Charles S. Mueller

USGS National Seismic Hazard Maps

The U.S. Geological Survey (USGS) recently completed new probabilistic seismic hazard maps for the United States, including Alaska and Hawaii. These hazard maps form the basis of the probabilistic component of the design maps used in the 1997 edition of the NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures, prepared by the Building Seismic Safety Council and published by FEMA. The hazard maps depict peak horizontal ground acceleration and spectral response at 0.2, 0.3, and 1.0 sec periods, with 10%, 5%, and 2% probabilities of exceedance in 50 years, corresponding to return times of about 500, 1000, and 2500 years, respectively. In this paper we outline the methodology used to construct the hazard maps. There are three basic components to the maps. First, we use spatially smoothed historic seismicity as one portion of the hazard calculation. In this model, we apply the general observation that moderate and large earthquakes tend to occur near areas of previous small or moderate events, with some notable exceptions. Second, we consider large background source zones based on broad geologic criteria to quantify hazard in areas with little or no historic seismicity, but with the potential for generating large events. Third, we include the hazard from specific fault sources. We use about 450 faults in the western United States (WUS) and derive recurrence times from either geologic slip rates or the dating of pre-historic earthquakes from trenching of faults or other paleoseismic methods. Recurrence estimates for large earthquakes in New Madrid and Charleston, South Carolina, were taken from recent paleoliquefaction studies. We used logic trees to incorporate different seismicity models, fault recurrence models, Cascadia great earthquake scenarios, and ground-motion attenuation relations. We present disaggregation plots showing the contribution to hazard at four cities from potential earthquakes with various magnitudes and distances.

The 2014 United States National Seismic Hazard Model

New seismic hazard maps have been developed for the conterminous United States using the latest data, models, and methods available for assessing earthquake hazard. The hazard models incorporate new information on earthquake rupture behavior observed in recent earthquakes; fault studies that use both geologic and geodetic strain rate data; earthquake catalogs through 2012 that include new assessments of locations and magnitudes; earthquake adaptive smoothing models that more fully account for the spatial clustering of earthquakes; and 22 ground motion models, some of which consider more than double the shaking data applied previously. Alternative input models account for larger earthquakes, more complicated ruptures, and more varied ground shaking estimates than assumed in earlier models. The ground motions, for levels applied in building codes, differ from the previous version by less than ±10% over 60% of the country, but can differ by ±50% in localized areas. The models are incorporated in insurance rates, risk assessments, and as input into the U.S. building code provisions for earthquake ground shaking.

Seismic Hazard in Hawaii: High Rate of Large Earthquakes and Probabilistic Ground-Motion Maps

The seismic hazard and earthquake occurrence rates in Hawaii are locally as high as that near the most hazardous faults elsewhere in the United States. We have generated maps of peak ground acceleration (PGA) and spectral acceleration (SA) (at 0.2, 0.3 and 1.0 sec, 5% critical damping) at 2% and 10% exceedance probabilities in 50 years. The highest hazard is on the south side of Hawaii Island, as indicated by the M I 7.0, M S 7.2, and M I 7.9 earthquakes, which occurred there since 1868. Probabilistic values of horizontal PGA (2% in 50 years) on Hawaiis south coast exceed 1.75 g . Because some large earthquake aftershock zones and the geometry of flank blocks slipping on subhorizontal decollement faults are known, we use a combination of spatially uniform sources in active flank blocks and smoothed seismicity in other areas to model seismicity. Rates of earthquakes are derived from magnitude distributions of the modern (1959–1997) catalog of the Hawaiian Volcano Observatorys seismic network supplemented by the historic (1868–1959) catalog. Modern magnitudes are M L measured on a Wood-Anderson seismograph or M S. Historic magnitudes may add M L measured on a Milne-Shaw or Bosch-Omori seismograph or M I derived from calibrated areas of MM intensities. Active flank areas, which by far account for the highest hazard, are characterized by distributions with b slopes of about 1.0 below M 5.0 and about 0.6 above M 5.0. The kinked distribution means that large earthquake rates would be grossly underestimated by extrapolating small earthquake rates, and that longer catalogs are essential for estimating or verifying the rates of large earthquakes. Flank earthquakes thus follow a semicharacteristic model, which is a combination of background seismicity and an excess number of large earthquakes. Flank earthquakes are geometrically confined to rupture zones on the volcano flanks by barriers such as rift zones and the seaward edge of the volcano, which may be expressed by a magnitude distribution similar to that including characteristic earthquakes. The island chain northwest of Hawaii Island is seismically and volcanically much less active. We model its seismic hazard with a combination of a linearly decaying ramp fit to the cataloged seismicity and spatially smoothed seismicity with a smoothing half-width of 10 km. We use a combination of up to four attenuation relations for each map because for either PGA or SA, there is no single relation that represents ground motion for all distance and magnitude ranges. Great slumps and landslides visible on the ocean floor correspond to catastrophes with effective energy magnitudes M E above 8.0. A crude estimate of their frequency suggests that the probabilistic earthquake hazard is at least an order of magnitude higher for flank earthquakes than that from submarine slumps.

