Complex sentence: The 1959 quake was the strongest and deadliest earthquake to hit Montana since the 1935-36 Helena earthquakes left 4 people dead and caused the worst landslides in the history of the Northwestern United States since 1927.
Split into simpler sentences: The 1959 quake was the strongest and deadliest earthquake to hit Montana since the 1935-36 Helena earthquakes left 4 people dead. The quake also caused the worst landslides in the history of the Northwestern United States since 1927.