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The day the Great Crater formed...by Sean Meaney
You likely know of the great crater. During the wrath of the immortals when the armies of the master sought to destroy all of darokin by bringing an asteroid (meteorite) from the sky, it is diverted north into the borderlands between glantri and darokin impacting in southern glantri creating 'a crater over twenty miles in diameter', the humanoids migrate in, and that would be that.
A crater over twenty miles in diameter requires an iron meteorite 6000 feet in diameter at an angle of 38 degrees and an impact velocity of 10.563 miles per second.
The initial fireball is 19 miles in radius. Large rocks from 1000 feet diameter near the crater rim out to 8 feet in diameter at over sixty miles from centre are deposited as ejecta. A quake in the 8.6 magnitude hits the known world. Thermal radiation is igniting trees out to two hundred miles radius (which takes in the western forest of alfhiem) with third degree burns for every living thing out to two hundred and fifty miles (3d6/round). That drops away to first degree burns at three hundred miles. Alfhiem city, glantri city, darokin city are wiped from the map by a wind blast that begins at the crater rim at over five thousand miles an hour and drops mildly cyclonic by the time it reaches the mountains north of the five shires.
Total loss of livestock, peasants out to the wendarian ranges. Stone buildings beyond two hundred and fifty miles are spared. Trees inside two hundred and fifty miles are 90% uprooted, destroyed. Trees at three hundred miles are 30% uprooted, destroyed.
A not-so-great crater
To reduce the destruction we need to reduce the meteorite to 1000 feet diameter, at 45 degrees entry, producing a crater 4.5 miles. A fireball of 1.92 miles radius. A 7.0 quake. This means much less destruction. Hill sized boulders at four miles radius with four thousand mph wind blast. Boulder strewn out to twelve miles with one thousand mph wind blast. Three foot rocks at twenty miles with a 537 mph wind...
So living things out to 36 miles take third degree radiation burns. Winds are catastrophic at 214 mph. 90 percent trees down, wood buildings, stone towers. At 44 miles radiation is second degree burns. Air blast of 154 mph downs wooden structures and 90 percent of trees which are still catching fire. At sixty miles the radiation drops to first degree burns. Gravel ejecta, airblast of 188 mph knocks down 30 percent of trees and removes wooden/tile rooftops. At 68 miles there is no radiation. The wind gust is 77.3 mph. Dust and gravel rain from the sky.
Thats far more manegable for the setting aftermath though the crater is far smaller.