About science; About microquakes

This is about science produced by the California Institute of Technology and originally broadcast by station K. PPC Pasadena California. The programs are made available to this station by national educational radio. This program is about micro quakes with host Dr. Peter less a man and his guest Dr. James Brown. Here now is Dr. Leslie Mann an earthquake is an awesome and destructive phenomenon and happily something which few of us are destined to experience most of mankind is happy to regard the earth as one fixed basis and as serving as an immutable and solid foundation for the many changes wrought on its surface by man the ancients spoke comfortably of terra firma. The first fixed earth. But in actuality the mantle of our earth is continuously moving shuddering sliding shrugging like the skin of a great

beast. In response to the vast forces within its shell. Much of this seismic activity is too gentle to be noticed or felt by the human inhabitants of our globe and is not recorded even by orthodox seismographs. But these small earth movements are very important. They may be less than one millionth of the size of a destructive earthquake and they are called micro earthquakes and fairly recently scientists have started studying their effects with highly sensitive seismographs. Dr. James in Broome is a specialist in these matters. He did his undergraduate work at the University of Nevada and then obtained his Ph.D. from Columbia. He has worked as a geophysicist with the US Coast and Geodetic service and is currently associate professor of geology at the California Institute of Technology. Jim would you care to give a somewhat more

precise scientific description of a micro earthquake. Well Dear Microsoft quake is really just a small earthquake in the broadest sense and only recently in the last couple of years have we had instruments operating in the field which are sensitive enough to record these smaller earthquakes. And so a general definition might be an earthquake which we need an instrument of again of the order of a million. That is it will magnify the ground motion a million times in order to see it. And generally speaking Jim these micro earthquakes are of no possible danger to mankind that that's true. The amount of energy released in the amount of motion on a fault is so small perhaps the order of hundreds of an inch for the small ones that they can't possibly do any danger.

On the other hand I suppose their characteristics are essentially the same as those of large destructive earthquakes. This is true as far as we know micro quakes are caused by similar forces have similar source sizes and shapes and except for their size apparently are caused by the same sort of mountain building forces that cause the larger earthquakes. So in a sense I suppose they provide what chemists would cool a test tube means studying the behavior of larger earthquakes. Yes of course the hypothesis that they are the same or they are caused by the same forces as the large earthquakes is not definitely proven we have not studied them long enough to be sure. But at the present stage of our knowledge of these earthquakes they don't show any characteristics different from the larger earthquakes. In Because of this it's very possible that ultimately they may tell us something about the forces which are causing the larger earthquakes do we have any

understanding of these forces which cause real and quakes Jim. Really we don't have any proven ideas or proven theories about the cause of earthquakes. There are many theories these theories usually involve forces such as the rotation of the earth the flow of heat out of the earth or some other force within the earth that has the tremendous amount of energy necessary for large earthquakes but none of these mechanisms has really been proven. Jim do these micro earthquakes occur in any special place on the face of the earth. Yes just as large earthquakes are concentrated in belts around the Pacific and in a belt extending from Europe to the Himalayan mountains to near Formosa. So we find that micro quakes are concentrated along the same belt and.

In particular they're concentrated along large Jackie faults although not right on the fault. For example we find there are very few micro quakes in the middle of the of Canada where there are no large earthquakes there are no volcanoes on the other hand in all areas where there are volcanoes and large earthquakes we find many many micro quakes also. Now you also mention Jim that these earthquakes minor micro quakes are centered around the famous old faults of the earth. I expect that this is part of the reason that we are concerned with it so much at the California Institute of Technology because this institute is so close to the famous Senate and drinks for. Is this correct. This is correct and at present we are concentrating our studies on micro earthquakes on a Senate race fall in Southern California. We've occupied a number of sites along the San Andreas Fault extending from

halfway between San Francisco and Los Angeles down to the Mexican border and we've counted numerous micro quakes thousands of them and attempted to find out which spots along the San Andreas are now active as far as my career quakes are concerned. When you talk about thousands of these micro quakes Jim does this mean to imply that many of these may occur in a single day and given areas. Yes as we find that as the size of an earthquake is taken smaller that larger and larger numbers occur. In fact for each factor of 10 that we decrease the size of the earthquake we find that there are ten times as many of them occurring. So along the San Andreas fault if we take micro quakes which can be as much as a billion times smaller than the larger earthquakes we find that within 20 miles of any given spot along with Senate race we can expect 20 to 100 of these to occur every

day. Jim in San Andreas in such a famous old fault maybe you could say a few words about its geography and the history of this fault. Well this fault became first noted during the great 1906 earthquake in which there was a lot of property damage and destruction in San Francisco as a result of motion on this fault. The fall itself as far as we can trace it extends from Cape Mendocino. On the coast of California Northern California down through San Francisco and down down past 40 miles east of Los Angeles and into Mexico. And then it continues on into the Gulf of California and out to sea as far as we know its not known beyond there. So the San Francisco 4 earthquake occurred in many respects because San Francisco was right on top of the fault.

