5.1.4 Magnitudes in SEISAN

Magnitudes are dealt with in many different programs in SEISAN and this section intends to give an overview.

SEISAN can calculate most standard international magnitudes: Local magnitude Ml, coda magnitude Mc, body wave magnitudes mb and mB (broadband), surface wave magnitudes Ms and MS (broad band) and moment magnitude Mw. The magnitudes are calculated in HYP as median of the station magntudes. Optionally, also averages can be calculated. For more details on the parameters used and the exact formulas, see program HYP.

Parameters for magnitudes

The input parameters used for calculating magnitudes are amplitudes and periods, code length and seismic moment M0. Amplitude and coda length can be obtained manually from MULPLT while automatic amplitudes for Ml can be obtained with programs EEV (command am running AUTOMAG), programs AUTOSIG and AUTOMAG. Seismic moments are obtained from spectra manually with MULPLT or automatically with AUTOSIG and AUTOMAG. Moments can also be obtained from moment tensor inversion using EEV. The amplitudes are stored as phases with the following names Ml: IAML mb IAmb mB IVmB BB Ms IAMs 20 MS IVMs BB

While the moments obtained by spectral analysis are stored in the SPEC lines and in the MOM lines for moment obtained by moment tensor inversion. While the magnitudes are calculated for all channels, only the average magnitude is stored on the header lines (except for Mw from spectral analysis, where magnitudes corresponding to each moment is stored on the SPEC line). The parameters to calculated Ml and Mc are given in STATION0.HYP file. The parameters for calculating spectral magnitude are given in MULPLT.DEF and/or SEISAN.DEF. Parameters for calculating Mw from moment tensor inversion is generated by EEV (using the model in STATION0.HYP) and stored in the S-file. The remaining magnitudes have fixed parameters.

Changing Ml and Mc parameters will result in new magnitudes being calculated with the next update of the S-file. For the spectral magnitudes, only changes caused by hypocentral distance, density and velocity will affect the recalculation of magnitude. If the attenuation change, the spectra must be recalculated.

Storing magnitudes

Magnitudes are written on the type one header line. On the main first header line there is room for 3 magnitudes, see example below

1998	812 1410 26.8 L  36.755-121.462  8.0FFBGS  4 1.3 4.6LTES 4.9WTES 5.0WHRV

where magnitude is given by type (e.g. L for Ml), size and a 3 letter agency. When the location program HYP is operated, the magnitudes are overwritten EXCEPT for the 3. magnitude on the header line which never will be deleted by any program. The intention with this is to be able to store an external reference magnitude. If more than 3 magnitudes are calculated (2 if a magnitude in 3. position), a new type one line is written with exactly the same origin time and location agency

1998  812 1410 27.0 L  36.755-121.462  8.0FFBGS  4 1.7 4.6LTES 4.7CTES 5.0WHRV1
1998  812 1410 27.0 L                       BGS        4.9WTES                1

There is thus room for only 6 magnitudes calculated by SEISAN. More magnitudes can be stored on more type 1 lines which must have different location agency and/or origin time compared to the first header line.

Searching for events using magnitude criteria

Program SELECT can search for a combination of magnitudes of different type, size and agency. SELECT can also optionally search for magnitudes on all header lines. SeisanEplorer (SE) can also search for a magnitude combination, but only among the 6 prime magnitudes. Manipulating magnitudes

There can be a need to convert one magnitude to another. The MAG program can make magnitude relations (also using all header lines). This magnitude relation can then be used, also by MAG, to convert one magnitude to another and write it back to the S-file. Since some programs (like EPIMAP) uses the magnitude in the first position on the header line, MAG can also be used to move any one of the magnitudes to that position. Program NORHEAD can move magnitudes from following header lines up to empty magnitude spaces on the first line. Program REPORT can move magnitudes around on the header line according to user choices. This moving around of magnitudes is now not needed for some programs. There is now a magnitude in SEISAN simply called M, which is unique magnitude returned according to priorities given in SEISAN.DEF

MAGNITUDE_ORDER                                               WGCM
MAGNITUDE_ORDER                                               LBER
MAGNITUDE_ORDER                                               WBER
where the order to select magnitudes as given here from top (high priority) to bottom (low priority). Only a few programs use this facility: SE, CLUSTER and ASSO. Plotting magnitude information

Magnitude sizes are plotted on all epicentral and hypocentral plotting programs. Relations between magnitudes can be plotted with MAG, b-value can be calculated and plotted with SE (standard method and Wiechert method) and BVALUE. A completeness check can be plotted with SE.

