Climbers reported steam eruptions in the summit craters for decades after activity apparently ceased in 1921, and the naturalist Paul Schulz documented 30 steam vents at the summit in the 1950s. A report from the United States Geological Survey declared that No one can say when, but it is almost certain that the Lassen area will experience volcanic eruptions again. Similarly, the California Volcano Observatory lists its threat level as Very High. At the time of the early 20th century eruptions, the area surrounding the volcano was only sparsely populated, but a similar eruption today would threaten many lives and the northern Californian economy. Volcanic eruptions occur with similar frequency to major earthquakes from the San Andreas Fault, and at least 10 eruptions have taken place within the state during the past 1,000 years, the most recent at Lassen Peak. Under 1 percent of the state's population lives within hazard zones that could be affected by an eruption, but collectively hazard zones are visited by more than 20 million people each year. Moreover, a number of the potentially active Californian volcanoes reside less than from highly populated areas, and explosive eruptions could produce ash that travels for several hundred miles. In the case of signs that suggest impending volcanic activity, the United States Geological Survey has a plan in place to utilize portable monitoring instruments, deploy scientists to the area, and implement an emergency response plan developed by the National Park Service if an eruption were imminent.