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Local Earthquake Effects

Everyone understands the destructive effects that occur as a major, Richter Scale 8+ earthquake shakes the land and brings down structures.

There are two other less obvious but still very destructive effects that a major earthquake will have in Madison, Iron, St. Francois and Washington Counties, and along the Mississippi river, particularly in the Bootheel. The local effects include mine collapse in the Park Hills area, and thixotropy in unconsolidated river sediments and mill tailings.

Mine collapse is easily understood. A large network of mined-out ground extends under Park Hills and surrounding towns. In some areas, mined-out ground comes very close to the surface and unsupported mine roof rock spans tens to a hundred or more feet of width. Vertically, some of the mined-out ground extends a hundred or more feet from top to bottom. Some areas are flooded with water, some are filled with air. Tree roots extend into mine in at least one area under downtown Bonne Terre, illustrating the thinness of remaining rock cover holding up parts of the town.

Places where mined out areas being hundreds of feet deeper in the earth are not likely to suffer earthquake collapse damage because the roof rock overlying the mine is much thicker and therefore much stronger. Mines near Viburnum are examples.

The most recent major earthquake along the New Madrid Fault in extreme southeastern Missouri happened during 1811-1812, which pre-dates the start of large scale lead mining in southeastern Missouri. The mines, the mined land, and the tailings piles have never been exposed to a major earthquake, so there is no direct historical experience to predict what will happen during the next major earthquake.

However, reasonable predictions can be made by observing the effects of major earthquakes under similar conditions in other places.

Consider an idealized earthquake in which rock plates slip one time, during a few seconds, along a geological fault, radiating earthquake energy into the surroundings.

Earthquakes travel through the ground as Primary energy waves (P) and Secondary energy waves (S). It’s easy to remember which is which if you think of Primary (P) waves as &#8220Push” and Secondary (S) waves as &#8220Shake,” which describes what the waves do as they pass through an area.

P-waves travel faster than S-waves and are the first to arrive. As the P-wave passes, a loud BANG is usually heard, similar to a powerful sonic boom from an aircraft. Within seconds to minutes later, the slower S-wave arrives. S-waves are similar to waves on water. S-waves cause the ground to rise and fall, and begin shaking apart buildings and other structures. S-waves can continue for some time after the initial earthquake.

Ground motion resulting from S-waves will shake, fracture and displace surface materials, including a thin roof-rock cap remaining between mined-out areas and the surface. Many supporting underground mine pillars will be destroyed and will collapse. Unsupported mine roof rock will collapse into some of the underground mined-out areas, dropping buildings, streets, utilities, roads and other structures down into the mines. Surviving parts of town may become islands of demolished buildings atop tall vertical cliffs, and some islands will be surrounded by water. On the bright side, the unique landscape eventually could become a major tourist attraction, a sort of Venice In The Ozarks and underwater historical site.

Adding insult to injury, collapsed areas will soon be inhabited by bats, including many &#8220endangered” Indiana and other bats, stopping land use and property reconstruction under Draconian restrictions and penalties imposed by the expired Endangered Species Act.

A real, not idealized, earthquake is usually not a single event, but is caused by multiple rock slippage along a fault over a period of minutes. Overlapping P and S waves are created by each slippage. During the 1811-1812 event on the New Madrid fault, earthquakes were more or less continuous for three days and the last aftershock happened a year later. That much ground motion, over that much time, will certainly collapse all that is collapsible including the land over large underground mines in the Park Hills area. Mines at Fredericktown are smaller and have more rock cover, and are unlikely to suffer major collapse.

Thixotropy describes the conversion of certain kinds of gel into a liquid when shaken. In terms of earthquake relevance, &#8220gel” can be water-saturated, fine-grained sediment such as mud, silt, even fine-grained sand, and mill tailings. &#8220Gellin’ may be comfortable in a shoe insert, but can be a disaster during an earthquake.

It’s a matter of scale. Mud in your back yard is unlikely to liquefy and cause damage during an earthquake. But on the scale of miles to tens of miles, layers of waterlogged sediment can and do turn from solid to liquid during prolonged shaking from a powerful earthquake. There are historical records of that happening during the 1811-1812 earthquake.

When the land liquefies, heavy objects sink and sometimes disappear. Buildings, cars, bridges and other structures and objects are no longer supported by the ground they stand on, which has essentially become quicksand, and they sink into the earth.

Thixotropic effects will be greatest in unconsolidated and semi-consolidated, fine-grained sediments, primarily along the Mississippi River floodplain and along former floodplains of the abandoned river channels that form much of the land in the Missouri Bootheel. Upland streams in our area typically have bedrock close to the surface under the stream and a stream bed made of gravel. Solid rock and coarse gravel will not liquefy and will not become quicksand during an earthquake. Structures built on bedrock will not be subject to thixotropism and will suffer minimal damage compared with structures built on large floodplains.

What will be a problem locally is mill tailings. Mill tailings are sand and silt discarded from the milling process that separates desired metallic minerals from waste rock.

As S-waves begin to arrive, tailings undergo thixotropic liquefaction. As S-waves continue to arrive, the pile of liquefied tailings begins to move, similar to waves on water. Those waves can become a huge battering ram smashing into the tailings retaining dam with explosive force, wave after wave, causing dam failure. Tailings then become a mudflow roaring downstream just as lake water could flow downstream from a dam break. Houses and other structures will be carried away and buried in mud.

Thixotrophy is reversible. If the tailings mudflow stops moving, the mud will return to a semi-solid condition. If the mudflow blocks the stream into which it flowed, and the mud reverts to semi-solid form, the mudflow will create a dam across the stream. Water will rise behind the mud dam, creating a lake which could be many miles long.

Eventually the lake will rise to the mud dam and begin spilling over. Because the mud is unconsolidated and weak, water flow will take out even a large mud dam very quickly, perhaps in as little as ten minutes. The lake will roar downstream as a wall of water, a flash-flood washing away everything in its path and spreading mill tailings loaded with lead and other &#8220heavy metals” tens of miles downstream.

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