Body-wave Magnitude Research Articles

First Application of the New IASPEI Teleseismic Magnitude Standards to Data of the China National Seismographic Network

This article presents the results of the application of the new measurement standards for teleseismic magnitudes. They have been applied in parallel with traditional Chinese magnitude procedures to more than 14,000 digital broadband-velocity seismograms of 531 earthquakes in the magnitude range 4–9. The records were made between 2001 and 2007 at stations of the China National Seismographic Network (CNSN) and analyzed at the China Earthquake Network Center (CENC). The regression relations between different types of standard magnitudes, traditional Chinese magnitudes, and the respective values of m b, M S, and M e published by the National Earthquake Information Center (NEIC), as well as Global Centroid Moment Tensor (GCMT) moment magnitudes M w, are presented. The new broadband body-wave magnitude m B(BB) is measured at dominant periods in the range 0.2< T <30 sec that on average increase exponentially with magnitude, as expected by seismic scaling laws. The broadband surface-wave magnitude M S(BB) measures the maximum Rayleigh-wave amplitudes in a wide range of periods (3≤ T <60 sec) and epicentral distances (2°≤ Δ ≤160°), as originally proposed for the current International Association of Seismology and Physics of the Earth’s Interior (IASPEI) standard calibration function for M S determination. The article analyzes and disproves with rich data several widespread prejudices or misunderstandings concerning the applicability, stability, and accuracy of magnitudes based on readings from unfiltered broadband records in general and the IASPEI M S formula in particular. It is shown that the use of surface-wave amplitudes in a broad period range removes most of the systematic distance dependence that is observed when the IASPEI formula is restricted to periods around 20 sec. In the distance range between 2° and 103°, we have investigated the trend between M S(BB) and M S(20), which is the common surface-wave magnitude determined by the NEIC for periods between 18 and 22 sec and at teleseismic distances only. The two magnitudes agree rather well and have comparable measurement errors, even when M S(BB) is measured at regional distances of less than 20°. On average, however, M S(BB) is slightly larger than M S(20) at distances less than 45° with a small and consistent distance-dependent trend in the residuals M S(BB)- M S(20) of -0.0029 magnitude units per degree in both the regional and teleseismic distance range. The analyzed regression relations further reveal the following: (1) the new standard for m b delays saturation and yields about 0.5 units larger m b values for great earthquakes; (2) broadband m B(BB) moves the saturation limit for body-wave magnitudes further up to about 8.3. This makes m B(BB) a good candidate for providing rapid magnitude estimates of strong earthquakes well ahead of M w; (3) standard M S(20) scales perfectly with the M S published by the NEIC; (4) M S(BB) reduces the underestimation of surface-wave magnitude by M S(20) for weaker earthquakes and those recorded at regional distances for which the maximum surface-wave amplitudes occur mostly at periods well below 16 sec; and (5) new m B(BB) and M S(BB), that is, the body-wave and surface-wave magnitudes directly measured on velocity-broadband records, reproduce well the classical Gutenberg–Richter relation between m B and M S, which—together with the relation between seismic energy E S and m B—formed the basis for deriving the modern moment- and energy-magnitude scales M w and M e.

Comparison of Magnitude Estimates for New Zealand Earthquakes: Moment Magnitude, Local Magnitude, and Teleseismic Body-Wave Magnitude

New Zealand is one of the more seismically active countries in the world, with more than 15,000 earthquakes located each year. Routine moment tensor analysis of regional seismic data for earthquakes with moment magnitude M w>∼3.5–4.0 has recently been implemented in New Zealand. Nearly 330 regional moment tensor (RMT) solutions have been calculated for earthquakes in the New Zealand region dating back to late 2003. This complements local magnitude ( M L), the primary magnitude calculated by GeoNet in New Zealand. The RMT catalog, along with 155 Global Centroid Moment Tensor (Global CMT) catalog solutions, is used to compare M w with M L for New Zealand earthquakes. In addition to M w and M L, there are more than 330 teleseismic body-wave magnitude ( m b) values available from the United States Geological Survey and International Seismological Center catalogs for events that also have an M w. These are used to examine the relationship between M w, M L, and m b for New Zealand earthquakes. There is a clear distinction in the relationship of M L to M w for shallow focus (≤33 km depth) and deep focus (>33 km depth) earthquakes. Shallow focus earthquakes show M L to be fairly consistent with M w, particularly for events with M w>∼4.5 and with ![Graphic][1] . Deep focus earthquakes have M L consistently larger than M w (more than a full magnitude unit for some events) with ![Graphic][2] . M w and m b are in fairly good agreement regardless of the depth, whereas m b estimates are consistently smaller than M L for deep events. This suggests that M L is overestimated for deep focus events and that the large M Ls are not the result of physical properties of the source. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif

