Abstract: Earthquake-induced landslides cause a significant portion of earthquake-related fatalities and economic losses, and can have long-lasting negative societal impacts. We present a method to estimate the impact of seismically induced landslides on affected populations using the output of the USGS near-real-time earthquake products. Using a newly developed, comprehensive dataset of 196 historical earthquakes, including 127 events with known landslide fatality counts, we develop an empirical model that estimates the order of magnitude of potential fatalities based on the exposure of population to expected landslide occurrence. Using the grid of landslide probabilities output by the USGS Ground Failure earthquake product, we estimate population exposure by multiplying the predicted probability grid with a gridded global population database adjusted for population growth. We then sum over the entire grid for each event, and term this the predicted ‘landslide exposure index.’ We examine income level of the country of each earthquake as a secondary factor to represent relative vulnerability of the surrounding area. We compare these values to the number of actual fatalities for 91 training events in order to calibrate a model that can be used to predict the order of magnitude of potential fatalities due to plausible future earthquakes using scenario earthquakes. We observe a significant positive correlation between predicted and observed fatalities, with high variability in fatality rates for similar exposure levels. This suggests that other factors (e.g., building type, time of day, landslide density, effect of urbanization on population) may improve this estimate. Ultimately, the outputs of this method can be integrated into the USGS near-real-time earthquake information system. The outputs can provide input for use in the Ground Failure alert level designation and also an estimate of landslide fatalities, which are currently not included in PAGER’s loss estimates.
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Abstract: In October 2014, a 2-week long slow slip event (SSE) occurred near Gisborne at the northern Hikurangi Margin, New Zealand. It was recorded by offshore instruments, deployed by the Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip (HOBITSS) project. This study uses data compiled from May 2014 to July 2015, recorded on 15 HOBITSS ocean bottom seismometers as well as 12 Geonet stations around the time of this uniquely recorded SSE. We use the S-wave splitting technique to detect stress and fluid changes associated with slow slip. The shear wave splitting fast polarization direction is often inferred to represent the maximum horizontal stress directions (SHmax) in crustal studies and has been shown to vary temporally at volcanoes and in association with large earthquakes. Using Multiple Filter Automatic Splitting Technique (MFAST) we analyse more than 3000 local earthquakes to look for temporal changes in S-wave fast polarization directions and delay times during the Gisborne 2014 SSE. Because S-wave splitting results are sensitive to variations in earthquake locations, we also analyse results from individual spatial earthquake clusters to test the robustness of temporal changes and to better indicate where the measured anisotropy originates. The mean fast directions in the area show a NE-SW trending fast polarization direction similar to local SHmax directions mapped by previous studies and to local fault trends. Preliminary SWS results at LOBS stations show approximately 5-10 degrees of change in fast polarization direction from before to after the SSE. We also observe increased delay times (~ 0.5 seconds) occurring around the time of the SSE with a slow decrease in delay time in the following months. We hypothesize an initial movement of fluids in cracks due to changes in stress during the SSE, followed by a relaxation period where cracks slowly return to their previous state.
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Abstract: Refraction seismic profiles and Ground Penetration Radar (GPR) lines were acquired at Billecocha´s High Plateau (BHP), Imbabura-Ecuador, to visualize a fault zone at the near-surface. The faults present at BHP occur about 4000 meters above sea level (masl) in a nature reserve in an area four km wide by ten km long. The processes responsible for the faults still remain in discussion. However they are located near densely populated urban centers, such as Ibarra, destroyed in 1868 by an earthquake. In this context, our work aims to fill the gap left by previous regional tectonic studies by providing an accurate near-surface image of the zone and evidence of the faults´ kinematics. The design of the seismic experiment comprised dense profiles, using 4.5 Hz geophones. Both source points and receivers were spaced at 1 m interval along 3 profiles of 48 m long. The GPR reflection radargrams were acquired along ten 50 m long lines using two different antenna frequencies (250 and 100 MHz). Generalized Reciprocal Method and Seismic Traveltime Tomography were used to provide a preliminary image of the faults. The average velocity of the radar impulse was obtained with ground markers and was used to correct the radargrams. A multichannel analysis of surface waves (MASW) was also carried out to complement the study. Thanks to these technological applications at the study site a conspicuous normal fault zone is synergistically imaged. The thickness of the sedimentary layers were obtained comparing the images from the refraction and GPR surveys. An average for both P and S wave velocities was estimated for the layers involved in the fault mechanism using the analysis of the refraction profiles and MASW. The 2D near-surface final image obtained provides a deeper insight into the architecture and kinematics of the fault system at BHP.
