This episode like many others in Season 1 really rocked so hard. In this one, Monk has to prove that Henry Rutherford's wife Christina killed him in an earthquake. Now there's really no mystery for Christina's shown openly doing the crime in the pre-credits scene and Darryl's true colors are shown early on. However the rest of the episode makes up for it and it's quite hilarious with practically one laugh after another. It's a nice introduction to Sharona's sister Gail who has the spunk of Sharona, but is definitely not as sweet. In fact, she pretty much despises Monk especially as he has to stay with her. Monk is pretty funny especially with the jumbled up phrases due to the earthquake and aftershocks. Also, the Charades bit is hilarious as Monk keeps guessing everything correctly. In addition, there's the bathing in mineral water which is just like him. Oh boy, there's too many funny scenes to name. It's also amazing to see Monk jump on the villain to protect Sharona which proves the strong relationship they have with each other. Sharona gets some excellent screen-time as she and Gail bicker over who did what first. Also, the fact that she was working with Henry Rutherford for charity and her kindness to Christina and also to Monk as she checks on him and manages to let him get the guest room at Gail's. So she's not the self-absorbed whore that some people make her out to be...at least not in this season. Of course there's the bickering with Monk over her boyfriend and then at the end the money which the brief moments are great too.
Stottlemeyer and Disher don't really do much in this episode, but they do throw in a bit of support for Monk at least as much as they can along with dealing with the earthquake situation.
The supporting cast is great too with Gail, Darryl, and Christina specifically along with the others doing fine too. So despite the lack of a mystery, "Earthquake" surely is a highlight of Season 1