Frequency of Occurrence of Earthquakes

Descriptor	Magnitude	Average Annually
Great	8 and higher	1 ¹
Major	7 - 7.9	17 ²
Strong	6 - 6.9	134 ²
Moderate	5 - 5.9	1319 ²
Light	4 - 4.9	13,000 (estimated)
Minor	3 - 3.9	130,000 (estimated)
Very Minor	2 - 2.9	1,300,000 (estimated)
¹ Based on observations since 1900. ² Based on onservations since 1990.

The USGS estimates that several million earthquakes occur in the world each year. Many go undetected because they hit remote areas or have very small magnitudes. The NEIC now locates about 50 earthquakes each day, or about 20,000 a year.

Number of Earthquakes Worldwide for 2000 - 2004 Located by the US Geological Survey National Earthquake Information Center Magnitude 2000 2001 2002 2003 2004 8.0 to 9.9 1 1 0 1 0 7.0 to 7.9 14 15 13 14 7 6.0 to 6.9 158 126 130 140 97 5.0 to 5.9 1345 1243 1218 1203 884 4.0 to 4.9 8045 8084 8584 8462 7464 3.0 to 3.9 4784 6151 7005 7624 5314 2.0 to 2.9 3758 4162 6419 7727 5190 1.0 to 1.9 1026 944 1137 2506 1332 0.1 to 0.9 5 1 10 134 103 No Magnitude 3120 2938 2937 3608 8437 Total 22256 23534 27454 31419 *28828 Estimated Deaths 231 21357 1685 33819 787

Number of Earthquakes in the United States for 2000 - 2004 Located by the US Geological Survey National Earthquake Information Center Magnitude 2000 2001 2002 2003 2004 8.0 to 9.9 0 0 0 0 0 7.0 to 7.9 0 1 1 2 0 6.0 to 6.9 10 5 5 7 2 5.0 to 5.9 60 45 70 54 15 4.0 to 4.9 287 294 538 541 226 3.0 to 3.9 913 834 1525 1303 1114 2.0 to 2.9 657 646 1228 704 1092 1.0 to 1.9 0 2 2 2 1 0.1 to 0.9 0 0 0 0 0 No Magnitude 415 434 507 333 884 Total 2342 2261 3876 2946 *3334 Estimated Deaths 0 0 0 2 0

Red values indicate the earthquakes occurred in Alaska.
Blue values indicate the earthquakes occurred in California.

* As of 22 October 2004

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Number of Earthquakes Worldwide for 1990 - 1999 Located by the US Geological Survey National Earthquake Information Center Magnitude 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 8.0 to 9.9 0 0 0 0 2 2 1 0 1 0 7.0 to 7.9 12 11 13 12 11 18 14 16 11 18 6.0 to 6.9 115 105 166 137 146 183 149 120 117 116 5.0 to 5.9 1635 1469 1498 1426 1542 1318 1222 1113 979 1104 4.0 to 4.9 4493 4372 5128 4999 4518 8003 8756 7903 7303 6972 3.0 to 3.9 2457 2952 4692 4326 5041 5151 4923 4513 5945 5605 2.0 to 2.9 2364 2927 3066 5393 5371 3842 2391 2400 4091 4201 1.0 to 1.9 474 801 886 1170 779 645 295 388 805 715 0.1 to 0.9 0 1 3 9 17 19 1 4 10 5 No Magnitude 5062 3878 4072 3997 1944 1826 2186 3415 2426 2096 Total 16612 16516 19524 21476 19371 21007 19938 19872 21688 20832 Estimated Deaths 51916 2326 3920 10096 1634 7980 589 3069 9430 22662

Number of Earthquakes in the United States for 1990 - 1999 Located by the US Geological Survey National Earthquake Information Center Magnitude 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 8.0 to 9.9 0 0 0 0 0 0 0 0 0 0 7.0 to 7.9 0 1 2 0 1 0 2 0 0 1 + 1 6.0 to 6.9 3 6 15 9 4 6 4 6 3 6 5.0 to 5.9 72 50 72 62 64 45 100 63 62 50 4.0 to 4.9 283 255 404 270 333 350 612 362 411 352 3.0 to 3.9 621 701 1717 1119 1543 1058 1060 1072 1053 1398 2.0 to 2.9 411 555 998 1009 1196 822 654 759 742 814 1.0 to 1.9 1 3 5 7 2 0 0 2 0 0 0.1 to 0.9 0 0 0 0 0 0 0 0 0 0 No Magnitude 877 599 368 457 444 444 375 575 508 381 Total 2268 2170 3581 2933 3587 2725 2807 2839 2779 3003 Estimated Deaths 0 2 3 2 60 1 0 0 0 0

Red values indicate the earthquakes occurred in Alaska.
Blue values indicate the earthquakes occurred in California.

As more and more seismographs are installed in the world, more earthquakes can be and have been located. However, the number of large earthquakes (magnitude 6.0 and greater) have stayed relatively constant.

Graphs

TABLE 4 - Magnitude vs. Ground Motion and Energy

Magnitude Change Ground Motion Change (Displacement) Energy Change 1.0 10.0 times about 32 times 0.5 3.2 times about 5.5 times 0.3 2.0 times about 3 times 0.1 1.3 times about 1.4 times

TABLE 4 shows, for example, that a magnitude 7.2 earthquake produces 10 times more ground motion that a magnitude 6.2 earthquake, but it releases about 32 times more energy. The energy release best indicates the destructive power of an earthquake.

See also: Are Earthquakes Really on the Increase?

Question:
How much bigger is a magnitude 9.7 earthquake than a 6.8 earthquake?

A magnitude 9.7 earthquake is 794 times BIGGER on a seismogram than a magnitude 6.8 earthquake. The magnitude scale is logarithmic, so

       (10**9.7)/(10**6.8) = (5.01*10**9)/(6.31*10**6) = .794*10**3 = 794
			  OR
			   = 10**(9.7-6.8) = 10**2.9 = 794.328

Another way to get about the same answer without using a calculator is that since 1 unit of magnitude is 10 times the amplitude on a seismogram and 0.1 unit of magnitude is about 1.3 times the amplitude, we can get,

	10 * 10 * 10 / 1.3 = 769 times [not exact, but a decent approximation]

The magnitude scale is really comparing amplitudes of waves on a seismogram, not the STRENGTH (energy) of the quakes. So, a magnitude 9.7 is 794 times bigger than a 6.8 quake as measured on seismograms, but the 9.7 quake is about 23,000 times STRONGER than the 6.8! Since it is really the energy or strength that knocks down buildings, this is really the more important comparison. This means that it would take about 23,000 quakes of magnitude 6.8 to equal the energy released by one magnitude 9.7 event. Here's how we get that number:

One whole unit of magnitude represents approximately 32 times (actually 10**1.5 times) the energy, based on a long-standing empirical formula that says log(E) is proportional to 1.5M, where E is energy and M is magnitude. This means that a change of 0.1 in magnitude is about 1.4 times the energy release. Therefore, using the shortcut shown eartlier for the amplitude calculation, the energy is,

	32 * 32 * 32 / 1.4 = 23,405 or about 23,000

The actual formula would be:

	((10**1.5)**9.7)/((10**1.5)**6.8)

	= 10**(1.5*(9.7-6.8)) = 10**(1.5*2.9) = 22,387

This explains why big quakes are so much more devastating than small ones. The amplitude ("size") differences are big enough, but the energy ("strength") differences are huge. The amplitude numbers are neater and a little easier to explain, which is why those are used more often in publications. But it's the energy that does the damage.