ALBERT

Beschreibung: Crystal Growth & Design DOI: 10.1021/cg401230a

Beschreibung: The long and complex history of southern Africa makes it a geological nexus for understanding how crust forms, evolves and survives plate tectonic processes over billions of years. The goal of this study is to characterize the crustal thickness, composition, and Moho impedance contrasts across the Kaapvaal and Zimbabwe Cratons and surrounding mobile belts, which range in age from Archean to Palaeozoic. We use data gathered from the 1997–1999 Southern Africa Seismic Experiment, the Africa Array (2006–2007) and the Global Seismographic Network (1993–2009) to generate P -wave receiver function Gaussian-weighted common conversion point stacks across the region in order to provide a continuous 3-D image of crustal variations throughout southern Africa. We observe thickened crust associated with mobile belts and the intrusion of the Bushveld Complex relative to the less-deformed cratons. The southern Kaapvaal and eastern Zimbabwe Cratons have a well-defined Moho with an average depth of ~34 km and Vp / Vs of ~1.73, indicative of felsic average crustal composition. We explain the felsic composition observed in the Kaapvaal Craton in the context of significant crustal modification related to the deposition of the Ventersdorp lavas. We find that the Bushveld Province, the site of the world's largest layered mafic intrusion, has a thick (〉40 km) crust with a Vp / Vs 〉 1.8, indicative of a mafic average crustal composition. The magnitude of Moho conversions beneath the Bushveld Province is variable, with the lowest amplitude conversion appearing between the eastern and western limbs of the Bushveld Complex, indicative of mafic underplating beneath the region. In the Limpopo Belt and western Zimbabwe Craton, we observe low amplitude Moho conversions beneath the Okavango Dyke Swarm, and attribute this to the reworking of the crust by mafic underplating and intrusion during the Jurassic rifting of Gondwanaland. The Namaqua-Natal event thickened the crust and created a gradational transition from crust to mantle as seen by low amplitude Ps arrivals from receiver functions. Evidence for the presence of a mafic lower crust beneath the Namaqua-Natal Belt is observed in high Vp / Vs values (~1.8) and a high concentration of granulite xenoliths in kimberlite intrusions. In contrast to past interpretations for craton formation that suggest sharp Moho boundaries and low Vp / Vs ratios are characteristic of undisturbed cratons, we propose that these crustal properties are more controlled by tectonic events that later modify the existing cratonic crust. We cannot rule out secular crustal formation variations in the early Earth, but we propose that the southern African cratonic crust has been too heavily modified by later tectonic events to be used in arguments for secular variation, as may be the case for other cratons as well. Thus, it is important to consider the regional geological history of cratons to ensure that secular variation is not confused with the effects of later tectonic deformation and crustal modification.

Beschreibung: The Central Andes of southern Peru, Bolivia, Argentina and Chile (between 12°S and 42°S) comprise the largest orogenic plateau in the world associated with abundant arc volcanism, the Central Andean Plateau, as well as multiple segments of flat-slab subduction making this part of the Earth a unique place to study various aspects of active plate tectonics. The goal of this continental-scale ambient noise tomography study is to incorporate broad-band seismic data from 20 seismic networks deployed incrementally in the Central Andes from 1994 May to 2012 August, to image the vertically polarized shear wave velocity ( V sv ) structure of the South American Cordillera. Using dispersion measurements calculated from the cross-correlation of 330 broad-band seismic stations, we construct Rayleigh wave phase velocity maps in the period range of 8–40 s and invert these for the shear wave velocity ( V sv ) structure of the Andean crust. We provide a dispersion misfit map as well as uncertainty envelopes for our V sv model and observe striking first-order correlations with our shallow results (~5 km) and the morphotectonic provinces as well as subtler geological features indicating our results are robust. Our results reveal for the first time the full extent of the mid-crustal Andean low-velocity zone that we tentatively interpret as the signature of a very large volume Neogene batholith. This study demonstrates the efficacy of integrating seismic data from numerous regional broad-band seismic networks to approximate the high-resolution coverage previously only available though larger networks such as the EarthScope USArray Transportable Array in the United States.

