The paper presents the results of investigations of deformation processes in the near-surface sedimentary rocks, which have been carried out in a seismically active region of the Kamchatka peninsula since 2007. The peculiarity of the experiments on registration of geodeformations is the application of a laser strainmeter–interferometer constructed according to the Michelson interferometer scheme. Besides rock deformations, geoacoustic emission in the frequency range from several hertz to the first tens of kilohertz is under investigation. Piezoceramic hydrophones installed in artificial water reservoirs are applied. It is shown that periods of primary rock compression and tension with a duration of up to several months are distinguished in the geodeformation process at the observation site. During the direction change in the deformations, when the geodeformation process rate grows, an increase in geoacoustic radiation is observed.
The Kamchatka peninsula, one of the seismically active regions of the planet, is a natural test ground for investigation of seismo-tectonic processes that appear as a result of stress accumulation and relaxation in the lithosphere. It is a natural geodeformation process accompanying the movement and interaction of continental and oceanic plates. The topicality of its investigation is determined by the fact that it plays an important role in many geophysical processes that are discussed in seismology, mining and other spheres of science and engineering. Acoustic emissions are elastic oscillations occurring as a result of dislocation changes in a medium. They are often used to make diagnostics of deformations, since the characteristics of the excited radiation are directly associated with deformation process features. The phenomenon of acoustic emission is observed in a wide range of materials, structures and processes. The largest-scale acoustic emission is associated with seismic waves, whereas the least-scale level is caused by dislocation movement in crystals. Between these two types of acoustic emission is a wide range of scales, from laboratory tests and natural experiments to industrial control (Pollock, 1970, 1989). The mesoscale range, corresponding to sound vibrations, has an intermediate position according to wavelength, and plays an important role in the interaction of macro and micro dislocations. Hardness of landscapes, mountain slopes, glaciers, snow covers and large technical constructions are associated with mesoscale deformation processes. An increase in regional mesoscale deformations is observed at the final stage of earthquake preparation (Agnew and Wyatt, 2003; Berardino et al., 2002; Dolgikh et al., 2007; Sasorova et al., 2008). As a result, local effects of earthquake precursors of different natures appear, including those in acoustic signals of the sound range (Dolgikh et al., 2007; Gregori et al., 2005, 2010; Kuptsov, 2005; Levin et al., 2010; Morgunov et al., 1991; Paparo et al., 2002; Sasorova et al., 2008).
During the development of acoustic methods for investigation of mesoscale deformations, the principal difficulties appear due to the significant inhomogeneity of natural media and hard propagation conditions for elastic oscillations, particularly in the frequency range of the first kilohertz. Strong distortion and weakening of a signal restrict the possibilities of remote methods and require the development of distributed measuring systems applying modern data-processing technologies that have reached the required level only during recent years. Investigation of the relation of geoacoustic emission to regional deformation disturbances needs the organization of long distributed observations, construction of specialized systems for data acquisition and processing, and development of models adopted to real conditions for solving inverse problems to determine the regions of deformation disturbances.
It is reasonable to carry out investigations of mesoscale deformations in seismically active regions. A seismotectonic process is constantly going on there, accompanied by stronger rock deformations; thus, stronger effects in geoacoustic emissions should be registered. It is confirmed by the results of investigations in different seismically active regions (Gregori et al., 2005, 2010; Kuptsov, 2005; Levin et al., 2010; Morgunov et al., 1991; Paparo et al., 2002; Sasorova et al., 2008), where geoacoustic emission anomalies in the frequency range of the first kilohertz, which preceded strong earthquakes, were determined. The papers (Alekseev et al., 2001; Dobrovolsky, 2000; Okada, 1985; Vodinchar et al., 2007) present the models, which show the deformation nature of the appearance of such anomalies, and the paper (Dolgikh et al., 2007) experimentally confirms the relation of the geoacoustic emission anomaly to the dynamics of geodeformation processes before an earthquake. Near-surface sedimentary rocks, characterized by low strength and high plasticity, are the most suitable for investigation of deformations. Even a small stress change there causes geoacoustic emission. It should be taken into account that changes in sedimentary rock deformations may be determined both by the dynamics of a regional seismotectonic process and the local peculiarities of a registration site. Rock plastic flows from near mountain slopes, soil seasonal freezing and defrosting; sharp changes in atmospheric pressure during cyclones may also contribute. In all these cases, anomalous behavior of geodeformation processes and geoacoustic emission responses will be registered. In the present paper, the authors did not aim at the classification of anomalies in deformations, but they tried to analyze the peculiarities of a geodeformation process registered at one measurement site within a long period of time and to determine the peculiarities of its relation to geoacoustic emission.
