rus
Issue #7/2022
G. I. Dolgikh, S. S. Budrin, A. V. Davydov, S. G. Dolgikh, A. V. Mishakov, V. A. Chupin, V. A. Shvets
Study of Intergeospheric Interaction in the Microseismic Range Using the Laser Interferential Station
Study of Intergeospheric Interaction in the Microseismic Range Using the Laser Interferential Station
DOI: 10.22184/1993-7296.FRos.2022.16.7.540.550
The generation regularities of primary and secondary microseisms in the measuring ground location area have been determined by processing synchronous data of a two-coordinate laser strainmeter, a laser nanobarograph and a laser meter of hydrospheric pressure variations. The magnitude of atmospheric oscillations caused by the secondary microseisms propagating in the Earth’s crust has been obtained. The transfer factor of this interaction is about 0.023 Pa / nm that is three to four times greater than the same factor for the Rayleigh wave propagation in the Earth’s crust, generated by the earthquakes. The work results confirm the high efficiency of synchronous application of all measuring installations that allows us to accurately establish the origin of the registered geosphere disturbances.
The generation regularities of primary and secondary microseisms in the measuring ground location area have been determined by processing synchronous data of a two-coordinate laser strainmeter, a laser nanobarograph and a laser meter of hydrospheric pressure variations. The magnitude of atmospheric oscillations caused by the secondary microseisms propagating in the Earth’s crust has been obtained. The transfer factor of this interaction is about 0.023 Pa / nm that is three to four times greater than the same factor for the Rayleigh wave propagation in the Earth’s crust, generated by the earthquakes. The work results confirm the high efficiency of synchronous application of all measuring installations that allows us to accurately establish the origin of the registered geosphere disturbances.
Теги: laser meter of hydrospheric pressure variations laser nanobarograph microseisms sea waves two-coordinate laser strainmeter двухкоординатный лазерный деформограф лазерный измеритель вариаций гидросферного давления лазерный нанобарограф микросейсмы морские волны
Study of Intergeospheric Interaction in the Microseismic Range Using the Laser Interferential Station
G. I. Dolgikh1, S. S. Budrin1, A. V. Davydov1, S. G. Dolgikh1, A. V. Mishakov2, V. A. Chupin1, V. A. Shvets1
Ilichev Pacific Oceanological Institute, Far Eastern Branch of the Russian Academy of Sciences, Vladivostok, Russia
Far Eastern Federal University, Vladivostok, Russia
The generation regularities of primary and secondary microseisms in the measuring ground location area have been determined by processing synchronous data of a two-coordinate laser strainmeter, a laser nanobarograph and a laser meter of hydrospheric pressure variations. The magnitude of atmospheric oscillations caused by the secondary microseisms propagating in the Earth’s crust has been obtained. The transfer factor of this interaction is about 0.023 Pa / nm that is three to four times greater than the same factor for the Rayleigh wave propagation in the Earth’s crust, generated by the earthquakes. The work results confirm the high efficiency of synchronous application of all measuring installations that allows us to accurately establish the origin of the registered geosphere disturbances.
