The Modified Mercalli Intensity (MMI) estimates the intensity of shaking from an earthquake at a specific location or over a specific area by considering its effects on people, objects, and buildings. At high intensities (MMI ≥ 6), earthquake shaking damages buildings. The severity of the damage depends on the building type, the age of the building, and the quality of the construction. Masonry and non-ductile concrete buildings can be more severely damaged than wood-frame or engineered buildings. Buildings built to older building codes can be more severely damaged than recently constructed buildings using newer codes.
The chart below describes the damage that three common building types can experience at the five highest intensity levels expected in the San Francisco Bay region. Note that the expected damage varies with quality of construction and level of retrofitting. To accurately evaluate how your house will perform in an earthquake, it should be evaluated by a structural engineer or an appropriately certified contractor.
Larger buildings that do not fit the categories described below, such as multi-unit high-rise apartment buildings, typically undergo a higher level of engineering design prior to construction, but older buildings of this type can be very vulnerable to damage in earthquakes. Evaluation of larger structures for earthquake safety and the need for retrofitting is far more complicated than can be addressed here and an engineer should perform a building evaluation. More detail on specific structures is included on ABAG’s Guide to Vulnerable Housing Types.
Residents of any of these structures can take positive actions by securing building contents and/or insuring those contents to limit their earthquake losses. Methods exist to retrofit structures and to secure building contents to prevent damage and loss in future earthquakes. Earthquake insurance is an option to cover catastrophic losses.
Colors correspond to MMI values shown on the Shaking Hazard Map.
For complete MMI descriptions not specific to building types, click here.
These descriptions were developed by ABAG and the Earthquake Mapping Update Advisory Committee. The project was generously funded with a grant from the USGS Earthquake Hazards Program. These descriptions were developed in 2013.
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