The damage concentrated itself in Irondale with an almost surgical precision. Within a two-block area near the epicenter, fourteen chimneys toppled, including six at a single brick store that collapsed simultaneously. Cracked windows spider-webbed across storefronts. In Shelby and Jefferson counties, the shaking reached intensities of VI and VII on the Mercalli scale -- "Strong" to "Very strong" -- enough to knock objects from shelves, wreck plaster, and send people stumbling. Further out, the tremor faded to a subtle vibration, felt across seven states but causing no damage beyond the immediate vicinity. The earthquake struck before Alabama had a seismograph network, so the 5.1 magnitude is a retroactive estimate. But residents of Irondale and nearby Pell City did not need instruments to measure what had happened: their wells told the story. Water levels dropped dramatically, and in Pell City, one well lost its water supply by a measurable depth.

The 1916 earthquake originated in the Eastern Tennessee seismic zone, one of the most active seismic areas in the Southeastern United States, though most people who live above it have never felt a tremor. The zone runs roughly parallel to the Alabama-New York Lineament, a deep structural feature in the Earth's crust. Faulting here is strike-slip -- two blocks of rock grinding sideways past each other, deep beneath the surface. Scientists believe the earthquakes from this zone signal the reactivation of ancient basement faults, buried far below the younger Paleozoic rock layers visible at the surface. These deep, old fractures converge on one another, and the interplay of stronger and weaker faults produces the occasional jolt that reaches the surface. The zone generates roughly one non-damaging earthquake per year, many so faint only seismographs detect them. But every few decades, something stronger breaks through.

Alabama's seismic record reads like a slow-burning series of reminders. Following the 1916 Irondale event, minor earthquakes struck in 1917, 1927, 1931, and 1939. In 1957, a tremor near Birmingham sent vibrations into Georgia and Tennessee, cracking and displacing small objects. Two years later, a Huntsville earthquake damaged chimneys and wrecked plaster. Another event rattled Alabama in 1997. Then in 2003, a magnitude 4.6 earthquake near Fort Payne renewed attention to the state's seismic vulnerability. Over a 91-year period, Alabama recorded 19 earthquakes ranging from near-imperceptible to damaging. The pattern is consistent: decades of quiet punctuated by brief, startling reminders that the ground is not as stable as it appears.

The 1916 earthquake caused minor damage in a sparsely built area. But Birmingham has grown enormously since then. A 2007 United States Geological Survey study estimated that an earthquake of similar size striking a heavily populated area of Alabama today could damage thousands of buildings and cause up to one billion dollars in losses. The threat extends beyond Alabama's own faults. The New Madrid seismic zone, centered along the Mississippi River in Missouri, Tennessee, and Arkansas, produced catastrophic earthquakes in 1811 and 1812 that rang church bells as far away as Boston. A repeat of those events would send powerful seismic waves through Alabama's infrastructure. The Irondale earthquake of 1916 was a warning written in toppled chimneys and dry wells. More than a century later, the faults beneath Alabama remain active, and the cities above them have grown far more fragile.