Pure and Applied Geophysics

Liu, L.

doi: 10.1007/PL00001235pmid: N/A

— This paper reviews some remarkable characteristics of earthquakes in a Stable Continental Region (SCR) of the South China Block (SCB). The kernel of the SCB is the Yangtze platform solidified in late Proterozoic time, with continental growth to the southeast by a series of fold belts in Paleozoic time. The facts that the deviatoric stress is low, the orientations of the major tectonic features in the SCB are substantially normal to the maximum horizontal principal stress, and a relatively uniform crust, seem to be the major reasons for lack of significant seismicity in most regions of the SCB. Earthquakes in this region are mainly associated with three seismic zones: (1) the Southeast China Coast seismic zone related to Guangdong-Fujian coastal folding belt (associated with Eurasia-Philippine Sea plate collision); (2) the Southern Yellow Sea seismic zone associated with continental shelf rifts and basins; and (3) the Downstream Yangtze River seismic zone spatially coinciding with Tertiary rifts and basin development. All three seismic zones are close to one or two major economic and population centers in the SCB so that they pose significant seismic hazards. Earthquake focal mechanisms in the SCB are consistent with strike-slip to normal faulting stress regimes. Because of the global and national economic significance of the SCB and its dense population, the seismic hazard of the region is of outstanding importance. Comparing the SCB with another less developed region, a pending earthquake with the same size and tectonic setting would cause substantially more severe social and economic losses in the SCB. This paper also compiles an inventory of historic moderate to great earthquakes in the SCB; most of the data are not widely available in English literature.

Louvari, E.; Kiratzi, A.; Papazachos, B.; Hatzidimitriou, P.

doi: 10.1007/PL00001236pmid: N/A

 — Source parameters for thirteen earthquakes in the SE Adriatic region have been determined using P and SH body-waveform inversion. The results of this modeling are combined with eleven other earthquakes with M ≥ 5 whose focal mechanisms have been determined mainly by waveform modeling. The results confirm that movement on mainly low-angle reverse faults causes the deformation in coastal southern Yugoslavia through Albania up to the Lefkada Island in NW Greece. This zone of thrusting has a NW–SE trend (N34°W), follows the coastline, and dips towards the continent. The slip vectors of these events trend at ∼N229° along the Dalmatian coasts, to ∼N247° along Albania and NW Greece. The deformation is attributed to the continental collision between the Adriatic block to the west and Eurasia to the east. Along the mountain line in eastern Albania (Albanides Mts.) and in NW Greece (Hellenides Mts.), E–W extension is occurring. The E–W extension associated with the orogenic belt could be attributed to a variety of models such as: gravity, internal deformation of the thrust wedge, a probable down bulge of the dense lithosphere of the Adriatic block beneath the Eurasian lithospheric plate in combination with the compressional stresses applied along the collision belt.

Jechumtálová, Z.; Šílený, J.

doi: 10.1007/PL00001237pmid: N/A

 — We introduce an approach to estimate the error in the determination of the point-source mechanism and corresponding source-time function by inverting waveform data contaminated by a noise. No a priori assumptions about the statistical characteristics of the noise are needed. The estimate of the confidence region of the retrieved source is constructed as a set of solutions of the inverse problem for individual realizations of noisy data. They are generated from a sample of observed noise by Monte-Carlo simulation: white noise is convolved with the observed noise sample yielding a particular realization of the noise, which is random but retains the spectral characteristics of the observed sample. This approach allows us to consider even the situations where the noise varies from station to station. However, the basic assumption is that the noise is random, thus the method cannot handle systematic errors, e.g., due to mislocation or incorrect model of the medium.¶Synthetic tests with noisy data simulating the configuration of the seismic network in Friuli, NE Italy, show that determination of the source-time function is rather uncertain even with low-noise contamination but that orientation of the mechanism is recovered very well. As an example, local waveforms of a magnitude 3 event from the 1988 Moggio Udinese swarm, Northern Italy, are processed.

