What Happened to Sarah's House
Sarah was at work when Northridge hit. Her husband Mike was home, upstairs. He thought the house was going to fall down around him. It didn't, but it came closer than they realized.
The Sliding Problem
Sarah's house was built in the 1950s. Back then, they didn't always bolt houses to their foundations. The house just sat on top of the concrete. Gravity kept it in place.
During the shaking, the house slid about two inches off the foundation. Not completely, but enough. That two inches of movement cracked the foundation walls, broke plumbing connections, and racked the frame enough to jam half the doors.
If the shaking had lasted longer or been stronger, the house might have slid completely off. People in the neighborhood had houses that did.
The Cripple Wall Failure
Sarah's house has a crawl space with short wooden walls, maybe 2 feet tall, between the foundation and the floor. These are called cripple walls. They're weak points in earthquakes.
Her cripple walls partially collapsed. The sheathing broke away from the studs. The house dropped a couple inches on one side before the framing caught against the foundation. That drop is what caused the corner damage, the visible daylight around the window.
The Diagonal Cracks
The foundation walls themselves had new diagonal cracks after the earthquake. These formed from the shaking. Concrete is strong in compression but weak in shear. The back-and-forth motion creates shear stress. The concrete cracked along diagonal lines to relieve it.
Some of the cracks formed X patterns where stress went both directions during the shaking cycle.
How Earthquakes Damage Foundations
I asked Sarah's contractor to explain what happens. It's different from the slow damage I deal with in the midwest.
The Sudden Force
My foundation problems develop over years. Water pressure, settlement, freeze-thaw. Slow accumulation. Earthquake damage happens in seconds.
The ground moves violently. The foundation moves with it. The house, with all its mass, wants to stay in place. That mismatch between what the foundation is doing and what the house wants to do creates enormous stress. Something has to give.
The Back-and-Forth
Earthquake shaking isn't one big shove. It's rapid back-and-forth motion, sometimes for 30 seconds or more. Each cycle stresses the structure in opposite directions. The concrete that cracked one direction gets pushed the other way. Cracks propagate. Damage accumulates with each oscillation.
The Weak Links
Earthquake damage concentrates at weak points. The connection between house and foundation. Cripple walls. Openings like garage doors. Unreinforced corners.
Sarah's house had multiple weak links. Unbolted connection. Flimsy cripple walls. The earthquake found every one of them.
Soil Matters Too
Sarah's neighborhood is built on fill, soft soil that used to be marshland before development. Soft soil amplifies earthquake waves. Houses on rock feel the shaking. Houses on fill feel it worse.
The contractor said Sarah's house probably experienced 50% stronger shaking than houses on bedrock the same distance from the epicenter. Her soil literally magnified the earthquake.
Sarah's Repair and Retrofit
The $18,000 covered both repairing the damage and retrofitting the house so it wouldn't happen again.
Foundation Bolting
First thing: bolt the house to the foundation. They drilled through the wood sill plate and into the concrete, installed anchor bolts every 4 feet around the perimeter. Now the house can't slide off.
This is such a basic retrofit that some areas of California now require it when houses sell. Sarah paid about $3,500 for this part alone.
Cripple Wall Bracing
They reinforced the cripple walls with plywood sheathing, properly nailed to the studs and the sill plates. The previously flimsy cripple walls became shear walls that can resist horizontal forces.
This is the other common retrofit for older California homes. Sarah's cost about $5,000.
Crack Repair
The diagonal cracks in the foundation walls got epoxy injection. The contractor said epoxy was appropriate here because the foundation wasn't experiencing ongoing movement. The cracks were from a one-time event. Epoxy would restore strength and seal the cracks permanently.
Cost: about $2,000 for all the cracks.
Corner and Connection Repairs
The shifted corner took the most work. They had to jack the house back into position, repair the foundation wall where it had cracked, and rebuild part of the window framing. About $7,500.
They also fixed broken plumbing, adjusted doors, and dealt with interior damage. That was another $5,000 or so on top of the foundation work.
What Sarah Checks Now
After Northridge, Sarah became a foundation hawk. She checks things regularly that she never thought about before.
Bolt Inspection
Every year, she looks at the anchor bolts they installed. Making sure they're still tight, not rusted, not pulled. One bolt had worked slightly loose after a 4.5 magnitude quake a few years ago. She had it retightened. Caught it early.
Crack Monitoring
Just like I track my cracks here in the midwest, Sarah tracks hers. Any new crack after an earthquake gets documented. Any growth in existing cracks gets noted. She has 25 years of records now.
Post-Earthquake Inspections
After any earthquake she feels, even a small one, Sarah does a walkthrough. Looks at the foundation, the cripple walls, the connections. Checks for new cracks. Opens and closes doors. Looks for any change.
Most of the time, everything is fine. But she's caught minor damage from smaller quakes that she was able to address before it became bigger damage.
What This Taught Me
I don't live in earthquake country, but Sarah's experience taught me things that apply everywhere.
Know Your Weak Points
Sarah's house had unbolted connections and weak cripple walls. She didn't know until the earthquake found them. I didn't know my downspout drainage was failing until my basement flooded.
Every house has vulnerabilities. Better to find them through inspection than through failure.
Act Before the Event
Sarah could have retrofitted her house before Northridge for maybe $8,000. Instead she paid $18,000 after, including damage repair. The retrofit alone is always cheaper than the retrofit plus the damage it would have prevented.
That's why I fix my drainage problems proactively instead of waiting for a flood. The lesson translates.
Document Everything
Sarah's insurance claim went smoothly because she had photos of her foundation before the earthquake. She could prove what was new damage versus what was preexisting. Without that documentation, claims get complicated.
I photograph my foundation annually now, partly because of Sarah. If something ever happens, I have a record of what was there before.