NEW TECHNOLOGY IN SEISMIC MEASUREMENTS

ABSTRACT. There are results of development in the field of automations of seismometry with using the new technology and new approach to fast processing seismic information for the short-term forecast of strong earthquakes; described system of real-time automatic processing of seismic signals, allowing get parameters of earthquakes immediately on receipts of body waves.

BASIC IDEA

For a realization of the short-term forecasting of earthquakes it is necessary to have an information about all earthquakes occurred on territory of region within last day - hours. Foreshocks - the forerunners of destructive earthquakes are among these small and microearthquakes. It is necessary to determine their geographical coordinates, energy (Rautian's class), spectra and other parameters for the short-term prognosis.

HISTORY OF A PROBLEM

From a beginning of seismic observations on the Earth, i.e. since a 1891, when the first seismogram of strong Japanese earthquake was obtained in Potsdam, there is a problem on the determination of geographical coordinates of a source. Since that time coordinates of earthquake discover, using data of several seismic stations, which the event was fixed by [1].

The typical algorithm of these measurements consists of the following. From a difference of times of arrival of a P-wave and S-wave, have a smaller velocity, determine of epicentral distance on a hodograph and determine a location of epicentre by overlapping epicentral distances from the remote places of observation.

In this mode it is necessary to have at least three different places of observation and channels of connection for transmission of an information in a center of handling, i.e. unique seismic station be not capable univalently to locate events.

At the same time, majority of events are fixed only on one - two stations. It is connected to that which earthquakes of various magnitudes are distributed statistically under the law of Gutenberg-Richter, defining frequency of earthquakes:

Lg (N) = 7 - 0.75 Ms,

where N - amount of earthquakes one range of magnitude ±0.5;

Ms - magnitude of earthquake on surface waves.

From this formula it is visible, that an overwhelming majority of earthquakes are weak and microearthquakes.

For the purposes of earthquake forecasting, when the statistical data processing is made and the prediction can be done only on a representative statistics, it is very important to have an information about weak and microearthquakes.

MODERN BACKGROUND

Now there are global, regional and local nets of seismic observation, on which data the coordinates of earthquakes are determined. Though the world catalogues (IRIS, NEIC, WWSN) contain coordinates about 20 thousands earthquakes annualy, it is not enough of this data for midle- and short-term forecasting of earthquakes, because for earthquakes have a class lower 10 (Ms < 4) there is no representative statistics. Besides this data even been arriving from Internet, have the large delay (8 and more hours).

The regional and local seismic nets, such as a net of the Institute of Seismology of Kazakh Academy of Sciences have no a modern technology of data acquisition and processing permitting fast enough to treat data for the purposes of the short-term earthquakes forecasting. Besides this, a technology of handling of events excluding from reviewing data obtained less than from three seismic stations, because here is applied "triangle" algorithm. At the second, this net uses a "handmade" technology of handling of seismic signals on seismograms which are not permitting to have exact evaluations of parameters of earthquakes.

THE NEW METHOD OF SEISMICITY MONITORING

A new method of monitoring of seismicity are developed for a measurement of geographical coordinates of earthquakes the observation from a single point, diminution of an error of a measurement of coordinates, and also for acceleration of process of a measurement. It is patented in Republic of Kazakhstan [2].

Idea of this method is in following. After arrivaling of a P-wave to observation point determine an apparent azimuth of a wave direction. On a arrival of a S-wave or the waves of other type determine a distance up to epicentre based on a measuring difference of times of arrival of waves and applying of a hodograph. After that obtained values of an apparent azimuth and hodographial distance determine a true azimuth and epicentral distance, using the correction table made beforehand. Then, using the standard formulas of a spherics, on a true azimuth and epicentral distance determine a geographical longitude and latitude of earthquake.

The correction table is istablished by retrospective data processing, i.e. measurement of an apparent azimuth and interval of time between arrival of waves of a different type for earthquakes with known coordinates on entries, being available in archive, of these waves in a point of observation, for which there is a table. Then on an obtained interval of time discover hodographial epicentral distance. From geographical coordinates of earthquake calculate a real (geographical) azimuth and epicentral distance. After that subtract value of a real azimuth from apparent, obtained from a seismic entry of P-wave (by results of retrospective handling).

Accordingly subtract true epicentral distance from hodographial. The obtained differences are a basis of single-error corrections, of which the table will consist. For deriving a stability of outcomes (minimization of an average error) the values of single-error corrections for the same elements of a table, i.e. average the same values of an apparent azimuth and hodographial epicentral distance.