Keywords: two-coordinate laser strainmeter, laser nanobarograph, laser meter of hydrospheric pressure variations, microseisms, sea waves
Received on: 10.10.2022
Accepted on: 01.11.2022
INTRODUCTION
For historical reasons, the concept of «microseisms» was introduced in an attempt to interpret the Earth’s background oscillations in the period range of 2–20 s, the origin of which was attributed to the frequent weak earthquakes of small magnitude. Subsequently, it was found that these disturbances are caused not by earthquakes, but by the sea waves that generate oscillations in the Earth’s crust in the frequency range of 2–20 s (conditionally), when interacting with the bottom and breaking in the reference zone. The benchmark works devoted to the origin and development of microseismic waves (2–20 s) are the articles of Hasselman and Longuet-Higgins [1, 2] stating that the progressive and standing wind-induced sea waves excite the primary and secondary microseisms, respectively, while interacting with the seabed. The periods of primary microseisms are equal to the periods of progressive wind-induced waves, and the periods of secondary microseisms are equal to half the period of progressive sea waves due to the fact that changes in hydrostatic pressure of a standing sea wave occur twice during one period of a surface sea wave. The periods of primary and secondary microseisms depend on the periods of wind-induced sea waves that are related to the wind velocity and duration, the area and depth of the water zone above which the wind is available. Moreover, for example, [3] shows that formation of the largest spectral maximum in the microseismic range with a peak frequency of 0.14–0.22 Hz (7.1–4.5 s) is related to the low-frequency elastic energy scattering in the rock formations. Moreover, this paper argues that «the ocean waves should not be construed as the cause of low-frequency seismic noise, according to the Longuet-Higgins theory, but rather as its consequence». It is possible that microseismic waves are excited by the atmospheric processes similar to the excitation of the «Earth infragravitational noise» by atmospheric pressure fluctuations in the resonant and near-resonant cases [4].
In view of the foregoing, an important task is to determine the primary source of Earth oscillations in the period range of 2–20 s, i. e. in the microseismic range. It is desirable to perform such studies on one measuring ground using the equipment that simultaneously measures variations in the atmospheric and hydrospheric pressures, microfluctuations of the Earth’s crust. The equipment shall be preferably developed based on the same measuring principles, and shall have the unique amplitude and frequency specifications (wide operating frequency range, high sensitivity). At present, such requirements are met by the one-coordinate and two-coordinate laser strainmeters [5, 6], laser nanobarographs [7], and laser meters of hydrospheric pressure variations [8] that are currently installed on one measuring ground «Cape Schultz» of the Ilichev Pacific Oceanological Institute, Far Eastern Branch of the Russian Academy of Sciences [9].
However, it is not possible just to determine the primary sources of various wave and non-wave processes, since we are extremely interested in the issues related to the study of transformation regularities of these processes at the geosphere boundaries, with their interaction with other multi-scale processes and phenomena. Thus, one of the areas is related to the excitation of atmospheric waves by transmitted Rayleigh waves in the period range from 1 to 20 s generated by the earthquakes [10]. However, the same waves are excited by the gravitational sea waves in the same range of periods, i. e. from 2 to 20 s. The second area that is rather popular now, is the global temperature rise. For some reason, every researcher considers only the impact of greenhouse gases, but does not take into account the energy dissipation in the Earth’s crust or in the marine Earth’s crust due to which the temperature of the world ocean and atmosphere is increased. There are some papers where these effects are studied at the initial level, for example, [11]. With an intensification in the storm activity, an increase in the total energy of typhoons / cyclones, an increase in the total power of earthquakes that depend, among other things, on solar activity, the dissipative energy is raised entailing a global temperature rise.
This paper will pay some attention to solving the problems described above.
LASER INTERFERENTIAL MEASURING SYSTEMS
The article analyzes the data obtained when performing synchronous measurements of variations in the atmospheric and hydrospheric pressures, crustal micromovements at the seismoacoustic and hydrophysical test site «Cape Schultz» of the Ilichev Pacific Oceanological Institute, Far Eastern Branch of the Russian Academy of Sciences using a laser nanobarograph, a laser meter of hydrospheric pressure variations, and a two-coordinate laser strainmeter consisting of two unequal-arm laser strainmeters with the measuring arm lengths of 52.5 and 17.5 m, located relative to each other at an angle of 92°. All laser and interferential devices are designed based on the Michelson interferometer scheme using the frequency-stabilized helium-neon lasers as the radiation sources.
Figure 1 shows the appearance of a laser nanobarograph based on an equal-arm Michelson interferometer, a frequency-stabilized helium-neon laser produced by Melles Griot that ensures frequency stability in the ninth place, a set of aneroid chambers with the mirror-like coating, a digital recording system, and a transmission device for the obtained experimental data to the experimental database. Most of the laser nanobarographs developed by us record the atmospheric pressure variations in the frequency range from 0 (conditionally) to 10,000 Hz with an accuracy of 50 μPa [7].
