1,720,969 research outputs found
An Investigation of the Seismic Site Response of the Area Surrounding the New Student Housing at Cal Poly Pomona Using Spectral Ratio Analysis
No full textA seismic investigation of the response parameters of the area near the new student housing units on the Cal Poly Pomona campus was carried out. Double resonance will occur at a site if the underlying soil column produces amplification at a specific frequency that matches up with the resonance of the building itself. Under these conditions, greater motions are more likely to occur during an earthquake and thus a higher likelihood of damage exists. Resonance period and ground motion amplification values at five sites around the area of the new student housing units were estimated in this project. The study involved deploying five seismometers around the construction site. Ambient noise, which represents the small continuous ground vibrations present in the environment, was recorded for several hours and then analyzed using the H/V spectral ratio technique with the Geopsy software to determine the resonance period and amplification values. This method requires no permit and has no environmental impact, so it is especially useful in well developed areas. The Site Effects Assessment using Ambient Excitations (SESAME) project provides guidelines for implementing the technique. Data was collected in areas selected according to SESAME guidelines. The reliability of the curves and the clarity of the peaks for each site were also determined using the SESAME guidelines. The construction site overlies alluvial silt with the San Jose Fault trending east-west at the north end. With the found resonance frequency of ~1 Hz, buildings near 10 stories may experience double resonance during a seismic event
Locations of New Craters Produced by Observed Lunar Impacts
No full textThe Meteoroid Environment Office (MEO) has been observing lunar flashes which may produce impact craters detectable by Lunar Reconnaissance Orbiter (LRO). There have been over 300 flashes observed in an 8+ year time span of routine observations from MSFC and one new crater has been correlated with a flash recorded in March of 2013. Using ArcMap's tools to georeference lunar images of the impact flashes caused by meteoroids impacting the Moon's surface we are able to determine the selenographic latitude and longitude of each impact crater. Georeferencing associates the images of the impact flashes with a lunar base map by assigning control points that correlate distinguishing features on both the Moon image and the base map. The coordinates that are obtained from my work will be sent to researchers using LRO, which is a satellite currently in orbit around the Moon. This data will guide LRO to search for additional and/or new impact craters
Locating Archaeological Artifacts from San Salvador in Colton, CA, Buried During the Great Flood of 1862, Using Ground Penetrating Radar
No full textSan Salvador was the largest settlement along the Old Spanish Trail between New Mexico and Los Angeles during the early 1840s. In the flood of 1862, the Santa Ana River washed away or buried settlers' adobe homes and belongings. Our goal is to use geophysical subsurface imaging techniques to help the Spanish Town Heritage Foundation in assessing the potential for buried artifacts and possibly documenting the cultural importance of this site, before development is implemented. For our research, we are conducting surveys using Ground Penetrating Radar (GPR). This equipment allows us to image or detect buried adobe structures, household cookware, and metallic farming tools in the shallow subsurface. Artifacts are anticipated to be at a depth of 4 meters based on historical archives. The GPR antenna allows imaging to a depth of approximately 7 meters. Since we began our survey in April 2018, 60 profiles have been recorded, each with an approximate length of 100 meters. The site has little topography and is mostly covered in sand, minimizing data collection and processing issues. We expected GPR to be successful because it can penetrate this surface material, and because the physical properties of the buried artifacts that we expect to image are distinct from those of the soil in which they are currently buried. We have detected several anomalies within 500 meters of the Santa Ana River. We plan to focus additional investigation on this part of the site and use different geophysical techniques to confirm and further image these anomalies
Measuring Deformation Due to the San Jose Fault Using GPS Monuments Near Cal Poly Pomona Campus
