Automatic Fault Plane Solution for the Provision of Rapid Earthquake Information in South Korea
Round 1
Reviewer 1 Report
The authors developed a real-time automated moment tensor inversion using TDMT_INV to provide quick source parameters for local and regional earthquakes in the vicinity of the Korean Peninsula. Overall, this study is well-written and organized generally. Before recommending publication, some minor issues need to improve. Details are listed as follows.
1) Is there technology using actual services?
2) If there is a service case for TDMT_INV, where to find it?
3) What is the meaning of "VAR" in this paper?
4) Figure 2, Figure 3b, and Figure 5 should be shown on a larger scale.
5) There are a few English typos. Please go over and correct them.
Author Response
We thank the Associate Editor and the reviewer for their comprehensive evaluations of our work. We have carefully considered the feedback and revised the manuscript. The modifications to the manuscript addressing all comments are marked with red font color.
attached file
Author Response File: 
Author Response.pdf
Reviewer 2 Report
I think that this paper is good and the methodology for automatic fault plane solution for the provision of rapid earthquake information in South Korea is very promising. However, some remarks can be taken into account to improve the article:
· · The English is correct in general. Paper is well written.
· · Green's function calculation method was not explain in the paper. Add a paragraph to explain the method is very appreciate (analytical, semi-analytical or numerical).
· · Details on the calculation code do not exist in the paper. The addition of a paragraph presenting the calculation code is essential.
· · This procedure also makes it possible to obtain solutions using a minimum number of stations. Add a reference to confirm this hypothesis.
· · If several stations are used, is it that the impact of the geometry of the network of stations has been considered? Add a paragraph to explain whether you have tested this parameter (the geometry of the network).
· · Only 03 recent references mentioned in the paper, and the rest are more than 10 years old. Add at least 5 more recent ones between 2017-2022.
· It would be interesting to see how this methodology performs for other sites in the world!!
For this, I recommend this paper be accept for publication with revision.
Author Response
Response to reviewer
[Reviewer 2]
I think that this paper is good and the methodology for automatic fault plane solution for the provision of rapid earthquake information in South Korea is very promising. However, some remarks can be taken into account to improve the article:
We thank the Associate Editor and the reviewer for their comprehensive evaluations of our work. We have carefully considered the feedback and revised the manuscript. The modifications to the manuscript addressing all comments are marked with red font color.
- The English is correct in general. Paper is well written.
Thank you.
(2) Green's function calculation method was not explain in the paper. Add a paragraph to explain the method is very appreciate (analytical, semi-analytical or numerical).
Appropriate revisions have been made to lines 76–88.
Moment tensor analysis involves fitting theoretical waveforms with observed signals and inverting for the moment tensor elements [17]. The applied methodology is based on a simplified general representation of the seismic sources by considering both a spatial and temporal point-source [18]. This method can be summarized by the equation:
|
|
(1) |
where Un is the observed nth of displacement, Gni,j is the nth of Green’s function for specific force-couple orientations, x is the distance to source from seismic station, z is the depth. We calculated Green's function, which is the impulse response of the observed seismic data. The empirical Green's function is obtained between path- and site-specific effects because these constitute the earth's response between the source and receiver [19, 20]. Therefore, Green's function can be used to retrieve earthquake source properties or estimate theoretical waveforms. Mij is the scalar seismic moment tensor. In addition, the indices i and j refer to geographical directions. The general force-couples for a deviatoric moment tensor may be represented by three fundamental-faults, namely a vertical strike-slip, a vertical dip-slip, or a 45 ° dip-slip [21]. Equation (1) can be calculated by assuming the source depth for each inversion using linear least squares. The estimated scalar moment tensor, Mij, can be decomposed into the scalar seismic moment, a compensated linear vector dipole moment tensor (CLVD), and a double-couple moment tensor (DC) [22]. This decomposition is represented as percent DC (PDC) and CLVD (PCLVD), respectively. A percent isotropic (PISO) is assumed as zero for this deviatoric application. The strike, rake, and dip of fault planes can be obtained by DC.
(3) Details on the calculation code do not exist in the paper. The addition of a paragraph presenting the calculation code is essential.
We believe the algorithm for this technology can be developed based on the schematic diagram in Figure 1. Unfortunately, this algorithm is confidential owing to the institutional policy. However, we can provide it after reviewing it when anyone requests it.
