erc-log-match-format

erc-log-match-format is a variable defined in `erc-match.el'.
Its value is

"%t<%n:%c> %m"

Documentation:
Format for matched Messages.
This variable specifies how messages in the corresponding log buffers will
be formatted. The various format specs are:

%t Timestamp (uses `erc-timestamp-format' if non-nil or "[%Y-%m-%d %H:%M] ")
%n Nickname of sender
%u Nickname!user@host of sender
%c Channel in which this was received
%m Message

You can customize this variable.