The Influence of Maximum Magnitude on Seismic-Hazard Estimates in the Central and Eastern United States

The purpose of this erratum is to correct several mistakes and to clarify the rationale for the analysis presented in Mueller (2010). These corrections and clarifications do not affect the quantitative results of the maximum-magnitude (mmax) sensitivity analysis that was presented in the article. In two places (first paragraph of the EPRI/SOG and USGS mmax Distributions for Two CEUS Sites section and last paragraph of the Summary and Discussion section) the author incorrectly states that the U. S. Nuclear Regulatory Commission (NRC) “required” updates to the mmax distributions in the application for a site. In fact, the NRC “requested” additional information about the updated mmax distribution used by the South Texas combined-license applicant as part of NRC’s ongoing review. The applicant recently (March 2010) replied to the request for additional information, and the NRC is currently reviewing the response. The author acknowledged support by the NRC (Mueller, 2010; acknowledgements section). More correctly, the work wassupportedbytheNRC’sOfficeofNuclearRegulatoryResearch,whichsupportscollaborative,long-rangeresearchthat is designed to inform future assessments of seismic hazards. This support does not imply endorsement of the results of the study by the NRC, especially in the context of seismic-hazard assessments at specific sites. The Electric Power Research Institute/Seismicity Owners Group (EPRI/SOG) mmax data from two nuclear power plantsiteswereusedtoinform thechoicesofalternativemodels for the sensitivity study. One of the sites was still under review by the NRC at the time of publication. The EPRI/ SOG mmax models are complex and difficult to summarize; broad distributions are used to reflect diverse approaches and large uncertainties. The reader should expect to be convinced that the range of alternative mmax models used in the analysis is reasonable, and scientific discussion is often improved through the use of site-specific examples. The two sites chosen represented two mmax “types” that helped to define the range of alternative models. The primary rationale for Mueller (2010) is to present the objective results of themmax sensitivity analysis. As noted in the article, there are other modeling differences that can obscure direct comparisons between U. S. Geological Survey (USGS)–based and EPRI/SOG-based hazard estimates. The article should not be read as expressing any preference for one mmax approach over another. This topic is beyond the scope of a sensitivity study (and the author resisted the suggestions of some reviewers to take the article more strongly in this direction). In particular, the last two paragraphs of Mueller(2010)illustrate thekindsofdifficultiesthatcanarise in defining—and especially in updating—seismic-hazard models. The final sentence is not meant to imply a preference for any particular mmax approach or model.

Southern surface rupture associated with the 1992 M7.4 Landers Earthquake: Did it all happen during the mainshock?

Approximately three minutes after the magnitude 7.4 Landers mainshock on 28 June 1992, a M5.7 aftershock occurred south of the mainshock epicenter, with a location of 34° 7.65′N, 116° 23.82′W. This aftershock was recorded on an array of portable digital seismic instruments deployed in Morongo Valley, 21 km southwest of the event. Although peak accelerations are found to differ by approximately 50% at stations with similar general site conditions within 500 m of each other, there is good coherence of arrivals across the array for frequencies ≤1 Hz. We use the recordings to determine the apparent phase velocity and azimuth of propagation across the array, and show that the event clearly ruptured to the south, with a rupture length of ∼11 km and a rupture velocity of approximately 3.0 km/s. Our results suggest that at least some of the mapped surface rupture south of the town of Yucca Valley (∼11 km in extent; maximum displacement of 20 cm) may have been associated with this aftershock. If this is the case, then the fault that produced the southern end of the Landers mainshock (the Johnson Valley fault) need not continue at depth across the active left-lateral, east-west trending Pinto Mountain fault.

