Three-dimensional lithospheric structure below the New Zealand Southern Alps

Uppermost mantle seismic structure below the Southern Alps in South Island, New Zealand, is investigated by teleseismic P wave travel time residual inversion. The three-dimensional tomographic images show a near-vertical, high-velocity (2–4%) structure in the uppermost mantle that directly underlies thickened crust along the NNESSW axis of the Southern Alps. The center of the high-velocity anomaly lies to the east of the Alpine fault which bounds Pacific and Australian plate rocks. The oblique collision of these plates resulted in the uplift of the Southern Alps during the past 5–7 m.y. Also, a high-velocity anomaly (3–5%) corresponding to the Hikurangi subduction zone lies to the northeast of the Southern Alps anomaly, and low-velocity anomalies (-3%) underlying parts of northwestern and southern South Island may be signatures of late Tertiary extension and volcanism. The data consist of teleseismic arrival times from the New Zealand National Seismograph Network and arrival times recorded during the 1995–1996 Southern Alps Passive Seismic Experiment. Crustal heterogeneity was accounted for by back projecting the rays through an independently obtained three-dimensional crustal velocity and Moho depth model. The Southern Alps uppermost mantle velocity anomalies are most simply explained by lithospheric thickening below the center of convergence accompanied by thinning and asthenospheric upwelling adjacent to the region of convergence

Intermediate-Depth Earthquakes in a Region of Continental Convergence: South Island, New Zealand

It is rare to find earthquakes with depths greater than 30 km in continent–continent collision zones because the mantle lithosphere is usually too hot to enable brittle failure. However, a handful of small, intermediate-depth earthquakes (30–97 km) have been recorded in the continental collision region in central South Island, New Zealand. The earthquakes are not associated with subduction but all lie within or on the margins of thickened crust or uppermost mantle seismic high-velocity anomalies. The largest of the earthquakes has M_L 4.0 corresponding to a rupture radius of between 100 and 800 m, providing bounds on the upper limit to the rupture length over which brittle failure is taking place in the deep brittle–plastic transition zone. The earthquake sources may be controlled by large shear strain gradients associated with viscous deformation processes in addition to depressed geotherms

Creating realistic models based on combined forward modeling and tomographic inversion of seismic profiling data

Amplitudes and shapes of seismic patterns derived from tomographic images often are strongly biased with respect to real structures in the earth. In particular, tomography usually provides continuous velocity distributions, whereas major velocity changes in the earth often occur on first-order interfaces. We propose an approach that constructs a realistic structure of the earth that combines forward modeling and tomographic inversion (FM&TI). Using available a priori information, we first construct a synthetic model with realistic patterns. Then we compute synthetic times and invert them using the same tomographic code and the same parameters as in the case of observed data processing. We compare the reconstruction result with the tomographicimage of observed data inversion. If a discrepancy is observed, we correct the synthetic model and repeat the FM&TI process. After several trials, we obtain similar results of synthetic and observed data inversion. In this case, the derived synthetic model adequately represents the real structure of the earth. In a working scheme of this approach, we three authors used two different synthetic models with a realistic setup. One of us created models, but the other two performed the reconstruction with no knowledge of the models. We discovered that the synthetic models derived by FM&TI were closer to the true model than the tomographic inversion result. Our reconstruction results from modeling marine data acquired in the Musicians Seamount Province in the Pacific Ocean indicate the capacity and limitations of FM&TI

Campylobacteriosis, Eastern Townships, Québec

Independent risk factors for campylobacteriosis (eating raw, rare, or undercooked poultry; consuming raw milk or raw milk products; and eating chicken or turkey in a commercial establishment) account for <50% of cases in Québec. Substantial regional and seasonal variations in campylobacteriosis were not correlated with campylobacter in chickens and suggested environmental sources of infection, such as drinking water

Geophysical structure of the Southern Alps orogen, South Island, New Zealand

The central part of the South Island of New Zealand is a product of the transpressive continental collision of the Pacific and Australian plates during the past 5 million years, prior to which the plate boundary was largely transcurrent for over 10 My. Subduction occurs at the north (west dipping) and south (east dipping) of South Island. The deformation is largely accommodated by the ramping up of the Pacific plate over the Australian plate and near-symmetric mantle shortening. The initial asymmetric crustal deformation may be the result of an initial difference in lithospheric strength or an inherited suture resulting from earlier plate motions. Delamination of the Pacific plate occurs resulting in the uplift and exposure of mid-crustal rocks at the plate boundary fault (Alpine fault) to form a foreland mountain chain. In addition, an asymmetric crustal root (additional 8 - 17 km) is formed, with an underlying mantle downwarp. The crustal root, which thickens southwards, comprises the delaminated lower crust and a thickened overlying middle crust. Lower crust is variable in thickness along the orogen, which may arise from convergence in and lower lithosphere extrusion along the orogen. Low velocity zones in the crust occur adjacent to the plate boundary (Alpine fault) in the Australian and Pacific plates, where they are attributed to fracturing of the upper crust as a result of flexural bending for the Australian plate and to high pressure fluids in the crust derived from prograde metamorphism of the crustal rocks for the Pacific plate

Population-Attributable Risk Estimates for Risk Factors Associated with Campylobacter Infection, Australia

One-sentence summary: Each year, an estimated 50,500 cases in persons >5 years of age can be directly attributed to consumption of chicken