Enlarged seismogram of earthquake in the Atlantic Ocean, 13 October 2025

Photoenlargement of a seismogram of an earthquake with an epicentre in the Atlantic Ocean, recorded on a Milne-Shaw seismograph at Helwan, Cairo, 13 October 1925. Mounted on display board with caption: ‘No.3. Earthquake in the bed of the Atlantic Ocean at 42W. 10N. recorded at Cairo October 13th 1925. S – P = 9 minutes 14 seconds = 5000 miles distant. Note the waves marked PR and SR. These are the Primary and Secondary waves after reflection from the earth’s surface at the mid-position between the origin and the observatory.’

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.

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 to have occurred on the northern Mid-Atlantic Ridge, and calculate its magnitude as 7.2 on the Richter scale.