Body-wave Magnitude Research Articles

Station factors of Khonsa and Yaongyimsen seismograph stations

Body wave magnitudes of 384 and 440 teleseismic events in the distance range 9–100° are determined using short-period P-wave data obtained from the vertical component seismograms of Khonsa (Tirap district, Arunachal Pradesh) and Yaongyimsen (Mokukchung district, Nagaland) seismic stations. These magnitudes are compared with the corresponding body wave magnitudes reported by the National Earthquake Information Service of the United States Geological Survey. Average residuals for Khonsa and Yaongyimsen stations are found to be +0.09 and +0.48, respectively. It is observed that the average residuals (Δ7) for both the stations decrease with respect to epicentral distance (Δ), focal depth (h) and body wave magnitude (mb). For the Khonsa station, the respective linear relations are: Δ=0.12−1×10−5 Δ, δM=0.22−7××10−5 h, ΔM=1.96−0.356mb and similarly, for the Yaongyimsen station the relations are ΔM=0.59−2×10−5 Δ, ΔM=0.54−19×10−5 h, ΔM=2.56−0.391mb. The nature of the variation of residuals is found to be nearly similar for both the stations.

Re-evaluation of the large deep earthquake of January 21, 1906

The large deep earthquake of January 21, 1906 is re-evaluated using old seismogram data and updated analysis techniques. From the P and pP-P time data the hypocentre parameters are determined as follows: origin time, 13h 49min 35s; latitude, 33.8°N; longitude, 137.5°E; depth, 340 km. The body-wave magnitude m B is re-evaluated from the amplitude and periods of P, PP and S waves. The average value of 7.4 is obtained. This value is the smallest among any values assigned previously to this shock, and it is denied that the earthquake is the world's largest deep shock in this century. The focal mechanism is estimated from the P-wave first motions and amplitude distribution of P and S waves. Synthetic body waves are used to constrain the mechanism and to determine the seismic moment. The mechanism solution suggests the down-dip compression typical of this region. A seismic moment of 1.5 × 10 27 dyn · cm is obtained. This value and the re-evaluated value of m B are consistent with the moment- B relation obtained for other deep earthquakes.

Turkish strong ground motion data acquisition and analysis

The installation of a strong-motion network in Turkey was initiated in 1973 under the sponsorship of the Earthquake Research Institute. Today, 61 accelerographs and 40 seismoscope stations have been installed along the most active seismic zones, namely the North Anatolian zone and in the graben systems of western Anatolia. All the strong ground motion records, which were obtained during the period 1973–1983, are listed in the tables. The date, origin time and body-wave magnitude of the events are given together with the local intensity, focal distance of the recording station and maximum acceleration registered by the instrument. Information about the significant earthquakes, which were recorded by SMA-1 Kinemetrics type accelerographs, are also given in the following. August 19, 1976 Denizli earthquake in western Anatolia with a magnitude 4.7, caused considerable damage in the city. It is interesting to note that this was the first earthquake whose strong motion was recorded by an accelerograph located within the epicentral area. The baseline corrected acceleration and velocity values for the N-S component is calculated to be 34.21% g and 21.35 cm s −1. November 24, 1976 Çaldiran earthquake in eastern Anatolia showed the spectral acceleration of 11.2% g for a natural period of 0.73 s. December 9 and 16, 1977 İzmir earthquakes with magnitudes 4.8 and 5.3 caused damage at various locations in İzmir. The two earthquakes were recorded by a SMA-1 and by two seismoscopes, and showed the acceleration values reaching 21% g in the E-W component. The strong-motion accelerographs of the other earthquakes that occurred in the 1973–1982 period were also processed and preliminary evaluation of the data was studied taking the damage distribution and the site conditions into consideration.

