NS21A-01 INVITED
Shallow Geophysical Characterization of the Mount St. Helens Edifice
Multiple Refraction Microtremor (ReMi) transects were completed on Mount St. Helens, Washington to obtain shear-velocity soundings valuable to the generation of a shallow velocity model for earthquake location. Prior to the 2004 eruption of Mount St. Helens, the 1-D velocity model used to locate earthquakes used a 4.6 km/s velocity to depths of 2.2 km. Given the volume of pyroclastic flows and altered dacite domes exposed in the crater walls, the velocity model was overestimating the shallow velocities. The result was reduced, or no depth constraint for shallow earthquakes preceding the 2004 eruption, which greatly hindered early interpretations. Surface-wave sources included off-end, shallow (<1 km) volcanic earthquakes and rockfalls from the over-steepened horseshoe crater. Transects with 15-20 channels and transect lengths of 300-400m constrained shear velocities in excess of 100 m. These transects were used on erosional scarps and in areas with known geology to test the precision of the ReMi technique in a volcanic setting. Our transects sampled andesite, basalt, pyroclastic flows, landslide deposits, and dacitic domes. The correlation between the mechanical strength of the observed geology on erosional scarps, and modeled shear-velocity structure was generally good. A 36 channel, 4 km long transect was conducted, with a combination of 1 Hz and 4.5 Hz sensors, to determine a suitable velocity model for locating shallow earthquakes. Results show that the transect constrained shear velocities to depths below 2 km depth. The averaging quality of surface-wave techniques is advantageous for the construction of 1-D velocity models for earthquake location since a velocity model averaged over the entire edifice reduces the use of large static station corrections at any one station. Indeed, when the surface-wave derived velocity model is applied to shallow earthquakes at Mount St. Helens, the station corrections decrease and the depth errors are improved. This application of surface-wave dispersion with passive sources is potentially applicable on volcanoes worldwide, where accurate velocity models are lacking. An additional utility of this technique may be in the use of landslide hazard studies, where potentially weak rock is obscured below the surface. ReMi data collected on a hazardous volcano could be combined with geologic observations and geotechnical testing to provide a more thorough analysis of edifice stability.
NS21A-02
Near-Surface Factors Affecting Earthquake Ground-Motion Amplification Due to Basin Edges: Modeling of the Haifa Urban Basin, Israel
A fault-bounded basin up to 2 km deep underlies the low-elevation sections of the city of Haifa. Extensive seismic- reflection sections and an unusually dense H/V data set of single-station microtremor recordings define basin geometry and stratigraphy in detail and allow constraint of shear velocities. This basin appears as a contrast in shear velocity of 2:1 against surrounding older sedimentary rocks, even in its deeper reaches. We conducted 2-D and 3-D finite-difference modeling studies of the basin using S. Larsen's E3D code up to frequencies of 6 Hz. Our modeling included sensitivity studies using simple cross sections, and comparisons of 2-D results against 1-D modeling conducted in ProShake. A prominent 2-D effect not seen in the 1-D modeling is a narrow zone of high PGV amplification a few hundred meters inside the basin from its edges, which also appears in spectral accelerations at shorter periods. Spectral accelerations at longer periods are more amplified where the basin is deeper. Tracking wave propagation within the synthetic sections, this "basin-edge effect" appears where two wave phases interfere constructively: 1) the Rayleigh wave propagating slowly horizontally into the basin from the basin's edge, after conversion from the upgoing S wave outside the basin, at its edge; and 2) the upgoing S wave within the basin, delayed by the low shear velocities in the basin. This effect is not related to any upward refraction or propagation along the (often) steep wall of the basin, nor to any 1-D or 2-D resonances. The edge effect locates instead within the basin at the distance from the edge where the delay of slow, shallow Rayleigh-wave propagation equals the delay of the upgoing S wave in the basin, relative to the S arrival outside the basin. In Haifa, with 0.3 km/s shear velocities near the surface, the basin-edge effect locates to several hundred meters from the edge. With this explanation for the basin-edge effect, we surmise that it will appear only at periods T < t0, where t0 is the one-way vertical shear-wave travel time through the basin. Our synthetic spectral accelerations confirm this, with the basin-edge effect appearing only at periods T < 1.6 sec.
