Jesse F. Lawrence

Crust and upper mantle structure of the Transantarctic Mountains and surrounding regions from receiver functions, surface waves, and gravity: Implications for uplift models

[1] This study uses seismic receiver functions, surface wave phase velocities, and airborne gravity measurements to investigate the structure of the Transantarctic Mountains (TAM) and adjacent regions of the Ross Sea (RS) and East Antarctica (EA). Forty-one broadband seismometers deployed during the Transantarctic Mountain Seismic Experiment provide new insight into the differences between the TAM, RS, and EA crust and mantle. Combined receiver function and phase velocity inversion with niching genetic algorithms produces accurate crustal and upper mantle seismic velocity models. The crustal thickness increases from 20 ± 2 km in the RS to a maximum of 40 ± 2 km beneath the crest of the TAM at 110 ± 10 km inland. Farther inland, the crust of EA is uniformly 35 ± 3 km thick over a lateral distance greater than 1300 km. Upper mantle shear wave velocities vary from 4.5 km s � 1 beneath EA to 4.2 km s � 1 beneath RS, with a transition between the two at 100 ± 50 km inland near the crest of the TAM. The � 5k m thick crustal root beneath the TAM has an insufficient buoyant load to explain the entire TAM uplift, suggesting some portion of the uplift may result from flexure associated with a buoyant thermal load in the mantle beneath the edge of the TAM lithosphere.

Attenuation tomography of the western United States from ambient seismic noise

[1] We show that the spatial coherency of the ambient seismic field can be used for attenuation tomography in the western United States. We evaluate the real portion of the spatial coherency with an elastic geometric spreading term (a Bessel function) and a distance dependent decay (an attenuation coefficient). In order to invert the spatial coherency, a weight stack inversion technique is applied. We recover phase velocity and attenuation coefficient maps at periods of 8–32s, which correspond to the elastic and anelastic structure at crustal and upper mantle depths. The phase velocity maps obtained by this method are of similar resolution to more standard two‐station methods. The attenuation results provide an important complement to the information extracted from earthquake‐based tomography. Several geological features are easily identifiable in the attenuation coefficient maps, such as the highly attenuating sedimentary basins along the West Coast of the United States, and the highly attenuating Yellowstone region, and the boundaries of the Snake River Plains.

Combined Receiver-Function and Surface Wave Phase-Velocity Inversion Using a Niching Genetic Algorithm: Application to Patagonia

A combined inversion of body wave receiver functions and Rayleigh wave phase velocities using a niching genetic algorithm (NGA) increases the unique- ness of the solution over separate inversions and also facilitates explicit parameteri- zation of layer thickness in the model space. This parameterization requires fewer layers and a priori constraints for modeling more complex structures than traditional linearized inversion. NGAs solve for a suite of locally optimal solutions in addition to isolating the globally optimal solution by using an evolutionary paradigm. This nonlinear examination of the error space provides an opportunity to examine trade- off within the model space. The method, when applied to synthetic data, locates interfaces within 2 km of the test structure and identifies appropriate structure and velocity. Applying this method to data from a deployment of five broadband stations in Chilean Patagonia yields a regional model of crustal structure that is consistent with the geological history of the region. Sediment thickness is inversely proportional to crustal thickness, with sediment thicknesses reaching more than 4 km, where the crustal thickness thins to 28 km. This is consistent with previous geological studies, which suggests that the Rocas Verdes basin in western Patagonia formed by crustal thinning, isostatic compensation, and subsequent sedimentation.

Body wave extraction and tomography at Long Beach, California, with ambient‐noise interferometry

We retrieve P diving waves by applying seismic interferometry to ambient-noise records observed at Long Beach, California, and invert travel times of these waves to estimate 3-D P wave velocity structure. The ambient noise is recorded by a 2-D dense and large network, which has about 2500 receivers with 100 m spacing. Compared to surface wave extraction, body wave extraction is a much greater challenge because ambient noise is typically dominated by surface wave energy. For each individual receiver pair, the cross-correlation function obtained from ambient-noise data does not show clear body waves. Although we can reconstruct body waves when we stack correlation functions over all receiver pairs, we need to extract body waves at each receiver pair separately for imaging spatial heterogeneity of subsurface structure. Therefore, we employ two filters after correlation to seek body waves between individual receiver pairs. The first filter is a selection filter based on the similarity between each correlation function and the stacked function. After selecting traces containing stronger body waves, we retain about two million correlation functions (35% of all correlation functions) and successfully preserve most of body wave energy in the retained traces. The second filter is a noise suppression filter to enhance coherent energy (body waves here) and suppress incoherent noise in each trace. After applying these filters, we can reconstruct clear body waves from each virtual source. As an application of using extracted body waves, we estimate 3-D P wave velocities from these waves with travel time tomography. This study is the first body wave tomography result obtained from only ambient noise recorded at the ground surface. The velocity structure estimated from body waves has higher resolution than estimated from surface waves.

Constraining seismic velocity and density for the mantle transition zone with reflected and transmitted waveforms

We examine stacks of several seismic phases having different sensitivities to mantle transition zone structure. When analyzed separately, underside P and S reflections (PdP and SdS) are suggestive of very different structures despite similar raypaths and data coverage. By stacking the radial component of PdP rather than the vertical PdP, we show that this difference does not result from interference from other more steeply inclined phases such as PKP and Ppdpdiff. In general, stacks of P-to-S converted phases (Pds) appear to lack evidence of a 520-km discontinuity when examined without other phases. When these phases and stacked topside P reflections (Ppdp) are analyzed jointly using a nonlinear inversion method, consistent but nonunique, seismological models emerge. These models show that a discontinuity at ∼653 km depth has smaller contrasts in density and velocity than found in most previous studies. A sub-660 gradient can account for the majority of this difference. A 1.6 ± 0.5% P-velocity contrast and a 2.2 ± 0.3% density contrast at ∼518 km depth without a S-velocity contrast can explain the lack of a P520s, together with robust Pp520p and S520S phases. For models parameterized with a finite thickness for each discontinuity, the 410-km discontinuity is consistently ∼3 times thicker than the 660-km discontinuity.

