The 1935 Helena earthquake occurred at in

Montana Montana () is a state in the Mountain West division of the Western United States. It is bordered by Idaho to the west, North Dakota and South Dakota to the east, Wyoming to the south, and the Canadian provinces of Alberta, British Columbi ...

, with an

epicenter The epicenter, epicentre () or epicentrum in seismology is the point on the Earth's surface directly above a hypocenter or focus, the point where an earthquake or an underground explosion originates. Surface damage Before the instrumental pe ...

near Helena. It had a magnitude of 6.2 on the

surface wave magnitude The surface wave magnitude (M_s) scale is one of the magnitude scales used in seismology to describe the size of an earthquake. It is based on measurements of Rayleigh surface waves that travel along the uppermost layers of the Earth. This ma ...

scale and a maximum perceived intensity of VIII (''Severe'') on the

Mercalli intensity scale The Modified Mercalli intensity scale (MM, MMI, or MCS), developed from Giuseppe Mercalli's Mercalli intensity scale of 1902, is a seismic intensity scale used for measuring the intensity of shaking produced by an earthquake. It measures the eff ...

. The temblor on that date was the largest of a series of earthquakes that also included a large

aftershock In seismology, an aftershock is a smaller earthquake that follows a larger earthquake, in the same area of the main shock, caused as the displaced crust adjusts to the effects of the main shock. Large earthquakes can have hundreds to thousand ...

on October 31 of magnitude 6.0 and a maximum intensity of VIII. Two people died in the mainshock and two others died as a result of the October 31 aftershock. Property damage was over $4 million.

Tectonic setting

Helena lies within the northern part of the ''Intermountain Seismic Belt'' (ISB). This area of relatively intense seismicity runs roughly north-south from northwestern Arizona, through Utah, Idaho, and Wyoming, before dying out in northwestern Montana. It shows a marked deflection to the northwest near Helena, where it intersects with the Lewis and Clark fault zone, a zone of older WNW-trending faults. The ISB is characterized by normal faulting of late

Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...

age, indicating active

extensional tectonics Extensional tectonics is concerned with the structures formed by, and the tectonic processes associated with, the stretching of a planetary body's crust or lithosphere. Deformation styles The types of structure and the geometries formed depend on ...

, as recognized throughout the

Basin and Range province The Basin and Range Province is a vast physiographic region covering much of the inland Western United States and northwestern Mexico. It is defined by unique basin and range topography, characterized by abrupt changes in elevation, alternating ...

to the south. The

focal mechanism The focal mechanism of an earthquake describes the deformation in the source region that generates the seismic waves. In the case of a fault-related event it refers to the orientation of the fault plane that slipped and the slip vector and is ...

of the two largest earthquakes of the 1935 sequence are consistent with right lateral

strike-slip faulting In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

on faults similar in trend to two major mapped NW-SE faults, the Bald Butte and

Helena Valley Helena may refer to: People *Helena (given name), a given name (including a list of people and characters with the name) *Katri Helena (born 1945), Finnish singer *Helena, mother of Constantine I Places Greece * Helena (island) Guyana * ...

Faults.

Earthquake sequence

The sequence began with a small earthquake on October 3. It was followed by a damaging earthquake on October 12 (magnitude 5.9, intensity VII), the mainshock on October 18 (magnitude 6.2, intensity VIII), the large aftershock on October 31 (magnitude 6.0, intensity VIII) and a further large aftershock on November 28 (magnitude 5.5, intensity VI). There were a total of 1800 temblors recorded between October 4, 1935, and April 30, 1936.

Damage

There was widespread damage to unreinforced masonry structures in the Helena area with more than 200 chimneys being destroyed. The strongest effects were seen in the northeast, where buildings were constructed on

alluvial Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. Alluv ...

soil, and in the southern business district, which contained many brick buildings. The most extensively damaged building was the

Helena High School Helena High School is a public high school for grades 9 through 12 located in Helena, Montana, United States. It is part of the Helena Public School District. Founded in September 1876, it is the oldest high school in the state of Montana.

, which was completed in August 1935 and had just been dedicated in early October. The school buildings, which had cost $500,000, had not been designed to be earthquake resistant. Another building that was totally destroyed and had to be rebuilt was the Lewis and Clark County Hospital. The October 18 earthquake caused an estimated $3 million of damage to property. The aftershock of October 31 caused an estimated $1 million of further property damage, particularly to structures already weakened by the October 18 shock. Two people were killed by falling bricks in Helena during the October 18 shock. Two brick masons died as a result of the October 31 aftershock, while removing a brick tower.

Aftermath

A temporary camp was set up by the Red Cross and

Federal Emergency Relief Administration The Federal Emergency Relief Administration (FERA) was a program established by President Franklin Roosevelt in 1933, building on the Hoover administration's Emergency Relief and Construction Act. It was replaced in 1935 by the Works Progress Adm ...

on land at the

Montana Army National Guard The Montana Army National Guard is a component of the United States Army and the United States National Guard. Nationwide, the Army National Guard comprises approximately one half of the US Army's available combat forces and approximately one t ...

's Camp Cooney. The first night, 400 people stayed there, but most had found space with friends or family outside of the damaged area by the end of the week. Many other people lived in tents around the town for the next few weeks, either due to damaged property or for fear of aftershocks. The National Guard were deployed in Helena to keep sightseers away from the damaged buildings, there was no looting.

Modern-day concerns

A computer simulation performed for the Lewis and Clark County office of Disaster and Emergency Services estimated that a 6.3 magnitude earthquake today would result in property damage over $500 million. Another concern for future earthquakes in the area is

soil liquefaction Soil liquefaction occurs when a cohesionless saturated or partially saturated soil substantially loses strength and stiffness in response to an applied stress such as shaking during an earthquake or other sudden change in stress condition, in ...

, especially in the area north of the city where there are alluvial soils and a high water table.

References

External links

* {{DEFAULTSORT:1935 Helena earthquake Helena earthquake, 1935 Earthquakes in the United States Natural disasters in Montana 1935 in Montana Helena earthquake, 1935