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S-wave attenuation in northeastern Sonora, Mexico, near the faults that ruptured during the earthquake of 3 May 1887 Mw 7.5.

Villalobos-Escobar GP, Castro RR - Springerplus (2014)

Bottom Line: The attenuation functions obtained for 23 frequencies (0.4 ≤ f ≤ 63.1 Hz) permit us estimating the average quality factor Q S  = (141 ± 1.1 )f ((0.74 ± 0.04)) and a geometrical spreading term G(r) = 1/r (0.21).These results indicate that near the fault zone S waves attenuate considerably more than at regional scale, particularly at low frequencies.This may be the result of strong scattering near the faults due to the fractured upper crust and higher intrinsic attenuation due to stress concentration near the faults.

View Article: PubMed Central - PubMed

Affiliation: División Ciencias de la Tierra, Departamento de Sismología, Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), Carretera Ensenada-Tijuana No. 3918, Ensenada, Baja California 22860 México.

ABSTRACT
We used a new data set of relocated earthquakes recorded by the Seismic Network of Northeastern Sonora, Mexico (RESNES) to characterize the attenuation of S-waves in the fault zone of the 1887 Sonora earthquake (M w 7.5). We determined spectral attenuation functions for hypocentral distances (r) between 10 and 140 km using a nonparametric approach and found that in this fault zone the spectral amplitudes decay slower with distance at low frequencies (f < 4 Hz) compared to those reported in previous studies in the region using more distant recordings. The attenuation functions obtained for 23 frequencies (0.4 ≤ f ≤ 63.1 Hz) permit us estimating the average quality factor Q S  = (141 ± 1.1 )f ((0.74 ± 0.04)) and a geometrical spreading term G(r) = 1/r (0.21). The values of Q estimated for S-wave paths traveling along the fault system that rupture during the 1887 event, in the north-south direction, are considerably lower than the average Q estimated using source-station paths from multiple stations and directions. These results indicate that near the fault zone S waves attenuate considerably more than at regional scale, particularly at low frequencies. This may be the result of strong scattering near the faults due to the fractured upper crust and higher intrinsic attenuation due to stress concentration near the faults.

No MeSH data available.


Related in: MedlinePlus

Examples of signal and noise spectra from stations ELO (located on bedrock) and MOR (located on conglomerate). Left panels are spectra from local events (r < 20 km) and right panels spectra from regional events (r = 138.9 km). Continuous lines represent the 80% energy S-wave spectra (both horizontal components) and discontinuous lines are the noise spectra of a 6-seconds time window choose previous to the first P-wave arrival.
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Fig3: Examples of signal and noise spectra from stations ELO (located on bedrock) and MOR (located on conglomerate). Left panels are spectra from local events (r < 20 km) and right panels spectra from regional events (r = 138.9 km). Continuous lines represent the 80% energy S-wave spectra (both horizontal components) and discontinuous lines are the noise spectra of a 6-seconds time window choose previous to the first P-wave arrival.

Mentions: The records were baseline corrected, subtracting the mean, to remove long-period biases. We chose time windows starting before the first S-wave arrival and containing 80% of the S-wave energy to calculate the Fourier acceleration spectra of the north–south (NS) and east–west (EW) components. We verify visually that the windows contain the strong-ground motions including the peak acceleration. The first and last 5% of the window are cosine tapered and the spectral amplitudes smoothed averaging within a variable frequency band of ±25% of 23 predefined central frequencies between 0.4 and 63.1 Hz. The spectral amplitudes were smoothed using a variable frequency band of +/- 25% over the 23 predefined central frequencies. For each spectral record, we selected the signal frequency band above noise level by visual inspection of the spectrum. Figure 3 shows examples of signal and noise spectra from stations ELO and MOR located on bedrock and conglomerates, respectively. This figure compares noise and signal spectral levels recorded at both horizontal components from events recorded at epicentral distances less than 20 km (left frames) and at 138.9 km (right frames). The signal spectra (solid lines) correspond to the 80% of the energy of the S-waves and the noise spectra (dashed lines) to 6-seconds time windows starting before the first P-wave arrivals. Examples of the acceleration spectra calculated for two events with magnitudes ML 3.5 and 1.9 are also shown in Figures 4 and 5, respectively. The signal to noise ratio for these stations is above one in the frequency band 1.0 – 63.1 Hz for local events but increases for events with lager magnitude (M > 1.7). For regional events the signal to noise ratio of stations ELO and MOR is above one in the band 1.0 – 40 Hz.Figure 3


S-wave attenuation in northeastern Sonora, Mexico, near the faults that ruptured during the earthquake of 3 May 1887 Mw 7.5.

