Wednesday, January 18, 2017

St. Tammany Visited by Earthquake

On November 3, 2002, I was visiting friends on Bayou Monga (off the Tchefuncte River just south of I-12) , when suddenly the water in the bayou began sloshing back and forth and the masts on the sailboats started swaying back and forth, ropes slapping up against them. If I hadn't known any better, I would have said it was an earthquake. (Actually I think I did tell someone it looked like an earthquake.)

In fact, it was. The Denali earthquake in Alaska had found fault with the ways things were going and generated a 7.9 sized earthquake, one in which even St. Tammany Parish felt the effects. All across America, people were feeling the aftermath of the largest inland American earthquake in recorded history. 

However, some experts think that St. Tammany may be overdue for a sizeable earthquake of its own. In an article posted on the WAFB Channel 9 website in 2012, Thomas Morrison wrote that geologists think that Louisiana may be overdue for earthquake

"As far as natural disasters go, earthquakes are not high on the list of concerns for Louisiana residents. However, LSU geologists say the state may be overdue and the results could be disastrous.

"Based on earthquakes in the past, we will probably get a magnitude 5 (on the Richter scale) earthquake every 15 to 20 years," said Juan Lorenzo, geology professor at LSU.

"Fault lines can be found in northwestern and southern Louisiana in a series of faults referred to as the "Baton Rouge fault system." Lorenzo notes earthquakes that begin along these fault lines would be on par with what hit the East Coast last month.

"You would feel the shaking, and it would knock some things over, but there should only be minor damage," Lorenzo said.

"The wild card is if we have another huge event in the New Madrid area," said Richard McCulloh, research assistant with the Louisiana Geological Survey at LSU. "What would the consequences be in Louisiana?"

"The New Madrid earthquakes were a series of earthquakes in 1811 along fault lines in Arkansas, Missouri and Tennessee. According to the United States Geological Survey, the effects were felt across the nation and were so powerful they changed the flow of the Mississippi river and all but destroyed Charleston, SC.

"The damage was minimal in Louisiana, but Lorenzo said next time could be different.

"The possibility is there (for damage), but we just don't know because it hasn't happened before."

"An AP analysis on a recent report by the Nuclear Regulatory Commission found that the River Bend 1 nuclear plant near the Mississippi River at Baton Rouge fault lines was "nine times" more likely to receive core damage due to an earthquake. The report found that "a quarter of America's reactors may need modifications to make them safer."

"Because of where the fault lines are, and the levees have been around for a long time, an earthquake could (but is not likely to) cause damage," Lorenzo said.

Lorenzo clarified that the levee system then was not nearly as extensive as it is now, and the results could be different, but there is no concrete evidence to suggest it would.

"Another worst case scenario is if we have a level 5 earthquake," Lorenzo said. "That could cause an underwater landslide of sediments carried by the Mississippi River that could potentially bring down an oil well or platform."

According to McCholloh, the recent earthquakes on the East Coast should be a "wake up call" for the rest of America. "If it had been one magnitude higher on the Richter scale, it could have been very destructive."

End of WAFB Article



The image above shows how the Baton Rouge earthquake fault line extends eastward along the Lake Pontchartrain shoreline in St. Tammany Parish. While the fault isn't suspected of being a "seismic" fault, it is subject to pressures caused by subsidence (sinking) of the area below New Orleans. In other words, some experts fear that the southern Louisiana coastline is in danger of sliding into the Gulf of Mexico, and the line where that separation may occur runs from Madisonville to Slidell. 

More information about that may be found AT THIS LINK. 

Here is some information from the USGS:

Subsurface geologic faulting is common throughout most of the Louisiana coast due to the underlying presence of a thick Tertiary Basin. Lake Pontchartrain is located on the northern margin of the South Louisiana Tertiary Basin and faults in the southern portion of the Lake have a style typical of the Tertiary Basin. The northern and western portion of the Lake is underlain by a regional fault system associated with the Lower Cretaceous margin, which is also the major boundary for the Tertiary Basin (Lopez, 1997). The fault system is composed of mostly sub-parallel or en echelon faults which step southward as the fault system continues eastward from Baton Rouge through the area north of Lake Maurepas and into Lake Pontchartrain.

