How many earthquakes are recorded in California every day, week, and month?

This question was submitted to us by a 4th grade class in Southern California. We thought it was a great question and have made it into a FAQ! Dr. Robert Uhrhammer at the Berkeley Seismological Laboratory provided this answer.

Using the CNSS earthquake catalog, we retrieved earthquake data for 1988-1997 for the area of California. 372,826 earthquakes were retrieved. Thus, on an average year, approximately 37,300 earthquakes are recorded and analyzed. Then the rate of earthquake occurrence in California is:

	Time	    Number
		of earthquakes

	Year	    37,283

	Month	     3,107

	Week	       714

	Day	       102

Thus we record and analyze about 100 earthquakes per day on average. Most of the analysis is now done by automated computer algorithms so that seismologists no longer need to manually determine the location and magnitude of each earthquake. In California, earthquake monitoring is the shared responsibility of UC Berkeley, Caltech, and the United States Geological Survey.

As a function of magnitude, the number of events analyzed per year is:

	Magnitude	-------------Rate---------------
			(eq/yr)	 (eq/mo)   (eq/wk)   (eq/day)	

	>= 1		29545	 2462	   568	        81
	>= 1.5		16038	 1336	   308	        44	

	>= 2		 6076	  506	   117	        17
	>= 2.5		 1950	  163	    38	         5.3

	>= 3		  604	   50	    12	         1.7
	>= 3.5		  200	   17	     3.8	 0.55

	>= 4		   65	    5.4	     1.3	 0.18
	>= 4.5		   20	    1.7	     0.38	 0.055

	>= 5		  6.8	    0.57     0.13	 0.019	
	>= 5.5		  2.2	    0.18     0.042	 0.0060
 
	>= 6		  1.2	    0.10     0.023	 0.0033
	>= 6.5		  0.6	    0.05     0.012	 0.0016

where ">=" means "greater than or equal to". Note that the above table is not reliable at magnitudes above 6 because the 10 year seismicity sample is not sufficiently long to include a lot of magnitude 6+ earthquakes. In generating the above table, we assumed that the rate at which earthquakes occur does not vary with time. Large aftershock sequences violate this assumption.

To obtain a clearer picture of the seismicity with time, we prepared these histograms of annual seismicity from 1970 through 1997.

The threshold at which people report feeling an earthquake is approximately magnitude 2 (under ideal conditions, ie, not moving and in the immediate vicinity of the epicenter). The threshold at which some damage is reported (such as broken windows and objects knocked off shelves) is approximately magnitude 4. The threshold at which damage to weak structures (unreinforced masonary) occurs is approximately magnitude 5.5.