However we have another famous earthquake in Southern California which was the Long Beach quake. Was that related to the center and transformer. Only indirectly it evidently was caused by the same sort of forces that caused the San Andreas fault and it occurred they could along on a fault which is parallel to San Andreas. However it is not it. An earthquake along me exactly along the senators fault itself. Now this is not a dream is for Jim is there. Is there any way in which one can describe what exactly the character of this fault is. Has the earth slipped on one side of the fault in one direction or is the vertical movement or what is the character of the fort. The former is what we call a strike slip fault. And by that I mean it has predominately horizontal motion on it. That is one side of the fault moves horizontally with respect to the other but there is no vertical motion and the evidence that

we find for this type of motion varies depending on me the age of the evidence we see for example. There is evidence that the fault was moving right laterally and when I say right laterally I mean that. If for example a giant were sitting astride the fault and emotional in the fall in a right lateral sense would cause his right foot to go backwards. And you can see that this does not depend on which direction he was sitting on the fault. Another way of saying it is that it would have rotated a stick across the fault clockwise. I see and earthquakes are bigger Pradhan faults. Some of them are right handed faults and some of them a left handed Fultz Is this the case. This is right and in California we have. Also very important left lateral or left handed faults for example a guard lock big pine fault intersects the San Andreas and is left handed.

Now I was mentioning before the types of evidence we might find which would tell us that a fault had moved. For very old rocks we find as we cross a fault that two types of rocks on each side of the fault never match up on the San Andreas. We may be in Iraq on one side which was deposited under the ocean millions of years ago we step a few feet across a fault we find perhaps granite which was and chewed in a mountain range and was molten at one time and we find this repeated again and again along the fault and it's obvious that the two sides of the fall have moved and as we look at older rocks we find the displacement indicated is greater. Whereas when we look at younger rocks we find less and less displacement up until the present day we find evidence of displacement which has occurred say in the last thousand years or so or in historic time. But you are still speaking about thousands of years in your discussion

about the movement of the earth on either side of the fault. Is there anything that happens in smaller time scales I mean the matter of years or weeks for example. Could one observe that the earth had moved across the fault. Yes Peter on a shorter time scale we have the fact that some streams which are running are being deflected. One side is being deflected relative to the other by the earthquake and even on a shorter time scale. We actually find observations of displacements of the fall in the field. These occur very spectacularly during larger earthquakes for example during the 1006 earthquake. In one case a farmer's walkway to his house was shifted 15 feet to the right. That's a very very interesting I believe that there are also many aerial photographs which show farmers fences which have been shifted and shared by the motion

of the the earth. Have you ever come across any let me say direct evidence yourself of this type of motion. Join the movement to the fold. Yes it just happens in the last year since I've been studying the microscope wakes. I've had two occasions to observe such motions in the field. In both cases I was going out into the field with a set of instruments to record the small micro earthquakes and in both cases this occurred after a fairly large earthquake. And in these cases I happened to accidentally look at the road and see that the white line which was painted down the center of the road had been offset by a crack. In other words this route Jim cut almost at right angles across the Fort Hood. Yes the road cut at right angles across the fog and in one case in a fall down by the Mexican border. A large motion had occurred on this fault in one thousand forty one cars the destructive 940 Imperial

