WEBVTT 00:00.000 --> 00:03.980 Many ways to express the extent of an earthquake are used today in 00:03.980 --> 00:06.240 newspapers, on TV and on the internet. 00:06.840 --> 00:11.280 People speak of its strength, its magnitude or the open-ended Richter 00:11.280 --> 00:11.600 scale. 00:12.780 --> 00:18.000 When Charles Francis Richter published his scale in 1935, his main 00:18.000 --> 00:21.380 goal was to better understand the seismic activity in California. 00:22.820 --> 00:26.820 He wanted to introduce a measurable quantity for earthquakes that did 00:26.820 --> 00:30.040 not describe long-distance effects, but the strength of the earthquake 00:30.040 --> 00:31.060 source itself. 00:31.680 --> 00:35.000 He called this new quantity the magnitude of the earthquake. 00:36.040 --> 00:40.380 In an interview in 1980, Richter said that he was glad that the 00:40.380 --> 00:43.140 expression open-ended is now used in newspapers. 00:44.080 --> 00:48.120 This is because since the introduction of earthquake magnitude, people 00:48.120 --> 00:51.880 had been confusing it with the closed-ended intensity scale, which 00:51.880 --> 00:54.980 describes the damage and other effects of earthquakes. 01:03.970 --> 01:05.350 Hello and welcome. 01:06.530 --> 01:10.190 In this video, I will be showing you how the strength of earthquakes 01:10.190 --> 01:13.030 was measured in the past and how it is determined today. 01:13.370 --> 01:17.050 I will show you what different types of earthquake magnitudes exist 01:17.730 --> 01:19.690 and how they differ from each other. 01:20.610 --> 01:24.570 During his research, Richter noticed that the earthquakes occurring in 01:24.570 --> 01:28.390 California differed greatly in the maximum ground motion they caused. 01:28.950 --> 01:32.530 In order to be able to describe these differences across multiple 01:32.530 --> 01:35.790 orders of magnitude, Richter used a logarithmic scale. 01:37.170 --> 01:41.170 According to this scale, the strength of the quake is calculated from 01:41.170 --> 01:45.310 the logarithm of the maximum amplitude A measured on the seismogram, a 01:45.310 --> 01:50.010 correction factor for the distance delta times 2.76 and an adjustment 01:50.010 --> 01:52.170 factor of minus 2.48. 01:53.450 --> 01:57.190 The distance factor is necessary, because the greater the distance 01:57.190 --> 02:00.310 from a quake, the smaller the ground motions experienced. 02:01.370 --> 02:05.990 For the magnitude itself, the formula means that an amplitude that is 02:05.990 --> 02:09.710 10 times higher results in a magnitude that is greater by 1. 02:10.690 --> 02:15.090 And an amplitude that is 100 times higher results in a magnitude that 02:15.090 --> 02:15.970 is greater by 2. 02:17.230 --> 02:21.470 This new physical quantity was developed and measured by Richter with 02:21.470 --> 02:25.090 a historic Wood-Anderson seismometer initially only for California. 02:25.090 --> 02:28.050 The depth of the quake was not taken into account. 02:29.950 --> 02:33.510 However, because earthquakes do not only occur in California but 02:33.510 --> 02:37.430 worldwide and other, more modern measuring instruments are used today, 02:37.670 --> 02:40.410 the Richter scale is no longer valid in a strict sense. 02:41.430 --> 02:45.790 Despite this, magnitudes for local earthquakes are calculated using a 02:45.790 --> 02:49.550 formula similar to the original one by Charles Richter and called the 02:49.550 --> 02:50.770 local magnitude ML. 02:51.550 --> 02:56.030 Today, it is calculated as the average of all stations that record an 02:56.030 --> 02:56.390 earthquake. 02:57.150 --> 03:01.050 Richter and his colleague Benno Guttenberg soon realized that the 03:01.050 --> 03:04.950 local magnitude was unable to assess earthquakes recorded globally. 