June——A smart shock at midnight at San Francisco.
August 1st.—Two light shocks at San Francisco, at 10h. 48m. P.M., and at 11h. 6m. P.M., another shock.
December 19th.—At 2h. 38m. P.M. a smart shock was felt throughout the city; directly afterwards another and more severe one occurred. The first was a short, sudden jar, while the second was undulatory. The accuracy of the telegraph operator at Santa Clara has enabled us to form a correct idea of the course of this shock, and to correct to some extent the popular errors relating to the direction of the seismic wave. His time was 2h. 44m. 31s., being within 29 seconds of true time, which would then be 2.45. This gives us only 6½ minutes difference in elapsed time, and gives for the direction of the wave an east course in place of north and south, as reported, which corresponds to our measured observations here. I take this opportunity of expressing the thanks of the Academy to this operator for his accuracy and kindness in furnishing us data in this and other phenomena of scientific and public interest.
February and March, 1864.
During the present year, 1864, we have had two smart earthquakes at the date of writing this report.
February 26th.—At San Francisco a light shock at 0h. 40m. M., and another at 2h. 10m. M. These were reported to me by persons who were up in the south and west parts of the city. At 5h. 47m. a very smart shock occurred, having three distinct vibrations, which induced many to rise somewhat earlier than was usual. This earthquake was preceded by a strong electric storm (so called here), between this city and Visalia; the particulars of which have not as yet all come to hand. It was followed next day by one of those severe “northers” with which the people of this State are very familiar. Barometer very low. This fact is mentioned only on account of the unseasonable period at which the gale occurred.
This earthquake was felt more severe at San José and Santa Clara than at this place, and occurred at nearly the same hour.
March 5th.—A severe shock of earthquake at San Francisco at 8h. 49m. M. The first wave had a north and south direction and continued 1¾ seconds; nearly 1¾ seconds elapsed before the second shock, which was at 8h. 49m. 3s., and continued 1½ seconds. The second shock was rotatory; the pendulum swinging north and south from the first shock, began and continued to describe a short oval or nearly a circle from the effects of the second shock, and continued thus for more than half an hour, until stopped and brought to rest. Magnetism was not suspended in this earthquake, nor any other that has occurred since my instrument was suspended. These observations were made at the height of twenty feet four inches from the ground. The total of time included in the shock was nearly five seconds. The farthest point south to which I have been able to trace its effects is the Mission San Juan, and north to Sacramento, a distance inclusive of 177 miles. In an easterly direction we have not traced it east of Stockton, about 60 miles. It was felt at Santa Clara, Santa Cruz, Gilroys, south of San Francisco, and at Santa Rosa and Petaluma to the north.
Since the above was in type, advices have been received from Visalia. At that locality the shock was very smart. The first shock took place at 8h. 45m. M., being four minutes earlier than that at San Francisco. This gives for the entire distance, north and south (over which the earthquake extended), 257 miles. This extent of latitude, and the almost simultaneous period of time at which it was observed along the entire line of distance so far as heard from, leads to the conclusion that its probable centre was south of San Francisco and nearly in the same longitude. From the violence of its action I feel inclined to the belief that its centre was in the neighborhood of San José, for all accounts agree in this one point, that more turbulence of the earth’s surface occurred in this region during its continuance than at any other place yet known. At Visalia the first shock was a mere tremor, but was followed by an undulatory movement when the second shock occurred, which was some three or four seconds later. This wave moved in an east and west direction at that place.