The AS-1 Seismograph – Operation, Filtering, S-P Distance Calculation, and Ideas for Classroom Use 1   L. Braile, November, 2002; Updated November, 2004 http://web.ics.purdue.edu/~braile

Examples of Recorded Seismograms and Filtering of Seismograms:  The AS-1 seismometer (Figure 1) has a natural period of oscillation of about 1 s.  To make the seismometer more useful for recording local as well as distant earthquakes, the amplifier unit supplied with the AS-1 uses strong filtering to enhance the long period (greater than 10 s period) response of the instrument.  The combined response of the seismograph is more broadband and can record signals ranging from about 30 s period to 2 Hz (0.5 s period).  Local and regional (within about 1200 km or 10 degrees distance from the seismograph location) earthquake seismograms are usually dominated by short period (near 1 s period) energy.  Teleseismic (distant earthquakes, 10 to 180 degrees) recordings usually consist of lower frequency (longer period) energy and often have prominent surface wave arrivals of 12 – 30 s period.  Microseisms are a nearly universal source of ground noise in seismic recording and have periods of about 3 – 8 s.  Because this period range is amplified strongly by the AS-1 seismograph, filtering of the records is often desirable to enhance the earthquake signals.  A useful filter for local and regional events (short period) is a bandpass filter with frequency cutoffs of 0.5 Hz to 3 Hz (frequency in cycles per second, or Hertz, is equivalent to 1/Period in seconds, so these cutoffs are the same as 2 s period and 0.333 s period; in the AmaSeis control menu, after the seismogram is selected or extracted, enter the two frequencies, lowest frequency is cutoff one, highest frequency is cutoff two).  An example of use of a short period filter is shown in Figures 2 and 3 for an earthquake that was located 2.63 degrees (~292 km) from the station.

The web page for this document is:

Partial funding for this development provided by IRIS and the National Science Foundation.

ã Copyright 2002-4.  L. Braile.  Permission granted for reproduction for non-commercial uses.

Figure 1.  The AS-1 seismometer showing the main components of the instrument.  The AS-1 is a vertical component seismometer.  Up and down motions of the ground, and therefore of the base and frame of the seismometer, cause the coil to move relative to the magnet that is suspended by the spring and boom assembly.  The mass of the seismometer, consisting primarily of the magnet and the washers, tends to remain steady because of inertia when the base moves.  The motion of the coil relative to the magnet generates a small current in the coil.  The current is amplified and digitized by an amplifier unit (not shown) and connected to the computer for recording and display.  The damping (using oil in the container and a washer mounted to a bolt extending downward from the boom into the oil) reduces the tendency for the mass and spring system to oscillate for long duration from a single source of ground motion (arrival of seismic waves at the location).

Figure 2.  AS-1 seismogram recorded at West Lafayette, Indiana from an earthquake located near Evansville, Indiana, December 7, 2000.  The earthquake epicenter was about 292 km away from the seismograph and had a magnitude of about 3.9 (mbLg).  Microseisms of about 3-6 second period are visible before the first arrival (the compressional or P wave) that is located at about 1.1 minutes (relative time).  The S (Shear) wave and surface waves are the largest arrivals following the P wave.

Figure 3.  Seismogram for the Evansville earthquake (Figure 2) after filtering (0.5 – 3 Hz cutoffs) to enhance the high frequency energy of the earthquake signal.  The P, and S and surface wave arrivals are recognized partly by the frequency contrast and are slightly enhanced, as compared to the background microseism noise and the unfiltered seismogram shown in Figure 2, on the filtered seismogram.

