- Education and Science»
- Geology & Atmospheric Science
California Earthquakes: They Should Not Terrify You!
Today, (this article was written on 7-7-10), the TV news reported on an earthquake in Southern California. It was a magnitude 5.4. As a California Native, and amateur geology enthusiast, I am often amused by out-of-state folks who claim to be 'terrified' of the earthquakes experienced here in California. Let me assure you, there is no reason for such terror.
For starters, if you are nervous simply because earthquakes cannot be predicted, don't be. 99% of the hundreds or even thousands that occur daily all over the world are too small to even be felt except by the sophisticated machines (seismographs ) designed expressly for the purpose.
Prediction, in and of itself can induce quite a bit of stress, as in the case with weather prediction, the science for which has gotten quite accurate. It will be all over the news that a hurricane or tornado is on the way, leaving you, yes, possibly time to prepare, but not always. You will most certainly have plenty of time to stew and worry over whether or not it will pass over you and inflict severe damage including loss of lives, or pass you by entirely. That cannot be predicted.
The neat thing about earthquakes by contrast, is this: while the geologists may not yet be able to forecast them, they are usually of very brief duration, and by the time you figure out it was an earthquake, it is over with. A matter of seconds, unlike major storms which can go on for several hours.
The next thing to remember about earthquakes is that they are not seasonal. They happen irregularly, and therefore, 'happen when they happen,' but it has nothing to do with the time of year. There is no such thing as 'earthquake weather.'
There is, by contrast, a 'hurricane season' and a 'tornado season.' Every year the folks who live in areas of the country subject to those storms must worry about having their roofs ripped off, or worse, losing their entire house to a severe storm.
When these storms strike a populated area, massive destruction is guaranteed. However, even a 'moderate' earthquake usually does little more than rattle a few nerves of those inclined to be nervous. Minor damage along the lines of goods falling from store shelves is usually the extent of the problems.
Measurements--The Richter Scale
There are hundreds in a week, probably, and no one notices. If you live in or near Hollister, California, that is facetiously known as "the Earthquake Capitol of the World," for its almost daily but mostly unnoticeable temblors. There are a lot of earthquakes in that area, but again, little of note. Nearby Parkfield is the less populated area of choice for the Northern California branch of the United States Geological Survey (USGS) to have set up all kinds of monitoring equipment in the hopes of advancing the science of prediction.
You will hear quakes referred to by some decimal number, say a 5.4, as in the case of today's temblor. Anything less than about a 3.0 or so is too small to feel. This is the Richter Scale, ( further explained at the USGS website).
It measures the magnitude, or intensity of the quake, and is the most frequently heard and used measurement. Because it is a logarithmic scale, each whole number represents a 10-fold increase in power. Ergo, a 5.0 is ten times greater in intensity than a 4.0, and so forth. (See chart, below.)
Illustration of Logarithmic Scale
As you can see in the map below, the rupture which caused the massive destruction of the famous 1906 San Francisco Earthquake was far larger. The 1906 quake is represented by the red line, while the much smaller blue segment of the line shows the Loma Prieta rupture of 1989.
Although the Richter Scale had not yet been conceived in 1906, estimates place it at about an 8 or 8.1. The Loma Prieta was a 7.1. By comparing the two in this visual representation, the ten-fold increase per scale number is easily understood.
1906 Compared to Loma Prieta
Measurements--The Modified Mercalli Scale
The next number, much, much less frequently heard or used by newscasters, would be the Modified Mercalli Scale. These are whole numbers, expressed in Roman Numerals, and refer to the sensation of what the quake felt like in a given area.
This has to do with the structure of the ground at that point. It could possibly be felt much more strongly at a point further from the epicenter (the point on the map where the earthquake was centered), but perhaps the ground there was very sandy and loose, while at the epicenter, there may be bedrock near the surface. In the first case, the earthquake waves essentially get 'trapped' in the sand, and it will vibrate and shake much more vigorously than in bedrock, where the waves simply pass through the rock, and all that might be felt is a single sharp jolt or two.
Modified Mercalli Intensity Map
Terminology--Epicenter and Focus
The news reports have made 'epicenter' a word of fairly common knowledge, showing it as they do by way of graphic map overlays. The epicenter is the place on the surface above the actual location of the quake.
