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letter from Alaska
Jul. 11th, 1999 12:00 amSunday, July 11, 1999
College, Alaska
Dear Mom & Dad, Siblings, and Pets,
I received your letter which Magie picked up for me from the Post office in Cantwell. It had arrived just hours after I left, on the 21st. First, to answer your questions: Bugs? Yes, they do exist, sometimes in prodigious numbers; othertimes they merely serve as a lite backgrond radiation. Most of the time they do not live up to the reports I received before coming to Alaska. Othertimes, such as at the apty-named "Swampbuggy Lake" on the Denali highway, they cover your body, more than one per square inch - maybe even 1/cm^2. It was at swampbuggy ("swamp, buggy," maybe) lake that the BLM agent determined that our proposed digging site was virtually on top of an ancient native Alaskan burial area. We were more than happy to move elsewhere to dig, although Doug (not present at the time) was a bit perturbed by the discontinuity in our seismic line. At the cabin the mosquitos had the potential to be quite viscious, but Brad kept them at bay via an array of smoldering mosquito coils (i.e. wood chips and pyretheiums) supported atop sections of PVC pipe, creating the image of some Tibetan shrine. (Aside: across the Parks highway from our Cantwell "subdivision," the proprietors of a "trading post" kept two tibetan yaks. The Yaks butt heads whenever they needed to resolve a dispute... Throw them food, they turn to each other and butt their heads; eventually they know who's won, and the winner takes the food.)
Bears? We saw six (or so) grizzlies in Denali N.P. Some other, future interns are going to get to dig holes in Denali. We were on a scouting trip, to show the NPS where we wanted to dig, and thus initiate the 1-year approval process. Thus we got our very own VIP tour of the park, in two suburbans. Normal tourists are constrained to tour busses at a cost of $50/person. We also saw two young black bears in Yukon territory.
Our work in Cantwell consisted mainly of the digging of holes. In these holes we buried 55 gallon trashcans. Later, seismometers would be installed. How they plan on locating these buried vaults later, especially in winter, is unknown to me. I think it is unknown to them too.
BEAAR is the Broadband Experiment Across the Alaska Range. The object is to find out what's under Mt. McKinley (Denali). They're going to do that by looking at the signals (seismograms) received at the stations from teleseisms (earthquakes far away). These waves reflect from layers (interfaces), and by reconstructing the reflections you can learn things about the structure of the earth. Or something like that. (The reflected waves are called "receiver functions.") Maggie is working on that project—I'm doing something else.
The simplest methods for interpreting seismograms involve the time of arrival of the P and S waves. When an earthquake occurs, it releases P and S waves. P waves are compression waves (sound, [drawing]); S waves are sheer waves ([drawing of a transverse wiggle]). The P waves travel faster than the S waves, so the difference in arrival times between P and S waves tells you approximately how far away the event originated. For instance, Geoff said that multiplying the S-P time by 8 km/s provides a good estimate. There are charts and models for better estimates.
You can't tell anything about the direction to an earthquake with only a single instrument. With multiple instruments ther are a variety of methods which may be employed in order to solve for a direction (back azimuth).
You can determine magnitude using one station, as a function of the distance to the event (computed via S minus P times) and the maximum amplitude of the event on the seismogram.
One way to determine the direction to an event is to use three seismometers at the same place, one oriented north-south, one east-west, one vertical.
By looking at the direction of movement when the P waves come in on each element, you can form a vector which will either point towards or away from the direction from which the wave arrived (depending on whether the first thrust of the P wave was a "push" or a "pull.") Obviously the earthquake must have originated inside the earth, so it's easy to resolve this ambiguity.
The other (main) technique is simply to use multiple, widely-spaced stations. Using the time difference between P and S phases on each station, and then triangulate / use intersecting circles/spheres to solve for the point on/in the Earth that is the proper distance from each station.
My project involves seismic arrays. An array is a collection of seismometers clumped within a few kilometers of each other:
Based on the differnce in arrival times between the stations, you can determine the angle at which the wave arrives. Knowing the station locations, you can also determine the speed at which the wave is moving [propagating].
Because you're using multiple stations and some cool techniques, you can "tune" the array like a giant earthquake-detecting antenna. Arrays such as this were developed to detect nuclear explosions and are used to enforce the Comprehensive Test Ban treaty. I can routinely detect magnitude 4's from a distance of 10,000 km (an entire hemisphere of the globe).
My project is to automate this process and make it work in realtime.
