In today's activity we will revisit some ideas about frequency content and filtering using some real seismic data, including how continuous ground vibrations are digitized with a particular sampling rate when they are recorded into computer memory.
\nFor this activity, we will revisit the September 12, 2007 Magnitude 8.5 earthquake in southern Sumatra that we examined in the earlier Earthquake Source Directivity assignment. We will learn how to request that data with the dataselect web service and then work on filtering the retrieved seismogram in SAC.
\nYou can start with finding the details about this earthquake using the event web service. You can use the URL Builder like we did in the last assignment or you can modify the URL you used last time manually. The key information you will need to use to search for this event is the date (search from September 12 to September 13) and the magnitude (try a minimum magnitude of 5). What is the exact day and time information for this earthquake?
{ =2007-09-12T11\:10\:26 ~2007-09-12T23\:49\:04#This is the time of a different earthquake, not the magnitude 8.5 event. If you are having trouble getting the web service to work, this is the link that I used https\://service.iris.edu/fdsnws/event/1/query?starttime\=2007-09-12T00\:00\:00&endtime\=2007-09-13T00\:00\:00&minmag\=5 ~2007-09-12T01\:55\:40#This is the time of a different earthquake, not the magnitude 8.5 event. If you are having trouble getting the web service to work, this is the link that I used https\://service.iris.edu/fdsnws/event/1/query?starttime\=2007-09-12T00\:00\:00&endtime\=2007-09-13T00\:00\:00&minmag\=5 ~2007-09-12T11\:23\:09#This is the time of a different earthquake, not the magnitude 8.5 event. If you are having trouble getting the web service to work, this is the link that I used https\://service.iris.edu/fdsnws/event/1/query?starttime\=2007-09-12T00\:00\:00&endtime\=2007-09-13T00\:00\:00&minmag\=5 ~2007-09-12T14\:40\:07#This is the time of a different earthquake, not the magnitude 8.5 event. If you are having trouble getting the web service to work, this is the link that I used https\://service.iris.edu/fdsnws/event/1/query?starttime\=2007-09-12T00\:00\:00&endtime\=2007-09-13T00\:00\:00&minmag\=5 } // question: 81081 name: IRISDMC.Tutor.3.Filt.02 ::IRISDMC.Tutor.3.Filt.02::Based on this information, how many aftershocks magnitude 5 and larger occurred on the same day as the mainshock?{# =%100%17:0# =%50%18:0#A bit too large. You may have counted the mainshock, the header line, or an event from a different region. =%25%19.5:0.5#Too large. You may have counted the mainshock, the header line, or an event from a different region. =%0%500010:499989#Too large. I think there is something wrong with your request if you see this many events. This is the link that I used https\://service.iris.edu/fdsnws/event/1/query?starttime\=2007-09-12T00\:00\:00&endtime\=2007-09-13T00\:00\:00&minmag\=5 =%50%16:0#A bit too small. You may have miscounted. =%25%15:0#Too small. You may have miscounted. =%0%7:7#Too small. I think there is something wrong with your request if you see this many events. This is the link that I used https\://service.iris.edu/fdsnws/event/1/query?starttime\=2007-09-12T00\:00\:00&endtime\=2007-09-13T00\:00\:00&minmag\=5 } // question: 81082 name: IRISDMC.Tutor.3.Filt.03 ::IRISDMC.Tutor.3.Filt.03::What was the magnitude of the largest aftershock on the same day as the mainshock?{# =%100%7.9:0.5# =%0%500003.45:499995.55#Too large. If you are having trouble getting the web service to work, this is the link that I used https\://service.iris.edu/fdsnws/event/1/query?starttime\=2007-09-12T00\:00\:00&endtime\=2007-09-13T00\:00\:00&minmag\=5 =%0%4.3:3.6#Too small. If you are having trouble getting the web service to work, this is the link that I used https\://service.iris.edu/fdsnws/event/1/query?starttime\=2007-09-12T00\:00\:00&endtime\=2007-09-13T00\:00\:00&minmag\=5 } // question: 81083 name: IRISDMC.Tutor.3.Filt.04 ::IRISDMC.Tutor.3.Filt.04::[html]One of the stations we looked at during the Earthquake Source Directivity assignment was SUR in Sutherland, South Africa. In order to request data from this station, you will need to know the network and location code of this instrument. You can find this information with the station web service. Enter the formatted day and time information from the previous question for both the start and end time, and then SUR in for the name. What is the 2-character network code for this station?