The earthquake itself, where the initial stress movement actually takes place, is far below the surface, maybe miles down. This location is called the focus, or hypocenter. See the graphic here: http://earthquake.usgs.gov/learn/glossary/?termID=98
Terminology--P Waves and S Waves
There are two main types of earthquake motions that will be felt by people; P Waves, and S Waves. The P-Wave, or Primary wave is often felt as an initial jolt, while the S-Wave follows, and is more responsible for the characteristic shaking motion.
If you imagine two people holding opposite ends of a Slinky toy, and one holds his end still while the other pushes the other end backward and forward toward the other person, you have a great graphic example of a P-Wave. If you take that same Slinky toy, and both people slide their ends from side to side, you have your S-Wave illustration.
Types of Faults
In addition to all of this, there are also many different types of faults. There are side-slip faults (such as California's infamous San Andreas) in which one side moves to the right or left with respect to the other side.
There are subduction zone faults, in which the crust of the earth moves underneath another section of crust. Subduction faults typically result in the strongest of earthquakes, and these faults are the type found off the coast of Japan.
There are also dip-slip, also-called 'normal' faults, and 'thrust' or 'reverse' faults, both of which involve uplift of one section of ground above the other, or, depending on your perspective, the dropping-down or depression of a section below another. See the animation here for an example: http://earthquake.usgs.gov/learn/glossary/?termID=59
There are, of course, all sorts of additional technicalities involved, and I won't go into them here. However, it is interesting to note that mine shafts frequently follow thrust or reverse faults...this is certainly the case in much of the California gold mines of the 49er Gold Rush days.
Role of the Media in Scaring People
Now, on to the 'scary' parts. First and foremost, "if it bleeds, it leads" is the main motto in the world of TV news. The same is pretty much true for newspapers and the internet news services. Sensationalism sells. People remaining safe is 'ho-hum,' and does not sell ads.
It's All About Building Codes
The terrible images we've all seen of massive death and destruction following earthquakes around the world (Turkey, Mexico City, Peru, etc.) have a good deal less to do with the earthquake itself than with the lack of building codes in those places. Take a close look at how those dwellings are constructed, compared to structures in most of the U.S.A. There is no comparison. Furthermore, not only do we have stringent building codes in effect, especially in earthquake-prone areas of the country such as California, we also have zones in which it is illegal to build habitation simply because of the proximity to an earthquake fault.
(Japan, by contrast, has building codes at least as stringent as our own, and has done an excellent job of retrofitting older buildings in its big cities.)
Engineering--Things We've Learned the Hard Way
And the Bay Bridge collapse? Blame those S-Waves. Pulling in opposite directions, the bridge was simply stretched beyond its engineered tolerances. Not, however, a common scenario. The same was true for the collapse of the Cypress Structure freeway section in Oakland. In that case, it was poor engineering to blame in combination with those pesky S-Waves.
All in all, here in the USA, earthquakes are more of a minor nuisance, and topic of conversation much like the weather. It is not that often that true disasters happen a la Northridge or Loma Prieta. They don't bother me: I don't let them.
It's Also About Location
In some cases, there are dwellings in these areas that were 'grandfathered in' prior to the enactment of those laws. If you buy a home in these areas, you are required to sign a statement known as the Alquist-Priolo disclosure. It means you cannot sue the realtor or seller for selling you a house in that area should the worst happen. However, 'the worst' is still rather unlikely for normal single-family wood-frame homes. I lived in just such an area, and had signed that disclosure years before the 1989 Loma Prieta earthquake. You know, the one that was on the national and even world news for knocking down part of the San Francisco-Oakland Bay Bridge and setting the Marina district ablaze?
Dual Culprits: Liquefaction And Soft First Story
About that: again, it was location, location, location. The Marina district was built on poorly engineered landfill. It was subject to liquefaction. Did you ever stand on the beach within a few yards of the tide, then make a jumping-up-and-down motion without actually lifting your feet? Notice how you start to sink in, and water begins to puddle around your feet? Yep. That's liquefaction at work. See the animation here: http://earthquake.usgs.gov/learn/glossary/?term=liquefaction
That's what happened in the Marina. To complicate matters, the buildings were old, built prior to earthquake code, and had so-called 'soft' first stories, meaning parking garages and the like with no walls to compartmentalize the space. These are prime candidates for collapse. By contrast, where I lived, even though I was within walking distance of part of that infamous San Andreas Fault, nothing happened at my house.