That's all for now.
Tobin
College, Alaska
Dear Mom & Dad, Siblings, and Pets,
I received your letter which Magie picked up for me from the Post office in Cantwell. It had arrived just hours after I left, on the 21st. First, to answer your questions: Bugs? Yes, they do exist, sometimes in prodigious numbers; othertimes they merely serve as a lite backgrond radiation. Most of the time they do not live up to the reports I received before coming to Alaska. Othertimes, such as at the apty-named "Swampbuggy Lake" on the Denali highway, they cover your body, more than one per square inch - maybe even 1/cm^2. It was at swampbuggy ("swamp, buggy," maybe) lake that the BLM agent determined that our proposed digging site was virtually on top of an ancient native Alaskan burial area. We were more than happy to move elsewhere to dig, although Doug (not present at the time) was a bit perturbed by the discontinuity in our seismic line. At the cabin the mosquitos had the potential to be quite viscious, but Brad kept them at bay via an array of smoldering mosquito coils (i.e. wood chips and pyretheiums) supported atop sections of PVC pipe, creating the image of some Tibetan shrine. (Aside: across the Parks highway from our Cantwell "subdivision," the proprietors of a "trading post" kept two tibetan yaks. The Yaks butt heads whenever they needed to resolve a dispute... Throw them food, they turn to each other and butt their heads; eventually they know who's won, and the winner takes the food.)
Bears? We saw six (or so) grizzlies in Denali N.P. Some other, future interns are going to get to dig holes in Denali. We were on a scouting trip, to show the NPS where we wanted to dig, and thus initiate the 1-year approval process. Thus we got our very own VIP tour of the park, in two suburbans. Normal tourists are constrained to tour busses at a cost of $50/person. We also saw two young black bears in Yukon territory.
Our work in Cantwell consisted mainly of the digging of holes. In these holes we buried 55 gallon trashcans. Later, seismometers would be installed. How they plan on locating these buried vaults later, especially in winter, is unknown to me. I think it is unknown to them too.
BEAAR is the Broadband Experiment Across the Alaska Range. The object is to find out what's under Mt. McKinley (Denali). They're going to do that by looking at the signals (seismograms) received at the stations from teleseisms (earthquakes far away). These waves reflect from layers (interfaces), and by reconstructing the reflections you can learn things about the structure of the earth. Or something like that. (The reflected waves are called "receiver functions.") Maggie is working on that project—I'm doing something else.
The simplest methods for interpreting seismograms involve the time of arrival of the P and S waves. When an earthquake occurs, it releases P and S waves. P waves are compression waves (sound, [drawing]); S waves are sheer waves ([drawing of a transverse wiggle]). The P waves travel faster than the S waves, so the difference in arrival times between P and S waves tells you approximately how far away the event originated. For instance, Geoff said that multiplying the S-P time by 8 km/s provides a good estimate. There are charts and models for better estimates.
You can't tell anything about the direction to an earthquake with only a single instrument. With multiple instruments ther are a variety of methods which may be employed in order to solve for a direction (back azimuth).
You can determine magnitude using one station, as a function of the distance to the event (computed via S minus P times) and the maximum amplitude of the event on the seismogram.
One way to determine the direction to an event is to use three seismometers at the same place, one oriented north-south, one east-west, one vertical.
By looking at the direction of movement when the P waves come in on each element, you can form a vector which will either point towards or away from the direction from which the wave arrived (depending on whether the first thrust of the P wave was a "push" or a "pull.") Obviously the earthquake must have originated inside the earth, so it's easy to resolve this ambiguity.
The other (main) technique is simply to use multiple, widely-spaced stations. Using the time difference between P and S phases on each station, and then triangulate / use intersecting circles/spheres to solve for the point on/in the Earth that is the proper distance from each station.
My project involves seismic arrays. An array is a collection of seismometers clumped within a few kilometers of each other:
Based on the differnce in arrival times between the stations, you can determine the angle at which the wave arrives. Knowing the station locations, you can also determine the speed at which the wave is moving [propagating].
Because you're using multiple stations and some cool techniques, you can "tune" the array like a giant earthquake-detecting antenna. Arrays such as this were developed to detect nuclear explosions and are used to enforce the Comprehensive Test Ban treaty. I can routinely detect magnitude 4's from a distance of 10,000 km (an entire hemisphere of the globe).
My project is to automate this process and make it work in realtime.
That's all for now.
Tobin