Excitation of short-period body-waves by great earthquakes

Short-period seismograms from large earthquakes are generally complex and are hard to Fourier-analyze. An innovation of spectral analysis has been made for WWSSN short-period seismograms. The parameters necessary for the analysis are: dominant frequency ƒ 0; maximum amplitude A max; and a characteristic duration σ of complicated wave-packets. Spectrum of short-period seismograms, Y(ƒ), is shown to be approximated as Y(ƒ)=1.07 σ ƒ0 A max The spectrum, after correcting for the effects of geometrical spreading, unelastic attenuation instrumental response, and crustal magnifications, is used to retrieve a short-period seismic moment M 1 of earthquake sources. The method has been applied to more than 900 short-period seismograms from 79 large earthquakes throughout the world. These earthquakes cover the seismic moment, M 0, from 7.5 × 10 29 to 7.5 × 10 24 dyne.cm. M 1 would not be saturated in this range, although body-wave magnitude usually is. A relation of M 0 ∞ M 1 2 is obtained for the present data set. A relation between M 1 and redetermined body-wave magnitude m∗ b is also shown log M 1=1.24 m ∗ b+17.9 where m b ∗ is obtained from maximum amplitudes of P-wavetrains. The analysis method is based on an extension of algorithm of earthquake magnitude determination, however, it has a potential beyond the body-wave magnitude. As M 1 is related closely to the source excitation of short-period seismic waves, it is one of the key parameters in understanding the fracture mechanisms of irregular faults.

Seismic risk in Turkey, the Aegean and the eastern Mediterranean: the occurrence of large magnitude earthquakes

Summary. The file of Turkish seismicity developed by Kandilli Observatory, Istanbul, for earthquakes to 1970 is extended here up to 1978 using 1SC and PDE data. Entries into this file are maintained on the surface wave magnitude scale M,, and conversion of body wave magnitude mb to M, has been carried out where necessary using a formula derived for Turkish earthquakes. Completeness analysis suggests that magnitudes M,> 4.5 may be used for statistical evaluation of seismic risk. This file is analysed by a range of methods to provide a suite of risk forecasts. Forecasting results from least squares and maximum likelihood estimates of the whole process Gutenberg-Richter cumulative frequency law of earthquake magnitude occurrence, and from the part process of Gumbel’s first extreme value distribution, all show small systematic differences in forecasts, but all three methods lead to magnitude forecasts which fall well within the range of standard deviation. However, these forecasts are obtained by subdividing Turkish seismicity in a cellular manner, and many of these cells show curvature of the earthquake frequency magnitude distribution curve concave with respect to the origin: Gumbel’s third asymptotic distribution of extreme values is chosen as an appropriate statistical description. Approximate upper bounds o to earthquake surface wave magnitude occurrence are evaluated and estimates of largest magnitudes expected over an interval of 75yr are forecast with uncertainties. Values of w are asymptotic, uncertain, and theoretically correspond to infinite return periods. Strain energy release diagrams are then invoked to estimate empirically the large magnitude M, which is equivalent to the total strain energy which may be accumulated in a region. This equivalent magnitude M, is consistently less than w and there is a finite ‘waiting time’, typically ranging from about 15 to 70 yr, during which the energy equivalent to M3 may be accumulated.

Estimates of magnitudes and short-period wave attenuation of Chinese earthquakes from Modified Mercalli intensity data

AbstractIntensity data for Chinese earthquakes are used to estimate the body-wave magnitude, mb, of selected historical earthquakes and to estimate Q0, the 1-sec period Q value of Lg waves for various geographical areas of China. In order to derive the necessary empirical relation between the intensity distribution and mb, data are used from recent earthquakes, for which instrumentally obtained mb values as well as isoseismal maps are available. Average Qo values are approximately 175 for the mountainous regions of southwest China, 550 for southeastern China, and 150 for Taiwan. These values agree qualitatively with those obtained by Evernden (1983) and Chen et al. (1983), who utilized a different method of analysis of the intensity data

Teleseismic observations of aftershocks immediately following an underground explosion

