How do scientists measure earthquakes?

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1 Name: Source: How do scientists measure earthquakes? An earthquake, one of the most dreaded natural disasters, occurs as a result of a sudden release of energy in the Earth s crust creating seismic waves. We all are aware of the fact that high-intensity earthquakes can result in a massive destruction of life and property. 1. Predict: How are earthquakes detected and measured? Earthquakes are detected using a seismometer. A seismometer, also known as a seismograph, is an instrument that records movements of the ground. It is used to detect seismic waves generated by earthquakes. A seismoscope can also be used for the detection of underground movements. It indicates that motion has occurred, but does not give a continuous record of the ground movements. Think of a seismograph as a kind of sensitive pendulum that records the shaking of the Earth. The output of a seismograph is known as a seismogram. In the early days, seismograms were produced using ink pens on paper or beams of light on photographic paper, but now it's most often done digitally using computers. 2. Sketch a pendulum instrument that could record the movement of the Earth s surface.

2 There are two ways in which scientists quantify the size of earthquakes: magnitude and intensity. Magnitude is a measure of the amount of energy released during an earthquake, and you've probably heard news reports about earthquake magnitudes measured using a scale. Something like, "A magnitude 7.3 earthquake struck Japan today. Details at ten." Did you ever wonder why, if it's that important, they just don't tell you right away? 3. Why is there a delay in the information? The Richter scale was invented, in the 1930s by Dr. Charles Richter, a seismologist at the California Institute of Technology. It is a measure of the largest seismic wave recorded on a particular kind of seismograph located 100 kilometers (about 62 miles) from the epicenter of the earthquake and was a way for scientists to compare earthquakes of different sizes. 4. What is the root word, prefixes/suffixes in seismologist, that help us determine the meaning of the word? The reason the Richter scale was so popular was that the earthquake's magnitude could be determined by looking at the largest wave recorded on a seismogram right after the earthquake. News stories about the size of the earthquake could then be quickly given to the public on the radio and television stations.

3 Richter scale's faults So what's wrong with the good old Richter scale? Nothing, really. It was a great tool for its time. But like last year's computer, it's been nudged aside by new technology. It isn't straightforward, like a ruler, but logarithmic. In theory, for each increase of one full point on the Richter scale, the ground shakes 10 times harder, and 32 times as much energy is released. The Richter magnitude is based on the size of the largest waves arriving at a particular type of instrument, the Wood Anderson seismograph. It's known as a local magnitude scale because it considers only waves arriving within 370 miles of the epicenter -- the spot on the ground directly above where the earthquake started. If there are no instruments within this distance, the size is estimated from readings taken closer and farther away. However, each earthquake is a rich blend of seismic waves. If you imagine the ground as the surface of a sea, these waves range from ripples to choppy whitecaps to long, slow swells.the Richter scale measures only the chop -- high-frequency waves that travel through the body of the rock, rather than at the surface. These are the ones most damaging to small structures, from homes to buildings a few stories tall. But the bigger the earthquake, the more long, slow waves it produces. In fact, this turns out to be a key difference between major earthquakes and the more moderate kind. The big ones don't necessarily shake the ground 10 times harder; but they do shake it longer, and they put out more long, slow swells of the type that can resonate with -- and damage -- big structures like high-rises and bridges. When it came to measuring these big earthquakes, the Richter scale fell short. So more than a decade ago, scientists started to use the moment magnitude scale. It's based on the full range of waves coming from an earthquake, including those that take up to 100 seconds to pass a given spot. This type of analysis has made possible for new seismic instruments that capture everything instead of only part of the energy released. 5. Why is the Richter scale no longer used? Explain your answer using evidence from the text. (Hint: There should be 3 reasons)

4 Moment Magnitude Scale Measures Great Earthquakes The moment magnitude scale (MMS) was devised by scientists after the 1960 Chilean earthquake and the 1964 Alaskan earthquake that were so large that the Richter scale did not adequately show the magnitude and size of what happened during these great earthquakes of the last century. The moment magnitude scale includes the area of the fault's rupture and slippage along the fault. It also includes the size of the seismic waves recorded on the seismograms. Today the MMS is used to estimate the magnitudes for all moderate to large earthquakes. Other scales are used for earthquakes less than 3.5 magnitude, which is the majority of earthquakes felt worldwide. 6. What incident led scientists to develop a new scale? Why? The MMS measures movement in three directions: north/south, east/west and, and vertical (up/down). Like the Richter scale, it's logarithmic, which, in English means that a small numeric bump equates to a serious increase in the violence with which the Earth shakes. It works in orders of magnitude where a 5.0 earthquake is not 20% stronger than a 4.0 quake; it is 10 times as strong One-thousand percent. There is a massive difference between a 6.0 and a 7.0 quake. 7. Explain the difference between a 4.0 and 5.0 earthquake.