{ =%100%II# =%50%SY#This network is for synthetic seismograms calculated for the SUR site location. It is not the network that contains recorded data at the SUR site.
} // question: 81084 name: IRISDMC.Tutor.3.Filt.05 ::IRISDMC.Tutor.3.Filt.05::Which of the following location codes are available for recorded seismometer data (not synthetic seismograms)?{ ~%50%00#1 of 2 correct answers. This is for a STS-1 seismometer, often considered to be the best instrument available. You will want to use this location code when requesting data. ~%50%10#1 of 2 correct answers. This is for a STS-2 seismometer, often considered to be the next best thing relative to a STS-1. ~%-25%S1#This is a synthetic seismogram location code. If you are having trouble getting the web service to work, this is the link that I used https\://service.iris.edu/fdsnws/station/1/query?sta\=SUR&starttime\=2007-09-12T11\:10\:26+&endtime\=2007-09-12T11\:10\:26+&level\=channel&format\=text ~%-25%S3#This is a synthetic seismogram location code. If you are having trouble getting the web service to work, this is the link that I used https\://service.iris.edu/fdsnws/station/1/query?sta\=SUR&starttime\=2007-09-12T11\:10\:26+&endtime\=2007-09-12T11\:10\:26+&level\=channel&format\=text } // question: 81085 name: IRISDMC.Tutor.3.Filt.06 ::IRISDMC.Tutor.3.Filt.06::When you request data, you will want to request the 3-character channel for broadband, high-gain, vertical component data. Which channel available for station SUR would match this?{ =%100%BHZ# } // question: 81086 name: IRISDMC.Tutor.3.Filt.07 ::IRISDMC.Tutor.3.Filt.07::[html]You now have almost enough information to use the dataselect web service. However, we need to determine the start and end time of the seismogram we are requesting. I will ask that you request data from 640 seconds after the origin time to 830 seconds after the origin time to capture the P wave that we examined to determine the rupture directivity. What is the exact day and time information for the start time you will input to dataselect?
{ =%100%2007-09-12T11\:21\:06# =%0%2007-09-12T11\:10\:26#This is the origin time of the earthquake. You need to add 640 seconds to this time. =%0%2007-09-12T10\:59\:14#You need to ADD 640 seconds to the origin time, not subtract.
} // question: 81087 name: IRISDMC.Tutor.3.Filt.08 ::IRISDMC.Tutor.3.Filt.08::What is the exact day and time information for the end time you will input to dataselect?{ =%100%2007-09-12T11\:24\:16# =%0%2007-09-12T11\:21\:06#This is the start time I requested. You need to add 830 seconds to the origin time. =%0%2007-09-12T11\:10\:26#This is the origin time of the earthquake. You need to add 830 seconds to this time. } // question: 81088 name: IRISDMC.Tutor.3.Filt.09 ::IRISDMC.Tutor.3.Filt.09::The last piece of information needed to make dataselect request is the data Format. Which option would allow you to receive the data in a format for use in SAC?{ =%100%*SAC*zip*# =%100%*zip*SAC*# =%0%*#[html]The dataselect web service explanation page lists the different options, and the URL Builder page for dataselect should have a drop-down box for the different format options. } // question: 81176 name: IRISDMC.Tutor.3.Filt.10 ::IRISDMC.Tutor.3.Filt.10::[html]Before we request the data, I would suggest you create a directory called irisdmc inside your home directory, and then create another directory called filt inside the irisdmc directory. What command should ensure that you are in the new filt directory?
