9000 Years of Movement

In 2006, a large pit was trenched in this area on the Hayward Fault. The rocks revealed that about 9000 years ago there was a large stream flowing about 150 feet from the current location. Creeping movement of the fault over the years, about 5 millimeters per year, has offset the sediments over time. On the ground surface, this type of active movement can still be seen in the parking lot with many offset curbs and en echelon structures (cracks from fault creep) due to the stress. To learn more about en echelon structures, refer to the adjacent sign “En Echelon Structures”.

see a virtual tour of the trench click here

Offset Layers of Rock and Fault Trace

To the west of the fault the sediment is dark in color, gravelly and sandy, and has visible layers (these are the 9000 year-old creek bed for ancient Alameda Creek. To the east of the fault the sediment is tan, and has little gravel or visible layers (these are flood sediments from Alameda Creek 4000 years ago).  They are different because due to fault movement, two parts of the earth from different areas have been brought next to each other.

En echelon fractures are tension fractures that form ribbons of migrating parallel cracks. These fractures are seen throughout the parking lot. The cause of these fractures are from strike-slip movement of rigid blocks in opposite directions.