Alfalfa County feels recent earthquake

A 4.2 magnitude earthquake shook the Alfalfa County area a little before 8 a.m., Friday, Feb. 19, when an earthquake was recorded 3.728 miles southwest of Manchester.

In response to the 4.2 magnitude earthquake that occurred in the Manchester area this morning, the Induced Seismicity Department (ISD) of the Oklahoma Corporation Commission has directed operators of specified disposal wells in the area to cease operation or reduce volume.

The following applies to area oil and gas wastewater disposal wells that inject into the Arbuckle formation:

• Arbuckle disposal wells within 3 miles of the earthquake’s epicenter: Cease operation.

• Arbuckle disposal wells within 3 to 10 miles of the earthquake’s epicenter: Reduce disposed volume to no more than 50 percent of most current 30 day average.

It is estimated that these actions will reduce disposed volumes by about 7,000 barrels a day.

Researchers are in broad agreement that disposal into the Arbuckle formation in parts of Oklahoma can raise risk of induced seismicity. Commission ISD directives since 2015 limiting or eliminating such disposal are among the actions credited with the sharp decrease in Oklahoma’s seismicity rate since 2015.

Oklahoma is within the stable interior of the Meers Fault.

Approximately 1,200 to 1,300 years ago a very large earthquake produced a sixteen-mile-long and fifteen-foot-high fault scarp north of the present town of Meers in Comanche County.

The New Madrid, Missouri, earthquakes of 1811 and 1812 probably were the earliest historical tremors felt in the area that is now eastern Oklahoma (then part of Arkansas Territory).

Before Oklahoma became a state, the earliest documented earthquake epicenter within its current boundaries occurred on October 22, 1882, probably near Fort Gibson, Indian Territory, although the exact location cannot be precisely determined.

The Cherokee Advocate reported that at Fort Gibson “the trembling and vibrating were so severe as to cause door and window shutters to open and shut, hogs in pens to fall and squeal, poultry to run and hide, the tops of weeds to dip, [and] cattle to lowe.”

The first locatable earthquake in Oklahoma occurred near Jefferson in Grant County on Dec. 2, 1897.

Until 2011, the largest known Oklahoma earthquake (with the possible exception of the Fort Gibson 1882 event) had occurred near El Reno in Canadian County on April 9, 1952.

This magnitude 5.5 earthquake, as measured on the Richter Scale, caused a fifty-foot-long crack in the Capitol Building in Oklahoma City.

It was felt throughout Oklahoma and in parts of seven other states. The total felt area was approximately 140,000 square miles. Des Moines, Iowa, and Austin, Texas, were at the northern and southern limits, respectively.

On Nov. 5, 2011, a magnitude 5.6 earthquake was centered near Prague, damaging homes and the Benedictine Hall at St. Gregory’s University in Shawnee.

In 2016 the United States Geological Survey upgraded the Prague event to magnitude 5.7.

On Sept. 3, 2016, a magnitude 5.8 earthquake occurred, centering at Pawnee, in Pawnee County, damaging a dozen buildings. To date, this was the state’s largest quake.

Prior to 1962, all fifty-nine earthquakes in Oklahoma were known either from historical accounts or from seismograph stations outside the state.

Oklahoma’s first seismographs were installed in late 1961. From 1962 through 1976, seventy additional earthquakes were added to the earthquake data base.

In 1977 a statewide network of seismograph stations was installed to improve earthquake detection and location. More than 1,550 additional earthquakes occurred in Oklahoma from 1977 through 2002.

Typical Oklahoma earthquake magnitude values range from 1.8 to 2.5, and focal depths are generally shallow (less than three miles). Earthquakes have occurred in seventy-three of Oklahoma’s seventy-seven counties. Only Washington, Nowata, Craig, and Adair counties have had no known earthquakes.

An analysis of Oklahoma earthquake data indicates at least four principal seismic areas, based upon a consistent pattern of recurrence. These areas include eastern McClain and Garvin counties and southeastern Grady County; Canadian County; south-central Oklahoma, including Love, Carter, and Jefferson counties; and the Arkoma Basin in southeastern Oklahoma.

According to the Oklahoma Geological Survey at the University of Oklahoma from 2010 to 2020 Alfalfa County recorded 2,474 earthquakes ranging from 0.1 magnitude to bigger.

The most recorded in a year during that time frame came in 2015 when 961 were recorded.

Here is a breakdown by year of how many were recorded from 2010-2019:

• 2010: one.

• 2011: one.

• 2012: zero.

• 2013: 114.

• 2014: 381.

• 2015: 961.

• 2016: 324.

• 2017: 184.

• 2018: 80.

• 2019: 168.

• 2020: 260.

How are earthquakes

recorded?

How are earthquakes measured? How is the magnitude of an earthquake determined?

Earthquakes are recorded by a seismographic network. Each seismic station in the network measures the movement of the ground at the site.

The slip of one block of rock over another in an earthquake releases energy that makes the ground vibrate.

That vibration pushes the adjoining piece of ground causing it to vibrate and the energy travels outward from the earthquake in a wave.

There are many different ways to measure the numerous aspects of an earthquake.

Magnitude is the most common measure of an earthquake’s size. It’s a measurement of the size of the earthquake source and is the same number no matter where you are or what the shaking feels like.

The Richter scale is a method that is no longer used to measure the largest wiggle on the recording.

Other magnitude scales measure different parts of the earthquake.

Intensity is a measurement of the shaking and damage caused by the earthquake and this value changes from location to location.

Seismologists have observed that for every magnitude 6 earthquake it would take 32 magnitude 5’s; 1,000 magnitude 4’s; 32,000 magnitude 3’s (and so on for smaller earthquakes) to equal the energy of one magnitude 6 event.

Even though we always record many more small events than large ones, there are never enough to eliminate the need for the occasional large earthquake.

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