{ =cd \~/irisdmc/filt
~cd \~/irisdmc/
#You want to be inside of the filt directory, not the irisdmc directory.
~cd \~/
#You want to be inside of the filt directory that is inside your irisdmc directory.
~cd \~/filt
#The filt directory should be inside the irisdmc directory, not your home directory
} // question: 81089 name: IRISDMC.Tutor.3.Filt.11 ::IRISDMC.Tutor.3.Filt.11::[html]Now you are ready to create the link for the dataselect request. You are welcome to use the URL Builder for dataselect, but note that you will need to use the link you build on your linux virtual machine, not in your regular web browser. The reason is because submitting the link will result in data being immediately sent back to the application you submit it from. We do not want the data to return to your web browser, we want the data on the linux virtual machine so we can open it up in SAC.
\nAt the command line of the linux machine, you can use the command wget to retrieve the output of the http link. For this case, you should use the wget command with this format\:
\n[seismo@MUL16-VirtualBox filt] wget "link" -O sac.zip
\nwhere the link you build with the URL Builder has quotation marks around it. The -O option tells wget to send the output to a file, sac.zip in this case. Is this the correct link to use? (the O is a Capital o)
\nhttp\://service.iris.edu/fdsnws/dataselect/1/query?net\=II&sta\=SUR&loc\=00&cha\=BHZ&starttime\=2007-09-12T11\:21\:06&endtime\=2007-09-12T11\:24\:16&format\=sac.zip&nodata\=404
Once the sac.zip file downloads, you will need to unpack the SAC file using this command\:
\n[seismo@MUL16-VirtualBox filt] unzip sac.zip
\nand then
\n[seismo@MUL16-VirtualBox filt] ls *SAC
\nWhat is the name of the file unpacked by unzip?
{ =%100%II.SUR.00.BHZ.M.2007.255.112106.SAC*# =%75%II.SUR.00.BHZ.M.2007.255.*.SAC#Close, but it appears you had the hour, minute, or second wrong when requesting the data.
=%100%II.SUR.10.BHZ.M.2007.255.112106.SAC#Ok, but it appears you had the wrong location code when requesting the data - it should be 00 instead of 10.
=%0%*#It would help if you could copy and paste the file name into this answer.
} // question: 80821 name: SAC.Tutorial.9.1.03 ::SAC.Tutorial.9.1.03::[html]First, let's make sure the .SAC files from a previous tutorial are in the rupture directory.
\n[seismo@MUL16-VirtualBox rupture] ls *.SAC
\nYou should see multiple .SAC files from a previous tutorial. If you do not, you can run the following command to copy them to the rupture directory.
\n[seismo@MUL16-VirtualBox rupture] cp /usr/local/database/seismograms/*.SAC .
\nOnce you are certain the .SAC files are in the rupture directory, go ahead and start sac, and then read the data file for station SUR. Which of the following commands would read the seismograms for station SUR?
{ =r *SUR*.SAC
#Correct, please run this command if you have not already.
~r SUR.SAC
#There is no file named SUR.SAC so please use the
\nlscommand to list the files and see what their filenames are. ~r SUR*.SAC
#There is no file that begins with SUR and end in .SAC so please use the
\nlscommand to list the files and see what their filenames are. ~r *SUR.SAC
#There is no file that ends in SUR.SAC so please use the
\nlscommand to list the files and see what their filenames are. ~r *SUR*.SAC.*
#There is no file that has .SAC. in the file name so please use the
\nlscommand to list the files and see what their filenames are. } // question: 80815 name: SAC.Tutorial.9.1.04 ::SAC.Tutorial.9.1.04::[html]Now plot this seismogram. You should notice that the data shows the earthquake prominently, but there is also some "noise" that is present before the earthquake begins, making it more difficult to see the initiation of the earthquake on the seismogram. In this activity we will work to remove this noise, by learning how the seismogram is sampled to figure out the range of recorded frequencies, what the frequency content of the earthquake is, and then what the frequency content of the noise is. With all of that information, we will able to devise a filter to remove the noise and preserve the earthquake signal.
