Klaus H. Jacob

The theoretical response of sedimentary layers to ambient seismic noise

For over thirty years, attempts have been made to gain information about sediment amplification during earthquakes from observations of ambient seismic noise. While the results of several feasibility studies have been encouraging, theoretical support for the technique is scant. We present a model for the response of sedimentary layers to ambient seismic noise. The noise sources are modeled as a random distribution (in time and space) of point forces located on the Earths free surface. This model is applied to a site where observed noise spectral ratios, relative to a rock site, have previously been shown to reveal the fundamental resonant frequency of a soft clay layer. Approximating the sediment site as a single layer over a half-space, the horizontal noise spectrum predicted by our model reveals the fundamental resonance and first harmonic of the layer. We also examine an estimate of site response proposed by Nakamura (1989), which is formed by dividing the horizontal-component noise spectrum by that of the vertical component. Nakamuras estimate applied to both observed and predicted noise-spectra was also successful in identifying the fundamental resonance, with a slight (<10%) shift toward lower frequencies. Future work is needed to determine the generality of our results, and to elucidate the influence of the simplifying assumptions.

Volcanoes as Possible Indicators of Tectonic Stress Orientation—Aleutians and Alaska

A new method for obtaining from volcanic surface features the orientations of the principal tectonic stresses is applied to Aleutian and Alaskan volcanoes. The underlying concept for this method is that flank eruptions for polygenetic volcanoes can be regarded as the result of a large-scale natural magmafracturing experiment. The method essentially relies on the recognition of the preferred orientation of radial and parallel dike swarms, primarily using the distribution of monogenetic craters including flank volcanoes. Since dikes tend to propagate in a direction normal to the minimum principal stress (T-axis), the method primarily yields the direction of the maximum horizontal compression (MHC) of regional origin. The direction of the MHC may correspond to either the maximum (P-axis) or intermediate (B-axis) principal stress.The direction of MHC obtained at 20 volcanoes in the Aleutian arc coincides well with the direction of convergence between the Pacific and North American plates. This result provides evidence that in the island arc the inferred direction of MHC is parallel to the maximum principal tectonic stress. In the back-arc region, general E-W trends of MHC are obtained from seven volcanic fields on islands on the Bering Sea shelf and the mainland coast of Alaska. These volcanic fields consist mostly of clusters of monogenetic volcanoes of alkali basalt. In the back-arc region, the trends of MHC may correspond to an E-W intermediate, a vertical maximum, and a N-S minimum principal stress.Implications for the tectonics of island arcs and back-arc regions are: (1) volcanic belts of some island arcs, including the Aleutian arc, are under compressional deviatoric stress in the direction of plate convergence. It is improbable that such arcs would split along the volcanic axis to form actively spreading marginal basins. (2) This compressional stress at the arc, probably generated by underthrusting, appears to be transmitted across the entire arc structure, but is apparently replaced within several hundred kilometers by a stress system characterized by horizontal extension (tensional deviatoric stress) in the back-arc region. (3) The volcanoes associated with these two stress systems differ in type (polygenetic vs. monogenetic) and in the chemistry of their magmas (andesitic vs. basaltic). These differences and the regional differences in orientation of the principal tectonic stresses suggest that the back-arc stress system has its own source at considerable depth beneath the crust.

Body wave and surface wave analysis of large and great earthquakes along the Eastern Aleutian Arc, 1923–1993: Implications for future events

