The Morgan Hill Earthquake of 1984

Two houses in Morgan Hill. A landslide caused the house in the foreground to move to the left. Notice the crack in the wall, next to the chimney.

Image: Karl Steinbrugge
Courtesy of the National Information Service for Earthquake Engineering, EERC, University of California, Berkeley

The Morgan Hill earthquake, also called the Halls Valley earthquake, occurred on 24 April 1984 along the Calaveras fault 18 km east of San Jose and 22 km north of Morgan Hill. It had a magnitude of ML=6.2 and a focal depth of 8 km. The earthquake was felt in California and Nevada over an area of 120,000 km2 and caused damage estimated to be worth $7.5 million. After the earthquake, many cracks were found in the ground, but no unequivocal evidence of surface faulting was found. The aftershocks define an area that extends approximately 30 km southeast of the epicenter along the Calaveras fault. This is the section of the fault that has generally regarded as having ruptured at depth during the earthquake.

The USGS community intensity map shows the location of the epicenter, and the intensity of shaking based on survey responses collected after the event. You can mouse over each zip code to see the total number of responses. If you happen to have lived in the Bay Area in 1984, and you remember this earthquake, you can still fill out a report.

The following table lists reports of the effects of the Morgan Hill earthquake in different places. Complete the table by assigning MMI intensities for all of the locations. Intensity is usually given in Roman numerals, but please don't use Roman numerals for this exercise, the program that will grade your work isn't capable of handling them.

You can give intermediate values (e.g., 4.5) if you feel it is appropriate. You may wish to print a copy of this page and write down your answers, so that you can quickly re-enter them if your work is interrupted.

LocationDistance to EpicenterDistance to FaultDescription of ShakingMMI
Antioch72 km72 km Felt by several; frightened a few; hanging objects swung slightly.
Berkeley70 km70 kmFelt by many and frightened some; small objects moved but did not overturn; hanging pictures swung out of place.
Coyote32 km8 kmFelt by all; some windows broken out; light and heavy furniture was overturned; many objects thrown from store shelves; chimneys fell.
Gilroy36 km 12 km Many items thrown from store shelves; many cracks in walls; one wardrobe toppled; several chimneys fell.
Hollister58 km 32 km Plate glass window jumped from the frame and shattered; several windows broken out; many dishes broken; light furniture overturned; few items thrown from shelves; hairline cracks in walls.
Jackson Oaks subdivision 18 km 2 km At least four houses knocked off foundations; walls shifted; many chimneys down; pictures fell off walls; refrigerator toppled and furniture broke.
Los Angeles530 km 500 km Not felt.
Morgan Hill22 km 6 km Felt by all; light damage to 47 homes; a few chimneys fell; mobile homes fell off supports; many items thrown from store shelves.
Sacramento 145 km 145 km Felt by a few; slight swaying of curtains on the 16th floor of downtown building.
Salinas75 km49 km Hanging pictures fell; windows cracked; light and heavy furniture overturned.
San Jose 18 km 18 km Cracks in some walls and plaster fell; no glass broken; dishes broken; many items thrown from store shelves; pictures fell; some chimneys cracked.
San Martin27 km 6 km Felt by all; many cans of paint fell from shelves; many dishes broken; heavy furniture overturned; many items thrown from store shelves; some chimneys fell; cracks in walls.