We will start with learning how the seismogram is sampled. Since sac data files have a variety of information stored in a "header" at the beginning of the file, we can use the LISTHDR command to see this information, which can be abbreviated to lh. Let's run this command on the data file you just read into sac. Note that since there is a pretty long list of information about this data file, you will need to hit the enter key when sac gives you the "Waiting" prompt.
SAC> lh
Hopefully you can see there is a lot of metadata contained in a SAC file about how the data was recorded. Here is a list of what the different header variables represent\:
NPTS \= number of data points
B \= begin time
E \= end time
IFTYPE \= file type
LEVEN \= evenly sampled time series
DELTA \= spacing in time of data points
IDEP \= physical unit of the data
DEPMIN \= minimum amplitude
DEPMAX \= maximum amplitude
DEPMEN \= mean amplitude
OMARKER \= event origin marker
AMARKER \= first arrival (P) marker
T0MARKER \= t0 (S) marker
KZDATE \= reference date
KZTIME \= reference time
IZTYPE \= type of reference time
KSTNM \= station name
CMPAZ \= component azimuth relative to north
CMPINC \= component "incidence angle" reletive to the vertical
STLA \= station latitude
STLO \= station longitude
STEL \= station elevation
STDP \= station depth below surface (meters)
EVLA \= event latitude
EVLO \= event longitude
EVDP \= event depth
DIST \= source receiver distance in km
AZ \= azimuth
BAZ \= back azimuth
GCARC \= great circle distance
LOVROK \= TRUE if it is okay to overwrite this file on disk
NVHDR \= Header version number. Current value is the integer 6.
SCALE \= Multiplying scale factor for dependent variable
NORID \= Origin ID (CSS 3.0)
NEVID \= Event ID (CSS 3.0)
NWFID \= Waveform ID (CSS 3.0)
LPSPOL \= TRUE if station components have a positive polarity (left-hand rule)
LCALDA \= TRUE if DIST, AZ, BAZ, and GCARC are to be calculated from station and event coordinates
KCMPNM \= Component name
KNETWK \= Network name
MAG \= Event magnitude
Using the output of your LISTHDR command, what is the spacing between data points in the seismogram you read into SAC? Please include a unit.
{# =%100%0.05:0# =%0%0.005:0#An order of magnitude too small =%0%0.5:0#An order of magnitude too large =%0%5:0#Two orders of magnitude too large =%0%0.025:0.025#Too small. You need to find the DELTA value in the LISTHDR command output. =%0%4999.525:4999.475#Too large. You need to find the DELTA value in the LISTHDR command output. } // question: 80566 name: SAC.Tutorial.9.1.05 ::SAC.Tutorial.9.1.05::[html]Based on the answer to the previous question, what is the sampling rate (number of samples per second)?
{# =%100%20:0# =%100%20.000012:0# =%0%10:10#Too small. The sampling rate is 1/DELTA.
=%0%5009.5:4989.499988#Too large. The sampling rate is 1/DELTA.
} // question: 80702 name: SAC.Tutorial.9.1.06 ::SAC.Tutorial.9.1.06::[html]What is the maximum frequency one can observe is this data? This is often referred to as the Nyquist frequency. If you are unfamiliar with the electronics term Nyquist frequency, you can read about how it is determined here\: http\://mathworld.wolfram.com/NyquistFrequency.html
{# =%100%10:0# =%100%10.000006:0# =%0%5:5#Too small. The Nyquist frequency is the sample rate divided by two.
