Validation of Shear‐Wave Velocity Models of the Pacific Northwest

Four surface‐wave tomographic models in the Pacific Northwest and a combined CRUST2.0 and AK135 model are tested and validated systematically. Synthetic Green’s functions calculated with the models using a finite‐difference method are compared with empirical Green’s functions at periods of 7–50 s. To ensure high‐quality signals, empirical Green’s functions are extracted from the ambient noise cross correlation of vertical‐to‐vertical components between station pairs that have up to a decade of recorded data. The observed and synthetic Green’s functions are cross correlated at multiple frequency bands to determine phase delay times and cross‐correlation coefficients. The delay time predicted by the CRUST2.0 and AK135 model is predominantly positive and is linearly dependent on interstation distance, indicating that the combined model is, on average, too fast for the Pacific Northwest. Among the four shear‐wave velocity models, CUB and one model derived from regional tomography exhibit moderately and weakly negative linear trends, respectively, between the delay time and interstation distance, a result indicative of a slower‐than‐actual velocity. The delay times of the other two models are normally distributed with an approximately zero mean and without any apparent relationship with interstation distance. The cross‐correlation coefficients are more scattered at short periods, reflecting unresolved heterogeneities of the crust structure in these models. The misfit between the empirical Green’s functions and synthetic waveforms suggests the need for a better‐resolved crust and uppermost mantle velocity model, which is critical for the precise estimate of ground motion for seismic hazard evaluation and understanding of the tectonic processes of the Pacific Northwest

Stress Drop Variation of Deep‐Focus Earthquakes Based on Empirical Green’s Functions

We analyze source characteristics of global, deep‐focus (>350 km) earthquakes with moment magnitudes (Mw) larger than 6.0–8.2 using teleseismic P‐wave and S‐wave spectra and an empirical Green’s functions approach. We estimate the corner frequency assuming Brune’s source model and calculate stress drops assuming a circular crack model. Based on P‐wave and S‐wave spectra, the one standard deviation ranges are 3.5–369.8 and 8.2–328.9 MPa, respectively. Based on the P‐wave analysis, the median of our stress drop estimates is about a factor of 10 higher than the median stress drop of shallow earthquakes with the same magnitude estimated by Allmann and Shearer (2009, https://doi.org/10.1029/2008JB005821). This suggests that, on average, the shear stress of deep faults in the mantle transition zone is an order of magnitude higher than the shear stress of faults in the crust. The wide range of stress drops implies coexistence of multiple physical mechanisms.Plain Language SummaryThe change of shear stress (i.e., stress drop) during an earthquake is thought to be larger for deeper earthquakes than shallow earthquakes because of higher overburden pressure. However, the observational evidence for stress drop dependence on depth is still inconclusive. We estimate stress drops of earthquakes deeper than 400 km from recorded ground motion spectra. We find that the median stress drop of deep earthquakes is about one order of magnitude higher than the stress drop of shallow (<50 km) earthquakes. This implies that the shear stress of deep faults is moderately higher than of faults in the crust. The wide range of our stress drop estimates suggests that various mechanisms producing deep earthquakes coexist.Key PointsEmpiricalGreen’s functions are applied to analyze stress drops of deep‐focus earthquakesOne standard deviation ranges are 3.5–369.8 MPa for P waves and 8.2–328.9 MPa for S wavesThe median stress drops suggest that fault shear stress is an order of magnitude higher in the mantle than in the crustPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154937/1/grl60493_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154937/2/grl60493.pd

A window into the complexity of the dynamic rupture of the 2011 Mw 9 Tohoku-Oki earthquake

The 2011 Mw 9 Tohoku-Oki earthquake, recorded by over 1000 near-field stations and multiple large-aperture arrays, is by far the best recorded earthquake in the history of seismology and provides unique opportunities to address fundamental issues in earthquake source dynamics. Here we conduct a high resolution array analysis based on recordings from the USarray and the European network. The mutually consistent results from both arrays reveal rupture complexity with unprecedented resolution, involving phases of diverse rupture speed and intermittent high frequency bursts within slow speed phases, which suggests spatially heterogeneous material properties. The earthquake initially propagates down-dip, with a slow initiation phase followed by sustained propagation at speeds of 3 km/s. The rupture then slows down to 1.5 km/s for 60 seconds. A rich sequence of bursts is generated along the down-dip rim of this slow and roughly circular rupture front. Before the end of the slow phase an extremely fast rupture front detaches at about 5 km/s towards the North. Finally a rupture front propagates towards the south running at about 2.5 km/s for over 100 km. Key features of the rupture process are confirmed by the strong motion data recorded by K-net and KIK-net. The energetic high frequency radiation episodes within a slow rupture phase suggests a patchy image of the brittle-ductile transition zone, composed of discrete brittle asperities within a ductile matrix. The high frequency is generated mainly at the down-dip edge of the principal slip regions constrained by geodesy, suggesting a variation along dip of the mechanical properties of the mega thrust fault or their spatial heterogeneity that affects rise time

Source Mechanism and Rupture Directivity of the 18 May 2009 M_W 4.6 Inglewood, California, Earthquake