Why? We were essentially on bedrock, and the earthquake waves just passed on through, with no more shaking than a heavy truck going by. In fact, at first, that's what I thought it was. Glasses did not even rattle in the cupboard.
Now, nearly everyone has seen images of San Francisco's skyline as it exists in modern days. This famous pyramidal structure surely stands out.
The Transamerica Pyramid was specifically designed to withstand earthquakes. The first precaution that was taken was to excavate the soil in an amount equivalent to the weight of the finished structure, so no additional stresses were placed upon the ground.
Next, the building rests on shock absorbers, on each of the support pilings. The technical name for these over-sized shock absorbers is "base isolators." Shock absorbers, as found on your car, is the best analogy. Mounted to both the building and its foundation, they act as a separator, keeping the building out of direct contact with the foundation and thereby any violent earth movements.
There are also seismographs in various locations within the building to monitor movement. During the Loma Prieta Quake, the building moved (swayed) approximately one foot, but sustained no damage at all. It did just what it was designed to do.
Emergency preparedness is no different than for any other kind of emergency, which is to say, be prepared to be on your own for at least 2 or 3 days before help can get to you. Store water, store and rotate batteries for freshness, have food stocked up, and don't wait until the disaster is upon you and try to battle your way to a store. Camping gear is probably the very best emergency kit you can have. That's why I don't worry. I am a well-equipped and experienced camper.
Worrying Is Wasted Energy and Needless Stress
If you heard today's TV reports of the Borrego Springs area quake I mentioned at the outset, and paid close attention, you'd have heard all the interviewees saying that they felt it, but there was no damage, or only minimal damage with a few things falling from shelves. Even the lady with the retro-decor store right at the epicenter reported no damage or broken merchandise.
You may sometimes hear of a 'swarm' of 3.5 to 5.0 quakes happening in California (largely in Southern California, by the way...) .. 3.5? I sneeze harder than that!
See? No worries. It doesn't pay to waste engery worrying in advance about something that may or may not happen. Geologists will constantly say we are due for a big one "sometime in the next 30 years." It's always 'in the next 30 years.' They said that 30 years ago...still waiting...wondering why there isn't a countdown going on...if it was 'in the next 30 years' 30 years ago..shouldn't we be at 3-2-1 by now? Apparently earthquakes don't pay attention to human counting or time frames. So I don't pay attention to them.
Thanks, but I'll keep my infrequent California earthquakes, and not worry about having my roof torn off or worse on a seasonal basis.
An Added Note:
This hub was originally published well before either the 6.3 earthquake of February 21, 2011 that devastated parts of Christchurch, New Zeland, or the March 11, 2011, 8.9 disaster that struck Honshu, Japan (which, incidentally, is the strongest earthquake ever recorded there).
It should be noted, however, that the terrifying earthquake and subsequent tsunami which devastated Japan is not something to worry about here in California. For one thing, this was an offshore earthquake, and it is the location under the sea floor that displaces the water and causes the tsunami. California's faults are on dry land, so do not generate tsunamis.
Secondly, it is the type of fault that is responsible for the severity of the quake itself. Japan's earthquakes are caused by a subduction zone fault, in which one edge of a tectonic plate dives beneath another. California's quakes are largely strike-slip faults, which shift horizontally. The thrust faults that do cause uplift, are simply strike faults turned sideways, if you will, to visualize; they just move past each other, and one does not dive underneath the other. None of these are capable of producing the magnitude of quake seen in the Japanese disaster.
Since this article was written, in fact, there has been an increasing number of more violent quakes in scattered parts of the world, such as the 7.0 temblor in southeastern Pakistan on January 18, 2011, or the 7.0 New Year's Day 2011 shaker in Santiago Del Estero, Argentina.
This hub was originally written to address the moderate earthquake, and offer information to help my readers understand the mechanics behind these common events.
In no way do I wish to minimize the suffering and horror of these truly terrifying displays of nature's fury that have befallen these countries and their people.
© 2010 Liz Elias