3. Steeper mountains
The peaks of old East Coast mountains are generally not as steep than those out west, largely because they have been exposed to millions more years of erosion. Just as river stones become more rounded and less jagged over time as the force of water and other rocks smooth them down, mountains give way to the elements and become less steep through time.
Not only have West Coast mountains experienced less long-term erosion, but some are also still tectonically active today, producing fresh, steep cliff faces that continue to grow upward.
4. More intense wet season
The National Weather Service has reported that, in the past 45 days, the region in which the Washington mudslide took place has experienced double its normal rainfall for this time of year, according to USA Today.
Such rapid delivery of precipitation does not allow time for water to flow deep underground, and causes sediment to become saturated much faster than it would if rainfall were distributed less intensely over a wider timeframe, as is more common on the East Coast.
"Rainfall is key," Finnegan told Live Science, explaining that large slides, like the one in Washington, respond to rainfall over weeks to months, whereas small slides respond over hours to days.
5. Tectonically active
While heavy rainfall alone can trigger mudslides, earthquakes also often instigate the flow, and tend to be more common on the more tectonically active West Coast than the East Coast. Officials in Snohomish County, Wash., where the mudslide occurred last week, reported that a 1.1 magnitude earthquake occurred about two weeks before the slide; even so, the U.S. Geological Survey has stated that the earthquake did not cause this event. More likely, recent rain and soil saturation triggered the lethal landslide, the USGS said.
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