=%0%50004.5:49994.499994#Too large. The Nyquist frequency is the sample rate divided by two.
} // question: 80568 name: SAC.Tutorial.9.1.07 ::SAC.Tutorial.9.1.07::[html]What is the unit for this frequency value?{ =Hz ~seconds#Frequency is samples per second. ~samples#Frequency is samples per second. ~cycles#Frequency is cycles per second. ~nanometers#This is a unit for spatial distance. } // question: 80819 name: SAC.Tutorial.9.2.01 ::SAC.Tutorial.9.2.01::[html]
Next we will examine the frequency content of this earthquake recording. Recall from our earlier SAC tutorial on frequency content that we use FFT command to perform the fast-fourier transform. Then the time series will be converted to the frequency domain, so the regular PLOT command will not work. Instead, we will use the PLOTSP to plot the frequency spectrum (abbreviation PSP). For this activity, we can use the AM and LOGLOG options to just plot the amplitude part of the frequency component and do a logarithmic X-axis and Y-axis.
SAC> fft
SAC> psp am loglog
What is the general trend in the amplitude vs. frequency plot?{ =Decreasing amplitude with increasing frequency ~Increasing amplitude with increasing frequency#You should see the Y-values steadily decrease as the X-values increase. ~Increasing amplitude with increasing frequency, then decreasing amplitude with increasing frequency#You should see the Y-values steadily decrease as the X-values increase. ~Decreasing amplitude with increasing frequency, then increasing amplitude with increasing frequency#You should see the Y-values steadily decrease as the X-values increase. ~Essentially no change in amplitude with increasing frequency#You should definitely see a change in amplitude with increasing frequency. } // question: 80569 name: SAC.Tutorial.9.2.02 ::SAC.Tutorial.9.2.02::[html]
At what frequencies do we see the largest amplitudes?{ ~.001-.01#Too small. There are no amplitudes shown for this frequency range. =.01-.1 ~.1-1#Too large. The amplitudes shown for this frequency range are not the largest. Please review your answer to the previous question. ~1-10#Too large. The amplitudes shown for this frequency range are not the largest. Please review your answer to the previous question. ~10-100#Too large. The amplitudes shown for this frequency range are not the largest. Please review your answer to the previous question. } // question: 80570 name: SAC.Tutorial.9.2.03 ::SAC.Tutorial.9.2.03::[html]
How big do the amplitudes get?{ =100,000 – 1,000,000 ~10,000 – 100,000#Not quite. Note that the amplitude versus frequency plot should have shown the amplitudes reach between t10^5 and 10^6 powers. ~1,000 – 10,000#Too small. Note that the amplitude versus frequency plot should have shown the amplitudes reach between 10^5 and 10^6 powers. ~100 – 1,000#Too small. Note that the amplitude versus frequency plot should have shown the amplitudes reach between 10^5 and 10^6 powers. ~10 – 100#Too small. Note that the amplitude versus frequency plot should have shown the amplitudes reach between 10^5 and 10^6 powers. ~1 – 10#Too small. Note that the amplitude versus frequency plot should have shown the amplitudes reach between 10^5 and 10^6 powers. } // question: 80571 name: SAC.Tutorial.9.2.04 ::SAC.Tutorial.9.2.04::[html]
How small do the amplitudes get?
{ ~100,000 – 1,000,000
#Too big. Note that the amplitude versus frequency plot should have shown the amplitudes get down between 10^2 and 10^3 powers.
~10,000 – 100,000
#Too big. Note that the amplitude versus frequency plot should have shown the amplitudes get down between 10^2 and 10^3 powers.
~1,000 – 10,000
#Too big. Note that the amplitude versus frequency plot should have shown the amplitudes get down between 10^2 and 10^3 powers.
=100 – 1,000
~10 – 100
#Too small. Note that the amplitude versus frequency plot should have shown the amplitudes get down between 10^2 and 10^3 powers.