NS21A-03 INVITED
Ground-Truthing Seismic Refraction Tomography for Sinkhole Detection in Florida
In order to provide effective return of storm water runoff to the subsurface aquifer, the Florida Department of Transportation (FDOT) constructs detention basins adjacent to its transportation facilities. These basins serve as a collection point for runoff within a local drainage area, and the overburden soil above the aquifer provides a natural filter for contaminants in the surface runoff water. However, the geologic setting for many of these basins in Florida is karst, limestone bedrock at shallow depth, and the concentration of water flow in these basins leads to frequent development of sinkholes. These sinkholes are an environmental hazard, as they provide a direct, open conduit for contaminant-laden runoff water to return to the aquifer rather than percolate through the overburden soil. Consequently, FDOT is keenly interested in all aspects of sinkholes, including factors leading to formation, methods of early detection, and effective methods for rapid repair. Recently, FDOT has engaged in a research effort to evaluate the capabilities of a wide range of geophysical investigation tools with regard to detection of sinkhole-prone areas within sites being considered for construction of detention ponds. The geophysical techniques evaluated have included ground penetrating radar (GPR), multi- electrode electrical resistivity (MER), seismic MASW, and seismic refraction tomography. In addition to geophysical testing at the research sites, extensive traditional geotechnical site characterization has been conducted, including boring and sampling of soil and rock, standard penetration tests (SPT), and cone penetration tests (CPT). The proposed paper will evaluate the capabilities of seismic refraction tomography. Comparisons between refraction tomograms and borehole logs, SPT soundings, and CPT soundings suggest that the refraction method can map the laterally-variable top of bedrock surface typical of karst terrane. During a recent ground proving exercise at the University of North Florida/University of Florida karstic limestone geophysical/ground proving test site in central Florida, the limestone bedrock surface was mapped along several survey lines using both intrusive and geophysical techniques. Analyses of site data revealed a highly erratic limestone bedrock surface. Analysis of seismic refraction data demonstrated that the refraction tomography software system was able to reveal the undulating bedrock surface. However, the tomography data revealed marked differences in the compression wave velocities at the top of the bedrock surface at various locations along one of the survey lines. Compression wave velocities were highest within slots or valleys and lowest at the tops of blocks or pinnacles. Ground proving via cone penetration tests and geotechnical borings appears to corroborate this finding, and demonstrates the importance of measuring multiple material parameters during site characterization activities in complex terrane. Finally, two sinkholes formed in the detention pond directly over two valley/bowl features after refraction testing was completed, demonstrating that refraction tomography has potential in identifying areas at risk for sinkhole development.
NS21A-04
Simultaneous ReMi and MSASW Technique
This paper discusses a procedure to estimate shear wave velocity of a soil profile using a simultaneous Refraction Microtremor (ReMi) and Multi-channel Spectral Analysis of Surface Waves (MSASW) technique. MSASW and ReMi differ fundamentally in their recorded source signal type. MSASW is an active-source technique requiring signal from swept vibratory signal such as Vibroseis to generate surface waves. Vs profile is obtained from the Raleigh wave field generated by the source. Conversely, ReMi is a passive technique, recording ambient noise or microtremors being present in an urban environment. The shear wave velocity profile is obtained by identifying the fundamental mode Raleigh wave field within the microtremors. Advantage of using a simultaneous ReMi and MSASW technique is that one can obtain a much more accurate dispersion curve. This in turn will result in more accurate estimation of shear wave velocity. This paper will discuss the technique and will provide several experimental site results.
NS21A-05
A Microtremor Study in the New Madrid Seismic Zone
We present results of a microtremor study in the New Madrid seismic zone (NMSZ). The study utilizes Nakamura's method to investigate horizontal to vertical (H/V) spectral ratios across the central Mississippi embayment, in areas that experienced strong ground shaking and widespread soil liquefaction during the 1811 and 1812 earthquake sequence. Results from 20 sites show that embayment thicknesses ranging from 100 to 200 m and 500 to 900 m have fundamental periods ranging from 1.5 to 1.9 s and 3.0 to 4.1 s, respectively. The average shear-wave velocities for these two stratigraphic intervals are calculated at 325 m/s and 830 m/s, respectively. Most pronounced in the frequency spectra are peaks associated with a strong impedance contrast at the interface between embayment sediments and underlying basement rocks, which deepen towards the central embayment and to the south. The impedance contrast coupled with the basin configuration could contribute significantly to wave amplification at these fundamental periods. These results are consistent with similar studies conducted in other areas of the seismic zone.
