This is the 16023rd most frequent Polish word.
trzęsieniu
"Trzęsieniu" means "shaking" or "tremor" in English.
In this sentence, 'trzęsienia' refers to the noun 'earthquake', modifying 'ziemi', meaning 'earth', creating the term 'earthquake'.
W wyniku trzęsienia ziemi wiele budynków zostało zniszczonych.
As a result of the earthquake, many buildings were destroyed.
Here, 'trzęsieniu' is in the locative case of the noun 'trzęsienie', indicating the time frame after the event.
Po trzęsieniu ziemi ludzie zaczęli odbudowywać swoje domy.
After the earthquake, people began rebuilding their homes.
In this context, 'trzęsieniu' in the locative case describes the earthquake as an identifiable event being discussed.