Is a 4.0 Earthquake Strong?

Where a 4.0 fits on the magnitude scale

Magnitude measures the size of an earthquake at its source. The scale is logarithmic, which means each whole number step is a big jump. A 5.0 produces about 10 times more ground motion than a 4.0 on a seismograph and releases roughly 32 times more energy. Going the other way, a 4.0 releases roughly 32 times less energy than a 5.0 and about 1,000 times less than a 6.0.

So while 4 and 6 look close together, they describe very different events. Seismologists usually call magnitude 4 to 4.9 earthquakes light, magnitude 5 moderate, magnitude 6 strong and magnitude 7 major. Our article on how powerful a 7.0 earthquake is walks through the scale in more detail.

What people typically notice

How an earthquake feels at a particular spot is called Intensity, and it is described with the Modified Mercalli Intensity (MMI) scale. Near the epicenter, a shallow 4.0 commonly produces intensity around IV, sometimes V. At those levels, reports usually describe the following:

• A sudden jolt or a short rolling motion, often lasting only a few seconds.
• Windows, dishes and doors rattling, and walls making creaking sounds.
• Hanging objects such as lamps or plants swinging.
• Parked cars rocking slightly.
• A sound like a heavy truck passing or something heavy hitting the building.
• Near the epicenter, a few small objects may fall off shelves.

How far away a 4.0 can be felt

A moderate-depth 4.0 is typically felt within a few tens of kilometers of the Epicentre, and weakly beyond that. People on upper floors of tall buildings, or those sitting or lying still, are more likely to notice it than people walking or driving.

The felt area depends strongly on the region. In the central and eastern United States, the older, colder and less fractured crust carries seismic energy efficiently, so a 4.0 there may be felt over a much wider area than a similar earthquake in California. This is one reason a modest earthquake in the eastern U.S. can generate felt reports from several states.

Can a 4.0 cause damage?

Most of the time, no. Damage from a 4.0 is usually limited to things like items falling from shelves, broken glassware or, occasionally, small cracks in plaster. Structural damage is uncommon.

There are exceptions. A very shallow 4.0 directly beneath a town can produce stronger local shaking. Old unreinforced brick buildings, loose chimneys and poorly built structures are the most vulnerable. Soft soils can amplify shaking. These cases are unusual, but they are why emergency managers still pay attention to a 4.0 in a populated area.

How often 4.0 earthquakes happen

Magnitude 4 earthquakes are very common. The U.S. Geological Survey (USGS) estimates that roughly 13,000 earthquakes of magnitude 4.0 to 4.9 occur worldwide each year. Many are offshore or in remote areas, so only a fraction are felt by people.

For comparison, USGS estimates roughly 1,300 earthquakes of magnitude 5 to 5.9 each year and about 134 of magnitude 6 to 6.9. Each step up is roughly ten times rarer. You can see recent magnitude 4 earthquakes near you on the EQRadar live map, which shows earthquakes reported by USGS.

What a 4.0 can and cannot tell you

Sometimes a 4.0 is followed by smaller aftershocks over the next hours or days. Less often, it turns out to be a Foreshock of a larger earthquake, but that can only be known afterward. Most magnitude 4 earthquakes are not followed by anything bigger, so there is no need to panic. It is a good moment to check that your home is prepared and that you know how to Drop, Cover and Hold On.

Early reported magnitudes can change by a few tenths as more data arrives, so a 4.2 might become a 3.9 or a 4.4. You can follow the details on the event updates page.

Frequently asked questions