Calculate PRFs for a station

Prepare seismic records order by stations

Seismic data including direct P arrival should be trimmed according earthquakes. Meanwhile, the data should be order by stations instead of events. For example, YN001 and YN002 are stations; the SAC files are teleseismic data records of these stations.

event_data/
├── YN001
│   ├── 2018.229.15.35.02.1.sac
│   ├── 2018.229.15.35.02.2.sac
│   ├── 2018.229.15.35.02.3.sac
│   ├── 2018.229.22.06.55.1.sac
│   ├── 2018.229.22.06.55.2.sac
│   │......
├── YN002
│   ├── 2018.229.15.35.02.1.sac
│   ├── 2018.229.15.35.02.2.sac
│   ├── 2018.229.15.35.02.3.sac
│   ├── 2018.229.22.06.55.1.sac
│   ├── 2018.229.22.06.55.2.sac
│   │......

Calculate PRFs using a simple command

Prepare a configure file

We should prepare a configure file as following including all parameters that will be set during the calculation. The format is following Python module of configparser

[path]
datapath = Data/Path/to/station_name
rfpath = Result/Path/to/station_name
imagepath = Path/to/images
catalogpath = Path/to/catalog

[search_eq]
date_begin = 20110701
date_end = 20111101
catalog_server = IRIS
magmin = 5.5
magmax = 10
dismin = 30
dismax = 90

[match_eq]
dateformat = %Y.%j.%H.%M.%S
ref_comp = .1.
suffix = SAC
tolerance = 60
offset =

[snr]
noisegate = 5
noiselen = 50

[filter]
freqmin = 0.05
freqmax = 2

[trim]
time_before = 10
time_after = 120

[deconv]
gauss = 2
itmax= 400
minderr = 0.001

[save]
target_dt = 0.01
only_r = no
criterion = crust

Run in command line

We have provided the command prf. The usage is shown as:

usage: prf [-h] [-l] [-r COMP] [-s] [-b [BAZ]] cfg_file

Calculating RFs for single station

positional arguments:
  cfg_file    Path to RF configure file

optional arguments:
  -h, --help  show this help message and exit
  -l          use local catalog. Default is false
  -r COMP     Reverse components: EN, E or N
  -s          Switch East and North components
  -b [BAZ]    Correct back-azimuth with minimal energy of T component. "baz"
              is specified as half-searching range. Default value is 90 deg

  • cfg_file: configure file shown above.

  • -l if the argument was specified, a local file of catalog would be used in searching earthquakes.

  • -r Reverse the horizontal components. The arguments should be in EN, E or N.

  • -s If this option is specified, the East and North components would be changed.

  • -b Correct azimuth of horizontal components with minimal energy of T component. the argument would be specified as half-searching range. Default value is 90 deg.

Initialize a project instance

To further understand the procedure of the command prf, we recommend calculating PRFs with writing a Python script as following steps.

First, let’s initialize a RFinstance. In this instance, we can set parameters, match earthquakes from catalog and calculate PRFs.

from seispy.rf import RF
from os.path import join
from obspy import UTCDateTime

pjt = RF()

Set parameters

Most of parameters are saved in the class pjt.para. All default parameters are shown as following

print(pjt.para.__dict__)

{'_datapath': '/Users/xumj',
 '_rfpath': '/Users/xumj',
 '_imagepath': '/Users/xumj',
 '_catalogpath': '/Users/xumj/.pyenv/versions/anaconda3-5.3.1/lib/python3.7/site-packages/seispy-1.1.8-py3.7.egg/seispy/data/EventCMT.dat',
 'offset': None,
 'tolerance': 210,
 'dateformat': '%Y.%j.%H.%M.%S',
 'date_begin': 1976-01-01T00:00:00.000000Z,
 'date_end': 2019-07-11T14:04:15.365860Z,
 'magmin': 5.5,
 'magmax': 10,
 'dismin': 30,
 'dismax': 90,
 'ref_comp': 'BHZ',
 'suffix': 'SAC',
 'noisegate': 5,
 'gauss': 2,
 'target_dt': 0.01,
 'phase': 'P',
 'time_before': 10,
 'time_after': 120,
 'only_r': False}

Thus, we can set them in our scripts

pjt.para.datapath = 'Data/Path/to/station_name'
pjt.para.rfpath = 'Result/Path/to/station_name'
pjt.para.suffix = 'sac'
pjt.para.ref_comp = ".1."
pjt.date_begin = UTCDataTime('20180701')
pjt.date_end = UTCDataTime('20190701')
pjt.para.offset = 0
pjt.para.tolerance = 60

or in a configure file as above. When you want to initialize an instance using this configure file, please add the path to RF() as:

pjt = RF(cfg_file='path/to/config')

Search earthquakes from catalog

We use the same process as the SplitRFLab. To match the data records and events, we should search earthquakes with some criteria (period, epicentral distance and magnitude).

