Results
Asymmetric Impulses
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Figure 5
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[35] Method 1 was used to separate possible high-amplitude impulses in the minute range of
periods. In Figure 5 the results are given of processing initial 20 Hz records of five stations
(Figure 1) before Kronotskoe earthquake in which components were separated with periods ranging
from 8 to 128 minutes. ARU data were not used because of the noise of technogenic origin in
the day time. From the figure, it follows that in the time interval under investigation 84 hours
before the earthquake no considerable variation of noise level was noted in any of the five stations.
The amplitude of impulses at each of the stations only makes several percent of the level of common
microseisms in the range of 2-6 seconds. We studied the variation of noise level with the period
increase in all of the stations. It was revealed that microseisms have maximum amplitude in the
range of 3-5 s, which is in agreement with the model of their oceanic origin. As the period
lengthens the amplitude drops, so that with period of 1 minute it, on the average, decreases by
20 times. Then the decrease of amplitude slows down and after a period of 5-10 minutes it gradually
starts growing. In the qualitative sense, the spectra of oscillations intensity have the same form as
the spectrum for PET station that is shown in Figure 4. However, the farther is the station from PET,
the slower is the decrease with lengthening of periods in the range from seconds to first minutes.
Thus values of oscillations velocities shown in
y -axis of plots in Figure 5 allow us to draw the
following preliminary conclusion. The fact that their value gradually decreases as the distance of
the station from Kamchatka (PET) increases and their level is higher for an order of magnitude
in PET station as compared to other stations suggests that their source was located in the Pacific
seismically active zone.
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Figure 6
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[36] The feature of PET station is asymmetric impulses of negative polarity. The dynamics of the
impulses quantity change automatically separated by the program in 30-day interval before
Kronotskoe earthquake is shown in Figure 6. Curve 1 shows the level of "common'' microseisms
of second range, where
D is dispersion of initial record in successive windows of length of
4 seconds (80 initial counts with sampling frequency of 20 Hz). Two upper curves show
variations of the amount of impulses found by the program and having positive and negative
polarity of minute range for 1 day with a step of 0.1 day. Gradual increase in the amount of
impulses is noted especially of negative polarity
N(-) before the earthquake.
[37] We compared oscillations of the amount of asymmetric impulses and the level of
microseisms of second range of periods; no correlation between them was established. It means
that impulses were not caused by storms in the seas and oceanic areas. The influence of more
distant meteorological phenomena, for example in the Atlantic regions, is not excluded, but this
possibility was not checked. Correlations between the quantity of impulses and semidiurnal,
diurnal and two-week earth tides was not revealed either.
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Figure 7
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[38] Let us consider the structure of individual impulses and their sequence. One and the same
section of PET station record of 3 December of about 3-hour duration is shown in plots 1 and 2
(Figure 7). The lower plot is the record with discreteness 1 s filled with microseism variations of
the second range of periods. Plot 2 is the low-frequency constituent after high frequencies were
rejected with smoothing by Gaussian kernels with averaging radius of 100 s. In this plot you can
see impulses with the following features: 1) duration of individual impulse is ~10-15 min,
and intervals between them make ~40 min; 2) The impulse form changes from almost symmetrical
to one-polar with gradual suppression of positive phase. Note that impulse amplitude in plot 2 is
an order less than microseisms level in plot 1 and therefore they cannot be seen in the lower plot.
A record fragment of the same time and processed in a similar way from Obninsk station located
at the East European platform at a distance of 6500 km from Petropavlovsk station (see Figure 1) is
shown in plot 3 (Figure 6) for comparison. Impulses of the type of plot 2 are not detected and the
amplitude of oscillations is two less orders.
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Figure 8
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[39] Similar results were obtained before Neftegorskoe earthquake for Yuzhno-Sakhalinsk station
(YSS), which was the nearest to the epicenter. In Figure 8, record fragments of microseisms (plot 1)
and of low-frequency component (plot 2) of this station and of Obninsk station (plot 3) for
comparative purposes are shown. In plot 2 both asymmetric and symmetric impulses of duration
~10-15 minutes are separated. Comparing Figure 8 and Figure 7 we note the following: durations
of individual impulses (plots 2) coincide in both cases; as distinct from Kronotskoe earthquake,
intervals between sequential impulses before Neftegorskoe earthquake are not regular; impulse
amplitude in Figure 8 is approximately one twentieth as much as compared to Figure 7; microseisms
amplitude (plot 1 Figure 8) is approximately one twenty-fifth as much as the one in Figure 7;
oscillations amplitude at OBN station (plots 3) is comparable in the both cases (~5-7 nM s
-1 ),
and their structure differs from that of stations YSS and PET. Before Neftegorskoe earthquake the
amount of negative polarity impulses increased in the last 5 days against the undisturbed
background of second-range microseisms.
