NS11D-0788
Bedrock topography in a buried karstic area by applying multielectrode measurements completed with "pricking probe"
Formation of surface depressions is a significant geological hazard. Prediction of future sinkholes in buried karstic areas needs a knowledge about the subsurface. In order to determine the varying topography of the karstifiable bedrock (in Bakony Mts, Hungary), we carried out multielectrode measurements, but among the hard field conditions the bedrock depth could not be reached. The resistivity anomalies in some places had a seasonal variation (low-resistivity in springtime, high-resistivity in the end of summer), therefore we interpreted the resistivity lows as indicators of locations with high water content. At the same time, when pushing the current- and potential electrodes into the ground, we discovered a regularity in the areal distribution of the soil's rock debris content. Therefore we carried out a systematic electrode-pricking experiment, and categorised the soil's "toughness" as 0, 0.5 or 1 (corresponding to soft penetration, scratching or blockage within the upper 30 cm). We have found a relationship between the locations of resistivity- and the soil's toughness extrema. From some epikarstic features we think that high "pricking probe" values indicate smaller depths of the intact bedrock. The corresponding resistivity minima may indicate originally intact horsts of the carbonate rock, either already collapsed or going to be collapsed.
NS11D-0789
A long-term monitoring of resistivity variation at the Wushangting mud volcano site in Southwestern Taiwan
We used a two-dimensional (2D) time-lapse electrical resistivity method in monitoring the activities of mud volcanoes. The mud volcano is a geomorphic feature formed by gases and fluid gushing through fault fissures at the mudstone area. The extensively eruption of fluid in mud volcanoes in the Yanchao area of southwestern Taiwan reveals the activities of the active Chishan Fault. However, there are no direct evidence showing the relationships between erupted gas volumes and fault activities. Through the time-lapse resistivity monitoring in the area, we hope to provide useful information to evaluate the fault activities. Our in-field monitoring site is located at the Wushangting Preservation Zone, which are on the Chishan Fault line in Yanchao. The measuring period is from July 2006 to May 2007, measuring frequency is once a week in the first month and is decreased to about once a month in the following months. The average resistivity at the research site is between 3.36 to 9.43 Ohm-m. During the period, the major changes of resistivity are located between the surface and a depth of 3-m. On December 26th, 2006, three earthquakes occurred as high as 6.7, 6.4, 5.2 at the Richter scale outside the sea of PingTung county, about 100 km southwest from the monitoring site. After the earthquakes, the resistivity is found to be raised up 2 to 5 Ohm-m between the surface and a depth of 3-m. There are a lot of reasons that may cause the changes of resistivity, for example, the temperature, the humidity, the earthquake activities and its subsequent influence, i.e., gas or fluid emission from the subsurface. After examine the weather records during the monitoring period, we suggest that the decrease of resitivity in the monitoring site is most likely to be the subsequent influence of the earthquake activities. Currently, we are continuing the resistivity monitoring surveys and hope to provide more data in order to be compared with the previous observation records.
NS11D-0790
3-D Resistivity Tomography for Cliff Stability Study at the D-Day Pointe du Hoc Historic Site in Normandy, France
The D-Day invasion site at Pointe du Hoc in Normandy, France is one of the most important World War II battlefields. The site remains today a valuable historic cultural resource. However the site is vulnerable to cliff collapses that could endanger the observation post building and U.S. Ranger memorial located just landward of the sea stack, and an anti-aircraft gun emplacement, Col. Rudder's command post, located on the cliff edge about 200 m east of the observation post. A 3-D resistivity tomography incorporating extreme topography is used in this study to provide a detailed site stability assessment with special attention to these two buildings. Multi-electrode resistivity measurements were made across the cliff face and along the top of the cliff around the two at-risk buildings to map major subsurface fracture zones and void spaces that could indicate possible accumulations and pathways of groundwater. The ingress of acidic groundwater through the underlying carbonate formations enlarges pre-existing tectonic fractures via limestone dissolution and weakens the overall structural integrity of the cliff. The achieved 3-D resistivity tomograms provide diagnostic subsurface resistivity distributions. Resistive zones associated with subsurface void spaces have been located. These void spaces constitute a stability geohazard as they become significant drainage routes during and after periods of heavy rainfalls.
