Upper crust structure under CTBTO station «Petropavlovsk-Kamchatsky» by endogenic

microseismic activity

Chebrov V.N., Kugaenko Yu.A., Saltykov V.A.

Geophysical Survey of Russia, Kamchatkan Branch, Petropavlovsk-Kamchatsky

A S I AKa

mch

atkaSea of

Okhotsk

Bering Sea

P A C I F I C

O C E A N

CTBTO station «Petropavlovsk-

amchatsky» K

PS36 is located in central part of Kamchatka peninsula in the area with low human activity.

Distance to Petropavlovsk-Kamchatsky (main town of Kamchatka) is about 70 km.

Petropavlovsk-Kamchatskyand «domestic» volcanoes

Petropavlovsk-Kamchatsky

Airport “Petropavlovsk” and Avachinsky

Volcano

Population: 195,000 (2010)

Avachinsky Bay and Vilyuchik Volcano

Gate of Avachinsky Bay

On the way from Petropavlovsk to PS36

Avachinsky Volcano

Vilyuchik Volcano

Koryaksky Volcano

Kronotsky Volcano

Gorely Volcano.Asid lake.

Panoramic view from Gorely Volcano

Current eruption of Kizimen Volcano

Road through snow

The Valley of the GeysersGiant Geyser

Primary seismic station PS36 consists of an 11-element array and one Central Recording Facility (CRF), which receives data from the remote collection arrays and transmits it to International Data Centre IDC (Vienna, Austria).

PEA2

PEB3

PEB4

PEB5

PES2

PEB2

PEA3

PEB1

PEA1

PEA0, PA0B

N

2 km

CRF

PS36

PEA2

PEB3

PEB4

PEB5

PES2

PEB2

PEA3

PEB1

PEA1

PEA0, PA0B

N

2 km

CRF

PS36 configuration:Small aperture short-period seismic array of nine registration points (two concentric circles with the diameters 4000 m and 1000 m and joint center) and one additional point S2 on the bedrock within array pattern. Small ring – stations PEA1, PEA2, PEA3, large ring – stations PEB1, PEB2, PEB3, PEB4, PEB5, central point - stations PEA0, PEA0B. Broadband three-component seismometer (point PA0B) was installed in central of the array.

PS36 seismic equipment:ten borehole short-period vertical seismometers GS-21 (velocity output bandwidth - 1-20 Hz) and one broadband three-component borehole seismometer Guralp CMG-3TB (0.02-50 Hz). Seismometers were installed to the boreholes at a depth 50 meters.

PEA0

PEA1

PEA2

PEA3

PEB1

PEB2

PEB3

PEB4

PEB5

PES2

-120 dB

-150 dB

-180 dB

-210 dB

-240 dB

Example of PS36 original seismic record(background microseismic signal, short-period vertical cannels)

1 Hz 10 Hz

Main foreshock of Tohoku EQ

09/03/2011 02:44 Mw=7.2

Distance ~ 2,000 km

Example of PS36 seismic

record

Sho

rt-pe

riod

verti

cal

cann

els

Bro

adba

nd 3

-com

pone

nt s

eism

omet

er

Tohoku EQ

11/03/2011 05:49 Mw=8.9

Distance ~ 2,000 km

off-scale reading

Example of PS36 seismic

record

Sho

rt-pe

riod

verti

cal

cann

els

Bro

adba

nd 3

-com

pone

nt s

eism

omet

er

For investigation of crust structure and heterogeneity detection we used microseismic emission as sensitive indicator of the stress distribution in the medium and short-period vertical cannels of PS36 as multichannel array.

The construction of images of deep noise sources reduces to the analysis of the spatial distribution of their intensity.

The method estimates the energy of weak coherent radiation from various points of the medium.

53.6

52.6

51.6

50.6

PS36

Nachikinsky hydrothermal deposite

PS36 on the map of the thermo-mineral springs and geothermal resources of Southern Kamchatka.

At early stages, emplacement of large intrusions and an intenive Paleogene-Neogene hydrothermal activity were associated with this zone.

Fractures of this zone control outflows of the Nachikinsky hydrothermal deposit in near-field region of PS36.

