The name for the newer scale of measurement is The Momentary Magnitude Scale.
The Mercalli Scale measures Intensity. Both the American Military Man, Richter, and the Italian Priest Mercalli, were Seismologists. They were geologists with a specialty. Their disciplines were the study of Earthquakes.
The Richter Scale is a method of computations from specialized Seismographs that assign a number to the Magnitude of the event. “The Richter Scale”, as a descriptor, is on it’s way out.
* ”What’s going on is that we’re just recovering from decades of telling a white lie, that’s all.”
Thomas H. Heaton, President of the Seismological Society of America says, “You probably wouldn’t catch us using the term “Richter Magnitude” around here even though this was the home of Richter.”
Rodger Musson of the British Geological Survey in Edinburgh adds, “It seems to be a popular misconception that it’s actually a piece of equipment, like a bathroom scale. Things have come to such a pass in today’s press, that I had an inquiry recently from the Sunday Times, no less, asking for a picture of the Richter scale. I said this was a bit like asking for a picture of kilometers.”
The terms of measurement in use now are modifications of the numeric environment of the Richter scale, but, in 1995, the “correct” terminology is still in flux. Associated Press has recently dropped “Richter” terminology and now discusses “Momentary Magnitude” and “Preliminary Magnitude.” Because the scales are logarithmic, each Point of Increase is Ten Times the amount of strength.
Thus, a 7.0 is one hundred times the power of a 6.0, and an 8.0 is one thousand times that of a 6.0! We can see that little things mean a lot when we note that a 6.1 is ten times as strong as a 6.0.
THE MERCALLI SCALE
The Mercalli Scale was introduced in 1902. Since then, refinements in practice have resulted in a standard Modified Mercalli Scale based on interviews and effects.
The Scale goes from Roman Numerals One through Twelve.
THE MODIFIED MERCALLI SCALE
l. Not felt, except by a few people under especially favorable conditions.
ll. Felt only by a few people at rest. Delicately suspended objects may swing.
lll. Felt quite noticeably under favorable conditions. Parked automobiles may rock slightly.
lV. Felt by most people; some awakened. Dishes, windows, doors disturbed, walls cracked. Sensation like that of a heavy truck striking a building.
V. Some dishes, windows, and so on, broken; a few instances of cracked plaster; unstable objects moved.
Vl. Felt by all; many frightened and run outdoors. Some heavy furniture moved; a few instances of fallen plaster or cracked chimneys.
Vll. Everyone runs outdoors. Damage negligible in buildings of good design and construction; slight to moderate in well built ordinary structures; some chimneys broken. Noticed by people driving cars.
Vlll. Panel walls thrown out of frame structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. Sand and mud ejected in small amounts.
lX. Buildings shifted off foundations. ground cracked conspicuously. Underground pipes broken.
X. Ground badly cracked, rails bent, landslides considerable from riverbanks and steep slopes. Water splashed over banks.
Xl. Few masonry structures remain standing. Bridges destroyed. Broad fissures in the ground. Earth slumps and land slips in soft ground.
Xll. Damage total.Waves seen on ground surface. Lines of sight and level distorted. Objects thrown into the air.
You can see that this is a subjective way to evaluate the effects of a concrete occurrence. It depends on wide investigation and many interviews, plus first hand observation of the aftermath of the event.
Because there is often discrepancy between the Mercalli rating of effects and the numbers of the Richter readings from the epicenter, a newer method of taking readings from both the Epicenter (The point of fracture below the surface) and the Focus (On the surface above the fracture) is in use.
The place where the Earthquake originates is called the Focus. The point on the ground directly above that is called the Epicenter. In the case of the Northridge / Reseda Quake, the Focus was eleven miles below the Epicenter of Lindley Street and Saticoy.
As a trivia aside, the largest quake ever recorded was a 9.5 in Chile in 1960. Chile is a strong candidate for an offshore quake and Tsunami similar to Japan, 2011, as is the Seattle region.
The weak blind thrust plates, like so many under the Los Angeles Basin, when they are under compression, form a thrust or overlapping sliding sheet. If it thrusts over the opposing sheet, it creates a hill or adds to a hill. If it thrusts under the opposing plate, the ground lowers into a valley effect. Very generally, all areas with hills and valleys are the result of under land tectonics.
The areas north of the San Fernando Valley, Canyon Country, Saugus, Valencia, especially the Vasquez Rocks area, until the flats going to Lancaster, were created by violent quakes in the past. Pretty much everywhere in the world where you see mountains and cliffs rising from a flat plain, plate movement in the distant past created them. From the Rocky Mountains through the Alps, New Zealand and even the Pacific Palisades, it took massive force to raise them and they wear their sharply defined faces with pride.
As I reference later, the Northridge quake raised the Santa Susana Mountains by 18 inches.
Much of the information about the fault systems under Los Angeles, until very recently, have been known and hidden by the oil and natural gas companies. They considered it “Proprietary Material.” Scientists now, in 2011, have access to that information and are adding up the material’s veracity.
The newer Los Angeles faults were identified by Dr. Peter Shearer, a seismologist at the Scripps Institute of Oceanography in San Diego and by Dr. John Shaw, a Structural Geologist at Harvard University .
Dr. Shaw states that the blind thrust faults of Los Angeles are caused by the relative motion of the North American and Pacific plates, two huge masses of crusts that are sliding past each other along the San Andreas Fault. Because of a dogleg in the fault, The Los Angeles basin is being squeezed like an accordion between these masses of earth.
The consensus is that the blind thrust faults under downtown L. A. can, and at some time will, produce a devastating