NS11D-0791
Using Artificially Generated Electromagnetic Signals, Produced by the High Frequency Active Auroral Research Program (HAARP) Full Transmitter Array, for Geophysical Probe Waves.
Field measurements were made to test the feasibility of using HAARP stimulated ionospheric emissions of Very Low Frequency (VLF) electromagnetic signals, as probe waves for geophysical studies. The test site chosen for the experiment was a mud volcano near Tolsona, Alaska on the Glenn Highway. Data acquisition was done with a Zonge GDP-32 Receiver, magnetic coils, and electric lines provided by UAF. HAARP was run at full power and modulated at varying frequencies in the VLF range at frequencies of: 1, 4, 8, 16, 32, 96, 4096 and 6144 Hz. Both natural and induced VLF signals were recorded, with the intention of seeing a stronger signal at HAARP-generated frequencies upon examination of the data. Initial analysis indicates HAARP- generated VLF signals have the signal strength necessary for use as probe waves in geophysical field studies. However it is also clear that power lines near the site had minor interference with the data and therefore further analysis is needed before any direct claims can be made. This experiment was part of and made possible by the PARS 2007 Summer School.
NS11D-0792
Detection of Wide-area Clustal Movement By DInSAR Using L-band SAR Data
There are many active volcanoes and active faults over Kyushu, which is the southwest island in Japan. Kyusyu area has the stress field, which tends to develop the normal faults. These crustal features are different from those of other region in Japan. Additionally, the natural hazards accompanied with geomorphic changes like landslides and debris flows occur at many places in Kyushu region. In order to learn the critical tendency of the crustal deformations in Kyushu, it is needed to investigate the amount of the geomorphic changes all over this area widely. The DInSAR is the powerful technique to measure the geomorphic changes at a time. Usually the target of DinSAR measurement is the local crustal movement in one scene, but the area of one scene is too narrow for covering all Kyushu island area. This study discusses the detection of the wide-area crustal movement in Kyushu area by DInSAR using JERS-1/SAR and ALOS/PALSAR data. The differential interferograms in 5 paths around Kyushu main land could be connected with the short-interval data pairs.
NS11D-0793
A Mystery Unraveled: Booming Sand Dunes
"Booming" sand dunes have intrigued travelers and scientist for centuries. These dunes emit a persistent, low-frequency sound during a slumping event or a natural avalanche on the leeward face of the dune. The sound can last for several minutes and be audible from miles away. The resulting acoustic emission is characterized by a dominant audible frequency (70 - 105 Hz) and several higher harmonics. \par In the work of Vriend et al. (2007), seismic refraction experiments proved the existence of a multi-layer internal structure in the dune that acts as a waveguide for the acoustic energy. Constructive interference between the reflecting waves enables the amplification and sets the frequency of each boom. A relationship was established that correctly predicts the measured frequency in terms of the thickness (~ 2.0 m) and the seismic body wave velocity of the loose, dry surficial layer (~ 240 m/s) and the substrate half-space (~ 350 m/s). \par The current work highlights additional measurements and simulations supporting the waveguide model for booming sand dunes. Experiments with ground penetrating radar continuously display the subsurface features which confirm the layered subsurface structure within the dune. Cross-correlation analysis shows that the booming sound propagates at speeds close to the measured body wave velocity. Squeaking sounds, which are generated during the onset of the slide and precede the sustained booming emission, have been found to have distinctly different characteristics. These short bursts of sound are emitted at a lower frequency (50 - 65 Hz) and propagate at a lower propagation speed (125 m/s) than the booming emission. \par The acoustic and elastic wave propagation in the dune has been simulated with a finite difference code. The interaction between the air and the ground produces a coupling wave along the surface. The reflections in the surficial layer propagate in a dispersive band at a group velocity that is slower than the phase velocity of the layer. Different source mechanisms are used in order to simulate the squeaking and booming emission within the dune. \vspace{.1in} \noindent{\footnotesize Vriend, N. M., M. L. Hunt, R. W. Clayton, C E. Brennen, K. S. Brantley, and A. Ruiz-Angulo (2007), Solving the mystery of booming sand dunes, Geophys. Res. Lett., 34, L16306, doi:10.1029/2007GL030276.} http://www.its.caltech.edu/~nmvriend/research/
NS11D-0794
Seismic Refraction Mapping of the K-T Boundary Complex Near the Brazos River, Falls County, Texas
The relationship between the Chicxulub impact and the paleontologically defined boundary between the Cretaceous and Tertiary periods is a hotly debated subject. The K-T complex, with its distinctive yellow spherule- rich clay layer, is capped by sandstone beds in streambed exposures along the Brazos River in Falls County, TX. Studies of the microfossils in the Falls Co. location show that the paleontological K-T boundary is stratigraphically removed from the yellow clay layer associated with Chicxulub. This study is an attempt to construct a 3-D image of the top of "event complex" by the use of geophysical methods. A near surface seismic refraction survey was completed in an area near the exposed K-T sections. Our seismic images may shed some light on competing theories as to the depositional environment at the time the sandstone bed was formed. Important seismically-resolved topographic features such as undulations and discontinuities could play a large role in shaping a better understanding of the K-T boundary complex origins.