The 500-km-long Alaska Peninsula and Shumagin Islands segments of the Aleutian arc have a moderate to high probability of rupturing in one or more large or great earthquakes in the next few decades. To understand the likely modes of rupture in the next sequence of large and great events and to delineate the current geometry of the plate interface, we determine focal mechanisms, depths, and source time functions from seismic records for the largest events since 1917: the great earthquake of November 10,1938, and seven events with surface wave magnitude (Ms) of 6.9 to 7.5. Teleseismic body waves and surface waves are used to estimate the seismic moment and gross rupture characteristics of the great earthquake of November 10,1938, along the Alaska Peninsula. Body wave inversion of five P and four SH waves gives a duration of about 110 s in which moment was released in two episodes, each of about 50 s duration, with the second being larger than the first. The first source was located in the general vicinity of the epicenter of the mainshock, and the second, which occurred about 60 s later, was centered about 180 km to the northeast. The body-wave-derived seismic moment is 3.7×1021 N m (Mw 8.3). We corroborate the body wave results by calculating surface waves by normal-mode summation and comparing them with data at periods greater than 50 s. An adequate fit to observed seismograms is obtained for either a single point source or two point sources, with one located about 180 km northeast of the mainshock epicenter. Rupture in 1938 appears to have been confined to the Alaska Peninsula segment; uniform rupture into the Shumagin region is not supported by the data. The Ms 6.9 earthquake of May 13,1993, ruptured a small portion of the Shumagin gap. The earthquake of May 14,1948, (Ms 7.5) occurred on a shallow dipping thrust fault with a depth of about 31 km, not 60 km as originally suggested. Five Ms ∼ 7 events in the 1938 rupture zone have locations, depths, and mechanisms that define a shallow dipping (16°–19°) plate interface which shallows to 8° under Kodiak Island. Subduction of seamounts of the Gulf of Alaska seamount province may explain the location, moment, focal mechanism, and depth of the five Ms ∼ 7 events. The plate interface dips nearly uniformly between the Alaska Peninsula and Shumagin segments, indicating that segmentation, if any, is not controlled by the orientation of the plate interface. A slight warp of the plate interface may form the boundary between rupture zones of great earthquakes along the Kodiak and Alaska Peninsula segments of the arc and may explain why rupture zones of large to great earthquakes rarely cross this tectonic boundary.

Tilt and seismicity changes in the Shumagin seismic gap

Changes in the ground surface tilt and in the rate of seismicity indicate that an aseismic deformation event may have occurred between 1978 and 1980 along the plate boundary in the eastern Aleutians, Alaska, within the Shumagin seismic gap. Pavlof Volcano was unusually quiescent during this period. The proposed event would cause an increase of stress on the shallow locked portion of the plate boundary, bringing it closer to rupture in a great earthquake.

New York City Panel on Climate Change 2015 Report Chapter 4: Dynamic Coastal Flood Modeling

Philip Orton,1,a Sergey Vinogradov,2,a Nickitas Georgas,1,a Alan Blumberg,1,a Ning Lin,3 Vivien Gornitz,4 Christopher Little,5 Klaus Jacob,6 and Radley Horton4 1Stevens Institute of Technology, Hoboken, NJ. 2Earth Resources Technology/National Atmospheric and Oceanic Administration, Silver Spring, MD. 3Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ. 4Columbia University Center for Climate Systems Research, New York, NY. 5Atmospheric and Environmental Research, Lexington, MA. 6Lamont-Doherty Earth Observatory, Palisades, NY.

Seismic waves generated by aircraft impacts and building collapses at World Trade Center, New York City

Seismologists sometimes do their work of data acquisition and analysis against a tragic background. Usually, the context is fieldwork far from home, in an area subjected to the natural but sometimes devastating effects of an earthquake. But in the present case, we are in our own New York City area; that is, the Lamont-Doherty Earth Observatory of Columbia University, in Palisades, N.Y; and the context is inhuman actions against people and the fabric of our society. As the appalling events of September 11 unfolded, we found that we had recorded numerous seismic signals from two plane impacts and building collapses of the two World Trade Center (WTC) towers, often at times different than those being reported elsewhere. Collapses of the two WTC towers generated large seismic waves, observed in five states and up to 428 km away The north tower collapse was the largest seismic source and had local magnitude ML 2.3. From this, we infer that ground shaking of the WTC towers was not a major contributor to the collapse or damage to surrounding buildings. But unfortunately, we also conclude that from the distance at which our own detections were made (the nearest station is 34 km away at Palisades) it is not possible to infer (with detail sufficient to meet the demands of civil engineers in an emergency situation) just what the near-in ground motions must have been.

Microearthquakes in the ahuachapan geothermal field, el salvador, central america.