Appropriate revisions have been made to lines 296–297
Data Availability Statement: Earthquake record data can be downloaded from https://necis.kma.go.kr, and algorithms data will be provided upon request.
(4) This procedure also makes it possible to obtain solutions using a minimum number of stations. Add a reference to confirm this hypothesis.
Appropriate revisions have been to made to lines 159–179.
To obtain the maximum reduction variance in the inversion, the epicentral distance range was tested from 50 to 250 km and was found to be reasonable up to 200 km for local events. The number of stations in quadrants, preventing the azimuth of stations focusing in specific directions, was dependent on the magnitude so the number in the case of magnitude less than 4 and greater than 4 is 6 and 4, respectively. Currently, the accuracy of the focal mechanism depends on the stations. Therefore, the reliability of the station used must be controlled, which is called the quality factor. The quality factor of 4 was enough in the automatic process, despite the magnitude being less than 4.
Fault mechanisms are more accurate when obtained using four-quadrant data. FOMEC requires both tensional and compressional wave information based on the epicenter to the double-couple plane, rendering its analysis difficult in earthquakes outside the seismic network. In addition, a reliable focal mechanism can be obtained if the AFM contains at least one station in every quadrant [28].
In contrast, AFM calculates tomographic information after selecting stations with high VR using theoretical synthetic waveforms. Compared with AFM, tomographic information can be calculated across a wider area with fewer stations. If we increase the number of AFM stations, data on the high relationship between theoretical synthetic wave and observed data will be required. When high VR stations were added to the previous test, there was no significant change in the focal mechanism form. However, increasing the number of stations is difficult when earthquakes occur at sea. Therefore, using at least four stations or more in two quadrants is recommended.
- Clinton, J. F.; Hauksson, E.; Solanki K. Automatically generatedmoment tensor solutions for southern California: robustness of the mw magnitude scale and style of faulting. Seism. Soc. Am. 2006, 96, 1689–1705.
(5) If several stations are used, is it that the impact of the geometry of the network of stations has been considered? Add a paragraph to explain whether you have tested this parameter (the geometry of the network).
Kindly check answer (5). I believe it is the same.
(6) Only 03 recent references mentioned in the paper, and the rest are more than 10 years old. Add at least 5 more recent ones between 2017-2022.
Appropriate revisions have been made to lines 230–258.
Currently, the EEW service in Korea provides cell phone messages to citizens about an incoming earthquake to respond to danger. The KMA EEW automatically calculates and spreads an earthquake warning message when an earthquake of ML > 3.5 occurs (or 4.0, if occurring over the sea). AFM can be used with a magnitude of ≥3.5, rendering it suitable for the notified earthquake analysis. The notified earthquake information service is provided through the KMA website (https://www.weather.go.kr/w/eqk-vol/search/korea.do), which began on July 22, 2019. The earthquake information service report comprises hypocenter, earthquake magnitude, seismic measurement intensity, waveform, and seismic analysis.
AFM covers only the notified area and depends on seismic network distribution. The number of Korean peninsula seismic stations has rapidly increased since 2016. Figure 6 shows the current status of observation networks operated by the KMA and KIGAM. There are 301 observatories operating at the KMA and 61 at KIGAM. The boundary line of the notification area is set based on the operating observations and is indicated using the black line in Figure 6. The KMA has been promoting the expansion and revision of the old stations. In addition, KMA will be able to add more observatories using other agencies. Therefore, we must estimate Green’s function for the additional stations. These service policies were designed based on the decisions made by the KMA in South Korea, which might differ in other countries.
Figure 6. Observation network in South Korea for earthquake detection. Black line: boundary line of notification for domestic earthquakes, blue triangle: seismic observations from the KMA, green triangle: seismic observations from KIGAM.
(7) It would be interesting to see how this methodology performs for other sites in the world!!
Thank you for your comment.
Author Response File: Author Response.pdf
Reviewer 3 Report
The manuscript entitled “Automatic fault plane solution for the provision of rapid earthquake information in South Korea” discusses the implementation of an automated procedure for calculating focal mechanisms after an earthquake early warning. The manuscript is well written, however, needs a few changes. Following are some of the observations:
1. The limitation of FOCMEC is stated that it needs an accurate region-specific velocity model. However, the new method developed by the authors also requires an accurate Green function. How this limitation of FOCMEC is overcome in the new method?