Seismic Evidence for Subduction-Transported Water in the Lower Mantle

We use seismic attenuation tomography to identify a region at the top of the lower mantle that displays very high attenuation consistent with an elevated water content. Tomography inversions with >80,000 differential travel-time and attenuation measurements yield 3D whole-mantle models of shear velocity (V S ) and shear quality factor (Q μ ). The global attenuation pattern is dominated by the location of subducting lithosphere. The lowest Q μ anomaly in the whole mantle is observed at the top of the lower mantle (660-1400 km depth) beneath eastern Asia. The anomaly occupies a large region overlying the high-Q μ sheet-like features interpreted as subducted oceanic lithosphere. Seismic velocities decrease only slightly in this region, suggesting that water content best explains the anomaly. The subducting of Pacific oceanic lithosphere beneath eastern Asia likely remains cold enough to transport stable dense hydrous mineral phase D well into the lower mantle. We propose that the eventual decomposition of phase D due to increased temperature or pressure within the lower mantle floods the mantle with water, yielding a large low-Q μ anomaly.

Impulse Response of Civil Structures from Ambient Noise Analysis

Increased monitoring of civil structures for response to earthquake motions is fundamental to reducing seismic risk. Seismic monitoring is difficult because typically onlya few useful, intermediate to large earthquakes occur per decade near instrumented structures. Here, we demonstrate that the impulse response function (IRF) of a multistory building can be generated from ambient noise. Estimated shear- wave velocity, attenuation values, and resonance frequencies from the IRF agree with previous estimates for the instrumented University of California, Los Angeles, Factor building. The accuracy of the approach is demonstrated by predicting the Factor build- ings response to an M 4.2 earthquake. The methodology described here allows for rapid, noninvasive determination of structural parameters from the IRFs within days and could be used for state-of-health monitoring of civil structures (buildings, bridges, etc.) before and/or after major earthquakes. Online Material: Movies of IRF and earthquake shaking.

A novel strong-motion seismic network for community participation in earthquake monitoring

The Quake-Catcher Network (QCN) is breaking new ground in seismology by combining new micro-electro-mechanical systems (MEMS) technology with volunteer seismic station distributed computing. Rather than distributing just computations, the QCN allows volunteers to participate in scientific data collection and computation. Using these innovative tools, QCN will increase the number of strong-motion observations for improved earthquake detection and analysis in California, and throughout the world. QCNs increased density of seismic measurements will revolutionize seismology. The QCN, in concert with current seismic networks, may soon provide advanced alerts when earthquakes occur, estimate the response of a building to earthquakes even before they happen, and generate a greater understanding of earthquakes for scientists and the general public alike. Details of how one can join the QCN are outlined. In addition, we have activities on our website that can be used in K-16 classrooms to teach students basic seismology and physics concepts.

Performance of Several Low-Cost Accelerometers

Several groups are implementing low-cost host-operated systems of strong-motion accelerographs to support the somewhat divergent needs of seismologists and earthquake engineers. The Advanced National Seismic System Technical Implementation Committee (ANSS TIC, 2002), managed by the U.S. Geological Survey (USGS) in cooperation with other network operators, is exploring the efficacy of such systems if used in ANSS networks. To this end, ANSS convened a working group to explore available Class C strong-motion accelerometers (defined later), and to consider operational and quality control issues, and the means of annotating, storing, and using such data in ANSS networks. The working group members are largely coincident with our author list, and this report informs instrument-performance matters in the working group’s report to ANSS. Present examples of operational networks of such devices are the Community Seismic Network (CSN; csn.caltech.edu), operated by the California Institute of Technology, and Quake-Catcher Network (QCN; Cochran et al., 2009; qcn.stanford.edu; November 2013), jointly operated by Stanford University and the USGS. Several similar efforts are in development at other institutions. The overarching goals of such efforts are to add spatial density to existing Class-A and Class-B (see next paragraph) networks at low cost, and to include many additional people so they become invested in the issues of earthquakes, their measurement, and the damage they cause.

Rapid Earthquake Characterization Using MEMS Accelerometers and Volunteer Hosts Following the M 7.2 Darfield, New Zealand, Earthquake

We test the feasibility of rapidly detecting and characterizing earthquakes with the Quake-Catcher Network (QCN) that connects low-cost microelectromechan- ical systems accelerometers to a network of volunteer-owned, Internet-connected com- puters. Following the 3 September 2010 M 7.2 Darfield, New Zealand, earthquake we installed over 180 QCN sensors in the Christchurch region to record the aftershock se- quence. The sensors are monitored continuously by the host computer and send trigger reports to the central server. The central server correlates incoming triggers to detect when an earthquake has occurred. The location and magnitude are then rapidly esti- mated from a minimal set of received ground-motion parameters. Full seismic time series are typically not retrieved for tens of minutes or even hours after an event. We benchmark the QCN real-time detection performance against the GNS Science GeoNet earthquake catalog. Under normal network operations, QCN detects and characterizes earthquakeswithin9.1softheearthquakeruptureanddeterminesthemagnitudewithin 1 magnitude unit of that reported in the GNS catalog for 90% of the detections.