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Early magnitude estimation for the MW7.9 Wenchuan earthquake using progressively expanded P-wave time window.

Peng C, Yang J, Zheng Y, Xu Z, Jiang X - Sci Rep (2014)

Bottom Line: This information would have been available 40 s after the earthquake origin time and could have been refined in the successive 20 s using data from more distant stations.The reason for the magnitude underestimation is in part a combined effect of high-pass filtering and frequency dependence of the main radiating source during the rupture process.Finally we suggest only using Pd alone for magnitude estimation because of its slight magnitude saturation compared to the τc magnitude.

View Article: PubMed Central - PubMed

Affiliation: Institute of Geophysics, China Earthquake Administration, No. 5 Minzudaxue South Road, Haidian District, Beijing 100081, China.

ABSTRACT
More and more earthquake early warning systems (EEWS) are developed or currently being tested in many active seismic regions of the world. A well-known problem with real-time procedures is the parameter saturation, which may lead to magnitude underestimation for large earthquakes. In this paper, the method used to the MW9.0 Tohoku-Oki earthquake is explored with strong-motion records of the MW7.9, 2008 Wenchuan earthquake. We measure two early warning parameters by progressively expanding the P-wave time window (PTW) and distance range, to provide early magnitude estimates and a rapid prediction of the potential damage area. This information would have been available 40 s after the earthquake origin time and could have been refined in the successive 20 s using data from more distant stations. We show the suitability of the existing regression relationships between early warning parameters and magnitude, provided that an appropriate PTW is used for parameter estimation. The reason for the magnitude underestimation is in part a combined effect of high-pass filtering and frequency dependence of the main radiating source during the rupture process. Finally we suggest only using Pd alone for magnitude estimation because of its slight magnitude saturation compared to the τc magnitude.

No MeSH data available.


Related in: MedlinePlus

Average values of τc as a function of the cut-off frequency and of PTW.The dashed lines are the best fit curves evaluated through a linear regression on the τc average values with PTW 3, 9, and 15 secs, respectively.
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f3: Average values of τc as a function of the cut-off frequency and of PTW.The dashed lines are the best fit curves evaluated through a linear regression on the τc average values with PTW 3, 9, and 15 secs, respectively.

Mentions: We tested 110 combinations of cut-off frequencies and PTWs, from 0.001 Hz to 0.07 Hz and from 3 to 30 s, respectively. To avoid the S-wave contamination while increasing the PTW we computed the theoretical S-wave arrival times and excluded from our analysis all the stations for which the estimated S-wave arrival occurred within the considered PTW. While expanding the PTW the closest stations are hence excluded one by one during the analysis. Furthermore, in order to compare the average Pd values for different stations, these have to be corrected for the geometrical attenuation effect. A global Pd-Magnitude scaling relationship obtained by Kuyuk & Allen27 was used to correct the displacement for the distance effect and to normalize all the measured values to a reference distance of 100 km. In this research, they used data from earthquakes of magnitude 3.0–8.0 recorded in California and Japan. In addition, many events with magnitudes from 0.2 to 3.0 which were detected by the ElarmS EEWS from 1 May 2012 to 10 June 2013 were also used for obtaining the Pd-Magnitude relationship. These data set provides a “real world” view of the events that any EEWS must handle, i.e., very small earthquakes in addition to the larger magnitudes that the warning system is designed for. The best fit regression relationship for the global data set is where MGPd is the global Pd-Magnitude, R is the epicentral distance in kilometers and Pd is in centimeters. This relationship can also be rewritten into the following form in order to make it in the same format as those in previous studies: Then we computed the average Pd and τc values as a function of the cut-off frequency for different PTWs. Results are shown in Figures 2 and 3. We found that both parameters vary with the cut-off frequency with a similar trend. Both Pd and τc assume the largest values for small cut-off frequencies and large PTWs. However, the relationships between these two parameters and the cut-off frequency for each considered PTW are gradually changed from linear to non-linear. For the parameters with PTW < 20 s, their logarithms decrease almost linearly with the cut-off frequency, but when the PTW expanding, the relationships are gradually changed into nonlinear, especially for the parameters with the cut-off frequency < 0.01 Hz. The reason is that the epicentral distances for stations used for calculating the Pd and τc with PTW ≥ 20 s are more than 150 km and many other stations within 150 km are excluded. For these used stations the low-frequency content are more and more dominant in the data as the low cut-off frequency progressively decreases. This result is not consistent with that obtained by Combellie et al.37 because most of the stations used in their study are located 150 km away from the epicentre and only data with PTW 30 s are used to calculate the two early parameters. If they use longer PTW, they should come to the same conclusion. However, from Figures 2 and 3 we can also find that there are linear regressions between these two parameters logarithms and the cut-off frequency for each considered PTW when the lower limit of the cut-off frequency is 0.02 Hz. This behavior suggests that, log(τc) [or log(Pd)] may be used for estimating the magnitude, provided that proper scaling coefficients are determined from the data with certain cut-off filtering frequency. For the sake of uniformity with the previous works and since the empirical relationships used in this work were obtained with a 0.075 Hz cut-off frequency, we decided to maintain this value for the filtering operation.


