October 25, 1918 Earthquake: Puerto Rico, US Territories

The after-shock of October 24th originated about where the main shock did and was probably due to an extension of the fault downwards... The most severe of these aftershocks occurred on October 24th at 11:43 p.m... Observers say that these strong aftershocks differed from the first disturbance in that they seemed to consist chiefly of horizontal oscillations. They apparently had a considerable amplitude, but the period of oscillation was relatively slow, and therefore they produced little damage, although some loose objects, not disturbed by the first shock, were thrown down... At the time of the earthquake on October 24th this cable was again broken in practically the same place as that of the first rupture described above [area bounded by the parallels 18 deg 25 min and 18 deg 35 min north and the meridians 67 deg 15 min and 67 deg 30 min west], and Captain Morrell states that "here again the cable was badly crushed and buried by a landslide, the break being due to the core having been spewed out and nipped by the outer sheathing wires." The evidence here given makes it fairly certain that the rupturing of the cables was due chiefly to submarine landslides on very steep slopes... Although the cables probably crossed several faults there is no definite evidence that the ruptures occurred directly above a fault and were caused by a displacement along it. Rather it would appear that the cables were ruptured by landslides precipitated down the steep slopes by the violent vibrations of the earthquake... The most severe of the after-shocks occurred on the night of October 24th, at 11:43 p.m., and had a maximum intensity, in the western part of Porto Rico of about VII R-F. This shock came when most people were asleep, and therefore it is difficult to obtain accurate estimates of the relative intensity at various points. Moreover, the effects of the disturbance on buildings and similar structures were largely masked by the greater effects of the earthquake of October 11th. The intensity "was greatest along the west coast of Porto Rico and decreased with the distance from that region. Bottles, lamps, and other loose objects were overturned at Aguadilla, Corsica, Rincon, Afiasco, Cabo Raja, and other towns in the western part of the island.

The shock was felt all over Porto Rico and even as far as St. Thomas, where many persons were awakened from sleep. Puerto Plata on the north coast of Santo Domingo, about 380 km west of the origin, was the most distant point reporting the shock. The earthquake was felt on board the steamboat Mariana, which at the time was about eleven kilometers southwest of Mona Island lighthouse. According to Captain Thebald the boat plunged and then rolled heavily. The cable of the French company was again broken and partly buried as though by a landslide, and this occurred in the same place as one of the breaks of October 11th. The seat of the disturbance was evidently located in the northeastern part of Mona Passage and in the same general area as that of the earthquake of October 11th, although the evidence in this case is not so abundant. But the breaking of the cable, the apparent similarity in the distribution of the intensity between this shock and the main shock of October 11th, the recorded times of the arrival of the waves at Viequez and Port-au-Prince, and the length of the first phase of the motion

at these two stations, all point to an origin practically the same as the main shock. The earthquake of October 24th was presumably due to a displacement along a fault, but the nature of the displacement seems to have been different from that which occurred at the time of the first earthquake. The displacement may have been horizontal

rather than vertical, or it may have occurred near the bottom of the fracture and without reaching the surface; certainly there was no important vertical displacement of the bed of the ocean, for there was no sea wave.

(above from reference #359)