Short-period Instruments Research Articles

Thirty years of seismic network recording in South Australia: selected results

The year 1993 marks 30 years of seismic network recording of earthquakes in South Australia. The network currently comprises 17 short-period instruments, of which six use digital recording and five employ triaxial sensors. Approximately 350 earthquakes are located within the State each year using a computerised seismic analysis system developed by the Phillip Institute of Technology. A duration-based magnitude scale, equivalent to the Richter M L scale, has been developed for most stations. The pre-network (historical) record of earthquake activity in South Australia dates back to 1837. Epicentres are available for just 45 earthquakes. Of these, ten are of magnitude five or greater. The instrumental data over the last 30 years show a similar trend in epicentral pattern to the historical earthquakes, with the major zones being the Flinders Ranges, Eyre Peninsula and the Southeast. There have been 40 earthquakes of magnitude four or greater since 1963, the largest earthquake being of magnitude six in the Musgrave Ranges during 1986. Mine and quarry blasts within the State have been useful in calibration of the network, both in terms of hypocentral location and amplitude studies. The explosions have also furnished valuable information on the crustal velocity distribution. Epicentral co-ordinates of mine blasts are generally good to within 3–5 km. The explosions yield Richter magnitudes of one (charge size 1 or 2 tonnes) to 2.5 (charge size 50 tonnes). The crustal S waves, used in earthquake magnitude determinations, exhibit an amplitude-distance decay coefficient of 1.2. The P-wave velocity function of the crust increases non-linearly with depth, from about 5.0 km s −1 near the surface to 6.4 km s −1 at 20 km depth, where the gradient reduces, with the velocity reaching a value of about 7.4 km s −1 at a depth of 40 km.

The Valencia trough: Coda-Q

The coda-Q method has been applied to a set of earthquakes located in the Mediterranean region, between the Iberian Peninsula and the Balearic Islands (the Valencia trough), and recorded at the short-period seismographic station of Ebro (EBR), located in Roquetes (Tarragona), to obtain the anelastic quality factor Q of coda waves. The applied method uses a set of master curves taking into account the frequency-dependence of the coda- Q values. To determine the master curves used to obtain the coda- Q values, non-linear inversion techniques have been applied to the short-period instruments. The coda- Q values and the frequency-dependence obtained for the region seem to indicate that the southern part of the Valencia trough tends to attenuate the seismic energy in the same manner as the Betic System, and the northern part seems to present higher Q values than the southern one. This fact may indicate different Q structures in both regions. A good correlation is observed among the obtained coda- Q values, their frequency-dependence, and known coda- Q values and L g wave attenuation in the coastal region of the Mediterranean Sea.

The 520 km discontinuity

Although some of the earliest suggestions of a seismic discontinuity near 520 km depth in the upper mantle came from seismic studies of the structure to the north of Australia, results from more recent deployments of portable arrays of short-period instruments are equivocal. Of the several hundred earthquakes recorded during these experiments, only a small fraction provide data that could help to constrain the properties of the discontinuity. Of the few earthquake recordings that can address the question of whether such a discontinuity exists, most indicate that it does not exist as a sharp feature (i.e., less than 50 km thick) at depths of less than 540 km. The data do not, however, constrain the existence of a discontinuity at depths greater than 540 km. Furthermore, the velocity increase could be distributed over a significant depth range; even a 50-km transition would not appear in the short-period observations and still provide sufficient reflectivity at near vertical incidence to be compatible with long-period observations for waves traveling steeply in the mantle.

The quantitative characteristics of earthquake sources as determined in the seismological practice of the U.S.S.R.

The source-parameter determinations in the U.S.S.R. from seismological observations using standard instruments, including a station with digital registration (STSR) and frequency-selective equipment (TCHISS) are discussed. To determine magnitude from P-waves with short-period instruments, a calibrating function of the Homogeneous Magnitude System (HMS) is recommended for seismological practice in the U.S.S.R. For assessment of source parameters (seismic moment, displacement and stress drop) a special multi-factor procedure for focal spectra analysis is developed. Magnitudes and focal dynamic parameters of 30 strong earthquakes are presented and compared with analogue parameters of the NEIS and Harvard solutions. It is proved that magnitudes determined by surface-wave M LH (ESSN, Edinaya Systema Seismicheskih Nabludenii—The Unified System of Seismic Observations) and M s (NEIS) practically coincide. Magnitude m b (NEIS) has been systematically underestimated as compared with M PV (ESSN). Seismic moments by ESSN ( M P o ) and NEIS ( M O CMT ) are linearly dependent. A comparison of fault plane solutions shows almost full agreement in about half the number of the earthquakes considered; the orientation of nodal planes and axes of principal stresses coincide despite their belonging to different stages of faulting. The time-frequency fields by TCHISS data for earthquakes with equal magnitudes and seismic moments show a different character of energy release.