~1 – 10
#Too small. Note that the amplitude versus frequency plot should have shown the amplitudes get down between 10^2 and 10^3 powers.
} // question: 80572 name: SAC.Tutorial.9.2.05 ::SAC.Tutorial.9.2.05::[html]How might this frequency pattern differ from a smaller earthquake?{ =A smaller earthquake would have the largest amplitudes at higher frequencies ~A smaller earthquake would have the largest amplitudes at lower frequencies#Smaller earthquakes tends to have more high frequency energy and less low frequency energy. ~A smaller earthquake would have the largest amplitudes at the same frequencies#Smaller earthquakes tends to have more high frequency energy and less low frequency energy. ~It is impossible to predict whether the frequency pattern would differ because it depends entirely on the roughness of the fault.#While fault roughness can have an effect on frequency content, we can reasonably predict how the largest amplitudes would change for a smaller earthquake. } // question: 80820 name: SAC.Tutorial.9.3.01 ::SAC.Tutorial.9.3.01::[html]
Our next task is to figure out the frequency content of the noise that is recorded before the earthquake on the seismogram. To isolate this part of the signal, we will use the CUT command to restrict the data to 30 seconds before the earthquake (A marks the first arrival of the earthquake waves) and then re-read the seismogram.
SAC> cut A -30 0
SAC> r *SUR*SAC
SAC> p
Which of the following describes this plot? Choose TWO answers that apply.{ ~%50%It looks like a sine wave ~%-25%It looks like a straight line#It should look more wavy than this. ~%-25%It looks like a boxcar function#It should look more wavy than this. ~%-25%It looks like an impulse function#It should look more wavy than this. ~%-25%There is a steep upward trend#The values at the end of the shortened seismogram should be slightly lower than those at the beginning. ~%-25%There is a steep downward trend#The values at the end of the shortened seismogram should be slightly lower than those at the beginning. ~%-25%There is a slight upward trend#The values at the end of the shortened seismogram should be slightly lower than those at the beginning. ~%50%There is a slight downward trend } // question: 80816 name: SAC.Tutorial.9.3.02 ::SAC.Tutorial.9.3.02::[html]
This trend in the shortened seismogram can unfairly influence our frequency determination, so we can use the RTREND command to remove this trend.
SAC> rtrend
SAC> p
Looking at the waves in the time domain of this seismogram, we will want to make an estimate of the dominant frequency of the noise. You may find it easiest to measure the period from peak to peak. What is the dominant period? Please make sure to include a unit.{# =%100%5:1# =%50%3.5:0.5#Close, but too small =%50%6.5:0.5#Close, but too large =%0%1.5:1.5#Too small. Please measure the time on the x-axis between one peak and the next peak. =%0%5003:4996#Too large. Please measure the time on the x-axis between one peak and the next peak. } // question: 80574 name: SAC.Tutorial.9.3.03 ::SAC.Tutorial.9.3.03::[html]
Now convert the dominant period of the noise from the previous question to the dominant frequency of the noise. Please make sure to include a unit.{# =%100%0.208:0.042# =%50%0.292:0.042#Close, but too large =%50%0.153:0.013#Close, but too small =%0%0.07:0.07#Too small. Frequency is 1/period. =%0%4999.625:4999.375#Too small. Frequency is 1/period. } // question: 80575 name: SAC.Tutorial.9.3.04 ::SAC.Tutorial.9.3.04::[html]