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Figure 9
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[40] Before Hokkaido earthquake the records of two stations shown in Figure 2 intense impulses
were revealed with amplitudes above diurnal and semidiurnal tidal oscillations. It is
demonstrated in Figure 9, where 4-day records (96 hours) are given for the period of 16-19
September 2003: plot ERM is of Erimo station located in the southeast of Hokkaido in
subduction zone and actually in the epicentral zone of the earthquake; plot PET is of station
Petropavlovsk on Kamchatka shore located in subduction zone; plot MDJ is of station
Mudanjiang located in the continent in northeastern China. In ERM and PET plots, individual
impulses of positive and negative polarity as well as series of impulses close in time can be
seen against diurnal and semidiurnal tides. Positive polarity series is noted in the area of
16-20 hours in ERM plot; negative polarity series in PET plot takes the interval of 82-96
hours. We note three features that in our opinion are significant: 1) impulses were only
manifested at stations ERM and PET located in subduction zone, 2) the times of both single
impulses and series of impulses do not coincide in ERM and PET plots, 3) impulses amplitude
in ERM plot is on the average greater as in magnitude and with respect to the span of tidal
variations both as compared to plot PET. From data given in Figure 9, it is reasonable to
assume that the sources of impulse oscillations are located in subduction zone. Since we
do not know the actual sensitivity of a number of stations in the minute and hour range of
periods, we give attention to relative amplitudes of oscillations in comparison with tides
or microseisms of second range.
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Figure 10
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[41] More detailed structure of impulses of stations ERM and PET is shown in Figure 10. We changed
impulses polarity in ERM plot artificially to inverse as compared to plot 9 so
that comparing is more convenient. Low-frequency oscillations of hour range of periods caused
by earth tides were rejected by deducting Gaussian trend with the radius of 1000 s. Intervals
between sequential impulses make first thousands of seconds. From more detailed time base of
the impulses 1, 2, 3, and 4, it can be seen that the time of impulse rise to extreme values
is not the same and lies in the interval from 100 to 200 seconds. This interval is within
standard frequency range of IRIS stations IRIS
[Starovoit and Mishatkin, 2001].
From Figure 10 it follows that the comparative amplitude of impulses with respect to high-frequency
noise of second-range microseisms is greater at ERM stations. Since BHZ measuring channels
of IRIS stations record the vertical component of displacement velocity, it is believed that
the presented impulses correspond to one-polar vertical step of the base movement.
Comparison with horizontal component record showed that horizontal component amplitudes of
N-S and E-W impulses are almost one less order as compared to vertical components and are
comparable to high-frequency noise amplitudes.
[42] Before Sumatra earthquake, quasi-sinusoidal symmetric oscillations in the minute range
of periods were only detected in the records of stations shown in Figure 3.
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Figure 11
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Periodic Oscillations
[43] As a result of applying method 2 to the analysis of microseism variations before
Kronotskoe earthquake of 5 December 1997 with
M = 7.8 and coordinates [54.64oN, 162.55oE],
spectrum-time diagrams of logarithmic likelihood function increment
D were obtained for six
seismic stations IRIS: PET, YAK, OBN, MAG, YSS, and ARU that have similar characteristics
and the location of which is shown in Figure 1. The stations are located in different seismic
geological settings at considerable distances from each other. Station PET is located in
subduction zone at a distance of
D = 310 km from the epicenter. Station MAG second
closest to the epicenter ( D = 900 km) is located in the north of the Sea of Okhotsk
characterized by deep earthquakes. In the records of those two stations, periodic oscillations
were revealed three hours before Kronotskoe earthquake (Figure 11). Red bands on the figure
indicate emergence of periodic oscillations in the period range from 20 to 60 minutes.
Increment
D was estimated for a sequence of time moments of seismogram maximums
exceeding the level equal to the mean value in the window plus sample standard deviation in
the same window. The count starts from 00 Greenwich time (UTC) 2 December 1997. Values
D were calculated in time window of duration
DT = 3 hours with a shift
DS = 1 hour,
thus diagrams are presented in the figure, beginning at 3 o'clock 2 December. Last points in
the time scale in diagrams (83 hours) correspond to 11 hours on 5 December (27 minutes before
the moment of earthquake).
[44] The beginning of the period under investigation was chosen because 2 December 1997
was characterized by quiescent seismic conditions were noted in the zones where the
enumerated-above stations are located and where local earthquakes of energy class
K>10 ( M>4 )
were not noted from Geophysical Service RAS data. Dramatic activation of the seismic process
in the future Kronotskoe earthquake epicentral area started in Kamchatka in the middle of
3 December. That day three earthquakes of
K>10 and three earthquakes of
K>11 occurred there.