Apparent Slow Oceanic Transform Earthquakes Due to Source Mechanism Bias

Slow earthquakes, characterized as ones whose magnitude increases with period, are often observed occurring on oceanic transform faults (Kanamori and Stewart 1976; Okal and Stewart 1982; Stein and Pelayo 1991; Choy and Boatwright 1995; Newman and Okal 1998; Perez-Campos et al. 2003). Previous work has found that oceanic transform earthquakes often have lower body wave magnitudes compared to ridge and oceanic intraplate earthquakes of comparable surface wave magnitude and seismic moment. This phenomenon might result from the slow events having lower stress drop. Alternatively, the effect could be due to a source mechanism bias resulting from the fact that transform earthquakes are primarily strike-slip events on steeply dipping faults. In this geometry, body waves arriving at teleseismic distances left the source near nodal planes, resulting in smaller amplitudes than for dip-slip events of the same moment. Slow earthquakes are ones that release energy more “slowly” than typical earthquakes and therefore radiate more energy at longer periods. Such earthquakes have been identified in various ways. One is from an anomalous ratio of the body wave magnitude mb , which reflects the energy release at a period of 1 s, to the surface wave magnitude MS , which reflects the energy release at a period of 20 s. Although slow earthquakes have been observed in various tectonic environments, they are especially common for oceanic transform earthquakes that have strike-slip mechanisms, as shown by analysis of a global catalog of oceanic earthquakes (Stein and Pelayo 1991). The transform earthquakes generally had lower body wave magnitudes compared to ridge and oceanic intraplate earthquakes of comparable surface wave magnitude and seismic moment. This effect is illustrated in Figure 1A, where a transform event with a moment larger than the ridge event shown has lower mb than …

Reply to Bayrak’s comments on “An investigation of seismicity for western anatolia” by Sayil and Osmanşahin (2008)

I looked over the comments of Mr. Critle, and can say that this is not a scientific ‘‘criticization’’. It is very clear that subject of our paper is one of the general seismicity studies based on statistical methods for a defined region. Besides, it has similar format with such a kind of studies in literature (Papazachos 1989; Papazachos and Papaioannou 1993; Sayil and Osmansahin 2004; Sayil and Osmansahin 2005a, b; Sayil and Osmansahin 2006; Sayil 2007) Mr. Critle says in his first criticism: ‘‘they did not form sub-regions suitable with geometrics of the fault systems in the region’’. We did not study on geometrics or mechanisms of the fault systems. We have used eliptic forms as usual (above references) to show the earthquakes occurred in each sub-region. Other criticisms are about our data. It is very clear that we have used the data of the most known and reliable centres and scientists. These catalogues (given in Tables 1 and 2 in our article, Sayil and Osmansahin 2008) had included intensity (Io), body wave magnitude (mb) and surface wave magnitude (Ms) of earthquakes. A new catalogue with main shocks (Ms C 4.0) has been developed from these catalogues for investigation area in our article. In this study (Sayil and Osmansahin 2008), the standard least square (linear regression) method has been used for obtaining the correlations of Ms-mb and Ms-Io. Standard deviations were found as r = 0.17 and r = 0.14, respectively (Fig. 3 in Sayil and Osmansahin 2008) and this method can be used in these error limits. The linear regression method is the most important for statistical investigation, and it is mainly used in literature (Gutenberg and Richter 1954; Alptekin 1978; Erdik et al. 1985; Erdik et al. 1999; Ambraseys 2001; Panza and Romanelli (2001) ; Kayabali 2002; other references in Sayil and Osmansahin 2008). Computed Ms-Io and Ms-mb relations are consistent with those of Ambraseys (2001) and Bath (1966). Other regression methods (polinomial regression,

