Emergency workers are converging on downtown Modesto this morning, but it's just for a drill.

A portion of I Street is closed until 1 p.m. for Operation “Shake, Rattle and Roll,” Stanislaus County’s version of a statewide medical health exercise.

Based on an earthquake scenario, the exercise consists of a building collapse drill at Ralston Tower and operating an evacuation shelter in Patterson.

Roughly 150 volunteers and staff from various agencies will participate in the exercise in various activities.

Fire and ambulance services will conduct search and rescue and field triage and treatment capabilities at Ralston Tower.

Law enforcement and public works personnel will manage security, and traffic operations and hospitals will exercise their ability to handle patient surge during a mass casualty incident.

The American Red Cross and Stanislaus County Community Services Agency will open and operate a shelter for “evacuees.”