No full textThe San Jose fault is a relatively little-known fault that runs from the eastern San Gabriel Valley towards the Chino Basin and through the Cal Poly Pomona campus, with a total length of 18 km. The fault is considered a significant hazard due to its potential to generate an earthquake of magnitude 6.0-6.5, as well as its location in a densely populated area. Although the 1990 Upland earthquakes are thought to have occurred on this fault, it has been the topic of little investigation. However, its destructive potential provides an important incentive to monitor the fault and learn more about its geometry and slip rate, which is the purpose of this study. In order to do this we had to find areas with exposed bedrock and good visibility of the sky, on and near Cal Poly Pomona campus where 10 distributed monuments could be installed for use in geodetic studies. In these studies precise measurements are made of Earth's surface deformation. The monument locations will be well documented for future reference and a first set of measurements will be made of their detailed absolute and relative locations. We will use total stations to determine the distance between the monuments, as well as Global Positioning System units to obtain accurate absolute locations of the monuments. Additional measurements will be conducted yearly to produce estimates of slip rate and direction of motion of the San Jose fault on CPP campus
Locations of New Craters Produced by Observed Lunar Impacts
No full textThe Meteoroid Environment Office (MEO) has been observing lunar flashes which may produce impact craters detectable by Lunar Reconnaissance Orbiter (LRO). There have been over 300 flashes observed in an 8+ year time span of routine observations from MSFC and one new crater has been correlated with a flash recorded in March of 2013. Using ArcMap's tools to georeference lunar images of the impact flashes caused by meteoroids impacting the Moon's surface we are able to determine the selenographic latitude and longitude of each impact crater. Georeferencing associates the images of the impact flashes with a lunar base map by assigning control points that correlate distinguishing features on both the Moon image and the base map. The coordinates that are obtained from my work will be sent to researchers using LRO, which is a satellite currently in orbit around the Moon. This data will guide LRO to search for additional and/or new impact craters
Geophysical Controls on Fault-Groundwater Interactions at San Andreas Oasis, Dos Palmas Preserve
No full textThe San Andreas Oasis has historically provided a reliable source of fresh water near the northeast margin of the Salton Sea; however, since the recent completion of the Coachella Canal Lining Project in 2007, surface water at the site has begun to disappear. This has hindered efforts by the Bureau of Land Management (BLM) to preserve and restore the unique environment created by the Oasis. The BLM has proposed the installation of a recharge pond near the site. Controls on groundwater dynamics and recharge are complicated by the presence of the Hidden Springs Fault (HSF), which trends near the Oasis. Its surface expression is apparent as a lineation against which all plant growth terminates, suggesting that it may form a partial barrier to subsurface groundwater flow. We present an application of several geophysical exploration techniques to constrain structural controls on groundwater recharge. A total of nine direct current (DC) resistivity surveys were performed to the east and west of the Hidden Springs Fault (HSF). Magnetic profiles were taken across the HSF to better define its trend and delineate additional faults. Five very low frequency (VLF) electromagnetic induction profiles were also conducted across the fault zone and in areas upgradient of the Oasis, where alluvium and poor coupling prevent ground-based resistivity surveys. Results suggest the existence of a previously unmapped fault to the northeast of San Andreas Oasis. Our measurements are consistent with the HSF acting as a barrier to lateral flow, while also exhibiting transport of water along the fault. Together these two faults channel southeast-directed flow, localizing groundwater and associated plant growth in a narrow, fault-bounded lineament. Based on this interpretation, we have recommended that a recharge pond be placed to the north of San Andreas Oasis, allowing recharge to flow between the two faults
Using Geophysical Methods to Locate Archaeological Features and Artifacts from La Placita de Los Trujillos, Buried by the Santa Ana River during the Great Flood of 1862
No full textIn the mid-1800s, La Placita de Los Trujillos (La Placita) was situated alongside the Santa Ana River (SAR) in what is now Colton, CA. The settlement had grown to be the largest non-native community between New Mexico and Los Angeles, until the river washed away or buried all adobe structures and settlers’ belongings during the Great Flood of 1862. Historical archives reveal that a significant portion of San Salvador is buried approximately 3 meters beneath sandy flood deposits of the vacant, 200-acre lot of Pellissier Ranch. The motivation for this study was to provide the Spanish Town Heritage Foundation with data they could use to advocate the cultural importance of Pellissier Ranch in their attempt to stop the city from developing the site for warehouses. From April 2018 to January 2020, we conducted eleven days of geophysical field surveys in search of buried adobe structural