The Surface Wave Magnitude for the 9 October 2006 North Korean Nuclear Explosion

Surface waves were generated by the North Korean nuclear explosion of 9 October 2006 and recorded at epicentral distances up to 34 degrees, from which we estimated a surface wave magnitude (M{sub s}) of 2.94 with an interstation standard deviation of 0.17 magnitude units. The International Data Centre estimated a body wave magnitude (m{sub b}) of 4.1. This is the only explosion we have analyzed that was not easily screened as an explosion based on the differences between the M{sub s} and m{sub b} estimates. Additionally, this M{sub s} predicts a yield, based on empirical M{sub s}/Yield relationships, that is almost an order of magnitude larger then the 0.5 to 1 kiloton reported for this explosion. We investigate how emplacement medium effects on surface wave moment and magnitude may have contributed to the yield discrepancy.

The New IASPEI Standard Broadband Magnitude mB

Research Article| September 01, 2008 The New IASPEI Standard Broadband Magnitude mB Peter Bormann; Peter Bormann Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences Telegrafenberg 14473 Potsdam, Germany pb65@gmx.net (P.B.) saul@gfz-potsdam.de (J.S.) Search for other works by this author on: GSW Google Scholar Joachim Saul Joachim Saul Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences Telegrafenberg 14473 Potsdam, Germany pb65@gmx.net (P.B.) saul@gfz-potsdam.de (J.S.) Search for other works by this author on: GSW Google Scholar Author and Article Information Peter Bormann Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences Telegrafenberg 14473 Potsdam, Germany pb65@gmx.net (P.B.) saul@gfz-potsdam.de (J.S.) Joachim Saul Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences Telegrafenberg 14473 Potsdam, Germany pb65@gmx.net (P.B.) saul@gfz-potsdam.de (J.S.) Publisher: Seismological Society of America First Online: 09 Mar 2017 Online ISSN: 1938-2057 Print ISSN: 0895-0695 © 2008 by the Seismological Society of America Seismological Research Letters (2008) 79 (5): 698–705. https://doi.org/10.1785/gssrl.79.5.698 Article history First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Peter Bormann, Joachim Saul; The New IASPEI Standard Broadband Magnitude mB. Seismological Research Letters 2008;; 79 (5): 698–705. doi: https://doi.org/10.1785/gssrl.79.5.698 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search Ten years after the development of a local magnitude scale, ML, by Richter (1935), Gutenberg (1945a, 1945b, 1945c) proposed to extend the magnitude concept to measurements of maximum amplitudes of body and surface waves at teleseismic distances. For the first time this made possible comparable estimates of earthquake size on a global scale (Gutenberg and Richter 1949, 1954). In addition, body waves allow magnitude measurements for earthquakes at all depths. Teleseismic magnitudes were originally determined for earthquakes with magnitudes mostly around 6 and larger on intermediate- to long-period displacement-proportional records... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Location and Magnitude Estimation of the 9 October 2006 Korean Nuclear Explosion Using the Southern Great Basin Digital Seismic Network as a Large-Aperture Array

The Southern Great Basin Digital Seismic Network (SGBDSN) has been designed for monitoring high-frequency (1–40-Hz) local events at and near Yucca Mountain, Nevada, the designated site for the national high-level nuclear-waste repository. We find that the network is also effective as a large-aperture teleseismic array for monitoring events in and close to North Korea, the recent location of an underground nuclear test that occurred on 9 October 2006, 01:35:28 coordinated universal time (UTC), National Earthquake Information Center (NEIC) m b 4.3. We explain this by (1) low ambient noise, (2) energy-efficient propagation paths (the nuclear explosion and deep earthquakes on nearly the same ray path show dominant frequencies between 0.9 and 2.5 Hz), and (3) coherent signals across the SGBDSN. The network, when used as an array, provides a particularly good beam signal-to-noise ratio (SNR) for the nuclear explosion. Estimated beam SNR is 20 dB at frequencies between 0.9–2.5 Hz. Between January 1996 and December 2006, 58% (11 of 19) of the events with 7.1> m b>3.3 located within 300 km of the North Korean nuclear explosion by NEIC are considered large enough to be confidently picked by an SGBDSN analyst. The first-arrival of the North Korean event itself is apparent on 25 of the 29 SGBDSN unfiltered recordings. The direct P phase is confidently identified using slowness and back azimuth estimated with cross-correlation and frequency-wavenumber methods. Static time corrections for beamforming are estimated using 11 deep earthquakes within 300 km of the nuclear explosion. With the statistical regression model named two-way layout, we estimate the event first-arrival time offset and the mean station effects. The resulting relative time delays are used to calibrate the nuclear explosion single-array location to the NEIC location, using the delays derived from 11 earthquakes. With magnitude substituted for time, the same method is used to estimate magnitude corrections. The SGBDSN magnitude estimate is 4.3 using the Veith–Clawson body-wave magnitude formula.