Valley earthquake. And since I had been new in California at the time I saw it as a crack in the road and I automatically assumed that all the seismologist in California would know about this but just to be safe I took a picture. And when I came back I talked to various geophysicists and lo and behold I found that this was a very new and startling thing that there was actually still some motion occurring along this crack. And so I said well I have a picture to prove it and I showed people the picture and they were somewhat skeptical. So had a leading doctor of scientific integrity did they do. Well not really. Well they certainly didn't doubt that picture they had a hard time doubting the picture. But anyway they wanted to go back down and check. So at a later date we all went back down and much to my surprise I couldn't find any evidence of the offset of the white line that I had seen before and you mean the walk on was not completely straight in the room. It appeared that way and I was quite embarrassed. However on more

detailed study of the white crack it appeared that possibly had been painted over and sure enough when we checked with the highway department we found that this was the case. They had come along and painted over the art painted over the offset about a week after I was there. So my observation was vindicated. So you of scientific integrity remained sonde and the evidence had simply been tampered with after the fact. That's an amusing. This is right and in fact now we've requested the the engineer that every time such a similar thing occurs please do not paint over the line and let us check it first. So we're using the Department of routes as I rather of course seismograph from some of our research purposes. Yes in fact it just. Recently when an earthquake occurred down there we called him up and asked him if he'd run out and look at the crack for us. You know Jim I understand that. In June 1966 there was a fairly significant earthquake in this

region. Could you elaborate on that. Yes this is this earthquake was the most recent fairly large earthquake that occurred on a section of fall about halfway between San Francisco and Los Angeles that on the Senate race for this section of the fault has had fairly large earthquakes in one thousand twenty two thousand nine hundred thirty four and now in one thousand sixty six. And this is the first time this part of the fault has moved since the United States has become intensely interested in the possibility of earthquake prediction. And so there were quite a number of people interested in this for this earthquake. Well the California Institute of Technology the Seismological Laboratory immediately went up to this does own of this earthquake and it was about four o'clock in the morning and everyone was tired when we got there but we decided at the suggestion of Dr. Clarence Allen to take a look at the road where the fault was known to cross the road. And so we went out and looked and sure enough the road was offset

about two inches at this time. And we were quite surprised but we then proceeded to set up our instruments in the area and while we were doing this we periodically check back and we found that the crack was extending itself. And as time went on the motion across across the fault which was displacing the white line in the pavement increased to about four and a half inches in about two days. Now Jim this was motion that occurred actually often the earthquake itself. This is right and this is a new result that we haven't known about before. Now when a large earthquake occurs there's generally disruption of communications. There's quite a bit of difficulty getting into the area often the police block the roads and so forth. And we really don't have any good observations immediately after an earthquake. Now this is one of the few cases where we were able to get right to the area immediately after the earthquake and we find that a great amount of the displacement occurred after the actual shaking of the

ground was over. And this may occur may have occurred on many other quakes that we don't know about. Jim talking about observations and and studies of earthquakes I expect that in this respect friends the Japanese who always seem to have bigger and better earthquakes and everyone else would have been studying these micro quakes rather extensively. Is this the case. Yes they they have been pioneers in the study of micro quakes as they have been pioneers in just about all the techniques that we use or intend to use in the prediction of earthquakes if that's possible. In fact the United States play in his model is modeled after the Japanese plant. Now there have been observations of earthquakes in Japan just about as long as any country in the world especially if we consider observations of sea level and distortions of the earth's crust which extend way back into the 16 hundreds and the

frequency of earthquakes in Japan I take it especially these micro quakes can be extremely high. Yes on the average the island of Japan the islands of Japan have a high rate of activity of earthquakes which is 10 or more times greater than we have here. In fact felled earthquakes in the region of Tokyo are daily almost daily occurrence. Not by felt earthquakes you mean something which would cause the cups and saucers to rattle in the lamps to sway. Or do you mean something we couldn't that well is somewhat weaker than that the human body is a very good sensor of such motions not of it's very poor at estimating the actual magnitude of of the ground motions however it is very sensitive in telling that there has been some sort of vibration. So people will feel earthquakes before the actual rock actually rattle dishes. But by a felled earthquake we simply mean an earthquake that is large enough to be reported by some

person. And what is what effect do these felt earthquakes the micro quakes have upon the earth itself. I mean I don't expect the ground opens up or that there are any large visible distortions in the earth. Well not for each one. For the very small earthquakes that as I mentioned earlier the energy is very small However just recently in near the town of matches here in Japan they have been having what's called an earthquake swarm where many thousands. And indeed tens of thousands of these small earthquakes are occurring and when so many occur they can add up to a lot of ground distortion. And in this case they are now in this particular swarm which started in around October 965 and reached a rate of about 600 failed earthquakes per day and several thousand recorded on instruments. The ground was