NS21A-06 INVITED
The use of Near-surface Geophysics in Evaluating and Assessing Natural Hazards
The list of natural hazards that transform the physical environmental is extensive: earthquakes, tsunamis, floods, volcanoes, lahars, landslides and debris flows, avalanches, karst/cavern collapse, heavy-metal contamination, permafrost, liquefaction, and magnetic storms. Because these events or conditions can have significant negative impact on health and infrastructure, the need for knowledge about and education of natural hazards is important. Near-surface geophysics can contribute in significant ways to both the knowledge base and wider understanding of these hazards. The discipline encompasses a wide range of methodologies, some of which are described below. A post-tsunami helicopter electromagnetic (EM) survey along the coasts of Aceh, northern Sumatra was used to discriminate between fresh-water and saltwater aquifers., saltwater intrusion occurred close to the coast as a result of the tsunami and deep saltwater occurrences particularly around 30 m depth were mapped up to several kilometers inland. Based on the survey results recommendations were made to locate shallow hand-dug wells and medium depth (60m) water wells. Utilizing airborne EM and magnetic measurements, a detailed assessment of the internal distribution of altered zones within an active volcano; Mount Rainier (NW USA) showed that alteration is much more restricted than had been inferred from surficial exposures alone. The study also suggested that the collapse of fresh, unaltered portions of the volcano is possible, and no flank of the volcano can be considered immune from lahars during eruption. Ground penetrating radar (GPR) has been used worldwide in a variety of applications from geotechnical investigations related to geologic hazards. These include assessment of transportation infrastructure, which maybe be damaged due to a natural hazard, study of the movement of rock glaciers in the Swiss Alps, and search and recovery of avalanche victims. Permafrost is widespread in polar areas and cold mountain terrain. GPR, electrical resistivity and EM methods have been used successfully to map permafrost and massive ground ice. The stability of these materials has impact on building and development within these regions. Mass movements in lowland permafrost terrain, which have implications for climate change, are being monitoring with thermal borehole measurements. Whether in times of flood or draught, understanding our watersheds is an important use of near surface geophysics. Satellite-based remote sensing methods are used to efficiently obtain soil moisture measurements over large regions. Ground-based conductivity meters are used to map soil types that play a fundamental role affecting the pattern of stream flow response. Synthetic Aperture Radar (SAR) is used to directly measure surface deformation, which can be related to subsurface hydrological conditions, aseismic deformation or landslides. Resolution of fine as 2mm/year can be obtained from satellite-based measurements. This level of resolutions aids in seismic risk assessment and allows the extent of landslides to be mapped and monitored efficiently. A series of national probabilistic seismic shaking hazard maps are being produced by the US Geological Survey using gravity, magnetic and seismic data in addition to other information. They can be used as input for many policy decisions on building codes and land use, and to estimate the probabilities of strong earthquakes, detailed maps of shaking amplification and susceptibility to liquefaction and landslides, and planning scenarios of large urban earthquakes.
NS21A-07
On the Seismic Hazard of Areas with Archaeological Sites.
A methodological approach which allows the actual level of seismic hazard of areas with archaeological sites to be evaluated is proposed. The procedure consists of a seismic, geological and geomorphological characterization of the area in study and a subsequent analysis of the observed damage in the archaeological site, arranged on the basis of specific protocols of seismic diagnostics, for the evaluation of the seismic evidence index. This index gives a numerical modelling of the seismic character of the observed damage within a correlation between collapsed structures in the archaeological site and specific endogenous and exogenous elements of the same area. An evaluation of the coherence of the actual level of seismic hazard of the areas which archaeological sites fall within, was done through a comparison between the strong ground motion which determined the observed damage and that one taken from the seismic hazard maps of the same area. A test of the procedure has been performed in different archaeological sites in Eastern Sicily (Italy).
NS21A-08
U.S. states and territories national tsunami hazard assessment, historic record and sources for waves
In 2005, the U.S. National Science and Technology Council (NSTC) released a joint report by the sub-committee on Disaster Reduction and the U.S. Group on Earth Observations titled Tsunami Risk Reduction for the United States: A Framework for Action (Framework). The Framework outlines the President's&pstrategy for reducing the United States tsunami risk. The first specific action called for in the Framework is to "Develop standardized and coordinated tsunami hazard and risk assessments for all coastal regions of the United States and its territories." Since NOAA is the lead agency for providing tsunami forecasts and warnings and NOAA's National Geophysical Data Center (NGDC) catalogs information on global historic tsunamis, NOAA/NGDC was asked to take the lead in conducting the first national tsunami hazard assessment. Earthquakes or earthquake-generated landslides caused more than 85% of the tsunamis in the NGDC tsunami database. Since the United States Geological Survey (USGS) conducts research on earthquake hazards facing all of the United States and its territories, NGDC and USGS partnered together to conduct the first tsunami hazard assessment for the United States and its territories. A complete tsunami hazard and risk assessment consists of a hazard assessment, exposure and vulnerability assessment of buildings and people, and loss assessment. This report is an interim step towards a tsunami risk assessment. The goal of this report is provide a qualitative assessment of the United States tsunami hazard at the national level. Two different methods are used to assess the U.S. tsunami hazard. The first method involves a careful examination of the NGDC historical tsunami database. This resulted in a qualitative national tsunami hazard assessment based on the distribution of runup heights and the frequency of runups. Although tsunami deaths are a measure of risk rather than hazard, the known tsunami deaths found in the NGDC database search were compared with the qualitative assessments based on frequency and amplitude. The second method to assess tsunami hazard involved using the USGS earthquake databases to search for possible earthquake sources near American coastlines to extend the NOAA/NGDC tsunami databases backward in time. The qualitative tsunami hazard assessment based on the results of the NGDC and USGS database searches will be presented.