So, a lecture about the Riga which mentioned neither "Riga," nor the most famous game with it (Capablanca-Ed. Lasker)?  Curious.

8. c4 is in fact another old book move ...

It's worth mentioning that this is usually known as the Riga variation, and there are several older threads in the forum devoted to it.

I haven't compared the analysis here with the older ones, since I don't play this from either side of the board.

Open Ruy Lopez - Seirawan Variation

1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O Nxe4 6. d4 

(6. Re1? Nc5 7. Bxc6 dxc6 8. Nxe5 Be7 9. d4 Ne6 - black has the bishop pair and a good endgame.) 

6... exd4 7. Re1 d5 8. Nxd4 Bd6 

At this point we have to cover all possible continuations, though, the main idea keeps repeating itself in most lines.

Lines covered:

A) 9. f3
B) 9. Nc3
C) 9. Rxe4
D) 9. Bxc6+
E) 9. h3
F) 9. g3
G) 9. Nd2

And the mainline:

H) 9. Nxc6



A) 9. f3
9... Qh4 (9... Bxh2+ would also work) 10. fxe4 Bg4 11. Bxc6+ (11. Qd3 Qxe1+) (11. Qd2 Qxh2+
12. Kf1 O-O-O 13. Nf3 Bxf3 14. gxf3 Qh1+ 15. Kf2 Qh4+ 16. Kf1 dxe4) 11... bxc6
12. exd5+ Kd7 13. dxc6+ Kc8 14. Qd2 Qxh2+ 15. Kf1 Qh1+ 16. Kf2 Qh4+ 17. Kg1
Bh2+ 18. Kf1 Bg3 19. Qe3 Qh1+ 20. Qg1 Qxg1+ 21. Kxg1 Bxe1 - black has a significant lead in both activity and material.

***

B) 9. Nc3 
9... Bxh2+ 10. Kxh2 Qh4+ 11. Kg1 Qxf2+ 12. Kh2 Qh4+ - with a perpetual draw.

***

C) 9. Rxe4+ 
9... dxe4 10. Qh5 (10. Nxc6 Bxh2+) 10... Bd7 11. Nxc6 Bxc6 12. Bxc6+ bxc6 - black is better.

***

D) 9. Bxc6+ 
9... bxc6 10. Nxc6 (10. f3 Qh4) 10... Bxh2+ 11. Kxh2 Qh4+ 12. Kg1 Qxf2+ 13. Kh2 Qh4+ - with a perpetual draw.

***

E) 9. h3
9... Qh4 10. Rxe4+ dxe4 11. Nxc6 O-O 12. Nd4 Bxh3 - black has a substantial attack against the white king.

***

F) 9. g3
- g2-g3 appears, at first glance, quite logical. White inhibits both ...Qh4 and ...Bxh2+. The problem here reveals that black’s positives are not only the attack against the white king but also the positioning of his pieces.
9... O-O 10. Nxc6 Qf6 11. Be3 bxc6 - with a very active and potent position for black.

***

G) 9. Nd2 
9... Bxh2+ 10. Kxh2 Qh4+ 11. Kg1 Qxf2+ 12. Kh2 Qxd4 13. Nxe4 Qxd1 14. Nf6+ Kd8 15. Rxd1 gxf6 16. Rxd5+ Ke7 17. Bxc6 bxc6 18. Rc5 Kd6 19. Be3 Bg4 - This endgame deserves to be studied. Whites stands better, has better pawn structure and his king is safer. However, he is also 2 pawns down in material with opposite coloured bishops and plenty of file space for both sides to utilize their rooks. According to Houdini (and I have a very strong computer!) this endgame is dynamically equal. To my mind, this is preferable for white but even so, it is certainly not winning and in any rate, one should also remember that at move 12 black could have played ...Qh4+ and take the thematic perpetual draw.

***

H) MAIN LINE 9. Nxc6
9... Bxh2+ 10. Kh1 

(GM Yasser Seirawan’s analysis claims that black is winning if white plays 10. Kf1, unfortunately, this is incorrect. True enough, white constantly needs to find best moves so practically speaking, OTB, this remains a valuable variation for black. Game continues: 10... Qh4 11. Be3 (11. Nd4+ c6 12. Be3 Bg4 - is similar to the mainline of 11. Be3) (11. Qf3? O-O 12. Nd4 Bg4 13. Rxe4 dxe4 14. Qxe4 Be5 15. Qxe5 Qh1#) 11... O-O 12. Nd4 Bg4 13. Nf3 Qh5 
- At this point GM Seirawan verbally declared that “this position sucks for white” and indeed it is not very appetising to say the least but unfortunately it is not losing either. Game continues: 
14. c3! (14. Nbd2 Be5) 14... Bd6 15. Ke2!! {a very hard move to find but it saves white’s game!} 15... Bc5 16. Bxc5 Bxf3+ 17. gxf3 Nxc5 18. Rh1 Rfe8+ 19. Bxe8 Rxe8+ 20. Kd2 Qg5+ 21. Kc2 Qf5+ 22. Kd2 Qg5+ 23. Kc2 Qf5+ - with a perpetual draw.) 

(We should also consider the capture: 10. Kxh2 Qh4+ 11. Kg1 Qxf2+ 12. Kh1 O-O!? {obviously, the complications are enormous and black could easily avoid them by playing ...Qh4+ and taking the perpetual draw} 13. Be3 Qh4+ 14. Kg1 Bg4 15. Qc1 Bh3 16. gxh3 Qg3+ 17. Kf1 bxc6 18. Nc3 Qxh3+ 19. Ke2 Qg4+ 20. Kd3 c5 21. Bc6 c4+ 22. Kd4 Rab8 23. Nxd5 Rfd8 24. Bf4 Qe6 25. Ke3 Rxd5 26. Bxd5 Qxd5 27. Qd1 Qc6 28. Qf3 Re8 29. Qg2 Re6 - this “idea variation” is not meant to be precise nor does it claim that any of the moves were forcing but it does bring to light the complications of the position and if one cares to study it and familiarise himself with its intrinsic complications he would definitely have a significant advantage over his dazzled opponent who hasn’t seen any of it before in his life and the clock is ticking! )

10... Qh4 11. Rxe4+ dxe4 12. Qd8+ Qxd8 13. Nxd8+ Kxd8 14. Kxh2 Be6 15. Bf4 f5 16. Nd2 Kc8 17. c3 h6 (17... Rg8 18. a3 b5 19. Bc2 Kb7 (19... g5 20. Be3 h5 21. Nb3 c6 22. Nc5 Bd5 23. a4) 20. Be3 Rgf8) 18. a3 g5 19. Be3 Rd8 20. g3 b6 21. Bc6 Ra7
- this is a fairly complicated endgame and one where white is certainly not worse but then, neither is black. Therefore, it stands to reason that the better prepared player will have advantage over his baffled opponent.

The Seirawan variation of the open Ruy Lopez appears to hold!