For teleseismic or distant earthquakes, lower frequency filtering often enhances the seismogram, especially the surface wave arrivals that are prominent for shallow focal depth (0 – 70 km depth) earthquakes.  Suggested filter cutoffs for lower frequency (longer period) signal enhancement are 0.01 Hz (cutoff one) to 0.2 Hz (cutoff two), equivalent to a 100 s to 5 s period range.  Because the seismograph has very low response at very low frequencies, the lowest frequency cutoff (0.01 Hz) is intended primarily to avoid amplitude distortion and offset of the zero level of the trace that is sometimes caused by filtering the extracted trace.  An example of the advantages of filtering seismograms to enhance the long period energy is shown in Figures 4, 5 and 6.  Figure 4 is the AmaSeis screen display for a 24 hour record (each horizontal trace is one hour long; the display is designed to look like the familiar drum recording on paper; hours scale is along the left side; minutes scale is at the bottom) including an earthquake that occurred at 11:41:47.9 (Greenwich Mean Time, GMT), August 9, 2000, in Michoacan, Mexico.  The event had a magnitude of 6.5 (surface wave magnitude, MS) and was located about 25.96 degrees (2887 km) from the seismograph at West Lafayette, Indiana.  The extracted seismogram (Figure 5) shows a close-up view of the earthquake signal.

Figure 4.  AS-1 seismograph recording for August 9, 2000, Michoacan earthquake.

Figure 5.  Extracted seismogram for the Michoacan earthquake.  Amplitude scale is in count or digital units.

The filtered seismogram (Figure 6) enhances the main arrivals and makes it much easier to determine arrival times.   Filtering using the control menu in AmaSeis can be performed more than once to further enhance the frequency range of interest.

Figure 6.  Extracted seismogram for the Michoacan earthquake after bandpass filtering (0.0001 Hz and 0.1 Hz cutoff frequencies) to enhance the long period (10,000 – 10 s) energy.  The P wave arrival (the first arriving energy is at about 3 minutes, relative time, followed about 40 s later by the PP arrival.  The PP phase is a P wave that reflects from the Earth’s surface once near the halfway distance between the epicenter and the seismograph station.  The S (shear) wave arrives at about 7.5 minutes and is visible partly because of the frequency change (lower frequencies often characterize the shear waves).  Prominent surface waves (Rayleigh waves) are visible at about 14 – 15 minutes relative time.

S-P Distance Calculation:  The AmaSeis software includes a useful tool for determining the epicenter-to-station distance from the S-P arrival time difference on seismograms.  For AS-1 seismograms extracted from a 24 hour record or saved as SAC files, or for SAC format seismograms downloaded from the internet, the AmaSeis arrival time picking tool can be used to mark the interpreted arrival times of the P and S waves (Figure 7).  Then, by selecting the travel time curve tool, the seismogram is displayed on standard travel time curves.  By moving the seismogram on the screen, the interpreted P and S times can be aligned with the arrivals on the travel time curves (Figure 8).  The S-P times can also be analyzed using standard travel time curves (Bolt, 1993, p 134; or from the internet at Once aligned, the epicenter to station distance is determined and displayed.

Figure 7.  Vertical component seismogram from GSN station KIP for the September 30, 1999 Oaxaca earthquake.  The SAC file was downloaded using the IRIS WILBER tool.  The seismogram was displayed using AmaSeis and the P and S arrival times (vertical red lines) picked.

Figure 8.  KIP seismogram from Figure 7 displayed with the travel time curve tool in AmaSeis.  In this tool, the seismogram can be moved around on the screen until the selected P and S arrival times are aligned with the travel time curves.  The epicenter-to-station distance corresponding to the observed S-P time is then determined by the position of the seismogram and the specific distance, in this case 58.72 degrees, is displayed adjacent to the distance axis.

Using seismograms from three or more stations, the location of the epicenter can be estimated by triangulation.  For example, for the September 30, 1999 Oaxaca earthquake, seismograms for stations KIP, BINY, COLA and AMNO were displayed in the AmaSeis program, the P and S arrival time picked, and the epicenter to station distances inferred.  Using an inflatable globe, circular arcs are drawn on the globe corresponding to the epicenter-to-station distance for each seismograph station (Figure 9).  When all arcs have been drawn, the approximate epicenter location is determined by the intersections of the arcs (Figure 10).  Although this triangulation method is not the most reliable or accurate technique for locating earthquakes, it is relatively easy to understand and illustrates concepts of the travel times of seismic waves at increasing distances and earthquake location methods.

