About science; About geologic history in the making

This is about science produced by the California Institute of Technology and originally broadcast by station KPP C. Pasadena California. The programs are made available to the station by national educational radio. This program is about geologic history in the making with host Dr. Albert Hibbs and his guest Dr. Stuart Smith associate professor of geophysics. Here now is Dr. him's the face of the earth changes steadily due to the action of waves against the shore or the action of a stream and cutting in a canyon. The spreading of the lava from a volcano over the surface. But deep underneath the surface in response to some of these changes as well as in addition to them the earth moves. Masses of rock slide against each other creating not only earthquakes but also fundamental changes in the shape of the surface of the earth. We have with us now Dr. Stuart Smith of Caltex seismo lab who has been watching very closely one special and convenient

example of the way in which an earthquake changes the surface of the earth. Convenient because it happened fairly close to Pasadena where Dr. Smith works. And fundamental because it's part of one of the biggest and most impressive earthquake faults it's known in North America or in fact even in the world. Dr. Smith has been with Cal Tech since he became a student there in 1957 he took his degree in one thousand sixty one and has since been with the Seismological Laboratory as a geophysicist. Well tell me first do to start out with where is this quake of yours that you've been watching with such a diligent care and when to just are watching it. It's about midway between Los Angeles and San Francisco and the San Andreas Fault between the towns of Parkfield and Chile. Why is June 28 of last summer that the earthquake occurred there so it's less than that particular earthquake and it's considerably less than a year old from how.

How soon after it happened did you get up to take a look at it. Well we heard about the earthquake about 10 o'clock at night and we drove all night and arrived at the site just shortly after sunrise and were met with quite a few features. As a result of the earthquake including large cracks in the ground offset fences. Unfortunately. The crack crossed the highway at one point and offset the white line which made a very convenient place time measuring how what do I mean by that time. What was the measurement you made by looking at this conveniently placed white line. Well when we arrived just after sunrise the total displacement was just slightly more than an inch and on checking the white line later in the afternoon it had increased to perhaps two and a half inches and that same day same day. That's why you were around or did you feel this motion going home the cause of sex to an inch and a half full motion.

Well we felt a number of aftershocks at the time. We didn't know that the displacement was continuing. It was a surprise to us two in the afternoon to find that the line had been displaced in fact we we ran into some of our colleagues from the U.S. Geological Survey who had also measured the white line somewhat later than we had and we disagreed on the measurements and both of us thought that what I could find out who was right. That's right and turned out we were both right and that the fog was still moving. Where was this particular earthquake in any way destructive to the nearby outpost of civilization I assume there were some in this vicinity houses or farms or something. No there are a number of branches in the area. There was very little damage done to any of the structures. A few cracks in the bridge here and there brick chimney fell down that kind of thing. A few dishes rattled here and there but it was very sharp very short duration shock and not much damage. So I really wasn't what would be called a major major earthquake all by itself with no no I'd done

this area has a history of this kind of earthquake. This is about the third repetition during the century of displacement on the fault and in this locality and this particular region strength. We located a number of ranchers who had experienced the previous earthquakes one thousand nine hundred forty one thousand twenty one who were living on the same land and they pointed out to us that the cracks recurred in almost the same spots and I sees them 20 and 30 years ago which serve the Continuum for a fairly repetitious menace. Yes and very very well defined one can point to the ground within a few feet say this is where the fault lies this is where it broke. How do you know exactly where it was when you started out your trip at 10 o'clock at night. It was well-spent to triangulate that closely on where this thing would happen. Yes fairly closely we have a network of seismographs stations in Southern California. And we conferred with our colleagues at Berkeley and compared times of arrival and were able to triangulate and establish that

it was somewhere in the vicinity of Parkfield and then of course knowing the senator has fought goes right through the town of Parkfield leave core suspected it was there. So you had a pretty one my old man by the time you got there what what. Did you do after you got there were sides measuring the white line as they were. I soon knew there was something more to it than you. Well of course when we went up we didn't know for sure that there would be any faults placement that one could see at the surface because many earthquakes in California simply shake the surrounding area and there is no known surface effects visible. As soon as we realized that the fall had broken the surface and that it was continuing to move this was perhaps the most interesting thing that the motion didn't stop with the earthquake but continued. We attempted to set up some special instrumentation along the fault and across it. And in addition we set up a number of of. Markers on opposite sides of the fault that we surveyed in

