scholarly journals Seismological Aspects of the December 2004 Great Sumatra-Andaman Earthquake

2006 ◽  
Vol 22 (3_suppl) ◽  
pp. 1-12 ◽  
Author(s):  
Hiroo Kanamori

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.

1983 ◽  
Vol 73 (4) ◽  
pp. 1063-1076
Author(s):  
Thorne Lay ◽  
Donald V. Helmberger

abstract Relationships between travel-time and amplitude station anomalies are examined for short- and long-period SH waves and short-period P waves recorded at North American WWSSN and Canadian Seismic Network stations. Data for two azimuths of approach to North America are analyzed. To facilitate intercomparison of the data, the S-wave travel times and amplitudes are measured from the same records, and the amplitude data processing is similar for both P and S waves. Short-period P- and S-wave amplitudes have similar regional variations, being relatively low in the western tectonic region and enhanced in the shield and mid-continental regions. The east coast has intermediate amplitude anomalies and systematic, large azimuthal travel-time variations. There is a general correlation between diminished short-period amplitudes and late S-wave arrival times, and enhanced amplitudes and early arrivals. However, this correlation is not obvious within the eastern and western provinces separately, and the data are consistent with a step-like shift in amplitude level across the Rocky Mountain front. Long-period S waves show no overall correlation between amplitude and travel-time anomalies.


1983 ◽  
Vol 73 (1) ◽  
pp. 83-96 ◽  
Author(s):  
Michel Campillo ◽  
Michel Bouchon

abstract We present a study of the seismic radiation of a physically realistic source model—the circular crack model of Madariaga—at close distance range and for vertically heterogeneous crustal structures. We use this model to represent the source of small strike-slip earthquakes. We show that the characteristics of the radiated seismic spectra, like the corner frequency, are strongly affected by the presence of the free surface and by crustal layering, and that they can be considerably different from the ones of the homogeneous-medium far-field solution. The vertical and radial displacement spectra are the most strongly affected. We use this source model to calculate the decay of peak ground velocity with epicentral distance and source depth for small strike-slip earthquakes in California. For distances between 10 and 80 km, the peak horizontal velocity decay is of the form r−1.25 for a 4-km hypocentral depth and r−1.65 for deeper sources. The predominance of supercritically reflected arrivals beyond epicentral distances of 70 to 80 km produces a sharp change in the rate of decay of the ground motion. For most of the cases considered, the peak ground velocity increases between 80 and 100 km. We also show that the S-wave velocity in the source layer is the lower limit of phase velocities associated with significant ground motion.


1983 ◽  
Vol 73 (6A) ◽  
pp. 1499-1511
Author(s):  
Paul Silver

Abstract A method is proposed for retrieving source-extent parameters from far-field body-wave data. At low frequency, the normalized P- or S-wave displacement amplitude spectrum can be approximated by |Ω^(r^,ω)| = 1 − τ2(r^)ω2/2 where r^ specifies a point on the focal sphere. For planar dislocation sources, τ2(r^) is linearly related to statistical measures of source dimension, source duration, and directivity. τ2(r^) can be measured as the curvature of |Ω^(r^,ω)| at ω = 0 or the variance of the pulse Ω^(r^,t). The quantity ωc=2τ−1(r^) is contrasted with the traditional corner frequency ω0, defined as the frequency at the intersection of the low- and high-frequency trends of |Ω^(r^,ω)|. For dislocation models without directivity, ωc(P) ≧ ωc(S) for any r^. A mean corner frequency defined by averaging τ2(r^) over the focal sphere, ω¯c=2<τ2(r^)>−1/2, satisfies ωc(P) > ωc(S) for any dislocation source. This behavior is not shared by ω0. It is shown that ω0 is most sensitive to critical times in the rupture history of the source, whereas ωc is determined by the basic parameters of source extent. Evidence is presented that ωc is the corner frequency measured on actual seismograms. Thus, the commonly observed corner frequency shift (P-wave corner greater than the S-wave corner), now viewed as a shift in ωc is simply a result of spatial finiteness and is expected to be a property of any dislocation source. As a result, the shift cannot be used as a criterion for rejecting particular dislocation models.