Figure 9.  Using a piece of string marked at the appropriate distance in degrees (measure along the equator) and a felt pen (water soluble ink), a circular arc is drawn on an inflatable globe using the seismograph station (in this case, station AMNO) as the center of the circle.  Arcs are also drawn for other stations using the appropriate distances inferred from the S-P times.

Figure 10.  After all arcs have been drawn, the epicenter is inferred by the approximate intersection of the arcs.  Because of inaccuracies in the estimation of the S-P times and drawing the arcs on a globe, the intersections of the arcs may not be exactly a single point.  However, a reasonably  good  location is determined by this method.

Because the S wave arrival is not always prominent on a vertical component seismogram, the epicenter-to-station distance cannot always be inferred from AS-1 seismograms.  Furthermore, for seismograms from earthquakes that are located greater than 105 degrees from the station, no direct S wave arrivals are present due to the Earth’s core.  For earthquakes recorded at the West Lafayette, Indiana AS-1 station since April, 2000 (Table 1), distances inferred from the AS-1 seismograms are compared with the calculated distances (from the USGS reported epicenter to the station location) in Figure 11.  Although the distances generally correspond, the AS-1 distance determination appears to be accurate to only about +/- 4 degrees distance for distances greater than about 20 degrees.

http://web.ics.purdue.edu/~braile/edumod/as1lessons/UsingAmaSeis/UsingAmaSeis.htm.  An S minus P earthquake location exercise is available at:

Figure 11.  Comparison of calculated distances (using station location and USGS epicenter location) with distance estimated from the S-P times on seismograms recorded by the AS-1 seismograph.  Distance estimates from the S-P times were determined using the travel time curve tool in AmaSeis.  Comparing actual and AS-1/AmaSeis S-P calculated distances.  N = 75; Standard Deviation = 2.51  degrees (November, 2004).

Ideas for Classroom Use:  Operating an AS-1 seismograph in a classroom or school building encourages awareness of earthquake activity around the world and provides an opportunity for teachers and students to work with real scientific data.  There is particular interest and excitement when a local or regional event occurs and is recorded by the seismograph or when a significant distant event happens.  Students will be interested in “checking the seismograph” each day and seeing how their records and magnitude estimates compare with the seismograms recorded at other stations and official magnitudes.  The occurrence of significant earthquakes around the world (there are about 20 events per year of magnitude of 7 or greater: many of these events cause significant damage) can be used to stimulate discussion, learning and research on geography, the causes of earthquakes, propagation of seismic waves, earthquake hazards, and earthquake safety.  Some specific suggestions for classroom use are:

1.      Maintain a catalog of seismograph recording.  Students can check the seismograph every day (or at regular intervals), perform routine maintenance (check that it is operating properly and perform a time check), identify possible recorded events and enter appropriate information into the catalog.  A hand written catalog is sufficient for recording such information as time corrections and approximate times of recorded events.  A more

complete catalog can be created using a spreadsheet (see Table 1, below) with information on earthquake to station distance and magnitudes.  Distance from the earthquake epicenter can be inferred from the P and S travel times as described above (AmaSeis provides a simple tool for performing this estimation; these times will not be able to be determined for all recorded events).  Magnitudes can be estimated using the procedures described in The AS-1 Seismograph – Magnitude Determination or using the online magnitude calculator at: http://web.ics.purdue.edu/~braile/edumod/MagCalc/MagCalc.htm.  A comparison between the AS-1 distance and magnitude information can be made by checking the USGS/NEIC, IRIS event search or IRIS seismic monitor sites on the Internet.  Instructions for accessing earthquake data on the Internet are included in

http://web.ics.purdue.edu/~braile/edumod/eqdata/eqdata.htm.  All data should be written in the catalog to provide a record of activity of your seismograph station and document the comparison of distance and magnitude determinations.