Beschreibung: In 2008, an earthquake-planning scenario document was released by the U.S. Geological Survey (USGS) and California Geological Survey that hypothesizes the occurrence and effects of a Mw7.8 earthquake on the southern San Andreas Fault. It was created by more than 300 scientists and engineers. Fault offsets reach 13 m and up to 8 m at lifeline crossings. Physics-based modeling was used to generate maps of shaking intensity, with peak ground velocities of 3 m/sec near the fault and exceeding 0.5 m/sec over 10,000 km2. A custom HAZUS®MH analysis and 18 special studies were performed to characterize the effects of the earthquake on the built environment. The scenario posits 1,800 deaths and 53,000 injuries requiring emergency room care. Approximately 1,600 fires are ignited, resulting in the destruction of 200 million square feet of the building stock, the equivalent of 133,000 single-family homes. Fire contributes $87 billion in property and business interruption loss, out of the total $191 billion in economic loss, with most of the rest coming from shake-related building and content damage ($46 billion) and business interruption loss from water outages ($24 billion). Emergency response activities are depicted in detail, in an innovative grid showing activities versus time, a new format introduced in this study.

Beschreibung: A study for the Multidisciplinary Center for Earthquake Engineering Research (MCEER) provides fragility functions for 52 varieties of mechanical, electrical, and plumbing (MEP) equipment commonly found in commercial and industrial buildings. For the majority of equipment categories, the MCEER study provides multiple fragility functions, reflecting important effects of bracing, anchorage, interaction, etc. The fragility functions express the probability that the component would be rendered inoperative as a function of floor acceleration. That work did not include the evidence underlying the fragility functions. As part of the ATC-58 effort to bring second-generation performance-based earthquake engineering to professional practice, we have compiled the original MCEER specimen-level performance data into a publicly accessible database and validate many of the original fragility functions. In some cases, new fragility functions derived by ATC-58 methods show somewhat closer agreement with the raw data. Average-condition fragility functions are developed here; we will address in subsequent work the effect of potentially important—arguably crucial—performance-modifying factors such as poor anchorage and interaction.

Beschreibung: The “cracking an open safe” methodology has been used to tabulate HAZUS-based seismic vulnerability as functions of structure-independent intensity, while avoiding iteration in the structural analysis. The vulnerability functions give mean damage factor (MDF, defined here as mean repair cost as a fraction of replacement cost) versus 5%-damped elastic spectral acceleration response at 0.3-second and 1.0-second periods, for every combination of occupancy type, model building type, design level, magnitude, distance, site soil classification, etc. Like HAZUS-MH, these prior seismic vulnerability functions give no estimate of uncertainty in damage factor. The coefficient of variation (COV) of damage factor is readily calculated by taking advantage of the fact that that at any level of excitation there is a probability mass function of damage state and an implicit distribution of repair cost conditioned on damage state. COV is calculated here for each combination of occupancy type, model building type, etc., tabulated alongside MDF, and the tables presented for public use at www.risk-agora.org . It is found that a HAZUS-based COV generally decreases with increasing MDF (as has been observed using other analytical vulnerability methods), and the standard deviation of damage factor generally increases with increasing MDF.

Beschreibung: The ShakeOut Scenario assessed earth-science impacts, physical damage, and socioeconomic impacts of a hypothetical M7.8 southern San Andreas Fault earthquake. Among many detailed studies were special studies of 12 lifelines, 7 of which were performed by panels of employees of the utilities at risk. Panels met for four hours. Panelists were presented with the scenario's earth science impacts and previously estimated damage to “upstream” lifelines. They then hypothesized a realistic outcome of the earthquake on damage and service restoration, identifying research needs and mitigation options. The panel process worked well: panelists were well qualified and seemed to fairly assess realistic earthquake impacts and restoration, probably more realistically than an outside consultant would have been able to do, thus improving the ShakeOut. Panelists gained insight into lifeline interaction, mutual-aid needs, communication capabilities, and backup supplies. Southern California Edison, for example, enhanced its planning and preparedness for a large Southern California earthquake.