8.7 Phase picking, amplitude, weight and polarity

Picking phases:

The plot will display any pick present in the database (current S-file). To pick new phases, position cursor at phase, and press the key as indicated on top of the screen (if in Single mode). E.g. pressing 1 will read IP. Pressing the same key again with the cursor at a different place will delete the old one (indicated with a D) and display the new one. Additional default phases, which can be picked, are i for I ans e for E (note upper or lower case). Keys for phases have default definitions, but can be redefined using the file MULPLT.DEF, see below. The end of the coda is picked as a phase (END) and the program calculates coda length IF AND ONLY IF A P-READING IS PRESENT.

Picking amplitudes:

Position the cursor at the bottom or top of a wave and press a, then at the other extreme (bottom or top) and press a (do not use upper case, see below). There is no requirement for going left to right or top to bottom, it can be done in any order as long as the two extremes are marked. At each press, a cross is marking where the pick was made. In case a filter, like WA, MS or Mb is applied, the program will associate the amplitude with the respective amplitude phase (IAML, IAMs_20,IVmB_BB, IVMs_BB or IAmb). Amplitude and period are calculated and stored with the phase. Otherwise, if none of these filters are applied, a menu pops up and the user needs to select an amplitude phases name phase name to which the amplitude and period readings are associated. If an attempt is made to pick amplitude on a trace which is not in nm or nm/s, the reading must be confirmed since SEISAN assumes all amplitudes to be in nm or nm/s (see section on instrument correction). If no amplitude phase is picked, no amplitude is stored. The amplitudes are always assumed to come in pairs so if e.g. 3 amplitude values have been picked, and the user tries to pick a phase or quit the program, it will appear frozen since the program is still waiting for the next amplitude measurement. It is always the last pair of amplitude measurements, which are used. Amplitudes can be picked on both corrected and uncorrected traces.

If A is pressed instead of a, the amplitude is read and marked automatically. It works in most cases, but sometimes two subsequent peaks are not correctly chosen and the amplitude reading has to be done manually. The method is to find the absolute extreme and then the largest amplitude before or after is selected in order to obtain the peak to peak amplitude, from which the amplitude is calculated by dividing by 2. For more information, see. ../LIB/auto_amp.for.

Manual and automatic amplitude picking can be done in both single trace and multi trace mode. In multi trace mode the amplitude will be picked at the trace nearest the cursor.

Component names when picking phases:

In the old Nordic format S-file (the default), the component only has 2 letters while in the waveform file it has 3 letters (in old SEISAN waveform files it can be 4, but rarely used). There must therefore be a unique translation between the two. This definition is given in the subroutine componen.for in LIB. Most common combinations are now defined, however if a new one is defined in the waveform file which does not exist in componen.for, the first and last letter of the input component will be used. If e.g. an input component is called SHZ, then the code in the S-file will be SZ. This means that picks for stations with components, which do not differ in first and last character, cannot be separated in the S-file. Component names for rotated channels will be e.g. SR and ST for short period radial and transverse components respectively.

In the new Nordic format, Nordic2, there is room for 3 character component names and in addtion network and locateion codes are stored. So there is no problem with identifying a channel. The complete station, component, network and location codes will be use to identify a reading.

Reading polarity:

If the cursor is above or below the trace at a distance marked by horizontal tics on the sides of the plot, the first motion is also picked and displayed when the next phase is picked. So if e.g. picking a P with dilatation, put cursor below the line and pick P. Do not use a filter if possible. Note that if you delete the phase later, the polarity is also deleted.

Assigning weight:

A phase can be assigned a weight. Move the cursor close to a pick and press one of the keys 0-9 in UPPER case thus using e.g. !"# (default, can also be changed), and a HYPO style weight is assigned and displayed. Although weights 0 to 9 can be put in, HYP only uses 0-4 and 9 (see section 7.1). Phases with associated amplitude, period, azimuth or apparent velocity are displayed with a hat below on the phase indicator line. The default keys for the weights might not be correct on all keyboards, if not, set keys in MULPLT.DEF using the parameter PHASE WEIGHT KEY.

The keys are defined as follows

Weight Linux Windows
1         !     !
2         @     "
3         #     #
4         $     $
5         %     %
6         ^     &
7         &     /
8         *     (
9         (     )
0         )     =

Automatic determination of coda length (C or c):

The coda length can be quite variable among different operators and a function has been made to automatically determine the coda length. The signal is bandpass filtered and the end of the coda is determined by a standard STA/LTA procedure. The parameters are set in the MULPLT.DEF file. Press C to find coda length automatically or c to determine manually. If parameter CODA AUTO is set in MULPLT.DEF, c is used.

In old Nordic format the position of the coda can only be determined if a P-phase is present sienc the coda lenght (time between coda pick and P) is stored together with the P. The location of the pick is indicated with Coda on the plot.

In new Nordic format, Nordic2, the coda can be picked even when there is no P since the coda is now an independent phase called END (IASPEI convention, indicating end of earthquake signal). However, the coda length is only given (on phase line END) if there is a P.

From SEISAN version 12, coda is no longer stored with the P but with the phase END. However, magnitude is still calculated when coda is stored with the P.

Below is shown an example of old and new format, Nordic2.

Old:

 STAT SP IPHASW D HRMM SECON CODA AMPLIT PERI AZIMU VELO AIN AR TRES W  DIS CAZ7 
 ASK  SZ IP        337 56.25  88                          62   0.38010  171 140 


New Nordic2:

 STAT COM NTLO IPHASE   W HHMM SS.SSS   PAR1  PAR2 AGA OPE  AIN  RES W  DIS CAZ7
 ASK  S Z      IP          337 56.250              RRR dd  62.00.38010  171 140  
 ASK  S Z       END        339 24.680   88.0                            171 140

Peter Voss : Fri Nov 12 10:33:10 UTC 2021