Comparison of Methods for Estimating Station Magnitude Corrections for Improved Seismologic Monitoring of the Comprehensive Nuclear-Test-Ban Treaty

Abstract Accurate estimation of the magnitude of seismic events is essential to effective monitoring of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). The standard estimator of body-wave magnitude in the International Data Center (IDC) bulletins is the average m b reading at the detecting stations in a prescribed distance window. Two sources of bias may affect these magnitudes. First, some stations may tend to give high readings and others low readings, so the average may be biased by the particular set of detecting stations. Second, the averaging method ignores eligible stations that could have, but failed to, detect an event. The first source of bias can be reduced by estimating station correction terms. The second source of bias can be handled by applying maximum likelihood methods that take into account both detecting and nondetecting stations. We compare several different methods that have been proposed to estimate station correction terms and show that a joint maximum likelihood method is clearly superior to other alternatives. A least-squares method that was used by Murphy and Barker (2003) to estimate correction terms leads to biased estimates of correction terms because it fails to take into account nondetecting stations. The joint method provides accurate estimates of station correction terms and is also superior to applying maximum likelihood to single events without estimating station corrections or with the least-squares corrections. We illustrate our ideas by analyzing data from about 15,000 seismic events recorded by the prototype IDC. We caution that our correction terms are for primary stations only and are not meant to be final estimates; to obtain final correction terms would require applying our methods to a complete data set including both primary and auxiliary stations.

Comparison between different earthquake magnitudes determined by China Seismograph Network

By linear regression and orthogonal regression methods, comparisons are made between different magnitudes (local magnitude ML, surface wave magnitudes MS and MS7, long-period body wave magnitude mB and short-period body wave magnitude mb) determined by Institute of Geophysics, China Earthquake Administration, on the basis of observation data collected by China Seismograph Network between 1983 and 2004. Empirical relations between different magnitudes have been obtained. The result shows that: ① As different magnitude scales reflect radiated energy by seismic waves within different periods, earthquake magnitudes can be described more objectively by using different scales for earthquakes of different magnitudes. When the epicentral distance is less than 1 000 km, local magnitude ML can be a preferable scale; In case M MS, i.e., MS underestimates magnitudes of such events, therefore, mB can be a better choice; In case M>6.0, MS>mB>mb, both mB and mb underestimate the magnitudes, so MS is a preferable scale for determining magnitudes of such events (6.0 8.5, a saturation phenomenon appears in MS, which cannot give an accurate reflection of the magnitudes of such large events; ② In China, when the epicentral distance is less than 1 000 km, there is almost no difference between ML and MS, and thus there is no need to convert between the two magnitudes in practice; ③ Although MS and MS7 are both surface wave magnitudes, MS is in general greater than MS7 by 0.2∼0.3 magnitude, because different instruments and calculation formulae are used; ④ mB is almost equal to mb for earthquakes around mB4.0, but mB is larger than mb for those of mB≥4.5, because the periods of seismic waves used for measuring mB and mb are different though the calculation formulae are the same.

An improved UK local magnitude scale from analysis of shear andLg-wave amplitudes

The amplitudes of shear and Lg waves recorded at UK seismograph stations from local earthquakes in the range 0–600 km have been subjected to an analysis of variance, allowing separation of the effects of earthquake size, distance and station corrections for each recording site. The propagation paths sample mainly the central to western part of the UK, with good north–south coverage. The analysis of 385 amplitude readings at 28 stations from 40 earthquakes showed that the effects of both distance and station (site) corrections were statistically significant. Tables of corrections for both distance and station effects have been derived to allow local magnitude ML to be determined more accurately from horizontal and vertical component records. One set of tables allows the estimation of an ML which is consistent with the original Richter definition of ML, with a standard deviation which is smaller than that produced by the theoretical attenuation curve, defined according to attenuation in Southern California, which has customarily been used to calculate ML in the UK. The reduction in standard deviation is mainly due to the incorporation of station terms to correct for station effects, since the theoretical and observed variations of attenuation with distance are similar. This similarity implies that Southern California and the UK show a similar variation of distance-dependent attenuation for Lg waves, which was not expected in view of the differences in geology and tectonics. Another set of tables is provided to calculate an ML which is consistent with the body wave magnitude mb determined by the International Data Centre from station records of the Comprehensive Test-Ban Treaty Organization’s global monitoring network. The decay with distance of the predominantly Lg-wave amplitude values gives a value of 440 ± 50 for Lg Q at a frequency of 3 Hz, in agreement with estimates obtained from spectral displacement amplitudes of Lg waves in the UK.

