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Calculate a P-wave Receiver Function (PRF)

0. Download SAC files for this example

Download SAC files

Download and unzip this .tar.gz file, which include three SAC files.

1. Import corresponding modules

import obspy
import seispy
import numpy as np
import matplotlib.pyplot as plt

2. Read SAC files with 3 components (ENZ)

You should perpare teleseismic data if SAC format (ENZ) and read them via obspy. To facilitate the follow-up, you'd better write positions of the station and the event into SAC header (i.e., stla, stlo, evla, evlo and evdp).

st = obspy.read('tele/*.101.*.SAC')

3. Pre-process for raw data

You should remove the mean offset and linear trend of the waveforms, then filtered them with a Butterworth bandpass filter in the range of 0.05–2 Hz. The figures show a comparison between the raw data and the data after pre-process

st_cmp = st.copy()

st.detrend()
st.filter("bandpass", freqmin=0.05, freqmax=2.0, zerophase=True)

# Plot for comparesion
st_cmp.plot()
st.plot()

png

png

4. Calculate the epicenter distance and back-azimuth

To trim the waveform or rotate the components, you can use the seispy.distaz to calculate the epicenter distance and back-azimuth.

da = seispy.distaz(st[0].stats.sac.stla, st[0].stats.sac.stlo, st[0].stats.sac.evla, st[0].stats.sac.evlo)
dis = da.delta
bazi = da.baz
ev_dep = st[0].stats.sac.evdp
print('Distance = %5.2f˚' % dis)
print('back-azimuth = %5.2f˚' % bazi)
Distance = 51.64˚
back-azimuth = 131.59˚

5. Rotation

Now you can rotate horizontal components (ENZ) into radial and transverse components (TRZ)

st_TRZ = st.copy().rotate('NE->RT', back_azimuth=bazi)

6. Estimating P arrival time and ray parameter by obspy.taup

from obspy.taup import TauPyModel

model = TauPyModel(model='iasp91')
arrivals = model.get_travel_times(ev_dep, dis, phase_list=['P'])
rayp = model.get_ray_paths(ev_dep, dis, phase_list=['P'])
P_arr = arrivals[0]
P_ray = rayp[0]
/Users/xumj/.pyenv/versions/anaconda3-5.3.1/lib/python3.7/site-packages/obspy/taup/tau_branch.py:496: UserWarning: Resizing a TauP array inplace failed due to the existence of other references to the array, creating a new array. See Obspy #2280.
  warnings.warn(msg)

7.Trim the waveforms for PRF

Then you cut 130 s long waveforms around P arrival time (from 10 s before to 120 s after theoretical P arrival times).

dt = st[0].stats.delta
shift = 10
time_after = 120

cut_pos_begin = int((P_arr.time - st[0].stats.sac.o - shift) / dt)
cut_pos_end = int((P_arr.time - st[0].stats.sac.o + time_after) / dt)

T = st_TRZ[0].data[cut_pos_begin:cut_pos_end+1]
R = st_TRZ[1].data[cut_pos_begin:cut_pos_end+1]
Z = st_TRZ[2].data[cut_pos_begin:cut_pos_end+1]

time_axis = np.linspace(-shift, time_after, T.shape[0])

ax1 = plt.subplot(3,1,1)
ax1.plot(time_axis, Z)
ax1.set_ylabel('Z')

ax2 = plt.subplot(3,1,2)
ax2.plot(time_axis, T)
ax2.set_ylabel('T')

ax3 = plt.subplot(3,1,3)
ax3.plot(time_axis, R)
ax3.set_ylabel('R')

Text(0,0.5,'R')

png

8. Calculate PRF

seispy.decov.decovit provide a function with an iterative time-domain deconvolution method. Now let's Calculate a PRF. In this example we assume:

  • Gauss factor = 2.0
  • The maximum number of iterations = 400
  • Minimum error = 0.001
f0 = 2.0
tmax = 400
minderr = 0.001

PRF_R, RMS, it = seispy.decov.decovit(R, Z, dt, R.shape[0], shift, f0, tmax, minderr)
PRF_T, RMS, it = seispy.decov.decovit(T, Z, dt, T.shape[0], shift, f0, tmax, minderr)

plt.plot(time_axis, PRF_R)
plt.plot(time_axis, PRF_T)
plt.xlim([-5, 30])
plt.legend(['R', 'T'])
<matplotlib.legend.Legend at 0x1c22688d30>

png

The figure shows PRFs in R and T components.

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