thumbnail of About science; About earthquakes and faults
Hide -
If this transcript has significant errors that should be corrected, let us know, so we can add it to FIX IT+
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 this station by national educational radio. This program is about earthquakes meeting to discuss this subject or Dr. Peter lissome 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. Alan. Dr. Allen was educated at Reed College in Portland and obtained his doctorate from Celtic 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 Clara Clarence what exactly is an earthquake. Oh Peter 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 us 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 courses what are the courses. 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 forces come from or why we have these forces and that's the question that we really don't know. And to some degree that's the same question as when you asked 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 are 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 if I told you that we had solved this problem. Do earthquakes occur all over the world or are there places where you are 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 and 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 our 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 bill extends from Southern Europe through eastward through Turkey on them audios and 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 agate 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. Does the fact that it is a rift fairly have something to do with the earthquake or probability there. Yes we think Salty the rift valleys are themselves affected valleys and when I say fact perhaps I should explain this better. 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 small earthquakes that occur every day let's say here in Southern California. This breaking 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 fault 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 and levels on the Earth's surface so Fault is a place where an earthquake has actually occurred. Is that true. Yes I think in general the facts are related to sudden displacements on the. Related to earthquakes it is true that sometimes we observe slow or gradual displacement along faults and
this is 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 during earthquakes. The San Andreas for Cairns is a very famous one. Would you like to describe a little bit. It's pots and and some details about it. Well the Senate race 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 fault 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 OBE likely by the Santa driest fall 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 characterizes a number of other areas around the world such as Chile in the Philippines and Turkey where there are faults very similar to the Senate. Those with horizontal rather than were tickling displacements. 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 in the topography. This is
particularly noticeable but 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 and the continuity 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. Whereas 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 trans would you feel you would find the pros on the one side moving north and there is on the other side moving slightly 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 and 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 causes fracturing. If one build a very very long fence across on a dry asphalt let's say 50 miles long and then watch this thing over a period of years one would find that fences gradually being deferment 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 rocks within the fault zone that eventually causes the rocks 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 in terms of who if we would paint two to write lines one on each side of the float. 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 you get 10 or 20 miles on each side of the fault and measure the relative displacement of points at some distance away from the fault it turns out to be at least in some parts of the state about two 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 a Hollister south of San Francisco where all roughly half an inch a year of slow gradual slippage is taking place without being directly associated with with earthquakes. Before we leave faults I must ask you to disabuse me. If this is not a popular misconception that seems to be set up by makers of horror movies and such talk of fiction in which one sees an earthquake in which you're full it opens up the earth gapes and all sorts of things fall
into it. 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 and certainly nothing of that magnitude has ever happened during an earthquake and basically the cause of the earthquake is the two walls of the front sliding. Her other one another I'm not opening up in 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 writers might imagine now locally and in particular in areas of heavy soil. One can have large cracks open up temporarily or permanently during an earthquake and occasionally these are large enough so that 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. Parents What is the lyrics carry.
When I say lurch I merely mean due to the shaking of the ground if you shake it hard enough you tend to make the ground split. So does it like it Jim Lehrer does that run into a commercial cabal of jelly hard enough and yellow it to presumably would split and this may be quite unrelated to the funding. This is primarily a matter of shaking of the ground reportedly and one thousand six a cow 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 of the towel the call's tail would have been twisted in a clockwise fashion because this is the sense of displacement that was observed at that time and is characteristic of the San Andreas fault in general. I like that analogy turns but I expect that today you are somewhat more sensitive and precise in scientific instruments than cause tails
to measure the movement tonight. 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 you used the word study rather than used because certainly earthquake prediction is something that has 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 and I think many seismologists sociologists are increasingly optimistic that that maybe someday we will be able to predict earthquakes. Because 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 many lives and back to. You could argue that under some circumstances a prediction of an earthquake might cause enough panic to to 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 or how intense they may be. And this is exactly the kind of information that engineers need if they are 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 actions and and 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 and the location of earthquakes hearts actually does that work. Well some of the seismographs now are pretty sophisticated instruments but but fundamentally they they 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 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 as I said graphic as although there are many variations on this theme now I suppose there at your size murder lab which incidentally is an eye not done on the regular campus of count here. Read up on that beautiful San Rafael Hill. Why do you have it up there.