Underground explosions often trigger a series of earthquakes (aftershocks) at least one magnitude unit less than the magnitude of the explosion. In the first one or two hours following an explosion the rate of occurrence of the aftershocks can be so high that the signals from separate shocks overlap. Few if any of the P-waves from these aftershocks have been observed at teleseismic distances. Here, however, evidence is presented that the aftershock sequence triggered by at least one explosion (the Nevada Test Site explosion, Greeley) commenced a second or two after the firing of the explosion and that the radiation from some of the first of these aftershocks was detected at teleseismic distances; the radiation being present in the coda of the P signal from the explosion. The evidence for the presence of the aftershocks is that for the Greeley explosion the coda of the P signal contains a much higher proportion of high-frequency energy than the initial P pulse and yet the absence of high-frequency energy in the P pulse seems to be a source effect and not the result of losses on the transmission path. So, as the explosion source radiated little high-frequency energy it is proposed that the high-frequency energy in the coda is from aftershocks which have P-wave spectra with corners well above the corner frequency of that of the explosion. The body-wave magnitude of the largest of the aftershocks is estimated to be about one unit less than that of the explosion. The low magnitude of the aftershocks does not, however, appear to rule out the possibility that the radiation from the aftershocks can account for all the radiation from the explosion source that is usually attributed to a double-couple type of source occurring simultaneously with or immediately following the explosion.

Ground Motion of Mississippi Valley Earthquakes

Three methods are used to estimate the dependence of strong ground motion on epicentral distance and magnitude for Mississippi Valley earthquakes. The first utilizes earthquake intensity attenuation relations, correlations between modified Mercalli (M.M.) intensity and ground acceleration, velocity and displacement, and relations between maximum M.M. intensity and earthquake magnitude. The second method used near-field strong-motion data from the western United States which is extrapolated to larger distances by using seismic wave attenuation that is appropriate to the central United States. The third method uses observed strong ground-motion values for the central and eastern United States to determine acceleration, velocity and displacement as a function of epicentral distance for an earthquake of body-wave magnitude (m\db) equal to 5. Theoretical scaling relations are employed to construct similar curves for other magnitudes. All three methods give similar results for earthquakes in the m\db range of 5.0–6.5. An m\db = 5.0 earthquake is about the lower level of damaging events. An m\db = 6.5 earthquake has a recurrence time of about one century in the central and eastern United States.

A test of foreshock occurrence in the central Aleutian Island Arc

AbstractA statistical method for estimating the occurrence of foreshocks to larger earthquakes using local seismicity data is applied to the catalog of earthquakes from a local seismograph network near Adak, Alaska. Foreshocks are defined in a restricted sense as earthquakes preceding larger earthquakes within a critical time calculated from the mean time between successive earthquakes in the volume surrounding the target event. The threshold of completeness of the earthquake catalog is estimated as duration magnitude Md2.2. A low-magnitude cutoff on the catalog of Md2.5 was used to minimize potential bias from variations in weather and instrumentation condition. About 6 to 10 per cent of 51 main shocks of mb ≧ 4.5 in the Adak thrust zone were preceded by detectable earthquakes satisfying this statistical definition for foreshocks. As many as 14 percent of mb ≧ 5.0 earthquakes may have been preceded by detectable earthquakes satisfying this definition. An estimate of foreshock occurrence using as background the seismicity rate in the 4 months preceding larger earthquakes indicates a slightly higher rate of occurrence. The volume around the target events which showed foreshock activity is used to infer the size of a zone of preparation for larger earthquakes in the Adak area. The data are consistent with a zone of preparation whose size is a linear function of target-event magnitude. The size of the zone of preparation ranges from 20 to 45 km in radius for body-wave magnitudes 4.0 to 5.7.

Effects of network-average magnitude bias on yield estimates for underground nuclear explosions

Summary. The ISC body-wave magnitude, mb ISC, of presumed underground nuclear explosions in Kazakhstan, USSR, is shown to be systematically biased, by comparison to that recorded at the array station EKA (mb EKA). Linear regression gives: This is found to be due in part to anelastic attenuation effects on mbEm, but several characteristics of the ISC data (in particular, histograms of all contributing individual station magnitudes) demonstrate that the bias is also due to network-averaging effects. For the smaller explosions, those stations with a positive mb bias dominate the data set, but the remainder of the network fails to detect the event. Conversely, for larger explosions, additional stations, with negative mb bias will detect. Accepted magnitude: yield (Y) relationships will thus be in error if applied with mb Isc. Use of published station corrections for EKA allows estimation of an mb EKA: Y relationship, and hence, from the mb 1sC: mb EKA comparison, a magnitude: yield relationship which takes account of network-average bias. This is:

Seismic rupture patterns in Oaxaca, Mexico

The spatio‐temporal patterns of seismic activity for events with body wave magnitude mb ≥ 4.0 are investigated for the three most recent large earthquakes in the Oaxaca region of southern Mexico (August 23, 1965, MS = 7.6, MW = 7.5; August 2, 1968, MS = 7.1, MW = 7.3; November 29, 1978, MS = 7.8, MW = 7.6). A master catalogue of earthquakes is compiled for the analyses, using the International Seismological Center (ISC) catalogue supplemented by the National Earthquake Information Service (NEIS) catalogue; the events are then relocated with the Joint Hypocenter Determination (JHD) method. At this magnitude threshold, we find that the aftershock zone of the 1965 earthquake was seismically quiescent for at least 20 months prior to the main event; the 1968 earthquake was preceded by 1 year of foreshock type activity clustered in the aftershock zone; the 1978 earthquake was preceded by 43 months of quiescence and one event (mb = 4.7) within the aftershock zone 4 months prior to the mainshock. We also find that a segment of the subduction zone in Oaxaca remains unbroken by these earthquakes. In addition, some catalogue problems are pointed out which are important to interpretations of seismicity patterns.

Empirical magnitude and spectral scaling relations for mid-plate and plate-margin earthquakes

Published values of body-wave magnitude m b , surface-wave magnitude, M s , and seismic moment, M o , for mid-plate and plate-margin earthquakes, are analyzed, to determine their inter-relationship and to develop scaling laws for earthquake spectra. All of the plate-margin earthquakes used in the study occurred near the border of the Pacific Ocean. The mid-plate earthquakes occurred in both continental plate interiors and oceanic plate interiors. For mid-plate earthquakes the m b - M s relation can be represented by two straight lines, of slope one and two. The intersection of the two lines corresponds to the body-wave magnitude for which the corner period of the spectrum is at 1 sec. The M b - M s relation for plate-margin earthquakes is more complicated, because the derived spectra exhibit two corner periods. A principal feature of the m b - M o and M s - M o relations is the fact that the large plate-margin earthquakes have a higher M o value than the mid-plate earthquakes, for the same m b or M s . Because m b is a measure of the spectral amplitude for frequencies at which damaging ground motion occurs, and M 0 is a measure of the spectrum at very long periods, it follows that mb is a good estimator of strong ground motion, whereas a moment-derived magnitude is not. The latter, however, is a good measure of fault rupture area. The derived spectra of the mid-plate earthquakes are flat at the long periods, and have a slope of two at the short periods. Their seismic moment varies as the fourth power of the corner period, implying that the stress drop increases as the moment increases. For the large plate-margin earthquakes the derived spectra are flat at the long periods, have a slope of one at intermediate periods, and a slope of two at short periods. The stress drops are almost independent of moment for the larger plate-margin earthquakes. The moment-magnitude relations for mid-plate earthquakes indicate that an M s = 8.7 event requires a fault rupture length of only 60 km, for a fault width of 20 km. A plate-margin earthquake of M s = 8.7 and fault width of 20 km would require a fault length of 6000 km, obviously impossible. Assuming a fault width of 20 km, an M s = 8.2 earthquake has a rupture length of about 850 km, which is as large as can be expected for strike-slip faults, such as the San Andreas of California. Subduction zone earthquakes, on the other hand, if they have fault widths as great as 200 km, can give rise to M s = 8.7 earthquakes.