The cause for recent activity of the BR-DS faults in Lake Pontchartrain is unclear. Faulting in Lake Pontchartrain and elsewhere along Louisiana's coast is currently being investigated at the UNO.

The BR-DS faults in Lake Pontchartrain have been identified through "conventional" oil and gas seismic profiles, high-resolution seismic and fault offset along bridge structures.

Fault offset on U.S. 11 Bridge south of Slidell
 
This BR-DS fault system in Lake Pontchartrain is composed of four faults. All of these faults are south dipping, normal faults which form a southeasterly trend of en echelon fault surface traces. All of these faults extend from the surface (i.e. the Lake's bottom) to a minimum depth of 20,000 ft. The faults generally dip 45 to 65 degrees from horizontal. 

The four BR-DS faults (designated here as South Point Fault, Goose Point Fault, Causeway Fault and Madisonville Point Fault) are generally moving with relative offset of 0.1 inch/yr (2.54 mm/yr). However, some evidence suggests movement rates as high as 0.4 inches/yr (10.16 mm/yr). The faults were probably active in the Miocene, Late Pleistocene and the Recent.


South Point Fault

The South Point Fault is the most eastern BR-DS fault located in Lake Pontchartrain. It is named after South Point, located on the eastern south shore of the Lake where the Norfolk-Southern Railroad Bridge makes landfall. The fault is at least 14.5 km (9 mi) long and may extend further east across a peninsula of land and into Lake Borgne. The fault visibly offsets the 1960 U.S. Interstate 10 twin bridges by approximately 7.6-10.2 cm (3-4 in) at 0.48 km (0.3 mi) north of the south shore. Westward 2,000 ft the fault offsets the 1928 Highway 11 Bridge by approximately 12.7-15.2 cm (5-6 in), seen 0.32 km (0.2 mi) from the south shore. The Norfolk-Southern Railroad is another 1.4 km (0.9 mi) west of the Highway 11 Bridge and was rebuilt here in 1987. The railroad has approximately 7.6 cm (3 in) of offset.

Goose Point Fault
The Goose Point Fault extends from Pass Rigolets westward past Goose Point for 27.4 km (17 mi) and is the longest BR-DS fault identified in Lake Pontchartrain. The fault can most clearly be seen 1.05 km (0.65 mi) from the north shore where it crosses the Highway 11 Bridge and offsets the structure by at least 15.2 cm (6 in) (Figure 28). The nearby Norfolk-Southern Bridge was rebuilt here in the late 1990's and the most recent effect from the fault is not yet determined on the new structure. The U.S. Interstate 10 Bridge does have a slight dip at the north base of the elevated boat crossing section which is likely due to the Goose Point Fault. Vibracores taken at Highway 11 indicate the Pleistocene is offset by approximately ten ft and the Holocene has significant sediment expansion across the fault.

Causeway Fault
The Causeway Fault is 11.3 km (7 mi) long and is the shortest BR-DS fault identified in the Lake. The fault can be seen on the older south-bound span of the Causeway Bridge approximately 5.0 km (3.1 mi) from the north shore. This span has at least 7.6 cm (3 in) of visible offset. The newer north-bound Causeway Bridge was built in 1969 and probably has slight offset, but has not been confirmed.

Madisonville Point Fault
The 14.5 km (9 mi) long Madisonville Point Fault is named after the slight point found west of the town on Madisonville. It has been identified on "conventional" and high resolution seismic and where the fault makes landfall in the northwest corner of Lake Pontchartrain. The fault does not appear to cross any bridges in Lake Pontchartrain, which might otherwise document its most recent activity. It is suspected that the fault is active since it persists up to the Lake bottom on high-resolution seismic. 

Then, back in 1811...

St. Tammany has been visited by the shock waves of many earthquakes from afar, but it was the 1811 New Madrid, MO, earthquake for which St. Tammany had a front row seat. Here's an article that was published in a July, 1976, News Banner edition, written, I would think, by Polly Morris, even though her name isn't on it. 

The Time St. Tammany Trembled

It was on a dark night of December in the year of 1811 when strange things began to happen in St. Tammany Parish. It was all the more frightening because there was no warning... no explanation.