remains, household items composed of wood and iron, and large farming equipment that would have existed in La Placita. Over 7,000 meters of ground-penetrating radar (GPR) profiles were acquired with both a GSSI-SIR 3000 and 4000, using the 400 MHz and 350 MHz HyperStacking antennas respectively, and 25 km of magnetic gradiometry data was acquired using a GEM GSM-19T Proton Precession Magnetic Gradiometer. GPR profiles imaged several 'adobe melt' signatures at a depth range of 1.4 - 3.8 meters, and two buried canals with surfaces reaching 1.3 meters below the surface, and bottom depths of 2.5 meters. Additional structural GPR anomalies vary in size from 9 - 70 meters at a depths of 1 - 3.8 meters. We hypothesize that some of the shallower buried (sub-2-meters) anomalies exist for a few reasons: the portion of the anomalous feature closer to the river absorbed the majority of the river's force, which shielded the more intact part, or the location did not experience the full strength of the river's force, causing less destruction and burial beneath the sandy flood deposits. Contemporary debris from trespassers resulted in many false-positives in gradiometer results that needed to be excluded from mapped vertical gradients. We therefore focused more heavily on GPR results, using the gradiometer as a complementary technique to attempt to verify anomalies we imaged with GPR. In two locations, we observed a positive correlation between the two data-sets, attributed to a sub-2-meter burial of anomalies. This confirms claims made by other researchers that features / artifacts with low magnetic content cannot be seen if buried deeper than 2 meters. From our results, we conclude with confidence that several anomalies we imaged in GPR profiles can be related to structures / features of La Placita, and the depth range of the settlement at Pellissier Ranch is primarily 2 - 3.8 meters. Based on our surveys, we suggest to focus any future investigation in the middle of Pellissier Ranch. There is one east-west dirt road that cuts through the middle of the 200-acre site. At about the halfway point of that east-west road, we found a collection of promising anomalies
Imaging the Los Angeles Basin by Travel-Time and Waveform Modeling of Data from a Temporary High-Density Broadband Experiment
Get PDFA total of 73 broadband seismometers were deployed in a dense (~1 km spacing) seismic array from Long Beach to Puente Hills for a passive source experiment called the Los Angeles Syncline Seismic Interferometry Experiment (LASSIE). The purpose of this experiment was to collect high-density seismic waveform data in the Los Angeles Basin (LAB) to better understand basin structure and response. This research leverages the data collected from LASSIE to assess a suite of six one-dimensional velocity models that are frequently used in Southern California source and structural studies on their ability to model LAB response, using approaches that include travel time comparisons, as well as full waveform modeling. To this end, we compare characteristics of seismograms recorded by LASSIE for small earthquakes that occurred within or near the LAB to broadband synthetic seismograms generated using the Frequency-Wavenumber approach developed by [Zhu and Rivera, 2002]. Six of the eight largest earthquakes during the LASSIE deployment provided sufficient data with clear P-and S-wave arrivals for analysis. Synthetics and LASSIE data for these events were compared based on P-and S-wave first arrival times, duration of significant ground motion, and anomalous high amplitude secondary phases that were initially visually noted on numerous seismograms. Arrival time comparisons show that the [Hauksson and Jones, 1989] model most accurately predicts the P-and S-wave arrival times for all events. This result was surprising, given that this model was created from arrival time data from the 1987 Whittier Narrows sequence, and the earthquakes used in this study were located throughout the LAB. The best performing model for predicting durations varied by event, but the [Magistrale et al., 1992] inversion model worked well for earthquakes located inside the LAB. The slightly more complex shallow structure and greater contrast in velocity between shallow and deeper layers within the basin for this model increases durations, which better replicated durations measured for stations and earthquakes inside the LAB. Both the investigation of the anomalous secondary arrivals, as well as the duration analysis showed evidence of the LAB acting as a waveguide, internally trapping seismic energy, in particular for events inside the basin. Both arrival times and durations showed significant variation for stations located in and near major structural features. The Newport-Inglewood Fault Zone produces delayed P-wave arrivals and significant scatter in the arrival time data. The Whittier Narrows region produces a sharp decrease in arrival times, most likely associated with a high velocity body that has been imaged in the subsurface at a depth of 3 to 5 km. Differences between