Question: What was the initial magnitude of the earthquake that formed Lake Isom?
Context: The 1811–12 New Madrid earthquakes were an intense intraplate earthquake series beginning with an initial earthquake of moment magnitude 7.5–7.9 on December 16, 1811, followed by a moment magnitude 7.4 aftershock on the same day.  They remain the most powerful earthquakes to hit the contiguous United States east of the Rocky Mountains in recorded history.  They, as well as the seismic zone of their occurrence, were named for the Mississippi River town of New Madrid, then part of the Louisiana Territory, now within the US state of Missouri. The 1968 Inangahua earthquake struck 25 km west of Murchison, New Zealand, near the small town of Inangahua Junction at 5:24 am NZDT on 24 May 1968.  The earthquake had a moment magnitude of 7.1, a local magnitude of 6.7, a surface wave magnitude of 7.4 and a maximum Mercalli intensity of X ("Very Destructive").  It occurred at a depth of only 12 km , being extremely shallow for an earthquake of its size.  It resulted in the deaths of three people, with a further 14 people injured, making it the fifth deadliest earthquake in New Zealand's recorded history.  Numerous aftershocks followed the quake, including 15 that were magnitude 5 or greater and occurred within a month. The 2011 Oklahoma earthquake was a 5.7 magnitude intraplate earthquake which occurred near Prague, Oklahoma on November 5 at 10:53 p.m. CDT (03:53 UTC November 6) in the U.S. state of Oklahoma.  The epicenter of the earthquake was in the vicinity of several active wastewater injection wells.  According to the United States Geological Survey (USGS), it was the most powerful earthquake ever recorded in Oklahoma; this record was surpassed by the 2016 Oklahoma earthquake.  The previous record was a 5.5 magnitude earthquake that struck near the town of El Reno in 1952.  The quake's epicenter was approximately 44 mi east-northeast of Oklahoma City, near the town of Sparks and was felt in the neighboring states of Texas, Arkansas, Kansas and Missouri and even as far away as Tennessee and Wisconsin.  The quake followed several minor quakes earlier in the day, including a 4.7 magnitude foreshock.  The quake had a maximum perceived intensity of VIII ("Severe") on the Mercalli intensity scale in the area closest to the epicenter.  Numerous aftershocks were detected after the main quake, with a few registering at 4.0 magnitude. The 1980 El Asnam earthquake occurred on October 10 at with a moment magnitude of 7.1 and a maximum Mercalli intensity of X ("Extreme").  The shock occurred in the Algerian town of El Asnam (now known as Chlef).  The shocks were felt over 550 km away, with the initial earthquake lasting 35 seconds.  It was the largest earthquake in Algeria, and was followed three hours later by a magnitude 6.2 aftershock.  The earthquake created about 42 km of surface rupture and had a vertical slip of up to 4.2 m .  No foreshocks were recorded.  The earthquake was found to have occurred very close to the epicenter of the 1954 earthquake using joint epicenter determination techniques.  It occurred at a previously unknown reverse fault. This is a list of earthquakes in 1974.  Only magnitude 6.0 or greater earthquakes appear on the list.  Lower magnitude events are included if they have caused death, injury or damage.  Events which occurred in remote areas will be excluded from the list as they wouldn't have generated significant media interest.  All dates are listed according to UTC time.  Maximum intensities are indicated on the Mercalli intensity scale and are sourced from United States Geological Survey (USGS) ShakeMap data.  Activity was once again below average with 11 events reaching magnitude 7+.  The largest came in October in Peru when a magnitude 7.6 earthquake struck Lima.  Other magnitude 7.0+ events shook the Caribbean, China and the southwest Pacific Islands.  Two events dominated the 25,000 deaths during 1974.  China was struck by a magnitude 6.8 event in May with upwards of 20,000 deaths.  At the end of the year, Pakistan was struck by a modest 6.2 magnitude earthquake causing 5,300 deaths.  Gabon in central Africa was hit by a rare earthquake in September. The 2014 Orkney earthquake occurred at 12:22:33 SAST on 5 August, with the epicentre near Orkney, a gold mining town in the Klerksdorp district in the North West province of South Africa.  The shock was assigned a magnitude of 5.5 on the Richter scale by the Council for Geoscience (CGS) in South Africa, making it the biggest earthquake in South Africa since the 1969 Tulbagh earthquake, which had a magnitude of 6.3 on the Richter scale.  The United States Geological Survey (USGS) estimated a focal depth of 5.0 km .  According to the USGS, the earthquake was "severely dangerous" as the epicentre was located near Orkney and Klerksdorp.  The CGS reported 84 aftershocks on 5 August and 31 aftershocks on 6 August, with a magnitude of 1.0 to 3.8 on the Richter scale.  According to the CGS, the earthquake is the biggest mining-related earthquake in South African history. Reelfoot Lake is a shallow natural lake located in the northwest portion of U.S. state of Tennessee, in Lake and Obion counties.  Much of it is really more of a swamp, with bayou-like ditches (some natural, some man-made) connecting more open bodies of water called basins, the largest of which is called Blue Basin.  Reelfoot Lake is noted for its bald cypress trees and its nesting pairs of bald eagles.  It is the site of Reelfoot Lake State Park.  Lake Isom, a similar, smaller lake to the immediate south, is a National Wildlife Refuge area. On 12 February 2013, North Korean state media announced it had conducted an underground nuclear test, its third in seven years.  A tremor that exhibited a nuclear bomb signature with an initial magnitude 4.9 (later revised to 5.1) was detected by the China Earthquake Networks Center, Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization and the United States Geological Survey.  In response, Japan summoned an emergency United Nations meeting for 12 February and South Korea raised its military alert status.  It is not known whether the explosion was nuclear or a conventional explosion designed to mimic a nuclear blast; as of two days after the blast, Chinese, Japanese, and South Korean investigators had failed to detect any radiation. Lake Isom is a small natural lake located in Lake County, Tennessee immediately south of Reelfoot Lake.  It is fed by Running Reelfoot Bayou, the outlet stream of Reelfoot Lake.  Like Reelfoot, it was formed in the 1811–12 New Madrid earthquakes and it is very shallow and swampy.  The entire lake and its environs, covering 1,846 acres (747 ha) comprise the Lake Isom National Wildlife Refuge and have been such since 1938. The 2003 Alabama earthquake took place on April 29 at 3:59 A.M. Central Daylight Time (local time when the event occurred) eight miles (13 km) east-northeast of Fort Payne, Alabama.  The number of people who felt this quake was exceptionally high as the earthquake could be felt in 11 states across the East Coast and as far north as southern Indiana.  The earthquake was strongly felt throughout metropolitan Atlanta.  The Georgia Building Authority was called out to inspect the historic Georgia State Capitol in downtown Atlanta and other state-owned buildings, but found no problems.  However, this is not out of the ordinary as earthquakes east of the Rocky Mountains can be felt several times the area felt on West Coast earthquakes.  The earthquake was given a magnitude 4.6 on the moment magnitude scale by the USGS (other sources reported as high a magnitude as 4.9) and reports of the duration of the shaking range from 10 seconds to as long as 45 seconds.  It is tied with a 1973 earthquake near Knoxville, Tennessee as the strongest earthquake ever to occur in the Eastern Tennessee Seismic Zone, which is the second most active seismic zone east of the Rocky Mountains, with the New Madrid Seismic Zone the most active.
Answer: 7.5–7.9