1) The Sefer Midrash Chidush (by R' Eliezer Nachman Puah,) writes that Hashem commanded Moshe to split the sea with the term Bekiah, as explained above, this denotes one division. However, as the various Midrashim bring the sea adamantly refused. Until "Hayam ra'ah vayanos” - “The sea saw (Hashem coming) and fled.” However, Hashem was not pleased and punished the sea by tearing it to (twelve) pieces, - "Legozer yam suf Iigzarim”. Therefore, although it is written in the Torah that the sea was split "Vayibaku”, Chazal are mechadesh to us that what actually took place in the end was keriah.
2) An answer (partially brought by the Likutei Yehuda) in the name of the Chidushei Harim. He explained that the term Keriah is used to denote two things: a) When two separate things have been made into one and then are separated. E.g. when one tears a garment which is (separate threads joined together) in mourning this is called "keriah". b) The term Keriah implies the possibility of the two now separate things being rejoined afterward. Again, with the example of the garment, in olden times it was customary to sew it back together after the appropriate time, (known as Me'acha).
Bekiah, however, refers to only one thing - Something that was one piece to begin with for example a tree or piece of wood. When divided it is called Bekiah - e.g. Bereishit 22, 3 "Vayevaka Atzeh Olah”- “And he (Avraham) cleaved the wood for the burnt-offering.”) When creating the sea, Hashem made its existence conditional upon it splitting when the Jews left Egypt.
Therefore, in reality the sea was made from two parts held together as one until that time, it only had the appearance of being one. Therefore, the correct term is for the split should be keriah, because the sea was joined back together afterward to kill the Egyptians.
Hashem called it bekiah. Bekiah refers to splitting only, because when Hashem instructed Moshe regarding the sea, he did not want to use the word keriah which would hint to the destruction that He had to bring upon His creatures when He would later reunite the sea.