Exercises that include magnitude calculations using AS-1 seismograms are available at:

Table 1.  Earthquake List -- Events recorded by AS-1 Seismograph, West Lafayette (40.44N, 86.90W) Indiana (L. Braile)

 DATE TIME(GMT) LAT LON DEP. USGS USGS USGS USGS AS-1 AS-1 AS-1 Calc. Dist AS-1 Dist Location Comments MM/DD/YY HR:MN:SS Deg. Deg. (km) mb MS Mw mbLg mb MS mbLg Deg. Deg. 01/01/00 11:22:57 46.888 -78.930 18 4.7 5.2 5.2 8.610 Quebec 05/04/00 4:21:16 -1.110 123.570 26 6.7 7.5 7.6 7.0 132.060 Salawesi 05/12/00 18:43:18 -23.550 -66.450 225 6.2 7.2 6.5 66.480 65.3 Argentina Int. depth, P,pP,PP,S 06/02/00 11:13:49 44.510 -130.080 10 5.8 5.7 6.2 5.8 5.5 31.830 32.9 Coast Oregon Good S 06/04/00 16:28:26 -4.720 102.090 33 8.0 7.6 143.490 So. Sumatera Long Surf wave train 06/21/00 0:51:47 63.980 -20.760 10 6.1 6.6 6.0 6.7 44.230 45.2 Iceland 07/11/00 1:32:29 57.370 -154.210 43 6.3 6.0 45.350 47.3 Kodiak Is., AK 08/04/00 21:13:03 48.790 142.250 10 6.3 7.1 6.1 7.1 81.080 81.6 Sakhalin Is. 08/06/00 7:27:13 28.860 139.560 394 6.3 7.3 6.3 98.660 Bonin Is. Region 08/07/00 14:33:56 -7.020 123.360 648 6.5 6.5 137.100 Banda Sea 08/09/00 11:41:48 18.200 -102.480 45 6.1 6.5 6.4 5.8 25.960 26.5 Michoacan, Mex. Good S 08/22/00 20:12:14 36.490 -91.110 10 3.9 3.6 5.180 New Madrid reg. 08/28/00 15:05:48 -4.110 127.390 16 6.5 6.8 6.8 6.6 132.440 Banda Sea 08/28/00 19:29:32 -4.120 127.030 33 6.5 6.4 6.4 132.650 Banda Sea 09/28/00 23:23:43 -0.220 -80.580 22 5.8 6.0 6.6 5.6 5.6 40.920 Coast Equador 10/02/00 2:25:31 -7.980 30.710 34 6.1 6.7 6.5 6.6 116.050 Lk Tanganyika 10/04/00 14:37:44 11.120 -62.560 110 6.1 6.3 36.280 Windward Is. 10/04/00 16:58:44 -15.420 166.910 23 6.1 6.9 6.8 6.9 112.100 Vanuatu Is. 10/25/00 9:32:24 -6.550 105.630 38 6.3 6.6 6.8 144.400 Sunda Arc 11/01/00 4:27:46 -7.910 -74.470 151 5.9 5.9 5.8 49.490 Peru-Brazil Bor. Reg. 11/06/00 11:40:29 56.300 -153.350 33 5.5 5.6 6.0 5.6 5.6 45.040 Kodiak Is. Reg. 11/16/00 4:54:56 -3.960 152.270 33 6.0 8.2 8.0 7.8 115.760 New Ireland, PNG 11/16/00 7:42:17 -5.180 152.050 33 6.2 7.8 7.8 7.3 116.020 New Ireland, PNG 11/17/00 21:01:56 -5.450 151.680 33 6.2 8.0 7.8 7.6 117.200 New Ireland, PNG 12/06/00 17:11:07 39.620 54.770 30 6.7 7.5 7.0 6.5 7.2 92.940 Turkmenestan 12/07/00 14:08:49 37.930 -87.710 11 3.9 3.7 2.630 near Evansville, IN 01/10/01 16:02:42 57.090 -153.620 33 6.2 6.8 6.8 5.8 6.4 45.070 Kodiak Is. Reg. 01/13/01 13:08:41 40.740 -125.330 5 5.3 5.2 5.6 5.7 29.040 Off coast N. California 01/13/01 