Qc Evaluation AU network

Introduction

The following is an evaluation of waveforms and metadata from selected stations of the AU network. This effort was prompted by prroblems in using data from two stations of the network for the earthquake of 2013/06/09 14:22.

This evaluation will consist of looking at teleseismic signals to check amplitudes and polarities.

QC Evaluation June 11, 2013

In the presentations to follow, waveform data were obtained using the IRIS WilberII interface. The SEED volumes were unpacked using the command

rdseed -f SEED_VOLUME -p -R -d -o 1

The instrument response was removed using the information in the RESP file and ultimately the waveforms were rotated to vertical, radial and transverse.

The first check was to plot the vertical component ground velocity in a record section plot as a function of arc-distance. For teleseismic P waves, the amplitude should not change much with respect to distance.

Marianas earthquake 2013/05/14

This event had the parameters

Event: 2013/05/14 00:32:25.8
Catalog: NEICALRT  Mag: 7.0  Type: MW  Contributor: NEIC  
Lat: 18.75  Lon: 145.26  Depth: 603.30
Description: MARIANA ISLANDS

Location of earthquake and stations

True amplitude record section

This plot was made using the gsac commands

GSAC> r *BHZ*
GSAC> hp c 0.02 n 3
GSAC> lp c 0.1 n 3
GSAC prs p 0.07 amp 0.2 ann sta tl 0 200 sa gcarc vl 20 65

The AS31 amplitudes are too low. Focusing in a time window +- 25 sec about the predicted P-wave first arrival, and using the same filter as above, the peak amplitudes are
```
NIUEBHZ.AU..sac (22):
       DEPMAX         8.484535e-07
MOOBHZ.AU..sac (18):
       DEPMAX         7.549526e-07
STKABHZ.AU..sac (38):
       DEPMAX         3.242334e-07
AS31BHZ.AU..sac (1):
       DEPMAX         6.289518e-08
```
Here is seems as if the amplitude differs by at least a factor of 10.
A problem was noted previously with the component orientation for QIS. The next image compares the vertical component ground velocity (m/s) (no filtering) in a window 20 seconds before the predicted P arrival to 60 seconds after. The QIS vertical motion agrees with other stations. Thus the vertical orientation is correct.

The back azimuth from QIS to the earthquake is 8.4 degrees. However the three component particle motion in the next plot indicates that the motion is a push to the north instead of the expected up to the south.

The component orientation for the NS component in inverted
The metadata indicates the orientation expected for a typical N component. Is there a wiring problem?.

Taiwan earthquake 2013/06/02

This event had the parameters

Event: 2013/06/02 05:43:03.8
Catalog: NEICALRT  Mag: 6.5  Type: MW  Contributor: NEIC  
Lat: 23.79  Lon: 121.13  Depth: 14.30
Description: TAIWAN

Location of earthquake and stations

True amplitude record section

This plot was made using the gsac commands

GSAC> r *BHZ*
GSAC> hp c 0.02 n 3
GSAC> lp c 0.1 n 3
GSAC> prs  amp 0.2 ann sta vl 30 75 p 0.07 tl 40 240 sa 0

In plot record section plots the amplitude of AS31 is too low, as is STKA. Focusing in a time window +- 25 sec about the predicted P-wave first arrival, and using the same filter as above, the peak amlitudes are
```
CNBBHZ.AU..sac   (5): DEPMAX          1.16306e-06
EIDSBHZ.AU..sac  (7): DEPMAX         1.045306e-06
KDUBHZ.AU..sac  (10): DEPMAX         9.438546e-07
MTNBHZ.AU..sac  (16): DEPMAX         9.437817e-07
STKABHZ.AU..sac (34): DEPMAX         2.852866e-07
AS31BHZ.AU..sac  (1): DEPMAX         4.017207e-08
```
We see that that the AS31 ground motions are about 20 times smaller than expected and that the STKA motions are about 4 times smaller.
Following up on the discussion concerning QIS, the next figure show the time window from 25 seconds before to 60 seconds after the expected P wave first arrival. The traces are filtered in the 0;02 - 0.10 Hz band.

The back azimuth from the station to the earthquake is 336 degrees. For compression on the vertical, we expect a negative amplitude on the NS and a positive on the EW.

Discussion

Focusing on QIS, after some experimentation, it seems as if the following set of orientations may be reasonable:

     Sac notations for direction of positive ground motion on component

QIC    BHN   CMPINC=90  CMPAZ=201  instead of 0
QIC    BHE   CMPINC=90  CMPAZ=111  instead of 90
QIC    BHZ   CMPINC=0   CMPAZ=0

To test this I made some synthetic teleseismic signals using GCMT MT parameters to compare first motions and waveforms shapes. This is not a perfect exercise because of limitations of the synthetics.

Ground velocities of the earthquake of 20130514003225 are comapred in the 0.02 - 0.05 Hz band in a window starting 80 seconds before the P arrival to 300 seco nds after.

Ground velocities of the earthquake of 20130602054303/ are comapred in the 0.02 - 0.05 Hz band in a window starting 80 seconds before the P arrival to 600 seconds after.

A problem with a comparison to synthetics is that the simple 1-D velocity model may never catch the complexity in the observed data. The waveform comparison for both earthquakes shows that arrivals at the P-wave time are fit well in polarity and amplitude. The comparison for the 2013/06/02 earthquake shows good agreement in the P-waves and in the transverse and even most of the radial.

Thus the original inference of a component orientation problem at QIS is validated.

last changed June 11, 2013