TREMR

September 19, 1985. A M8.1 earthquake strikes 350 kilometres off the Pacific coast of Mexico. At that distance, most of Mexico City should have been fine. Buildings on solid rock barely swayed. But in the old lakebed district — where the Aztecs built Tenochtitlán on a shallow lake, later drained by Spanish colonisers — hundreds of mid-rise buildings between eight and eighteen storeys simply collapsed. On the same blocks, taller towers and smaller buildings stood completely untouched.

Nearly 10,000 people died. The pattern of destruction was not random.

The answer lies in resonance. And understanding it explains not just what happened in Mexico City, but what happens inside every building during every significant earthquake — and what engineers have done about it since.

The Waves Arrive

When a fault ruptures, it releases energy as two main types of seismic body waves. P-waves — primary, compressional — travel fastest, arriving first as a brief vertical jolt, like someone pushing a spring end-on. S-waves — secondary, shear — arrive seconds later and shake the ground sideways. These are the destructive ones. They can accelerate the ground horizontally with a force measured as a fraction of gravity — and in extreme events, several times gravity.

A building doesn't experience an earthquake as a single pulse. The ground beneath it moves; the foundation moves with it; and the upper structure tries to follow. Depending on height and stiffness, the top of a building can swing many times further than the base — amplifying the input rather than simply transmitting it.

Every Building Has a Natural Frequency

Every structure has a natural period — the rate at which it prefers to oscillate when disturbed and released. Tap a ruler hanging off a desk and it vibrates at its natural frequency. So does a skyscraper, only much more slowly. The rough rule of thumb for buildings: 0.1 seconds per storey. A 5-storey building oscillates at around 0.5 seconds per cycle; a 15-storey building at roughly 1.5 seconds.

When earthquake ground motion arrives at a frequency close to a building's natural period, the result is resonance. Each shaking cycle reinforces the last rather than cancelling it. Like timing pushes on a child's swing, even modest inputs at the right frequency build to enormous amplitude.

In Mexico City in 1985, the ancient lakebed — up to 40 metres of soft clay and volcanic silt — transformed the earthquake's energy into long-period waves of around 2 seconds. Buildings of 12 to 20 storeys, with natural periods in exactly that range, coupled with the amplified waves and resonated catastrophically. Buildings of 5 storeys and buildings of 30 storeys, oscillating at different periods, did not. Some stood untouched metres from structures that had pancaked.

This effect — soft soil amplifying specific frequencies — is called site amplification. Mexico City is an extreme case, but the same phenomenon affects any building on loose sediment, landfill, or waterlogged ground.

How Buildings Actually Fail

The forces in an earthquake are primarily lateral — horizontal, not vertical. Buildings are designed primarily to resist gravity: the weight of floors, walls, contents, and occupants pushing straight down. They must also resist wind forces horizontally, but a major earthquake can produce lateral accelerations several times greater than any wind load a building was designed for. This gap is the fundamental vulnerability.

Three failure modes dominate earthquake collapses:

Soft-story collapse. Ground floors of urban buildings are often the most open: parking garages, glass-fronted shops, hotel lobbies, open-plan restaurants — spaces with minimal structural walls. When the ground floor is weaker than the floors above it, it becomes the hinge point. The heavy mass of the upper storeys keeps moving while the weak ground level buckles beneath them, and the building drops straight down onto its own footprint. In the Turkey–Syria earthquakes of February 2023, soft-story collapse was the dominant failure mode. More than 50,000 people died.

Pancake collapse. When the connections between floor slabs and supporting columns fail — rather than the columns themselves — each floor drops onto the one below. Pancake collapses are particularly lethal because they leave almost no survival voids. The 1995 Kobe earthquake killed 6,434 people across western Japan, many in pre-code concrete apartment blocks that pancaked.

Torsional failure. A building's centre of mass — where its weight is concentrated — rarely aligns perfectly with its centre of stiffness — where its structural resistance lies. An asymmetric building doesn't translate straight back and forth during shaking; it rotates. Corners, which travel the greatest arc during rotation, experience the highest stress and fail first.

What Engineers Do About It

Modern seismic engineering has three main strategies.

Ductility. Steel and carefully detailed reinforced concrete can be designed to flex rather than fracture — absorbing seismic energy through controlled yielding. A ductile moment-resisting steel frame bends at specifically engineered joints; when the shaking stops, the building has deformed but remains standing and evacuable. The goal isn't zero damage — it's that the structure survives long enough for everyone inside to get out. Ductility requirements are now fundamental in the seismic codes of Japan, California, New Zealand, and Chile.

Tuned mass dampers. Some tall buildings suspend a large mass near their top, connected to the structure by hydraulic actuators and cables. The mass is tuned to move at the building's natural period but out of phase with its oscillation: when the building sways one direction, the damper moves the other, cancelling a significant portion of the motion. Taipei 101 in Taiwan contains the world's most famous example — a 660-tonne steel sphere, 5.5 metres in diameter, hanging between the 92nd and 87th floors. Visitors can watch it move from a dedicated viewing platform. During strong typhoons, the sphere visibly sways while the occupied floors of the building remain comparatively still. Wind-induced motion is reduced by around 40%.

Base isolation. Rather than strengthening the building, isolation physically decouples it from the ground. The structure sits on a set of bearings — typically thick rubber pads reinforced with steel plates, or Teflon sliders around a lead core — that allow horizontal movement of up to half a metre while the building above remains nearly stationary. An isolated building has an effective natural period of 3 to 5 seconds, far outside the 0.1-to-2-second range where most earthquake energy concentrates. The ground moves; the building doesn't follow.

When the 1994 Northridge earthquake struck the Los Angeles basin, the USC University Hospital — built on a base isolation system — continued operating without interruption. Patients were not evacuated. Adjacent non-isolated hospitals sustained significant structural damage and had to close. The difference in outcome was not the earthquake; it was the design of the foundation.

The Gap Between Knowledge and Practice

Every post-earthquake investigation reaches the same conclusion: buildings that followed modern codes survived; buildings that didn't, collapsed.

After Mexico City 1985, Mexico overhauled its seismic design standards. After Turkey's 1999 Kocaeli earthquake killed 17,000 people, Turkey rewrote its code. After the 2023 Turkey–Syria earthquake killed more than 50,000, investigators found that most collapsed buildings had been constructed before modern codes existed — or had received construction amnesties, legal regularisations allowing illegal structures to remain standing without being rebuilt to code. Some nominally compliant new buildings had been built with inferior concrete, undersized reinforcement bars, or without accounting for the soil conditions beneath them.

There is a repeating pattern to this. After a major earthquake, codes improve and enforcement tightens — sometimes dramatically. A generation passes. The collective memory of the disaster fades. Construction costs become the primary concern again. Enforcement lapses. And the next earthquake finds buildings that look modern but aren't built to modern standards.

The shaking itself lasts seconds — rarely more than a minute, even for a major event. The damage is done in that time. But the vulnerability is built over years and decades beforehand: through shortcuts taken at the construction site, through amnesties granted to buildings that should have been demolished, through the slow erosion of urgency that settles in between one earthquake and the next.

A building's fate in an earthquake is largely decided before the ground starts moving.