17:33:31 12.830 -88.790 39 6.7 7.8 7.6 6.5 7.5 27.550 27.6 Off coast C. America 01/16/01 13:25:00 -3.970 101.660 33 6.7 6.8 6.6 142.880 So. Sumatera 01/18/01 1:40:48 25.880 -110.380 10 5.0 5.0 24.370 23.4 Gulf of California 01/23/01 5:23:37 13.920 -90.990 120 5.7 5.4 26.710 El Salvador 01/26/01 3:03:19 41.990 -80.830 5 4.2 4.1 4.800 Ohio Good S-P 01/26/01 3:16:41 23.400 70.320 24 7.1 7.9 7.7 7.8 112.990 So. India 02/13/01 14:22:08 13.650 -88.990 13 5.7 6.4 5.6 6.0 26.800 29.9 El Salvador 02/13/01 19:28:27 -5.080 102.440 36 6.7 7.2 7.1 143.710 So. Sumatera 02/17/01 20:11:28 54.250 -133.610 10 5.6 5.9 5.8 6.1 33.930 Queen Charlotte Islands 02/24/01 7:23:49 1.460 126.290 33 6.6 7.0 7.0 6.7 128.450 N. Molucca Sea 02/26/01 5:58:23 46.920 144.410 394 5.8 6.1 81.730 Sea of Okhotsk 02/28/01 18:54:33 47.150 -122.720 51 6.5 6.6 6.8 5.9 26.530 23.4 Nisqually (Seattle, WA) 03/15/01 13:02:42 -32.321 -71.492 37 6.1 5.6 6.0 ? ? 73.820 Near co Central Chile 04/09/01 9:00:57 -32.668 -73.109 11 6.1 6.3 6.7 73.900 75.3 Off co Central Chile 04/14/01 2:20:13 56.080 -119.810 10 5.3 4.7 5.3 5.2 26.620 28.5 Alberta 04/19/01 21:43:42 -7.410 155.865 17 6.0 6.6 6.7 6.7 115.420 Solomon Islands 04/21/01 17:18:57 42.925 -111.395 0 5.4 4.9 5.3 5.6 18.450 18.2 Idaho 04/29/01 21:26:55 18.736 -104.545 10 5.2 5.5 6.1 5.2 26.460 Near co Jalisco, MX 05/04/01 6:42:13 35.205 -92.194 10 4.5 4.4 4.9 6.740 7.0 Arkansas 05/20/01 4:21:44 18.816 -104.446 33 5.5 6.0 6.3 5.5 6.3 26.350 30.9 Jalisco, Mexico 05/25/01 0:00:51 44.268 148.393 33 6.1 6.7 6.7 6.2 6.3 82.090 79.0 Kuriles Dist from PP 05/25/01 5:06:11 -7.869 110.179 143 5.8 6.3 144.130 Jawa, Indonesia 06/03/01 2:41:57 -29.666 -178.633 178 6.8 7.1 109.770 107.0 Kermadec Is. Dist from PP 06/14/00 19:48:48 51.160 -179.828 18 6.0 6.3 6.5 6.2 6.3 61.490 Andreanof Is., Aleutians 06/18/01 19:56:56 -24.291 -69.173 89 5.5 5.8 5.9 66.480 N. Chile 06/19/01 9:32:25 -22.739 -67.877 147 5.5 6.1 ? 65.300 Chile-Bolivia border region 06/23/01 20:33:14 -16.265 -73.641 33 6.7 8.2 8.4 6.5 7.7 57.740 Near coast of Peru Good complete seismogram 06/24/01 1:22:53 -17.585 -71.958 33 5.4 5.5 5.5 59.370 Near coast of Peru 06/26/01 4:18:31 -17.745 -71.649 24 6.2 ? 