Seismic Source Migration During Strombolian Eruptions Inferred by Very‐Near‐Field Broadband Seismic Network
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Abstract We analyze seismic waves excited by small Strombolian explosions to understand the source process of volcanic explosions. We deployed five broadband seismometers at only 100–300 m away from the active craters of Stromboli volcano, Italy. Moment tensor inversion of the entire seismic signals in the 0.05–0.2 Hz band locates the source at a depth of 170 and 150–200 m west/southwest of the crater where acoustic waves are excited. Contrary, the sources of seismic waves in the 0.2–0.5 and 0.5–1.0 Hz bands are excited almost at the explosion onset and are located close to the crater. We show for the first time that explosions are preceded of about 10–20 s by a small amplitude seismic phase. Semblance analysis shows that this phase is radiated from a depth of 170 m beneath the western part of the crater area. Our analysis indicates that the source moves about 50 m toward the active crater 10–20 s before the explosion occurs at the surface. At the explosion onset, the source moves back to the same location of the small preceding phase. These lateral migrations of the seismic source are estimated by moment tensor inversion and semblance analysis. We suggest that migration reflects the bending of the shallow feeding system toward northeast. Seismic waves are thus reflecting the history pressure generated by the rising of a gas‐rich pocket in the very shallow portion of a magma mush and by the following restoring force occurring after the explosion.Keywords:
Strombolian eruption
Seismometer
Microseism
Seismic moment
A group of resonant vertical seismometers, each tuned to cover a part of the spectrum of microseism frequencies, has been operated for about one year. These instruments (a) clearly distinguish between simultaneous microseisms from two separate sources; (b) show an improved signal‐to‐noise ratio for microseisms from a single storm, permitting earlier detection of storm onsets; (c) show clearly the increase in period of frontal microseisms as cold fronts move seaward from the east coast of North America; (d) record only the envelope of the oscillations, which greatly facilitates measurement of intensity as a function of time; and (e) appear to be very useful tools in continued attempts at hurricane location by means of microseism amplitude studies. The performance of the instruments is demonstrated by seven case histories in which microseismic readings of seismometers tuned to different frequencies are related to the meteorological conditions which are apparently responsible for the microseismic activity.
Microseism
Seismometer
Seismic Noise
Envelope (radar)
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The design of horizontal and vertical component seismometers for recording ocean-generated microseisms is first described. Both instruments have a reasonably constant response to ground displacements in the 0·05 to 0·5 c/s range. Two three-component stations are in operation using these seismometers. The construction of the vaults and the methods of maintaining stable air conditions inside them are described. The seismometer outputs from the remote station are telemetered to the central recording laboratory by converting each to a frequency-modulated signal and transmitting over a G.P.O. landline. A brief description of the construction and performance of the equipment used is given. The six outputs are monitored on a graphic recorder and also sampled once per second for digital recording on punched paper tape. Some results of the analyses carried out during the winter of 1963/64 are presented. Auto spectra are computed and related to the spectra of sea waves, either deduced from the prevailing weather conditions over the North Atlantic or measured from weather ship wave recordings. The Rayleigh wave constant is evaluated for the two stations and related to local geology. The coherence and phase between components at one station and between stations as functions of frequency are explained with reference to the proportions of Rayleigh and Love waves present in microseisms and to the direction of arrival of the waves.
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Rayleigh Wave
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Seismogram
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abstract Enhancement of P waves is obtained by the suppression of microseisms with a combination of vertical linear strain seismograph and vertical inertial seismograph outputs. Significant suppression is observed at sites where the predominant microseisms consist of essentially single-mode Rayleigh waves in contrast to that observed at a site where the microseismic noise is compositionally more complex. A description is given of the vertical linear strain seismograph used in these investigations. Consideration is given to the use of a vertical transverse strain seismograph at sites where the microseismic noise field is complex.
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Seismometer
Strain (injury)
Rayleigh Wave
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Summary A comparison study between borehole arrays, broadband seismometers and surface geophones is undertaken using a new passive seismic dataset from the Fox Creek, Alberta, area. Induced seismicity and microseismic events are compared based on waveform character, arrival time accuracy, and frequency content. In addition, using cross-correlation for detection, the effectiveness of each of the instruments at picking up events is compared. It is concluded that the broadband seismometers are most effective for induced seismicity monitoring, while the borehole array is most suitable for the microseismic monitoring.
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Seismometer
Passive seismic
Vertical seismic profile
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abstract Seismometers in spherical aluminum pressure housings have been weighted to float stably at midwater depths in the ocean, and thus record water motions in a frequency band of 0.02 to 5 cps. Simultaneous records made with a midwater instrument at 1.2-km depth and a bottom instrument at 4.6-km depth showed coherence at spectral power peaks of leaky organ-pipe frequencies and additional coherence peaks at frequencies down to 0.025 cps. Twenty organ-pipe modes can be tentatively identified. The spectral power can be attributed almost entirely to microseismic motions in wave-guide modes. We conclude that the forcing functions for microseisms are broad enough so that deep ocean-bottom and midwater microseism spectral peak frequencies are characteristic of local bathymmetry.
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Seismometer
Forcing (mathematics)
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Abstract We performed a frequency‐dependent polarization analysis on ambient seismic energy recorded by 1768 USArray Transportable Array (TA) seismometers for the time period of 1 April 2004 through 31 October 2014. The seismic energy has strong seasonal variations in power and polarization at essentially all stations; however, the annual variation is much smaller. One year of data is sufficient to determine the average properties of the ambient seismic wavefield at a particular site. The average power and dominant period in the double‐frequency (DF) microseism band, defined here as periods of 2–10 s, vary significantly and coherently across North America. Proximity to a coastline generally leads to increased DF microseism amplitude, but site geology is much more important, with sedimentary basins having especially large DF amplitudes. The western U.S. as a whole has longer dominant DF periods than the central and eastern U.S., with the southeastern U.S. having the shortest dominant DF periods. Power spectral density estimates at many TA stations show a splitting of the DF microseism peak into two distinct subpeaks. This has been observed previously in data recorded by ocean bottom seismometers, with the shorter‐period DF peak attributed to the local sea and the longer‐period DF peak attributed to more distant, coastally generated microseisms. In the case of the land‐based TA data analyzed here, the DF splitting arises from simultaneous microseism generation at various source areas (Pacific Ocean, Atlantic Ocean, and Gulf of Mexico) with distinct, preferred excitation frequencies. DF microseism source properties derived from global models of ocean wave interaction support this interpretation.
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Seismic Noise
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Direction of Approach of microseismic waves was investigated in Kyushu by means of vector seismographs. It was found that no microseismic wave comes from the west direction even when typhoons were situated in the south-west direction. The frequency of arrival directions was distributed partially for the direction of the Hyuga Sea where the continental margin is near to the coast.From these distributions and those at the Abuyama Observatory, it is reasonably concluded that the nearer the continental margin is to the coast, the more frequently microseismic waves are generated.
Microseism
Seismometer
Typhoon
Margin (machine learning)
Continental Margin
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