Microearthquakes occur on a steeply dipping plane interpreted here as the fault that allows hot water to circulate to the surface in the geothermal region. These small earthquakes are common in many geothermal areas and may occur because of the physical or chemical effects of fluids and fluid pressure.

Chapter 7: Indicators and monitoring

A popular paradigm states: What cannot be measured cannot be managed. The programmatic and practical objectives of the city and members of the New York City Climate Change Adaptation Task Force are to develop Flexible Adaptation Pathways for the region’s critical infrastructure. These objectives will require ongoing and consistent monitoring of a set of climate change indicators. Monitoring of key indicators can help to initiate course corrections in adaptation policies and/or changes in timing of their implementation. The relevant indicators are related to changes in the climate, climate science, climate impacts, and adaptation activities. Thus, these indicators need to be devised and tracked over time to provide targeted quantitative measures of climate change impacts, and adaptation in order to provide useful information to decision makers in regard to timing and extent of adaptation actions.

Hazards and the built environment: attaining built-in resilience

This is an ambitious book well edited from solicited contributions by a wide range of authors covering the management of risks from natural hazards of the most common types of perils (typhoons, storms, floods and earthquakes), and even from planned terrorism. This book successfully brings together the current state of expert knowledge and the supporting facts. It breathes the spirit that it succinctly states by an aphoristic quote ascribed to president John F. Kennedy: ‘There are risks and costs to a program of action. But they are far less than the long-range risks and costs of comfortable inaction’. The book is a primer on sound disaster risk management, and motivator for a broad audience ranging from practising engineers, architects, employees of interand non-governmental organisations working on resilient and sustainable development of vulnerable communities; to policy makers, lawmakers and lawyers alike; economists; emergency and disaster risk managers; community activists; to academic teachers and their students of all the above disciplines and professions. Editions compiled of contributed papers, separately authored (for instance conference proceedings), suffer quite often from a lack of integration and coherence. They tend to leave gaps where topical coverage is needed to make a whole from an incomplete sum of their parts. In this case, the editor has made a thoughtful attempt, and I believe quite successfully, to make a universe out of diversity that reflects the real-life complexity of this multi-dimensional subject. This is not a textbook in engineering or of construction management, nor a textbook in the theory of sustainable development and community building. But it offers very thoughtful advise and insights into what constitutes good (and bad) practise for any and all professions involved in building communities that strive to be resilient to the perils of nature, i.e. of communities where extreme natural events can be experienced without turning each occurrence into a social disaster, over and over again. This timely and handy edition has four Parts broken down into a total of 17 Chapters, each chapter some 20 pages long, written by a total of 28 lead and contributing authors. Part I introduces, in two chapters, the need for built-in resilience (L. Bosher); for mainstreaming disaster risk management (D. Alexander). Part II addresses structural adaptation with seven contributing chapters that cover the range from good construction standards – or lack thereof, often in emerging economies – to outright latest engineering practise and architectural issues. Part III focuses on non-structural adaptation, meaning communityand policy-based, law-enhanced adaptation and risk management measures providing the social foundation for a hazard-resilient built environment. Part IV concludes with an afterword on integrating resilience into construction practise (A. Dainty and L. Bosher). Each chapter has its own list of references, and supporting graphs and tables. The book provides, for starts, an overview Table of

Did mud contribute to freeway collapse

At least 41 people were killed October 17 when the upper tier of the Nimitz Freeway in Oakland, Calif., collapsed during the Ms = 7.1 Loma Prieta earthquake. Seismologists studying aftershocks concluded that soil conditions and resulting ground motion amplification were important in the failure of the structure and should be considered in the reconstruction of the highway. Structural design weaknesses in the two-tiered freeway, known as the Cypress structure, had been identified before the tragedy. The seismologists, from Lamont Doherty Geological Observatory in Palisades, N.Y., and the U.S. Geological Survey in Menlo Park, Calif., found that the collapsed section was built on fill over Bay mud. A southern section of the Cypress structure built on alluvium of Quaternary age did not collapse (see Figure 1).