Abstract: We used seismic broadband records from the SIerras Pampeanas Experiment using a Multicomponent BRoadband Array and the Eastern Sierras Pampeanas array to analyze modern shallow seismicity using a new seismic velocity model for the continental crust of the Eastern Sierras Pampeanas (ESP). The ESP, located in the central part of Argentina, more than 700 km east from the trench, are the easternmost manifestation of crustal deformation in the Andean retroarc between 29 and 32ºS. We obtained a crustal velocity model using the joint inversion of teleseismic receiver functions and surface wave data, calibrated with gravity measurements. This model describes in detail the crustal seismic velocity variations beneath the ESP. Then, we analyzed crustal earthquakes recorded by the arrays that occurred during 2008-2010. An average of 49 P and S wave arrivals per event were picked and resulted in 31 new earthquake locations. Errors in location appeared to be improved with values on average of 1.2km in latitude, 1.7km in longitude and 2.7 km in focal depth. The average rms is 0.27s and the average azimuthal gap is 104º. In addition, we determined first motion focal mechanisms further constrained with amplitude ratios SH/P, SV/P and SV/SH. Finally, we inverted these focal mechanism solutions to estimate the regional stress tensor. We observe that most of the analyzed seismicity beneath the ESP is located between 10 and 25km depths in the central part between 30.5-31.5ºS. Our focal mechanisms are mainly reverse and consistent. These small to moderate size earthquakes should not be underestimated in this region of Argentina that concentrates 1.33 M inhabitants, one nuclear power plant, 12 large dams Historical and modern seismicity seem to agree with shallow focal depths, which enhance the seismic hazard in this region. Future work will include more seismicity and seismic sources, aftershocks and structure characterization.
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Abstract: Seismic gaps along the subduction zones are locations where large earthquakes have not occurred in a long time. These areas are considered locked and are accumulating large amounts of strain energy that will ultimately be released in major earthquake. The Nicoya Peninsula in northwestern Costa Rica was considered a zone with this type of seismic gap. The previous major earthquakes in Nicoya occurred on 1853, 1900 and 1950, which indicates about a 50-year recurrence interval for the characteristic earthquake cycle. With the goals to: 1) record and locate strong subduction zone mainshocks [and foreshocks, “early aftershocks”, and preshocks] in Nicoya Peninsula, at the entrance of the Nicoya Gulf, and in the Papagayo Gulf regions of Costa Rica, and 2) record and locate any moderate to strong upper plate earthquakes triggered by a large subduction zone earthquake in the above regions, a seismic strong motion array (SSMAP project) was installed in the Nicoya Peninsula, array composed of 10 sites with Geotech A900 accelerographs. Also, the OVSICORI-UNA network was upgraded with ES-T episensors that could record the large event. On September 5, 2012, a Mw=7.6 earthquake occurred in the seismic gap and appears to be the expected event based on the 50 years recurrence interval, but was instead 62 years later. The main shock focal mechanism was thrust faulting of the Cocos plate in the Middle America trench with strike N54W and dip 20 degrees NE. The mainshock location was 9.671 N and 85.878 W. The maximum accelerations from two A900 stations perpendicular to the trench, Fortuna (distance 112 km) and Pedernal (distance 128 km) were: 13.8% and 8.9 % g; although the main acceleration was recorded at Dulce Nombre de Nicoya 122% g. The October 10 (MW 5.3) and 24 (Mw 6.6) aftershocks recorded at Tamarindo were accelerations of 2.4% and 8.2% g. We also relocated 50 events from 2006 to 2016 for moderate magnitudes (4 < Mw < 6.5), mainly located in Nicoya Peninsula region.
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Abstract: Seismic surface waves are valuable for investigating subsurface structures. However, in many other applications such as in seismic reflection imaging, it is desirable to separate the surface waves from the data. We propose a data-driven approach to predict and separate surface waves from the data based on the nonlinear dispersion measurement. In addition, we can also separate the surface waves into fundamental mode and overtones. The procedure has two steps. We first estimate high-resolution surface wave phase velocities from the recorded data using our nonlinear signal comparison (NLSC) approach. This enables us to predict the surface waves at each receiver location. We then subtract the predicted surface waves from the input seismic data. We applied our approach on two synthetic datasets and one field active-source seismic gather. From these examples, we can see that our new approach could effectively predict and separate surface waves with high fidelity.