and continue to research every few days during the first few weeks of the market on the white line. Yes that was the purpose there. This was quite successful. What kind of instruments would you use when you when you did move an instrumentation set up in there. Well we did on the fault itself we dug a trench across it and laid a quartz tube some 25 feet long right across the trace and attached a very sensitive electronic transducer to the end of this quartz tube and proceeded to monitor continuously the slipping and creeping of the fault at this particular locality. So it quits troops going across the crack perpendicular to the crack I presume. That I'm both and you have some kind of a device that measures what is really meant. Well not exactly the fault is what we call the transfers fault or a strike slip fault one where you

stand on one side and look across and it is moved to the right. We put the tube across at about 45 degrees which would be the direction of minimizing tension isn't perpendicular. I don't know and I know where the concrete pier on either end of the courts is fastened to one pier and the other end is free and we attach the transducer to the other concrete pier and that's measuring the. Extension along the direction of the quartz tube. White cord 29 just a steel rod. Mainly because of the thermal properties of course. We had it buried some five feet below the surface of the ground but the daily temperature variations on a steel bar would be a very large effect as large as the displacements we wanted to measure. I'm quite as my son 13 and yes quartz is among the materials one can use in the field is perhaps

the least. Temperature sensitive it's also light and strong. I would think though under 5 feet of dirty already it would have insulated out most of the variation but there's still enough left so that the yes so the steel matters. How long did you monitor the behavior of the quartz rod or are you still monitoring behavior. Well the the great rains came in California here about a month ago hand washes out really. Earth quake measuring defeated by rain nature's conspired against the whole area was under about 18 inches of water and the installation was it was considered temporary but it was ruined. So we've recently reinstated it with a slightly different device this time with an eye to the future about doing this perhaps in some other localities. Where one has to be to measure over a distance much larger larger than 25 feet. Going to a device. Where we stretchy and environ across

the fault and hold it under constant tension via wire I assume in Viper the same reason temperature problem that's right that's right this special alloys can be treated so it has almost the. Zero temperature coefficient that is heated a small amount and it doesn't expand or contract at all. The use this also in a tube and buried under the ground is the seat of the service you know this is also buried inside a large water pipe 6 inch water pipe and has the capability of being extended for a much larger distance. What is a dissenter you. Well we're in the same locality we're still using 25 feet but we could easily go to distances of 300 400 feet between the something made up specially for this particular application I would use a standard instrument. No this is this is a rather simple rather crude instrument but it does the job it so I mentioned it's a taut wire stretched across the fault it's kept at constant tension and it actuates a

machinist's dial gauge micrometer the types that are used on a large machine is delayed. Which reads to thousands of inches and we can simply look down the hole with a little telescope and read the dial. What you have to go right up there every few days and look down the hall with a telescope. Well we would except that we have a very cooperative rancher who was actually here as a scientific assistant I must know. He said I've been very cooperative. Most of the the ranchers in an area like this are extremely interested in your quakes and many of them are quite well-read. I was surprised when visiting some of them to find many magazines and books on the subject of earthquakes in their libraries. Well I guess that's understandable if I live right on top of a fault and I feel. A certain kinship with the motion of the earth how much is it moved by now. Incidentally since you started the measurement as a continued it's continued in a ever decreasing rate and the total displacement in one place is now almost 8 inches which is perhaps four or

five times the displacement that actually took place during the earthquake itself. When has this happened by a series of subsequent quakes more and more but smaller jars are how do you how does this motion actually take place. It's gone both ways since the time of the earthquake and the last part of June. There's been a steady what we call creep where the fault moves smoothly one surface against the other. Without producing any noticeable earthquake type of effects of shaking or anything like this one. One can't time the motion is not perceptible to a person even standing in a fog. Then also during that time there's been a large number of perhaps thousands of small aftershocks. Ranging in size from those which will rattle dishes to those which are perceptible only to a very sensitive person standing very still on a quiet day.

And are sensitive instruments of showing that. The fault indeed was just continuously during some of these very small earthquakes and so we have a steady decrease in the displacement rate. What a creep. Plus occasional discontinuity as our jumps take place during these individual earthquakes then this means and you must have some instruments up there besides the environment quartz tubes to look at that behavior what else was well out of there immediately after the earthquake we installed six or seven portable seismograph stations around the area. These are very sensitive to high frequency seismometers they're much like a microphone. Either one would use to listen to the other if there are enough of these that one can triangulate and locate the vents and we found some of them were occurring directly beneath our stations. In addition

we had an experimental. Set up consisting of a pendulum type devices suspended on either side of the fault. See the courts ride already in barbed wire only measures the relative. Displacement between one side of the fault and the other. The pendulum devices will tell us exactly what kinds of accelerations have been experienced on the opposite side before they will in fact tell us whether one side is really remain stationary on the other side is moved or whether in fact they're both moving. What is the result by the way. Well the result is something that both sides at fault are moving but in perhaps a peculiar way and. That we're not at all sure that we understand exactly what's happening yet in the situation we have seen a number of instances where it appears like one side of a fault moves perhaps two or three times faster

