southwest of Socorro is a lavacapped hill; a comparatively recent fault has cut through the lava and lowered the eastern portion about one hundred feet relatively to the western. A little further east, near where the Magdalena railroad crosses the arroyo, there are two faults in a sandy mud flow. It is, therefore, clear that earth movements have occurred in this region in very recent geologic time, and are probably still continuing.
These various observations lead to the conclusion that the origin of the shocks was a short distance west, northwest or southwest of Socorro. It is not probable that all the shocks occurred at exactly the same point or necessarily on the same fault, but only within the same limited region. Mr. C. T. Brown, a well known mining engineer of Socorro, examined the mines in the Magdalena and in the Socorro mountains and states that no movement occurred on the faults there; this means either that the movement was more deep-seated than the mines, or that the origin was not in these mountains.
The shocks were not very severe, and on rock foundation would scarcely have reached an intensity of more than VII; the water-soaked alluvium near the Rio Grande intensified the damage, and the lack of foundation to the houses and the poor character of the construction (the majority of the houses were built of adobe without any cementing material) permitted greater damage than would otherwise have occurred; and an intensity and destructive power was ascribed to the shocks which they did not actually have.
It is very unfortunate that there were no seismographs in New Mexico, which might have yielded more definite information about the character of the shocks; the very simple devices used for a short time at Socorro and San Antonio have helped materially in pointing out the place of origin.
Mr. J. J. Leeson, voluntary weather observer at Socorro, has sent me a very complete list of the shocks felt in his town from July 2, 1906 to January 16, 1907, which, as already stated, occurred almost daily. Unfortunately his records after the latter date were destroyed by fire, so that we do not know exactly when the frequent tremors ceased.
Light shocks have been reported at Socorro on June 6th, 16th, 17th, 28th and 29th and on July 7th, 11th and 21st, 1907. That of June 28th was also felt at Magdalena. Several shocks occurred in 1908 and five fairly sharp ones in 1909, but none during the first five months of 1910.
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It seems characteristic of this region that earthquakes occur in groups. Dr. Rufus Bagg[*] has given some account of a series of thirty-four shocks which occurred between January 19th and March 8, 1904, and referred them to local `displacements in the Socorro Mountains and its outliers." A long series of shocks is reported to have occurred between 1898 and 1900, but I have been unable to get definite information about it. Strong shocks were reported by Dr. Bagg on April 29, 1868, in April 1869, on July 6, 1886, and in 1897; that of 1869 seems to have been the strongest shock recorded before 1906; none of them seem to have done any material damage.