Chile forward Sanchez was the star of the show in east London as Arsenal moved above Liverpool and Manchester City with a triumph.
Chile forward Sanchez was the star of the show in east London as Arsenal moved above Liverpool and Manchester City with a triumph that was every bit as emphatic as the scoreline suggests.
After setting up Mesut Ozil for the opening goal in the 24th minute, Sanchez then made the match his own.
He danced away from two defenders to make it 2-0, fired home from the edge for 3-0 and, after the sides had traded goals from Andy Carroll and Alex Oxlade-Chamberlain, respectively, impudently lobbed West Ham keeper Darren Randolph to seal the one-sided win.
It was hard to escape the feeling that here were two London clubs travelling in opposite directions.
Arsenal, now just three points behind leaders Chelsea, were superior in every department.
With space in which to operate with freedom in front of the West Ham back four, Sanchez and Ozil danced to their own tune.
On the other hand, fourth bottom West Ham appear to be having the breath sucked out of them by the twin failures of adapting to their new home and recapturing last year's compelling form.
The visitors moved forward at will. Ozil and Sanchez exchanged passes in the 13th minute before a low cross from the overlapping Nacho Monreal was poked out of the path of the incoming Sanchez by Winston Reid.
Next, from an inswinging Monreal corner, Shkodran Mustafi's downward header just evaded the outstretched leg of Laurent Koscielny.
By then, West Ham had already suffered the disruption of losing James Collins to a muscle injury in only the seventh minute.
The home side finally roused themselves after 18 minutes when Dimitri Payet played a delightful pass through the Arsenal defence to Manuel Lanzini, who dallied too long before shooting when the angle became too acute to beat Petr Cech in the Arsenal goal.
That was no more than an interlude in the game's motif and it came as no surprise when Arsenal took the lead.
Angelo Ogbonna's pass out of defence was cut out by Francis Coquelin.
The ball fell into the path of Sanchez who skipped past a West Ham defender before squaring for Ozil to side-foot home.
Chances came and went for Arsene Wenger's side, firstly for Oxlade-Chamberlain who then slipped Sanchez into space only for the Chilean to take an uncharacteristic heavy touch as he attempted to go round Randolph.
If the second half began at a slower pace which offered West Ham some hope, their best chance fell to Ashley Fletcher whose shot flew inches past the post.
Normal service was resumed when Monreal raced to the byline and cut the ball back for Arsenal substitute Aaron Ramsey, whose close-range shot was kicked away by Randolph.
It set the stage for Sanchez who, with the ball glued to his feet, turned Arthur Masuaku then sprinted away from him into the box and as Ogbonna stood off him, drilled a low shot across Randolph and into the far corner.
Eight minutes later he took his chance with a low shot from the edge of the box when the West Ham defence backed off him and Randolph went down too late to stop it.
That was part of a crazy finale. Within the space of 60 seconds, West Ham pulled one back when substitute Carroll reacted quickest to head home after a Payet free-kick had bounced down from the bar and then Oxlade-Chamberlain curled home from 20 yards to make it 4-1.
That only left Sanchez to seal his virtuoso display by latching on to an Oxlade-Chamberlain pass and lifting the ball over Randolph for his hat-trick.