How do you determine the magnitude for an earthquake that occurred prior to the creation of the magnitude scale?
For earthquakes that occurred between about 1890 (when modern seismographs came into use) and 1935 when Charles Richter developed the magnitude scale, people went back to the old records and compared the seismograms from those days with similar records for later earthquakes. For earthquakes prior to about 1890, magnitudes have been estimated by looking at the physical effects (such as amount of faulting, landslides, sandblows or river channel changes) plus the human effects (such as the area of damage or felt reports or how strongly a quake was felt) and comparing them to modern earthquakes.
Many assumptions have to be made when making these comparisons. For example, how do you compare the shaking for people living in log cabins or tents in the early 1800s with shaking for people living in high-rise steel and concrete buildings (with waterbeds!) in the 1990s? Because different researchers can get widely varying magnitudes from using different assumptions on how to make these comparisons, many of the old earthquakes have big differences in the magnitudes assigned to them. For example, magnitude estimates for the quakes that occurred near New Madrid, Missouri in 1811 and 1812 vary from the upper magnitude 6 range to as high as 8.8, all because of the choices the researchers made about how to compare the data.
Learn more: Earthquake Magnitude, Energy Release, and Shaking Intensity
Related
What does it mean that the earthquake occurred at a depth of 0 km? How can an earthquake have a negative depth; that would mean it’s in the air. What is the geoid, and what does it have to do with earthquake depth? What does it mean that the earthquake occurred at a depth of 0 km? How can an earthquake have a negative depth; that would mean it’s in the air. What is the geoid, and what does it have to do with earthquake depth?
An earthquake cannot physically occur at a depth of 0 km or -1km (above the surface of the earth). In order for an earthquake to occur, two blocks of crust must slip past one another, and it is impossible for this to happen at or above the surface of the earth. So why do we report that the earthquake occurred at a depth of 0 km or event as a negative depth sometimes? First of all, the depth of an...
Seismometers, seismographs, seismograms - what's the difference? How do they work? Seismometers, seismographs, seismograms - what's the difference? How do they work?
A seismometer is the internal part of the seismograph, which may be a pendulum or a mass mounted on a spring; however, it is often used synonymously with "seismograph". Seismographs are instruments used to record the motion of the ground during an earthquake. They are installed in the ground throughout the world and operated as part of a seismographic network. The earliest "seismoscope" was...
How can I make my own seismometer? How can I make my own seismometer?
It is relatively easy to acquire the necessary materials and build your own seismometer. The links here are to various sources with information on how to build a seismometer. They range from very simple and inexpensive to sophisticated and pricey. Model Seismograph - Classroom Demonstration Build your own Seismograph Station Build Your Own Seismograph Amateur Seismology Homebuilt Seismograph FAQ
What was the first instrument that actually recorded an earthquake? What was the first instrument that actually recorded an earthquake?
The earliest seismoscope was invented by the Chinese philosopher Chang Heng in A.D. 132. This was a large urn on the outside of which were eight dragon heads facing the eight principal directions of the compass. Below each dragon head was a toad with its mouth opened toward the dragon. When an earthquake occurred, one or more of the eight dragon-mouths would release a ball into the open mouth of...
What is a Geoid? Why do we use it and where does its shape come from? What is a Geoid? Why do we use it and where does its shape come from?
A geoid is the irregular-shaped “ball” that scientists use to more accurately calculate depths of earthquakes, or any other deep object beneath the earth’s surface. Currently, we use the “WGS84” version (World Geodetic System of 1984). If Earth were a perfect sphere, calculations of depth and distances would be easy because we know the equations for those calculations on a sphere. However, the...
Related
What does it mean that the earthquake occurred at a depth of 0 km? How can an earthquake have a negative depth; that would mean it’s in the air. What is the geoid, and what does it have to do with earthquake depth? What does it mean that the earthquake occurred at a depth of 0 km? How can an earthquake have a negative depth; that would mean it’s in the air. What is the geoid, and what does it have to do with earthquake depth?
An earthquake cannot physically occur at a depth of 0 km or -1km (above the surface of the earth). In order for an earthquake to occur, two blocks of crust must slip past one another, and it is impossible for this to happen at or above the surface of the earth. So why do we report that the earthquake occurred at a depth of 0 km or event as a negative depth sometimes? First of all, the depth of an...
Seismometers, seismographs, seismograms - what's the difference? How do they work? Seismometers, seismographs, seismograms - what's the difference? How do they work?
A seismometer is the internal part of the seismograph, which may be a pendulum or a mass mounted on a spring; however, it is often used synonymously with "seismograph". Seismographs are instruments used to record the motion of the ground during an earthquake. They are installed in the ground throughout the world and operated as part of a seismographic network. The earliest "seismoscope" was...
How can I make my own seismometer? How can I make my own seismometer?
It is relatively easy to acquire the necessary materials and build your own seismometer. The links here are to various sources with information on how to build a seismometer. They range from very simple and inexpensive to sophisticated and pricey. Model Seismograph - Classroom Demonstration Build your own Seismograph Station Build Your Own Seismograph Amateur Seismology Homebuilt Seismograph FAQ
What was the first instrument that actually recorded an earthquake? What was the first instrument that actually recorded an earthquake?
The earliest seismoscope was invented by the Chinese philosopher Chang Heng in A.D. 132. This was a large urn on the outside of which were eight dragon heads facing the eight principal directions of the compass. Below each dragon head was a toad with its mouth opened toward the dragon. When an earthquake occurred, one or more of the eight dragon-mouths would release a ball into the open mouth of...
What is a Geoid? Why do we use it and where does its shape come from? What is a Geoid? Why do we use it and where does its shape come from?
A geoid is the irregular-shaped “ball” that scientists use to more accurately calculate depths of earthquakes, or any other deep object beneath the earth’s surface. Currently, we use the “WGS84” version (World Geodetic System of 1984). If Earth were a perfect sphere, calculations of depth and distances would be easy because we know the equations for those calculations on a sphere. However, the...