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.
What was the duration of the earthquake? Why don't you report the duration of each earthquake? How does the duration affect the magnitude?
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?
How are earthquakes recorded? How are earthquakes measured? How is the magnitude of an earthquake determined?
Moment magnitude, Richter scale - what are the different magnitude scales, and why are there so many?
What is the difference between magnitude and intensity? What is the Modified Mercalli Intensity Scale?
The USGS Earthquake Hazards Program recently released a new strategic plan for earthquake monitoring entitled the “Advanced National Seismic System – Current Status, Development Opportunities, Priorities, 2017-2027.”
The Hawaiian Volcano Observatory’s 1912–2012 Centennial—100 Years of Tracking Eruptions and Earthquakes
HAWAI‘I ISLAND, Hawaii —The history of earthquakes and seismic monitoring in Hawai‘i during the past century will be the topic of a presentation at the University of Hawai‘i at Hilo on Thursday, January 26, at 7:00 p.m.
USGS will Grant Universities $5 Million to Beef Up Public Safety Grants totaling $5 million under the American Recovery and Reinvestment Act are being awarded to 13 universities nationwide to upgrade critical earthquake monitoring networks and increase public safety.
Title: The 150th Anniversary of the Damaging 1868 Hayward Earthquake: Why It Matters and How We Can Prepare for Its Repeat
- The Hayward Fault in the heart of the Bay Area is one of the most urbanized faults in the US.
- Studies of the fault reveal that it has produced 12 large earthquakes in the past 2000 years spaced 100-220 years apart.
USGS map displaying intensity of potential ground shaking from natural and human-induced earthquakes. There is a small chance (one percent) that ground shaking intensity will occur at this level or higher. There is a greater chance (99 percent) that ground shaking will be lower than what is displayed in these maps.
Detective Stories from North America and Japan
by Brian Atwater, USGS Seattle
- A tsunami from western North America entered Japanese written history in Jan 1700
- Decades of basic research on both sides of the Pacific led to this discovery
- The endings underpin public-safety measures in the United States and
by Morgan Page, USGS Research Geophysicist
- Scientists cannot currently predict the precise time, location, and size of future damaging earthquakes.
- Historical records of earthquakes in California date back over 150 years.
- Geologists have dug trenches to extend the known history on some faults back to around 1,000 years before
This photograph, taken by George Lawrence from a series of kites five weeks after the great earthquake of April 18, 1906, shows the devastation brought on the city of San Francisco by the quake and subsequent fire. The view is looking over Nob Hill toward business district, South of the Slot, and the distant Mission. The Fairmont Hotel, far left. dwarfs the Call Building...
This map shows earthquakes above magnitude 4.0 in the eastern United States since 1973, the first year with a complete catalog. There are 184 earthquakes recorded. An earthquake of magnitude 4.0 or greater can cause minor or more significant damage. The circle sizes correspond to earthquake magnitude, ranging from 4.0 to 5.9 (the largest was in the Gulf of Mexico).