5 The MMS measures the total energy of an earthquake, called the seismic moment. The seismic moment of an earthquake is determined based on three factors. The first factor is the distance that rock slides along a fault surface after it breaks, called the fault slip. The second factor is the area of the fault surface that is actually broken by the earthquake. And the third factor is the measurement of how rigid the rocks are near the broken fault. A strong rock, such as granite, cannot be broken easily. That makes it a highly rigid rock. 8. Make a list and sketch each of the three factors that determines the seismic moment. The most powerful seismic moment ever measured registered a 9.5 on the MMS it happened when an earthquake struck just off the coast of Chile on May 22, Here is an approximate and basic description of what happens when earthquakes of different strengths strike. You ll notice that the higher the number gets, the worse the damage becomes. 9.0 and above Causes complete devastation and large-scale loss of life. 8.0 Very few buildings stay up. Bridges fall down. Underground pipes burst. Railroad rails bend. Large rocks move. Smaller objects are tossed into the air. Some objects are swallowed up by the earth. 7.0 It is hard to keep your balance. The ground cracks. Roads shake. Weak buildings fall down. Other buildings are badly damaged. 6.0 Pictures can fall off walls. Furniture moves. In some buildings, walls may crack. 5.0 If you are in a car, it may rock. Glasses and dishes may rattle. Windows may break. 4.0 Buildings shake a little. It feels like a truck is passing by your house. 3.0 You may notice this quake if you are sitting still, or upstairs in a house. A hanging object, like a model airplane, may swing. 2.0 Trees sway. Small ponds ripple. Doors swing slowly. But you can't tell that an earthquake is to blame. 1.0 Earthquakes this small happen below ground. You can't feel them.

6 9. At what number would expect some casualties? Explain your answer. What if the earthquake was in Indonesia instead of in the United States, would there be the same number of casualties? Why or Why not? Reassessing quakes For instance, the 1906 San Francisco earthquake and the 1964 Good Friday quake in Alaska both had surface-wave magnitudes of 8.3. Yet the Alaskan quake involved a much bigger fault and released 100 times as much energy, so when the more accurate moment magnitude scale came along, San Francisco was downgraded to 7.8, and the Alaskan quake was boosted to 9.2. The 1989 Loma Prieta quake was demoted, too. Its official surface-wave magnitude is 7.1, but on the moment scale it's only a 6.9. After the 1960 Chile earthquake the Richter scale registered the earthquake size at 8.5 magnitude. Scientists using the MMS revised the earthquakes size up to a 9.5, the highest ever recorded since seismographs were invented. 10. Why are some earthquakes being reduced while others are increased on the scale?

7 This is a comparison between the two scales for four other large earthquakes that occurred in the past. New Madrid, MO Richter scale MMS 8.1 San Francisco, CA Richter scale MMS 7.7 Prince William, AK Richter scale MMS 9.2 Northridge, CA Richter scale MMS 6.7 Five earthquakes of magnitude 9 or above have been recorded during the past 45 years, which averages out to one every decade. It turns out that earthquake occurrences seem to follow what is called a power-law distribution, meaning that if there is on average on magnitude 9 earthquake every ten years somewhere in the world, then on average there should be one magnitude 8 earthquake every year, 10 magnitude 7 earthquakes every year, and 100 magnitude 6 earthquakes every year. So, if someone "predicts" that a magnitude 6 earthquake will occur somewhere in the world during the next week, don't be too impressed if it happens because random probability tells us that there should be a magnitude 6 earthquake somewhere in the world every 365/100 = 3.65 days! In reality, things are a little more complicated. But, you get the picture. 11. What is your opinion of this data? Give details to support this data or argue it.