Hydrologically-driven crustal stresses and seismicity in the New Madrid Seismic Zone

The degree to which short-term non-tectonic processes, either natural and anthropogenic, influence the occurrence of earthquakes in active tectonic settings or ‘stable’ plate interiors, remains a subject of debate. Recent work in plate-boundary regions demonstrates the capacity for long-wavelength changes in continental water storage to produce observable surface deformation, induce crustal stresses and modulate seismicity rates. Here we show that a significant variation in the rate of microearthquakes in the intraplate New Madrid Seismic Zone at annual and multi-annual timescales coincides with hydrological loading in the upper Mississippi embayment. We demonstrate that this loading, which results in geodetically observed surface deformation, induces stresses within the lithosphere that, although of small amplitude, modulate the ongoing seismicity of the New Madrid region. Correspondence between surface deformation, hydrological loading and seismicity rates at both annual and multi-annual timescales indicates that seismicity variations are the direct result of elastic stresses induced by the water load.

Large-scale changes in continental water storage have been shown to have an impact on seismicity. Here, the authors show that variation in the rate of microearthquakes in the New Madrid Seismic Zone coincides with hydrological loading in the Mississippi embayment at both annual and multi-annual timescales.

Physical applications of GPS geodesy: a review

Geodesy, the oldest science, has become an important discipline in the geosciences, in large part by enhancing Global Positioning System (GPS) capabilities over the last 35 years well beyond the satellite constellation's original design. The ability of GPS geodesy to estimate 3D positions with millimeter-level precision with respect to a global terrestrial reference frame has contributed to significant advances in geophysics, seismology, atmospheric science, hydrology, and natural hazard science. Monitoring the changes in the positions or trajectories of GPS instruments on the Earth's land and water surfaces, in the atmosphere, or in space, is important for both theory and applications, from an improved understanding of tectonic and magmatic processes to developing systems for mitigating the impact of natural hazards on society and the environment. Besides accurate positioning, all disturbances in the propagation of the transmitted GPS radio signals from satellite to receiver are mined for information, from troposphere and ionosphere delays for weather, climate, and natural hazard applications, to disturbances in the signals due to multipath reflections from the solid ground, water, and ice for environmental applications. We review the relevant concepts of geodetic theory, data analysis, and physical modeling for a myriad of processes at multiple spatial and temporal scales, and discuss the extensive global infrastructure that has been built to support GPS geodesy consisting of thousands of continuously operating stations. We also discuss the integration of heterogeneous and complementary data sets from geodesy, seismology, and geology, focusing on crustal deformation applications and early warning systems for natural hazards.

MIDAS robust trend estimator for accurate GPS station velocities without step detection

Automatic estimation of velocities from GPS coordinate time series is becoming required to cope with the exponentially increasing flood of available data, but problems detectable to the human eye are often overlooked. This motivates us to find an automatic and accurate estimator of trend that is resistant to common problems such as step discontinuities, outliers, seasonality, skewness, and heteroscedasticity. Developed here, Median Interannual Difference Adjusted for Skewness (MIDAS) is a variant of the Theil-Sen median trend estimator, for which the ordinary version is the median of slopes v_ij  = (x_j-x_i )/(t_j-t_i ) computed between all data pairs i > j. For normally distributed data, Theil-Sen and least squares trend estimates are statistically identical, but unlike least squares, Theil-Sen is resistant to undetected data problems. To mitigate both seasonality and step discontinuities, MIDAS selects data pairs separated by 1 year. This condition is relaxed for time series with gaps so that all data are used. Slopes from data pairs spanning a step function produce one-sided outliers that can bias the median. To reduce bias, MIDAS removes outliers and recomputes the median. MIDAS also computes a robust and realistic estimate of trend uncertainty. Statistical tests using GPS data in the rigid North American plate interior show ±0.23 mm/yr root-mean-square (RMS) accuracy in horizontal velocity. In blind tests using synthetic data, MIDAS velocities have an RMS accuracy of ±0.33 mm/yr horizontal, ±1.1 mm/yr up, with a 5th percentile range smaller than all 20 automatic estimators tested. Considering its general nature, MIDAS has the potential for broader application in the geosciences.

