ALBERT

Description: Estimates of magnitudes of large historical earthquakes are an essential input to and can seriously affect seismic-hazard estimates. The earthquake-intensity observations, modified Mercalli intensities (MMI), and assigned magnitudes M of the 1811–1812 New Madrid events have been reinterpreted several times in the last decade and have been a source of controversy in making seismic-hazard estimates in the central United States. Observations support the concept that the larger the earthquake, the greater the maximum-felt distance. For the same crustal attenuation and local soil conditions, magnitude should be the main influence on intensity values at large distances. We apply this concept by comparing the mean MMI at distances of 600–1200 km for each of the four largest New Madrid 1811–1812 earthquakes, the 1886 Charleston, South Carolina, earthquake, the 1929 M  7.2 Grand Banks earthquake, and the 2001 M  7.6 Bhuj, India, earthquake. We fit the intensity observations using the form MMI= A + C x dist–0.8 x log(dist) to better define intensity attenuation in eastern North America (ENA). The intensity attenuation in cratonic India differs from ENA and is corrected to ENA using both the above estimate and published intensity relations. We evaluate source, marine geophysical, Q , and stress-drop information, as well as a 1929 Milne–Shaw record at Chicago to confirm that the 1929 Grand Banks earthquake occurred in ENA crust. Our direct comparison of mean intensities beyond 600 km suggests M  7.5, 7.3, 7.7, and 6.9 for the three New Madrid 1811–1812 mainshocks and the largest aftershock and M  7.0 for the 1886 Charleston, South Carolina, earthquake, with an estimated uncertainty of 0.3 units at the 95% confidence level (based on a Monte Carlo analysis). Our mean New Madrid and Charleston mainshock magnitudes are similar to those of Bakun and Hopper (2004) and are much higher than those of Hough and Page (2011) for New Madrid. Online Material: Tables of mean modified Mercalli intensity for 800–1200 and 600–1000 km distance ranges, and figures of least-squares fit for all intensity measures used and for frequency-dependent Q in easternmost Canada.

Description: A new macroseismic intensity prediction equation is derived for the central and eastern United States and is used to estimate the magnitudes of the 1811–1812 New Madrid, Missouri, and 1886 Charleston, South Carolina, earthquakes. This work improves upon previous derivations of intensity prediction equations by including additional intensity data, correcting magnitudes in the intensity datasets to moment magnitude, and accounting for the spatial and temporal population distributions. The new relation leads to moment magnitude estimates for the New Madrid earthquakes that are toward the lower range of previous studies. Depending on the intensity dataset to which the new macroseismic intensity prediction equation is applied, mean estimates for the 16 December 1811, 23 January 1812, and 7 February 1812 mainshocks, and 16 December 1811 dawn aftershock range from 6.9 to 7.1, 6.8 to 7.1, 7.3 to 7.6, and 6.3 to 6.5, respectively. One-sigma uncertainties on any given estimate could be as high as 0.3–0.4 magnitude units. We also estimate a magnitude of 6.9±0.3 for the 1886 Charleston, South Carolina, earthquake. We find a greater range of magnitude estimates when also accounting for multiple macroseismic intensity prediction equations. The inability to accurately and precisely ascertain magnitude from intensities increases the uncertainty of the central United States earthquake hazard by nearly a factor of two. Relative to the 2008 national seismic hazard maps, our range of possible 1811–1812 New Madrid earthquake magnitudes increases the coefficient of variation of seismic hazard estimates for Memphis, Tennessee, by 35%–42% for ground motions expected to be exceeded with a 2% probability in 50 years and by 27%–35% for ground motions expected to be exceeded with a 10% probability in 50 years.

Description: I have estimated Brune (single-corner) stress parameters (stress drops) for several Oklahoma earthquakes, including the 2016 M w  5.8 Pawnee earthquake and the largest aftershocks. My approach is to estimate corner frequency from the peak of the tangential component of the velocity Fourier spectrum using recordings about 50 km or less from the epicenter, where possible. Because Brune stress parameter is for body waves, care has been taken to avoid contaminating spectral peaks from surface waves, nearby building interactions, and soil resonances. For shallow earthquakes, leaky-mode surface waves have been observed overlapping P and S waves, including in Oklahoma. Hence, spectral shape fitting has been avoided in this study. Oklahoma earthquakes (potentially induced) tend to be shallow (〈10 km and usually ~5 km or less). In general, Brune stress parameter for the M w  5.8 Pawnee and other Oklahoma mainshocks ranges between 14 and 22 MPa, whereas aftershocks and smaller magnitude earthquake stress parameters fall in the 1–11 MPa range. These values are typical for the south-central United States potentially induced and natural (shallow and deeper) earthquakes. Other eastern North America regions have natural earthquake stress parameters above 15 MPa, but shallow natural earthquakes can have values in the 2–12 MPa range, similar to potentially induced earthquakes. Comparisons with other estimates of corner frequency using spectral shape fitting suggest that these estimates can be biased by leaky-mode surface waves and/or systematic site responses among recording stations, which affects stress-parameter estimates and interpretations.