Kijko, A.; Lasocki, S.; Graham, G.

doi: 10.1007/PL00001238pmid: N/A

— Seismic hazard analysis methods in mines are reviewed for the purpose of selecting the best technique. To achieve this goal, the most often-used hazard analysis procedure, which is based on the classical frequency-magnitude Gutenberg-Richter relation, as well as alternative procedures are investigated.¶Since the maximum regional seismic event magnitude m max is of paramount importance in seismic hazard analysis, this work provides a generic formula for the evaluation of this important parameter. The formula is capable of generating solutions in different forms, depending on the assumptions of the model of the magnitude distribution and/or the available information regarding past seismicity. It includes the cases (i) in which seismic event magnitudes are distributed according to the truncated frequency-magnitude Gutenberg-Richter relation, and (ii) in which no specific model of the magnitude distribution is assumed.¶Both synthetic, Monte-Carlo simulated seismic event catalogues, and actual data from the copper mine in Poland and gold mine in South Africa, are used to demonstrate the discussed hazard analysis techniques.¶Our studies show that the non-parametric technique, which is independent of the assumed model of the distribution of magnitude, provides an appropriate tool for seismic hazard assessment in mines where the magnitude distribution can be very complex.

Hamdache, M.; Retief, S. J. P.

doi: 10.1007/PL00001239pmid: N/A

 — The procedure developed by Kijko and Sellevoll (1989, 1992) and Kijko and Graham (1998, 1999) is used to estimate seismic hazard parameters in north Algeria. The area-specific seismic hazard parameters that were calculated consist of the b value of the Gutenberg–Richter frequency–magnitude relation, the activity rate λ(M) for events above the magnitude M, and the maximum regional magnitude M max. These parameters were calculated for each of the six seismogenic zones of north Algeria. The site-specific seismic hazard was calculated in terms of the maximum possible PGA at hypothetical engineering structures (HES), situated in each of the six seismogenic zones with coordinates corresponding with those of the six most industrial and populated cities in Algeria.

Tsapanos, T. M.

doi: 10.1007/PL00001240pmid: N/A

 — Earthquake hazard parameters are estimated by the application of the maximum likelihood method. The technique is based on a procedure which utilizes data of different quality, e.g., those in which the uncertainty in the assessment of the magnitudes is great and those in which the magnitudes are computed with great precision. In other words the data were extracted from both historical (incomplete) and recorded (complete) files. The historical part of the catalogue contains only the strongest events, whereas the complete part can be divided into several sub-catalogues; each one assumed to be complete above a specified magnitude threshold. Uncertainty in the determination of magnitudes has also been taken into account. The method allows us to estimate the earthquake hazard parameters which are the maximum regional magnitude, M max, the activity rate, λ, of the seismic events and the well known value β (b=β log e), which is the slope of the magnitude-frequency relationship. All these parameters are of physical significance. The mean return periods, RP, of earthquakes with a certain lower magnitude M ≥ m are also determined. The method is applied in the Island of Crete and the adjacent area, where catastrophic earthquakes are known from the historical era. The earthquake hazard of the whole area is divided in a cellular manner which allow the analysis of the localized hazard parameters and the representation of their regional variation. The seismic hazard analysis, which is expressed by: (a) The annual probability of exceedance of a specified value of magnitude and (b) the return periods (in years) that are expected for given magnitudes, for shallow events is finally performed for shallow events. This hazard analysis is useful for both theoretical and practical reasons and provides a tool for earthquake resistant design in both areas of low and high seismicity.

Mandal, P.; Padhy, S.; Rastogi, B. K.; Satyanarayana, H. V. S.; Kousalya, M.; Vijayraghavan, R.; Srinivasan, A.

