Arsenal vs West Ham Live : Mesut Ozil opened the scoring with Arsenal’s first shot on target after Angelo Ogbonna’s mistake and Sanchez’s cut-back set up a simple finish.
The Hammers threatened to equalise after the break before Sanchez turned, drove into the box and finished brilliantly past Darren Randolph.
The Chilean added a second eight minutes later with a low strike into the corner, before Andy Carroll came on for a first appearance since 18 August and stooped to nod in the rebound from a Dimitri Payet free-kick that hit the bar.
However, any faint hopes of a comeback were extinguished by Alex Oxlade-Chamberlain’s excellent finish from range and Sanchez’s masterful dinked chip over the keeper.
The heavy defeat leaves Slaven Bilic’s side one point above the relegation zone after Sunderland beat Leicester 2-1 earlier on Saturday.
Arsenal trail leaders Chelsea by three points but will slip to third if Liverpool win at Bournemouth on Sunday.
Arsene Wenger’s side are now unbeaten in 13 league games, and their last away defeat was in February.
There was a 10-minute second-half spell during which many Arsenal fans might have recognised the familiar beginnings of a slump to draw or defeat despite having dominated for much of the game.
But Sanchez provided the match’s key moment when he killed Shkodran Mustafi’s booming pass from deep, turned past Anthony Masuaku and found the far angle of the net with brilliant ease with just under 20 minutes to play.
Arsenal defender Laurent Koscielny had been forced to clear a Manuel Lanzini cross bound for Ashley Fletcher from right underneath his own crossbar, West Ham’s record signing Andre Ayew had just come on, and Carroll – who scored a hat-trick against Arsenal in a 3-3 draw in April – was being readied for action.
However, it was Sanchez’s force that proved the most compelling. Each of his three goals were superbly taken but it was the final, hat-trick goal that will live longest in the memory.
Played through by Oxlade-Chamberlain, he feigned a dummy shot to fool already defeated Hammers keeper Randolph before delicately lifting the ball into the net for what must be one of the goals of the season so far.
‘A big humiliation’
West Ham were certainly not helped by losing James Collins to injury in the game’s early stages, and his replacement Alvaro Arbeloa looked off the pace.
But regardless of ill fortune, the Hammers remain without a win in the league since their home victory over Sunderland on 22 October.
Manager Slaven Bilic said this result, coming after Wednesday’s 4-1 defeat by Manchester United in the EFL Cup, was “a big humiliation”, and the fact that his players faded after Sanchez increased Arsenal’s lead will be another cause for concern.
A win for Hull in their match at Middlesbrough on Monday will send West Ham into the bottom three.
West Ham boss Slaven Bilic on BT Sport: “I am not looking for excuses. Yes, we had injuries and mistakes, but we were not good enough.
“It was a big humiliation. I feel sorry for the fans and the club. We played some good games in this season, and had some good moments today even at 0-0 and 1-0 down.
“But I have to be honest and say we don’t have the intensity and from the dedication comes the quality. We don’t have it the same as last season, even in training.
“We are in a serious situation before this game which we don’t like. The players know that, but it’s not only about knowing but sacrificing, and your dedication for the game.
Arsenal manager Arsene Wenger, speaking to BBC Sport: “We had a convincing performance and strong result.
“We created many chances and you could see West Ham are not a confident team at home. On top of that we had a strong performance.
“Alexis Sanchez – it took him a while mentally to get into the game, but he can always give you something special. He’s a fighter and a classy player. You don’t find many players like him. He can always surprise you. He also has great technique as well.
“I believe we are on a strong run and it’s down to us to be mentally ready to maintain that run.”
The stats behind Sanchez’s season
There were 14 minutes and 29 seconds between the first and third goals of Alexis Sanchez’s hat-trick
Sanchez has been directly involved in six of Arsenal’s past eight Premier League goals (five goals, one assist)
His tally for the season is 13 goals and nine assists from 19 matches in all competitions
No player has received more yellow cards this Premier League season than Winston Reid, with six
Ozil has equalled his Premier League goal tally of last season with eight goals
West Ham travel to Liverpool for a 16:30 GMT kick-off in the Premier League on Sunday, 11 December.
Arsenal play away to Swiss club Basel in the Champions League on Tuesday, before welcoming Stoke in the Premier League at 15:00 next Saturday.