<P> In May 1980 , a strong earthquake swarm that included four Richter magnitude 6 earthquakes struck the southern margin of the Long Valley Caldera . It was associated with a 10 inches ( 250 mm ) , dome - shaped uplift of the caldera floor . These events marked the onset of the latest period of caldera unrest that is ongoing . This ongoing unrest includes recurring earthquake swarms and continued dome - shaped uplift of the central section of the caldera accompanied by changes in thermal springs and gas emissions . After the quake , another road was created as an escape route . Its name at first was proposed as the `` Mammoth Escape Route '' but was changed to the Mammoth Scenic Loop after Mammoth area businesses and land owners complained . </P>
<P> In May 1980 , a strong earthquake swarm that included four Richter magnitude 6 earthquakes struck the southern margin of the Long Valley Caldera . It was associated with a 10 inches ( 250 mm ) , dome - shaped uplift of the caldera floor . These events marked the onset of the latest period of caldera unrest that is ongoing . This ongoing unrest includes recurring earthquake swarms and continued dome - shaped uplift of the central section of the caldera accompanied by changes in thermal springs and gas emissions . After the quake , another road was created as an escape route . Its name at first was proposed as the `` Mammoth Escape Route '' but was changed to the Mammoth Scenic Loop after Mammoth area businesses and land owners complained . </P>
when did the largest swarm of earthquakes at long valley take place
when did the largest swarm of earthquakes at long valley take place
May 1980
May 1980