science

Information on Hayward Fault

by Heidi Stenner, Consultant

1. Where is the fault and what exactly is a fault?

40 miles long and about 8 miles deep, the Hayward Fault traverses the entire East Bay area.
On the exhibit's trench walls, the fault is marked with vertical strings. The fault is defined by the different sediments on either side, showing where the earth has moved during earthquakes.
The land on the western side of the fault is sliding north and the eastern side is sliding south. The fault is where the sliding takes place, both during earthquakes and during fault creep.
When a portion of the fault breaks and allows the earth to slide, the energy released is what we call an earthquake.

2. How do we know that the fault is there?

In the big picture, scientists know where the fault is because of curbs and streets being offset from fault creep, pre-development aerial photos showing creeks shifted and other markers of fault movement, and from previous trench exposures showing the fault below ground (like at the exhibit).
In the exhibit's trench, we know where the fault is due to the contrast of sediments across a sharp line.
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.

3. What happened in the last big earthquake on the Hayward fault?

The last big earthquake along the Hayward Fault was in 1868. It was approximately a magnitude 6.9.
The fault broke for about 30 miles from around Milpitas to the Oakland-Berkeley border. The fault slipped 3 to 6 feet along the fault. The western side moved north and the eastern side moved south.
At the exhibit, the fault slid about 3 feet. It was closer to San Leandro where the fault slipped about 6 feet. This is what is likely to happen in the next big quake along the Hayward Fault.

4. What is a fault creep?

Fault creep is the slow, almost constant movement along a fault without large earthquakes. Creep is unusual.
Most faults around the world only slip during earthquakes generally larger than magnitude 6.7. The Hayward Fault slips both during those large earthquakes and also the rest of the time from fault creep.
The Hayward Fault in Fremont creeps about 3/16ths of an inch (5mm) per year. The fault needs to move about 6/16ths of an inch (9mm) per year to release the energy building up at the surface.
Fault creep is only happening in the upper couple of miles of the fault's total 8 mile depth, at least for most of the length of the fault. The lower part of the fault is locked and has to release energy during bigger earthquakes.
You can see results of fault creep near the exhibit. Both the edges of the parking lot island (next to the exhibit) and Sailway Drive (between Paseo Padre Parkway and the parking lot) are offset due to fault creep.

5. When is the next big earthquake?

Wouldn't we all love to know! Unfortunately, we cannot predict earthquakes.
Scientific studies have shown that over the last 2000 years, the Hayward fault has been causing large earthquakes (magnitude 6.7 or larger) every 125-175 years, on average. The last big earthquake, in 1868, was 138 years ago (2006).
Bottom line: we are in the time window when the next one can happen. This is why we need to be prepared!

6. What should I do to be prepared?

Keep emergency kits (water, radio, flashlight, etc.) at home, work/school, your car - anywhere you could likely be caught during an earthquake.
Develop a family plan. Identify the person everyone should call to stay updated (out-of-state number if possible). Identify where everyone should try to meet after the earthquake and how they could get there.
Prepare your house and office to reduce the likelihood of large objects hitting you where you frequently sit or sleep when strong shaking begins. Determine if your house needs strengthening to withstand heavy shaking.

Links for more detailed information on being prepared:

Prepare Bay Area - lots of detailed information on preparing (Red Cross)

Be Prepared for 72 Hours - easy to use information on preparing

Putting Down Roots in Earthquake Country - all around wonderful resource/reference for living with earthquakes (U.S. Geological Survey)

Understand What Could Happen at Your Home - shows different hazards for your address/area (Association of Bay Area Governments)