P-and S-wave differential arrival times revealed that Vp to Vs conversions should be calculated with a Poisson’s ratio based on lithology, with a higher value for the sediments and a lower value for the basement material. The general Southern California model of [Kanamori and Hadley, 1975] has arrival times differences two to three times larger and duration differences up to five times larger than LAB specific velocity models. These results show 1-D models with LAB structure are fundamental in explaining even basic waveform characteristics for events and stations in and near the LAB. Large lateral variations are difficult for 1-D models to handle, but, when used away from the major structural features of the basin they provide a fast and efficient means through which to predict arrival times and durations with reasonable accuracy
Seismological observations of upper mantle anisotropy [pt. 1] ; Source spectra of shallow subduction zone earthquakes and their tsunamigenic potential [pt. 2]
Get PDFOne of the most important developments in observational seismology in the last 10 years
is the worldwide increase in the number of broadband instruments and seismic networks,
as well as the improved access to the data-set that these seismometers provide. A data-set
of this magnitude offers nearly unlimited possibilities for research into earthquake source
processes and Earth structure. The work presented in this thesis involves the application of
different methods to seismological recordings, as well as an interpretation and discussion
of the results.
In Chapter I, I take advantage of the very broadband nature and small spacing of the
stations of TERRAscope, one of the first digital broadband seismic networks, to determine
dispersion curves for long period surface waves. This enables us to invert for an upper
mantle S-wave velocity model for southern California. The Rayleigh wave, SV, model is
about 4% slower than the model developed for tectonic north America. If the correction for
higher modes I performed on our Love wave data measurements is accurate, the resulting
SH velocity model shows about 5% anisotropy (transverse isotropy) in the upper mantle
beneath southern California.
In Chapter 2, I perform measurements of shear-wave splitting on a unique data-set
obtained from temporary arrays located above the Nazca subduction zone in South
America. Data from SKS, and local S-wave data from deep and intermediate depth
earthquakes, were used to develop a model of the anisotropy in this region. The above slab
component of anisotropy in the western region, where the slab is at a depth of about 300
km and up is oriented NS and its delay time is limited to about 0.3 sec. This direction
agrees with the shortening direction of the Andes and is orthogonal to that predicted by a
comer flow model. To the east, the stations have EW aligned fast directions and possibly
sample the Brazilian craton. The below slab component samples a zone of EW aligned
anisotropy, as well as trench parallel aligned anisotropy. The trench parallel directions can
be explained by the retrograde motion of the slab in south America, and I speculate that the
EW direction could suggest a tear in the slab, or a local EW orientation because of local
buckling of the slab under NS compression.
In Chapter 3 I use this same method for the data of the TriNet array. Here I find an overall
pattern of consistent directions of the polarization direction of the fast SKS waves, the
fastest P-wave velocities and the World Stress Map maximum horizontal compressive
stress directions. This suggest that the pattern of anisotropy is generally uniform in the
crust and lithospheric mantle, in a layer with an overall thickness of 100 to 150 km. The
alignment of most fast directions can be explained by plate-tectonic, extensional and compressional
events. We also examine the detailed lateral and vertical variations of anisotropy
in this region.
Chapter 4 is focussed on the differences in source spectra and tectonic setting between
tsunami earthquakes, which excite anomalously great tsunamis and 'regular' shallow
subduction earthquakes. We find that these unusual events have several characteristics in
common: low energy release at short periods, centroid location close to the trench, updip
rupture, relatively small accretionary prism, sediment subduction and a well-developed
horst and graben structure of the oceanic plate close to the trench. We speculate that these
events can nucleate in an unusually shallow part of the subduction zone, where sediments
normally exhibit stable sliding behavior, because of the contacts between the horsts and the
overriding plate. Because the earthquakes are so shallow, and there is some sediment being
subducted, part of the rupture goes through sediments, making the source process slow.
The true displacement (and thus the tsunami height) of these events may be underestimated
because the elastic constants of the fault zone are not taken into account when converting
seismic moment into displacement
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