than the other side. This is somewhat hard to understand but working on this now is it always the same side that don't seem to know. Another interesting thing. One side will move and then some later time and some later slip is the other side moves that it looks something like that yes. Curious. Another strange effect was revealed by the surveying that we've done. On these monuments that I mentioned we are markers that we put in across the floor. We see that one area of the fault will perhaps slow down in its rate of displacement almost as if it's become locked and another area will begin to creep rather rapidly and this will go on for some time perhaps a week or so and then the area that's locked will suddenly break loose again and begin to create faster and eventually catch up with the other section. When you say areas you mean up and down the floor you know somewhere along the line of the fall how far apart are

these areas which behave in this way as close as three or four miles. It's so implies you're it's getting squeezed Yes right in between these points the Earth is getting squeezed and the interesting thing is that no matter what happens at one locality in terms of the length of time that it's locked or how fast it moves it eventually catches up so the greater the total effect along the fault is very smooth and nicely behaved. And but piece by piece it's sort of a new climate right about security. This may in fact be partly responsible for some of the aftershocks are smaller earthquakes that occur these as you mentioned the Earth has got to be stretched and squeezed if this goes on and on. This is must be related to the earthquakes in some way. It's trying to relieve this squeezing my he said in jerks as what happens. When you mention that you had some of your instruments had shown that some of these aftershocks were almost directly under the instruments how deep under the instruments are that this is happening for that matter one. One thing I forgot to ask you is how deep was the

original shock which started this whole business. The main shock appeared to be at a depth of about seven or eight miles which is about typical for California earthquakes. And since that time there's been a distribution of earthquakes at the all depths ranging from that level right up to the surface. And in fact. Some of the earthquakes appear to occur right at the surface. Have any Have any been as deep as that one have any of the aftershocks been as deep as the original. Yes yes I see so it isn't just that the surface is it is readjusting to the original shock there's also been some further motion down to. That's right. When you say the right up at the surface how close is this you mean. Within a few hundred yards or well within the precision that we can have in locating events which means that perhaps a quarter of a mile. For those rich waves

which have actually produced an additional fault slippage at the surface. Look at the depths that we calculate from our side graph network. Ours essentially zero depth right answer. So comparatively compared to your measurement grid this is what you mean by that as a surface. Incidentally how how far along the fault is this motion extend. Many many miles in either direction or is it localized to just one region of it. The total extent of the cracks in the ground that we were able to walk out in and verify is more than 25 miles. And the way to put this into proper perspective the magnitude of the earthquake on the Richter magnitude scale was about five and a half. This fog bank for 25 miles is much larger than one would have expected for a

magnitude five and a half critically. Perhaps by a factor of two or three. So in a way it's an unusual request. You know the significance of the long volt link for the low energy released his is related to the the amount of stress that's released on the crack. Another way of stating that is if this earthquake was a low low stress drop originally did the material seem to yield at a lower level of stress because in nature the material itself. This this may be this may be the predominate effect yes in fact the drills are weaker here perhaps because they've been so often fractured in the past. We have spoken of the depth of the quake as being typical of California quakes and then you also spoke about the. Length compared to the strength was untypical. This brings up the question of how typical in general is this particular quake what. How broadly will you be able to

apply it in any direct sense. What you learn from this quake to others. Let's that's a difficult question. This earthquake has been studied in such great detail and perhaps we know more about it than than any earthquake of this magnitude out in my garage. It's were so intimate with it so to speak you know that we have no way of judging on whether it's really typical of. Many other earthquakes we think or other I think that it's typical of earthquakes at least in the eastern part of the Pacific region. So it would be typical for all earthquakes in the west coast of North America saying alongside Andreas and particularly it would resign under US since this time a number of other events have come to light which are also unusual in the same sense that this earthquake we're discussing which incidentally we've been referring to is the Parkfield likely

number of other smaller events have had this same feature of a a larger or a longer focal length than one would expect for the energy that was released in the earthquake and. They either What's that what's that piece of information based on earthquakes around the world or. California quakes so how do you relate fault likes to what kind of data do you use to relate fault like the strength of shock. Well this this is a collection of data wherever it's available to Liam. Unfortunately the samples we have of earthquakes that produce surface faulting are very few and far between. We don't often get a chance to see this kind of thing. And most of the data is on much larger earthquakes so this may be more typical than the people who thought before trying. Maybe just maybe actually more characteristic than the last. One to well limit it. We know for example that the larger it quakes we're able to determine