observed to start stretching between the town of Metz euro and the next town to the south about three kilometers away a kilometer is about a half a mile. And as the large numbers of micros quakes proceeded it was observed that the distance between these two towns increased and finally it had increased thirty three centimeters by the end of June of one thousand sixty six. Along with this fairly large tilts of the ground were observed these micro quakes swarms have occurred in the past in Japan and at times have preceded large earthquakes at other times have not. Honesty and big and the ground actually. How does one measure this tilting and displacement of the unit Jim. The the best technique for measuring this strain or displacement of the earth is to use the recently developed laser instruments

which are beams of very pure light. But before this there have been methods using light reflected light and various other delicate methods for measuring the distance between two points. I see and now this question of measuring the displacement of the earth or measuring the strain as I believe you refer to it technically comes zeroing in on the fascinating suggestion of is it possible in any way by measuring the strains or by measuring what is happening in the earth to get some sort of feeling about whether one can predict that an earthquake is about to occur. While there are a couple of ways that we can use the word predict here. If we talk about predicting the sight of an earthquake there is quite a body of information and data would suggest that we may be able to predict the

sites of earthquakes in other words one can say reasonably certainly that some places will have on like you to have earthquakes in other places almost on like to have them. This is correct for example. There is a fault in northern Turkey which extends several hundred kilometers and it has been found that successive sections of the fault have been breaking one after another in a nice continuous progression. In cases like this one could predict in a sense that the next earthquake would indeed start where the last one stopped and such occurrences are now being observed quite commonly However it is this is not a definite method of prediction. A Some other methods that might be used for prediction are observations of rapid distortion of the ground immediately preceding an earthquake. For example the Japanese are very optimistic about the possibility of earthquake prediction at least some of them are and they cite their long historic record of ground

distortion preceding earthquakes. For example the saga Wirth quake of 18 to the land rose one about three feet just prior to the earthquake four hours before the earthquake and in the Hamada earthquake of 1872. The land rose six feet. Only 30 minutes before the earthquake. Now unfortunately there are many other large earthquakes when we don't know whether the land changed its level or not. And there are cases as I mentioned net zero where the ground has changed its level and there has not been a major earthquake. Perhaps there will be but there have been cases where such distortions were not followed by earthquakes so the prediction is not definite. So the Japanese definitely have a plan and a program aimed at predicting earthquakes. Do we have anything like that in the United States Jim. Well just recently in the last year a group of geophysicists have made a proposal to the government. To

initiate a ten year plan very similar to the Japanese This plan would spend some hundred million dollars or more in one instrument. The sections of the fall which are potentially dangerous for example the San Andreas fault in California and hopefully pinned down in a little more detail some of these ground motions that precede large earthquakes. So it is possible that we might be able to get some advance warning of the earthquake. It is possible. Unfortunately we don't have the crucial measurements of how strained microburst quakes tilt of the ground and various other things immediately preceding a large earthquake and these large earthquakes are so rare that we have to if we want to carry this planet out we have to instrument the faults in preparation for the next big earthquake so that we can measure all these parameters immediately before the earthquake.

And yet earthquakes don't do large earthquakes don't occur very frequently in history do they. No. For example in the San Andreas fault it is roughly true that any given section of the fault will have a large earthquake on it about once every hundred years or perhaps once every 500 years. As we go to smaller earthquakes we find they occur more often but the large destructive ones will occur of the order of once every hundred years and thus we have to wait a long time to find if our experiment will work. None the less I imagine most people think that it is a worthwhile experiment to do. Most Jew physicists feel that it is a very worthwhile experiment especially since it will greatly lead to our understanding of the structure of the earth and the forces which cause defamation of the earth. And even a slight possibility that we could somehow reduce the millions of dollars of damage and the yearly

loss of life that occurs from earthquakes throughout the world is enough to give us incentive to study these faults more in more detail. So it definitely does seem like an entirely worthwhile thing and thus we see that the study of micro quakes presents the possibility only the possibility however but a very real one of rapidly obtaining a picture of the seismic character of an area. It gives us a means of understanding the behavior of large destructive seismic activity. And there is a splendid chance that this may give us the beginnings of our way towards the prediction of major earthquakes. Thank you Jim. This was about science with host Dr. Peter lesson and his guest Dr. James Bone of the California Institute of Technology. Join us again for our next program when two more members of the Cal Tech faculty will discuss a

subject of interest about science is produced by the California Institute of Technology and is originally broadcast by station KPP C. Pasadena California. The programs are made available to the station by national educational radio. This is the national educational radio network.