The Izu Peninsula sits on a geological peculiarity. It occupies the northern tip of the Philippine Sea Plate, yet GPS measurements show it moving almost due west -- a direction that diverges from the plate's overall northwestward drift. Geologists interpret this as evidence that the peninsula may actually ride its own fragment, an Izu microplate being shoved and rotated clockwise by the larger Philippine Sea Plate. The pivot point lies at the base of the peninsula, and the clockwise rotation drives the western side beneath Suruga Bay. This slow twisting motion loads stress onto a network of faults running through the peninsula's interior, and the most significant of these is the Tanna Fault, a left-lateral strike-slip fault trending roughly north-northeast through the mountains of northern Izu.

On that November morning, the Tanna Fault released its accumulated strain in a single violent rupture. The earthquake's focal mechanism was left-lateral strike-slip -- the western side lurching northward relative to the eastern side along a fault zone roughly aligned north-northeast to south-southwest. The Tanna Fault is part of the larger North Izu Fault Zone, or Kita-Izu Fault Zone, which stretches across the northern peninsula with an estimated slip rate of several millimeters per thousand years. It may not sound like much, but decades of locked stress unleashed in seconds produced devastation. The village of Kawanishi, now part of the city of Izunokuni, saw many of its buildings collapse outright. Damage rates were highest in communities built directly along the fault trace, where the ground itself shifted beneath foundations.

The shaking did not stop at toppling buildings. Across the peninsula's mountainous interior, saturated hillsides gave way. A landslide in the village of Nakakano killed 15 people, and another in the village of Kitakano took 8 more lives -- both locations now absorbed into the modern city of Izu. In total, 272 people died and 2,165 houses were totally destroyed. The seismic intensity reached shindo 6 in the city of Mishima, a level indicating violent shaking capable of knocking people off their feet. In Ito, fires broke out in the aftermath, compounding the destruction. The earthquake had been preceded months earlier by a swarm of smaller tremors near Ito, running from mid-February through May 1930, though seismologists did not consider the November event part of that swarm. Some researchers have also linked the earthquake to changes in Coulomb failure stress caused by the catastrophic 1923 Great Kanto earthquake, suggesting that the earlier disaster may have nudged the Tanna Fault closer to its breaking point.

The earthquake light observed during the 1930 North Izu event remains one of the most extensively documented cases of this rare and poorly understood phenomenon. Witnesses reported seeing luminous displays across a vast area encompassing the coasts of Suruga Bay, Sagami Bay, and Tokyo Bay, the Sagami River valley, and the entire Izu and Boso Peninsulas. The light appeared before the earthquake struck and persisted for at least an hour, ruling out explanations tied to simple electrical discharges from collapsing structures. Earthquake lights have been reported for centuries in seismically active regions, and hypotheses range from piezoelectric effects in stressed quartz-bearing rock to ionization of air by radon gas released from fractures. No single theory has achieved consensus, but the 1930 Izu event, with its broad geographic coverage and pre-seismic onset, remains a cornerstone dataset for researchers trying to solve the puzzle.

Today, the Tanna Fault is preserved as a geological landmark and educational site. Visitors can see the offset of a railway tunnel that was under construction when the 1930 earthquake struck, its alignment visibly displaced by the fault's lateral movement. The North Izu Fault Zone remains active, and the tectonic forces that produced the 1930 earthquake -- the rotation of the Izu microplate, the compression along Suruga Bay, the slow accumulation of strain on locked fault planes -- continue their patient work. The Izu Peninsula will shake again. The only questions are when, and whether the sky will glow first.