When English words are borrowed into Japanese, several phonological processes apply to nativize foreign sound strings, one of which is consonant gemination. It is generally observed that consonant gemination applies to strings of short vowels followed either by a plosive, an affricate or a fricative. Thus, words like cap are borrowed as [kjappu] with geminated /p/. Words like caption, capital, however, are borrowed as [kjapu∫ON] and [kjapitaru] without the expected gemination of /p/in both cases and of /t/ in the latter. The applicability of the gemination process inside a word seems to be more restricted than that found at word-final position.
Distribution of geminates shows three environments where consonant gemination is blocked word-internally:(i) where vowels are underlyingly followed by a consonant in a coda position. Compare caption/kjap. ∫ON/ ( . indicates a syllable boundary) with fashion/fa.∫ON/which surfaces as [fa∫∫0N]. (iii) where the vowels are not stressed as in the second syllable of capital.
Assuming that consonant gemination is a process that is triggered by a mora insertion in the prosodic structure which is further triggered by the rhythmic clash between two prominent units, we can find a reasonable explanation to the above restriction on word-internal gemination.
The Linguistic Society of Japan