Villalobos-Escobar GP, Castro RR - Springerplus (2014)

Examples of signal and noise spectra from stations ELO (located on bedrock) and MOR (located on conglomerate). Left panels are spectra from local events (r < 20 km) and right panels spectra from regional events (r = 138.9 km). Continuous lines represent the 80% energy S-wave spectra (both horizontal components) and discontinuous lines are the noise spectra of a 6-seconds time window choose previous to the first P-wave arrival.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4320219&req=5

Fig3: Examples of signal and noise spectra from stations ELO (located on bedrock) and MOR (located on conglomerate). Left panels are spectra from local events (r < 20 km) and right panels spectra from regional events (r = 138.9 km). Continuous lines represent the 80% energy S-wave spectra (both horizontal components) and discontinuous lines are the noise spectra of a 6-seconds time window choose previous to the first P-wave arrival.
Mentions: The records were baseline corrected, subtracting the mean, to remove long-period biases. We chose time windows starting before the first S-wave arrival and containing 80% of the S-wave energy to calculate the Fourier acceleration spectra of the north–south (NS) and east–west (EW) components. We verify visually that the windows contain the strong-ground motions including the peak acceleration. The first and last 5% of the window are cosine tapered and the spectral amplitudes smoothed averaging within a variable frequency band of ±25% of 23 predefined central frequencies between 0.4 and 63.1 Hz. The spectral amplitudes were smoothed using a variable frequency band of +/- 25% over the 23 predefined central frequencies. For each spectral record, we selected the signal frequency band above noise level by visual inspection of the spectrum. Figure 3 shows examples of signal and noise spectra from stations ELO and MOR located on bedrock and conglomerates, respectively. This figure compares noise and signal spectral levels recorded at both horizontal components from events recorded at epicentral distances less than 20 km (left frames) and at 138.9 km (right frames). The signal spectra (solid lines) correspond to the 80% of the energy of the S-waves and the noise spectra (dashed lines) to 6-seconds time windows starting before the first P-wave arrivals. Examples of the acceleration spectra calculated for two events with magnitudes ML 3.5 and 1.9 are also shown in Figures 4 and 5, respectively. The signal to noise ratio for these stations is above one in the frequency band 1.0 – 63.1 Hz for local events but increases for events with lager magnitude (M > 1.7). For regional events the signal to noise ratio of stations ELO and MOR is above one in the band 1.0 – 40 Hz.Figure 3

Bottom Line: The attenuation functions obtained for 23 frequencies (0.4 ≤ f ≤ 63.1 Hz) permit us estimating the average quality factor Q S  = (141 ± 1.1 )f ((0.74 ± 0.04)) and a geometrical spreading term G(r) = 1/r (0.21).These results indicate that near the fault zone S waves attenuate considerably more than at regional scale, particularly at low frequencies.This may be the result of strong scattering near the faults due to the fractured upper crust and higher intrinsic attenuation due to stress concentration near the faults.

View Article: PubMed Central - PubMed

Affiliation: División Ciencias de la Tierra, Departamento de Sismología, Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), Carretera Ensenada-Tijuana No. 3918, Ensenada, Baja California 22860 México.

ABSTRACT
We used a new data set of relocated earthquakes recorded by the Seismic Network of Northeastern Sonora, Mexico (RESNES) to characterize the attenuation of S-waves in the fault zone of the 1887 Sonora earthquake (M w 7.5). We determined spectral attenuation functions for hypocentral distances (r) between 10 and 140 km using a nonparametric approach and found that in this fault zone the spectral amplitudes decay slower with distance at low frequencies (f < 4 Hz) compared to those reported in previous studies in the region using more distant recordings. The attenuation functions obtained for 23 frequencies (0.4 ≤ f ≤ 63.1 Hz) permit us estimating the average quality factor Q S  = (141 ± 1.1 )f ((0.74 ± 0.04)) and a geometrical spreading term G(r) = 1/r (0.21). The values of Q estimated for S-wave paths traveling along the fault system that rupture during the 1887 event, in the north-south direction, are considerably lower than the average Q estimated using source-station paths from multiple stations and directions. These results indicate that near the fault zone S waves attenuate considerably more than at regional scale, particularly at low frequencies. This may be the result of strong scattering near the faults due to the fractured upper crust and higher intrinsic attenuation due to stress concentration near the faults.

No MeSH data available.


Related in: MedlinePlus