After making a time domain estimate, convert the time series to the frequency domain. Which command do we use to do that?{ =%100%fft# =%0%*#For a reminder of how we convert to the frequency domain, please see the earlier question that converted the shortened noise seismogram to the frequency domain to determine the dominant noise frequency. } // question: 80576 name: SAC.Tutorial.9.3.05 ::SAC.Tutorial.9.3.05::[html]
Now you should plot the results. Which command and options do we use to do that?{ ~plot am loglog#Since we have converted to the frequency domain with the fft command, the plot command will not work. ~psp am linlog#You need the plot to show log values on both axes because there are very large ranges of both X and Y values. ~plot am linlog#Since we have converted to the frequency domain with the fft command, the plot command will not work. ~psp am loglin#You need the plot to show log values on both axes because there are very large ranges of both X and Y values. ~plot am loglin#Since we have converted to the frequency domain with the fft command, the plot command will not work. =psp am loglog } // question: 80577 name: SAC.Tutorial.9.3.06 ::SAC.Tutorial.9.3.06::[html]
At what frequency do you see the largest amplitudes? Please make sure to include a unit.{# =%100%0.1655:0.0345#Correct =%50%0.115:0.015#Close but too small =%50%0.25:0.05#Close by too large =%0%0.01655:0.00345#You are an order of magnitude too small =%0%1.5:0.5#You are an order of magnitude too small =%0%0.0655:0.0655#Too small =%0%4999.6:4999.4#Too large } // question: 80578 name: SAC.Tutorial.9.3.07 ::SAC.Tutorial.9.3.07::[html]
How does the dominant noise frequency seen in the frequency domain (amplitude versus frequency plot) compare to the frequency you estimated in the time domain (seismogram peak-to-peak times)?{ =The frequency domain estimate was about the same as the time domain estimate ~The frequency domain estimate was about three times larger than the time domain estimate#Please compare the numbers again. ~The frequency domain estimate was about three times smaller than the time domain estimate#Please compare the numbers again. ~The frequency domain estimate was about ten times larger than the time domain estimate#Please compare the numbers again. ~The frequency domain estimate was about ten times smaller than the time domain estimate#Please compare the numbers again. } // question: 80579 name: SAC.Tutorial.9.3.08 ::SAC.Tutorial.9.3.08::[html]
How does the dominant noise frequency compare to the dominant earthquake frequency we found in the last section of this assignment?{ =The dominant noise frequency is higher than the dominant earthquake frequency ~The dominant noise frequency is lower than the dominant earthquake frequency#You should have found that the dominant earthquake frequency is between 0.01 and 0.1, which is not higher than the dominant noise frequency. ~The dominant noise frequency is about the same as with the dominant earthquake frequency#You should have found that the dominant earthquake frequency is between 0.01 and 0.1, which is not the same as the dominant noise frequency. } // question: 80817 name: SAC.Tutorial.9.3.09 ::SAC.Tutorial.9.3.09::[html]
Before we move on to the next section, please make sure that you stop the CUT function from trimming the data to before the earthquake. In other words, we want to make sure that we can read in the entire seismogram that has the earthquake in it and the time frame before it. You may want to use the following to help figure it out\:
SAC> help cut
Which command would prevent the cut function from operating on any new data read into SAC?