The MD Scale in Northern Morocco: A Comparative Study of Two Empirical Approaches

We have derived, evaluated, and compared two empirical methods for computing duration magnitude MD from 25 short-period vertical component stations of the Northern Morocco Seismic Network (NMSN). MD has been scaled to IGN (Insituto Geograpfico National, Spain) body-wave magnitude (mbIGN), using a set of 479 shallow (less than 30 km) earthquakes recorded from March 1992 to February 2001, with 2.5 ≤ mbIGN ≤ 5.4. In the first approach: Individual Network Calibration, we determined an individual MD formula for each station. In the second approach: Global Network Calibration, we used a single relationship to compute MDij (from the jth observation for the i-th earthquake) magnitudes at 25 selected stations as: MDij = − 0.14 +1.63 log10 (τij)+0.031(Δij)+cStaj. Residuals (MDij − mbIGN) for both techniques were thereafter deduced. Comparison between the two approaches provided the principal results: (1) The mean correlation between estimated magnitude; MDij and reference magnitude; mbIGN is about 89.9% for the individual calibration method, and near 95% for global calibration method in which station corrections cStaj were introduced, (2) Residuals (MDij − mbIGN) are relatively large, and are ranging between − 0.60 and 0.60 magnitude units, for the individual calibration method, whereas they vary in the range − 0.38 to 0.40, for the global calibration method with corrections; cStaj. (3) A random distribution of residuals (MDij − mbIGN) is observed for each station in the case of the individual approach. Thus, the resulting average of these residuals is almost equal to zero. Using a global calibration without corrections results in negative residuals for a group stations and positive residuals for another an group indicating respectively that sites corresponding to these groups have a tendency to underestimate, or overestimate observed magnitude values.

Research Note On the Amplitudes of Seismic Waves

Douglas, Hudson & Blamey (1972) suggest that the relative amplitudes of P and Rayleigh waves generated by underground explosions depend on the properties of the source medium. Calculations based on an assumption of a flat spectrum for P waves radiated at source, indicate that the relation between the body wave magnitude, mb (computed at 1 Hz) and the surface wave magnitude, M , (computed at 0.05 Hz) should be

Depth Determination by use of Teleseismic P-arrivals

Summary In order to get accurate depth estimates from teleseismic P-arrivals only an accurate travel-time model must be available. A model describing the travel times to a world-wide network from events within a specified region is presented. The following two assumptions are made. The crust and upper mantle are laterally homogeneous and the travel times from the region to each station are well approximated by a second-order polynomial in the distance. This model is applied to events in Japan. Most events lie at depths between 30 and 80 km. We obtained time residuals with a standard deviation of 0.60 seconds which is considerably less than the commonly achieved values of 0-8-1.0 seconds for global models. The RMS depth error when locating with this model is roughly 15 km if the body wave magnitude of the event is above 5.

Chinese National Network Magnitudes, Their Relation to NEIC Magnitudes, and Recommendations for New IASPEI Magnitude Standards