A two-coordinate laser strainmeter is used as a receiving system for the deformation processes of the Earth’s crust. It consists of an unequal-arm laser strainmeter with a measuring arm length of 52.5 m, placed at an angle of 18° relative to the north-south (N-S) line, and an unequal-arm laser strainmeter of the west-east (W-E) type with a measuring arm length of 17.5 m, placed relative to the 52.5‑m laser strainmeter at an angle of 92°. All laser strainmeters are based on an unequal-arm Michelson interferometer using the frequency-stabilized helium-neon lasers with a frequency stability in the 9–12th places as a light source. Figure 2 shows a general view of the underground beam waveguide of a 52.5‑meter laser strainmeter with a central interferential node and a frequency-stabilized laser providing the frequency stability in the 12th digits. Figure 3 demonstrates a schematic map of laser strainmeters on the measuring experimental test site «Cape Schultz» of the Ilichev Pacific Oceanological Institute, Far Eastern Branch of the Russian Academy of Sciences.
Figure 4 shows an internal view of a laser meter of hydrospheric pressure variations developed on the basis of a frequency-stabilized helium-neon laser that ensures the radiation frequency stability in the ninth place. It is placed in a cylindrical stainless-steel case fixed in a protection cage designed to secure the tool in the severe operating conditions (rock or muddy bottom). One side has a cable entry opening. The other side is tightly sealed with a lid. In addition to the protection cage, there is an elastic container with air outside the device, the outlet of which is connected through a tube to a balance chamber located in a removable cover. A Michelson interferometer, a balance chamber, a solenoid valve and a digital recording system are located in the case. The latest modification of the device [12] has made it possible to obtain the following technical specifications: operating range from 0 (conditionally) to 1000 Hz; measurement accuracy of hydrospheric pressure variations – 0.2 MPa; operating depths – up to 50 m that can be significantly improved by: 1) the use of a recording system with the best response time (up to 10–100 kHz); 2) the use of membranes of smaller thickness and / or larger diameter (up to 1 µPa); 3) the use of compensation systems with the greater potential (working depths up to 400 m).
The laser nanobarograph was placed in a small laboratory building (in Fig. 3 it is located in the middle of the line drawn from the laboratory building (point 3) to the north-south laser strainmeter (N-S, point 1). The laser meter of hydrospheric pressure variations was installed offshore in the southern part of Cape Schultz at a depth of 25 m.
All data obtained were transmitted via the cable lines to the laboratory premises (No. 3), where, after preliminary processing (filtration and decimation), it was recorded on hard media subsequently taken to Vladivostok. The data were then rewritten into the previously created digital experimental database. Depending on the tasks set, further processing of the obtained experimental data was performed. In this work, we will pay attention only to the microseismic range when solving some issues of the origin and transformation of hydrospheric, atmospheric, and lithospheric waves in this frequency range.
PROCESSING AND ANALYSIS
OF THE OBTAINED EXPERIMENTAL DATA
The experimental data at the above facilities were obtained during the advance of Typhoon Hagupit across the Sea of Japan. It was developed on July 31, 2020 in the Philippine Sea of the Pacific Ocean and, during its movement, caused storms in the seas of the Pacific Ocean. The storm intensity was peak when the pressure in the cyclone center dropped to 975 Pa. The cyclone passed along the east coast of China. When the cyclone entered the Yellow Sea, the cyclone’s rating dropped to a tropical depression, while entering the category of an extratropical cyclone. In this status, the cyclone entered the Sea of Japan on August 6, 2020. Moreover, despite the extratropical transition, the meteorological agencies continued to track Hagupit as a tropical storm until August 12, 2020 due to the vortex structure preservation and the remained energy balance from the heating water surface typical for the middle latitudes. We processed data received on August 6, 2020. Figure 5 shows the successive satellite images of wind-induced waves in the Sea of Japan, caused by the given typhoon, obtained on the date concerned.
