On April 13, 1992, at about 3:20 a.m. local time, an earthquake occurred in The Netherlands. Afterwards, interpretation of the seismic data afterwards showed that this earthquake was the largest ever recorded in The Netherlands. In this exercise you will answer the following questions: a. Where was the epicentre of the earthquake?
b. When exactly did the earthquake initiate?
To answer these questions, use the information below:
- 3:20 a.m. local time corresponds to 01:20 Universal Time (UT), or Greenwich Mean Time (GMT) (24-hour clock notation).
- Figure 3.6 shows five seismograms of the April 13 earthquake. The seismograms were recorded by seismographs in Germany, Belgium and The Netherlands. Figure 3.6 is only a selection of the many recordings done worldwide. For example, the earthquake was recorded in California and Australia as well. To accurately determine the earthquake’s epicentre, we use the recordings of the seismic stations closest to the earthquake.
- The seismograms in Figure 3.6 starts at reference time 01:20:15 GMT. Note that this time is arbitrary; it is not the actual origin time of the earthquake, because the actual moment it began is still unknown.
- Figure 3.8 shows the locations of the seismographs that produced the five seismograms.
- Figure 3.9 shows a graph of P- and S-wave travel times for places within a 500 km radius from the epicentre. This graph was made under the assumption that the earthquake took place at a depth of about 18 km. You can see that the time difference between P- and S-wave arrivals increases for seismometers farther from the epicentre. Thus, using the difference between the P- and S-wave arrival times, we can calculate the distance between seismograph and epicentre. This is called the epicentral distance.
Answer questions a. and b. If needed, you can use the hints below.
Hints to answer question a. Where was the epicentre of the earthquake?
i Mark the time of arrival of the P-wave in each of the seismograms and calculate the difference between P- and S-wave arrival times. Identification of S-wave arrival times is difficult so they have already been indicated.
ii Use Figure 3.9 to determine the epicentral distance for each seismic station.
iii Figure 3.8: Draw a circle around each station on the map with a radius equal to each station’s epicentral distance.
iv The epicentre of the earthquake lies where the different circles intersect.
Hints to answer question b. What was the origin time of the earthquake? i Determine the arrival time of the P-wave at one of the seismometers.
ii Calculate the travel time of the P-wave for that seismometer’s epicentral distance (Figure 3.9).
iii Subtract the travel time from the time of arrival. This will give the exact origin time of the earthquake.
Final questions
a. Why don’t the circles around the seismic stations intersect at one exact point? b. Why don’t the lines in the travel time graph go through the origin?
Figure 3.6: The seismograms belonging to Exercise 3-1.
Time from 01:20:15 GMT (s)
Time from 01:20:15 GMT (s) Time from 01:20:15 GMT (s)
A larger version of Figures 3-6 to 3-9 is available as a PDF-file. Print these figures out and use them to draw on.
Figure 3.7: The locations of the seismographs of Exercise 3-1.
Figure 3.8: The location of two large Sumatra earthquakes. The earthquake that caused a tsunami in December 2004 is denoted by a yellow star and a second large earthquake that occurred shortly after the first by a red star. The part of the fault plane that was active during each earthquake, called the rupture zone, is also indicated. (Source: US Geological Survey (USGS))
3.2 The relationship between plate tectonics and earthquakes.
Exercise 3-2*: Where do earthquakes occur? A, I
Read the newspaper article fragment below: Central America, October 9, 2005
The earthquake in El Salvador and Guatemala had a magnitude of at least 5.8 on the Richter scale. The epicentre of the tremor lies at a depth of 28 kilometres in the Pacific Ocean, about 51 kilometres from the Salvadorian city Barra Salada.
At least 610 people were killed.
a. Mark the location of the El Salvador and Guatemala earthquake on the map you used in Exercise 1-1. Mark the earthquakes studied in Exercise 3-1 and Optional Exercise 3-1 as well. b. Mark the location of more recent earthquakes. Use, for example, the website
http://www.earthweek.com.
c. Take a look at GB 192B (GB 174B). Compare the distribution of earthquakes with the location of the plate boundaries. Can you find a correlation between the two? Are there any exceptions? If so, how do you explain these exceptions?
d. What mistake is made in the newspaper article fragment above?
Earthquakes are most common along convergent plate boundaries (two plates moving towards each other) and transform plate boundaries (two plates sliding past each other). Little seismic activity is found along the third type of plate boundary, the spreading ridge, where plates move away from each other.
Exercise 3-3**: Plate motion near Japan A
From a Dutch final exam question in Geography (2007-II)
Many more earthquakes have occurred along the north-western boundary of the Philippine Plate than along the eastern boundary.
a. Use the data from map GB 157 (GB 140) and GB 192B (GB 174B) to explain the difference in the numbers of earthquakes between the Philippine Plate boundaries.
Figure 3.9: The fault planes to the southwest of Sumatra that were active during different earthquakes. For each fault plane, the date and the magnitude (in seismic moment, see Section 3.5) is given. (Source: K. Sieh, Caltech, USA)
It is evident from maps GB 157A and D (GB 140A and D) that the depth of the hypocentre of an earthquake in the vicinity of Japan and the distance of the earthquake from the Japan Trench are related.
b. What is the connection between the hypocentre depth of an earthquake and the distance to the Japan Trench? Do not consider earthquakes with a hypocentre depth less than 50 km.
Earthquakes along subduction zones are often very powerful, which makes them easier to study. We will investigate subduction zone earthquakes further by zooming in on earthquakes near Sumatra, Indonesia.
The large Sumatran earthquake of December 26, 2004, and the tsunami generated by it were the result of the active subduction zone along which Sumatra lies. The magnitude of the earthquake attracted a lot of attention from the scientific community; as did the enormous amount of data that modern technology was able to capture. The data record of the Sumatra earthquake helps us to learn more about earthquakes in general.
The Sumatran earthquake from December 2004 is one of the most powerful earthquakes that has occurred since seismographic recording began in around 1900. Like all other earthquakes, it was caused by the relative motion of parts of the Earth along a pre-existing fault plane. The Sumatran earthquake fault plane has been active for a very long time (millions of years). However, during the December 2004 earthquake over 1000 km of the plane was active.
The area where the Sumatran earthquake happened has a known pattern of plate motion: The Australian-Indian Plate subducts underneath the Eurasian Plate on which Indonesia lies. The Sumatra earthquake thus took place on a convergent plate boundary setting, in a subduction zone
The subduction zone runs along the south-western shore of Sumatra and the southern shore of Java (Figure 3.9 and Figure 3.10) south of the red line lies the deep-sea trench. At the latitude of northwest Sumatra, the Australian-Indian Plate subducts in a north-northeast direction underneath the Eurasian Plate with a velocity of 6 cm/year.
Figure 3.10: The motion of the Australian-Indian Plate with respect to the Eurasian Plate. The orange arrows indicate the direction of motion of the tectonic plates, the red arrows the direction of the December 2004 earthquake at Sumatra. (Source: Tsunami Laboratory, Novosibirsk, Russia)