Germán A. Prieto

Strong Ground Motion Prediction Using Virtual Earthquakes

Sedimentary basins increase the damaging effects of earthquakes by trapping and amplifying seismic waves. Simulations of seismic wave propagation in sedimentary basins capture this effect; however, there exists no method to validate these results for earthquakes that have not yet occurred. We present a new approach for ground motion prediction that uses the ambient seismic field. We apply our method to a suite of magnitude 7 scenario earthquakes on the southern San Andreas fault and compare our ground motion predictions with simulations. Both methods find strong amplification and coupling of source and structure effects, but they predict substantially different shaking patterns across the Los Angeles Basin. The virtual earthquake approach provides a new approach for predicting long-period strong ground motion. Ambient seismic noise helps predict the ground motion associated with future large earthquakes. Noise in Motion A large earthquake along the southern San Andreas Fault has the potential to cause serious damage to the city of Los Angeles, USA. Earthquake simulations in this region, which lies in a sedimentary basin capable of amplifying shaking, predict strong ground motion but they lack validation with observational data. Denolle et al. (p. 399) developed an independent method to predict ground motion using virtual earthquakes and information gleaned from background seismic noise. This ambient seismic field—generated by sources such as the oceans and atmosphere—produces differences in ground motion in the Los Angeles Basin compared to simulations, but suggests that locally shaking could on average be 3 times larger than the surrounding areas.

Subduction system and flat slab beneath the Eastern Cordillera of Colombia

Seismicity at the northern terminus of the Nazca subduction is diffused over a wide area containing the puzzling seismic feature known as the Bucaramanga nest. We relocate about 5000 earthquakes recorded by the Colombian national seismic network and produce the first 3-D velocity model of the area to define the geometry of the lithosphere subducting below the Colombian Andes. We found lateral velocity heterogeneities and an abrupt offset of the Wadati-Benioff zone at 5°N indicating that the Nazca plate is segmented by an E-W slab tear, that separates a steeper Nazca segment to the south from a flat subduction to the north. The flat Nazca slab extends eastward for about 400 km, before dip increases to ∼50° beneath the Eastern Cordillera, where it yields the Bucaramanga nest. We explain this puzzling locus of intermediate-depth seismicity located beneath the Eastern Cordillera of Colombia as due to a massive dehydration and eclogitization of a thickened oceanic crust. We relate the flat subducting geometry to the entrance at the trench at ca. 10 Ma of a thick - buoyant oceanic crust, likely a volcanic ridge, producing a high coupling with the overriding plate. Sub-horizontal plate subduction is consistent with the abrupt disappearance of volcanism in the Andes of South America at latitudes > 5°N.

Earthquake rupture below the brittle-ductile transition in continental lithospheric mantle

The slow and inefficient deep Wyoming earthquake ruptured in the ductile regime of the upper mantle. Earthquakes deep in the continental lithosphere are rare and hard to interpret in our current understanding of temperature control on brittle failure. The recent lithospheric mantle earthquake with a moment magnitude of 4.8 at a depth of ~75 km in the Wyoming Craton was exceptionally well recorded and thus enabled us to probe the cause of these unusual earthquakes. On the basis of complete earthquake energy balance estimates using broadband waveforms and temperature estimates using surface heat flow and shear wave velocities, we argue that this earthquake occurred in response to ductile deformation at temperatures above 750°C. The high stress drop, low rupture velocity, and low radiation efficiency are all consistent with a dissipative mechanism. Our results imply that earthquake nucleation in the lithospheric mantle is not exclusively limited to the brittle regime; weakening mechanisms in the ductile regime can allow earthquakes to initiate and propagate. This finding has significant implications for understanding deep earthquake rupture mechanics and rheology of the continental lithosphere.

Monitoring southwest Greenland’s ice sheet melt with ambient seismic noise

Researchers monitor southwest Greenland’s ice sheet mass changes by measuring seismic velocity variations in Greenland’s crust. The Greenland ice sheet presently accounts for ~70% of global ice sheet mass loss. Because this mass loss is associated with sea-level rise at a rate of 0.7 mm/year, the development of improved monitoring techniques to observe ongoing changes in ice sheet mass balance is of paramount concern. Spaceborne mass balance techniques are commonly used; however, they are inadequate for many purposes because of their low spatial and/or temporal resolution. We demonstrate that small variations in seismic wave speed in Earth’s crust, as measured with the correlation of seismic noise, may be used to infer seasonal ice sheet mass balance. Seasonal loading and unloading of glacial mass induces strain in the crust, and these strains then result in seismic velocity changes due to poroelastic processes. Our method provides a new and independent way of monitoring (in near real time) ice sheet mass balance, yielding new constraints on ice sheet evolution and its contribution to global sea-level changes. An increased number of seismic stations in the vicinity of ice sheets will enhance our ability to create detailed space-time records of ice mass variations.

Earthquakes initiation and thermal shear instability in the Hindu‐Kush intermediate‐depth nest.

Intermediate depth earthquakes often occur along subducting lithosphere, but despite their ubiquity the physical mechanism responsible for promoting brittle or brittle-like failure is not well constrained. Large concentrations of intermediate depth earthquakes have been found to be related to slab break-off, slab drip, and slab tears. The intermediate depth Hindu Kush nest is one of the most seismically active regions in the world and shows the correlation of a weak region associated with ongoing slab detachment process. Here we study relocated seismicity in the nest to constraint the geometry of the shear zone at the top of the detached slab. The analysis of the rupture process of the Mw 7.5 Afghanistan 2015 earthquake and other several well-recorded events over the past 25 years shows an initially slow, highly dissipative rupture, followed by a dramatic dynamic frictional stress reduction and corresponding large energy radiation. These properties are typical of thermal driven rupture processes. We infer that thermal shear instabilities are a leading mechanism for the generation of intermediated-depth earthquakes especially in presence of weak zone subjected to large strain accumulation, due to ongoing detachment process.

