Active Research

Our research projects span a wide geographic area and address a range of topics within the field of active tectonics. Below is a list of some of our active and recent projects subdivided into catagories based on the large-scale problem each addresses. Follow the links for further information, resources, and data for each project.

Repeating Earthquakes at the Mendocino Triple Junction

The Mendocino Triple Junction (MTJ) is a seismically active region at the transition between the San Andreas Fault system, the Mendocino Transform Fault, and the Cascadia Subduction Zone in Northern California. The triple junction itself is located offshore, making it difficult to study using traditional land-based methods. In this project, we document the occurrence of characteristically repeating earthquakes (CREs) near the MTJ and interpret them as indicators of aseismic creep. The CRE data implies a creep rate of ~3 cm/yr on the downgoing extension of the Mendocino Transform Fault, consistent with other estimates of low coupling on oceanic transform faults. We also find repeating earthquakes on the southern margin of the North American accretionary wedge. This project demonstrates the ability of CREs to monitor aseismic creep and its time-dependent features in a plate boundary region outside the coverage of current geodetic networks.

Earthquake Cycle Deformation (top)

Potential for larger earthquakes in the San Francisco Bay Area due to the direct connection between the Hayward and Calaveras Faults

The Hayward and Calaveras Faults, two strike-slip faults of the San Andreas Fault System passing through the East San Francisco Bay Area and accommodating ~30% of the relative motion between the North American and Pacific plates, are commonly considered independent structures for seismic hazard assessment. Here we use InSAR to show that surface creep on the Hayward Fault continues 15 km farther south than previously known, revealing new potential for rupture and damage south of the city of Fremont. The extended trace of the creeping Hayward Fault, also illuminated by shallow repeating micro-earthquakes, documents a surface connection between the Hayward and Calaveras Faults. At depths greater than 3-5 km, repeating micro-earthquakes located 10 km north-northwest of the surface connection highlight the 3-D wedge geometry of the junction. Our new model of the Hayward and Calaveras Faults argues that they should be treated as a single system with potential for earthquake ruptures generating events with magnitudes significantly greater that 7, posing a higher seismic hazard to the San Francisco Bay Area than previously considered in most assessments.

Interseismic deformation and refined earthquake potential on the Hayward-Calaveras Fault system

Evaluation of interseismic strain accumulation and creep traditionally relies on GPS, alignment arrays, and creepmeters, providing precise, but spatially sparse measurements. Here, we use InSAR to resolve interseismic deformation associated with the Hayward and Calaveras Faults, in the East San Francisco Bay Area, with a high spatial resolution. We keep InSAR independent from GPS and alignment arrays data and show that a large SAR dataset enables characterization of short and long-wavelength deformation signal as small as 2 mm/yr without the need of aligning the velocity field to GPS-based model of strain accumulation. By implementing an interferogram selection directly based on the spatial coherence, our modified time series approach enables deformation mapping in vegetated areas and leads to refined estimates of along fault-creep rates. Creep varies from 0 on north Calaveras to ~12 mm/yr on central Calaveras, the Calaveras Fault reaching its maximum creep rate south of the surface junction with the Hayward Fault. We estimate long-term slip rate on these faults by inverting the long-wavelength deformation, and shallow slip due to creep using the short-wavelength deformation. The slip distribution reveals locations of locked and slow creeping patches with capabilities of a M6.4 near San Leandro, a M6.5 near Livermore, a M6.3 south of Fremont, and a M6.1 near Morgan Hill. By considering the potential for cascading multi-segment ruptures the Hayward Fault could produce a M6.9, the north Calaveras Fault a M6.7, the central Calaveras Fault a M6.9, and a joined rupture involving the Calaveras Fault and the Hayward Fault could produce a M7.1.

Twenty-five years of postseismic viscous relaxation following the Mw 6.9 Loma Prieta earthquake