We investigate the linear regression relationships between common seismic magnitudes determined by the Chinese Earthquake Network Center (CENC) and compare them with related magnitude determinations for the same events at the U.S. Geological Survey's National Earthquake Information Center (NEIC). Despite their generally good agreement some systematic differences are revealed. These dif- ferences are due to differences in seismograph response (shape and bandwidth), the time window for measurement of maximum P-wave amplitudes, the period and dis- tance ranges used, and, in part, also the different calibration functions applied. Chi- nese broadband body-wave magnitude mB, compared with the NEIC short-period P- wave magnitude mb, is much less prone to magnitude saturation. Thus it is more suitable to assess the size of large earthquakes from P waves. Also, following Inter- national Association of Seismology and Physics of the Earth's Interior (IASPEI) recommendations of 1967, Chinese surface-wave magnitude MS is determined in a wider distance (1 D 180) and period range (3 sec T 30 sec) than MS(20) at NEIC (20 D 160 and 18 sec T 22 sec, respectively). Chinese MS for small and medium earthquakes at regional distances between 2 D 10 scales well with local magnitude ML. In contrast, NEIC MS(20) tends to underestimate the magnitude of regional events when the IASPEI-recommended MS calibration function by Vane yk et al. (1962) is used. These findings support some of the new standards for magnitude measurements from digital data adopted at the IASPEI meeting in 2005. They include, complementary to band-limited m b and M S (20), the determination of m B and M S (BB) measured on unfiltered broadband records.

Relationships Among Magnitudes and Seismic Moment of Earthquakes in the Taiwan Region

The seismic moments (M(subscript o)), body-wave magnitudes (m(subscript b)), and surfacewave magnitudes (M(subscript s)) of 201 Taiwan earthquakes with 4.8≤m(subscript b)≤6.6 published in the Global CMT catalog from 1976 to 2006 are used to study the correlations among the three source parameters. The resultant relationships are: log(M(subscript o))=(1.07±0.04) M(subscript s)+(18.72±0.20); log(M(subscript o))=(1.73±0.09) m(subscript b)+(15.09 ± 0.52); and M(subscript s) = (1.46±0.08) m(subscript b)-(2.52±0.43). The three relationships have high agreement with those of earthquakes in the circum-Pacific seismic belt. This might imply that the tectonic conditions and source properties of the Taiwan region behave like the average ones of the circum-Pacific seismic belt. The relationships between the three source parameters and local magnitude are: log(M(subscript o))=(1.27±0.06) M(subscript L)+(17.23±0.35); m(subscript b)=0.66±0.03) M(subscript L)+(1.69±0.17); and M(subscript s)=(1.03±0.06) M(subscript L)-(0.53±0.36).

Ms:mb Discrimination Study of Mining Explosions in Wyoming, USA, and in QianAn, China

We investigate the use of intermediate-period surface-wave magnitude, M s , and high-frequency body-wave magnitude, m b , from regional mining explosions for event discrimination by using techniques originally intended for separating earthquakes from explosions with teleseismic observations. The actual values of M s and m b suggest that the surface waves generated by long-duration mining explosions can make them appear earthquakelike. The data from the single anomalous shot indicate that if a significant part of the total explosives is simultaneously detonated the event will move into the explosion population. Data for this study are taken from a portable broadband deployment in Wyoming recording mining explosions in the Powder River Basin and a broadband network currently deployed in northeast China. The magnitudes, M s (vmax) and m b were estimated for five, kiloton-size mining explosions, four in Wyoming and one in QianAn, China. The resulting network M s : m b data were compared with data from a previous study that included earthquakes and contained single-fired explosions (Stevens and Day, 1985; Bonner et al. , 2003). Although the previous studies mostly examined events larger than those in this study, the Wyoming and China mining events plot in the earthquake population. Data from the anomalous Wyoming event, a blast in which a failure of the timing system caused a large portion of the blast pattern to simultaneously detonate, plot in the explosion population with m b 4.4. The simultaneous detonation of a large portion of blast array increased the body-wave magnitude but had little effect on surface-wave magnitude.

A Probability of Detection Method for Reducing Short-Period mb-Ms False Alarm Rates

