PEER has just published Report No. 2014/15 titled “NGA-East Regionalization Report: Comparison of Four Crustal Regions within Central and Eastern North America using Waveform Modeling and 5%-Damped Pseudo-Spectral Acceleration Response” as a new addition to the PEER Report Series. It was authored by Jennifer Dreiling, Marius P. Isken, Walter D. Mooney, Martin C. Chapman, and Richard W. Godbee.
An important aspect of the Next Generation Attenuation for Central and Eastern North America (NGA-East) project led by the Pacific Earthquake Engineering Research Center (PEER) entails assigning seismic wave attenuation values to major crustal regions. In this study, Central and Eastern North America (CENA) is subdivided into four regions based on the geologic and tectonic setting. The regions are the Central North America (CNA), the Appalachian Province (APP), the Atlantic Coastal Plain (ACP), and the Mississippi Embayment/Gulf Coast region (MEM). Each region is described by a statistically representative crustal seismic velocity-depth structure and Q-factor model. The crustal structure models are for very hard rock conditions and do not include any sediments. The four regions are shown in the figure below. The largest region is Central North America (CNA) and the others are, following a clockwise order, the Appalachian Province (APP), the Atlantic Coastal Plain (ACP), and the Mississippi Embayment/Gulf Coast region (MEM).
The purpose of this study was to evaluate similarities and differences in attenuation for these regions and to assess whether regions needed to remain separate or if they could be grouped based on their attenuation properties. This was achieved through a series of ground motion simulations. Seismic wave propagation was simulated for earthquakes at focal depths of 5, 10, 20, and 30 km, using two different ground motion simulation codes. Synthetic time series and the 5% damped pseudo-absolute response spectral acceleration (PSA) provide insight into the attenuation of ground motions that are typical for each region. The calculated PSA covers a hypocentral distance range of 7.5–500 km and oscillator frequencies ranging from 0.5 to 20 Hz. Spectral accelerations were compared both within and between regions.
The CNA is the biggest region geographically and offers the largest variety of crustal seismic velocity-depth structures associated with the unique geologic evolution of its sub-regions. We define CNA as our base region and use it for both comparisons and to estimate a reference range of within-region variability. After generalizing the 417 profiles available for CNA into one representative profile (CNARep), ground motions were calculated for the four aforementioned focal depths. The within-region variability was also assessed using ground motion simulations for a selected set of 18 alternative velocity models developed for the region (CNAAlt). We compared the PSA calculated for CNARep to the PSA values for the 18 alternative crustal structures, CNAAlt. We find that the representative crustal structure for CNA is reasonable based on the observation that the PSA matrix for CNARep and the mean PSA matrix of CNAAlt are almost identical. The range of uncertainties of ground motions for CNA may be estimated using the standard deviations of CNAAlt, which are dependent on the frequency and distance to the source as well on the focal depth of the event. While the standard deviations are relatively small for closer hypocentral distances, they increase at a distance of 45–85 km, which is an effect due to the time and space variations in arrival of strong Moho reflections.
To determine which of the four regions should be assigned to a common attenuation group, we compared the ACP, APP, and MEM regions to the CNA base region. Statistical distributions (histograms) of the PSAs for specific distance and frequency bands were used to show if there were significant differences between the regions. Additional analysis tools, such as moving window average of PSA versus distance for specific frequency bands, were also used in these comparisons.
This analysis demonstrates that there are two distinct attenuation groups:
- • GROUP 1: Central North America, Appalachians, Atlantic Coastal Plain
- • GROUP 2: Mississippi Embayment/Gulf Coast
We found that the seismic velocity structure of the crust, rather than the Q-factor, has the largest effect on the attenuation of ground motions for the earthquake-to-source distances considered here. The PSA values for the CNA and APP regions look very similar for all four focal depths. Their representative PSAs are highly comparable and only at larger distances from the epicenter do the values show significant differences. However, these differences are well within the range found in the CNA base region itself (i.e., within the range of possible ground motions for CNA). Thus, a clear majority of the PSAs for the APP region fits CNA’s ground motions or fall within its range of variability.
The PSA values for the ACP region are also very similar to the PSA values of the CNA representative model. This applies to all focal depths except the 20 km source depth, which was excluded because a layer boundary at 20.5 km produces unrealistically strong reflections for this source depth. Thus, the ACP region belongs to the same attenuation group as the CNA region, but with a statistical agreement that is somewhat lower than for the APP region.
The MEM region was found to clearly belong to a separate attenuation group. This result is in agreement with previous analyses that have found that the MEM region has unique attenuation characteristics.