Question

what is seismograph?
how is the magnitude of earthquake measured?

Answer 1

$\color{Red}{Hey Mate!}$

the answer is:

Seismograph is a device used to measure the vibrations of an earthquake.

It is usually measured on $\bf{Richter scale}$

Hope it helps you!!

$\huge{Thankyou!!}$

Answer 2

seismograph is a device/instrument is useful in measuring the magnitude of earthquake.

as the earthquake occurs, due to the vibration the point of the seismograph on a plane having scale such that the movement and the vibration will be marked on a plane. this how the magnitude of earthquake is measured.