{ =cut off ~cut stop#This is not an option for the cut command. ~read all *SUR*SAC#You need to use the cut command, not the read command. ~read no cut *SUR*SAC#You need to use the cut command, not the read command. ~cut a 0 100#With this command, cut will still operate on new data read into SAC. } // question: 80588 name: SAC.Tutorial.9.4.01 ::SAC.Tutorial.9.4.01::[html]Now that you have an idea what the frequency content of the earthquake and the noise are, we should be able to construct a filter that can remove the noise and preserve the earthquake signal. We will use the BANDPASS command to perform the filtering, which can be abbreviated bp. We then need to specify the frequency corners for this filter, which means the beginning and the end of the frequency band that we want to pass through the filter. Frequencies less the first corner and greater than the second corner will be restricted. Which of the following is true about the filter we want to construct? Choose all that apply.{ ~%50%The first filter corner should be at the bottom edge of the dominant earthquake frequencies ~%50%The second filter corner should be between the dominant earthquake frequencies and the dominant noise frequencies ~%-33.33333%The second filter corner should be at the bottom edge of the dominant earthquake frequencies#The second filter corner should be larger than the first, so it should be at the top edge of the dominant earthquake frequencies. ~%-33.33333%The first filter corner should be between the dominant earthquake frequencies and the dominant noise frequencies#In this case, the noise frequencies are larger than the earthquake frequencies, so the first filter corner will not be between them. ~%-33.33333%The first filter corner should be at the top edge of the dominant earthquake frequencies#The first filter corner should be smaller than the second, so it should be at the bottom edge of the dominant earthquake frequencies. } // question: 80818 name: SAC.Tutorial.9.4.02 ::SAC.Tutorial.9.4.02::[html]
To properly filter the seismogram and compare it to the original, we need to re-read the seismogram, apply the filter (but you need to specify numbers for FREQ1 and FREQ2), and then read the original seismogram back in, and finally plot the filtered (at the top) and original (at the bottom) seismograms for comparison. Here are the commands you should use\:
SAC> r *SUR*SAC
SAC> bp corner FREQ1 FREQ2
SAC> r more *SUR*SAC
SAC> p1
Using your answers to earlier questions, which filter should be try if we want to focus on preserving the dominant earthquake frequencies and removing the dominant noise frequencies?{ =bp corner 0.01 0.1 ~bp corner 0.16 0.25#This would filter everything except the dominant noise frequencies. You want to filter everything except the dominant earthquake frequencies. ~bp corner 0.1 1#This filter would focus around the dominant noise frequencies. You want a filter hat would focus around the dominant earthquake frequencies. ~bp corner 0.001 0.01#This filter would focus below the dominant earthquake frequencies. ~bp corner 1 10#This filter would focus above the dominant noise frequencies. You want a filter hat would focus around the dominant earthquake frequencies. } // question: 80583 name: SAC.Tutorial.9.4.04 ::SAC.Tutorial.9.4.04::[html]
Using the series of commands from two questions ago, re-read the seismogram, apply the filter from the previous question, and then read the original seismogram back in, and plot the seismograms for comparison. How do the filtered and original seismograms compare for this case?{ ~The filtered seismogram is about the same as the original seismogram#With this narrower range of FREQ1 and FREQ2 values for the filter, the noise level should change. =The filtered seismogram has smaller noise relative to the earthquake signal ~The filtered seismogram has larger noise relative to the earthquake signal#With this narrower range of FREQ1 and FREQ2 values for the filter focusing on the earthquake, the noise level should not increase. ~The filtered seismogram has a smaller earthquake signal relative to the noise#With this narrower range of FREQ1 and FREQ2 values for the filter focusing on the earthquake, the earthquake signals should not decrease. } // question: 80584 name: SAC.Tutorial.9.4.05 ::SAC.Tutorial.9.4.05::[html]
Using the same sequence of commands but with different FREQ1 and FREQ2 values, I would like you to experiment with the frequency range in your filter. Which of the following reduces the noise the most relative to the earthquake signal to improve our ability to pick the first arrival time?{ =bp corner .01 .03 ~bp corner .01 .06#Close, but there is still some noise near the first arrival time that would make it hard to see when the earthquake signals start. ~bp corner .01 .09#There is still some noise near the first arrival time that would make it hard to see when the earthquake signals start. ~bp corner .01 .12#There is still some noise near the first arrival time that would make it hard to see when the earthquake signals start. ~bp corner .01 .15#There is still some noise near the first arrival time that would make it hard to see when the earthquake signals start. ~bp corner .01 .18#There is still some noise near the first arrival time that would make it hard to see when the earthquake signals start. } // question: 80833 name: SAC.Tutorial.9.4.06 ::SAC.Tutorial.9.4.06::[html]
How does the filter that was the correct answer to the previous affect the earthquake signal?
{ =It becomes smoothed out.
~It becomes noisier.
~It did not change.
~There is no longer an earthquake signal.
}