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This is about science produced by the California Institute of Technology and originally broadcast by station KPCC in Pasadena California. The programs are made available to the station by a national educational radio. This program is about earthquakes meeting to discuss this subject. Dr. Peter less a man and his guest Dr. Clarence Allen professor of geology and geophysics Here now is Dr. Leslie Mann. Earthquakes mountains and faults of our Mother Earth have fascinated mankind since time immemorial. We have with us a man who has made this a lifelong study. Dr. Clarence Allen Dr. Allen was educated at Reed College in Portland and obtained his doctorate from Cal Tech where he has been since 1955. Working with the Seismological Lab where he holds many positions on committees concerned with
earthquakes their prevention or I shouldn't say that they're the study of earthquakes and how they may possibly occur and to open the discussion I'd like to ask you Kyra Clarence what exactly is an earthquake. Oh Peter I I suspect that most people associate an earthquake with the shaking or the quaking of the ground. And I would have to agree I think this is about as good a definition of an earthquake as one can give. I think most scientists would like to limit this to way shaking due to natural causes as opposed to artificial causes but nevertheless it is a shaking or a quaking of the ground where you say cruises What are the causes. Well you know you started right out by asking me. Perhaps the most difficult question of all because fundamentally we don't know. I wouldn't be honest if I if I stated that I knew anybody else really knew what the cause of earthquakes. Really was. There are many theories and many ideas
and we do have every reason to think that most earthquakes perhaps not all but most earthquakes are caused by the breaking. Of rocks at some depth in the earth's crust. A breaking that is caused by stresses and forces that gradually build up until the strength of the rocks is exceeded and they break. But if you ask me where those courses come from or why we have these forces that's the question that we really don't know. And to some degree that's the same question. As when you ask me what is the cause of mountains why are some parts of the earth higher than other parts. And again there are many theories and many ideas but the origin of these forces is something that we really don't know and something that we're working on. Many ideas on many research programs but I would not be honest as if I told you that we had solved this problem. Do X quakes occur all over the world or are there places where you expected to be more frequent than others. Well I'm. I think that you know and most people know that we tend to have more earthquakes here in California than let's say in New York City and indeed
around the world there is a wide variation in the frequency of earthquakes. There are two major earthquake belts where are the quakes occur more frequently than. In other parts of the world one of these is the is the circum pacific ring or those areas around the Pacific Ocean the so-called circus of a ring of fire which also has and is characterized by numerous volcanoes and young mountain chains. This is one of the major earthquake belts the other major earthquake belts extend the other earthquake belt extends from Southern Europe through eastward through Turkey on the M-Audio finally over into the Southeast Asia area where it joins the circumference of a ring of fire. Most earthquakes tend to occur in these two general belts although occasionally we do have earthquakes outside of these areas and sometimes these are very very large earthquakes. I notice that you exclude Africa from the discussion at least the southern parts. Is that relatively free. Well Peter I really shouldn't exclude all of Africa because certainly North Africa has had many
damaging and some disastrous earthquakes such as the agony earthquake that you remember. East Africa and the African river valleys down through Lake Tanganyika and down in that area does have some seismic activity. Generally speaking most of the southern part of the continent is relatively free of earthquakes at least as compared to the other areas that I was mentioning because the fact that it is a risk Sandy have something to do with the earthquake probability there. Yes we think so all the the rift valleys are themselves felt valleys and when I say fault perhaps I should explain this. Of that I spoke a moment ago about rocks breaking and this breaking causing the earthquake. Fundamentally when they break they break along the plane of failure and it is this plane or surface of failure that we know as a fault a fault surface is merely the surface along which sudden braking takes place during an earthquake and cause of the earthquake during the the many smaller earthquakes that occur every day let's say here in Southern California. This braking is limited to a
some area of many miles below the surface of the ground. But during a larger and larger earthquakes this braking extends over a larger and larger area until during the very large earthquakes a very great earthquakes. This breaking extends right to the ground surface of the fault plane intersects the ground surface displaces the ground surface and creates on a scar or what the geologists call a fall scarp. And. This sort of thing then tends to cause a small hill and if we have repeated earthquakes over periods of thousands of years they only grow intermittently until we have a mountain range and wells along the bases of these mountain ranges or within the rift valleys. Certainly one of the major reasons why we have differences in levels on the Earth's surface so Foti is a place where an earthquake has actually occurred. Is that. True. Yes I think in general the faults are related to sudden displacement some related to earthquakes. It is true that sometimes we observe slower gradual displacement along
