The 1991 Sierra Madre earthquake occurred on June 28 at 07:43:55 local time with a moment magnitude of 5.6 and a maximum Mercalli intensity of VII (Very strong).

[6] Geodetic research has shown that crustal shortening is occurring in the region of the Transverse Ranges at a rate of 0.33 inches (8.5 mm) per year.

The uplift initiated about two million years ago, and is a result of the convergence of the Pacific and North American plates near the restraining bend of the San Andreas Fault.

For example, both Monrovia and Pasadena each had more than a dozen buildings that were uninhabitable, but for the most part damage was limited to cracked plaster and broken windows.

The Pasadena City Hall had moderate damage consisting of a cracked wall, dislodged marble barriers in the restrooms, and an offset dome.

[5] With an area of perceptibility of around 58,500 km2, the shock was felt from Santa Barbara and Bakersfield to San Diego, Palm Springs, and Las Vegas.

Intensity VI (Strong) effects were observed over 1,100 km2, from Altadena and La Verne along the base of the mountains, to East Los Angeles.

Two independent reports suggested that if the complete (62 mi (100 km)) fault system were to rupture, it could result in a Ms 7.7 shock similar to the 1952 Kern County earthquake.