03:05.730 --> 03:09.310 That is why they introduced surface wave magnitude MS. 03:10.330 --> 03:13.970 Guttenberg himself established another measurement after that, the 03:13.970 --> 03:15.610 body wave magnitude MB. 03:16.270 --> 03:20.510 In both magnitudes, the dominant period T is included in the 03:20.510 --> 03:20.850 measurement. 03:21.810 --> 03:26.630 MS is, as the name says, determined from the maximum amplitude of the 03:26.630 --> 03:31.130 surface waves at periods of approximately 20 seconds and used for 03:31.130 --> 03:32.830 earthquakes recorded worldwide. 03:34.190 --> 03:39.090 MS is calculated from the logarithm of the maximum amplitude A divided 03:39.090 --> 03:44.490 by the period T plus 1.66 times the logarithm of the distance delta 03:44.490 --> 03:46.030 plus 3.3. 03:46.510 --> 03:50.070 Correspondingly, MB is determined from the maximum amplitude of the 03:50.070 --> 03:54.230 body waves at periods of around 1 Hz and is valid for epicenter 03:54.230 --> 03:56.390 distances of up to 100°. 03:57.490 --> 03:59.190 The formula is as follows. 03:59.530 --> 04:03.990 MB equals the logarithm of the amplitude divided by the period plus an 04:03.990 --> 04:07.650 empirically determined term q, which is dependent on the earthquake 04:07.650 --> 04:09.370 depth h and the distance delta. 04:09.830 --> 04:13.850 The advantage of all these magnitude scales is the rapid determination 04:13.850 --> 04:16.290 using the maximum amplitude and the seismogram. 04:17.690 --> 04:21.450 All scales mentioned so far are not suitable for measuring the 04:21.450 --> 04:23.090 magnitudes of very large quakes. 04:23.830 --> 04:27.870 The reason for this is that from a certain magnitude onwards, the 04:27.870 --> 04:31.110 maximum amplitudes in a seismogram no longer increase. 04:31.510 --> 04:35.110 This means that a magnitude does not exceed a certain threshold value 04:35.110 --> 04:36.670 even for larger quakes. 04:37.350 --> 04:40.570 This practical upper limit of the scale is called saturation. 04:42.590 --> 04:46.350 We can see this clearly in this illustration of theoretical earthquake 04:46.350 --> 04:47.170 source spectra. 04:47.670 --> 04:50.770 Shown here are the frequency spectra of various quakes. 04:51.430 --> 04:55.110 Plotted on the horizontal axis is the frequency and on the vertical 04:55.110 --> 04:56.070 one the amplitude. 04:57.550 --> 05:01.730 For each earthquake magnitude, we see a plateau range which falls 05:01.730 --> 05:03.090 after a certain frequency. 05:04.030 --> 05:07.150 We call this frequency the cut-off frequency. 05:08.070 --> 05:12.430 With the body wave magnitude as an example, we see that for magnitudes 05:12.430 --> 05:16.310 up to around 5½, the distances between the lines are approximately 05:16.310 --> 05:16.810 equal. 05:17.310 --> 05:21.370 For surface waves, this holds true for magnitudes of up to around 8. 05:22.030 --> 05:25.530 This means that for larger amplitudes, the magnitude no longer 05:25.530 --> 05:26.050 increases. 05:26.850 --> 05:30.650 In order to solve this problem, Thomas Hengs and Hiro Kanemori 05:30.650 --> 05:34.650 developed the moment magnitude scale, which performs calculations 05:34.650 --> 05:38.610 directly from the plateau area of the source spectrum, making its 05:38.610 --> 05:39.890 frequency independent. 05:40.890 --> 05:44.390 It is based on the seismic moment, which is calculated from the shear 05:44.390 --> 05:49.410 modulus mu, the area of the rupture plane S, and the displacement d on 05:49.410 --> 05:54.410 this area and is calculated as M0 equals mu times d times S. 05:55.210 --> 05:58.890 Practically speaking, M0 can be determined from the level of the 05:58.890 --> 06:00.190 plateau in the source spectrum. 