fogs and this has recently been shown along parts of the San Andreas fault here and in California. But I think we still believe that most of the displacement takes place suddenly and abruptly and during earthquakes. The San Andreas for Clarence is a very famous one. Would you like to describe a little bit. It's Ponson and some details about it. Well the sun address follows is of course well known to most of us here in California. And first came into prominence so to speak during the 1006 earthquake in San Francisco when it was recognized that the cause of that earthquake was slippage along a major fault system that broke for some 200 or more miles through the center city of San Francisco and for many miles farther north. Subsequently it was recognized that the same problem extended clear through the southern part of the state and we now know that the San Andreas Fault extends from north of San Francisco clear down through the Central Coast Ranges and north of the Los Angeles area and then splits into a number of branches and eventually goes into the Gulf of California. So essentially California
Split or blackly and via the San Andreas fault and the movements on this Fall to have predominantly been horizontal. A moment ago I was speaking of building a mountain where the one side of the park went up on the other side went relatively down. Strangely enough on the San Andreas Fault most of the movement appears to have been horizontal where one side is moved north on the other side is moved south and this now appears to be a pattern that is not only characteristic and unique of California as we once once thought but also all the characterizes a number of other areas around the world such as Chile in the Philippines and Turkey where there are calls very similar to the Senate. Those with horizontal rather than vertical displacement. I understand that from the air you can in certain spots you can crack distinctly see the center and various photos. Is this true. Well yes up eagerly. After all is an address pot represents a. A cut so to speak or a fracture that crosses the state and one gets high in the air one literally sees the fall as a scar. So to speak and look upon your face. This is
particularly noticeable towards sunset or sunrise when the shadows are low so one sees the detail topographic features as one goes along in the ground on the other hand it's rather difficult to see because the linearity in the continuity but are striking from the air tend to be lost when you're standing on the ground looking at it. So many of us cross the San Andreas Fault every day in automobiles and so forth and never know it. Or as if we were to do the same thing by air at the right time of day. It would be very very spectacular. Now if one were to for some reason build a fence across this front Karens would you feel you would find the pros on the one side moving north when there is on the other side groupings like the South would you. Well during the earthquake Yes and this happened literally during the 906 earthquake. Several fences in the San Francisco area were actually offset and one road was offset as much as 21 feet north of San Francisco. This was the maximum horizontal displacement that was observed at the time of that earthquake and I think the maximum displacement now
observed during any single earthquake was in Mongolia in 1957 when there was almost 30 feet of horizontal displacement during a single earthquake. So what. Yes if you happen to build a fence across the fault and we have a major earthquake on it it will be split. Actually. I mentioned at the very beginning that we think that the stresses gradually build up in the rocks and cause us fracturing. If one build a very very long fence across on a dry asphalt of say 50 miles long. And then watched this thing over a period of years one would find that fences gradually being deformed as one part of California is gradually moving with respect to another. The other part of California across the fault it is this distortion or strain of the rock within the fault zone that eventually causes the the rock to break. It gets to the point where this movement is more than the rocks can stand and so they break very suddenly and slip and cause the earthquake. What is the sort of rate of strain cancer. If we would pay into to write
lines one on each side of the photo would we be able to notice after a year that they weren't lined up any longer. Well if we if we get some distance from the fault let's say get 10 or 20 miles on each side of the fall and measure the relative displacement of points at some distance away from the fog it turns out to be at least in some parts of the state about 2 inches per year or something of that order of magnitude. Now we're not clear as it's not clear that this. Is the same throughout the length of the San Andreas fault but there are some areas where this kind of displacement is going on. I mention that also there was this problem of slow slippage along the fault. This is taking place up there on. The Hollister south of San Francisco where roughly half an inch a year of slow gradual slippage is taking place without being directly associated with with earthquakes. Before we leave for our time must I ask you to disabuse me. If this is about a popular misconception that seems to be set up by makers of horror