doi: 10.1007/PL00001241pmid: N/A

— On 28 March, 1999 (19:05:10.09, UT) a significant earthquake of M w 6.4 occurred in the Garhwal Himalaya (30.555°N, 79.424°E). One hundred and ten well-recorded aftershocks show a WNW-ESE trending northeasterly dipping seismic zone extending from a depth of 2 to 20 km. As the main shock hypocenter occurred at the northern end of this seismic zone and aftershocks extended updip, it is inferred that the main-shock rupture nucleated on the detachment plane at a depth of 15 km and then propagated updip along a NE-dipping thrust plane. Further, the epicentral distribution of aftershocks defines a marked concentration near a zone where main central thrust (MCT) takes a significant turn towards the north, which might be acting as an asperity in response to the NNE compression due to the underthrusting of Himalayan orogenic process prevalent in the entire region. Presence of high seismicity including five earthquakes of magnitude exceeding 6 and twelve earthquakes of magnitude exceeding 5 in the 20th century is presumed to have caused a higher level of shallow crustal heterogeneity in the Garhwal Himalaya, a site lying in the central gap zone of the Himalayan frontal arc. Attenuation property of the medium around the epicentral area of the 1999 Chamoli earthquake, covering a circular area of 61,500 km2 with a radius of 140 km, is studied by estimating the coda Q c from 48 local earthquakes of magnitudes varying from 2.5–4.8. These earthquakes were recorded at nine 24-bit REFTEK digital stations; two of which were equipped with three-component CMG40T broadband seismometers and others with three-component L4-3D short-period seismometers. The estimated Q o values at different stations suggest on average a low value of the order of (30 ± 0.8), indicating an attenuating crust beneath the entire region. The frequency-dependent relation indicates a relatively low Q c at lower frequencies (1–3 Hz) that can be attributed to the loss of energy due to scattering on heterogeneities and/or the presence of faults and cracks. The large Q c at higher frequencies may be related to the propagation of backscattered body waves through deeper parts of the lithosphere where less heterogeneities are expected. An important observation is that the region north of MCT (more rigid highly metamorphosed crystalline rocks) is less attenuative in comparison to the region south of MCT (less rigid slightly metamorphosed rocks (sedimentary wedge)). The acceleration decays to 50% at 20 km distance and to 7% at 100 km. Hence, even 1g acceleration at the source may not cause significant damage beyond 100 km in this region.

Singh, D. D.; Govoni, A.; Bragato, P. L.

doi: 10.1007/PL00022508pmid: N/A

— The digital data acquired by 16 short-period seismic stations of the Friuli-Venezia-Giulia seismic network for 56 earthquakes of magnitude 2.3–4.7 which occurred in and near NE Italy have been used to estimate the coda attenuation Q c and seismic source parameters. The entire area under study has been divided into five smaller regions, following a criterion of homogeneity in the geological characteristics and the constrains imposed by the distribution of available events. Standard IASPEI routines for coda Q c determination have been used for the analysis of attenuation in the different regions showing a marked anomaly in the values measured across the NE border between Friuli and Austria for Q 0 value. A large variation exists in the coda attenuation Q c for different regions, indicating the presence of great heterogeneities in the crust and upper mantle of the region. The mean value of Q c (f) increases from 154–203 at 1.5 Hz to 1947–2907 at 48 Hz frequency band with large standard deviation estimates.¶Using the same earthquake data, the seismic-moment, M 0, source radius, r and stress-drop, Δσ for 54 earthquakes have been estimated from P- and S-wave spectra using the Brune's seismic source model. The earthquakes with higher stress-drop (greater than 1 Kbar) occur at depths ranging from 8 to 14 km.

Alvarez, L.; Panza, G. F.; Vaccari, F.; González, B. E.

doi: 10.1007/PL00001242pmid: N/A

— We present the results of complete P-SV and SH waves modelling, up to a maximum frequency of 1 Hz, along two profiles in Santiago de Cuba city. The seismic sources are located in the depth range from 10 to 40 km on the Oriente fault zone at distances of several tens of kilometres from the city. The calculation has been made by a hybrid method: Modal summation in the regional anelastic model (one-dimensional) where the source is buried, and finite differences in the local sedimentary anelastic models (two-dimensional). The analysis of the influence of the depth and of the distance of the source on the site effects shows that standard traditional methods, based on the deconvolution analysis of the rock outcrop motion, can lead to erroneous results.

Butt, S. D.

doi: 10.1007/PL00001243pmid: N/A

— A series of experiments was conducted where rock core specimens were cyclically loaded to 50 MPa uniaxial stress while P waveforms were pulsed along the core axis. Some of these specimens were intact while others were prepared with a single through-going tensile fracture oriented perpendicular to the core axis. Recorded data was processed to determine Q, velocity and fracture closure. Results indicated that Q was constant over the studied bandwidth and Q for the fractured specimens decreased relative to the intact specimens as fracture stress decreased. Observed variations in static fracture stiffness among the tested specimens did not result in corresponding variations in Q. Velocity results showed similar trends. This work was done to provide comparative data for related field studies examining the feasibility of using attenuation measurements in comparable frequency bands to indicate the potential for roof failure during excavation in fractured rock masses.