how much stress is released in a larger earthquake and we extrapolate that result to smaller quakes and calculate what kind of stress we should get. And then I'm saying now that the particularly quake is is unusual in that it doesn't fit those extrapolations but perhaps that perhaps it shouldn't fit the extrapolation. You also pointed out that this is a sidewise motion a strike slip fault. So that this quake in general does not do too much in changing the contour of the land around it. What how much similarity is there between this kind of quake that you are monitoring and one that has a vertical motion. Can you can you use the data you are getting here do you believe to make some degree to gain more understanding of a vertical type of motion. Yes I think so. And Senator of course is

typified by horizontal motion at most of the places where we've seen it move right down. It does have interesting things that it has a few severe kinks in it particular north of Los Angeles so by air one can see there is a rather pronounced s curve in the sand under asphalt and where is that located. This is located near Gorman I said. Now if this if the San Andreas Fault is to move horizontally it's got a kink like this in it. Something unusual has to happen at the camp and in fact what will most likely happen will be vertical motion there. So in that sense the vertical horizontal motion are often closely related. There are other fundamental differences. For example in the Alaskan earthquake in 1954 in the predominant motion there was vertical and a very large area

of thousands and thousands of square miles was uplifted. What did this actually change of the shoreline around Alaska when this happened. Yes this changed the whole face of southeast Alaska. And I after this occurred. Geologists were able to establish it exactly this same kind of thing had occurred repeatedly over the last 10 to 20000 years and were able to trace back the history of the evolution of the shoreline there. Now Alaska you know asking. That's right. And. Parts of Alaska are are being uplifted and those regions the shoreline is. I gentle and broad large flat areas. I still deal seabed right in other areas it's sinking down and these are the areas where one has the. Fiord type structure with the mountains going right down into the water.

How much of what or how much motion was there in that Alaska quake. Was quite large the many islands were uplifted by as much as 12 to 15 feet and the. Maximum vertical displacement was close to 40 feet in some places. This qualifies as really a great earthquake and a significant change in the evolution of the face of the earth from this one event 50 times more than the motion that you have so far recorded in the right when you've been looking at right now of course there are maybe fifty times and maybe fifty times more earthquakes of this smaller size than the large was. Sure. It's hard to say we think however that the major evolution of the earth's surface is being controlled by the large events at large in the Alaskan quake of aftershocks which also continue the same ocean. Yes keep the details of how the motion went on are not nearly as well known but the sea level makes a very effective way

of. I stabbed machine with a vertical motion is going because in every place where people live by the sea they they watch the tides very closely and they know where the shore is tell you whether things are going up or down. It appears like most of the displacement occurred in Alaska during the main earthquake and not much occurred afterwards. Which is different than what we've been observing in California where you had quite a bit going on afterwards. Yes perhaps four times as much is actually creating uniquely. Do you think that your your measurements now are going to in any way enable you to make any kind of a prediction as when for example the parkin region might go again. Is there anything that's going to come out of this that will help you to estimate the build up of stress in this region. Well only in so far as we know. We think we're getting closer to an understanding of the stress fields that produce these earthquakes

but nothing that we're measuring. After an earthquake is directly related to our ability to predict one. That's something. Except in this just ical sense that there have been earthquakes and in that particular ranchers area were in the area between his farmhouse and his gas pump is cracked in one thousand or one in one hundred twenty two 934 now again 160. So you can keep a boxcar on it and I predict it will. Come out again before the turn of the century there. That's a very safe prediction. It seems like it's almost as regular as a clock in some ways. How do those from this just very briefly go back over something I've mentioned before when we started. Is there any sensible way of estimating the contribution of earthquake motion to other forms of geologic behavior that shaped the earth's surface. Well the.

The mountain ranges that we see for example California. Clearly been thrust up at some time in the past and we now think that they've been rather than have. Moved up continuously they've been pushed up sporadically and each time they're moved a small amount they are associated with a great earthquake. The the other geologic processes that are going on such as erosion and some of the things that you mentioned in beginning. Tend to ameliorate these effects of the earthquakes associated with raising up the mountain range in almost all the other facts tend to tear them down. This is this what you had in mind yes. So that the earthquake then becomes that the thing that makes the steep slopes. Yes what a stew thank you very much for joining us and telling us about how we can take a close look at how the surface of the earth is changing under our very eyes.

This was about science with host Dr. Albert Hibbs and his guest Dr. Stuart Smith join us again for our next program when Dr. Peter less a man will lead a discussion about current developments in family planning about science is produced by the California Institute of Technology and is originally broadcast by station KPCC Pasadena California. The programs are made available to the station by national educational radio. This is the national educational radio network.