it is .... that , it was.... that
Hello,Everyone !!!
I am wondering why it's not" it is ... that " but " it was.... that " in the following sentences.

When the instrument is situated at more that 700 miles from the earthquake centre, the graphic record shows three waves arriving one after the other at short intervals. The first records the arrival of longitudinal vibration. the second second marks the arrival of transverse vibrations which travel more slowly and arrive several minutes after the first. These two have travelled through the earth. It was from the study of these that so much was learn about the interior of the earth. The third, or main wave, is the slowest and has travelled round the earth through the surface rocks.

Thanks in advance : D

Question

it is .... that ,    it was....  that
Hello,Everyone !!!
I am wondering why it's not" it is ... that " but " it was.... that " in the following sentences.

When the instrument is situated at more that 700 miles from the earthquake centre, the graphic record shows three waves arriving one after the other at short intervals. The first records the arrival of longitudinal vibration. the second second marks the arrival of transverse vibrations which travel more slowly and arrive several minutes after the first. These two have travelled through the earth. It was from the study of these that so much was learn about the interior of the earth. The third, or main wave, is the slowest and has travelled round the earth through the surface rocks.

Thanks in advance : D

Answer

It's speaking of time. The study already happened  and this article is being written after the study took place. Maybe it took place a month or year ago. So, "it was" from that study they received the information they needed.

If the study is taking place now (present time) you would say or could say, "it is" from this study that we've received the information we need.