Scheme of a laser strainmeter–interferometer. 1 – He–Ne laser, 2 – collimator, 3 – flat-parallel plate, 4 – flat-parallel adjustment mirrors, 5 – photodiode, 6 – light guide, 7 – triple-prism reflector, 8 – registration system block.
A laser strainmeter–interferometer of an unequal-arm type, constructed
according to the scheme of Michelson interferometer (Fig. 1) and developed at
TOI FEB RAS (Dolgikh et al., 2007, 2012), is used to investigate
deformations. The principle of operation of a laser strainmeter is that
strainmeter basis change causes additional phase increment in a laser
radiation wave. The measurement method is the following. Shift of
interferometer mirrors, placed at the ends of basis
The advantage of a laser strainmeter against a mechanical one is the absence
of a mechanically sensitive element (Agnew and Wyatt, 2003; Amoruso and
Crescentini, 2009; Dolgikh et al., 2012). The effect of meteorological
parameter variations on the instrument is mainly the change in the laser beam
optical path. When a sealed or vacuum-treated light guide is used, the
measurement accuracy of the Earth crust relative deformations for the best
interferometer models is 10
A laser strainmeter–interferometer was installed on the ground surface on
case pipes of two 5 m dry wells 18 m spaced (interferometer measurement arm
length) at the Karymshina complex geophysical observation site in Kamchatka.
Figure 1 shows its structural scheme, where 1 is the frequency-stabilized
He–Ne laser with the frequency instability of 2
Structural scheme of the geoacoustic emission registration system.
Relative deformation
The system for geoacoustic emission measurement was realized by directed
broadband piezoceramic hydrophones installed in covered artificial reservoirs
with the size
Acoustic emission plots in seven frequency ranges on 22–24 August 2006. The arrow indicates the earthquake at 21:50 UTC.
At a distance of 15 m from the measuring systems, a Conrad WS-2103 digital weather station was installed. It measured air pressure, temperature, relative humidity, wind speed and rain intensity every 10 min. Wind speed measurements were carried out at the height of 4 m from the ground surface, whereas other parameters were measured at the height of 2.5 m.
Registration of near-surface sedimentary rock deformations has been carried
out since 2007. An example of the data is presented in Fig. 3. Rock relative
deformation
In the course of the investigation of geoacoustic emissions, it was
determined that anomalies in the kilohertz frequency range register 1–3 days
before strong earthquakes at distances of the first hundreds of kilometers
from an epicenter (Kuptsov et al., 2005). As an example, Fig. 4 illustrates a
nearly 1 day anomaly that was observed on 22–23 August 2006 before a group
of 15 seismic events registered on 24 August 2006 at a distance of about
200 km. The strongest earthquake with magnitude
Graphs of relative deformation
Graph of relative deformation
Cross-correlation function graphs between acoustic pressure
Cross-correlation function graphs between acoustic pressure
Examples of geoacoustic emission anomalies during near-surface rock
compression: rock relative deformation
The relation of geoacoustic disturbances, preceding seismic events, to
geodeformation changes was under investigation. In order to do that, a
piezoceramic hydrophone was temporally installed in a water reservoir on the
strainmeter base. In the case experiment on 1 May 2007, an anomalous
deformation pattern in comparison to the levels of calm diurnal variation was
registered. These sharp oscillations had quite a large amplitude of about
10
To estimate the relation between geoacoustic emission and rock deformations,
cross-correlation functions (CCF) between acoustic pressure second series
Examples of geoacoustic emission anomaly during near-surface rock
tension: rock relative deformation
Furthermore, the results of joint investigations of geoacoustic emission (the hydrophone is installed at a distance of 50 m from the strainmeter) and rock deformation confirmed that emission anomalies in the kilohertz frequency range are observed during a significant increase in the deformation rate, during both near-surface sedimentary rock compression (Fig. 9) and tension (Fig. 10).