I think the laboratory was originally put out there are only for one reason namely that in that area in the San Rafael hills we are on 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 loo VM which is not the most appropriate kind of environment in which to put a very sensitive seismographs where you're trying to record a very small way motions from earthquakes that are many thousands of miles away. How does a live VM differ actually from bedrock I mean these two are obviously quite different but what are the more general descriptions of those terms fare. CURRY Well I think the analogy that you yourself used a few minutes ago is perhaps a good one of the bowl of jelly or the bowl of Jell-O analogy if you have a substance which is a 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 it were is tied onto to the earth. Well it is that you are saying that it constitutes the earth right is the elusive Iam as is jello which fills some of the valley as the process depends on the particular kind of balloon. How much water there is within it and so forth but basically that's the idea. Yes I know one of the interesting things about this bedrock in the fact that it is a mountain pushed out from the earth. Is is the other part of your talk which is mountains and how they are related to earthquakes. Well I mentioned a few minutes ago that come in a major earthquake with were tical 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 bases of these mountains. Certainly the se the great Sierra Nevada and central California as 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 up last associated with major earthquakes. The last one of which occurred in 872 there and this is the way in which that mountain ranges come into existence. And I don't mean to imply that ALL mountain ranges are r r so c it would fall to 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 faults and it's 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 talk about 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 uplifted during one single earthquake. It was another case I'm curious because I think some people tend to want to make a mistake and 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 in and asked salmon India in 1907 where there was a 35 foot scar print 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 An 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 I 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 south west 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 is a suburb of 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. But I am fascinated by the feeling of the enormous forces inside the earth which bring about 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. Since the mountains on the surface of the earth tend to look like wrinkles when viewed from afar. Many people visualize 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 mountain 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 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 a. Daughter 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 as a liquid sort of material in the same sense that bouncing party of course seems like a liquid material when you want to form it slowly but if you form a very rapidly in boxes and consequently even the very hardest of rocks a given time will tend to form gracefully and slowly in the flow. So I don't mean to say the interior of the earth or at least not in the model is as liquid but nevertheless there is some suggestion at least many theories that this material is
flowing very very slowly. It's very interesting enough it's likely that trivia question people around Southern California occasionally talk about earthquake with or as though there was something in the atmosphere that could cause an earthquake. What do you know of years on that. Well I think one must people refer to where the quake whether this this must be placed in the category of US of A. 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 earthquakes but the kind of weather most people think about it or Quake whether that's something else again that's very
fast at the end of the last word. Terence Do you ever worry yourself about living in Southern California which is so prone to these smaller 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 were 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 as I say and will continue to be 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 reasons of earthquakes and that there are engineers who are designing buildings which will in effect be earthquake proof. Thank you very much.
This was about science with host Dr. Peter listen and his guest Dr. Clarence Allen professor of geology and geophysics joined us again for our next program when two more prominent scientists will discuss a subject of interest about science is produced by the California Institute of Technology and is originally broadcast by station KPCC in Pasadena California. The programs are made available to this station by national educational radio. This is the national educational radio network.
About science
About earthquakes and faults
Producing Organization
California Institute of Technology
Contributing Organization
University of Maryland (College Park, Maryland)
If you have more information about this item than what is given here, or if you have concerns about this record, we want to know! Contact us, indicating the AAPB ID (cpb-aacip/500-0k26f99g).
Episode Description
This program focuses on the science of earthquakes and faults. The guest with the program is Dr. Clarence Allen, director, Seismological Laboratory.
Series Description
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.
Broadcast Date
Media type
Embed Code
Copy and paste this HTML to include AAPB content on your blog or webpage.
Guest: Allen, Clarence R. (Clarence Roderic), 1925-
Host: Hibbs, Albert R.
Producing Organization: California Institute of Technology
Producing Organization: KPPC
AAPB Contributor Holdings
University of Maryland
Identifier: 66-40-8 (National Association of Educational Broadcasters)
Format: 1/4 inch audio tape
Duration: 00:27:35
If you have a copy of this asset and would like us to add it to our catalog, please contact us.
Chicago: “About science; About earthquakes and faults,” 1966-10-11, University of Maryland, American Archive of Public Broadcasting (GBH and the Library of Congress), Boston, MA and Washington, DC, accessed June 16, 2024,
MLA: “About science; About earthquakes and faults.” 1966-10-11. University of Maryland, American Archive of Public Broadcasting (GBH and the Library of Congress), Boston, MA and Washington, DC. Web. June 16, 2024. <>.
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