NS11D-0795
Characterization of Active Faults in the Algarve
The Carcavai fault zone is a NE-SW trending outcropping structure extending between S. Brás de Alportel (North) and Quarteira (south), with a total length of about 20 km, presenting reverse fault geometry with left-lateral strike- slip component. It is located close to the city of Loulé, in the Algarve region, southern Portugal. The fault zone has a complex surface course with two different sectors. The northern sector is characterized by several faults with NE-SW to E-NE-E-SW segmented by NW-SE trending faults. In the southern sector, trending NE-SE, there are several evidences of neotectonic deformation in sediments of Pio-Quaternary age. It is therefore considered to be an active fault zone and a probable source of the 1856 Loulé earthquake (intensity of VIII -MM). The study of this structure is of great importance for a proper evaluation of seismic hazard of the region. The location of some sectors of the fault zone is still to be confirmed, as well as the vertical offsets of the structure. In order to estimate these parameters, segment lengths and study the geometry of the fault zone at depth, geophysical data were acquired, as well as new geological data. In areas where the location of the fault was more uncertain, EM and seismic reflection profiles with a coarse spatial sampling were carried out. After the detailed location of the fault zone, and in places where geological evidence suggested the presence of the structure, seismic reflection profiles with a tighter spatial resolution were acquired. A standard processing flow was applied to the data and the refraction interpretation of the first breaks was also performed. The integrated interpretation of the geological and geophysical data confirmed the presence of a large reverse fault zone. The total fault length is still unknown since its prolongation offshore is still being studied. Together with vertical offsets estimates obtained, this data set will allow an improvement of the seismic hazard of the area, namely providing more refined estimates of co-seismic rupture, maximum expected earthquake and return periods.
NS11D-0796
Classification of earthquake site effects by shallow reflection seismics using a shear-wave land-streamer system
Touched in the SW by the Great Sumatra Fault, the densely populated delta of the Krueng Aceh River consists mainly of young alluvial sediments of clay, sand and gravel with partially high organic content. The depth of this sediment body and its internal structure are widely unknown. Whereas traditional timber constructed buildings are mostly unaffected by strong earthquakes, the change to concrete building techniques added a significant new and locally unknown seismic risk in this region. The classification of earthquake site effects in the city of Banda Aceh and the surrounding region of Aceh Besar was the aim of a high-resolution shear-wave reflection seismic survey in the Indonesian province Nanggroe Aceh Darussalam. In cooperation with the Government of Indonesia and local counterparts, this was part of the Project "Management of Georisk" of the Federal Institute for Geosciences and Natural Resources. Using shear-wave reflection seismics in combination with a land streamer has proven to be an enormously useful method in the sedimentary regions of the Aceh province with an easy and fast recording operation. In addition, the specialized seismic system accounts for compacted soil surfaces which allows a wide range of applications within cities, industrial sites, paved roads and also on small dirt roads. Using a vibrator seismic source, this technique was applied successfully also in areas of high building density in the city of Banda Aceh or in the surrounding mostly agricultural environment. Combined with standard geoengineering investigations like cone penetrometer tests, it was possible to evaluate the soil stiffness in populated urban areas down to 100 m depth in terms of the IBC2003. This is important for the exploration of new areas for save building foundation and groundwater aquifer detection in the tsunami-flooded region.