The New Madrid Seismic Zone: not dead yet

The extent to which ongoing seismicity in intraplate regions represents long-lived aftershock activity is unclear. We examined historical and instrumental seismicity in the New Madrid central U.S. region to determine whether present-day seismicity is composed predominantly of aftershocks of the 1811-1812 earthquake sequence. High aftershock productivity is required both to match the observation of multiple mainshocks and to explain the modern level of activity as aftershocks; synthetic sequences consistent with these observations substantially overpredict the number of events of magnitude ≥ 6 that were observed in the past 200 years. Our results imply that ongoing background seismicity in the New Madrid region is driven by ongoing strain accrual processes and that, despite low deformation rates, seismic activity in the zone is not decaying with time.

An Estimate of Increases in Storm Surge Risk to Property from Sea Level Rise in the First Half of the Twenty-First Century

Sea level is rising as the World Ocean warms and ice caps and glaciers melt. Published estimates based on data from satellite altimeters, beginning in late 1992, suggest that the global mean sea level has been rising on the order of 3 mm yr−1. Local processes, including ocean currents and land motions due to a variety of causes, modulate the global signal spatially and temporally. These local signals can be much larger than the global signal, and especially so on annual or shorter time scales.

Even increases on the order of 10 cm in sea level can amplify the already devastating losses that occur when a hurricane-driven storm surge coincides with an astronomical high tide. To quantify the sensitivity of property risk to increasing sea level, changes in expected annual losses to property along the U.S. Gulf and East Coasts are calculated as follows. First, observed trends in sea level rise from tide gauges are extrapolated to the year 2030, and these changes are interpolated to all coastal locations. Then a 10 000-yr catalog of simulated hurricanes is used to define critical wind parameters for each event. These wind parameters then drive a parametric time-evolving storm surge model that accounts for bathymetry, coastal geometry, surface roughness, and the phase of the astronomical tide. The impact of the maximum storm surge height on a comprehensive inventory of commercial and residential property is then calculated, using engineering models that take into account the characteristics of the full range of construction types.

Average annual losses projected to the year 2030 are presented for regions and key states and are normalized by aggregate property value on a zip code by zip code basis. Comparisons to the results of a control run reflecting the risk today quantify the change in risk per dollar of property on a percentage basis. Increases in expected losses due to the effect of sea level rise alone vary by region, with increases of 20% or more being common. Further sensitivity tests quantify the impact on the risk of sea level rise plus additional factors, such as changes in hurricane frequency and intensity as a result of rising sea surface temperatures.

Triggering of New Madrid seismicity by late-Pleistocene erosion

The spatiotemporal behaviour of earthquakes within continental plate interiors is different from that at plate boundaries. At plate margins, tectonic motions quickly reload earthquake ruptures, making the location of recent earthquakes and the average time between them consistent with the faults' geological, palaeoseismic and seismic histories. In contrast, what determines the activation of a particular mid-continental fault and controls the duration of its seismic activity remains poorly understood. Here we argue that the concentration of magnitude-7 or larger earthquakes in the New Madrid seismic zone of the central United States since the end of the last ice age results from the recent, climate-controlled, erosional history of the northern Mississippi embayment. We show that the upward flexure of the lithosphere caused by unloading from river incision between 16,000 and 10,000 years ago caused a reduction of normal stresses in the upper crust sufficient to unclamp pre-existing faults close to failure equilibrium. Models indicate that fault segments that have already ruptured are unlikely to fail again soon, but stress changes from sediment unloading and previous earthquakes may eventually be sufficient to bring to failure other nearby segments that have not yet ruptured.

Did earthquakes keep the early crust habitable?

The shallow habitable region of cratonal crust deforms with a strain rate on the order of approximately 10(19) s(1). This is rapid enough that small seismic events are expected on one-kilometer spatial scales and one-million-year timescales. Rock faulting has the potential to release batches of biological substrate, such as dissolved H(2), permitting transient blooms. In addition, the steady-state deformation of the brittle crust causes numerous small faults to be permeable enough (on the order of approximately 10(15) m(2)) for water to flow on a kilometer scale over relatively short geological times ( approximately 10(5) yr). Hence, active faults act as concentrated niches capable of episodically tapping resources in the bulk volume of the rock. Radiolysis and ferrous iron are potentially bases of sustainable hard-rock niches.