THE DATABASE OF DIGITIZED HISTORIC SEISMOGRAMS FOR NUCLEAR TESTS MONITORING TASKS

Sokolova I.N., Aleschenko I.B., Uzbekov A.N.

Institute of Geophysical Research NNC RK

In 1996 the Test Ban Treaty was opened for signature. Since that time the International Monitoring System has been created. Its task is detection and discrimination of nuclear explosions conducted in different mediums: atmosphere, underwater and underground. After 1996 6 nuclear explosions were conducted. This number is definitely small for testing the discrimination technology. Thus, for more detailed investigation it is necessary to use historical seismograms of nuclear explosions collected in archives of different Organizations beginning from the middle of the last century. These can be digital and analogue records. In the USSR nuclear tests started in 1949. That time continuous seismic observations in Kazakhstan were conducted by the stations located mainly on the territory of Northern Tien Shan. Northern Tian Shan stations had narrowband instruments SKM-3 with magnification V=20000-40000 [1]; in addition most of the stations had broadband instruments SKD with magnification 1000. Their task was earthquake monitoring at south and south-east of Kazakhstan. Other seismic stations (permanent and temporal on the whole USSR territory) belonged to the Complex Seismological Expedition (CSE) network created by the Institute of Physics of the Earth (IPE) AS USSR in Talgar town. The task of this network was not only earthquake monitoring and investigation of lithosphere structure, but detection and discrimination of nuclear tests at regional and teleseismic distances. All stations were equipped with sensitive instruments as SKM-3, USF, CSE and RVZT with magnification from V 40 000 to 120 000) [2]. The most of these stations are located at low seismic level places; this allowed to record even small underground nuclear explosions at teleseismic distances. Nuclear tests seismograms collected in Kazakhstan archives are unique and invaluable material for scientific investigations. However, with time the seismograms recorded on photo paper and stored for a long period at the archive become dark and fragile, ink records fade. In addition, analogue type of record makes difficult its use while solving the tasks requiring digital mathematical processing. Thus, currently it is urgent to digitize analogue seismograms and convert digitized data into advanced format convenient for data storage, use and exchange. Digitized seismograms can be successfully used for developing new technologies of nuclear explosions identification and improving and testing new monitoring methods of concealed nuclear tests, for calibrating the stations of the International Monitoring network of the CTBTO, for investigating the lithosphere structure and atmosphere where nuclear tests were conducted, to study geodynamics and underground nuclear tests influence on medium. Figure 1 shows location of seismic stations operated in different years and which seismograms were used for digitization. Seismograms of these stations are stored mainly in SEME MES RK archives in Almaty (Seismological Experience-Methodical Expedition), CSE IPE RAS in Talgar (Complex Seismological Expedition of the Institute of Physics of the Earth), IGR NNC RK at seismic stations Kurchatov, Borovoye, Aktyubinsk, Makanchi and IS KR in Bishkek (Institute of Seismology in Kyrgyzstan). In addition to the nuclear explosions seismograms, large chemical explosions with well known parameters are also of high interest. Figure 2 shows the map of nuclear and chemical explosions locations which records were used for digitization.

Figure 1. The map of analogue stations location

which seismograms were used for digitization.

Figure 2. The map of nuclear (red stars) and

chemical (purple stars) explosions which records

were used for digitization.

Figure 3 shows the main type of instruments (Table 1) which were used at the analogue stations of Kazakhstan and Central Asia since 1960-th. Although stations photopaper records were digitized, curvilinear seismograms recorded on usual paper were not digitized. Figure 4 shows a seismogram of digitized Uch-Terek chemical explosion of June 11, 1989, 6-59-52, recorded by SKM and SKD instruments, Alma-Ata station.

Figure 3. The main types of seismometers operated at analogue seismic stations of Kazakhstan and Central Asia: a) SGKM-3, b) SVKM-3, c) SGKD, d) SVKD, e) USF, f)RVZT, g) CSE.

Figure 4. An example of the digitized Uch-Terek chemical explosion of June 11, 1989, 6-59-52, recorded by SKM and SKD instruments. Alma-Ata station.

Station Natural

period T0,

s

Magnification Scanning

rate

Recording

type

Number of

channels

SKD 20 1.0К-1.5К 60 or 30 s photopaper 3

SKM 1.5, 2 25К-80К 120 or 60

s

photopaper 3

USF 1.5 50K-80K 240 s or

120 s

photopaper

3

RVZT 1.25, 1.5 100К-300К 120 s Ink pen 3

CSE 1.25, 1.5 300К-1000К 120 s Ink pen 1

Table 1. Analogue instruments characteristics

“NXSCAN” [4] software which allows in semi-automated mode to digitize previously scanned seismograms was used. The seismograms were digitized with 40 Hz frequency. Fragments of analogue seismograms digitized by NXSCAN are saved in SAC (Seismic Analysis Code) [5] format, and then converted into CSS 3.0 format (Center for Seismic Studies v.3.0) [5]. The database each record of which contains the following CSS 3.0 tables: wfdisc, site, sitechan, assoc, origin was created using digitized material. The testing results showed that digitized seismograms have good concordance with original visually and by kinematic and dynamic parameters. Thus, for instance, accuracy of arrival time picking depends much on clearness of phases arrival, scanning rate of seismogram and in average is ±7 %, accuracy of amplitudes estimations for low-sensitive channel is ±10 %, for high-sensitive channel ±5 %. Figure 5 shows an example of original and results of nuclear explosion seismogram digitization 02/01/1979, t0=04:13:00.2, =50.101◦, =78.863◦, mb=5.4.