NS11D-0797
Shear-Wave Structure Model of Venetian Plane, Italy, From Seismic Noise Measurements
We have measured fundamental mode Rayleigh wave group velocity dispersion from noise cross correlation between 2 stations. We focused on near surface shear wave structure, with short inter-distance and short time acquisition. The aim of the study was the use of seismic noise to define a realistic upper subsoil structure model, in order to compute a deterministic seismic hazard scenario. We considered different station location and the contribute of several noise sources like sea-wave and the anthropic ones. We analysed the Rayleigh group velocity dispersion with the FTAN method (Frequency-Time ANalysis, Levshin et al 1972,1992). The Venetian plane area (Italy) velocity model inverted is consistent with the known lithology, and in agreement with previous seismic measurements.
NS11D-0798
3D Geotechnical Soil Model of Nice, France, Inferred from Seismic Noise Measurements, for Seismic Hazard Assessment.
In seismic risk studies, the assessment of lithologic site effect is based on an accurate knowledge of mechanical properties and geometry of superficial geological formations. Therefore, we built a 3D subsurface model in the city of Nice, southeastern France, using not only geological and geotechnical data but also geophysical inputs. We used especially ambient vibration recordings to supply the lack of borehole data over the city. Nice spreads over 72 km2 and roughly 20% of the city is built upon recent alluvium deposits. Other parts of the city lie on Jurassic and Cretaceous rocks to the east and thick Pliocene conglomerates to the west. Nearly 450 boreholes located mainly in the alluvial valleys were used. Because they are essentially linked to previous planned constructions (such as road network or important building), their distribution is rather heterogeneous over the studied area. In the valleys moreover, less than 40% of the boreholes are reaching the rock basement. These boreholes have been analyzed and a representative soil column made of 9 sedimentary layers has been recognized. Shear wave velocity of these layers were obtained from Standard Penetration Test values using several empirical correlation law described in the literature. Because of its cost, an extended boring survey was not feasible to complete our data set. Traditional seismic profiling was also not intended, as it is not possible to use intensive explosive sources in town. Recent years have seen many studies using ambient vibration measurements for site effect estimation. Especially, the very simple H/V technique was proven to be suitable for microzoning studies although some limitation were pointed out when dealing with 2D or 3D structures. Nevertheless, this technique alone provides only the fundamental eigenfrequency of the site under investigation. But assuming the shear wave velocity in the sediment it can helps to constrain the depth of the bedrock thanks to the well known f0=VS/4H relationship. We measured ambient vibration at almost 500 points homogeneously distributed in the city of Nice. The recordings were processed according to the Nakamura technique (Nakamura, 1989) obtaining thus the fundamental resonance frequency of the sedimentary infill at each recording point. Adding to this data set some earthquake recordings from the French permanent accelerometric network (RAP) in Nice and a microtremor array recording (Bonnefoy-Claudet, 2004), we were able to better constrain the subsurface model of Nice as well as to extend it where no boreholes were existing. This paper shows the good agreement we observed between geotechnical data and ambient vibration recordings. It particularly points out the contribution of seismological surveys in the subsurface modeling.
NS11D-0799
Seismically-Induced Lateral Spreads Analyzed for Liquefaction and Cyclic Failure Mechanisms, Dixie Valley, NV
Paleo-lateral spreads, dated 2,000 to 2,500 years ago, have been identified by previous researchers in Dixie Valley, Nevada, a seismically active area of central Nevada which is bound to the west by the Stillwater Range and to the east by the Clan-Alpine Range. The study area is located between the Stillwater Range faults and the basin playa, and is composed of alluvial fan material washed down from the westward bounding Stillwater Range and of beach gravel, sands and fines from the eastward bounding pluvial Lake Dixie. Analysis of subsurface samples revealed that among the fines is black low-strength clays identified by XRD as primarily smectites. The clay quickly forms a beige oxidized rind and was identified by total organic carbon analyses to contain less than 1% organic material. While preliminary field work indicated the presence of sand boils and fissures consistent with seismically-induced lateral spreading due to liquefaction, the low-strength clay presents the possibility of seismically-induced lateral spreading due to cyclic failure. On-going lab analyses and modeling will identify the units most likely to fail during earthquake loading, and by which mechanism.
