Probabilistic seismic hazard assessment for Central Asia

<p>Central Asia is one of the seismically most active regions in the world. Its complex seismicity due to the collision of the Eurasian and Indian plates has resulted in some of the world’s largest intra-plate events over history. The region is dominated by reverse faulting over strike slip and normal faulting events. The GSHAP project (1999), aiming at a hazard assessment on a global scale, indicated that the region of Central Asia is characterized by peak ground accelerations for 10% probability of exceedance in 50 years as high as 9 m/s². In this study, carried out within the framework of the EMCA project (Earthquake Model Central Asia), the area source model and different kernel approaches are used for a probabilistic seismic hazard assessment (PSHA) for Central Asia. The seismic hazard is assessed considering shallow (depth &lt; 50 km) seismicity only and employs an updated (with respect to previous projects) earthquake catalog for the region. The seismic hazard is calculated in terms of macroseismic intensity (MSK-64), intended to be used for the seismic risk maps of the region. The hazard maps, shown in terms of 10% probability of exceedance in 50 years, are derived by using the OpenQuake software [Pagani et al. 2014], which is an open source software tool developed by the GEM (Global Earthquake Model) foundation. The maximum hazard observed in the region reaches an intensity of around 8 in southern Tien Shan for 475 years mean return period. The maximum hazard estimated for some of the cities in the region, Bishkek, Dushanbe, Tashkent and Almaty, is between 7 and 8 (7-8), 8.0, 7.0 and 8.0 macroseismic Intensity, respectively, for 475 years mean return period, using different approaches. The results of different methods for assessing the level of seismic hazard are compared and their underlying methodologies are discussed.</p>

Seven centuries of avalanche activity at Echalp (Queyras massif, southern French Alps) as inferred from tree rings

The purpose of this study was to reconstruct spatiotemporal patterns of avalanche events in a forested avalanche path of the Queyras massif (Echalp avalanche path, southeast French Alps). Analysis of past events was based on tree-ring series from 163 heavily affected multicentennial larch trees ( Larix decidua Mill.) growing near or next to the avalanche path. A total of 514 growth disturbances, such as tangential rows of traumatic resin ducts, the onset of compression wood as well as abrupt growth suppression or release, were identified in the samples indicating 38 destructive snow avalanches between 1338 and 2010. The mean return period of snow avalanches was 22 years with a 4% probability that an avalanche occurs in a particular year. On a temporal plan, three maxima in snow avalanche frequency were reconstructed at the beginning of the 16th and 19th centuries and around 1850, correlating with below-average winter temperatures and glacier advances. Analysis of the spatial distribution of disturbed trees contributed to the determination of four preferential patterns of avalanche events. The comparison of dendrogeomorphic data with historical records demonstrate that at least 18 events – six of which were undocumented – reached the hamlet of Echalp during the last seven centuries, but no significant temporal trend was detected concerning the frequency of these extreme events.

Seismic risk mapping for Germany

The aim of this study is to assess and map the seismic risk for Germany, restricted to the expected losses of damage to residential buildings. There are several earthquake prone regions in the country which have produced Mw magnitudes above 6 and up to 6.7 corresponding to observed ground shaking intensity up to VIII–IX (EMS-98). Combined with the fact that some of the earthquake prone areas are densely populated and highly industrialized and where therefore the hazard coincides with high concentration of exposed assets, the damaging implications from earthquakes must be taken seriously. In this study a methodology is presented and pursued to calculate the seismic risk from (1) intensity based probabilistic seismic hazard, (2) vulnerability composition models, which are based on the distribution of residential buildings of various structural types in representative communities and (3) the distribution of assets in terms of replacement costs for residential buildings. The estimates of the risk are treated as primary economic losses due to structural damage to residential buildings. The obtained results are presented as maps of the damage and risk distributions. For a probability level of 90% non-exceedence in 50 years (corresponding to a mean return period of 475 years) the mean damage ratio is up to 20% and the risk up to hundreds of millions of euro in the most endangered communities. The developed models have been calibrated with observed data from several damaging earthquakes in Germany and the nearby area in the past 30 years.

An alternative Bayesian statistics for probabilistic earthquake prediction in Mexico, Central and South America

The probabilities of occurrence of strong (M>6.5) earthquakes, in the seismically active regions of Mexico, central and south America, are estimated. The straightforward approach of Bayes statistics is applied in order to search for the inter-arrival times of strong earthquakes in predefined seismic zones of the above referred regions. The method introduced allows to determine the uncertainties involved, which are expressed as percentages of the earthquake mean return period. The determination in this way is very efficient because one may calculate uncertainties on the same time scale. It is also shown that the final maximum Bayesian probabilities of the inter-arrival times in the several seismic zones are dependent on the data set used and particularly on its time length. Comparisons between the predicted and the real time of earthquake occurrences are finally made in order to evaluate the correlation between them.

Quantitative evaluation of the seismicity in seismogenic sources of the Circum Pacific Rim

A quantitative measurement of the seismicity is undertaken along the seismogenic sources of the south and central America, as well as of Japan, Taiwan and Philippine islands. The mean return period, Τ , is considered as measure of seismicity. For this purpose the whole process (method of mean value) and the part process (first asymptotic distribution of Gumbel's extreme values) techniques are adopted. The seismicity is evaluated for both shallow and intermediate focal depth shocks which occurred in the examined sources. The a and b values of the magnitude-frequency relationship are estimated for each source. The obtained results show that large welldefine zones of the mean return periods are dominated in the central and south America, while this is not so strong in the areas of Japan, Taiwan and Philippine islands, which probably due to the different tectonic setting of the areas (Tsapanos, 1990).

Seismic Retrofit of 1958 Carquinez Bridge

The 1021-m (3,350-ft) long steel through truss bridge carrying the east-bound lanes of Interstate 80 across the western end of the Carquinez Strait about 40 km (25 mi) north of San Francisco opened to traffic in 1958. It was the first major highway bridge in the United States to use high-strength (T1) steel, the first to use welded built-up members, and the first to use high-strength bolted connections. These “firsts,” combined with the size of the bridge and the traffic demands, presented a formidable challenge to the retrofit design team. The retrofit objective was to prevent collapse of the bridge during an earthquake with an expected mean return period in the range of 1,000 to 2,000 years. Under this noncollapse criterion, significant damage to the bridge, such as yielding and buckling of members, was considered acceptable. It was important, then, that a measure of acceptable damage be defined and that the postyield behavior of the structure be both very predictable and very ductile. A preliminary design was prepared based on an elastic analysis with geometric non-linearities. The retrofit design was then examined by incorporating material nonlinearities into the model; adjustments to the retrofit design were required in some isolated areas. Design issues that the team addressed included connections for loads as high as 60 to 50 kN (13,500 kips); local buckling of thin-walled, high-strength members; postyield behavior of major load-carrying steel members; rocking of rigid A-frame tower assemblies; and an expansion joint for an 8-ft movement rating.