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Abstract: On 2 September 2017, an Mw 5.3 normal-faulting earthquake occurred in the Intermountain Seismic Belt about 15 km east of Soda Springs, ID, near Sulphur Peak. It was widely felt throughout southeastern Idaho, northern Utah, and western Wyoming, but caused little damage. It appears to be co-located with two earlier seismic sequences, one in 1960 and the other in 1982. To achieve high-accuracy locations, the University of Utah and the USGS partnered with the Idaho Geological Survey to deploy eight telemetered seismographs in the epicentral region. Using absolute and differential arrival times we determined high-accuracy locations for a catalog of ~1,100 aftershocks, complete to ML 2.5. Initial locations were determined with HYPOINVERSE using a local 1D velocity model, and further refined using joint hypocentral decomposition (MLOC) and cluster-based relative relocation (GrowClust). For 76 of the largest events in the sequence, we inverted regional distance waveforms for moment tensors. The small uncertainties in absolute hypocenters imply that two first-order observations about the 2017 Sulphur Peak sequence are robust. First, the events are to the east of the west-dipping Eastern Bear Lake Fault, perhaps in a complex fracture zone within the footwall. Second, the events are confined to the upper crust, with a maximum depth of ~10 km. Like previous seismicity in this region—such as the M4.7 Soda Springs sequence of 1982 and the M5.7 Draney Peak sequence of 1994—the 2017 Sulphur Peak sequence had an energetic aftershock sequence with the cumulative aftershock moment 0.7–1.4 times as large as the mainshock moment. The unusually high productivity of the sequence is evident from the fact that 17 aftershocks had magnitudes larger than the upper bound expected from Bath’s Law, and 16 of the 17 occurred within ten days of the mainshock. Following Reasenberg and Jones [1994], the expected number of such aftershocks is 0–4, and the probability of observing 16 is only 2.3×10-12.
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Abstract: Okmok volcano is a caldera in the Aleutian Arc that has produced at least 11 eruptions over the past century of observation. Between summers 2015 and 2016, we deployed a seismic array of 13 broadband instruments in and around the caldera. This array supplemented Okmok’s 12 permanent seismometers operated by the Alaska Volcano Observatory. Using these seismic data, we perform ambient noise tomography to image the subsurface velocity structure at Okmok. We obtain Rayleigh and Love wave group and phase velocity maps in the 0.2-0.7 Hz frequency band, with the latter being obtained via the spatial autocorrelation procedure. We then invert phase and group velocity maps for the shear-wave velocity structure. Relative to previous tomography studies of Okmok, our dense seismic array allows for enhanced imaging of the velocity structure at this volcano and therefore allows us to better constrain the distribution of magmatic fluids.
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Abstract: Consideration of vertical seismic design loads is important for long-span structural systems, short-period structures and for some non-structural components in the buildings. To this end, seismic design codes utilize alternative approaches to define vertical design spectrum at different levels of complexity: either as a fraction of horizontal design spectrum or using a separate functional form having features different than the horizontal spectrum. In all cases a consistency between the horizontal and vertical design spectral ordinates is sought. In this paper, we consider a suite of modern seismic design codes (e.g., Eurocode 8, ASCE 7-05, ASCE 7-10 and ASCE 7-16) as well as horizontal, vertical and vertical-to-horizontal (V/H) ground-motion predictive models to assess the properness of vertical design spectrum expressions in these codes. We generated earthquake scenarios to represent different magnitude, distance and site combinations. We estimated the horizontal and vertical spectra for each one of these scenarios from the selected horizontal, vertical and V/H ground-motion predictive models. We then compared the code-based (idealized) horizontal and vertical spectra of these cases with those obtained from the ground-motion predictive models. Our preliminary findings suggest that the current design code formulations are insufficient to properly address the vertical spectrum trends. We discussed the possible reasons behind the observed misrepresentation of idealized vertical spectra by the investigated codes. Our discussions may lead to the development of new rules for code-based vertical spectrum that is consistent with its horizontal counterpart.
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