The considered tropical cyclone caused the wind-induced waves in the Sea of Japan that, having left the typhoon effective area in the form of swell waves, excited the primary and secondary microseisms during their propagation and interaction with the offshore bed and in the reference zone. The primary microseisms are caused by the progressive swell waves, the period of which is equal to the period of progressive waves. The secondary microseisms are caused by the standing sea waves, the period of which is equal to half the period of progressive swell waves. During processing, we selected several synchronous recording areas of the laser strainmeters, a laser nanobarograph, and a laser meter of hydrospheric pressure variations.
In order to study the occurrence of disturbances in the atmosphere, the Earth’s crust and in the water within the microseismic range under consideration, caused by an active typhoon in the Sea of Japan, the synchronous measurements results of laser systems were subject to processing. The data are given in the table: N-S, W-E represent the results obtained from the laser strainmeters, Nan – from a laser nanobarograph and LMHPV – from a laser meter of hydrospheric pressure variations.
When analyzing the processing results, we have noted the following. 1) In the obtained spectra of the processed records of the 52.5‑meter laser strainmeter within the microseismic range, it is possible to determine the peaks corresponding to the primary and secondary microseisms. Moreover, the amplitudes of primary microseisms are 5–6 times higher than the amplitudes of secondary microseisms. 2) In the obtained spectra of the processed records of the 17.5‑meter laser strainmeter within the microseismic range, it is possible to determine the peaks corresponding to the primary and secondary microseisms. Morover, the amplitudes of secondary microseisms are more significant than the amplitudes of primary microseisms. 3) In the spectra of the laser nanobarograph records, it is possible to determine maxima corresponding to the secondary microseisms. The maxima corresponding to the primary microseisms are not identified in the laser nanobarograph records. 4) In the recording spectra of the laser meter of hydrospheric pressure variations, it is possible to determine maxima corresponding to the progressive waves, however, the maxima corresponding to the standing sea waves are not established. As an example, Figure 6 shows the spectra obtained by the spectral processing of synchronous experimental data from two laser strainmeters, a laser nanobarograph, and a laser meter of hydrospheric pressure variations, confirming the above-given details. Prior to the spectral processing, all records were processed using a Hamming bandpass filter in the frequency range of 0.01–2 Hz in order to suppress powerful spectral components beyond the microseismic range.
CONCLUSION
In the course of processing and analyzing the experimental data obtained from the laser strainmeters and a laser meter of hydrospheric pressure variations, it was found that a laser strainmeter with an arm length of 52.5 m, the axis of which is located perpendicular to the shore, steadily records the primary microseisms resulting from the transformation of progressive gravitational sea waves that are recorded by a laser meter of hydrospheric pressure variations installed on the bottom at a depth of 25 m not far from a 52.5‑meter laser strainmeter. The axis of the 17.5‑meter laser strainmeter is placed along Cape Schulz and at its point of location is almost perpendicular to the coastline. This place does not allow this laser strainmeter to reliably record primary microseisms in a similar way. This result confirms that the primary microseisms are P- or Rayleigh-type surface waves.
The secondary microseisms are accurately recorded by both laser strainmeters. In this case, the amplitudes of secondary microseisms determined by the laser strainmeters are comparable in magnitude. Having considered the fact that the laser strainmeter with an arm length of 52.5 m is three times larger than the laser strainmeter with an arm length of 17.5 m, it is possible to determine the approximate direction to the location of the source of secondary microseisms, with due regard to their classification as the transversal waves. According to the obtained experimental data of the 17.5‑meter and 52.5‑meter laser strainmeters and in consideration of the polarization of secondary microseisms, the direction to the supposed place of their generation was found. It is located at an angle of 22.4° relative to the axis of the 52.5‑meter laser strainmeter or 40.4° relative to the north-south line.
