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.

Copehill Down, UK

In 1943 the entire population of the British town of Copehill Down on Salisbury Plain was evacuated and they have never been allowed back since. The town was, apparently, converted to be a training facility for American troops preparing for the invasion into Europe as they had no experience of the more dense layout of European villages compared to those in America. The town is still owned by the MoD and has been expanded upon to recreate other village types across Europe, including eastern european towns in the 1980s.

But what caused them to pick this village in particular? Salisbury Plain  is home to a number of important mystical sites in the UK; most notably Stonehenge. It wouldn't be beyond the realms of possibility that Copehill Down was picked by a cult for their base of operations.  In order to protect the population from whatever was about to (or did) happen, the village was evacuated and turned over to Section D to protect. The village could be part of a gateway through to the Dreamlands, home to a band of ghouls or worse...

Banda Sea earthquake

The Banda Sea is an area of the Pacific ocean that is surrounded by hundreds of small islands, forming the Maluku Islands of Indonesia. It is about 1000 km (600 mi) east to west, and about 500 km (300 mi) north to south.

In 1938, on February the 1st, the Banda Sea was the site of the 9th strongest earthquake in the 20th century, generating massive tsunamis. Thankfully no-one was killed in the earthquake, but given the region and the chains of tiny islands it seems perfectly suited for some deep, dark mystery.

An obvious starting point is to link this to the Dagon story and plot hooks as the region is about right. The Dagon story does take place a bit further out into the pacific, but having a sunken realm in the Banda Sea makes some sense as we have all of the islands that are the peaks of something much much larger.

If you want to avoid raising a major deity and driving your players bat____ insane, you could set your adventures in a sunken city; a bit like Atlantis. Maybe a local cult, or a group of Nazi scientists have worked out it's location and have started to collect artifacts from it; maybe it's a city full of Deep Ones that needed cleansing...

For non-mythos related adventures, it could have even been the site of a secret German/Japanese submarine base... blowing up the base could cause a major collapse of a cave system, causing the earthquake.

Dagon

The next installment of the Lovecraft related plot ideas is based upon his second short story, Dagon.

In the story a sailor from WW1 gets shipwrecked in the Pacific and ends up on an exposed piece of land with strange topography. He wanders the land looking for a way off, but instead encounters a series of alien carvings and statues. He escapes the island, completely insane, in an attempt to escape from a monstrous being that rises form the deep...

Shifting this from WW1 to WW2 is simple enough, but having the players actually step foot on the island and encounter Dagon may be a bit too much unless you want to either kill them or drive them completely insane; so what can we do with it?

1) The sailor may have brought back rubbings of the carvings, or a small statue. This artifact could lead to a number of different missions; maybe the original visit to the island was during WW1 and the artifacts made their way into the British Museum. Perhaps they get stolen during the evacuation of the museum treasures during the Blitz and the investigators need to track it down and preventing a cult from using it to raise Dagon.

2) Alternatively, the investigators may be sent to a mysterious new island in the pacific and end up trying to prevent the Germans/Japanese from raising Dagon.