The Hayward Fault is a significant geological feature in the San Francisco Bay Area, traversing densely populated urban areas like Richmond, Oakland, Berkeley, Hayward, and Fremont. It shapes the East Bay landscape and is part of the broader San Andreas Fault system.
Understanding Fault Types
A geological fault is a fracture in the Earth’s crust where rock blocks have moved relative to each other. Scientists classify faults by the direction of this movement. Three main types exist: normal, reverse, and strike-slip.
Normal faults occur when the block of rock above the fault plane, known as the hanging wall, moves downward relative to the block below it, the footwall. This movement typically results from extensional forces, where the Earth’s crust is being pulled apart. Reverse faults involve the hanging wall moving upward relative to the footwall. These faults are formed by compressional forces that push rock masses together, often creating features like mountain ranges. When a reverse fault has a shallow dip angle, it is often referred to as a thrust fault.
A strike-slip fault is characterized by horizontal movement, where two blocks of the Earth’s crust slide past one another laterally. The fault plane is vertical or nearly vertical. This horizontal sliding motion results from shear stress. Strike-slip faults are categorized as right-lateral or left-lateral, depending on the direction of movement. The Hayward Fault is categorized as a right-lateral strike-slip fault.
Characteristics of the Hayward Fault
The Hayward Fault extends along the western base of the hills on the eastern side of San Francisco Bay. It stretches approximately 119 kilometers (74 miles) through East Bay cities like San Jose, Fremont, Hayward, Oakland, and Berkeley. The fault is oriented roughly parallel to the larger San Andreas Fault. Recent research links the Hayward Fault to the Rodgers Creek Fault to the north beneath San Pablo Bay, forming a combined fault system approximately 190 kilometers (120 miles) long.
A notable characteristic of the Hayward Fault is “fault creep,” a slow, continuous, and aseismic (non-earthquake producing) movement along the fault. The Hayward Fault is one of only a few dozen faults globally that exhibits this continuous creep. This creep occurs at an average rate of about 5 millimeters (0.2 inches) per year. While this slow movement alleviates some stress, the long-term slip rate is estimated at around 9 millimeters per year, indicating not all strain is released through creep.
This ongoing deformation can visibly offset structures built across the fault, such as curbs, sidewalks, and building foundations. The fault extends to a depth of about 8 miles; the upper two miles typically creep, while the lower six miles remain locked, accumulating stress for future seismic events.
Seismic History and Future Outlook
The Hayward Fault has a history of significant seismic activity. The most recent major earthquake occurred on October 21, 1868, with an estimated magnitude of 6.3 to 7.0. This event caused substantial damage throughout the San Francisco Bay Area and was known as the “Great San Francisco earthquake” before 1906. The 1868 earthquake resulted in ground rupture traced for about 20 miles (32 kilometers) from San Leandro to Fremont.
Paleoseismic studies, examining geological evidence, indicate the Hayward Fault has experienced at least 12 major earthquakes of magnitude 6.3 or greater over the past several hundred years. The average interval between these events is estimated at 140 to 150 years. Over 150 years have passed since the 1868 earthquake, suggesting the fault has accumulated sufficient strain for another major rupture.
The Hayward Fault poses a significant hazard in the United States due to its proximity to densely populated urban centers. The U.S. Geological Survey (USGS) estimates a significant probability of a magnitude 6.7 or greater earthquake on the Hayward Fault in the coming decades, with assessments as high as 33% by 2043. There is also a possibility that a simultaneous rupture of the linked Hayward-Rodgers Creek Fault could result in an even larger earthquake, potentially reaching magnitude 7.4.
Monitoring and Mitigation
Scientists employ technologies to monitor movement and strain accumulation along the Hayward Fault. These methods include creepmeters and alignment arrays, measuring subtle surface displacement. Satellite-based techniques like Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) also provide broader measurements of ground deformation. These efforts help researchers understand the fault’s behavior and assess future seismic hazards.
While precise earthquake prediction remains a challenge, research aims to develop early warning systems that could provide crucial seconds or minutes of notice before significant shaking begins. Public safety focuses on preparedness to minimize earthquake impact. Advice for earthquake-prone areas includes practicing “Drop, Cover, and Hold On”: dropping to the ground, taking cover under sturdy furniture, and holding on until shaking stops.
Secure heavy furniture and appliances to walls to prevent toppling during shaking. Prepare an emergency kit with water, food, and other supplies, and establish a family disaster plan to improve safety and recovery. After an earthquake, check for gas leaks and potential fires, and be prepared for aftershocks.