Of the May 7, 1986 Andreanof Islands Earthquake source parameters

Source characteristics of the May 7, 1986 Andreanof Islands earthquake (51.412°N, 174.830°W, NEIC) are investigated from WWSSN, GDSN and IDA records. First motions from over 60 stations determine one steeply dipping nodal plane. We constrained this nodal plane and inverted long‐period surface waves at a period of T=256 sec and determined the second nodal plane to be dip 18°, rake 116°, and strike 257°. This shallowly dipping thrust mechanism is consistent with plate motions in this region. Seismic moment from surface‐wave inversion is 1.3×1028 dyne‐cm corresponding to Mw=8.0. Amplitudes of body and surface waves from short‐period instruments yield magnitudes of and Ms=7.7. The teleseismic average P‐wave moment rate spectrum from 17 short‐ and intermediate‐period instruments is slightly lower than that of an average Mw=8.0 subduction‐zone event. We constrained the fault plane as determined above to deconvolve the first 90 secs of the long‐period body wave at 11 teleseismic stations to determine the source time function and the spatial distribution of moment release. The source time function consists of 4 moment‐releasing episodes which have a total moment release of 9.4×1027 dyne‐cm. The fault ruptured bilaterally with the largest moment releasing subevent occurring between 30‐45 sec. This subevent nucleates approximately 75‐90 km west of the determined epicenter. This region corresponds to the epicentral area of the 1957 Great Aleutian earthquake which is one of the largest earthquakes in recorded history.

Short-period P- and S-wave radiation from large earthquakes: Implications for spectral scaling relations

Recent measurements of peak P-wave amplitudes on World Wide Standardized Seismographic Network short-period instruments by Houston and Kanamori (1986) provided the opportunity to investigate source radiation from great earthquakes at higher frequencies than have previously been available. The dependence on moment magnitude (M) of the amplitude measurements (A) and the dominant period (T) in the P-wave seismograms are compared to predictions from several source-scaling relations. For all of the relations, the radiated energy was assumed to be randomly distributed over a duration proportional to the inverse corner frequency. An w-square source-scaling relation with a constant stress parameter of 50 bars gives a good fit to both observed quantities (A and T) for earthquakes up to M 9.5. This model, with the same stress parameter, also fits peak acceleration and peak velocity data for earthquakes with moment magnitude as low as 0.5. Predictions using the source spectra derived by Gusev (1983), which are representative of several published relations featuring regions of reduced spectral decay after an initial ~-2 attenuation beyond the corner frequency, do not fit the various high-frequency observations quite as well as do those using the ~-square model, although the differences between the predicted motions are generally within a factor of 2 to 3. Although the w-square model successfully predicts a wide variety of time-domain measures over an extraordinary magnitude range, it fails to fit the Ms, M correlation for large earthquakes; Gusev's spectral scaling relation, on the other hand, fits this correlation, but was constrained in advance to do so. This failure of the w-square model is of little practical concern, occurring as it does at periods longer than those of usual engineering importance. An ~-cube model fails completely to explain the seismic moment dependence of the observations.

Magnitude calibrating functions for a multidimensional homogeneous system of reference stations

Calibrating functions play a substantial role in the determination of earthquake magnitudes. Their shape and character is closely connected with the internal structure of the Earth, the distribution of velocities and attenuation of seismic waves in the Earth's body. These facts have been considered during the establishment of the Eurasian Homogeneous Magnitude System (EuHMS). The paper summarizes the procedures used for constructing the new complete set of calibrating functions for body and surface waves (vertical and horizontal components of P, S and L waves from medium period instruments, and vertical component of P waves from short period instruments). The calibrating functions were derived in the distance range between 20° and 100°, being based on several thousands of homogeneous observations for each wave type. The application of a HMS gives a possibility to derive the magnitude calibrating functions with a considerable degree of reliability due to the method of simultaneous optimization of the calibrating functions and the systems of station corrections. The magnitude calibrating functions for EuHMS represent the first complete set of calibrating functions, which are derived by the same standard method and initial observational material. A comparison with the existing calibrating functions shows that the application of σ and Q functions, presently used in the seismological practice for magnitude determinations, may lead to errors exceeding 1 4 of magnitude unit at certain epicentral distances, specific for each wave type. It appears that the internationally recommended Q functions are not admissible for magnitude determinations from short-period P waves.

