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I see a lot of different explanations online but I'm not sure if I understand the nuances correctly

From what I can gather:

仮定 is used when a situation is unclear and an assumption is made based on facts and logic

想定　is used when imagining a possible situation and assuming it happened

Considering this, I have trouble telling the difference between these sentences for example:

1. 今，大地震が起きたと仮定したらまずどうしますか




2. 今，大地震が起きたと想定したらまずどうしますか

I would like a more detailed explanation and if possible, some examples that show where one would be used and not the other.

Both are words that describe to the future ,but the conditions difference.

大地震が起きたと仮定したら、まずどうしますか？

In the case 仮定, we will think uncertain whether an earthquake will occur, but we will receive an attempt to let it happen.

大地震が起きたと想定したら、まずどうしますか？

In the case 想定, we will think in advance that an earthquake will occur.
The nuances of "plan" are just right.
And Japanese say "想定通り(as expected") = "計画通り(as planned)"

↓■Japanese■↓

どちらも未来について記す言葉になりますが、条件が変わります。

＞今，大地震が起きたと仮定したらまずどうしますか？

仮定の場合、地震が起こるか不確かではあるが、あえて起こるとしましょうと受け取ります。

＞今，大地震が起きたと想定したらまずどうしますか？

想定の場合、地震が起こることをあらかじめ考えておくことになります。
それは「計画」という言葉にニュアンスはちかいです。
私達日本人の場合、「想定通り」＝「計画通り」ともいいます。

Thank you.

仮定 is close to "hypothesis", and it is can be used with something unlikely or even impossible. 想定 is close to "anticipation", "expectation" or sometimes "assumption". It is usually used when you vividly imagine or anticipate something, or when you do a serious planning/drill/training on the assumption of something.

今、大地震が起きたと仮定したらまずどうしますか。

[?] 今、大地震が起きたと想定したらまずどうしますか。

This use of 想定 sounds odd to me because this is merely a hypothetical question, i.e., the speaker knows an earthquake is unlikely to happen today. You usually have to use 仮定 instead.

大地震が起きたと想定して、今から避難訓練を行います。

大地震が起きたと仮定して、今から避難訓練を行います。

Both are okay, but 想定 sounds the quake is more likely.

この地域で大地震が起きると10万名以上の死者が想定される。

You cannot use 仮定 here because the number of deaths is expected/anticipated, not hypothesized.

想定質問 anticipated/expected/potential question (i.e., likely to be asked in an upcoming presentation)

仮定の質問 hypothetical question (e.g., "what if you were a president of the US?")

この状況は想定できなかった。 The situation was beyond my expectation/imagination.

この状況は仮定できなかった。 (does not make sense)