movies and such type of fiction in which one sees an earthquake in which a fruit opens up the earth gapes and all sorts of things fall into it. Did does this ever happen. Well I I remember seeing a movie where I think an entire city or an entire town apparently fell in into a giant crack. Certainly nothing of that magnitude has ever happened during an earthquake and basically the cause of the earthquake is the two walls of the fault sliding curl of the one another not opening up and a great tension crack. So so fundamentally you can't have any great chasm opened up into the depths of the earth in the way that some people and some believe in the writers might imagine. Now locally in particular in areas of heavy soil one can have large cracks open up temporarily or permanently during an earthquake and occasionally of these are large enough so that the individuals or animals might fall into them. Insofar as I know however there is only one authenticated
instance of a person actually having fallen into one of these large cracks. And a crack which subsequently closed up and and engulfed the person cares what is a lurch. When I say lurch I merely mean. Due to the shaking of the ground if you shake it hard enough you tend to temp make the ground split. Sort of like a jail or a desert running a commercial cabal of jelly hard enough and yellow it presumably would split and this may be quite unrelated to the following. This is primarily a matter of shaking of the ground reportedly and 1000 sex cower fell into one of the cracks up north of San Francisco a crack which developed right along the fault trace and the crack then closed up leaving only its tail sticking out of the crack. If. As we know the the fault moved in a in a horizontal sense we have every reason to think about how the car's tail would have been twisted in a clockwise fashion because of the sense of displacement that it was observed at that time and is characteristic of the San Andreas fault in general.
I like that analogy. But I expected today you're somewhat more sensitive and precise a scientific instruments than cars. Tails to measure the movement and I also think and I wonder if you could tell us a little bit about some of the instruments you use and some of the techniques of earthquake prediction that are being studied now. I'm glad to use the word studied rather than used because certainly earthquake prediction is something that is not yet realized and we're not really sure that it ever will be. But there are a number of efforts being made that are at least aiming so to speak in this direction then I think many seismologists and the ologists are increasingly optimistic that that maybe someday we will be able to predict earthquakes. Of course I could always ask you what is really the point of predicting worms. Well this is a very good question and many people have argued that the prediction of an earthquake really might not save any lives an impact and you could argue that under some circumstances
a prediction of an earthquake might cause in a panic to a two to work in the other direction. My answer to that though I think would be that the very the very kinds of knowledge that may eventually lead us to predict earthquakes will also enable us to say where earthquakes are going to occur or how often they are going to occur. How intense they may be. And this is exactly the kind of information that engineers need a player to design buildings which will withstand earthquakes they need to know how often earthquakes are liable to occur they need to know what kinds of ground shaking will be associated with them. So I would I would argue very strongly that the same kinds of information that may eventually allow us to predict earthquakes will also permit the engineers to to take exactly the kind of action in the designing buildings and designing structures dams aqueducts so forth that will make. Make us completely safe so that come the earthquake we will not be worrying anyway. So I think there are very good reasons for working in this direction. You asked a moment ago about some of the
kinds of instruments and of course I can't tell you the kinds of instruments that might allow us to predict earthquakes because I don't know what they are but basically of course they are the fundamental tool that we have used for many years now and as the seismograph or the measuring of the location of earthquakes are expected does that work. Well some of the side graphs now are pretty sophisticated instruments but but fundamentally they they are all or at least most of them tend to operate on the same principle. And that is simply the principle of inertia. If one has a heavy mass it tends to stand still when you try to move it. And consequently if you take a very heavy weight or heavy mass and hang it from the spring then if you shake the world around it too the mass tends to stand still. And if we can measure the relative motion then between that that weight and the and the earth or the frame from which it hangs then we are centrally of. Measuring the kinds of displacements that a seismograph registers So fundamentally this is what a seismographic is although there are many variations on this theme.
