Kırşehir earthquake

On April 19th, 1938 the Kırşehir region of Turkey suffered a massive earthquake that killed 224 people. The epicentre is located near Kaman-Kalehöyük, an archeological site that has, since excavations in 1986, revealed artifacts from the Bronze, Iron and Ottoman eras. In 2005 archeologists discovered the world's earliest known example of steel manufacture in the form of a fragment of carbon steel.

We could go down the usual route of blaming the earthquake on a summoning, but there are other possible causes that have been covered up. The fact that we have this archeological treasure trove near by with items from multiple time periods makes me want to invoke some sort of time slip here that drags people into the wrong era. Alternatively we already have stories like The Tomb that create a link between current day (for the dreamer) and the past, allowing items and relationships to be formed. In either case, that carbon steel fragment may be a piece of world war 2 era equipment that had somehow made it's way back through the ages; whether it's wielder was also dragged back is another matter.

Anyway, back to the earthquake. The Tomb doesn't really give us a mythos cause for this as it's going to drive everyone mad. A gateway into a Dream realm however, could. Kaman-Kalehöyük would have also been on numerous caravan routes across Turkey and we all know how shifty some of those caravan owners were (well, they are in folk tales and holywood films, so that's enough right?). Some less scrupulous caravans may have either formed, or used, a gateway to a Dream realm in order to gain power over others, or strange artifacts to trade for profit. The gateway may have been covered over during some big storm in the past, only to be uncovered by a german archeological team in search of mystical powers. I can see this being another location for Walpurgen's experiments in breaching the veil between worlds.

Opening or closing the portal could have caused such a backlash as the fabric of reality is changed that it would cause the same effect as an earthquake.