Figure 5.а) The seismogram of nuclear explosion conducted at the

STS 02/01/1979, t0=04:13:00.2, =50.101, =78.863, mb=5.4.

Borovoye station. Analog record on photopaper. Figure 5.b) The results of seismogram digitization of

nuclear explosion conducted at the STS 02/01/1979,

t0=04:13:00.2, =50.101, =78.863, mb=5.4. Borovoye

station.

The database in CSS3.0 format contains more than 3000 seismic records of nuclear explosions

conducted at different world Test Sites (Table 2). Figure 6 shows a diagram of digitized seismograms

number distribution by the Test sites. The database allows the quick search by coordinates, magnitude,

source type and etc.

Test Site period

epicentral

distances

number of

digitized

seismogramms

STS 1961-1989 99-3493 1770

PNE USSR 1965-1990 607-3770 320

Novaya

Zemlya 1961-1990 2075-3826 290

Chemic

USSR 1957-2000 44-2648 327

LopNor 1965-1996 700-2006 463

Pokharan 1974-1998 1375-2970 45

Chagay 1998 1340-2224 31

Table 2. Characteristics of digitized seismograms by the Test Sites.

Figure 6. The diagram of digitized

seismograms by the Test Sites.

Nuclear Test Sites are located at different distances from the stations; in addition, geophysical environment and explosions carrying conditions differ significantly. Thus, each Test Site has magnitude threshold of recording, and it differs from station to station. Figure 7 shows distribution of digitized seismograms by magnitudes and distances using digitized records of Talgar station located in Northern Tien Shan. Thus, for the STS minimal mb=3.3, for LopNor minimal mb=3.7, for peaceful nuclear explosions at the USSR territory minimal mb=4.3. Other values of threshold magnitudes are observed for Borovoye station located in the north of Kazakhstan (Figure 8). Thus, for the STS minimal mb=4.3, for LopNor Test Site minimal mb=4, for peaceful nuclear explosions minimal mb=4.6, for Novaya Zemlya minimal mb=4.5. It is clear that both stations are sensitive, but for Novaya Zemlya Test Site the best station is Borovoye, and for the STS, LopNor Test Sites and peaceful nuclear explosions the best is Talgar station.

1000

3,0

3,5

4,0

4,5

5,0

5,5

6,0

6,5

LopNor

STS

PNE

Chem.

Novaya Zemlya

dist.,km

mb

Figure 7. Distribution of digitized

seismograms by magnitudes and distances.

Talgar station.

1000

4,0

4,5

5,0

5,5

6,0

6,5

7,0

LopNor

STS

PNE

chemic

Novaya Zemlya

dist.,km

mb

Figure 8. Distribution of digitized

seismograms by magnitudes and

distances. Borovoye station.

While analogue seismograms digitization the lack of source parameters for a range of small nuclear explosions in seismic catalogues was revealed. As a rule, for these explosions, there is no information about focal time, coordinates and magnitude, or there is data about explosion date and approximate coordinates. Much work on restoring parameters of small underground nuclear explosions at Semipalatinsk Test Site was conducted in [7] using records of analogue seismic stations of the USSR located at regional distances. However, UNE are well studied and described in literature [7, 8], but air and contact explosions were quite small and were not recorded by standard permanent seismic stations. In 1961-1962 at Opytnoye Polye (GZ) site of the STS (Figure 9) maximum number of air and contact explosions were conducted, and at that time IPE RAS installed high-sensitive stations along Pamir – Baykal profile to study structure of earth crust and upper mantle [9]; the profile extension was 3500 km. Epicentral distance from several stations of the profile to the Opytnoye Polye was 300 – 400 km. Parameters of more that 20 explosions were restored by digitized seismograms of air and contact explosions. Figure 10 shows seismograms of air explosion of October 20, 1962 conducted at 635 m height, having yield of 6.7 kT. Using the digitized records the explosion parameters were restored t0=09-21-45.6, =50.412, =77.776, mpva=3.0, energy class K=8.1. Records of air and contact explosions at Opytnoye Polye have all features of air explosions, powerful surface waves, some seismograms have a record of air wave. Thus, using historic seismograms it is possible to restore and replenish seismic catalogues of nuclear explosions for further use of explosion parameters in monitoring tasks.

Figure 9. The map of the STS sites location.

Figure 10. Seismograms of air explosion of

October 20, 1962, t0=09-21-45.6, =50.412,

=77.776, mpva=3.0. Z-component.

CONCLUSION A unique database of nuclear explosions seismograms has been created in CSS3.0 format using archive records

and is replenished constantly. Currently the database is actively used for solving different investigative tasks of seismology: for seismic discrimination of nuclear explosions and earthquakes; to study space-temporal variations of shear waves attenuation fields; to study long-period variations of seismic waves velocities; construction of regional travel-time curves of seismic waves; to study environment consequences of large explosions.

REFERENCE

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a) b) c) d)

e)

f)

g)