Regional relationships among earthquake magnitude scales

Various magnitude scales commonly used and the interrelationships among them are reviewed. It is shown that problems exist with all of the magnitude scales being used in the United States. When using regional catalogs, for example, it is often necessary to determine how the reported magnitudes were determined. Often such information is not available, although the potential errors are quite large. Both the MS and the mb scales were designed to be universal scales. However, both MS and mb magnitudes are often determined beyond the applicable range of the equations used to define the two scales. Furthermore, the MS magnitudes are not generally available for moderate to small earthquakes in most earthquake catalogs. The mb magnitudes are more generally available than MS values; however, there is also much greater variation in the way mb is determined. In particular, a significant change in the mb scale occurred in the early 1960’s when the World-Wide Standard Seismograph Network (WWSSN) was established. This change in instrumentation used to determine mb values had a significant effect on estimated magnitudes (post-1960 values are lower) and the saturation level of the mb scale. The older, longer-period instruments recorded larger mb magnitudes than can be recorded with the WWSSN instruments. In addition, great care must be taken when selecting the mb magnitudes of western U.S. earthquakes, because the values are often in considerable error owing to the fact that they were determined at distances less than 25° and were not properly corrected for attenuation in the upper mantle, or asthenosphere. The seismic body wave magnitude mb of an earthquake is strongly affected by regional variations in the Q structure, composition, and physical state within the earth. Therefore because of differences in attenuation of P waves between the western and eastern United States, a problem arises when comparing mb values for the two regions. A regional mb magnitude bias exists which, depending on where the earthquake occurs and where the P waves are recorded, can lead to magnitude errors as large as ⅓ unit. There is also a significant difference between mb and ML values for earthquakes in the western United States. An empirical link between the mb of an eastern U.S. earthquake and the ML of an equivalent western earthquake is given by ML = 0.57 + 0.92(mb)east. This result is important when comparing ground motion between the two regions and for choosing a set of real western U.S. earthquake records to represent eastern earthquakes.

Normal, blue and red earthquakes—A new way of earthquake classification on the basis of body-wave magnitudes

Monochromatic magnitudes, based on P- and on S-waves, provide a means to recognize differences in the spectral contents of body-waves radiated from earthquake foci. New, synthetic magnitude calibration functions taking into account periods of waves recorded, improve the consistency of magnitude figures assigned routinely to earthquakes. First results of a world-wide regionalization of earthquakes according to their spectral character are presented. Preponderance of short-period radiation in one class of earthquakes, and of long-period radiation in another is seen, if the radiation is compared with that of normal earthquakes.

Considerations for the body-wave magnitude determination in the recent earthquake data report of the United States Geological Survey

Considerations for the body-wave magnitude determination in the recent earthquake data report of the United States Geological Survey

Perceptible earthquakes in the Central and Eastern United States (Examined using Gumbel's third distribution of extreme values)

abstract Earthquake perceptibility P(l/m) in the Central and Eastern United States is defined as the joint probability that a point site perceives ground motion at least at intensity I in conjunction with an earthquake occurrence specifically of magnitude m . Regional seismicity expressed in the catalogs of Meyers and von Hake (1976) and Nuttli (1979) is examined in terms of magnitude frequency using Gumbel9s asymptotic extreme value distribution with an upper bound to magnitude (Gumbel III). The large “rogue” earthquakes associated particularly with the New Madrid seismic zone lead to upper bounded macroseismic body-wave magnitudes with large uncertainties around m b 7 3 4 ; but this approximate value is not contradicted by deterministic arguments. Examination of the regional earthquake perceptibility curves then shows a maximum perceptibility at several intensity levels for earthquakes in a relatively narrow band of magnitudes around m b 6 1 2 . For the Central and Eastern United States, this most perceptible magnitude is the earthquake which is most likely to be felt at any site, and this might be used in aseismic engineering design of noncritical structures. Extension of the analysis to integrated perceptibility then facilitates calculation of intensity expectation at any site.

Seismicity of the Colorado Lineament

The Colorado Lineament appears to be one of the source zones for the larger earthquakes of the west-central United States. As defined by Warner (1975), the lineament trends northeastward from northwestern Arizona to central Minnesota. Numerous Precambrian trends have been recognized along the lineament; some are reflected in the overlying strata. Mineralized areas such as the Colorado Mineral Belt, the Hartville iron deposits of Wyoming, and the Cuyuna Iron Range of Minnesota are associated with the lineament, which may represent a Precambrian continental plate boundary in the form of a wrench fault system (Warner, 1978). Between 1860 and 1875 about 18 earthquakes with epicentral intensity greater than or equal to VI and felt area greater than 25,000 km 2 (10,000 mi 2 ; equivalently a body-wave magnitude ges;4.5) had epicenters within the surface projection of the Colorado Lineament. All but a few of the remaining west-central earthquakes of this size can be associated with the Nemaha uplift, the Rio Grande Rift, the Wichita Mountain uplift and the Overthrust Belt. Although these latter structures have previously been recognized as source zones for larger earthquakes, the Colorado Lineament had not been so recognized.

