Hi Shan,
Please check whether log_archmeth1 parameter is set in your database.
This parameter should point to another partition log_archive.
When this parameter is active, at the time log_dir reaches threashold value , system will transfer the log files from log_dir to log_archive.
You need to actively take backup from log_archive and ensure this partition has enough space.
Hope this helps.
Regards,
Deepak Kori