The beginning of foreshock process is marked with an arrow and symbol F in the upper diagram
of Figure 11. The activation went on with increasing number of shocks on the following day; seven
events with
K>10, five events with
K>11
and one with
K=12.8 ( M = 5.5) were recorded on
4 December. The latter is indicated with an arrow and symbol
F a in Figure 5. On the
day of Kronotskoe earthquake, 13 foreshocks
K>10, 12 foreshocks of
K>11 and
4 foreshocks of
K>12 including one of
K=12.5 ( M = 5.3) were registered on 5 December
before the earthquake moment. The latter is indicated with an arrow and symbol
F b in
Figure 11. Altogether the foreshock series included 100 earthquakes of representative class
K>8.5 ( M>3 ).
[45] From Figure 11 it follows that the first series of periodic microseismic oscillations
manifested itself at the end of 2 December - beginning 3 December. The second series started
approximately at 2200 on 4 December (71 hours in diagrams) and was in progress until the time of
the Kronotskoe earthquake main shock. Comparison was made with data of stations YAK, YSS,
ARU, OBN, which are more remote from the source
[Sobolev et al., 2005].
It was revealed that the first series of periodic oscillations was appeared in diagrams of
D of 2-3 December. In the second series, oscillations at those stations were noted
immediately after foreshock
F a, but they were missing after foreshock
F b up
to the earthquake moment. Since periodic oscillations 3 hours before the earthquake were only
revealed at stations PET and MAG, which are the nearest to the epicenter, it is reasonable
to assume their relation to the process in the subduction zone adjacent to the epicenter.
The effect was most pronounced in PET station and maximum
D indicated a period
of ~37 minutes 1 hour before the earthquake.
[46] Emergence of periodic oscillations in minute range of microseisms was revealed before
Sumatra earthquake of 26 December 2004 with
M = 9.2 and epicenter coordinates [3.32oN, 95.85oE].
Preliminary analysis of records made by stations in Figure 3 showed that station MBWA in
Australia had not been operated in the period of Sumatra earthquake, there had been failures
and gaps in the records of DGAR and PALK stations and stations DAV and QIZ located in the
Pacific region had a different structure of microseismic oscillations as compared to stations
located in the Indian Ocean region. Therefore major studies were carried out from the data of
stations CHTO, KMI, XAN, COCO, and partially PALK. We used records of vertical
components with the exception of COCO station where this component had not been registered;
in the latter case, horizontal component data were processed. The base of data of those stations
that we used covered the interval from 6 to 26 December (00 hours 58 minutes) of that is to say
until the moment of Sumatra earthquake.
[47] A singular feature was the fact that 2.5 days before Sumatra earthquake in the southern
hemisphere another strong earthquake had occurred of
M = 7.9; the epicenter of the earthquake
with coordinates [49.31oS, 161.35oE] was located to the southwest of New Zealand (in Makkuori
ridge area). Oscillations of this earthquake were hundreds of times as much as microseisms level
in the above-mentioned stations.
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Figure 12
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[48] Time-frequency diagrams
D shown in Figure 12 for stations KMI and CHTO were
calculated with the use of method 2 described above. Arrows indicate the time when Sumatra
earthquake ( M = 9.2) and preceding Makkuori earthquake ( M = 7.9) occurred. Periodic oscillations
appeared after Makkuori and continued for one a day. Comparison with Figure 11 suggests an
effect similar to the one noted after the foreshock
F b of Kronotskoe earthquake.
In the records of stations XAN, COCO and PALK periodic calculations were not revealed.
Note that records of stations XAN and COCO were characterized with the noise increased level.
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Figure 13
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[49] In the last few days before Hokkaido earthquake of September 2003 with
M = 8.3 and
the epicenter coordinates [41.81oN-143.91oE] considerable foreshocks ( M>5
) or remote strong
earthquakes were not noted. However the appearance of periodic oscillations was detected. We
studied records of stations PET, YSS, OBN, ERM, MAJ, INC, MDJ, and BJT the location of
which is shown in Figure 2. From calculations of parameter
D it was revealed that
at three stations PET, YSS and MDJ oscillations were noted 16 hours before the earthquake,
which are presented in spectrum-time diagrams in Figure 13 (interval 4300-5100 minutes).
As distinct from Kronotskoe and Sumatra earthquakes, oscillations were revealed in more
low-frequency range with periods of 120-160 min. One more group of oscillations was revealed
fifty hours before the shock. Similar to Kronotskoe earthquake, periodic oscillations appeared
in the records of stations close to the epicenter. Unfortunately ERM station located in the epicentral
zone stopped recording 4 day before the earthquake.