For an elimination of the blank cells in the correction table, produce an interpolation of obtained values of corrections in the blank elements of a table.

With the purpose of liquidation of quantum noise for passage from one element of a table to other instead of the correction table it is possible to use smooth approximating function. For the definition of coordinates of depthy earthquakes determine at first their depth by known modes. It can be, for example, mode of the definition of depth of a seismic center on a difference of times of arrival of a P-wave and pP-waves.

After the determination of depth of a seismic center apparent azimuth and hodographial epicentral distance, using the three-dimensional correction table, the true values of an azimuth and epicentral distance discover. It is necessary to note, that for determination of epicentral distance the depth of a seismic center should be taken into account in this case. It is connected to that the waves of earthquakes of different depth have various hodographs. The three-dimensional table forms just so, as well as two-dimensional, except that in retrospective handling should participate and depth of earthquake.

Because at a seismic signal always there are microseismic noise, and the P-wave represents actually not one, but the set of imposed against each other waves, exact measurement of an azimuth of its arrival is rather inconvenient. For minimization of an error of the measuring of an azimuth in the given mode it is offered as one of possible variants to use only head plot of a wave - its front, in which least exchange and refracted waves. And, to use that plot of front, where the variance of instantaneous values of an azimuth is least, i.e. the microseismic noise is a minimum.

Thus, for a diminution of errors in the definition of an apparent azimuth, its measurement produce by an evaluation of an arctangent of a ratio of orthogonal horizontal components of a P-wave on a plot of front of a P-wave have a minimum variance of an azimuth. In an outcome it the drop of value of a root-mean-square scatter of values of azimuths for earthquakes from the same point of the Earth up to a level about 0,01 radians for want of precise entrances of a P-wave is possible.

The additional information about area of earthquake is kept in an angle of an exit of a P-wave, i.e. in angle between a vector of the arrival of a P-wave and horizon in a point of observation. It is possible With using of an exit angle shaping a three-dimensional table and consequent use it for definition of coordinates of earthquake is possible as the third coordinate of the correction table.

The seismic waves from earthquakes of various power classes have various spectral performances. That properties of a seismic trace "a seismic center - point of observation" frequently dependent, the arrival azimuth of the entry of a P-wave have some depends on a power class of earthquake. Therefore for magnification of precision of the measuring of coordinates it is possible to make the separate correction tables for earthquakes of different power classes and, determining a class of earthquake for a filing by known modes, to use the correction table, appropriate to a found power class.

For simplification of this variety of an offered method it is possible to be limited to make the separate correction tables of an azimuth and epicentral distance for P-waves of various phases, having in view of the correspondence of a phase of a P-wave to defined variant of a seismic trace (variation of an arrival azimuth of a P-wave for earthquakes of one area). The System of the Primary Analysis of a Seismic Information " (SPASI) was written on the Intel-X86 chips assembler. Its generalized algorithm is shown on figure 1.

 

 Fig. 1

THE SOFTWARE OF A SEISMIC SERVICE

SPASI consists of five basic subsystems:

- Subsystem of automatic registering of seismic oscillations;

- Subsystem of visualization of seismic oscillations;

- Subsystem of an automatic detection of groups of seismic waves;

- Subsystem of the analysis and determination of parameters of earthquakes;

- Subsystem of keyboard control of conditions and parameters of processes of registering, visualization and detection.

The subsystem of automatic registration works in an foreground mode (condition of the highest priority) and realizes acquisition of information from a device of parallel input in the computer, decoding of this information, writing to files. The subsystem of registration ensures access to flowing seismic data to subsystems of a detection, analysis and visualization.

The subsystem of a detection works in a condition of lower priority, than the subsystem of registration and realizes the following:

- detection of groups of seismic waves on a threshold, established by an operator;

- entry of a plot of a seismogram containing this group waves including lengths of a previous history and codas, established by an operator, in to archival file.

The subsystem of the analysis and determination of parameters of earthquakes is a recognizing system, which using a information theory approach to pattern recognition [3,4,5,6], works in lower priority. It realizes the determination of entry time, maximum amplitude of a P-wave, its phase, magnitude, azimuth, angle of an exit of a P-wave, Rautian's class, epicentral distance, geographical coordinates and time of the origin. In case of a detection only P-waves from earthquake, coordinates are determined approximately, and in case of a detection of S-wave - are more exact on a difference S - P.

The subsystem of visualization works in a background mode (condition of a low priority) and show seismograms from decoded information on the display of the computer in a graphic mode.