The generation point of secondary microseisms developed as a result of the loading effect of standing gravitational sea waves on the bottom, is located outside the measuring site area. Moreover, this conclusion is confirmed by the fact that the records of the laser meter of hydrospheric pressure variations do not demonstrate maxima corresponding to the standing sea waves, the periods of which are two times less than the periods of progressive gravitational sea waves.
The maxima in the range of secondary microseisms emphasized during processing of the laser nanobarograph records, but not highlighted in the records of the laser meter of hydrospheric pressure variations, indicate that they are caused by the secondary microseisms obtained from their generation area and are recorded by the laser strainmeters. Based on the data of a laser nanobarograph and laser strainmeters, it is possible to determine the ratio of atmospheric pressure amplitudes and micromovements. On average, it is about 0.023 Pa / nm that is approximately three to four times greater than the value obtained in [10] when registering the Rayleigh waves resulting from the earthquakes and atmospheric disturbances caused by them.
Source of funding
The work was partially supported by the Russian Science Foundation, project No. 22–17–00121 «Emergence, development and transformation of geospheric processes in the infrasonic range».
G. I. Dolgikh1, S. S. Budrin1, A. V. Davydov1, S. G. Dolgikh1, A. V. Mishakov2, V. A. Chupin1, V. A. Shvets1
Ilichev Pacific Oceanological Institute, Far Eastern Branch of the Russian Academy of Sciences, Vladivostok, Russia
Far Eastern Federal University, Vladivostok, Russia
The generation regularities of primary and secondary microseisms in the measuring ground location area have been determined by processing synchronous data of a two-coordinate laser strainmeter, a laser nanobarograph and a laser meter of hydrospheric pressure variations. The magnitude of atmospheric oscillations caused by the secondary microseisms propagating in the Earth’s crust has been obtained. The transfer factor of this interaction is about 0.023 Pa / nm that is three to four times greater than the same factor for the Rayleigh wave propagation in the Earth’s crust, generated by the earthquakes. The work results confirm the high efficiency of synchronous application of all measuring installations that allows us to accurately establish the origin of the registered geosphere disturbances.
Keywords: two-coordinate laser strainmeter, laser nanobarograph, laser meter of hydrospheric pressure variations, microseisms, sea waves
Received on: 10.10.2022
Accepted on: 01.11.2022
INTRODUCTION
For historical reasons, the concept of «microseisms» was introduced in an attempt to interpret the Earth’s background oscillations in the period range of 2–20 s, the origin of which was attributed to the frequent weak earthquakes of small magnitude. Subsequently, it was found that these disturbances are caused not by earthquakes, but by the sea waves that generate oscillations in the Earth’s crust in the frequency range of 2–20 s (conditionally), when interacting with the bottom and breaking in the reference zone. The benchmark works devoted to the origin and development of microseismic waves (2–20 s) are the articles of Hasselman and Longuet-Higgins [1, 2] stating that the progressive and standing wind-induced sea waves excite the primary and secondary microseisms, respectively, while interacting with the seabed. The periods of primary microseisms are equal to the periods of progressive wind-induced waves, and the periods of secondary microseisms are equal to half the period of progressive sea waves due to the fact that changes in hydrostatic pressure of a standing sea wave occur twice during one period of a surface sea wave. The periods of primary and secondary microseisms depend on the periods of wind-induced sea waves that are related to the wind velocity and duration, the area and depth of the water zone above which the wind is available. Moreover, for example, [3] shows that formation of the largest spectral maximum in the microseismic range with a peak frequency of 0.14–0.22 Hz (7.1–4.5 s) is related to the low-frequency elastic energy scattering in the rock formations. Moreover, this paper argues that «the ocean waves should not be construed as the cause of low-frequency seismic noise, according to the Longuet-Higgins theory, but rather as its consequence». It is possible that microseismic waves are excited by the atmospheric processes similar to the excitation of the «Earth infragravitational noise» by atmospheric pressure fluctuations in the resonant and near-resonant cases [4].
