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 seconds" they are making a statement about the duration of shaking). The duration of fault rupture is related to both how long it takes for a spot on the fault to slip (which seems to be quite fast) and the time it takes rupture to proceed along a fault.
You have to think of an earthquake as an area on a fault rather than just a point. It starts at a point and then the rupture propagates along the fault at around 2 kilometers or so per second. So the larger the area of the fault that ruptures, the longer the duration of the earthquake. And larger magnitude earthquakes have larger fault areas. So there is a general relationship between duration and magnitude.
The reason we don't list this sort of duration on the Latest Earthquake website is that figuring out how long an earthquake took to rupture is still a research project that takes some time rather than an automated process. The duration of shaking at a point on the ground depends on how long the earthquake took to occur and how the waves move through the ground to that point. If there are a lot of reflections and resonances near the point (for instance in a sedimentary valley), the shaking will last longer. In an area without resonances (for instance on a hard block of rock), it will last a shorter time.
You must also specify a duration of shaking over a given level. We can actually detect the shaking from the very largest earthquakes for weeks after they occur, but no one would say that they felt it for that long. So the duration of shaking is a very complex topic. We actually do use the duration of shaking to estimate the magnitude for some small earthquakes. If you see a "Md" or "duration magnitude" on the Latest Earthquake webpages, this is what has been done.
For an explanation see Magnitude. This is much like having someone yell, counting the echos, and then estimating how loud they yelled from how many echos you could hear. Finally, the damage to a given structure will depend both on the amplitude of the shaking and its duration. How to best combine these quantities into an estimate of the amount of damage is ongoing research. (contributed by Andy Michael)
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?
How do you determine the magnitude for an earthquake that occurred prior to the creation of the magnitude 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.
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.
USGS map showing (1) the locations of major populations and (2) the intensity of potential earthquake ground shaking that has a 2% chance of occurring in 50 years.
Shake map for a magnitude 7.0 earthquake on the Hayward fault with the eipicenter of earthquake near Oakland, Calfornia.
This graphic demonstrates that ground shaking from earthquakes is amplified at sites with sediment compared to those with harder bedrock. The upper panel shows ground shaking at bedrock and sediment sites in Washington, DC, from an earthquake in North Carolina. The lower panel shows ground shaking in DC from an earthquake in Alaska.
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).