Earthquake questions are raised whenever an event has happened, or one predicted.
Earthquakes have enthralled both young and old, from the “great San Francisco” earthquake of 1906, or the tsunami of 2007, to the latest headlines from Haiti and Chile.
The word “earthquake” is generally used to refer to any event caused naturally or by man that creates seismic (shaking) waves generated at geological fault lines but also by volcanic activity and man-made mine blasts and nuclear experiments.
This initial rupture is called the hypocenter. If you throw a rock into a lake, the waves ripple outwards from where the rock entered the water, getting weaker and weaker the farther from the center. This ground level center, in earthquake terms, is called the epicenter which is directly about the hypocenter.
During an earthquake, typically the first seismic wave is a compression wave, which is fast moving and hits with a sudden thump. They are followed a few seconds later by a second kind of seismic wave, a transverse wave, and cause the rocking, swaying, and shaking motions. Quakes are often preceded by foreshocks, or followed by aftershocks, which can be quite damaging and dangerous. They can occur hours or even days before or after the actual earthquake itself.
The size of earthquakes is usually measured using the “Richter” scale (Charles Richter developed this in the 1920s at the California Institute of Technology). It compares the heights of the seismic waves out from the epicenter and quantifies the amount of seismic energy by assigning a single number.
Following is a list of Richter scale numbers with related explanations of their power and an estimate of occurrence (source Wikipedia):
- 2.0 Richter Magnitude: Micro earthquakes, not felt. Approximately 8,000 a day.
- 2.0-2.9 Richter Magnitude: Usually not felt, but recorded. About 1,000 a day
- 3.0-3.9 Richter Magnitude: Often felt, but infrequently cause damage. 49,000 a year.
- 4.0-4.9 Richter Magnitude: Noticeable shaking of indoor items, rattling noises but little likelihood of significant damage. 6,200 a year.
- 5.0-5.9 Richter Magnitude: Can cause major damage to poorly-constructed buildings over restricted regions (unlikely to cause more than slight damage to well-designed buildings). 800 a year.
- 6.0-6.9 Richter Magnitude: Destructive in areas up to about 100 miles across in populated areas. 120 a year.
- 7.0-7.9 Richter Magnitude: Usually cause serious damage over larger areas. 18 a year.
- 8.0-8.9 Richter Magnitude: Can cause very serious damage in areas several hundred miles across. 1 a year.
- 9.0-9.9 Richter Magnitude: Devastating in areas several thousand miles across. 1 per 20 years.
- 10.0+ Richter Magnitude: Of epic proportions. Never recorded.
Here are some well-known earthquakes, and their approximate Richter scale magnitudes:
- 0.5 Magnitude Equals The Power Of A Large Hand Grenade (approx.)
- 3.5 Magnitude Chernobyl Nuclear Disaster, 1986
- 6.9 Magnitude San Francisco Bay Area Earthquake (CA, USA), 1989
- 8.0 Magnitude San Francisco Earthquake (CA, USA), 1906
- 8.5 Magnitude Sumatra Earthquake (Indonesia), 2007
- 9.5 Magnitude Valdivia Earthquake (Chile), 1960 (251 ZJ in this case)
- 10.0 Magnitude Never Recorded
This site will address other questions and answers about earthquakes.
In The News:
Santa Clarita Homeowners Have Questions After Sunday’s Earthquake
How many people where killed by earthquake? What was the cost of the damage? What caused the earthquake? What is the fault?
After earthquake, Italians ask questions about building codes
Critics say that lax compliance with existing regulations contributed to L’Aquila’s high death toll.
Will the right questions be asked about the earthquake in l’Aquila?
A state of emergency has been declared, funds are being allocated, and politics has been set aside momentarily in order to respond to the devestating earthquake which hit Abruzzo yesterday.
Just had a 5.0 earthquake a few minutes ago here in southern california. I was sitting here and it felt like somebody had came up behind me and shook my chair.
The San Andreas Fault and the San Francisco Bay Area
Here is the USGS’s current best guess at the risks. Something to ponder when you go to Monte Bello Open Space Preserve to enjoy nature some weekend.
USGS Volcano Hazards Program Volcano Update
The number of RB2S2BL earthquakes returned to background levels. Of the earthquakes that were strong enough to be l fef ocated, two were located beneath the south summit caldera.
Special Issue on Rotational Seismology
Rotational seismology is an emerging field for studying all aspects of rotational ground motions induced by earthquakes, explosions, and ambient vibrations.
Hear why you should have quake insurance
If you factor in the probability of major vs. moderate earthquake damages, the cost of annual insurance premiums and the availability of government disaster loans, I think it makes more economic sense.