In view of the foregoing, an important task is to determine the primary source of Earth oscillations in the period range of 2–20 s, i. e. in the microseismic range. It is desirable to perform such studies on one measuring ground using the equipment that simultaneously measures variations in the atmospheric and hydrospheric pressures, microfluctuations of the Earth’s crust. The equipment shall be preferably developed based on the same measuring principles, and shall have the unique amplitude and frequency specifications (wide operating frequency range, high sensitivity). At present, such requirements are met by the one-coordinate and two-coordinate laser strainmeters [5, 6], laser nanobarographs [7], and laser meters of hydrospheric pressure variations [8] that are currently installed on one measuring ground «Cape Schultz» of the Ilichev Pacific Oceanological Institute, Far Eastern Branch of the Russian Academy of Sciences [9].
However, it is not possible just to determine the primary sources of various wave and non-wave processes, since we are extremely interested in the issues related to the study of transformation regularities of these processes at the geosphere boundaries, with their interaction with other multi-scale processes and phenomena. Thus, one of the areas is related to the excitation of atmospheric waves by transmitted Rayleigh waves in the period range from 1 to 20 s generated by the earthquakes [10]. However, the same waves are excited by the gravitational sea waves in the same range of periods, i. e. from 2 to 20 s. The second area that is rather popular now, is the global temperature rise. For some reason, every researcher considers only the impact of greenhouse gases, but does not take into account the energy dissipation in the Earth’s crust or in the marine Earth’s crust due to which the temperature of the world ocean and atmosphere is increased. There are some papers where these effects are studied at the initial level, for example, [11]. With an intensification in the storm activity, an increase in the total energy of typhoons / cyclones, an increase in the total power of earthquakes that depend, among other things, on solar activity, the dissipative energy is raised entailing a global temperature rise.
This paper will pay some attention to solving the problems described above.
LASER INTERFERENTIAL MEASURING SYSTEMS
The article analyzes the data obtained when performing synchronous measurements of variations in the atmospheric and hydrospheric pressures, crustal micromovements at the seismoacoustic and hydrophysical test site «Cape Schultz» of the Ilichev Pacific Oceanological Institute, Far Eastern Branch of the Russian Academy of Sciences using a laser nanobarograph, a laser meter of hydrospheric pressure variations, and a two-coordinate laser strainmeter consisting of two unequal-arm laser strainmeters with the measuring arm lengths of 52.5 and 17.5 m, located relative to each other at an angle of 92°. All laser and interferential devices are designed based on the Michelson interferometer scheme using the frequency-stabilized helium-neon lasers as the radiation sources.
Figure 1 shows the appearance of a laser nanobarograph based on an equal-arm Michelson interferometer, a frequency-stabilized helium-neon laser produced by Melles Griot that ensures frequency stability in the ninth place, a set of aneroid chambers with the mirror-like coating, a digital recording system, and a transmission device for the obtained experimental data to the experimental database. Most of the laser nanobarographs developed by us record the atmospheric pressure variations in the frequency range from 0 (conditionally) to 10,000 Hz with an accuracy of 50 μPa [7].
A two-coordinate laser strainmeter is used as a receiving system for the deformation processes of the Earth’s crust. It consists of an unequal-arm laser strainmeter with a measuring arm length of 52.5 m, placed at an angle of 18° relative to the north-south (N-S) line, and an unequal-arm laser strainmeter of the west-east (W-E) type with a measuring arm length of 17.5 m, placed relative to the 52.5‑m laser strainmeter at an angle of 92°. All laser strainmeters are based on an unequal-arm Michelson interferometer using the frequency-stabilized helium-neon lasers with a frequency stability in the 9–12th places as a light source. Figure 2 shows a general view of the underground beam waveguide of a 52.5‑meter laser strainmeter with a central interferential node and a frequency-stabilized laser providing the frequency stability in the 12th digits. Figure 3 demonstrates a schematic map of laser strainmeters on the measuring experimental test site «Cape Schultz» of the Ilichev Pacific Oceanological Institute, Far Eastern Branch of the Russian Academy of Sciences.
