Repair disk from single user mode (fsck fails)

Earlier today my 2009 iMac emitted I/O errors trying to copy files. So I ran disk utility, verify disk to see if anything was amiss. It was. But as it's my startup disk I couldn't repair it, so I tried safe mode. Safe mode worked the very first time, but when I rebooted it shows the progress bar filling up, before halting. Every time I turn it on now, the same progress bar then crash happens. Booting in verbose mode reveals fsck is failing with signal 8. Using single-user mode, as that's as far as I'm aware the only way I can get any access to the system, I've been running fsck_hfs. However, it always fails. Below is the output from fsck_hfs -d /dev/disk0s2. ** /dev/rdisk0s2 Using cacheBlockSize=32K cacheTotalBlock=65536 cacheSize=2097152K. ** Root file system Executing fsck_hfs (version diskdev_cmds-491.6~3). ** Verifying volume when it is mounted with write access. ** Checking Journaled HFT Plus volume. ** Checking extents overflow file. ** Checking catalog file. Missing thread record (id = 9931104) Missing thread record (id = 16434146) Missing thread record (id = 19370458) disk0s2: I/O error hfs_swap_BTNode: offsets 10 and 11 out of order (0x0000, 0x0000) Invalid node structure (4, 115897) ** The volume Macintosh HD could not be verified completely. volume check failed with error 7 volume type is pure HFS+ primary MDB is at block 0 0x00 alternate MDB is at block 0 0x00 primary VHB is at block 2 0x02 alternate VHB is at block 1952491198 0x74622d5e sector size = 512 0x200 VolumeObject flags = 0x07 total sectors for volume = 1952591200 0x74622d60 total sectors for embedded volume = 0 0x00 I absolutely need to be able to boot to use it as an external monitor.