2. What is the magnitude threshold for earthquake early warning in Korea?
3. The entire results are discussed in reference to EEW. Please discuss what distance range is kept for regional earthquakes.
4. The figure resolution is very low.
5. Authors should try to report the results for magnitude <3.5 earthquakes (at least one earthquake).
Author Response
Response to reviewer
[Reviewer 3]
The manuscript entitled “Automatic fault plane solution for the provision of rapid earthquake information in South Korea” discusses the implementation of an automated procedure for calculating focal mechanisms after an earthquake early warning. The manuscript is well written, however, needs a few changes. Following are some of the observations:
We thank you for your comprehensive evaluation of our work. We have carefully considered the feedback and revised the manuscript. The modifications to the manuscript addressing all comments are marked with red font color.
(1) The limitation of FOCMEC is stated that it needs an accurate region-specific velocity model. However, the new method developed by the authors also requires an accurate Green function. How this limitation of FOCMEC is overcome in the new method?
FOMEC uses the initial P wave, making the examination of site properties (or Green’s function) unfeasible.
Appropriate revisions have been made to lines 162–182.
To obtain the maximum reduction variance in the inversion, the epicentral distance range was tested from 50 to 250 km and was found to be reasonable up to 200 km for local events. The number of stations in quadrants, preventing the azimuth of stations focusing in specific directions, was dependent on the magnitude so the number in the case of magnitude less than 4 and greater than 4 is 6 and 4, respectively. Currently, the accuracy of the focal mechanism depends on the stations. Therefore, the reliability of the station used must be controlled, which is called the quality factor. The quality factor of 4 was enough in the automatic process, despite the magnitude being less than 4.
Fault mechanisms are more accurate when obtained using four-quadrant data. FOMEC requires both tensional and compressional wave information based on the epicenter to the double-couple plane, rendering its analysis difficult in earthquakes outside the seismic network. In addition, a reliable focal mechanism can be obtained if the AFM contains at least one station in every quadrant [28].
- Clinton, J. F.; Hauksson, E.; Solanki K. Automatically generatedmoment tensor solutions for southern California: robustness of the mw magnitude scale and style of faulting. Seism. Soc. Am. 2006, 96, 1689–1705.
- What is the magnitude threshold for earthquake early warning in Korea?
We have addressed all your comments below and made suitable revisions in lines 230–258 in the manuscript accordingly, with a description of the KMA service policy.
Currently, the EEW service in Korea provides cell phone messages to citizens about an incoming earthquake to respond to danger. The KMA EEW automatically calculates and spreads an earthquake warning message when an earthquake of ML > 3.5 occurs (or 4.0, if occurring over the sea). AFM can be used with a magnitude of ≥3.5, rendering it suitable for the notified earthquake analysis. The notified earthquake information service is provided through the KMA website (https://www.weather.go.kr/w/eqk-vol/search/korea.do), which began on July 22, 2019. The earthquake information service report comprises hypocenter, earthquake magnitude, seismic measurement intensity, waveform, and seismic analysis.
AFM covers only the notified area and depends on seismic network distribution. The number of Korean peninsula seismic stations has rapidly increased since 2016. Figure 6 shows the current status of observation networks operated by the KMA and KIGAM. There are 301 observatories operating at the KMA and 61 at KIGAM. The boundary line of the notification area is set based on the operating observations and is indicated using the black line in Figure 6. The KMA has been promoting the expansion and revision of the old stations. In addition, KMA will be able to add more observatories using other agencies. Therefore, we must estimate Green’s function for the additional stations. These service policies were designed based on the decisions made by the KMA in South Korea, which might differ in other countries.
Figure 6. Observation network in South Korea for earthquake detection. Black line: boundary line of notification for domestic earthquakes, blue triangle: seismic observations from the KMA, green triangle: seismic observations from KIGAM.
(3) The entire results are discussed in reference to EEW. Please discuss what distance range is kept for regional earthquakes.
It is the same as answer (2).
(4) The figure resolution is very low.
We will submit high-resolution image files to the editorial team.
(5) Authors should try to report the results for magnitude <3.5 earthquakes (at least one earthquake).
Kindly check answer (2). I believe it is the answer your question.
Author Response File: Author Response.pdf