The preceding day had been a serene Sunday. The people had attended church services with thanksgiving in their hearts. No longer were they tossed from country to country and from king to king.

They belonged to the United States, and less than a year before Governor Claiborne had named their parish after an Indian chief. They went to bed early that night,  and as they banked their fires and blew out their myrtle candles, they felt a peace and very secure.

Sound and Fury

They were sleeping soundly when the thing began. They were awakened by the shaking of their beds. They reached for their candles and ran for the fireplace to relight them. But the whole house was rocking back and forth as though a giant was outside jostling it with mighty hands.

Dishes in the cupboard rattled and a few crashed to the floor and shattered. Iron pots swung like pendulums from their iron hooks and pictures tilted crazily on the walls.

The people lurched from their homes and stood barefooted on the cold ground that rolled and trembled under their feet. Women screamed or fainted. Or prayed as they tried to comfort hysterical children who cried in anguish and fear of the invisible thing out there somewhere in the darkness.

Men cursed and shouted. Or stood speechless with alarm and bewilderment. It was eerie and agonizing to stand in the blackness between midnight and dawn with the whole world twisting and writhing, and the creatures of the farm and forest crying out their fear of the unknown.

Some of the people said the Devil was abroad. Some said the Lord was coming and the end of the world was near. That the dead would rise from their graves. Others said it was a natural phenomenon that should not happen in Louisiana.

It would be days before the people of St. Tammany knew what had happened and that their frightening experience was mind indeed to the terror that took place in a little town 500 miles away.

It was 2 o'clock in the morning in New Madrid, Missouri. The day of December 16 was only two hours old. There was a sound similar to the distant rumble of thunder that quickly became a roar.

The ground heaved and buckled violently. It cracked open and then closed again, forcing water and sand 40 feet into the air like awful fountains of death.

Building were leveled, and it was said that the mighty Mississippi River was thrown back by the incredible upheaval and flowed northward briefly.

Then it came tumbling back again in terrible magnificent fury, sweeping everything before it into oblivion.

It seemed that there was no end to the violence, for there were aftershocks of the past upheaval and foreshocks of the coming torments.

In all, New Madrid was the victim of three separate earthquakes. One December 15, 1811, and on January 23 and February 7 of the next year. For three months the citizens literally shook in their boots.

The earthquake that centered at New Madrid was probably the worst in the United States since the coming of the white man. Across the border into Tennessee, land sunk suddenly and the water rushed in to form a lake. This is the largest lake in the state, for Reelfoot Lake is 18 miles long and three miles wide.

The damage in St. Tammany was no slight that there has been no mention of the earthquake in St. Tammany histories. However, the intensity can be reckoned by historians who wrote about New Orleans.

It is said that a ball was in progress when the crystal chandeliers started swinging and stopped the dance. It is impossible to determine the number of shocks felt in Louisiana, but it is safe to say that they were strong enough to frighten people and tear down a few shacks that were wobbly anyhow.

But who can say whether there was any sinking or rising of land? Only a hunter or fisherman would notice that a bayou had changed course or that an island seemed larger or smaller. Or that there was more or less marsh in a certain area.

It would have made no real difference. But a historian who was familiar with St. Tammany terrain might be puzzled in locating physical features on old maps or in old journals.

If the historian tried to follow in Iberville's footprints, he would be disappointed. Where would he find the River of Pearls that flowed into Lake Pontchartrain from the Honey Island area of Lake Borgne?

Where would be some little rigolets from this great river to near Bay Saint Louis? Where are the many streams of Father Paul Du Ru that were between Bay St. Louis on an overland road to Indian Village near Slidell?

A stream that he described as large as the Marne? And nowhere on modern maps is the Isle of Peas as described by Penicaut.

Were all the early explorers wrong, or has the land changed so much since 1699 that islands and rivers were lost? And was the earthquake one of the things that changed it?

End of article
 



Morris pointed out that St. Tammany was still just as prone to experiencing an earthquake as it always has been, and that, according to modern day geologists, it was pretty close to the nearest known earthquake fault. In fact, it is sitting right on top of it. Have a nice day.