Ars Adriatica ◽  
2012 ◽  
pp. 41
Author(s):  
Pavuša Vežić

The discussion emphasizes the peculiarity and individuality of both the shape and style of Dalmatian hexaconchs. Together with the rotunda of Holy Trinity at Zadar, they surely represent the most original architectural creation of early medieval Dalmatia and its specific cultural milieu which grew from a twofold tradition in a true symbiosis of the European East and West in the Adriatic area. Their mutual interdependence in Dalmatia was articulated through the individual shapes of religious architecture. These hexaconchs are a form specific to only the innermost part of Dalmatia, centred on the area between Zadar and Split, and deep into the hinterland of these towns, which corresponded to the Croatian principality.Certainly, buildings as special as this had their own original matrix - an individual spatial composition and a specific structure which formed their body. Without this, the hexaconchs would not have possessed the originality which has been observed by all the scholars who have written about them. Indeed, they have their own shape and style. By analyzing and interpreting the legacy of Dalmatian religious architecture, it seems plausible to assume that the early Christian baptistery of Zadar Cathedral may have served as a model not only for their hexaconchal shape and spatial structure but also for their dimensions and proportions. In the regional architecture prior to the period when the hexaconchs were built, no other building, aside from the Zadar baptistery, had such a shape and such a compositional compatibility with the hexaconchs; the very structure and measurements of their interior space. However, the architectural style of the hexaconchs, which display pilaster strips on their exteriors, and their vocabulary of pre-Romanesque language find their parallels on the monumental rotunda of Holy Trinity - a chapel adjacent to the baptistery itself, located nearby in the same episcopal complex - more than on any other late Antique or early medieval building both in the immediate region and in the whole Adriatic basin. For this reason, the search for the origin of the shape and style of Dalmatian hexaconchs leads us to Zadar and it is no wonder that almost every scholar who has studied this group of buildings has pointed to this fact. Their geographical distribution also witnesses this influence in its own way: two hexaconchs can be found at Zadar, while four or even five more are located in the wider Zadar area, adding up to seven out of the ten Dalmatian hexaconchs in total.This number implies that this group of rotundas, being characteristic for a specific period in Dalmatia, was created in a relatively short period of time. Moreover, it points to the building and carving workshops which, drawing upon the same source model, constructed the hexaconchs and provided them with stone liturgical furnnishings. In particular, further indications can be found in the production of the socalled Benedictine carving workshop, probably located at Zadar, a workshop from the time of Prince Trpimir which produced the furnishings for the hexaconchs at Pridraga and Kašić, and the carving workshop from Trogir which was responsible for the carvings at Trogir and Brnaze. All of these, with regard to the hexaconchs, testify to predominantly early ninth-century production, and represent the main argument for the dating of these interesting Dalmatian rotundas to the same time. Apart from their original pre-Romanesque shape, the majority of the free-standing hexaconchal rotundas were provided with early Romanesque additions during the course of time, and these additions turned these hexaconchs into small complexes of sorts. Vestibules created in this period suggest two possiblities: according to one, the vestibules added in this manner were actually a kind of exterior crypt, spaces where sarcophagi could be housed, and according to the other, some of these vestibules were also provided with bell-towers built on top of them. The latter possibility is implied by the dispositions of the suggested bell-towers and the strength of the supporting substructions (e.g. the Stomorica church at Zadar or the hexaconch at Kašić), but also by the stylistic elements which point to the early Romanesque, and architectural details, the function of which indicates a bell-tower (e.g. impost capitals of the Stomorica church or St Chrysogonus at Zadar, and an octogonal colonette from Kašić).


Geophysics ◽  
2018 ◽  
Vol 83 (1) ◽  
pp. R1-R11 ◽  
Author(s):  
Dmitry Borisov ◽  
Ryan Modrak ◽  
Fuchun Gao ◽  
Jeroen Tromp

Full-waveform inversion (FWI) is a powerful method for estimating the earth’s material properties. We demonstrate that surface-wave-driven FWI is well-suited to recovering near-surface structures and effective at providing S-wave speed starting models for use in conventional body-wave FWI. Using a synthetic example based on the SEG Advanced Modeling phase II foothills model, we started with an envelope-based objective function to invert for shallow large-scale heterogeneities. Then we used a waveform-difference objective function to obtain a higher-resolution model. To accurately model surface waves in the presence of complex tomography, we used a spectral-element wave-propagation solver. Envelope misfit functions are found to be effective at minimizing cycle-skipping issues in surface-wave inversions, and surface waves themselves are found to be useful for constraining complex near-surface features.


1969 ◽  
Vol 59 (2) ◽  
pp. 503-519
Author(s):  
Agustin Udias ◽  
Dieter Baumann

abstract A computer program has been developed to find the orientation of a double couple source model for the mechanism of an earthquake which best satisfies the data from P and S waves. The relationship between the two axes of the solution given by the equations for the polarization angle of S is used in order to rapidly find the orientation of the source model for which a total error value involving the error of S and P data is a minimum. The program gives best results for data from homogeneous instruments of similar period range. Solutions for three earthquakes, selected because of the orientation of the source, are presented and the reliability of their solutions under ideal conditions is discussed.


1970 ◽  
Vol 60 (3) ◽  
pp. 917-937 ◽  
Author(s):  
B. F. Howell ◽  
G. M. Lundquist ◽  
S. K. Yiu

Abstract Integrated magnitude substitutes the r.m.s. average amplitude over a pre-selected interval for the peak amplitude in the conventional body-wave magnitude formula. Frequency-band magnitude uses an equivalent quantity in the frequency domain. Integrated magnitude exhibits less scatter than conventional body-wave magnitude for short-period seismograms. Frequency-band magnitude exhibits less scatter than body-wave magnitude or integrated magnitude for both long- and short-period seismograms. The scatter of frequency-band magnitude is probably due to real azimuthal effects, crustal-transfer-function variations, errors in compensation for seismograph response, microseismic moise and uncertainties in the compensation for attenuation with distance. To observe azimuthal variations clearly, the crustal-transfer functions and seismograph response need to be known more precisely than was the case in this experiment, because these two sources of scatter can be large enough to explain all of the observed variations.


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