Earth Impact Effects Program

Gareth Collins, H. Jay Melosh and Robert Marcus

What are Transient and Final Crater Diameters?

When the projectile strikes the surface, it ejects rock from the surface, resulting in a crater. Shortly after the impact, the ground around the initially formed crater begins to collapse inward.

The Transient Crater Diameter is the distance from rim to rim of the crater that forms immediately after the impact, before the collapse begins.

The Final Crater Diameter is the distance from rim to rim of the crater once collapse has completed.

What is Thermal Radiation?

Terrestrial meteoroid impacts produce a cloud of hot vapor, termed the "fireball," which expands away from the impact point. The bulk of the thermal energy from this cloud is emitted when the fireball has expanded and cooled to a surface temperature of about 3000 K.

Thermal Exposure is the thermal energy per unit area that arrives at the user specified distance. To provide the user with a reference as to how damaging the thermal radiation is, several text descriptions of the damage are provided if the thermal energy is great enough to cause the described damage.

These descriptions include:
Clothing ignites
Much of the body suffers third degree burns
Much of the body suffers second degree burns
Much of the body suffers first degree burns
Newspaper ignites
Plywood flames
Deciduous trees ignite
Grass ignites

What is Mercalli Scale Intensity?

The Mercalli Scale is a set of 12 descriptions of the damage due to ground acceleration.

The descriptions are:
I. Not felt. Marginal and long-period effects of large earthquakes.
II. Felt by persons at rest, on upper floors, or favorably placed.
III. Felt indoors. Hanging objects swing. Vibration like passing of light trucks. Duration estimated. may not be recognized as an earthquake.
IV. Hanging objects swing. Vibration like passing of heavy trucks; or sensation of a jolt like a heavy ball striking the walls. Standing motor cars rock. Windows, dishes, doors rattle. Glasses clink. Crockery clashes. In the upper range of IV wooden walls and frame creak.
V. Felt outdoors; direction estimated. Sleepers wakened. Liquids disturbed, some spilled. Small unstable objects displaced or upset. Doors swing, close, open. Shutters, pictures move. Pendulum clocks stop, start, change rate.
VI. Felt by all. Many frightened and run outdoors. Persons walk unsteadily. Windows, dishes, glassware broken. Knickknacks, books, etc., off shelves. Pictures off walls. Furniture moved or overturned. Weak plaster and masonry D cracked. Small bells ring (church, school). Trees, bushes shaken (visibly, or heard to rustle).
VII. Difficult to stand. Noticed by drivers of motor cars. Hanging objects quiver. Furniture broken. Damage to masonry D, including cracks. Weak chimneys broken at roof line. Fall of plaster, loose bricks, stones, tiles, cornices (also unbraced parapets and architectural ornaments). Some cracks in masonry C. Waves on ponds; water turbid with mud. Small slides and caving in along sand or gravel banks. Large bells ring. Concrete irrigation ditches damaged.
VIII. Steering of motor cars affected. Damage to masonry C; partial collapse. Some damage to masonry B; none to masonry A. Fall of stucco and some masonry walls. Twisting, fall of chimneys, factory stacks, monuments, towers, elevated tanks. Frame houses moved on foundations if not bolted down; loose panel walls thrown out. Decayed piling broken off. Branches broken from trees. Changes in flow or temperature of springs and wells. Cracks in wet ground and on steep slopes.
IX. General panic. Masonry D destroyed; masonry C heavily damaged, sometimes with complete collapse; masonry B seriously damaged. (General damage to foundations) Frame structures, if not bolted, shifted off foundations. Frames racked. Serious damage to reservoirs. Underground pipes broken. Conspicuous cracks in ground. In alluviated areas sand and mud ejected, earthquake fountains, sand craters.
X. Most masonry and frame structures destroyed with their foundations. Some well-built wooden structures and bridges destroyed. Serious damage to dams, dikes, embankments. Large landslides. Water thrown on banks of canals, rivers, lakes, etc. Sand and mud shifted horizontally on beaches and flat land. Rails bent slightly.
XI. Rails bent greatly. Underground pipelines completely out of service.
XII. Damage nearly total. Large rock masses displaced. Lines of sight and level distorted. Objects thrown into the air.
Masonry A. Good workmanship, mortar, and design; reinforced, especially laterally, and bound together using steel, concrete, etc.; designed to resist lateral forces.
Masonry B. Good workmanship and mortar; reinforced, but not designed in detail to resist lateral forces.
Masonry C. Ordinary workmanship and mortar; no extreme weaknesses like failing to tie in at corners, but neither reinforced nor designed against horizontal forces.
Masonry D. Weak materials, such as adobe; poor mortar; low standards of workmanship; weak horizontally.

What is ejecta?

Debris excavated during the impact is deposited on the Earth's surface around the impact crater. This debris is called ejecta.

Ejecta is formed of particles (rock fragments) of a range of sizes. The average fragment size that we report is the average particle size at the given distance, but a range of particle sizes will land at any given location. Average fragment size decreases with distance from the crater, because smaller fragments tend to be ejected faster and from closer to the impact point.

When the ejecta lands it forms a layer of ejecta called an ejecta deposit or ejecta blanket. The average ejecta thickness is greatest at the transient crater rim and decreases as one over the distance from the rim cubed. Near the crater rim, this deposit is relatively thick and "continuous". It is often referred to as the continuous ejecta blanket. Further away, the deposit is much thinner and may be patchy (discontinuous).

At a given distance from the crater, the ejecta deposit may have a range of thicknesses, depending on the direction, particularly at ranges where the ejecta is discontinuous. Hence the estimate of ejecta thickness we give is an "average" around the crater at the given distance.

What is the air blast?

The energy due to the impact causes a distortion in the air. This distortion travels in the form of a wave. If the energy of the impact is very high, the wave may initially be a shock wave, travelling at a velocity greater than the speed of sound in air. The wave eventually decays into a sound wave travelling at 300 m/s (671 mph).

Peak overpressure is a measure of how much the pressure in the blast wave exceeds the atmospheric pressure of 105 Pa (1 bar). The air blast caused by the impact can cause a great deal of damage.

The damage due to the air blast may include the following:
Multistory wall-bearing buildings will collapse.
Multistory wall-bearing buildings will experience severe cracking and interior partitions will be blown down.
Wood frame buildings will almost completely collapse.
Interior partitions of wood frame buildings will be blown down. Roof will be severely damaged.
Multistory steel-framed office-type buildings will suffer extreme frame distortion, incipient collapse.
Highway truss bridges will collapse.
Highway truss bridges will suffer substantial distortion of bracing.
Highway girder bridges will collapse.
Cars and trucks will be largely displaced and grossly distorted and will require rebuilding before use.
Cars and trucks will be overturned and displaced, requiring major repairs.
Glass windows will shatter
About 30 percent of trees blown down; remainder have some branches and leaves blown off
Up to 90 percent of trees blown down; remainder stripped of branches and leaves.

What is a tsunami?

Impact-generated tsunami explained here...