Monday’s earthquake reminds us to take precautions

An earthquake rolled through Southern California Monday morning at 9:55AM but there were no immediate reports of damage.
The quake had an estimated magnitude of 4.7 a seismic analyst at the California Institute of Technology’s seismological laboratory. The initial temblor was reported as a 5.1 and about 50 ‘aftershocks’ were also reported.
The epicenter was about a dozen miles from the desert town of Anza, about 100 miles southeast of Los Angeles.
The temblor was felt sharply in the local area and caused a swaying or rolling motion in Los Angeles and San Diego.
Defining Quakes:
To understand earthquakes, here is a short primer on some of the terms scientists use and what they mean:
Earthquake: An earthquake is the sudden slip of one block of the earth’s crust past another that produces shaking as one of its effects. Just like the slip of one finger past another when snapping your fingers produces a sound wave, the slip along a fault produces waves that are perceived as earthquake shaking.
Magnitude is a number that represents the total energy released during an earthquake. The smallest earthquake ever recorded is about magnitude -2 (yes, like temperature magnitudes can be negative), and the largest historical event was magnitude 9.5. Although there is no theoretical limit to magnitude, it is unlikely that an earthquake much larger than 9.5 will occur. Each unit of magnitude represents a 32 times increase in the energy released by the fault. So a magnitude 7 earthquake has 32 times more energy than a magnitude 6 earthquake, and more than thousand times (32 x 32) more energy than a magnitude 5.0 earthquake, and a million times more energy than a magnitude 3.0 earthquake. There are no “points on the scale”. When seismologists say “point” it is to express the decimal point – “magnitude 6 point 5” means magnitude 6.5.
Intensity is a number (written as a Roman numeral) describing the severity of an earthquake in terms of its effects on the earth’s surface and on humans and their structures. Several scales exist, but the ones most commonly used in the United States is the Modified Mercalli Intensity scale sometimes written “MMI”. Unlike the magnitude, which has one value for each earthquake, the intensity depends on your distance from the earthquake and decreases with distance from the event.
The fault is the surface across which two blocks of crust slip in an earthquake. This planar surface may intersect the earth’s surface as an identifiable fault trace. Faults vary in size from centimeters to thousands of kilometers long. A fault zone may be a complicated set of fractures up to hundreds of kilometers wide. The magnitude of an earthquake is proportional to the area of the fault that slips and how much it slips. A magnitude 3.0 happens over a fault surface of 1-10 square meters. A magnitude 5.0 requires slip on a fault a few kilometers across, while a magnitude 8.0 needs a fault several hundreds of kilometers long. Big earthquakes occur only on big faults, but a little earthquake could occur on a big fault if only part of it slips. Small quakes may also happen on a little “secondary” fault near a big fault or on a tiny fault.
The slip is the amount of movement that occurs between the two sides of the fault surface during an earthquake. The amount of slip can range from a few centimeters for a magnitude 4.0 up to 10 meters or more for a magnitude 8.0. For smaller quakes this slip may all occur miles deep in the earth and not reach the surface.
The epicenter is the point on the earth’s surface above the hypocenter, which is the point at depth on the fault where the earthquake begins. When an earthquake occurs the slip doesn’t happen all at once. The earthquake begins at a point and ruptures across the fault. The rupture moves at about 3 kilometers per second, so a bigger earthquake lasts for a longer time.
An earthquake cluster, or earthquake sequence, is a group of earthquakes that are close in time and space. Every earthquake changes the stress in the surrounding rock and increases the probability that another earthquake will occur nearby. This probability dies off quickly with both time and distance, so mostly they are near the fault surface that has been moving. A big earthquake is on a big fault and therefore produces more aftershocks.
A mainshock is the largest earthquake in a sequence. A foreshock is any earthquake that happens near and before the mainshock. An aftershock is any earthquake that happens near and after the mainshock. Foreshocks, mainshocks, and aftershocks are all earthquakes and these terms simply describe the relationship between events in a sequence. For example, as a sequence progresses a quake dubbed a mainshock may have its status changed to foreshock if it is followed by an even bigger quake. Sometimes the largest aftershock or largest foreshock is so close in size to the mainshock (exactly the same magnitude or only 0.1 or 0.2 units apart) that the two events are called a doublet. However, generally the largest aftershock is about one magnitude unit smaller than the mainshock.
Triggered earthquakes are earthquakes that occur right after a big earthquake but are too far away from the mainshock fault to be called aftershocks. The first time we observed this clearly were earthquakes triggered by the magnitude 7.3 1992 Landers earthquake, which included a magnitude 5.7 earthquake in Nevada (over 200 miles away).
An earthquake swarm is an earthquake cluster that has several earthquakes close to the largest size (rather than a mainshock or a doublet). Unlike typical mainshock/aftershock sequences where the number of quakes dies off rapidly with time swarms may persist for longer periods of time. Swarms are characteristic of certain locations in California, especially volcanic and geothermal areas such as the Imperial Valley and Mammoth Lakes.

March 12, 2013

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