Enlarged seismogram of Taiwan earthquake, 15 October 1922

Photoenlargement of a seismogram of an earthquake with an epicentre near Formosa (Taiwan), recorded on a Milne-Shaw seismograph at Hong Kong Observatory, 15 October 1922. Mounted on display board with caption: ‘No 1. MILNE-SHAW SEISMOGRAM. Recorded at Hong Kong October 15th 1922. An earthquake sends out, and different speeds, three principal types of wave, (designated P, S and L, Primary, Secondary and Long). The first two are transmitted through the earth, & the third series along the surface. The time interval between their inception in the seismogram is a measure of the distance of the shock from the observatory. The above is a record of a disturbance relatively near the instrument, where S and L follow closely upon P, and are consequently more difficult to identify. S – P = 1 minute 44 seconds = 600 miles distant.’

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 as having originated in Hualien, Taiwan, and having a magnitude of 5.9 on the Richter scale.