Figure 4 shows an internal view of a laser meter of hydrospheric pressure variations developed on the basis of a frequency-stabilized helium-neon laser that ensures the radiation frequency stability in the ninth place. It is placed in a cylindrical stainless-steel case fixed in a protection cage designed to secure the tool in the severe operating conditions (rock or muddy bottom). One side has a cable entry opening. The other side is tightly sealed with a lid. In addition to the protection cage, there is an elastic container with air outside the device, the outlet of which is connected through a tube to a balance chamber located in a removable cover. A Michelson interferometer, a balance chamber, a solenoid valve and a digital recording system are located in the case. The latest modification of the device [12] has made it possible to obtain the following technical specifications: operating range from 0 (conditionally) to 1000 Hz; measurement accuracy of hydrospheric pressure variations – 0.2 MPa; operating depths – up to 50 m that can be significantly improved by: 1) the use of a recording system with the best response time (up to 10–100 kHz); 2) the use of membranes of smaller thickness and / or larger diameter (up to 1 µPa); 3) the use of compensation systems with the greater potential (working depths up to 400 m).
The laser nanobarograph was placed in a small laboratory building (in Fig. 3 it is located in the middle of the line drawn from the laboratory building (point 3) to the north-south laser strainmeter (N-S, point 1). The laser meter of hydrospheric pressure variations was installed offshore in the southern part of Cape Schultz at a depth of 25 m.
All data obtained were transmitted via the cable lines to the laboratory premises (No. 3), where, after preliminary processing (filtration and decimation), it was recorded on hard media subsequently taken to Vladivostok. The data were then rewritten into the previously created digital experimental database. Depending on the tasks set, further processing of the obtained experimental data was performed. In this work, we will pay attention only to the microseismic range when solving some issues of the origin and transformation of hydrospheric, atmospheric, and lithospheric waves in this frequency range.
PROCESSING AND ANALYSIS
OF THE OBTAINED EXPERIMENTAL DATA
The experimental data at the above facilities were obtained during the advance of Typhoon Hagupit across the Sea of Japan. It was developed on July 31, 2020 in the Philippine Sea of the Pacific Ocean and, during its movement, caused storms in the seas of the Pacific Ocean. The storm intensity was peak when the pressure in the cyclone center dropped to 975 Pa. The cyclone passed along the east coast of China. When the cyclone entered the Yellow Sea, the cyclone’s rating dropped to a tropical depression, while entering the category of an extratropical cyclone. In this status, the cyclone entered the Sea of Japan on August 6, 2020. Moreover, despite the extratropical transition, the meteorological agencies continued to track Hagupit as a tropical storm until August 12, 2020 due to the vortex structure preservation and the remained energy balance from the heating water surface typical for the middle latitudes. We processed data received on August 6, 2020. Figure 5 shows the successive satellite images of wind-induced waves in the Sea of Japan, caused by the given typhoon, obtained on the date concerned.
The considered tropical cyclone caused the wind-induced waves in the Sea of Japan that, having left the typhoon effective area in the form of swell waves, excited the primary and secondary microseisms during their propagation and interaction with the offshore bed and in the reference zone. The primary microseisms are caused by the progressive swell waves, the period of which is equal to the period of progressive waves. The secondary microseisms are caused by the standing sea waves, the period of which is equal to half the period of progressive swell waves. During processing, we selected several synchronous recording areas of the laser strainmeters, a laser nanobarograph, and a laser meter of hydrospheric pressure variations.
In order to study the occurrence of disturbances in the atmosphere, the Earth’s crust and in the water within the microseismic range under consideration, caused by an active typhoon in the Sea of Japan, the synchronous measurements results of laser systems were subject to processing. The data are given in the table: N-S, W-E represent the results obtained from the laser strainmeters, Nan – from a laser nanobarograph and LMHPV – from a laser meter of hydrospheric pressure variations.