Synchronization of Oscillations
[50] Methods 3 and 4 were used to search for oscillations synchronization effects at stations
spaced apart.
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Figure 14
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[51] In Figure 14, current values of Hurst generalized index
a
are shown,
which realizes the maximum of microseismic field singularity spectrum at 5 stations,
the location of which is shown in Figure 1. We studied the interval of 30 days before
Kronotskoe earthquake (309th-339th days in 1997). We used the window of 12-hour duration
with a shift of one hour. With considerable variations of amplitude
a we failed
to reliably separate intervals of synchronous oscillations at different stations.
But coherence spectral measure calculation
l(t, w)
as the modulus of canonical
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Figure 15
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coherences product component by component (formula (9)) allows detecting them (Figure 15). The major
burst of coherence is centered in the vicinity of time markers of 40000-42000
minutes (several days before the earthquake). As the moving time window is approaching
the earthquake moment, coherence level of
a-variations drops although it
remains higher than background statistical fluctuations. Diagrams in Figure 15 testify
to the increase of the time duration of low frequency "coherence spot'' as the number
of stations increases.
[52] To compare coherence level with different sets of stations their number should be
constant according to formula (9). Sorting all possible combinations of 3 stations lead us
to the conclusion that the set of stations PET, MAG, YAK have the highest value of
a
= 0.65 and the distance from Kronotskoe earthquake epicenter to those stations
is 350, 900 and 2050 km; stations OBN, ARU and YAK have the least value (0.32) and they are
the farthest from the earthquake, located at 6800, 5900 and 2050 km. In all variants
the coherence measure has a burst in the vicinity of the marker of 40000 minutes, which
corresponds to observation time interval of 29.11-03.12.1997 that is 3-7 days before the shock.
We note that on 03.12.1997 an intense series of foreshock activation started before Kronotskoe
earthquake. Besides in this interval the increase of asymmetric impulses number was observed at
PET station, which is the nearest to the earthquake epicenter (Figure 6). Just how random may be
these coincidences is difficult to judge.
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Figure 16
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[53] In the analysis of the situation before Hokkaido earthquake, the largest number of
stations participating in the computation was six: YSS, MDJ, INC, BJT, PET, OBN (Figure 2).
Records of stations ERM and MAJ were not used because the former, as it was mentioned
above, did not register microseisms in the last four days before the earthquake and the latter
was not operated for 7 days two weeks before the earthquake. It was established that synchronization
was manifested 2 days before the earthquake (interval 33000-35000 minutes).
It encompassed
time periods from 3 hours (frequency 0.005 1 min-1 ) and longer ones. As the number of stations
decreased, the amplitude
l(t, w) increased in accordance with formula (9).
It was significant that with complete sorting by 3 stations the most vivid effect was observed
for stations nearest to the epicenter of Hokkaido earthquake. Spectrum-time diagram for such
stations YSS, MDJ, INC is shown in Figure 16. Three features may be noted: 1) synchronization
with period ~3 hours (frequency ~0.005 1 min-1 ) started 9 days before the earthquake
(23000 minutes); 2) most vividly and in a wide range of periods it was manifested 2 days before
the earthquake (33000-35000 minutes); 3) a break in synchronization in the interval of 29000-31000
minutes was evidently associated with two remote strong earthquakes (shown with arrows) with magnitude 6.6.
The first of them with the epicenter coordinates [19.72oN-95.46oE] occurred on 21 September and
the second one with coordinates [21.16oN-71.67oW] occurred 10 hours later on 22 September.
Of course, the arrival of seismic waves to stations at different times disturbed synchronization.
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Figure 17
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[54] In Figure 17, frequency-time diagram
l(t, w) is given that was obtained from
record processing of stations CHTO, KMI, XAN, COCO before Sumatra earthquake. Beginning with
the time marker of 12800 minutes, coherence measure rise occurs with gradual lengthening of
prevailing periods of oscillations from several minutes to tens of minutes. In the assumption of
intra-terrestrial mechanism of the oscillations, they may be associated with resonance effects in
the blocks increasing in scale and/or in the lithosphere layers and deeper layers of the earth.
The analysis of microseism amplitude in the second range of periods showed that at all the above-mentioned
stations in the interval of 16-26 December their level was practically stationary,
which eliminates the atmosphere effects. Such phenomenon was previously noted in both the
range of very long periods of the order of 1 year in seismic catalog studies and laboratory
experiment of deforming and destructing a sample
[Sobolev, 2003].
Apparently it is a fundamental characteristic of non-equilibrium system approaching instability.
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