Global and along‐strike variations of source duration and scaling for intermediate‐depth and deep‐focus earthquakes

The systematic behavior of earthquake rupture as a function of earthquake magnitude and/or tectonic setting is a key in our understanding of the physical mechanisms involved during earthquake rupture. Geophysical evidence suggests that although deep earthquakes—including intermediate-depth and deep—are similar to shallow ones, the mechanism involved during deep earthquakes is different from that of shallow ones. In particular, the magnitude and depth dependence of scaled duration, a measure of earthquake rupture duration, has led to controversy of what controls deep earthquake behavior. Here we estimate scaled source durations for 600 intermediate-depth and deep-focus earthquakes recorded at teleseismic distances and show deviation from self-similar scaling. No depth dependence is observed which we interpret as due to little differences between intermediate-depth and deep-focus earthquake mechanisms. The data show no correlation between durations and plate age or thermal parameters, suggesting that the thermal properties of the plate have little effect on source durations. We nevertheless report differences in average source duration and scaling between subduction zones and along-strike variations of source durations that more closely resemble the geometry of subduction (flat or steep subduction) rather than plate age.

Global rupture parameters for deep and intermediate depth earthquakes

We investigate the global rupture parameters for deep and intermediate-depth earthquakes. From measurements of rupture duration and radiated seismic energy we estimate stress drop, apparent stress and radiation efficiency, and obtain a detailed earthquake energy budget. From scaling of the source parameters we highlight differences between crustal and deep seismicity, with the latter showing larger fracture energies. The observed increase of radiation efficiency with depth suggests that rupture mechanism for deep and intermediate depth events differs. In agreement with previous studies we observe along strike variability of rupture properties for deep and intermediate-depth earthquakes, correlating with slab morphology, plate age or presence of volcanic structures.

Continuous Monitoring of High‐Rise Buildings Using Seismic InterferometryContinuous Monitoring of High‐Rise Buildings Using Seismic Interferometry

The linear seismic response of a building is commonly extracted from ambient vibration measurements. Seismic deconvolution interferometry performed on ambient vibrations can be used to estimate the dynamic characteristics of a building, such as its shear-wave velocity and its damping. The continuous nature of the ambient vibrations allows us to measure these parameters repeatedly and to observe their temporal variations. We used 2 weeks of ambient vibrations, recorded by 36 accelerometers that were installed in the Green Building at the Massachusetts Institute of Technology campus, to monitor the shear wavespeed and the apparent attenuation factor of the building. Because of the low strain of the ambient vibrations, we observed small speed changes followed by recoveries. We showed that measuring the velocity variations for the deconvolution functions, filtered around the fundamental-mode frequency, is equivalent to measuring the wandering of the fundamental frequency in the raw ambient vibration data. By comparing these results with local weather parameters, we showed that the air humidity is the dominating factor in the velocity variations of the waves in the Green Building, as well as the main force behind the wandering of the fundamental mode. The one-day periodic variations are affected by both the temperature and the humidity. The apparent attenuation, measured as the exponential decay of the fundamental-mode waveforms, is strongly biased due to the amplitude of the raw vibrations and shows a more complex behavior with respect to the weather measurements. We have also detected normal-mode nonlinear interaction for the Green Building, likely due to heterogeneity or anisotropy of its structure. We found that the temporal behavior of the frequency singlets may be used for monitoring.

Earthquake source properties of a shallow induced seismic sequence in SE Brazil

We study source parameters of a cluster of 21 very shallow (<1 km depth) small-magnitude (MW < 2) earthquakes induced by percolation of water by gravity in SE Brazil. Using a multiple empirical Greens functions (meGf) approach, we estimate seismic moments, corner frequencies and static stress drops of these events by inversion of their spectral ratios. For the studied magnitude range (-0.3 < MW < 1.9), we found an increase of stress drop with seismic moment. We assess associated uncertainties by considering different signal time-windows, and by performing a jackknife resampling of the spectral ratios. We also calculate seismic moments by full waveform inversion to independently validate our moments from spectral analysis. We propose repeated rupture on a fault patch at shallow depth, following continuous inflow of water, as the cause for the observed low absolute stress drop values (<1 MPa) and earthquake size dependency. To our knowledge, no other study on earthquake source properties of shallow events induced by water injection with no added pressure is available in the literature. Our study suggests that source parameter characterization may provide additional information of induced seismicity by hydraulic stimulation.

Towards ultrasound travel time tomography for quantifying human limb geometry and material properties

Sound speed inversions made using simulated time of flight data from a numerical limb-mimicking phantom comprised of soft tissue and a bone inclusion demonstrate that wave front tracking forward modeling combined with L1 regularization may lead to accurate estimates of bone sound speed. Ultrasonic tomographic imaging of limbs has the potential to impact prosthetic socket fitting, as well as detect and track muscular dystrophy diseases, osteoporosis and bone fractures at low cost and without radiation exposure. Research in ultrasound tomography of bones has increased in the last 10 years, however, methods delivering clinically useful sound speed inversions are lacking.