PEA2

PEB3

PEB4

PEB5

PES2

PEB2

PEA3

PEB1

PEA1

PEA0, PA0B

N

2 km

I

I

I

II

III

Main peculiarities of shallow geology

I – the fault in the Plotnikova River valley;

II – Nachikinsky hydrothermal field

III – intrusion of granodiorites and quartz diorites (Miocene age).

PEA2

PEB3

PEB4

PEB5

PES2

PEB2

PEA3

PEB1

PEA1

PEA0, PA0B

N

2 km

Nachikinsky hydrothermal deposit

Point of long-termseismic emissionregistration

Why we are interested in crust structure of this area?

1/ Study of hydrothermal field configuration without expensive drilling;

2/ We are carrying out long-term investigation of seismic emission in this area, remote from anthropogenic activity. We have interesting results indicating high sensitivity of microseismic radiation to earth tides just here.

We have calculated, that area of microseismic data acquisition has radius only about 7 km.

So we want to find main source of seismic emission and their space distribution.

Technique

The method estimates the energy of weak coherent radiation from various points of the medium.

For the reconstruction of microseismic emission field, we calculated the measure of the similarity of seismic signals (Semblace, [Neidel and Taner, 1971]) as a ratio of the signals summarized over all sensors of the group to the sum of the energies of each sensor separately calculated for each sampling point covered by array.

This can be done through the focusing of the total wavefield recorded by a seismic array on internal points of the medium with scanning in given cross sections.

Semblace - measure of the similarity of seismic signals

M - the number of seismic channels;N - the signal length;n - the current sample number;X m(tn) - signal amplitude in the channel m;

βijkm - the weighting coefficient accounting for the geometric divergence of the wavefront, i.e., the emergence angle of the seismic ray at the recording point; τijkm - the synchronizing time shift of the signal.

Original waveforms Shifted waveforms

∆t=-t 1

∆t=-t 2

∆t=-t 3

s 1 s 2 s 3

Σ

Aggregate signal

Scheme of the aggregated signal (signal summed over all sensors after temporal shift) construction

Σ

To eliminate storm microseisms and local high-frequency surface interferences, bandpass filtering with a window of 2–6 Hz was applied.

The field was focused using longitudinal waves.

In order to reveal regions of the most stable radiation, we averaged the results over several frames after calculating the Semblance for each selected fragment of the record separately.

PEA2

PEB3

PEB4

PEB5

PES2

PEB2

PEA3

PEB1

PEA1

PEA0, PA0B

N

2 km

Nachikinsky hydrothermal deposit

S c a n n e d a r e a

Example of filtered seismic records ( 2 - 6 Hz)

PEA0

PEA1

PEA2

PEA3

PEB1

PEB2

PEB3

PEB4

PEB5

PES2

Sem

blan

ce0.

11

0.14

1 km 1 km 1 km

1 km1 km1 km

NN

NN N

N

Horizontal cross sections of the scanning area for different depths

All values were normalized to the maximum Semblanse value. The values S < 0.1 indicate the absence of any spatial correlation between the channels of the array.

Distribution of the microseismic radiation intensity for the area of Nachikinskii hydrothermal field.

N

SS c a n n e d a r e a

S N

0 1 2 3 4 5 6 km

0

1

2

3

4

5

D e

p t

h, k

m

D I s t a c e a l o n g p r o f i l e, km

Sem

blan

ce0.

11

0.14

Vertical N–S cross section across the thermal water occurrence area of the Nachikinskii hydrothermal field

The most intense anomaly is observed in the northeastern part of the area. Its radiation intensity is highest at depths of 2–4 km. The spatial position of the anomaly remains stable with depth. The anomaly is correlatable with the position of the thermal water occurrence area of the hydrothermal field.

On the base of seismic records of multichannel array PS36 we developed the model of upper crust structure in the area of Nachikinskii hydrothermal field (in central part of Kamchatka peninsula).

For investigation of crust structure we used microseismic emission as sensitive indicator of the stress distribution in the medium.

The detected anomaly can be interpreted as the most fractured zone extending along the boundary of the large diorite intrusion and serving as a passageway for thermal waters.

Our results are in good correlation with geological data, magnetotelluric and deep seismic sounding.

Final noteNext topic of our PS36 records study is tidal modulation of microseismic signals. For the future investigation we need real calibration parameters for each seismometers of PS36 instead of some summarized characteristics, which we have now.

Conclusion

Kliuchevskoy Volcano4750 m

Thank you for your attention

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