The 20-sec m b− M s discriminant is one of the most reliable and best understood methods for identifying underground nuclear explosions. To extend the discriminant to lower magnitude thresholds at regional distances it is necessary to perform the M s measurement at shorter periods. Although short-period Rayleigh- wave measurements have lower detection thresholds than those at 20 sec, they are more influenced by the effects of earthquake depth that cause overlap of earthquake and explosion populations. We present a technique using a probability of detection model (pxd) to estimate the probability that a surface-wave detection came from an underground explosion. The key to the method is the development of a simple analytic model to predict the maximum expected amplitude probability distribution (upper tail) from an underground explosion of a given body-wave magnitude recorded at a specified distance. The model assumes full coupling and accounts for material effects, attenuation, and amplification from tectonic release. The detection classifier is applicable when there is a signal detection above a specified signal-to-noise cutoff and the detection is of greater amplitude than the maximum expected explosion amplitude. We assume that in general, earthquakes generate larger 6- to 12-sec Rayleigh-wave amplitudes than explosions for a given body-wave magnitude. For a given sensor we define the probability of detection given that the source was an explosion. Using Bayes’ Rule we determine the probability that the signal detection originated from an explosion. The surface-wave probability of detection curve for a given period and the prior probability that an explosion occurred can also be included in the formulation. We compute the conditional probability (represented as a p value) that the detected signal originated from an explosion. No assumptions regarding the non-explosion source are necessary other than the fact that the maximum amplitudes are expected to be greater than those from the explosion under full coupling conditions. Under the specified conditions, the p value will always be a small value indicating a low probability that the detected signal originated from an explosion. The p value can also be thought of as a random variable that can be combined with other discriminants in a multivariate setting. We show results of the signal detection formulation using short- period Rayleigh waves from earthquakes and explosions in Eurasia and compare to the traditional m b− M s at different periods. For a set of earthquakes and explosions recorded at wmq measured at a 6- to 12-sec period, false alarm rates are reduced from 28.3% for m b− M s to 18.6% using the probability of detection model. Using pxd by itself as an earthquake identifier, 78% of all events are assigned a p value resulting in a false alarm rate of 22% that is better than m b− M s alone for this dataset.

Seismological Aspects of the December 2004 Great Sumatra-Andaman Earthquake

The 2004 Great Sumatra-Andaman earthquake had an average source duration of about 500 sec. and a rupture length of 1,200–1,300 km. The seismic moment, M0, determined with a finite source model, was 6.5×1022 N- m, which corresponds to Mw=9.18. Allowing for the uncertainties in the current M0 determinations, Mw is in the range of 9.1 to 9.3. The tsunami magnitude Mt is 9.1, suggesting that the overall size of the tsunami is consistent with what is expected of an earthquake with Mw=9.1 to 9.3. The short-period body-wave magnitude m^ b is 7.25, which is considerably smaller than that of large earthquakes with a comparable Mw. The m^ b versus Mw relationship indicates that, overall, the Great Sumatra-Andaman earthquake is not a tsunami earthquake. The tectonic environment of the rupture zone of the Great Sumatra-Andaman earthquake is very different from that of other great earthquakes, such as the 1960 Chile and the 1964 Alaska earthquakes. This difference may be responsible for the unique source characteristics of this earthquake. The extremely large size of the Great Sumatra-Andaman earthquake is reflected in the large amplitude of the long-period phase, the W phase, even in the early part of the seismograms before the arrival of the S wave. This information could be used for various early warning purposes.

Empirical Global Relations Converting M S and m b to Moment Magnitude

The existence of several magnitude scales used by seismological centers all over the world and the compilation of earthquake catalogs by many authors have rendered globally valid relations connecting magnitude scales a necessity. This would allow the creation of a homogeneous global earthquake catalog, a useful tool for earthquake research. Of special interest is the definition of global relations converting different magnitude scales to the most reliable and useful scale of magnitude, the moment magnitude, MW. In order to accomplish this, a very large sample of data from international seismological sources (ISC, NEIC, HRVD, etc.) has been collected and processed. The magnitude scales tested against MW are the surface wave magnitude, MS, the body wave magnitude, mb, and the local magnitude, ML. The moment magnitudes adopted have been taken from the CMT solutions of HRVD and USGS. The data set used in this study contains 20,407 earthquakes, which occurred all over the world during the time period 1.1.1976–31.5.2003, for which moment magnitudes are available. It is shown that well-defined relations hold between MW and mb and MS and that these relations can be reliably used for compiling homogeneous, with respect to magnitude, earthquake catalogs.

Quick estimator of the size of great earthquakes

The reintroduction of the body wave magnitude mB based on broadband records (i.e., records in a wide frequency range) of the first arriving seismic P‐waves into international seismological practice could result in better quick estimation of the size of large earthquakes.This recommendation, made by the Working Group for Magnitude Measurements of the International Association of Seismology and Physics of the Earth's Interior (IASPEI), was adopted—along with other recommendations related to future standards of magnitude measurements—by the IASPEI Commission on Seismic Observation and Interpretation at the IASPEI General Assembly in Chile, 2–8 October 2005.