Thanks for attaching derby.log, Deepika. The corruption first appeared with an INSERT INTO EVENTS_TABLE statement, as you note above. Before that statement, all I see in derby.log are boot and shutdown diagnostics. It appears that the installation was using Derby 10.2.2.0 at first on 2011-07-21. About 8 hours later the installation was brought down and rebooted with Derby 10.6.1.0. The installation was rebooted a couple times and ran fine with 10.6.1.0 until the first error occurred on 2011-11-03.
After that, the corruption choked a database backup on 2011-11-16. The INSERT statement failed again on 2011-11-17, 2011-11-22, 2011-11-23, 2012-02-06, and 2012-02-21. The following corrupted pages (all in the same conglomerate) appear in derby.log: 2953, 1936, 2914, 2125. I do not see any evidence that any other conglomerates are corrupted. It is possible that you may be able to repair the database (at least to the point that you can dump and reload it) by dropping and recreating the indexes on EVENTS_TABLE.
The initial corruption occurred several months ago. This long afterward, we generally don't have much success in figuring out what caused the original problem. Did anything special happen on 2011-11-03 that you can remember?