Magnitude
Seismologists indicate the size of an earthquake in units of magnitude. There are many different ways that magnitude is measured from seismograms because each method only works over a limited range of magnitudes and with different types of seismometers. Some methods are based on body waves (which travel deep within the structure of the earth), some based on surface waves (which primarily travel along the uppermost layers of the earth), and some based on completely different methodologies. However, all of the methods are designed to agree well over the range of magnitudes where they are reliable.
Preliminary magnitudes based on incomplete but available data are sometimes estimated and reported. For example, the Tsunami Centers will calculate a preliminary magnitude and location for an event as soon as sufficient data is available to make an estimate. In this case, time is of the essence in order to broadcast a warning if tsunami waves are likely to be generated by the event. Such preliminary magnitudes, which may be off by one-half magnitude unit or more, are sufficient for the purpose at hand, and are superseded by more exact estimates of magnitude as more data become available.
Earthquake magnitude is a logarithmic measure of earthquake size. In simple terms, this means that at the same distance from the earthquake, the shaking will be 10 times as large during a magnitude 5 earthquake as during a magnitude 4 earthquake. The total amount of energy released by the earthquake, however, goes up by a factor of 32.
Magnitudes commonly used by seismic networks include:
| Based on the duration of shaking as measured by the time decay of the amplitude of the seismogram. Often used to compute magnitude from seismograms with "clipped" waveforms due to limited dynamic recording range of analog instrumentation, which makes it impossible to measure peak amplitudes. | |||
| The original magnitude relationship defined by Richter and Gutenberg for local earthquakes in 1935. It is based on the maximum amplitude of a seismogram recorded on a Wood-Anderson torsion seismograph. Although these instruments are no longer widely in use, ML values are calculated using modern instrumentation with appropriate adjustments. | |||
| A magnitude for distant earthquakes based on the amplitude of Rayleigh surface waves measured at a period near 20 sec. | |||
| Based on the moment of the earthquake, which is equal to the rigidity of the earth times the average amount of slip on the fault times the amount of fault area that slipped. | |||
| Based on the amount of recorded seismic energy radiated by the earthquake. | |||
| Based on the integral of the first few seconds of P wave on broadband instruments (Tsuboi method). | |||
| Based on the amplitude of P body-waves. This scale is most appropriate for deep-focus earthquakes. | |||
| A magnitude for distant earthquakes based on the amplitude of the Lg surface waves. |
Parameters
These parameters provide information on the reliability of the earthquake location. Zero values usually indicate that the contributing seismic network did not supply the information.| Nst | Number of seismic stations which reported P- and S-arrival times for this earthquake. This number may be larger than Nph if arrival times are rejected because the distance to a seismic station exceeds the maximum allowable distance or because the arrival-time observation is inconsistent with the solution. |
| Nph | Number of P and S arrival-time observations used to compute the hypocenter location. Increased numbers of arrival-time observations generally result in improved earthquake locations. |
| Dmin | Horizontal distance from the epicenter to the nearest station (in km). In general, the smaller this number, the more reliable is the calculated depth of the earthquake. |
| Rmss | The root-mean-square (RMS) travel time residual, in sec, using all weights. This parameter provides a measure of the fit of the observed arrival times to the predicted arrival times for this location. Smaller numbers reflect a better fit of the data. The value is dependent on the accuracy of the velocity model used to compute the earthquake location, the quality weights assigned to the arrival time data, and the procedure used to locate the earthquake. |
| Erho | The horizontal location error, in km, defined as the length of the largest projection of the three principal errors on a horizontal plane. The principal errors are the major axes of the error ellipsoid, and are mutually perpendicular. Erho thus approximates the major axis of the epicenter's error ellipse. |
| Erzz | The depth error, in km, defined as the largest projection of the three principal errors on a vertical line. See Erho |
| Gp | The largest azimuthal gap between azimuthally adjacent stations (in degrees). In general, the smaller this number, the more reliable is the calculated horizontal position of the earthquake. Earthquake locations in which the azimuthal gap exceeds 180 degrees typically have large Erho and Erzz values. |
| M-type | Magnitude type, discussed at greater length above under Magnitude |
| Version | Computers automatically update the WWW pages as more reliable information about the earthquake is computed, particularly in the first 10 minutes following the earthquake. The highest version number is always considered authoritative. |
Comments
It was a 7.2 Mw