Early magnitude estimation for the MW7.9 Wenchuan earthquake using progressively expanded P-wave time window.

Peng C, Yang J, Zheng Y, Xu Z, Jiang X - Sci Rep (2014)

Average values of τc as a function of the cut-off frequency and of PTW.The dashed lines are the best fit curves evaluated through a linear regression on the τc average values with PTW 3, 9, and 15 secs, respectively.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4209451&req=5

f3: Average values of τc as a function of the cut-off frequency and of PTW.The dashed lines are the best fit curves evaluated through a linear regression on the τc average values with PTW 3, 9, and 15 secs, respectively.
Mentions: We tested 110 combinations of cut-off frequencies and PTWs, from 0.001 Hz to 0.07 Hz and from 3 to 30 s, respectively. To avoid the S-wave contamination while increasing the PTW we computed the theoretical S-wave arrival times and excluded from our analysis all the stations for which the estimated S-wave arrival occurred within the considered PTW. While expanding the PTW the closest stations are hence excluded one by one during the analysis. Furthermore, in order to compare the average Pd values for different stations, these have to be corrected for the geometrical attenuation effect. A global Pd-Magnitude scaling relationship obtained by Kuyuk & Allen27 was used to correct the displacement for the distance effect and to normalize all the measured values to a reference distance of 100 km. In this research, they used data from earthquakes of magnitude 3.0–8.0 recorded in California and Japan. In addition, many events with magnitudes from 0.2 to 3.0 which were detected by the ElarmS EEWS from 1 May 2012 to 10 June 2013 were also used for obtaining the Pd-Magnitude relationship. These data set provides a “real world” view of the events that any EEWS must handle, i.e., very small earthquakes in addition to the larger magnitudes that the warning system is designed for. The best fit regression relationship for the global data set is where MGPd is the global Pd-Magnitude, R is the epicentral distance in kilometers and Pd is in centimeters. This relationship can also be rewritten into the following form in order to make it in the same format as those in previous studies: Then we computed the average Pd and τc values as a function of the cut-off frequency for different PTWs. Results are shown in Figures 2 and 3. We found that both parameters vary with the cut-off frequency with a similar trend. Both Pd and τc assume the largest values for small cut-off frequencies and large PTWs. However, the relationships between these two parameters and the cut-off frequency for each considered PTW are gradually changed from linear to non-linear. For the parameters with PTW < 20 s, their logarithms decrease almost linearly with the cut-off frequency, but when the PTW expanding, the relationships are gradually changed into nonlinear, especially for the parameters with the cut-off frequency < 0.01 Hz. The reason is that the epicentral distances for stations used for calculating the Pd and τc with PTW ≥ 20 s are more than 150 km and many other stations within 150 km are excluded. For these used stations the low-frequency content are more and more dominant in the data as the low cut-off frequency progressively decreases. This result is not consistent with that obtained by Combellie et al.37 because most of the stations used in their study are located 150 km away from the epicentre and only data with PTW 30 s are used to calculate the two early parameters. If they use longer PTW, they should come to the same conclusion. However, from Figures 2 and 3 we can also find that there are linear regressions between these two parameters logarithms and the cut-off frequency for each considered PTW when the lower limit of the cut-off frequency is 0.02 Hz. This behavior suggests that, log(τc) [or log(Pd)] may be used for estimating the magnitude, provided that proper scaling coefficients are determined from the data with certain cut-off filtering frequency. For the sake of uniformity with the previous works and since the empirical relationships used in this work were obtained with a 0.075 Hz cut-off frequency, we decided to maintain this value for the filtering operation.

Bottom Line: This information would have been available 40 s after the earthquake origin time and could have been refined in the successive 20 s using data from more distant stations.The reason for the magnitude underestimation is in part a combined effect of high-pass filtering and frequency dependence of the main radiating source during the rupture process.Finally we suggest only using Pd alone for magnitude estimation because of its slight magnitude saturation compared to the τc magnitude.

View Article: PubMed Central - PubMed

Affiliation: Institute of Geophysics, China Earthquake Administration, No. 5 Minzudaxue South Road, Haidian District, Beijing 100081, China.

ABSTRACT
More and more earthquake early warning systems (EEWS) are developed or currently being tested in many active seismic regions of the world. A well-known problem with real-time procedures is the parameter saturation, which may lead to magnitude underestimation for large earthquakes. In this paper, the method used to the MW9.0 Tohoku-Oki earthquake is explored with strong-motion records of the MW7.9, 2008 Wenchuan earthquake. We measure two early warning parameters by progressively expanding the P-wave time window (PTW) and distance range, to provide early magnitude estimates and a rapid prediction of the potential damage area. This information would have been available 40 s after the earthquake origin time and could have been refined in the successive 20 s using data from more distant stations. We show the suitability of the existing regression relationships between early warning parameters and magnitude, provided that an appropriate PTW is used for parameter estimation. The reason for the magnitude underestimation is in part a combined effect of high-pass filtering and frequency dependence of the main radiating source during the rupture process. Finally we suggest only using Pd alone for magnitude estimation because of its slight magnitude saturation compared to the τc magnitude.

No MeSH data available.


Related in: MedlinePlus