NS11D-0800
Microtremor array measurement in Metropolitan Beijing
High-resolution near-surface geologic information is essential for seismic hazard reduction in urban areas. Traditionally, a seismic source within the area of study is always desirable for getting the high-resolution information. However, natural earthquake does not usually conveniently occur during periods of seismic observation. Meanwhile, explosive and/or vibrating sources are prohibited to be used in cities. To overcome these limitations for addressing urban seismic hazard reduction, microtremors, the ambient noise of micro-motions in the ground, are becoming valuable sources for extracting the much needed earthquake engineering parameters in urban settings. Using 30 three-component seismometers we have collected microtremor data at more than 1,000 sites in metropolitan Beijing area in the summer of 2007. The data were collected in two ways: 1) along two major roads running roughly orthogonal in NW and NE directions centered in the city center with 200-m seismometer spacing and minimum measurement duration of 1 hour; and 2) seismometer spacing of approximately 1-km that covered the entire area inside Beijing's 5th Beltway. The data are processed with both the horizontal to vertical spectral ratio method (H/V) and space correlation method. The delineated shear wave velocity and the bedrock interface mapping are compared with results by other means. Meanwhile, the newly obtained information on ground motion fundamental frequency and amplification factor will provide further constraints on seismic hazard analysis to those critical structures of the 2008 Summer Olympic Game. This project is supported by the Ministry of Science and Technology of China.
NS11D-0801
Evidence of Recent Slope Failures on the Scotian Slope
The Logan Canyon debris-flow corridor, a 5-20 km wide region of the Scotian Slope, offshore southeastern Canada, consisting of numerous mass-wasting deposits, was analyzed for slope stability. This area is representative of the class of seafloor that lies in the northeastern half of the Scotian Slope where canyons heavily dissect the seafloor. Slope failure features are ubiquitous on this margin and include slumps, slides, debris flows and turbidity currents. In the Logan Canyon area, a corridor dominated by debris flow deposits is observed from multibeam seafloor imagery. Earlier studies suggest that most of the mass wasting features on the Scotian Slope occurred in the early Holocene or earlier. In this study, multibeam, high resolution seismic reflection, and piston core data are used to identify and analyze large- and small- scale failures and to reconstruct the series of mass wasting events in the debris flow corridor. Slope stability analyses combined with seismostratigraphy and multibeam imagery were used to interpret the sequential events of Quaternary continental slope mass wasting processes and to determine the most likely triggering mechanism(s). This reconstruction reveals a recent mass-wasting event, suggesting active mass wasting on the Scotian margin.
NS11D-0802
The October 11, 1918 Mona Passage tsunami modeled using new submarine landslide evidence.
The October 11, 1918 ML 7.5 earthquake in the Mona Passage betweeen Hispaniola and Puerto Rico generated a local tsunami that claimed approximately 100 lives along the western coast of Puerto Rico. The area affected by this tsunami is now many-fold more populated. Although the exact cause of the tsunami is still unclear, newly-acquired high-resolution bathymetry of the Mona Passage and seismic reflection lines show a fresh submarine landslide 12 km northwest of Rincón in northwestern Puerto Rico and in the vicinity of the earthquake epicenter determined by Doser et al., (2005). The landslide area is approximately 76 km2 and probably displaced a total volume of 10 km3. The landslide's head scarp is at a water depth of 1.2 km, with the debris flow extending down to a water depth of 4.5 km. The seismic profiles and multibeam bathymetry indicate that the previously suggested source of the 1918 tsunami, a normal fault along the east side of Mona Rift (Mercado and McCann, 1998), was not active recently. The fault escarpment along Desecheo Ridge, which is near the Doser et al., (2005) epicenter, and our landslide appear, on the other hand, to be rather fresh. Using the extended, weakly non-linear hydrodynamic equations implemented in the program COULWAVE (Lynett and Liu, 2002), we modeled the tsunami by a landslide with a finite duration and with the observed dimensions and location. Marigrams (time series of sea level) were calculated at locations near to reported locations of runup. The marigrams show a leading depression wave followed by a maximum positive amplitude in good agreement with the reported polarity, relative amplitudes, and arrival times. Our results suggest this newly-identified landslide, which was likely triggered by the 1918 earthquake, was the probable cause of the October 11, 1918 tsunami and not a normal fault rupture as previously suggested.