On 18 May 2009, an M_w 4.6 earthquake occurred beneath Inglewood, California, and was widely felt. Though source mechanism and its location suggest that the Newport–Inglewood fault (NIF) may be involved in generating the earthquake, rupture directivity must be modeled to establish the connection between the fault and the earthquake. We first invert for the event’s source mechanism and depth with the cut-and-paste method in the long-period band (>5 s). Because of the low velocity shallow sediments in the Los Angeles (LA) basin, we use two velocity models in the inversion for stations inside and outside the LA basin. However, little difference is observed in the resolved source mechanism (M_w 4.6, strike 246°/145°, dip 50°/77°, rake 17°/138°) and depth (7 to ~9 km), compared to an inversion using the standard southern Calfornia model. With the resolved source parameters, we calibrate the amplitude anomaly of the short-period (0.5–2 Hz) P waves with amplitude adjustment factors (AAF). These AAFs are used as corrections when retrieving source mechanisms of the smaller aftershocks using short-period P waves alone. Most of the aftershocks show similar source mechanisms as that of the mainshock, providing ideal empirical Green’s functions (EGFs) for studying its rupture process. We use a forward modeling approach to retrieve rupture directivity of the mainshock, consistent with movement on the NIF with rupture toward the southeast. Although we focus on P waves for analyzing rupture directivity, the resolved unilateral pattern is also confirmed with the azimuthal variation of the duration of SH waves observed in the basin. The high rupture velocity near the shear velocity and relatively low stress drop are consistent with the hypothesis of rupture on a mature fault

3-D SHEAR WAVE VELOCITY STRUCTURES OF THE CRUST AND UPPER MANTLE BENEATH CASCADIA AND NEW ZEALAND FROM FULL-WAVE AMBIENT NOISE TOMOGRAPHY

The (de)hydration process and the amount of hydrated sediment carried by the downgoing oceanic plate play a key role in the subduction dynamics. The deformation and (de)hydration of the downgoing tectonic plates, as well as the seismic, tsunami, volcanic hazards, in Cascadia and the New Zealand regions are not fully understood, partly due to a lack of combined studies of onshore and offshore data. In order to address these questions, we developed a 3-D high-resolution shear wave velocity model beneath Cascadia, the North and the South Islands of New Zealand, extending from offshore to onshore, with the use of full-wave ambient noise tomographic method and joint inversion with active seismic data. We extracted the empirical Green’s functions from continuous seismic records on the vertical components between each station pair that provide high-quality Rayleigh-wave signals at periods of 4-50 s for Cascadia and 5-150 s for the New Zealand. We simulate wave propagation using finite-difference method to generate station Strain Green’s Tensors and synthetic waveforms. The phase delays of Rayleigh waves between the observed and synthetic data are measured at multiple period ranges. We then invert for the velocity perturbations from the reference model and progressively improve the model resolution. Our tomographic imaging shows many regional- and local-scale low-velocity features, which are possibly related to slab (de)hydration from the oceanic plate to the overriding plate. Moreover, seismic lows atop the plate interface beneath the forearc, indicating fluid-rich sediments subducted and overthrusted at the accretionary wedge. Furthermore, high shear wave velocity patches at the depths of 35-100 km beneath South Island of New Zealand, indicating the Pacific slab being segmented due to stretch during initiation of plate margins and due to current plate motions. Finally, the relatively low shear wave velocities at 100- 200 km depth beneath the South Island of New Zealand, indicating possible asthenospheric upwelling. Our full-wave tomographic models provide new constraints on our understanding of plate deformation, (de)hydration, slab segmentation, and asthenospheric upwelling beneath Cascadia and the New Zealand regions

Determination of earthquake focal depths and source time functions in central Asia using teleseismic P waveforms

We developed a new method to determine earthquake source time functions and focal depths. It uses theoretical Green's function and a time-domain deconvolution with positivity constraint to estimate the source time function from the teleseismic P waveforms. The earthquake focal depth is also determined in the process by using the time separations of the direct P and depth phases. We applied this method to 606 earthquakes between 1990 and 2005 in Central Asia. The results show that the Centroid Moment Tensor solutions, which are routinely computed for earthquake larger than M5.0 globally using very long period body and surface waves, systematically over-estimated the source depths and durations, especially for shallow events. Away from the subduction zone, most of the 606 earthquakes occurred within the top 20 km of crust. This shallow distribution of earthquakes suggests a high geotherm and a weak ductile lower crust in the region

Three-dimensional variations of the slab geometry correlate with earthquake distributions at the Cascadia subduction system

Significant along-strike variations of seismicity are observed at subduction zones, which are strongly influenced by physical properties of the plate interface and rheology of the crust and mantle lithosphere. However, the role of the oceanic side of the plate boundary on seismicity is poorly understood due to the lack of offshore instrumentations. Here tomographic results of the Cascadia subduction system, resolved with full-wave ambient noise simulation and inversion by integrating dense offshore and onshore seismic datasets, show significant variations of the oceanic lithosphere along strike and down dip from spreading centers to subduction. In central Cascadia, where seismicity is sparse, the slab is imaged as a large-scale low-velocity feature near the trench, which is attributed to a highly hydrated and strained oceanic lithosphere underlain by a layer of melts or fluids. The strong correlation suggests that the properties of the incoming oceanic plate play a significant role on seismicity

Laboratory simulations of fluid/gas induced micro-earthquakes:application to volcano seismology

Understanding different seismic signals recorded in active volcanic regions allows geoscientists to derive insight into the processes that generate them. A key type is known as Low Frequency or Long Period (LP) event, generally understood to be generated by different fluid types resonating in cracks and faults. The physical mechanisms of these signals have been linked to either resonance/turbulence within fluids, or as a result of fluids ‘sloshing’ due to a mixture of gas and fluid being present in the system. Less well understood, however, is the effect of the fluid type (phase) on the measured signal. To explore this, we designed an experiment in which we generated a precisely controlled liquid to gas transition in a closed system by inducing rapid decompression of fluid-filled fault zones in a sample of basalt from Mt. Etna Volcano, Italy. We find that fluid phase transition is accompanied by a marked frequency shift in the accompanying microseismic dataset that can be compared to volcano seismic data. Moreover, our induced seismic activity occurs at pressure conditions equivalent to hydrostatic depths of 200 to 750 meters. This is consistent with recently measured dominant frequencies of LP events and with numerous models