Kaye M. Shedlock

Near-surface structural model for deformation associated with the February 7, 1812, New Madrid, Missouri, earthquake

The February 7, 1812, New Madrid, Missouri, earthquake (M [moment magnitude] 8) was the third and final large-magnitude event to rock the northern Mississippi Embayment during the winter of 1811–1812. Although ground shaking was so strong that it rang church bells, stopped clocks, buckled pavement, and rocked buildings up and down the eastern seaboard, little coseismic surface deformation exists today in the New Madrid area. The fault(s) that ruptured during this event have remained enigmatic. We have integrated geomorphic data documenting differential surficial deformation (supplemented by historical accounts of surficial deformation and earthquake-induced Mississippi River waterfalls and rapids) with the interpretation of existing and recently acquired seismic reflection data, to develop a tectonic model of the near-surface structures in the New Madrid, Missouri, area. This model consists of two primary components: a north-northwest–trending thrust fault and a series of northeast-trending, strike-slip, tear faults. We conclude that the Reelfoot fault is a thrust fault that is at least 30 km long. We also infer that tear faults in the near surface partitioned the hanging wall into subparallel blocks that have undergone differential displacement during episodes of faulting. The northeast-trending tear faults bound an area documented to have been uplifted at least 0.5 m during the February 7, 1812, earthquake. These faults also appear to bound changes in the surface density of epicenters that are within the modern seismicity, which is occurring in the stepover zone of the left-stepping right-lateral strike-slip fault system of the modern New Madrid seismic zone.

74 – The GSHAP Global Seismic Hazard Map

This chapter summarizes the development and achievements of the GSHAP and the compilation of the Global Seismic Hazard Map. The Global Seismic Hazard Assessment Program (GSHAP) was designed to provide a useful global seismic hazard framework and to serve as a resource for national and regional agencies, by coordinating national efforts in multi-national regional-scale projects, by reaching a consensus on the scientific methodologies for seismic hazard evaluation and by ensuring that the most advanced methodologies are available worldwide through technology transfer and educational programs. Seismic hazard is defined as a probabilistic measure of earthquake ground shaking at a given location. The assessment of seismic hazard is the first step in the evaluation of the seismic risk, obtained by combining the seismic hazard with local site effects (anomalous amplifications tied to soil conditions, local geology, and topography) and with the vulnerability factors (type, value, and age of buildings and infrastructures, population density, land use, date and time of the day). The GSHAP Global Seismic Hazard Map is the first reference map for seismic hazard on a global scale, expressing the probability of ground shaking in a parameter of engineering interest (PGA), and the first obtained by the close collaboration of the scientists responsible for national seismic hazard zonations.

Delineation of intrabasin structure in a dilational jog of the San Jacinto Fault Zone, southern California

Three high-resolution seismic reflection lines were acquired in the northern part of the San Jacinto graben. The graben, a pull-apart basin formed by a dilational right step of the San Jacinto fault zone, has been previously interpreted as a simple rhombochasm. The reflection survey located at least one significant and previously unidentified intrabasin fault, referred to here as the Farm Road strand. This fault lies approximately halfway between the Claremont and Casa Loma strands of the San Jacinto fault zone. At the north end of the basin, the southwestern boundary of the graben is interpreted to be the newly identified Farm Road strand and not the Casa Loma strand as was previously thought. The identification of this intrabasin fault allows us to infer that the San Jacinto basin comprises coalescing subbasins and is not a simple pull-apart basin with an unusually large length:width ratio. The distances between the en echelon Casa Loma, Farm Road, and Claremont strands are between 1 and 2 km. This close spacing would likely permit an earthquake rupture to jump between strands and thus propagate through the San Jacinto basin.

Mini‐Sosie High‐Resolution Seismic Method aids hazards studies

A dramatic example of just how catastrophic earthquake damage can be occurred in 1989, when a nationally televised World Series game in San Francisco was preempted by the M 7.1 Loma Prieta earthquake. The surprising amount and distribution of damage reinforce the importance of seismic-hazard studies in urban areas, where potential for damage and loss of life is greatest. Unfortunately, many large urban centers developed before the advent of seismic-hazard microzonation mapping, ground-response building codes, and standardized air-photo reconnaissance. Decades of building, paving, and utility installation have modified the land surface to the extent that surficial geologic expression of faults and evidence of prehistoric earthquakes are either inaccessible or totally obliterated.

Toward a Comprehensive Catalog of Global Historical Seismicity

The United States Geological Survey (USGS) and the Colorado School of Mines (CSM) have initiated a project to locate more accurately all earthquakes recorded by instruments during the period 1900 to 1963. Seismicity for this period (hereafter referred to as historical seismicity, following Lee et al. [1988]) is still poorly understood, even for basic parameters such as earthquake locations (see Figure 1). In some cases this is the result of inherent limitations in the distribution, response characteristics, and timing of the instruments. However, locations for most of the pre-1964 earthquakes are poorly determined simply because modern data analysis techniques have yet to be applied to the available arrival-time observations, which are mainly preserved as printed bulletins and not in a computer-ready digital format. The arduous task of hand-entering these data has prevented the systematic analysis and relocation of historical seismicity.

Seismic surveys assess earthquake hazard in the New Madrid area

Identification of active faults and understanding their potential hazard to the population within the New Madrid seismic zone (NMSZ) is a major goal of the US National Earthquake Hazard Reduction Program (Nehrp). With primary funding provided by Nehrp, the NMSZ (Figure 1) located near the large population centers of Memphis and Saint Louis is targeted for earthquake hazards studies because of (1) its relatively high rate of earthquake activity (it is considered the highest seismic hazard zone in the US east of the Rocky Mountains) and (2) the occurrence of three major earthquakes (magnitude estimates 6.9–8.3) and thousands of accompanying aftershocks in this region in the years 1811–12.

California Earthquakes: Science, Risks, and the Politics of Hazard Mitigation

“Politics” should be the lead word in the sub-title of this engrossing study of the emergence and growth of the California and federal earthquake hazard reduction infrastructures. Beginning primarily with the 1906 San Francisco earthquake, scientists, engineers, and other professionals cooperated and clashed with state and federal officials, the business community, “ boosters,” and the general public to create programs, agencies, and commissions to support earthquake research and hazards mitigation. Moreover, they created a “regulatory-state” apparatus that governs human behavior without sustained public support for its creation. The public readily accepts that earthquake research and mitigation are government responsibilities. The government employs or funds the scientists, engineers, emergency response personnel, safety officials, building inspectors, and others who are instrumental in reducing earthquake hazards. This book clearly illustrates how, and why all of this came to pass.