Benjamin A. Brooks

The 2010 Mw 8.8 Maule Megathrust Earthquake of Central Chile, Monitored by GPS

Rupture kinematics of this very large earthquake were obtained from high-resolution Global Positioning System data. Large earthquakes produce crustal deformation that can be quantified by geodetic measurements, allowing for the determination of the slip distribution on the fault. We used data from Global Positioning System (GPS) networks in Central Chile to infer the static deformation and the kinematics of the 2010 moment magnitude (Mw) 8.8 Maule megathrust earthquake. From elastic modeling, we found a total rupture length of ~500 kilometers where slip (up to 15 meters) concentrated on two main asperities situated on both sides of the epicenter. We found that rupture reached shallow depths, probably extending up to the trench. Resolvable afterslip occurred in regions of low coseismic slip. The low-frequency hypocenter is relocated 40 kilometers southwest of initial estimates. Rupture propagated bilaterally at about 3.1 kilometers per second, with possible but not fully resolved velocity variations.

The Nazca -South America Euler vector and its rate of change

We present velocities relative to the South American plate for five GPS stations on the Nazca plate and use these measurements to estimate the modern Euler vector. We find a pole at 55.88N, 92.58W with a rotation rate of 0.60 8/Myr. Because the GPS station at Easter Island appears to be moving at approximately 6.6 mm/yr relative to the other Nazca stations, we repeat our analysis with this station excluded from the inversion to obtain a second and preferred result (called CAP10) with a pole at 61.08N, 94.48W and a rate of 0.57 8/Myr. We compare these results with published finite rotation vectors and infer that during the past 10 – 20 Myrs, the Nazca – South America rotation rate has decelerated by 0.048 – 0.06 8/Myr 2 .

Accuracy and Resolution of ALOS Interferometry: Vector Deformation Maps of the Father's Day Intrusion at Kilauea

We assess the spatial resolution and phase noise of interferograms made from L-band Advanced Land Observing Satellite (ALOS) synthetic-aperture-radar (SAR) data and compare these results with corresponding C-band measurements from European Space Agency Remote Sensing Satellite (ERS). Based on cross-spectral analysis of phase gradients, we find that the spatial resolution of ALOS interferograms is 1.3times better than ERS interferograms. The phase noise of ALOS (i.e., line-of-sight precision in the 100-5000-m wavelength band) is 1.6times worse than ERS (3.3 mm versus 2.1 mm). In both cases, the largest source of error is tropospheric phase delay. Vector deformation maps associated with the June 17, 2007 (Fathers day) intrusion along the east rift zone of the Kilauea Volcano were recovered using just four ALOS SAR images from two look directions. Comparisons with deformation vectors from 19 continuous GPS sites show rms line-of-site precision of 14 mm and rms azimuth precision (flight direction) of 71 mm. This azimuth precision is at least 4times better than the corresponding measurements made at C-band. Phase coherence is high even in heavily vegetated areas in agreement with previous results. This improved coherence combined with similar or better accuracy and resolution suggests that L-band ALOS will outperform C-band ERS in the recovery of slow crustal deformation.

The 2010 Maule, Chile earthquake: Downdip rupture limit revealed by space geodesy

Radar interferometry from the ALOS satellite captured the coseismic ground deformation associated with the 2010 Mw 8.8 Maule, Chile earthquake. The ALOS interferograms reveal a sharp transition in fringe pattern at ~150 km from the trench axis that is diagnostic of the downdip rupture limit of the Maule earthquake. An elastic dislocation model based on ascending and descending ALOS interferograms and 13 near-field 3-component GPS measurements reveals that the coseismic slip decreases more or less linearly from a maximum of 17 m (along-strike average of 6.5 m) at 18 km depth to near zero at 43–48 km depth, quantitatively indicating the downdip limit of the seismogenic zone. The depth at which slip drops to near zero appears to be at the intersection of the subducting plate with the continental Moho. Our model also suggests that the depth where coseismic slip vanishes is nearly uniform along the strike direction for a rupture length of ~600 km. The average coseismic slip vector and the interseismic velocity vector are not parallel, which can be interpreted as a deficit in strike-slip moment release.

Mid-Atlantic Ridge volcanism from deep-towed side-scan sonar images, 25°-29°N

Abstract We present deep-towed side-scan sonar mosaics of the inner valley floor of eight spreading segments at the slow-spreading Mid-Atlantic Ridge between 25 ° and 29 °N. An analysis of these images, which well-resolve features a few tens of meters in size, confirms that the multitude of small seamounts, with diameters between 0.5 and 3 km identified on the inner valley floor from previously collected multibeam bathymetry data, are volcanically constructed. Moreover, these images reveal that these volcanoes have distinct surface morphologies not evident in the coarser resolution multibeam bathymetry maps: 83% of the seamounts have a hummocky (bulbous) morphology; the other 17% have a smooth morphology. In addition to near-circular seamounts, small (1–2 km long) volcanic ridges are abundant in our study regions, and are not, in general, seen in the bathymetry maps. We combine these new morphological data with existing models for the construction of the shallow oceanic crust to obtain a better understanding of the melt delivery system that builds the distinctive seafloor topography at the slow-spreading Mid-Atlantic Ridge.

