Location: almost directly beneath Helena. Affected area: 363,000 square kilometers. Damage: $4 million. A series of earthquakes beginning on October 3, 1935, shook the area. The strongest of the shocks was on October 18. Several shocks of lesser intensity were followed by a second strong earthquake on October 31 that destroyed many buildings that had been previously damaged. The shocks continued with additional strong shocks on November 21 and November 28.