A CRJ200 AT FL240 EXPERIENCED N1 ENG VIBRATIONS. UNABLE TO MAINTAIN ALT; THE CREW DECLARED AN EMERGENCY AND LANDED AT THE NEAREST SUITABLE ARPT.

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LEVEL FLT AT FL240 IN IMC. OCCASIONAL MODERATE TURB AND ICING WAS RPTED. A SMALL PATCH OF WHAT LOOKED LIKE FROST (3-4 INCHES IN DIAMETER) FORMED ON MY (CAPT) FRONT WINDOW. NO ICE WAS OBSERVED ON THE FO'S OR MY WINDSHIELD WIPERS OR WIPERS NUT. WING INSPECTION LIGHT WAS TURNED ON AND NO ICE OBSERVED. WE BOTH COMMENTED ON IT SEEMING PECULIAR. WE ASKED FOR FL260 TO GET OUT OF THE CLOUDS BUT OUR REQUEST WAS DENIED BY ATC. SHORTLY AFTER THAT REQUEST; THE #1 ENG N1 VIBRATION PEGGED OFF THE CHART; ACCOMPANIED BY AN AURAL AND ACFT VIBRATION. THRUST WAS REDUCED ON OPERATING ENG AND I IMMEDIATELY DECLARED AN EMER AND REQUESTED FL260 AGAIN. WHILE CLBING TO FL260; WE ATTEMPTED TO BRING PWR UP ON THE #1 ENG AND EACH TIME WE WOULD GET EXCESSIVE ENG VIBRATION UP TO 4.0; ALONG WITH VIBRATION FELT THROUGHOUT THE ACFT AND AUDIO VIBRATION SOUNDS. WE WERE BOTH CONVINCED THAT THE ENG WAS HEADING TOWARD CATASTROPHIC FAILURE. ALL THIS TIME I HAD THE QRH OPENED AND WAS ANTICIPATING USING THE SEVERE ENG DAMAGE CHKLIST. AT FL260 WE WERE OUT OF THE CLOUDS BUT COULD NOT MAINTAIN ALT WITHOUT A DEGRADATION IN AIRSPD; SO AN IMMEDIATE REQUEST TO 16000 FT WAS ASKED FOR AND GIVEN. WE ASKED ATC OF ANY NEAREST SUITABLE ARPTS AND WE WERE TOLD ZZZ WAS 40 NM TO THE SW. AT THAT TIME WE CAME TO THE AGREEMENT THAT WE HAD AN ENG WE BELIEVED WAS IN A LOT OF TROUBLE AND THE INABILITY TO MAINTAIN ALT SO DIVERTING AND GETTING THE ACFT ON THE GND WOULD BE THE SAFEST OPTION. DISPATCH; MAINT; FLT ATTENDANT; PAX; AND CFR WERE ALL INFORMED OF OUR DECISION TO DIVERT. THE FO AND I DISCUSSED SHUTTING DOWN THE ENG AND CAME TO THE CONCLUSION THAT AT IDLE; THE ITT WAS NORMAL AND IT WAS NOT VIBRATING. SO WE LEFT IT RUNNING IN CASE IT WAS NEEDED FOR A GAR. WITH THE QRH IN MY LAP; WE RAN THE ABNORMAL SINGLE ENG PROC ALONG WITH OUR OTHER NORMAL CHKLIST ITEMS. JUST PRIOR TO TURNING BASE LEG; WE SLOWLY ADVANCED THE L MOTOR TO SEE ITS REACTION AND IT SEEMED LIKE IT WAS DOING MUCH BETTER; BUT WE DIDN'T TRUST IT BECAUSE OF WHAT WE HAD JUST SEEN. A SAFE LNDG WAS MADE AND THE ACFT WAS BROUGHT TO THE GATE. AT THE GATE I CONTACTED MAINT AND WAS TOLD THAT WHAT HE THOUGHT WAS THE CULPRIT WAS ICE. MAINT AND I DISCUSSED AND WENT THROUGH AN ABNORMAL CHKLIST I MISSED IN THE HEAT OF THE BATTLE. IT IS ABNORMAL N1 ENG VIBRATION (ICON). AT THIS TIME; I REALIZED I HAD MISSED SOMETHING. THE LACK OF VISUAL ICE AND THE SEVERITY OF THE SOUND; ACFT VIBRATION; AND PEGGED VIBRATION METER PLACED US IN A NARROW MINDSET OF 'WE ARE GOING TO HAVE AN ENG FAILURE;' AND FIXING IT NEVER REALLY OCCURRED TO ME. THE RJ'S WARNING MESSAGES; STATUS MESSAGES; AND ABNORMAL MESSAGES HAVE ALWAYS GUIDED ME TO THE CORRECT CHKLIST IN THE QRH. HOWEVER; THAT NIGHT I HAD NO WRITTEN GUIDANCE ON ANY MESSAGES. I FEEL WE SHOULD ALL EDUCATE OURSELVES ON CHKLISTS THAT DON'T WORK VIA THE EICAS SYS. ALL AND ALL; I'M NOT SURE THAT CHKLIST WOULD HAVE FIXED THE ENG. WHO KNOWS; IT COULD HAVE LED TO US DAMAGING IT. AT THE TIME; ALL WE WANTED WAS THE SAFEST OPTION FOR ALL OF US AND THAT WAS TO GET THE AIRPLANE ON THE GND. IN THE PREVIOUS 4 DAYS I HAVE HAD AN ACFT FRAME VIBRATION; WING ANTI-ICE SENSOR MESSAGE; FAULTY WING OVERHEAT; AC BUS #1 AUTO XFER; AC BUS #2 AUTO XFER; OVERTEMP; GND LIFT DUMPING THAT WOULD NOT RESET; DIVERSION TO AN ARPT WITHOUT ICING; CANCELLATIONS; DELAYS; AND UPSET PAX. THIS WAS JUST THE ICING ON THE CAKE. HOWEVER; I LEARNED AND HAD AN EXPERIENCE OF MISSING THAT ABNORMAL CHKLIST WHICH WILL IN THE LONG RUN BETTER ME AS A PLT. I THINK MORE EMPHASIS ON QRH PROCS THAT DON'T INVOLVE EICAS MESSAGES WOULD HELP TO PREVENT AN OCCURRENCE LIKE WE HAD. AS SAID EARLIER; I WANT YOU TO KNOW MY MAINT COORDINATOR DID A GREAT JOB OF EXPLAINING AND GOING THROUGH THE PROC I HAD MISSED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THAT THE ATMOSPHERIC CONDITIONS THAT NIGHT WERE STRANGE. THE ACFT WAS EXPERIENCING CONTINUOUS MODERATE CHOP BUT NO VISIBLE AIRFRAME ICING. THE VERY MINOR WINDSHIELD ICE DID NOT CONCERN THE CREW BECAUSE THEY HAD WING AND ENGINE ANTI ICE ON WELL PRIOR TO THIS EVENT. THEY ARE STILL AT A LOSS ABOUT THE VIBRATION'S CAUSE BUT IT DISSIPATED PRIOR TO LANDING AND THERE WAS NO INDICATION OF ENGINE DAMAGE. THE REPORTER STATED THAT IF THE ENGINE ANTI ICE WERE MALFUNCTIONING AN EICAS MSG SHOULD HAVE ALERTED THEM ABOUT IT. THE REPORTER HAS NOT HAD ANY FEEDBACK FROM HIS ACR ABOUT THIS EVENT.