The Things We Don't Think About: The Aftermath of a Major Earthquake from a Great Valley Perspective

The Great Shakeout is planned for Oct. 16, 2025, at 10:16 AM. 

Source: U.S. Geological Survey

We Californians live in earthquake country. The state is riddled with fault zones, and many of them are more than capable of producing damaging quakes. Some of the state's biggest cities are built on or adjacent to active faults, and much attention has been devoted to the probabilities and outcomes of major quakes in those cities, with predictions of possibly thousands of fatalities and hundreds of thousands of injuries. The dystopian models predict legions of newly homeless people, and badly disrupted services and ruptured transportation lines. It's grim reading. Government organizations are cognizant of the danger, and billions of dollars have been spent on preparing for such quakes, including reinforcement of vulnerable infrastructure, and educating the population about the dangers.

But what about the outlying regions of the state, somewhat removed from the most damaging aspects of the coming quakes? What is life going to be like after the fabled "BIG ONE" strikes? That's what I want to write about, since that's where I and many of my friends and acquaintances live. What's going to happen to us here in the Great Valley, in towns like Modesto and Turlock?

Source: Earthquake Country Alliance

First, the good news: the threat of severe shaking is considerably less in much of the Great Valley, with certain important exceptions (see the map above). Much of the valley floor is located tens of miles east of the most dangerous faults like the Hayward or San Andreas. The expected shaking will lead to some damage, but for the most part, buildings and infrastructure (roads, rails, pipelines) should fare reasonably well.

Liquefaction damage in San Francisco, 1906. Source: USGS

The sediments of the Great Valley can have a tendency towards liquefaction, the phenomenon of turning into "quicksand" during shaking, causing buildings to partially sink and severing buried pipelines and cables. This is especially true along river floodplains, including the San Joaquin River and within the Sacramento Delta (citizens of Tracy and Patterson need to take special note). This is the result of a shallow groundwater table. Luckily, much of middle and eastern parts of the valley have a deep water table, and liquefaction is less likely in the dry sediments.

Here's what happened last time in 1906. Source: Wikipedia open domain

So...let's imagine the worst has happened. A 7.8 magnitude earthquake has struck in the Bay Area, and devastation is widespread throughout the region. Hundreds, perhaps even several thousand people have died, and many tens of thousands have been injured. There are widespread power outages, and pipelines have been broken in many places, leaving many without water. Streets and freeways are blocked by debris and collapsed bridges, and emergency services are overwhelmed. Several hundred thousand homes have been rendered uninhabitable. Fires are burning in many areas. Dozens of aftershocks, some as great as magnitude 6, cause additional damage and panic. Life has taken a catastrophic turn, and millions are affected in profound ways. There's much uncertainty now as the population faces concerns of water, food, shelter, and danger from further seismic activity. It's the time that we find out if years of planning and preparation have been effective. Emergency services swing into action.

Agricultural fields in the western part of the Great Valley near Patterson (my photo)

Meanwhile in Modesto the situation is quite different. Although heavy shaking is felt by literally everyone, the sensation is more of a rolling motion, not the fierce movements that brought down bridges and unreinforced buildings in the Bay Area. Some chimneys are shaken loose, cracks occur in some older buildings, and items fall from shelves throughout the city. Some people are hurt by falling debris, but deaths are probably few in number. Electricity goes out due to the disruption statewide to the state power grid. The outages mean that water pressure can't be maintained, so faucets are dry throughout the city. Cell phone towers may have been damaged, and those that aren't are soon overwhelmed by phone traffic (it has been said that social media spreads earthquake news faster than the seismic waves themselves).

In the immediate aftermath, life has become inconvenient. The power is out, the water is off, sirens are blaring all over the city, but for the most part it's a time of waiting. Waiting for the power to come back on, the water to flow, and the phones to start ringing again. It could take days, maybe even a week or two. All things considered, for most of us it is an interruption.

But there are larger consequences with this incredible earthquake in places like Modesto. 

Emergency services will be largely unavailable in town. Ambulance and EMT crews will pressed into service in the Bay Area, and local hospitals will be filled to overflowing with the injured (many Bay Area hospitals will be overwhelmed and maybe even unusable). The same will be true of firefighters and perhaps law enforcement personnel as well.

Many Bay Area freeways will be impassable, but emergency supplies must still reach the region. Interstate 5 may still be open, but Highway 99 will become a more critical lifeline than it is today. Traffic will increase. The same will be true of the rail system.

As much as I don't like to think about it, most people will not have prepared for the eventuality of earthquakes. They won't have supplies of water and food and other materials on hand, so local store shelves will quickly empty out, and with a severely disrupted supply chain, those shelves will not quickly refill. There will be shortages. Restaurants and fast-food outlets will likely be closed for extended periods.

It's a given that our local community, our officials and authorities and our relatively undamaged infrastructure will be an important part of recovery efforts in Central California. My own institution, Modesto Junior College, has plans in place for sheltering and supporting thousands of people made homeless by the quake. They will likely be our guests for weeks or months. Many other schools and institutions have similar plans. The influx of hurting and anxious people will no doubt place a huge strain on our infrastructure, and I fervently hope we can rise to the occasion, just as I would hope that others will look upon us with compassion in case of catastrophic flooding in our valley (that's an entirely different subject for a future blog).

It's going to be a challenging time, and we can respond to it in two ways:

We can ignore the entire possibility of large earthquakes, and go on living our lives as if nothing will ever happen. That is frankly what most of us are doing now. It's seemingly worked well so far. We are of course living among fault zones that have been building up stress for a century or two, and they are all capable of producing damaging earthquakes right now. I don't think of this as a good choice. Not at all.

An aerial view of a devastated San Francisco shortly after the earthquake in 1906 (USGS)

Or...we could prepare. Take the possibility of a major quake with the intensity you would give it if you knew it was happening a few days. Gather supplies of food, water, and first aid supplies right now, before the shelves emptied by desperate quake refugees and victims. Have emergency go-bags ready in case you have to evacuate your home or community. Develop an emergency plan for your family, and maybe your neighborhood. Learn about the known seismic hazards in the place where you live. Check out my previous blog for more links and ideas. There is a great resource centered on Central California published by the California Earthquake Authority, the Earthquake Country Alliance, and the California Governor's Office of Emergency Services, that can be reached by clicking here.

It would be nice if we could know in advance that a quake is about to hit, but we don't. Since that's the case, we should be as ready as possible.

Source: USGS