It is clear from the comparison of the graphs of emission and the deformation
rate that geoacoustic disturbances occur during numerous sign-changing rock
shifts of different amplitude. Relative deformations of some shifts are small
enough; even at comparatively large amplitudes, they are not more than
10
During the data analysis for the whole period of observations since 2007, diurnal data, when registration was stopped for different technical reasons, were removed from consideration. For this reason, during the first 2 years of the experiment, the period of adjustment of the measurement, it was impossible to obtain deformation long data series to estimate the annual scale pattern. During the following period, the number of gaps decreased significantly, and it allowed us to consider the geodeformation process behavior within long time periods.
Rock relative deformation
Figure 11a shows an example of deformation change from March 2010 till
February 2012. When constructing a graph, data gaps, which occurred as a
result of the complex shutdown or detection of bad weather conditions, were
replaced by median values. Due to considerable oscillations of deformations
on the annual timescale, diurnal variations turned out to be smoothed. To
make objective estimations, graphs of median values and the mean square
deviation (MSD) of the difference between diurnal relative deformation
maximal and minimal values
As follows from Fig. 11, a significant shift in the base line by a value of several centimeters occurred over the 2 years of registration, which seems to be erroneous. The authors think that the obtained results show a real change in the base line and are not determined by errors as long as the trend determined by hardware peculiarities of the registration system was removed during the data analysis. When a failure occurred in the strainmeter operation, or the meteorological parameters had a significant impact, the data were removed from consideration. The main reasons for the significant shift in the base line are the following. Surface sedimentary rocks on the bank mountain slope of the Karymshina River are constantly in motion; it is a shear plastic flow, the velocity of which exceeds the velocities of regional plastic deformation in Kamchatka by several orders. The rocks on the steep slopes are always close to an almost critical, avalanche-prone state, so they are so sensitive to weak deformation disturbances that occur locally in close proximity and as a result of remote earthquake precursors. It should also be considered that the strainmeter is installed in the zone of different-rank tectonic faults. Their dynamics may cause significant changes in surface sedimentary rocks.
As follows from Fig. 11, during long periods, rock primary compression or tension is observed, but the most interesting regions are those where geodeformation direction change occurred. For example, in July–November 2010 in the deformation process, the primary compression is changed by primary tension, and the median values and the MSD show an average value increase and a relatively average value peak in relative deformation diurnal variations. From October 2011 till February 2012, deformation direction change occurred, the rock primary compression rate grew sharply, as well as the intensity of relative deformation per day. During this period, the most significant amplitude disturbance of geoacoustic emission was determined. It should be noted that such a strong compression for a short enough time period was registered for the first time.
Primary rock compression or tension, which lasts for several months, is observed in the deformation process registered at the observation site in Kamchatka. Similar results were obtained in the paper (Agnew and Wyatt, 2003). It allows us to suggest that similar effects are typical for the local deformation process. Geoacoustic anomalies are mainly registered during deformation direction change when the deformation process rate increases.
When deformations become more active, geoacoustic emission anomalies are observed in the form of a sharp and long increase in the level in the frequency range from hundreds of hertz to the units of kilohertz. During these periods, the deformation rate grows, and rock slips appear that result in the generation of the emission of increased intensity. The most vivid such effects are observed at the final stage of earthquake preparation. This result agrees well with the results of mathematical models (Alekseev et al., 2001; Dobrovolsky, 2000; Okada, 1985; Vodinchar et al., 2007) and natural experiments (Agnew and Wyatt, 2003; Berardino et al., 2002; Dolgikh et al., 2007; Sasorova et al., 2008). These authors showed that amplification of a deformation process occurs during earthquake preparation in the regions of their epicenters at distances of up to several hundreds of kilometers. Thus, anomalies of geoacoustic emission in the frequency range from hundreds of hertz to the units of hertz may be considered operative precursors of strong earthquakes.
The authors are very grateful to G. P. Gregori and H. I. Koulakov for their attention to the article and constructive discussion that helped to improve the manuscript. Edited by: H. I. Koulakov