Now I suppose your science murder lab which incidentally isn't a Knox gun on the regular campus of county. Read up on that beautiful San Rafael Hill. Why do you have it up there. I think the laboratory was originally put out there soley for one reason namely that in that area in the San Rafael hills we are on the granite bedrock. And we need that kind of firm bedrock in order to plant our seismographs onto the solid ground or as down on the Caltech campus or on many hundreds of feet of a little VM which is not the most appropriate kind of environment in which to put a very sensitive seismographs where you're trying to record very small wave motions from earth quakes that are many thousands of miles away. How does a Lluvia differ actually from bedrock I mean these two are obviously quite different but what are these more general descriptions of those terms. Well I think the analogy that you yourself used a few minutes ago is perhaps a good one of the bowl of jelly in the bowl of Jell-O analogy if you have a
substance which is. Sort of soupy and tends to form elastically very easily then then you tend to have a very different kind of response and you. And you're not measuring you're measuring the response of that particular kind of material rather than what's causing the movement. So in effect the bedrock as Aquarius tied onto to the earth. Well it is they are saying the same constitutes the earth prior is the Libyan as is jello which fills some of the valley as the horse depends on the particular kind of aluminum on how much water there is within it and so forth but basically loves the idea. Yes. Now one of the interesting things about this bedrock in the fact that it is a mountain pushed up from the earth is is the other part of your talk which is mountains and how they are related to earthquakes. While I mentioned a few minutes ago that. Company a major earthquake
with vertical displacement and a very large earthquake which displaces the ground surface one side of the fault ends up higher than the other side. And if this is then repeated every few thousand years with a major earthquake Eventually after a few thousand or millions of years one ends up with a mountain range and most of our mountain ranges here in Southern California have come into existence because of intermittent uplift associated with major earthquakes along faults which lie at the base the base of these mountains. Certainly the se the great Sierra Nevada and central California is a tilted block. It is tilted up toward the toward the west and the great eastern of Scotland of the Sierra Nevada is a fault scarp that has been caused by intermittent uplifts associated with major earthquakes. The last one of which occurred in 872 there and this is the way in which that mountain has come into existence. I don't mean to imply that ALL mountain ranges are are. Are so see it with faults or earthquakes. I'm sure you know that the volcanoes represent quite a different sort of thing and some mountain ranges are full of mountain ranges but
nevertheless a good proportion of our mountains here in California are associated with Falls and ends and obviously then also associated with earthquakes. It would seem that the sort of earthquakes that could raise a mountain range would be enormously stronger than the ones we took today was this many. Billions of years ago that that these very large earthquakes that could and are earthquakes getting weaker as the earth grows older. No I think we have every reason to believe Peter that the processes we see going on in the earth right now within the historic record are quite adequate and quite logical to explain what's happened within the geologic past. That is if we have a major earthquake once every 500 years which after all is a pretty long time span in terms of our lifetimes. Nevertheless that happens every 500 years with maybe 10 feet of displacement each time. Before long at least in terms of geological time one has a major mountain system. So I don't think there is any reason
to believe that earthquakes have necessarily been much larger in the past than they are right now. It would certainly be a mistake to assume that the entire Sierra Nevada was up with the during one single earthquake. I love the song seriously as I think some people tend to want to make a mistake in visualizing a mountain system or the rains as coming into existence during one earthquake. There is no evidence that this. Has happened in the past or certainly it has not happened during the historical record. How much vertical displacement can take place during a single earthquake. It is something on which we have some evidence because we have seen many great earthquakes during the historic record. There was one earthquake and in the last salmon India in 1897 where there was a 35 foot Scotland created during a single earthquake. And in so far as I know this holds the record for vertical displacement in the same sense that that 30 feet that I spoke of in Mongolia holds the record for horizontal displacement. During the last going to earthquake in 1964 which was a very great earthquake. There were changes an elevation of as much as perhaps 50 feet. These
these were associated with broad warping not discrete displacement across a fault but there were some areas that went up probably as much as 50 feet during this particular earthquake and other areas of course that went down. So we see that the geological features of the earth which are caused by earthquakes are really happening in a very very slow time scale compared with human beings. Yes although. We spoke about this two inches a year of gradual build up of strain across the San Andreas fault of course San Francisco is on one side of the fault on the north side or the North East Side and Los Angeles is on the west or southwest side and really in terms of geological time it won't be very long before if this movement continue in San Francisco and Los Angeles will be juxtaposed and then of course the argument will arise as to which city has a suburb or which city that's still off in the future insofar as you and I are concerned because that's not something that I think we'll have to worry about here. But I am fascinated by the feeling of the enormous