Description: We present probabilistic and deterministic seismic and liquefaction hazard maps for the densely populated St. Louis metropolitan area that account for the expected effects of surficial geology on earthquake ground shaking. Hazard calculations were based on a map grid of 0.005°, or about every 500 m, and are thus higher in resolution than any earlier studies. To estimate ground motions at the surface of the model (e.g., site amplification), we used a new detailed near-surface shear-wave velocity model in a 1D equivalent-linear response analysis. When compared with the 2014 U.S. Geological Survey (USGS) National Seismic Hazard Model, which uses a uniform firm-rock-site condition, the new probabilistic seismic-hazard estimates document much more variability. Hazard levels for upland sites (consisting of bedrock and weathered bedrock overlain by loess-covered till and drift deposits), show up to twice the ground-motion values for peak ground acceleration (PGA), and similar ground-motion values for 1.0 s spectral acceleration (SA). Probabilistic ground-motion levels for lowland alluvial floodplain sites (generally the 20–40-m-thick modern Mississippi and Missouri River floodplain deposits overlying bedrock) exhibit up to twice the ground-motion levels for PGA, and up to three times the ground-motion levels for 1.0 s SA. Liquefaction probability curves were developed from available standard penetration test data assuming typical lowland and upland water table levels. A simplified liquefaction hazard map was created from the 5%-in-50-year probabilistic ground-shaking model. The liquefaction hazard ranges from low (〈40% of area expected to liquefy) in the uplands to severe (〉60% of area expected to liquefy) in the lowlands. Because many transportation routes, power and gas transmission lines, and population centers exist in or on the highly susceptible lowland alluvium, these areas in the St. Louis region are at significant potential risk from seismically induced liquefaction and associated ground deformation.

Description: Comparisons between peak ground motions (PGMs) and intensities are mainly based on the regression of felt intensity with peak ground acceleration (PGA), peak ground velocity (PGV), response spectral values, and occasionally with maximum displacement. Using the Next Generation Attenuation-East database, we update and extend the relations of Dangkua and Cramer (2011) . We start by developing magnitude and distance-independent linear regression relationships between modified Mercalli intensity (MMI) in the range of I≤MMI≤VII and the ground-motion parameters (GMPs) of PGA, PGV, and 21 pseudospectral accelerations for central and eastern North America (CENA). We correct for recently acknowledged differences between community Internet intensities (CIIs) and MMI ( Hough, 2013 , 2014 ; Boyd and Cramer, 2014 ; Tosi et al. , 2015 ). We then perform residual analysis to evaluate whether there are any discrepancies and dependency, first with magnitude and then distance. For the magnitude-dependent analysis, we use a bilinear fit. This bilinear magnitude dependence is especially important for longer periods. The residuals show homoskedasticity with zero mean and are serially correlated. We correct for the serial correlation using the Cochrane–Orcutt procedure. Our new ground-motion intensity correlation equations for CENA have the form log 10 ( Y )= c 1 + c 2 x I + c 3 x min{ M w , M t }+ c 4 x log 10 (Dist), in which log 10 ( Y ) is the GMP, M w is the moment magnitude, M t is the magnitude of the bend in the bilinear magnitude dependence, log 10 (Dist) is the distance term, I is the intensity of interest (MMI or CII), and c 1 , c 2 , c 3 , and c 4 are the coefficients of regression. The equation is invertible for obtaining both CII and MMI from the PGM of interest and vice versa. Electronic Supplement: Figures of data for intensity prediction equation (IPE) evaluation, additional residual analyses, and evaluations based on other performance metrics. Tables that list the events and details of the earthquakes used in the analysis and the means, standard errors, and standard deviations of modified Mercalli intensity (MMI)/community Internet intensity (CII) values of the selected ground-motion parameters.

Description: We performed a suite of numerical simulations based on the 1811–1812 New Madrid seismic zone (NMSZ) earthquakes, which demonstrate the importance of 3D geologic structure and rupture directivity on the ground-motion response throughout a broad region of the central United States (CUS) for these events. Our simulation set consists of 20 hypothetical earthquakes located along two faults associated with the current seismicity trends in the NMSZ. The hypothetical scenarios range in magnitude from M  7.0 to 7.7 and consider various epicenters, slip distributions, and rupture characterization approaches. The low-frequency component of our simulations was computed deterministically up to a frequency of 1 Hz using a regional 3D seismic velocity model and was combined with higher-frequency motions calculated for a 1D medium to generate broadband synthetics (0–40 Hz in some cases). For strike-slip earthquakes located on the southwest–northeast-striking NMSZ axial arm of seismicity, our simulations show 2–10 s period energy channeling along the trend of the Reelfoot rift and focusing strong shaking northeast toward Paducah, Kentucky, and Evansville, Indiana, and southwest toward Little Rock, Arkansas. These waveguide effects are further accentuated by rupture directivity such that an event with a western epicenter creates strong amplification toward the northeast, whereas an eastern epicenter creates strong amplification toward the southwest. These effects are not as prevalent for simulations on the reverse-mechanism Reelfoot fault, and large peak ground velocities (〉40 cm/s) are typically confined to the near-source region along the up-dip projection of the fault. Nonetheless, these basin response and rupture directivity effects have a significant impact on the pattern and level of the estimated intensities, which leads to additional uncertainty not previously considered in magnitude estimates of the 1811–1812 sequence based only on historical reports. The region covered by our simulation domain encompasses a large portion of the CUS centered on the NMSZ, including several major metropolitan areas. Based on our simulations, more than eight million people living and working near the NMSZ would experience potentially damaging ground motion and modified Mercalli intensities ranging from VI to VIII if a repeat of the 1811–1812 earthquakes occurred today. Moreover, the duration of strong ground shaking in the greater Memphis metropolitan area could last from 30 to more than 60 s, depending on the magnitude and epicenter. Online Material: Tables of 1D velocity models used to generate the high-frequency synthetics, and figures of source models and peak ground motion synthetics.