Seismic Hazard Maps for Haiti

We have produced probabilistic seismic hazard maps of Haiti for peak ground acceleration and response spectral accelerations that include the hazard from the major crustal faults, subduction zones, and background earthquakes. The hazard from the Enriquillo-Plantain Garden, Septentrional, and Matheux-Neiba fault zones was estimated using fault slip rates determined from GPS measurements. The hazard from the subduction zones along the northern and southeastern coasts of Hispaniola was calculated from slip rates derived from GPS data and the overall plate motion. Hazard maps were made for a firm-rock site condition and for a grid of shallow shear-wave velocities estimated from topographic slope. The maps show substantial hazard throughout Haiti, with the highest hazard in Haiti along the Enriquillo-Plantain Garden and Septentrional fault zones. The Matheux-Neiba Fault exhibits high hazard in the maps for 2% probability of exceedance in 50 years, although its slip rate is poorly constrained.

Seismic Source Characterization for the 2014 Update of the U.S. National Seismic Hazard Model

We present the updated seismic source characterization (SSC) for the 2014 update of the National Seismic Hazard Model (NSHM) for the conterminous United States. Construction of the seismic source models employs the methodology that was developed for the 1996 NSHM but includes new and updated data, data types, source models, and source parameters that reflect the current state of knowledge of earthquake occurrence and state of practice for seismic hazard analyses. We review the SSC parameterization and describe the methods used to estimate earthquake rates, magnitudes, locations, and geometries for all seismic source models, with an emphasis on new source model components. We highlight the effects that two new model components—incorporation of slip rates from combined geodetic-geologic inversions and the incorporation of adaptively smoothed seismicity models—have on probabilistic ground motions, because these sources span multiple regions of the conterminous United States and provide important additional epistemic uncertainty for the 2014 NSHM.

Seismic hazard in the Nation's breadbasket

The USGS National Seismic Hazard Maps were updated in 2014 and included several important changes for the central United States (CUS). Background seismicity sources were improved using a new moment-magnitude-based catalog; a new adaptive, nearest-neighbor smoothing kernel was implemented; and maximum magnitudes for background sources were updated. Areal source zones developed by the Central and Eastern United States Seismic Source Characterization for Nuclear Facilities project were simplified and adopted. The weighting scheme for ground motion models was updated, giving more weight to models with a faster attenuation with distance compared to the previous maps. Overall, hazard changes (2% probability of exceedance in 50 years, across a range of ground-motion frequencies) were smaller than 10% in most of the CUS relative to the 2008 USGS maps despite new ground motion models and their assigned logic tree weights that reduced the probabilistic ground motions by 5–20%.

Seismic hazard in the eastern United States

The U.S. Geological Survey seismic hazard maps for the central and eastern United States were updated in 2014. We analyze results and changes for the eastern part of the region. Ratio maps are presented, along with tables of ground motions and deaggregations for selected cities. The Charleston fault model was revised, and a new fault source for Charlevoix was added. Background seismicity sources utilized an updated catalog, revised completeness and recurrence models, and a new adaptive smoothing procedure. Maximum-magnitude models and ground motion models were also updated. Broad, regional hazard reductions of 5%–20% are mostly attributed to new ground motion models with stronger near-source attenuation. The revised Charleston fault geometry redistributes local hazard, and the new Charlevoix source increases hazard in northern New England. Strong increases in mid- to high-frequency hazard at some locations—for example, southern New Hampshire, central Virginia, and eastern Tennessee—are attributed to updat...

Seismic Hazard in the Intermountain West

The 2014 national seismic-hazard model for the conterminous United States incorporates new scientific results and important model adjustments. The current model includes updates to the historical catalog, which is spatially smoothed using both fixed-length and adaptive-length smoothing kernels. Fault-source characterization improved by adding faults, revising rates of activity, and incorporating new results from combined inversions of geologic and geodetic data. The update also includes a new suite of published ground motion models. Changes in probabilistic ground motion are generally less than 10% in most of the Intermountain West compared to the prior assessment, and ground-motion hazard in four Intermountain West cities illustrates the range and magnitude of change in the region. Seismic hazard at reference sites in Boise and Reno increased as much as 10%, whereas hazard in Salt Lake City decreased 5–6%. The largest change was in Las Vegas, where hazard increased 32–35%.