When analyzing the processing results, we have noted the following. 1) In the obtained spectra of the processed records of the 52.5‑meter laser strainmeter within the microseismic range, it is possible to determine the peaks corresponding to the primary and secondary microseisms. Moreover, the amplitudes of primary microseisms are 5–6 times higher than the amplitudes of secondary microseisms. 2) In the obtained spectra of the processed records of the 17.5‑meter laser strainmeter within the microseismic range, it is possible to determine the peaks corresponding to the primary and secondary microseisms. Morover, the amplitudes of secondary microseisms are more significant than the amplitudes of primary microseisms. 3) In the spectra of the laser nanobarograph records, it is possible to determine maxima corresponding to the secondary microseisms. The maxima corresponding to the primary microseisms are not identified in the laser nanobarograph records. 4) In the recording spectra of the laser meter of hydrospheric pressure variations, it is possible to determine maxima corresponding to the progressive waves, however, the maxima corresponding to the standing sea waves are not established. As an example, Figure 6 shows the spectra obtained by the spectral processing of synchronous experimental data from two laser strainmeters, a laser nanobarograph, and a laser meter of hydrospheric pressure variations, confirming the above-given details. Prior to the spectral processing, all records were processed using a Hamming bandpass filter in the frequency range of 0.01–2 Hz in order to suppress powerful spectral components beyond the microseismic range.
CONCLUSION
In the course of processing and analyzing the experimental data obtained from the laser strainmeters and a laser meter of hydrospheric pressure variations, it was found that a laser strainmeter with an arm length of 52.5 m, the axis of which is located perpendicular to the shore, steadily records the primary microseisms resulting from the transformation of progressive gravitational sea waves that are recorded by a laser meter of hydrospheric pressure variations installed on the bottom at a depth of 25 m not far from a 52.5‑meter laser strainmeter. The axis of the 17.5‑meter laser strainmeter is placed along Cape Schulz and at its point of location is almost perpendicular to the coastline. This place does not allow this laser strainmeter to reliably record primary microseisms in a similar way. This result confirms that the primary microseisms are P- or Rayleigh-type surface waves.
The secondary microseisms are accurately recorded by both laser strainmeters. In this case, the amplitudes of secondary microseisms determined by the laser strainmeters are comparable in magnitude. Having considered the fact that the laser strainmeter with an arm length of 52.5 m is three times larger than the laser strainmeter with an arm length of 17.5 m, it is possible to determine the approximate direction to the location of the source of secondary microseisms, with due regard to their classification as the transversal waves. According to the obtained experimental data of the 17.5‑meter and 52.5‑meter laser strainmeters and in consideration of the polarization of secondary microseisms, the direction to the supposed place of their generation was found. It is located at an angle of 22.4° relative to the axis of the 52.5‑meter laser strainmeter or 40.4° relative to the north-south line.
The generation point of secondary microseisms developed as a result of the loading effect of standing gravitational sea waves on the bottom, is located outside the measuring site area. Moreover, this conclusion is confirmed by the fact that the records of the laser meter of hydrospheric pressure variations do not demonstrate maxima corresponding to the standing sea waves, the periods of which are two times less than the periods of progressive gravitational sea waves.
The maxima in the range of secondary microseisms emphasized during processing of the laser nanobarograph records, but not highlighted in the records of the laser meter of hydrospheric pressure variations, indicate that they are caused by the secondary microseisms obtained from their generation area and are recorded by the laser strainmeters. Based on the data of a laser nanobarograph and laser strainmeters, it is possible to determine the ratio of atmospheric pressure amplitudes and micromovements. On average, it is about 0.023 Pa / nm that is approximately three to four times greater than the value obtained in [10] when registering the Rayleigh waves resulting from the earthquakes and atmospheric disturbances caused by them.
Source of funding
The work was partially supported by the Russian Science Foundation, project No. 22–17–00121 «Emergence, development and transformation of geospheric processes in the infrasonic range».
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