Enlarged seismogram of Puerto Rico earthquake, 11 October 1918

Photoenlargement of a seismogram of an earthquake with an epicentre in Puerto Rico, recorded on a Milne-Shaw seismograph at Shide, Isle of Wight, England, 11 October 1918. Mounted on display board with caption: ‘No 2. Earthquake at PORTO RICO, recorded at Shide, Isle of Wight, October 11th 1918. S – P = 8 minutes 22 seconds = 4350 miles distant. Note the greater separation of the Primary, Secondary, and Long waves (P, S and L).’ This record was intended to be compared with 1926-660, showing an earthquake recorded on a seismograph located relatively close to an earthquake epicentre.

Milne-Shaw seismographs were amongst several designs of mechanical, pendulum-based seismograph available in the early 1900s that could detect the vibrations from distant earthquakes. They were modified versions of a device originally designed by seismologist John Milne around 1895, but incorporating a system of electromagnetic damping (to prevent the horizontal pendulum acquiring its own swing) and providing records of greater magnification. These modifications were designed by John Johnson Shaw, a seismology enthusiast who built the instruments himself for despatch to observatories and stations around the world.

This recording was made at Shide on the Isle of Wight, Milne’s former home and earthquake observatory, which continued to be used as a seismological site for several years after Milne’s death in 1913.

Analysing the arrival times different types of earthquake waves enabled geophysicists to locate earthquake epicentres, and also to infer the interior structure of the earth.

Seismologists now identify the earthquake recorded in this seismogram as the San Fermín or Puerto Rico earthquake of 1918, and calculate its magnitude as 7.1 on the Richter scale. It triggered a tsunami which, in combination with the earthquake itself, caused considerable damage and loss of life in nearby cities.