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Abstract: The 2017-09-08 earthquake M8.2 located 98 km SSW of Tres Picos, Mexico and 2018-01-23 M 7.9 earthquake located 280km SE of Kodiak, Alaska are the first great earthquakes to occur within the UNAVCO RT-GNSS footprint, which allows for a rigorous analysis of our dynamic and static processing methods. The need for rapid geodetic solutions ranges from seconds (EEW systems) to several minutes (Tsunami Warning and NEIC moment tensor and finite fault models). Here, we compare and quantify the relative processing strategies for producing static offsets, moment tensors and geodetically determined finite fault models using data recorded during this event. We also compare the geodetic solutions with the USGS NEIC seismically derived moment tensors and finite fault models, including displacement waveforms generated from these models. We define kinematic post-processed solutions from GIPSY-OASISII (v6.4) with final orbits and clocks as a “best” case reference to evaluate the performance of our different processing strategies. We find that static displacements of a few centimeters or less are difficult to resolve in the real-time GNSS position estimates, including the 5-min arc averages produced by UNR. The standard daily 24-hour solutions provide the highest-quality data-set to determine coseismic offsets, but these solutions are delayed by up to 48 hours after the event. Dynamic displacements, estimated in real-time, however, show reasonable agreement with final, post-processed position estimates, and while individual position estimates have large errors, the real-time solutions offer an excellent operational option for EEW systems, including the use of estimated peak-ground displacements and directly inverting for finite-fault solutions. In the near-field, we find that the geodetically-derived moment tensors and finite fault models differ significantly with seismically-derived models, highlighting the utility of using geodetic data in hazard applications.
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Abstract: A sinkhole collapse on 14 July 2017 destroyed two homes and required evacuation of 9 additional residences in Land O’Lakes, Florida. The sinkhole formed rapidly and within 10 hours reached most of its final size of approximately 40 by 50 m and a maximum depth of about 15 m. According to Pasco County officials, this is the largest sinkhole to have formed in the county during the last 30 years. The site is located near two natural lakes and occurs within an area of well-developed karst. The county allowed USF geoscientists to study the sinkhole resulting in an extensive collection of LiDAR, GPR, and lake-bottom profiling data. The seismology group installed one broadband 3-component seismometer on 25 July ~20 m W of the sinkhole edge and a second one on 27 September about 10 m E of the sinkhole edge. Data are recorded continuously at 200 Hz. Drilling to understand soil structure integrity and remediation work to stabilize the sinkhole led to extreme noise levels during daytime operations through most of August 2017. Nighttime noise is lower, but due to nearby houses, sensors pick up strong 60 Hz noise caused by AC units. The only signal unequivocally attributable to the sinkhole occurred on 5 August just after stabilization work, consisting of partial backfilling with limestone rocks, began that caused the sinkhole to widen by about 3 m along its entire western edge. After completion of stabilization work, no further signals have been detected from the sinkhole. Besides sinkhole monitoring, we recorded several large global earthquakes, e.g., the Mw=8.2 and Mw=7.1 Mexico earthquakes on September 8 and 17, respectively, as well as Hurricane Irma as it passed within 20 miles as a quickly weakening category 1 hurricane. In addition to outreach and sinkhole documentation as part of a multi-sensor approach, we use the site to train seismology graduate students in field practices and conducted a class field trip.
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Abstract: Increased seismicity in the Oklahoma and surrounding regions since 2008 and the passing of the EarthScope USArray through the region provides an opportunity to better define Lg attenuation in that region. Initial tomography results at 1 Hz by Gallegos et al. (2014) and a study of the location of the boundary between mid-continental low attenuation and Gulf Coast higher attenuation regions at 1.0 and 5.0Hz by Cramer and Al Noman (2016) and Cramer (2017) provide insight into the problem of defining Lg attenuation in the Oklahoma region. The location of the mid-continental/Gulf Coast Q boundary in Oklahoma is complicated by other crustal structures, such as the Southern Oklahoma Rift. We address this problem using Lg Q tomography and increased raypath coverage from M3 earthquakes surrounding the study region. We determine frequency dependent attenuation models Q(f) for the Oklahoma and surrounding region using direct Lg waves at regional distances of 200 to 1500 km. Using automated processing we extracted the Lg-wave amplitude spectra between 0.05 and 10 Hz from vertical waveforms recorded at 314 TA stations from 207 crustal earthquakes (M>3.0). We simultaneously determined the QLg distribution, source and site functions at different frequencies from the geometrical spreading corrected Lg amplitudes using a tomographic regularized inversion. The 1°x1° checkerboard test with synthetic data shows good resolution coverage over the region. The results at 1.0 Hz and 5.0 Hz show a trend of lower Q estimates near the Gulf Coast region while the central Oklahoma part reveals a higher Q estimate. Initial investigation of Gulf-Coast regional attenuation pattern suggests a good agreement with the defined Q boundaries by Cramer (2017). We expect to investigate the attenuation estimates in more detail, and at other frequencies to develop a frequency dependent Lg attenuation map for this region.
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