“The Big One” refers to an anticipated, high-magnitude earthquake (M8.0 or greater) along a major fault system. Key scenarios involve the Cascadia Subduction Zone (M9.0+ megathrust rupture) and the San Andreas Fault (M7.8 to M8.3 event). The disaster’s timeline extends far beyond the moment of rupture, encompassing initial ground motion, secondary hazards, the prolonged aftershock sequence, and the arduous process of societal recovery. Understanding the duration of each phase is necessary to comprehend the full scale of this multi-phase crisis.
The Duration of Ground Shaking
The physical ground shaking is the most intense and immediate phase, with duration proportional to the earthquake’s magnitude and the length of the fault that ruptures. A moderate earthquake (e.g., M6.9) might produce significant shaking for only 15 to 20 seconds. Even the 1906 San Francisco earthquake (M7.8) saw intense shaking for less than 42 seconds, but a megathrust event is fundamentally different due to the immense scale of the rupture.
A M9.0 earthquake, such as the one modeled for the Cascadia Subduction Zone, involves a fault rupture extending hundreds of miles. This massive rupture zone causes the ground to shake for several minutes as the rupture propagates along the fault line. Historical examples include the 2011 M9.0 Tohoku earthquake (six minutes) and the 1964 M9.2 Great Alaskan earthquake (four and a half minutes). This extended duration increases the time structures are under stress, leading to the failure of infrastructure designed for only brief shaking.
Immediate Secondary Hazards Timeline
The time immediately following the cessation of ground motion marks a transition to a cascading natural disaster defined by non-seismic hazards. This period spans the first 12 hours, during which the affected area is rapidly redefined by the aftermath of the shaking.
For coastal areas near a megathrust fault, the most time-sensitive secondary hazard is the tsunami. Unlike distant tsunamis that allow for hours of warning, a locally generated wave from a Cascadia-style event can arrive at the coast within 10 to 20 minutes of the shaking stopping. This rapid arrival makes immediate self-evacuation the only viable response.
Inland areas face immediate threats, including landslides and soil liquefaction, which begin during or immediately after the strong motion. Liquefaction occurs when saturated soil temporarily loses its strength, causing the ground to behave like a liquid and leading to the collapse of structures. Fires also ignite quickly due to ruptured natural gas lines and downed electrical wires, often starting within minutes and defining the initial hours of the crisis.
The Extended Aftershock Sequence
The geological duration of the event is significantly extended by the aftershock sequence, which maintains a state of seismic crisis long after the main shock has ended. Aftershocks are smaller earthquakes that occur within the same fault zone, representing the crust adjusting to the massive redistribution of stress. These subsequent events can be substantial; a M9.0 mainshock is capable of producing aftershocks of M7.0 or higher, which can cause additional damage to already compromised structures.
The rate and magnitude of aftershocks generally follow Omori’s Law, stating that the frequency of these quakes decays over time. While the decay is rapid at first, the sequence can persist for an extended period. Depending on the mainshock’s magnitude, aftershock activity can last for weeks to months, and sometimes for years. For example, the aftershock duration of a major historical earthquake was estimated to last for over two years.
Infrastructure Restoration and Recovery Timelines
For the population, the true duration of the disaster is measured not by ground motion but by the time it takes for essential services to return, marking the transition from emergency to recovery. The timelines for restoring infrastructure reflect the scale of the damage expected from a massive earthquake.
Power and communication systems, which rely on above-ground lines and vulnerable substations, are often the first to fail. Restoration is expected to take days to weeks, though localized outages in heavily damaged areas may last longer until access and repairs are completed.
Water and sewage systems, which involve vast networks of buried pipes, are the most difficult to repair, particularly in areas subjected to liquefaction and surface rupture. Engineering estimates suggest that some of the hardest-hit areas could be without full water service for weeks to several months.
Major transportation routes, including highways, bridges, and rail lines, will face significant damage, halting the movement of people and emergency supplies. While initial debris clearance may happen quickly, the full restoration of these major arteries to pre-event capacity is a multi-month logistical undertaking.