6.8 6.2 59.600 64.1 Near coast of Peru 06/26/01 14:05:07 61.340 -140.070 10 5.8 5.8 6.0 38.130 42.6 S. Yukon territory P,PP,S,SS 06/29/01 18:35:52 -19.522 -66.254 274 5.7 6.1 6.1 62.670 S. Bolivia 07/07/01 9:38:44 -17.543 -72.077 33 6.6 7.3 7.6 6.5 6.8 59.310 68.3 Near coast of Peru 07/19/01 18:00:40 57.203 -151.036 33 5.9 5.9 5.6 43.680 Kodiak Island area 07/24/01 5:00:09 -19.448 -69.255 33 5.7 6.2 6.3 5.9 61.800 N. Chile 07/26/01 0:21:37 39.059 24.244 10 6.0 6.6 7.0 6.1 6.4 78.980 Aegean Sea 07/28/01 7:32:43 59.025 -155.116 131 5.7 5.7 6.6 6.1 45.620 43.9 S. Alaska 08/02/01 23:41:06 56.260 163.790 14 5.9 6.2 6.3 5.7 6.2 66.720 Near E co Kamchatka 08/21/01 6:52:06 -36.813 -179.575 33 6.4 7.1 7.2 7.0 114.460 E of N Is. New Zealand 08/25/01 2:02:03 7.633 -82.766 25 5.9 5.7 6.1 5.9 32.870 33.6 S. of Panama 08/28/01 6:56:09 -21.722 -70.108 66 5.8 5.9 6.2 63.790 Near co of N Chile 09/04/01 12:45:53 37.143 -104.650 5 4.0 4.1 14.230 So. Colorado 09/05/01 10:52:08 37.143 -104.618 5 4.5 4.5 14.210 So. Colorado 09/07/01 2:45:59 -13.166 97.297 10 6.2 5.8 6.0 ? 152.580 S. Indian Ocean 09/11/01 9:13:26 14.995 -91.571 161 5.0 5.7 25.680 Guatemala 09/14/01 4:45:12 48.904 -128.317 10 5.5 5.8 6.0 5.5 30.330 34.1 Off coast Vancouver Is. 09/16/01 23:20:09 48.540 -128.600 10 5.1 5.4 5.6 5.4 30.510 Off coast Vancouver Is. 10/12/01 5:02:34 52.850 -132.110 10 5.5 5.7 5.7 5.6 32.880 Queen Charlotte Islands 10/12/01 15:02:16 12.590 144.940 37 6.6 7.0 7.0 6.9 108.690 S. of Marianas Islands 10/14/01 1:10:45 -8.590 144.660 68 5.9 4.9 5.9 144.660 Jawa, Indonesia 10/17/01 11:29:09 19.313 -64.912 33 5.3 5.6 6.0 5.5 28.260 35.3 Virgin Islands 10/19/01 3:28:41 -4.043 123.933 10 6.3 7.3 7.5 6.8 134.350 Banda Sea 10/21/01 0:29:24 -36.992 179.030 33 6.8 6.7 115.500 Near E co N Is New Zeal 10/30/01 3:24:28 24.066 -109.038 10 5.3 4.9 5.5 5.4 24.740 Gulf of California 11/13/01 9:47:36 22.264 -106.949 33 5.5 5.8 6.1 5.5 24.830 Near co Cent Mexico 11/14/01 9:26:10 35.945 90.535 10 6.1 8.0 7.7 7.5 103.960 Qinghai Prov., China 11/28/01 14:32:32 15.560 -93.090 85 5.7 6.3 5.9 25.370 Near co Chiapas, Mexico 12/02/01 13:01:53 39.530 141.067 122 6.1 6.5 6.4 89.180 Honshu, Japan 12/12/01 14:02:35 -42.813 124.688 10 6.5 6.7 7.0 ? 156.340 S of Australia 12/23/01 22:52:54 -9.613 159.530 16 6.2 7.0 6.7 6.9 114.090 Solomon Islands 01/02/02 17:22:48 -17.600 167.856 10 6.3 7.5 7.1 7.1 112.680 Vanuatu Islands 01/12/02 8:26:53 28.279 -69.566 10 5.7 4.9 5.4 5.7 18.720 19.0 Off E co United States 01/16/02 23:09:52 15.502 -93.133 80 5.8 6.4 5.8 25.430 Chiapas, Mexico 01/30/02 8:42:03 18.194 -95.908 108 5.6 5.9 5.7 23.490 30.3 Vera Cruz, MX, P,pP,S 02/14/02 23:23:13 14.975 -92.470 74 5.3 5.8 5.7 25.930 Chiapas, Mexico 02/22/02 19:32:42 32.375 -115.352 10 5.3 5.5 5.7 5.4 24.200 