EMB ERJ 145 ER/LR
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Took Evasive Action
Aircraft
As we were approaching [destination] on the arrival; we were instructed to vacate FL310 and descend to FL260. After I set 26;000 in the altitude selector and set VS for the descent; I noticed that on the EICAS the cabin altitude was displayed in amber and indicated 9200 feet. I brought the CA's attention to it and we double-checked the pressurization settings. Glancing back at the EICAS; the cabin altitude has risen to 9600 feet. We donned our Oxygen masks and [advised ATC]; stating we needed to descend to 10;000 immediately. Without clearance; but under Captain's authority; we executed a rapid descent to 10;000 feet; with an initial turn to the right (south). We were shortly given the clearance to 10;000 feet and cleared direct to the airport. Once we reached 10;000 feet; we stowed the speed brakes; slowed to 200 KIAS and put the landing gear back up. We then ran the QRH checklist for rapid cabin decompression and then an approach check. During the emergency descent; the indicated cabin pressure altitude got as high as 11;800. Not enough for the passenger masks to auto-deploy. Without the emergency descent; the cabin pressure altitude may have gotten much higher than we experienced. We landed under a 'priority' status with ATC; as the situation had been dealt with.An uncommon situation is probably the biggest threat that we face as pilots. Obviously; the decompression and the resultant need for a rapid descent put the aircraft in an undesired state. Even though we were caught off-guard by the situation; I believe this one to be as close to 'by-the-book' as an event can be. Our FA performed fantastically; and ATC was a great help in accommodating our needs as well.  I think this went as smoothly as it could. The constant need to include 'Rapid Descent' in our Memory Items for each training cycle certainly helps rote and muscle memory take over when we need it.
EMB-145 First Officer reported a loss of pressurization at FL310 resulting in a rapid descent down to 10;000 ft.