The Start of an Earthquake
Copyright 1999, Sam Penny

 

T+0 sec: The New Madrid Seismic Zone runs from the south-southwest to the north-northeast. To the west of the fault, forces within the earth are trying to move the basement rock to the north. To the east, opposing forces are moving the rock to the south. For centuries the stress from these forces has been accumulating in the form of added torsional pressure at the asperity, a slight bend, on the New Madrid fault directly beneath NSTS, the New Simon Truck Stop, at the Arkansas and Missouri border near Interstate 55.

As slippage occurs elsewhere along the fault, more and more strain in the form of distortion to the rock matrix is added at this rough point where the fault bends. There had been major relief in 1811 when a very large seismic event along the entire fault had shaken the knot enough to let it slip and relieve most of its stress. But the stress started accumulating again and the strain had started to rebuild. Over 250 seismic events per year along the fault passed the accumulating strain from one point to another along the system. It had now reached the time when that particular knot under New Simon must give way again.

The weakest point on the asperity that has prevented the slippage is at a depth of 17.8 kilometers beneath the Mississippi River valley, nearly eleven miles down into the earth. There is no visible light at that depth. There is no direct way for man to observe the actual events that occur that deep into the earth's crust. Gas well drillers have drilled holes to depths of over eleven kilometers in West Texas, but that is little more than half way to the depth where the greatest strain has accumulated in the basement rock of the New Madrid Seismic Zone.

Even though we cannot directly observe the events leading up to the actual earthquake, nor the actual seismic event itself, instruments sensing other earthquakes in the world are giving us an increasingly clear picture of what happens far below the surface when seismic events do occur. For now, only in our mind's eye can we observe the quake as it happens.

 

At the depth of 17.8 kilometers the temperature of the basement rock is over 200 degrees Celsius, 390 degrees Fahrenheit. The overburden pressure resulting from the weight of the rock above that point is almost 60,000 pounds per square inch. Under those conditions, rocks are more pliable and better able to deform to accommodate increased stress than rocks nearer the surface. But the rock is not hot enough to flow. In some cases the strain within the rock becomes enough for the rock faces to slide past each other creating micro-fractures, moving one molecule at a time. But the slip at the bend had been miniscule compared to the movement required to fully relieve its stress.

Micro-fractures and deformation have occurred with increasing frequency on both sides of the asperity as the rock matrix tried to accommodate the increased stress. The area around the knot has grown more and more strained. Then at the appointed time it can hold no longer. A small slip along the fault a few kilometers away a week ago has transferred the proverbial last straw that breaks the camel's back, and the fabric of the rock at the asperity starts to tear apart.

In the beginning a microscopic crack forms between the two sides of the fault at the weakest point on the knot and expands into a small fracture. The friction that is holding the two sides of the fault in place can no longer overcome the shear stress, and the two sides slip past each other, right to left, along the plane of the fault. The initial failure is at the molecular level, then it grows to the microscopic level, then it quickly expands to the macroscopic level. Seismologists identify such points of initial failure as the focus or hypocenter of the earthquake that results from the fracture.

The fracturing of the rock produces pressure waves, compressions and dilations of the molecules in the rock much like sound. Under New Simon these P or Primary waves move outward in all directions from the fracture point at a speed of 5,950 meters per second. When the P waves finally reaches the surface three seconds later, they are the first harbinger of the seismic event to the animals, people, and seismic sensors around New Simon.

The actual tear in the rock spreads outward from the focus in a rough circle along the vertical strike plane of the fault, aligned by the shear forces to be parallel to the fault. In only ten millionths of a second the circle of fracture spreads to an area 6 centimeters in diameter, about the size of the palm of your hand. The edges of the tear are moving outward at a speed of 2,950 meters per second as the S or Secondary shear wave initiated by the first fracture and amplified by the subsequent tears transfers the stress from the failed region to the unfractured matrix.

Within one thousand microseconds, or one millisecond, the leading edge of the P wave reaches 6 meters from the focus. The edge of the circular-shaped tear following the front of the S wave has moved outward 3 meters and begins to become ragged. The strength of the rock matrix varies across the area and some places rupture more easily and quickly than others. It sometimes takes several microseconds for the transferred stress to collect sufficiently to break stronger points in the rock matrix.

At the end of one millisecond the torn area of the fault is roughly a circle of 28 square meters, about the size of a large circular trampoline. As the circle continues to grow, the greatest transfer of stress is horizontal, and the fracture begins taking on more and more of an oval shape, spreading faster in the horizontal direction than in the vertical direction. The rock surfaces at the focus where the event started slide horizontally with a physical displacement approaching half a millimeter. These dimensions may seem small, but in our minds we must realize they all occur within just one millisecond.

Within three milliseconds, the fracture covers over 250 square meters. The P wave front is out to 18 meters, the S wave to 9 meters. Over 1,500 tons of rock are in motion, and though the maximum displacement at the focus is only two millimeters, that tonnage moved that distance in only three milliseconds. The energy required to accomplish this movement seems awesome, but small in comparison the energy that will ultimately be released. And once moving, the rock does not want to stop. It will send vibrations throughout the rest of the formation for some time to come.

After seven milliseconds, the fractured area is an oval 13 meters high and 41 meters long, involving an area of 500 square meters and a maximum displacement of seven millimeters. By this time enough energy has been released that if the quake stops immediately, the seismic event will still register on instruments at the surface as an earthquake of magnitude 1.0.

By the time 65 milliseconds pass the fractured area is an oval 400 meters in length by 130 meters in height with a surface area of 50,000 square meters, room enough for six full-sized baseball fields. The P wave climbs past 400 meters towards the surface, but still has 17.4 kilometers to go. If the quake stops at this point it registers as an earthquake of magnitude 3.0. On the average, eight quakes of this magnitude or higher strike the New Madrid Seismic Zone each year.

Half a second into the event, the fracture line around the oval reaches more and more strong points in the rock matrix that resist the tearing force of the S wave front. The edge of the P wave has advanced almost 3,000 meters, and the S wave to 1,500 meters. Over 2.5 million square meters of fault have fractured with a maximum displacement near the center of the oval of over half a meter, and the quake is now a magnitude 4.7 earthquake, equal to the quake that stuck near Marked Tree, Arkansas in 1987.

As the propagation of the fracture line runs into more and more resistance, more and more stress must be accumulated to extend the fracture. There is a chance the earthquake will stop within the next tenth of a second.

But then, as the S wave passes through the second asperity of the fault 1,550 meters to the south of the focus, another fracture starts. The second asperity fails along with the first. The fracture from that point propagates back towards the malingering points that had resisted the first passage of the wave front. The second lurch is too much, and those points resisting the advance of the fracture plane give way, adding their energy to the ever-expanding seismic event. There is now no stronger asperity within 45 kilometers to the north or 53 kilometers to the south that can stop the spread of the fracture. A major earthquake is underway.