forces inside the earth which bring it back to the strains that you talk of. How are they coolest and what do they constitute really. Well again Peter you've come back to the fundamental question that that I must plead ignorance to. There are many theories on and just let me mention let's say one that that you may remember from your early textbooks. And since the mountains on the surface of the earth tend to look like wrinkles when viewed from afar many people visualized at least in the early days that this was analogous to the shrinking of an orange where the skin tends to become wrinkled as the orange shrinks. And the analogy was then drawn and they thought that as the earth was cooling the shrinkage would cause the modern systems. This sounds good except for the fact that the evidence today suggests that if anything the earth is heating up and not cooling down and consequently there is no indication that really the earth is shrinking. There are many other reasons why this particular argument doesn't sound so good. I think many geologists and geophysicists now
would like to feel that there are very great currents deep within the earth convection currents very slow convective currents in the Earth's mantle and the drag of these currents as they approach the surface. That may be very responsible for the mountains we see from these great horizontal displacements on fault and the but still the driving mechanism of these currents why the differences in heat temperature should exist as in. Matter of considerable controversy what is actually flowing in these currents that you speak of Cairns. Well this would be rock and of course rock that you and I know as a solid one spoken of in terms of million years millions of years can be visualized as a thing as a liquid sort of material. In the same sense that bouncing punny of course seems like a liquid material when you work to form it slowly but if you form a very rapidly in boxes and consequently even the very hardest of rocks a given time or tended to form gracefully and slowly and flow. So I don't mean to say the
interior of the earth or at least not in the model is liquid but nevertheless there is some suggestion and at least many theories that this material is flowing very very slowly. It's very interesting you know it's likely a trivial question people around Southern California Casey talk about earthquake weather as there was something in the atmosphere that could cause an earthquake. What do you know of years on that. Well I think when most people refer to worst quake weather this this must be placed in the category of off. Of a off. An old wives tale. There simply is no evidence that earthquakes are associated with one type of weather versus another. On the other hand I should point out that when a large high pressure system moves in from the ocean the total force exerted on the continent could be very great and as a possible triggering mechanism one cannot rule out major barometric disturbances. Now it's never been proved by anybody that this indeed is a cause of triggering of earthquakes but it perhaps could be. So I don't
I don't want to make the flat statement that weather is necessarily unrelated to earth quakes but the kind of weather most people think about is the earthquake weather. That's something else again that's very fascinating as a last word. Terence Do you ever worry yourself about living in Southern California which is so prone to these small earthquakes. Well let me say quite firmly that I wouldn't live here and I wouldn't ask other people in their families to live here if I didn't feel this was safe and one reason I say this is because I worked a great deal with engineers. I'm not an engineer myself. You are. But the engineers know that we're going to have large earthquakes they know what's going to happen during these earthquakes. And buildings are being designed with this in mind our new buildings are much safer than our old buildings are. And I have every confidence in the engineering profession is indeed meeting this challenge and that indeed California is a and will continue to be a safe and as the years go by a safer place to live. Well that's very interesting. Parents and most gratifying to know that there are people like yourself studying and searching and probing for the causes and the
About science
About earthquakes and faults
Producing Organization
California Institute of Technology
KPCC-FM (Radio station : Pasadena, Calif.)
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University of Maryland (College Park, Maryland)
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This program focuses on the science of earthquakes and faults. The guest with the program is Dr. Clarence Allen, director, Seismological Laboratory.
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Interview series on variety of science-related subjects, produced by the California Institute of Technology. Features three Cal Tech faculty members: Dr. Peter Lissaman, Dr. Albert R. Hibbs, and Dr. Robert Meghreblian.
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Guest: Allen, Clarence R. (Clarence Roderic), 1925-
Host: Hibbs, Albert R.
Producing Organization: California Institute of Technology
Producing Organization: KPCC-FM (Radio station : Pasadena, Calif.)
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University of Maryland
Identifier: 66-40-61 (National Association of Educational Broadcasters)
Format: 1/4 inch audio tape
Duration: 00:27:42
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Chicago: “About science; About earthquakes and faults,” 1967-11-06, University of Maryland, American Archive of Public Broadcasting (GBH and the Library of Congress), Boston, MA and Washington, DC, accessed August 12, 2022,
MLA: “About science; About earthquakes and faults.” 1967-11-06. University of Maryland, American Archive of Public Broadcasting (GBH and the Library of Congress), Boston, MA and Washington, DC. Web. August 12, 2022. <>.
APA: About science; About earthquakes and faults. Boston, MA: University of Maryland, American Archive of Public Broadcasting (GBH and the Library of Congress), Boston, MA and Washington, DC. Retrieved from