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Questions and answer

1. How much does the moon's gravitational pull affect earthquakes?

1. the effect of the moon's gravitational pull on  earthquakes is one that has been investigated by seismologists for many years.

The short answer is that while the moon does deform the earth slightly in a continuous 12 hour cycle called the solid earth tides, it doesn't seem to have an effect on the time of occurrence of large earthquakes. There does seem to be a small correlation between the earth tides and aftershocks in some volcanic regions, such as Mammoth Lakes in California.

There is a much better correlation between the earth's gravitational pull on the moon and moonquakes.

For more information, we suggest "Earthquakes" by Bruce Bolt, or any other general book on earthquakes.

There are many web pages devoted to earthquake triggering by the moon. The University of California at Berkeley page ( gives the seismologist's point of view. You will find many more pages that claim to successfully predict earthquakes using the phases of the moon, for example). The problem with most of them is that they can't prove that they are doing significantly better than random chance. If they make enough predictions, some of them are likely to come true, because earthquakes are happening all the time.

2. Can earthquakes be predicted?

2. It is possible to estimate where big earthquakes are likely in the next 50-100 years, based on geologically observed motions on faults and the historic record of earthquakes. However it is not yet possible to accurately to predict the time of the next earthquake in a particular area.
A number of physical changes have been observed before some earthquakes, but the problem is that so far, no particular change has been noted consistently. Most of the observed changes are probably related to the high levels of stress in rocks just before an earthquake. Some of these effects include changes in the magnetic and electric fields, gas emissions, changes in water well levels and changes in the velocity of seismic waves. Other scientists look for changes in the frequency and location of small earthquakes.
A very small number of earthquakes have been successfully predicted. The most impressive prediction was near Haicheng, China in 1975, where a city of 90,000 people were told to move out of their houses a few hours before an earthquake destroyed 90% of the buildings. The prediction was based on a greatly increased number of small earthquakes that suddenly stopped, and unusual animal behaviour: Rats and mice left their holes and were weaving around like drunks, while shakes crawled out of winter hibernation and lay frozen on the ground. However the following year the city of Tangshan was destroyed by a magnitude 8 earthquake that was not predicted.

3. What is the relationship between past and future earthquakes in the same place?

3. About 10,000-15,000 earthquakes occur in New Zealand every year. Most of these earthquakes are too small to be felt and we only know they happened because seismologists operate delicate seismographs that can measure very small ground movements.

4. How many earthquakes happen in New Zealand each year?

4. This question has been heavily debated over the past few years. It had been thought that the same size earthquake happens on the same fault at regular intervals, ie if there had been a magnitude 6 earthquake 30 years ago and again 10 years ago at the same place, another magnitude 6 earthquake could be expected in 10 years.  Now it appears that the problem is not that simple. Earthquakes do occur repeatedly at nearly the same place, but the time between earthquakes and the magnitude of the earthquakes is not always the same.

5. Pictures of seismograms always show the S wave to be bigger than the P wave, but we have recorded several earthquakes from Fiordland where the P wave is bigger. Why is that?

5. The energy that is transformed into seismic energy during a fault slip is not radiated uniformly in space. Both p- and s-waves have their characteristic radiation pattern. Their maximum amplitudes are in different directions. Averaged over the whole sphere, the s-wave has a bigger amplitude. Therefore the s-wave is usually larger on the seismogram. However, in the direction where the p-wave has its maximum, the p-wave is bigger than the s-wave. The ratio of p- and s- wave amplitude depends on the orientation of the slip and on your position relative to the source. Therefore the same earthquake can cause very different seismograms depending on where it is recorded.

6. What is the largest earthquake to be recorded in New Zealand (and worldwide)?

6. The largest earthquake recorded in New Zealand was the 1960 magnitude 9.5 Chile earthquake. This earthquake caused 5700 deaths and was recorded all around the world. The largest New Zealand earthquake to be recorded was the 1931 7.8 Napier earthquake. Larger earthquakes have occurred in New Zealand before instrumentation and so there are no actual records.

7. Are earthquakes related to other natural disasters?

7. Earthquakes can trigger a tsunami, landslides and volcanic eruptions. In Chile on 22 May 1960 an earthquake with a magnitude of 9.5 (the world's largest historic earthquake) caused a tsunami which swept across the Pacific, reaching almost all the countries around the Pacific Ocean, including New Zealand. The combination of New Zealand topography and shallow earthquakes often produces spectacular landslides. The 1968 magnitude 7.2 Inangahua earthquake triggered a landslide that temporarily dammed the Buller river, creating a further hazard down stream. Earthquakes can also play an active part in volcanic processes helping magma, water vapour and various gases escape from the volcano.

8. What is a seismograph made out of? Approximately how much does it weigh?Would it be able to stand extreme conditions like those on Mars?

8. Seismographs usually have a metal case (often aluminium) and inside there is a small weight hanging from a spring. The weight can vary from a few ounces to tens of pounds, though there was an early seismograph that weighed several tons! Seismographs have operated on the moon (recording 600-300 moonquakes per year), and believe it or not, they have also already operated on Mars.Only one marsquake was recorded and it isn't known how frequently they occur.

9. Do earthquakes only occur along fault lines?

9. As far as seismologists understand, all but the very deepest earthquakes (ones that happen at depths of greater than 600 km) occur on faults. Earthquake waves are created when the two sides of a fault rapidly grind past each other. For most earthquakes the faults do not break the surface, so the faults can be "seen" only with seismic waves. The faults can be anywhere from metres to thousands of kilometres long.The mechanism that causes the deepest earthquakes isn't well understood.At 600 km the earth is probably too warm for faults to be brittle like glass, so some sort of chemical change might occur very rapidly.


10. Is a fault line always one big long line?

10. Faults can be as short as metres and as long as a thousand kilometers. The fault from one earthquake isn't always a straight line (the Edgecumbe earthquake, for example) and sometimes there can even be short offsets between parts of the fault.

11. What was the furthest away that an earthquake was felt?

  11. An earthquake in 1994 was felt over 8600 km away! The earthquake was centred 640 km beneath Bolivia and had a magnitude of 8.3 and was felt in tall buildings as far away as Seattle Washington.The earthquake was felt at such a large distance because seismic waves aren't damped out nearly as fast from earthquakes deep in the earth as they are from near-surface earthquakes.


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