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PNSN > Catalogs and Data > Catalogs > Basic Info

Basic Catalog Information - magnitudes, quality factors, and UTC time, and Catalog updating frequency

MAGNITUDES - The PNSN routinely uses the duration of the seismic signal, measured on several different stations, to determine magnitude. The formula is: Magnitude = -2.46 + 2.82 log(T); where T is the duration of the seismic signal in seconds (measured on a standard high-magnification seismogram). The duration of a seismic signal on a seismogram does not correspond to the time that a human being would feel the earthquake. For example, an observer might feel a magnitude 5 earthquake for just a few seconds.

In real life this translates to approximately: 

Magnitude       Signal Duration
2               35 sec.
3               85 sec.
4               190 sec.
5               500 sec.
Duration magnitudes are approximately equivalent to Richter magnitude.

QUALITY FACTORS - The short story about quality factors is that "A" is good, and "D" is bad.

There are actually two quality factors. The first is the "RMS" time residual. To locate an earthquake, we:

Our location program iteratively adjusts location, recalculates, etc. After several iterations, the location estimate "converges" to a "best" solution. The RMS residual is an estimate of how much misfit there is between the observed arrival times and those that are calculated. The misfit arises mainly from the fact that the crude velocity model that we use to compute arrival times is not a true picture of the real earth. Our model is a simple layered model, with constant velocity layers. It is only crudely accurate.

The second quality factor relates to how well our stations surround the earthquake, and to whether the nearest station is close enough to constrain depth.

A "CC or better" quality solution means that the event is reasonably well surrounded by stations, and that observed arrival-times of the waves at the various stations are reasonably consistent.

Universal Time (Greenwich Mean Time)

Universal Time (UTC) is 8 hours ahead of Pacific Standard Time, and 7 hours ahead of Pacific Daylight Time. Universal time is used in seismological observatories world-wide in order to coordinate readings for large earthquakes. 

UTC	PST	PDT
 0 	16	17
 1	17	18
 2	18	19
 3	19	20
 4	20	21
 5	21	22
 6	22	23
 7	23	00
 8	00	 1
 9	 1	 2
10	 2	 3
11	 3	 4
12	 4	 5
13	 5	 6
14	 6	 7
15	 7	 8
16	 8	 9
17	 9	10
18	10	11
19	11	12
20	12	13
21	13	14
22	14	15
23	15	16

UPDATES - How PNSN earthquake lists are updated

Routine procedure
The catalog is updated each time our data analyst processes a batch of data. Normally, this is about twice a day on weekdays. Sometimes our analyst works on the weekend, or is not at work on a weekday. Activity is constantly monitored, but a few days delay in processing small events occurs from time to time. If a vigorous sequence occurs, the analyst will first locate the largest events, then complete processing smaller events when time permits.

Larger events
When the PNSN preliminary processing system detects an event of magnitude 2.9 or larger, the following events occur:

The seismologist arrives as soon as possible (almost always within the hour), to finalize the event location and magnitude. Once the information is finalized, the seismologist:
This is file /SEIS/PNSN/INFO_GENERAL/basic.html, last modified 7/31/02
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