Broadband time domain modeling of earthquakes from Friuli, Italy

abstractShort-period (SP) and long-period (LP) seismograms written by the main shock and two principal aftershocks of the 1976 Friuli, Italy, earthquake sequence are modeled in the time domain using synthetic seismograms. The main shock occurred on 6 May 1976 (20h 00m, Ms = 6.5) and both aftershocks on 15 September 1976 (03h 15m, Ms = 6.0 and 09h 21m, Ms = 5.9). Source models were determined initially by trial and error and then refined using a waveform inversion program. Two point sources of radiation are required to adequately model the aftershock short-period records. For the 09h 21m aftershock, the model derived from short-period records also produces good fits to the long-period data. The seismic moment of this earthquake is found to be 0.8 to 1.0 × 1025 dyne-cm. The SP model for the 03h 15m aftershock, on the other hand, predicts long-period synthetics which do not agree with the observations. In particular, the SP moment (0.37 × 1025 dyne-cm) is about 212 times smaller than the LP moment (1 × 1025 dyne-cm). Adding a long-period component to the SP model considerably improves LP waveform and moment agreement. In the case of the main shock, a reasonable fit to the observed SP data is obtained using three point sources of radiation. However, LP synthetics computed using this model do not agree with the observations, and the SP moment (0.65 × 1025 dyne-cm) is a small fraction of the LP moment (3 to 5 × 1025 dyne-cm). Time function durations indicate that the individual events inferred from the SP records are radiated from patches of the fault having radii of 2 to 4 km and stress drops in the range 35 to 276 bars. In comparison, stress drops estimated from LP data are found to be 12 bars (main shock) and 24 bars (09h 21m aftershock). These observations suggest that the short-period instruments are sensitive to the high-frequency radiation emitted from small, high-stress drop areas on the fault plane whereas the long-period instruments record the overall motion during the earthquake.

To the problem of earthquake magnitudes determination

Causes of inconsistency in magnitudes determined by broad-band and short-period instruments and by different systems of observations are elucidated. In so doing, both observations of standard stations of USSO of the USSR and those of Soviet frequency selecting seismic stations (чисс) are employed. On the basis of large statistic material of the stationare network of USSO, a dependence of spectral constitution ofP waves and frequency-temporal fields constructed for some earthquakes have been revealed as well as a dependence of magnitude values upon an arrival time of maximal particle velocity. It has been shown that the range of measuring (A/T)max should be enlarged at least to 15 sec. A conclusion is drawn that the most stable magnitude consistent with the Catalogue by Gutenberg is the one determined by medium-period instruments.

The NORSAR Array and Preliminary Results of Data Analysis

Summary A large aperture seismic array, NORSAR, has been constructed in Norway. The project, which started in the summer of 1967, is a joint undertaking by the governments of Norway and the United States of America. NORSAR consists of 22 subarrays, each equipped with one three-component long-period and six short-period instruments. The array diameter is around 110 km, while that of a subarray is approximately 8 km. In the data centre, which is located just outside Oslo, are installed 2 IBM 360/40 computers with peripheral equipment, a special-purpose computer, and an experimental operations console. Routine tasks performed at the data centre comprise array monitoring and calibration, data acquisition, on-line event detection and off-line event analysis. In this paper we give a technical description of NORSAR, emphasizing the software aspects of the array operation, and present some analysis results of P waves recorded at NORSAR. For example, we have found that signal power and spectral characteristics vary across the array and seem to reflect local differences in the geological structures at the subarray sites. The recorded signals are found to be broadband and to contain significant energy at higher frequencies. Observed signal coherencies vary considerably across the array and are usually independent of station separation. Within the subarrays signal coherence is high and the waveforms exhibit little scattering.

SKP and related phases*

Abstract For shallow shocks, multiple SKP phases are observed after the initial SKP motion as long as 54 seconds on short-period instruments and as long as 87 seconds on long-period instruments. Amplitude data indicate that each multiple phase has a focal point similar to that of the initial SKP phase. The focal point for waves having periods of 1 to 5 seconds occurs at 131½°, and that for waves having periods of 5 to 10 seconds is broadly defined between 130° and 131°. Short-period SKP waves extend from 129° to at least 140°; long-period SKP waves, from 125° to 145°. The long-period waves are believed to be diffracted from the caustic in accordance with Airy's hypothesis. For all types of SKP phases the energy content of the short-period waves is several times less than that of the long-period waves. For the vertical component the agreement between theoretical and observed values of energy of long-period waves is good. For the horizontal component the observation of too little energy is not satisfactorily explained. SKP″, the SKP phase associated with the inner core, is observed between 114° and 125°. It records with very short periods. The observations of SKP″ present additional support for the hypothesis of large but continuous increase of velocity at the transitional boundary of the inner core.