Bending the Bolivian orocline in real time

Global positioning system (GPS) data from the central Andes record vertical axis rotations that are consistently counterclockwise in Peru and Bolivia north of the bend in the mountain belt, and clockwise to the south in southern Bolivia, Argentina, and Chile. These geologically instantaneous rotations have the same sense as rotations that have accrued over millions of years and are recorded by paleomagnetic and geologic indicators. The change in sign of the rotation at both decadal and million-year time scales occurs across the axis of topographic symmetry that defines the Bolivian orocline. When extrapolated to a common time interval, the magnitudes of rotation from geologic features and from GPS are surprisingly similar, given that a significant part of the instantaneous deformation field is probably elastic and due to interseismic locking of the plate boundary. Some of the interseismic deformation field must reflect permanent deformation, and/or some of the current elastic deformation will be converted to upper-plate permanent deformation over time rather than be recovered by elastic rebound during interplate earthquakes. We suggest that the spatial patterns of the elastic and the permanent modes of bending are similar because they are driven by the same stress field.

Magmatically Triggered Slow Slip at Kilauea Volcano, Hawaii

We demonstrate that a recent dike intrusion probably triggered a slow fault-slip event (SSE) on Kilauea volcanos mobile south flank. Our analysis combined models of Advanced Land Observing Satellite interferometric dike-intrusion displacement maps with continuous Global Positioning System (GPS) displacement vectors to show that deformation nearly identical to four previous SSEs at Kilauea occurred at far-field sites shortly after the intrusion. We model stress changes because of both secular deformation and the intrusion and find that both would increase the Coulomb failure stress on possible SSE slip surfaces by roughly the same amount. These results, in concert with the observation that none of the previous SSEs at Kilauea was directly preceded by intrusions but rather occurred during times of normal background deformation, suggest that both extrinsic (intrusion-triggering) and intrinsic (secular fault creep) fault processes can lead to SSEs.

Geodetic Constraints on the 2014 M 6.0 South Napa Earthquake

On 24 August 2014, the M 6.0 South Napa earthquake shook much of the San Francisco Bay area, leading to significant damage in the Napa Valley. The earthquake occurred in the vicinity of the West Napa fault (122.313° W, 38.22° N, 11.3 km), a mapped structure located between the Rodger’s Creek and Green Valley faults, with nearly pure right‐lateral strike‐slip motion (strike 157°, dip 77°, rake –169°; http://comcat.cr.usgs.gov/earthquakes/eventpage/nc72282711#summary, last accessed December 2014) (Fig. 1). The West Napa fault previously experienced an M 5 strike‐slip event in 2000 but otherwise exhibited no previous definitive evidence of historic earthquake rupture (Rodgers et al., 2008; Wesling and Hanson, 2008). Evans et al. (2012) found slip rates of ∼9.5  mm/yr along the West Napa fault, with most slip rate models for the Bay area placing higher slip rates and greater earthquake potential on the Rodger’s Creek and Green Valley faults, respectively (e.g., Savage et al., 1999; d’Alessio et al., 2005; Funning et al., 2007).

Crowdsourced earthquake early warning

Consumer devices and real and simulated earthquake data demonstrate that earthquake early warning can be achieved via crowdsourcing. Earthquake early warning (EEW) can reduce harm to people and infrastructure from earthquakes and tsunamis, but it has not been implemented in most high earthquake-risk regions because of prohibitive cost. Common consumer devices such as smartphones contain low-cost versions of the sensors used in EEW. Although less accurate than scientific-grade instruments, these sensors are globally ubiquitous. Through controlled tests of consumer devices, simulation of an Mw (moment magnitude) 7 earthquake on California’s Hayward fault, and real data from the Mw 9 Tohoku-oki earthquake, we demonstrate that EEW could be achieved via crowdsourcing.

Compact Multipurpose Mobile Laser Scanning System — Initial Tests and Results

We describe a prototype compact mobile laser scanning system that may be operated from a backpack or unmanned aerial vehicle. The system is small, self-contained, relatively inexpensive, and easy to deploy. A description of system components is presented, along with the initial calibration of the multi-sensor platform. The first field tests of the system, both in backpack mode and mounted on a helium balloon for real-world applications are presented. For both field tests, the acquired kinematic LiDAR data are compared with highly accurate static terrestrial laser scanning point clouds. These initial results show that the vertical accuracy of the point cloud for the prototype system is approximately 4 cm (1σ) in balloon mode, and 3 cm (1σ) in backpack mode while horizontal accuracy was approximately 17 cm (1σ) for the balloon tests. Results from selected study areas on the Sacramento River Delta and San Andreas Fault in California demonstrate system performance, deployment agility and flexibility, and potential for operational production of high density and highly accurate point cloud data. Cost and production rate trade-offs place this system in the niche between existing airborne and tripod mounted LiDAR systems.