Analysis procedures at the International Seismological Centre

Analysis at the International Seismological Centre (ISC) falls into three main categories—association, location and quantification. Difficulties of association on a global scale are not always appreciated. The ISC currently analyses 40–50 events a day, and as the first arrivals from any particular earthquake may span a time interval of up to 20 min, readings from events occurring in different parts of the world may overlap considerably in time. The present association algorithm depends on time only, and results in many chance mis-associations, or even the synthesis of fictitious events. At present, these mis-associations are rectified by seismologists' intervention, but full use of amplitude and period information could help to detect these errors automatically. Revision of locations follows standard least-squares procedures, based on Jeffrey's method of uniform reduction and Jeffreys-Bullen travel times. Locations are made from P phases only (including crustal phases), but other first arrivals and secondary phases are identified and residuals calculated. If depth cannot be determined by geometric means a search is made for depth phases, or failing this, the depth is restrained to that given by another agency, or to a conventional value. No provision is made for local variations in travel time. Body-wave magnitudes are allocated within the distance range 21–100° from reported readings of A and T, or their ratio, using Gutenberg-Richter calibration curves. Surface-wave magnitudes are calculated from the “Prague” formula, using reports of long-period A and T in the distance range 5–160°, but only values from stations at distances of 20° or more are used to determine an average M s for a particular event. There is no provision for the determination of local magnitude other than to reproduce values assigned by local agencies. Improvements in these procedures could be made through the automatic association of station readings, the introduction of local travel times, and better determination of earthquake size, particularly for local events.

On the possibilities of premonitory swarms for three sequences of earthquakes of the Burma—Szechwan region

Short term spatial and temporal variations in seismicity prior to the three sequences of earthquakes of m b ⩾ 5.8 of the Burma—Szechwan region are studied. Six years (1971–1976) of ISC seismicity data, as reported in the Regional Catalogue of Earthquakes, are considered. During the period, six earthquakes of body wave magnitude m b ⩾ 5.8 occurred in four sequences. Of these, three sequences are preceded by swarm activity in the epicentral regions. Evison (1977b) suggested that the swarm before the sequences of large shocks is a possible long-term precursor. He derived the conclusion by analyzing earthquakes in New Zealand and California. The analysis of the seismicity data for the region under investigation supports Evison's view and suggests that a relation between swarms and sequences of large events exists. The precursory time period (i.e. the time from beginning of the swarm to the main shock) for the Szechwan earthquakes of m b = 5.9 (Feb. 6, 1973) and m b = 5.8 (May 10, 1974) and the Burma earthquake of m b = 6.2 (Aug. 12, 1976) are 305, 317 and 440 days, respectively.

The amplitude spectra of P- and S-waves and the body-wave magnitude of earthquakes

Synthetic calibration functions for body-wave magnitudes are presented. They have been obtained from computations made under the assumption that the variation of amplitude spectra along the earth's surface is due to radial velocity heterogeneity and radial changes of the dissipation factor Q inside the earth only. The new calibration functions depend on the epicentral distance and the focal depth of the earthquake, as well as on the period of the body-wave under consideration. No single calibration function exists which would produce magnitude figures identical for all Fourier components of a body-wave. Observations on several hundreds of strong earthquakes in recent years show that the P-wave magnitudes vary systematically with the period of the wave. With the application of the synthetic calibration function a general increase of the teleseismically determined P-wave magnitudes with frequency is observed. This fact reconciles acceleration response spectra from narrowband strong-motion analyzers in the near-field of an earthquake and spectral magnitudes determined from far-field observations by sensitive narrow-band seismographs. The strength of high-frequent spectral components generally exceeds the strength of low-frequent components determined in both cases.