06:00.190 --> 06:07.690 Mw is then given by 2⁄3 times the logarithm of M0 in joules minus 9.1. 06:08.370 --> 06:12.610 Today, the moment magnitude Mw is determined globally for earthquakes 06:12.610 --> 06:16.350 with a magnitude greater than 5 and generally corresponds to what is 06:16.350 --> 06:19.850 described as strength, magnitude or Richter scale in the media. 06:22.570 --> 06:26.490 Hence, we see that the expression Richter scale has not only been 06:26.490 --> 06:30.550 retained for historical reasons, but is it really open-ended? 06:31.250 --> 06:34.810 Theoretically, yes, because the amplitude of the underlying source 06:34.810 --> 06:37.110 spectrum does not have a maximum limit. 06:38.350 --> 06:42.070 Practically speaking, however, the possible rupture planes on the 06:42.070 --> 06:45.950 earth and the displacement occurring along them limit the magnitude to 06:45.950 --> 06:47.410 values less than 10. 06:47.790 --> 06:53.010 The strongest earthquake ever measured occurred on May 22, 1960 in 06:53.010 --> 06:56.330 Chile and had a moment magnitude of 9.5. 06:57.630 --> 07:03.110 This corresponds to a rupture plane of 800 km by 200 km and a 07:03.110 --> 07:04.590 displacement of 21 m. 07:06.470 --> 07:10.690 Today, local networks are able to measure magnitudes less than zero. 07:11.290 --> 07:15.270 A magnitude minus 2 quake, for example, has a rupture plane measuring 07:15.270 --> 07:19.590 a good square meter or the size of an average tabletop, which is 07:19.590 --> 07:22.750 displaced by less than 1 mm during the rupture. 07:23.430 --> 07:27.590 You have now learned about magnitudes ranging from 9 to less than 07:27.590 --> 07:27.970 zero. 07:28.250 --> 07:32.670 One order of magnitude equals to a tenfold increase in amplitude. 07:33.690 --> 07:36.230 But how does the energy released behave? 07:37.110 --> 07:41.490 In this animation, you can see the energy ratios of quakes of various 07:41.490 --> 07:42.170 magnitudes. 07:43.730 --> 07:48.570 An earthquake with a magnitude that is two values higher releases 1000 07:48.570 --> 07:53.470 times as much energy, which is represented in this animation by the 07:53.470 --> 07:54.810 surface of the squares. 07:55.450 --> 08:00.190 The energy difference between magnitude 2 and 6 is a factor of 1,000 08:00.190 --> 08:00.190 ,000. 08:03.700 --> 08:08.020 In this video, I introduce to you the various earthquake scales. 08:08.600 --> 08:12.180 Local magnitudes are highly dependent on the location where they are 08:12.180 --> 08:15.860 recorded, making them difficult to compare with those from other 08:15.860 --> 08:16.280 regions. 08:16.280 --> 08:20.820 Magnitudes are typically measured using maximum amplitudes of certain 08:20.820 --> 08:21.500 wave types. 08:22.720 --> 08:26.760 The advantage is that they can be quickly and directly determined from 08:26.760 --> 08:30.780 the seismogram, but the disadvantage is that they become saturated for 08:30.780 --> 08:31.720 larger magnitudes. 08:32.100 --> 08:35.620 The moment magnitude, on the other hand, does not become saturated, as 08:35.620 --> 08:37.280 it is derived from the source spectrum. 08:38.280 --> 08:42.120 Today, it is used in a standardized form for earthquakes worldwide 08:42.120 --> 08:44.100 with a magnitude of five and greater. 08:44.820 --> 08:49.080 Magnitude scales are logarithmic and the energy released increases 08:49.080 --> 08:51.580 additionally by a factor of 1.5. 08:52.000 --> 08:56.320 This means that a magnitude 4 earthquake differs from a magnitude 2 08:56.320 --> 09:01.680 quake by a 100-fold maximum amplitude in the seismogram and a factor 09:01.680 --> 09:04.640 of 1000 in the amount of energy released.