By default, the logs are archived once a week via a crontab entry (Sunday, 0300, by default) - this option is set at install time, can be changed by running tarantella setup, or you can alter the crontab entry directly.
See "126.96.36.199 Automatic Log Archives" in the admin guide - http://docs.oracle.com/cd/E26362_01/E26354/html/sgd-installation.html#sgd-installation-full-restore and
http://docs.oracle.com/cd/E26362_01/E26354/html/tta-archive.html for a description of the archive command.
The archive command closes out the current log files, and compresses/summarizes them, and stores them in a numbered sub-directory under the "logs" directory, and opens news ones. Once closed out, you can no longer "query" the logfiles from the command line. Note that there are a limited number of archives maintained - 7 sets, I believe, is the default.
Have a look at the "archive" command under .../bin/scripts to see how it works.