N. Baja 03/03/02 12:08:23 36.543 70.424 256 6.6 7.3 100.610 Hindu Kush, Afganistan 03/05/02 21:16:10 6.171 124.284 31 6.3 7.2 7.2 6.9 125.470 Mindanao, Philippines 03/27/02 11:18:04 30.138 -114.070 10 4.7 5.1 24.360 Gulf of California 03/28/02 4:56:21 -21.590 -68.170 122 6.2 6.5 6.4 64.130 Chile-Bolivia border region P and pP phases 03/31/02 6:52:50 24.410 122.210 33 6.4 7.2 7.1 7.1 110.030 Taiwan PP,SS, Good 20s surf waves 04/01/02 19:59:32 -29.490 -71.070 69 6.2 5.7 6.4 6.4 71.110 71.0 Near co. Central Chile 04/18/02 5:02:47 16.950 -100.820 33 5.3 6.0 6.3 5.6 26.300 28.9 Near co. Guerrero, MX Good S-P 04/18/02 16:08:36 -27.350 -70.050 62 6.3 6.2 6.6 6.6 69.240 Near co. N. Chile not shallow, weak surf. Waves 04/18/02 17:57:23 16.800 -101.020 33 4.9 4.9 5.4 26.540 30.3 Near co. Guerrero, MX Also 4.9 event 3 min. later 04/20/02 10:50:45 44.510 -73.660 5 5.1 5.1 5.7 5.7 5.7 10.590 9.9 Plattsburgh, NY Good S-P, Large Lg arrival 04/26/02 16:06:07 13.114 144.562 85 6.6 7.0 7.1 6.9 108.520 Near Guam, Mariana Is. 05/17/02 10:40:10 48.100 -27.829 10 5.6 5.4 5.7 5.6 42.060 48.0 No. Mid Atlanatic Ridge PP-P 05/21/02 6:03:00 17.780 -81.916 10 5.3 5.0 5.7 5.5 22.990 N. of Honduras 05/25/02 5:36:31 53.930 -161.270 33 5.4 6.0 6.0 5.5 6.6 50.010 57.0 Aleutians S-P 05/28/02 4:04:22 -28.890 -66.570 21 6.1 5.7 6.0 6.3 71.510 Argentina 06/05/02 12:45:16 35.347 -36.17 10 5.5 5.1 5.7 5.5 39.890 N. Mid-Atlantic ridge 06/05/02 20:17:38 52.92 -74.42 18 4.5 5.2 4.2 15.060 N. Quebec 06/07/02 17:00:51 16.091 -96.637 33 5.5 5.0 5.6 5.8 25.740 Oaxaca, MX 06/13/02 1:27:18 -47.776 99.561 10 5.4 6.6 6.6 171.370 SE Indian Ridge 06/16/02 2:46:13 8.740 -84.026 33 5.4 6.2 6.4 5.6 31.710 Off Coast of Costa Rica 06/17/02 21:26:23 -12.581 166.349 33 6.0 6.6 6.7 6.5 110.780 Santa Cruz Islands Reg. 06/18/02 13:56:22 -30.754 -70.964 53 6.0 6.6 6.3 72.400 Chile-Argentina bor. Reg. 06/18/02 17:37:15 37.985 -87.778 5 4.4 5.0 5.3 2.590 2.8 near Evansville, IN Right lateral strike slip 06/22/02 2:58:21 35.616 49.050 10 6.3 6.4 6.5 6.3 94.110 Western Iran 06/27/02 5:50:33 -6.990 103.790 10 6.1 6.9 6.6 6.8 145.310 SW of Sumatra, Indo. 06/28/02 17:19:30 43.770 130.720 564 6.8 7.3 7.2 89.540 E Russia -NE China Bor. 07/09/02 18:40:35 43.560 -127.227 10 5.6 5.5 6.0 5.7 29.930 31.7 Off coast of Oregon 07/31/02 0:16:45 7.929 -82.793 10 6.0 6.4 6.5 6.1 32.620 35.3 S. of Panama 08/14/02 13:57:56 14.190 146.130 65 6.1 6.4 106.710 Mariana Islands 09/06/02 1:21:27 38.371 13.698 10 5.8 5.6