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.

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What is a Geoid? Why do we use it and where does its shape come from?

Geoid illustration Contrast of the Geoid model with an Ellipsoid and cross-section of the Earth's surface. (Public domain.) 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...

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...

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...

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-...

What was the duration of the earthquake? Why don't you report the duration of each earthquake? How does the duration affect the magnitude?

The duration of an earthquake is related to its magnitude but not in a perfectly strict sense. There are two ways to think about the duration of an earthquake. The first is the length of time it takes for the fault to rupture and the second is the length of time shaking is felt at any given point (e.g. when someone says "I felt it shake for 10...

What are UTC and GMT (in reference to the time of an earthquake)?

UTC stands for Coordinated Universal Time, and GMT stands for Greenwich Mean Time. The time of day that earthquakes occur around the world is reported in UTC or GMT, which are essentially the same. Learn more: Time Information

How can an earthquake have a negative magnitude?

Magnitude calculations are based on a logarithmic scale, so a ten-fold drop in amplitude decreases the magnitude by 1. If an amplitude of 20 millimetres as measured on a seismic signal corresponds to a magnitude 2 earthquake, then: 10 times less (2 millimetres) corresponds to a magnitude of 1; 100 times less (0.2 millimetres) corresponds to...

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...

How do seismologists locate an earthquake?

When an earthquake occurs, one of the first questions is "where was it?" The location may tell us what fault it was on and where damage (if any) most likely occurred. Unfortunately, Earth is not transparent and we can't just see or photograph the earthquake disturbance like meteorologists can photograph clouds. When an earthquake occurs, it...

How are earthquakes recorded? How are earthquakes measured? How is the magnitude of an earthquake determined?

Earthquakes are recorded by a seismographic network. Each seismic station in the network measures the movement of the ground at the site. The slip of one block of rock over another in an earthquake releases energy that makes the ground vibrate. That vibration pushes the adjoining piece of ground and causes it to vibrate, and thus the energy...

Moment magnitude, Richter scale - what are the different magnitude scales, and why are there so many?

Earthquake size, as measured by the Richter Scale is a well known, but not well understood, concept. The idea of a logarithmic earthquake magnitude scale was first developed by Charles Richter in the 1930's for measuring the size of earthquakes occurring in southern California using relatively high-frequency data from nearby seismograph stations...

What is the difference between magnitude and intensity? What is the Modified Mercalli Intensity Scale?

Magnitude scales, like the moment magnitude , measure the size of the earthquake at its source. An earthquake has one magnitude. The magnitude do not depend on where the measurement is made. Often, several slightly different magnitudes are reported for an earthquake. This happens because the relation between the seismic measurements and the...
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Date published: May 25, 2017

Updated USGS Earthquake Monitoring Strategy Released

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.”

Date published: January 23, 2012

A 100-year-long History of Earthquakes and Seismic Monitoring in Hawaii

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. 

Date published: September 24, 2009

Recovery Act Funds Will Upgrade Earthquake Monitoring

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.

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September 27, 2018

PubTalk 9/2018 - Hayward Earthquake

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.
  • There
Map of Active Faults and Historic Earthquakes in California
December 31, 2017

California Seismicity

Map of historic seismicity, major faults, and paleoseismic summary of San Andreas Fault system.

Attribution: Earthquake Hazards
USGS Forecast for Ground Shaking Intensity from Natural and Induced Earthquakes in 2017
February 28, 2017

USGS Forecast for Ground Shaking Intensity from Earthquakes in 2017

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.

Latest Earthquakes
August 31, 2016

Latest Earthquakes

July 30, 2015

PubTalk 7/2015 — The Giant Cascadia Earthquake of January 26, 1700

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
May 21, 2015

PubTalk 5/2015 — Breaking Badly:Forecasting California Earthquakes

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
Devastation of 1906 San Francisco Earthquake
May 31, 1906

Devastation of 1906 San Francisco Earthquake

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

Eastern earthquakes

Eastern Earthquakes

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).

Attribution: Natural Hazards
East versus west earthquakes

East vs West Coast Earthquakes

Map of USGS “Did You Feel It?” data shows that earthquakes east of the Rocky Mountains are felt over larger areas than earthquakes in the West.

Interactive map showing earthquake scenario data

Scenario Earthquake Map

Interactive map showing earthquake scenario data

Attribution: Natural Hazards