The subsystem of control works in a background condition and realizes a modification (set-up) of conditions and parameters of work of subsystems of registration, detection and visualization in according to commands of an operator. The interface of a subsystem of control is carried out as the menu on the display (menu of conditions in a Fig. 1). Its block-scheme is shown on Figure 2.

The menu of a subsystem of control allows to establish the following parameters of SPASI's operation in a basic mode:

Fig. 2

 Components of SPASI work in two modes:

For a realization of work in the first mode - real time - the program substitutes system subprograms of handling of interruptions on own. Besides the operator can at any moment change parameters of SPASI operation by setup menu. These operations are carried out the program in a background mode.

SPASI in according to a level of a threshold of a detection established by operator, works in two following modes:

The toggling from one mode to another is carried out smoothly, - in an assotiation from a drop of a threshold the segments of records of a seismic regime take the increasing and large long of all time, until merge to one continuous record.

The program writes seismograms in two basic types of files:

The real digital seismogram represents a linear digital record of a three-component seismic signal in word (two-byte) format, i.e. by a range 96 dB (±32767).

Visual seismogram is a logarithmicly compressed in pair of bytes same 96 dB of a record. In an outcome of such compression it is suit only the visual review and only for a qualitative evaluation of parameters of a three-component seismic signal with an error ±5 % in amplitude.

For simplification of a data access all kinds of seismograms (daily visual, hour visual, real digital) have identical length of blocks - 150 bytes. Besides each real digital record is written by the whole second packages, which length following:

The first block of a file of a seismogram (both real, and visual) is two textual ASCII-lines of fixed length - 73 characters plus two control bytes - CR, LF. It is a header of data.

The first line is a stamp of a seismogram, in which its parameters are indicated. Second line is ruler of the help for a stamp. To look parameters of a seismogram (stamp) it is possible to use usual textual viewer. The value of fields of a stamp is shown in a table 1.

There is a "tail" after a body of a seismogram, wich is a part of a file intended for nonstandart entries of the user, for example, comments can be noted. It is recommended, that length of a tail should be in the whole blocks, i.e. is multiple to 150 bytes. It will simplify programmers' manipulation with data.

TABLE 1

Structure of a stamp in a file of a seismogram

N of byte

Value in a stamp

Semantics

1 - 6

SVR98a (SDR98a)

The identifier of the standard of a record

7 - 9

ZNE

Sequence of components (normal)

10 - 13

XXXX

Sensitivity of the channel 1 [nm/d]

14 - 17

XXXX

Sensitivity of the channel 2 [nm/d]

18 - 21

XXXX

Sensitivity of the channel 3 [nm/d]

22 - 23

0.. 58

Previous history [s]

24 - 28

10 (20, 40, 500, 12000)

Step of sample [ms]

29 - 32

1998..

Year of a recording of a seismogram

33 - 34

01.. 12

Month of a recording of a seismogram

35 - 36

01.. 31

Day of a recording of a seismogram

37 - 39

GMT

Type of zone (Greenwich mean time)

40 - 45

00000... 235959

Time of a record's beginning[hhmmss]

46 - 50

150 x Nrec

Length of a record of a seismogram [bytes]

51 - 55

0 (150 x Nt)

Length of a tail [bytes] of Nt-blocks

56 - 58

ALM

Abbreviation of seismic station

59 - 66

+0763500

Longitude of a station [deg-min-sec]

67 - 73

+430900

Latitude of a station

74

CR (13)

Carriage return

75

LF (10)

Line feed

 

Thus, the structure of a file is a following:

Stamp

(header)

Body of a seismogram

Tail

0.... 150 N x 150 M x 150

For convenience of searching and sortings of seismograms they have time-ordering names, i.e. if the list of files of seismograms is ordered under the alphabet, it is ordered and on time.

PROSPECTS

Installed in SEFC "PROGNOZ" the new technology of seismic recording and monitoring will allow:

 Fig. 3

REFERENCES

  1. Khaidarov K.A. P. Patent of Republic of Kazakhstan N 4781 " The Method of the determination of earthquake's coordinates", date of priority: 28.04.93.
  2. Khaidarov K.A. Patent of Republic of Kazakhstan N 717 "The method of transformation of analog signals", date of priority: 06.05.87.
  3. Khaidarov K.A. Methodical problems of development perceptrons for systems of automatic monitoring and control. Alma-Ata, 1983.
  4. Khaidarov K.A. About properties of discriminant functions based on a measure of information. Alma-Ata, 1984.
  5. Khaidarov K.A. Application of information measures as a criterion for pattern recognition. Alma-Ata, 1982.
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