Update with inner join to same table optimizations

The query is:

CREATE TABLE logfile (
   sequence_number BIGINT,
   start_time DECIMAL,
   finish_time DECIMAL,
   irp_major VARCHAR(100),
   offset BIGINT,
   length BIGINT,
   filename VARCHAR(2000),
   is_seq BIT 
   );
//bulk insert 120 000 rows
CREATE CLUSTERED INDEX IX_logfile ON logfile(sequence_number); 
UPDATE dbo.logfile SET is_seq = (
SELECT  CASE WHEN a.offset = (b.offset + b.length) THEN 1 ELSE 0 END AS seq
FROM    dbo.logfile a
        LEFT JOIN dbo.logfile b
            ON a.sequence_number = b.sequence_number + 1
WHERE a.sequence_number = logfile.sequence_number)

Is there anyway I can improve the performance of the update query?

From stackoverflow

• MERGE
  INTO logfile l
  USING (
    SELECT sequence_number + 1 AS pseq,
           p.*
    FROM logfile p
  ) pl
  ON l.sequence_number = pseq
  WHEN MATCHED THEN
    UPDATE
    SET is_seq = CASE WHEN l.offset = (pl.offset + pl.length) THEN 1 ELSE 0 END
  

  Lieven : @Quassnoi. Different result or not, I did (again) learned something new. Thx.

  devzero : ahh found the error: sequence_number - 1 should be sequence_number + 1. Works great now, speed is more or less equivalent to @lievens, both are order of magnitude better than mine :)

  Quassnoi : Sure, missed it.

• Perhaps this update would be faster

  UPDATE dbo.LogFile SET is_seq = 0
  UPDATE dbo.LogFile SET is_seq = 1
  FROM dbo.LogFile f
       INNER JOIN (
         SELECT f2.sequence_number
         FROM dbo.LogFile f1
              INNER JOIN dbo.LogFile f2 ON f2.sequence_number = f1.sequence_number + 1     
         WHERE f1.Offset <> f2.Offset + f2.Length
       ) f2 ON f2.sequence_number = f.sequence_number