Load station information

the The station latitude and longitude are absolutely necessary when we are used to search earthquakes. the function will read stla and stlo of SAC header from files in pjt.para.datapath.

pjt.load_stainfo()

Search earthquakes

the function provide two method to search earthquakes. use

pjt.search_eq()

to search earthquakes in IRIS Web service with the CMT catalog.

In addition, the function allow to prepare earthquakes from a CMT catalog file (saved to seispy/seispy/data/EventCMT.dat). Use command updatecatalog to update the catalog file.

pjt.search_eq(local=True)

Associate SAC files with events

This is a important step, which allow to link SAC files and earthquakes in catalog. The pjt.para.dateformat, that is a format string as in time.strftime, including datetime infomation will allow to match events in catalog. For example, assuming the filename is 2018.229.15.35.02.1.sac. the pjt.para.dateformat should be %Y.%j.%H.%M.%S.

A reference sac file will read to Associate with events. Thus, file-search-string will help to find real SAC files in data path. In this program file-search-string composed of pjt.para.ref_comp and pjt.para.suffix. The presence of *pjt.para.ref_comp*pjt.para.suffix, such as *.1.*sac in this example, for SAC files will be found.

the pjt.para.offset and pjt.para.tolerance are used to match the origin time from catalog. The definition are the same as those in SplitRFLab.

  • The “offset” is the time duration between the event time and the starting time of your seismograms. Ideally, this offset should be identical to the “request start time” defined in the previous window but the data management center may have sent you data beginning later than requested. The offset value represents this difference.

  • The “Tolerance” value in seconds will define the time window within which the program will try to associate a seismic file to an event file, by using either its name or the information contained in the header. It is up to the user to find the best compromise: a value too small will let orphans and a value too large will bring confusion since several files could be associated to a seismic event. ../_images/offset.png

After setting up these parameters, use following command to match data records to the catalog:

pjt.match_eq()

Pretreatment

The process of pretreatment include detrend, bandpass filter, calculating arrival time, reject bad record with low SNR, trim records and rotate components from NE to RT.

Filter

We will aply a bandpass filter on seismic records. Two corners should be specified.

  • para.freqmin: Pass band low corner frequency.

  • para.freqmax: Pass band high corner frequency.

Signal-noise-ratios (SNR) calculation

seismic records with poor quality will be rejected in this step. We will reject records with SNR < para.noisegate. The SNR was calculated as:

$$ SNR = 10log_{10}\left(\frac{A_S}{A_N}\right) $$

where $A_N$ and $A_N$ are root mean squares (RMS) of the waveform trimmed with time length of para.noiselen before and after theoretical P arrival times, respectively.

Trim

The waveforms will be cut in this step before para.time_before and after para.time_after theoretical P arrival times, respectively.

pjt.detrend()
pjt.filter() # default using 'para.freqmin' and 'para.freqmax'
pjt.cal_phase()
pjt.drop_eq_snr() # The threshold used as 'para.noisegate'
pjt.trim() # from 'para.time_before' before P to 'para.time_after' after P
pjt.rotate()

PRFs calculation

We need parameters of pjt.para.gauss, pjt.para.itmax and pjt.para.minderr to calculate PRFs using iterative time-domain deconvolution method

  • pjt.para.gauss: Gauss factor. Default is 2.0.

  • pjt.para.itmax: The maximum number of iterations. Default is 400.

  • pjt.para.minderr: The minimum misfit. Default is 0.001.

pjt.deconv()

Save PRFs

Save the PRFs to pjt.para.rfpath with some criteria. Two kind of criteria allow to set (i.e., crust or mtz). if the parameter set as None, all of PRFs will be saved.

crust

  • The maximum peak should appear berween -2s and 2s.

mtz

  • The maximum peak should appear between -5s and 5s.

  • the maximum amplitudes of PRFs in a 30–120 s window after the direct P are smaller than 30% of the maximum amplitudes of the direct P phases.

pjt.saverf(criterion='crust')