Other programs using magnitudes

NORHEAD can take magnitudes from following header lines and move them up to empty magnitude spaces on first line.

CLUSTER cleans a catalog for foreshocks and aftershocks, partly based on magnitudes. ASSO is a program that merges events in two catalogs based on both time and magnitudes.

Calculating magnitude relations

The Ml attenuation function can be calculated by inverting amplitude reading from many events suing program MAG2. A coda wave relation can be made with MAG. Attenuation to be used with spectral Mw can be determined using QLG, SPEC and CODAQ. QLG is the recommended program. AUTOMAG can also be made to make some initial tests for the best Q to fit the Brune spectrum.

The magnitude implementation in SEISAN has been adjusted (version 8.3) to the new IASPEI standard.

Amplitude based magnitude identification

Over the years there has been different ways of identifying the amplitudes used for magnitudes. Originally, amplitudes were not identified with any specific magnitude and only the period and distance was used to find out which kind of magnitude should be calculated, like Ml or Ms. Later different amplitude type phase names were used to identify which type of magnitude should be calculated and now only one IASPEI name is used for the amplitude phase name. However, SEISAN is backwards compatible and will use all the old amplitude phase identifiers. Below is given all identifiers used.

Local magnitude ML:

Body wave magnitude mb:

Surface wave magnitude Ms (Ms_20):

Broad band body wave magnitude mB (mB_BB):

Broad band surface magnitude MS (Ms_BB):

Several parameters used for magnitude determination is controlled by the TEST values set in the STATION0.HYP (local events), STATIOND.HYP (distant event) or other model dependent STATION file. See the HYP chapter 7.1 for default TEST values. The above values are:
        TEST(57) 1500km
        TEST(113) 20 \bgroup\color{black}$^{\circ}$\egroup
        TEST(114) 20 \bgroup\color{black}$^{\circ}$\egroup
        TEST(115) 60 km

The underlined values are hardcoded in SEISAN.

Example showing mb, mB, Ms and MS amplitude and period readings with estimated magnitudes, on the BSD station respectively:

 In old Nordic:

 2015 1117 0710 30.8 D  39.919  20.421136.3  DNK 21 0.6 6.5sDNK 6.1SDNK 5.8bDNK1
 BSD  HZ  IAmb     714 18.53       194.3 0.98                          1739 348 
 BSD  HZ  IVmB_BB  714 14.54      2995.7 3.96                          1739 348 
 BSD  HZ  IAMs_20  722 15.83     97917.0 18.0                          1739 348 
 BSD  HZ  IVMs_BB  722 13.06     43913.8 12.3                          1739 348 

 In new Nordic2:

 2015 1117  710 30.8 D  39.919  20.421136.3  DNK 210.60 6.5sDNK 6.1SDNK 5.8bDNK1
 BSD  HHZ DK00  IAmb       714 18.530  194.3  0.98 DNK pv              1739 348 
 BSD  HHZ DK00  IVmB_BB    714 14.540 2995.7  3.96 DNK pv              1739 348 
 BSD  HHZ DK00  IAMs_20    722 15.83097917.0  18.0 DNK pv              1739 348 
 BSD  HHZ DK00  IVMs_BB    722 13.06043913.8  12.3 DNK pv              1739 348 
Hyp output:                                                                                
 BSD  HZ    dist:    1740.0   amp:         194.3 T:   1.0   mb =   5.4
 BSD  HZ    dist:    1740.0   amp:        2995.7 T:   4.0   mB =   5.7
 BSD  HZ    dist:    1740.0   amp:       97917.0 T:  18.0   Ms =   6.0
 BSD  HZ    dist:    1740.0   amp:       43913.8 T:  12.3   MS =   6.1