The 1989 Mw 6.9 Loma Prieta earthquake provides the first opportunity of probing the crustal and upper mantle rheology in the San Francisco Bay Area since the 1906 Mw 7.9 San Francisco earthquake. Here we use geodetic observations including GPS and InSAR to characterize the 1989 Mw 6.9 Loma Prieta earthquake postseismic displacement from 1989.8 to 2013. Pre-earthquake deformation rates are constrained by nearly 20 years of USGS trilateration measurements and removed from the postseismic measurements prior to the analysis. We observe 1-4 mm/yr GPS horizontal displacement toward the Loma Prieta epicenter until 2000, and ~2 mm/yr surface subsidence in southern San Jose between 1992 and 2002, which is not associated with the seasonal hydrological deformation in the central Santa Clara Valley. Previous work indicates afterslip dominated in the early (1989-1994) postseismic period, so we model the postseismic viscoelastic relaxation between 1994 and 2010 based on geodetic observations. The best fitting result predicts a 11-km-thick viscoelastic lower crust with viscosity of 1019 Pa s below a 19-km-thick elastic upper crust, underlain by a bi-viscous upper mantle with transient viscosity of 10^17 and long-term viscosity of 10^18 Pa s. The sub-millimeter scale postseismic may not well resolve the viscosity in different layers, but the inferred lithospheric rheology is consistent with estimation in South California. Besides, the viscoelastic relaxation may also contribute to the increase of repeating earthquake activity on the San Andreas Fault near San Juan Bautista after the Loma Prieta event.

Viscoelastic Postseismic Deformation Following the 2011 Mw9.0 Tohoku Earthquake

The surprisingly large Mw9.0 Tohoku earthquake ruptured the interface of the subducting Pacific Plate over an approximately 400-km-long and 200-km-wide area and produced a devastating tsunami, on March 11th, 2011. Land GPS stations have recorded up to more than one meter postseismic displacements in two years since the earthquake. Viscoelastic relaxation in the upper mantle of the shear stresses induced by the earthquake and aseismic afterslip of the megathrust both contribute to the very rapid crustal deformation observed since the earthquake. In this research, we integrate the wealth of geodetic data from NE Japan and modeling experiences developed at other margins to investigate the effects of mantle rheology on ollowing the 2011 earthquake.

Postseismic: Probing deep rheology across the eastern margin of the Tibetan plateau: Constraints from the 2008 Mw 7.9 Wenchuan earthquake

The fundamental geological structure and rheology of the Tibetan plateau have been debated for decades. Two major models have been proposed: (1) the deformation in Tibet is distributed, and associated with ductile flow in the mantle or lower crustal flow (LCF); (2) the Tibetan plateau was formed during interactions among rigid blocks with localization of deformation along major faults. On 12 May, 2008, a Mw 7.9 earthquake occurred on the Longmen Shan that separates the eastern Tibetan plateau and the Sichuan basin. The earthquake ruptured ~235 km of the Beichuan fault (BCF) and the entire Pengguan fault (PGF). Geodetic inversions show more than 5 slip asperities and ~16 m peak slip on SW BCF. All of the slip models show oblique thrusting along the SW BCF and a right-slip component gradually increases towards the NE end of the BCF. The postseismic displacement is a response to the redistribution of stresses induced by the earthquake and can be used to probe the deep rheologic properties underneath the surface (Wang et al., 2012). Here we incorporate two-year long geodetic measurements and numerical modeling to examine two end-member hypotheses to provide further evidence to the deep rheology in eastern Tibetan plateau.

Postseismic: The far reach of megathrust earthquakes: Evolution of stress, deformation and seismicity following the 2004 Sumatra-Andaman rupture

The December 26, 2004 Sumatra-Andaman event was the largest earthquake since the 1960s and the first Mw 9.2 event to occur in the era of modern space geodesy and broadband seismology. It produced measurable static surface displacements at distances as large as 4500 km [e.g., Banerjee et al., 2007]. Three months later, it was followed by a second great Mw 8.7 event on an adjoining segment of the subduction zone. These infrequent events provide the rare opportunity to fundamentally improve our understanding of the earthquake cycle of megathrust ruptures and the constitutive properties of the adjoining oceanic and continental crust and upper mantle. Far-reaching deformation and stress transients following these events will impact other faults in the region for decades to come, and either enhance or diminish their likelihood to rupture. We seek as complete an understanding as possible of the processes contributing to this deformation and the resulting stress and seismicity changes throughout southeast Asia.

Interseismic and Creep: Bay Area Velocity Unification

What is the rate of strain accumulation on faults in the San Francisco Bay Area? We have compiled the most up-to-date velocity field for the Bay Area and use 3-D elastic models to determine the slip rates on Bay Area faults most consistent with our data. The project includes abundant field work collecting campaign style GPS data in urban and suburban settings. We combine our own data with all known geodetic grade GPS data collected over the last decade by six different agencies. When combined using a uniform methodology, we call this new data set "BAVU" (pronounced "Bay View", or the Bay Area Velocity Unification).

Fault Interaction and Seismicity (top)

Temporal Variations of Intraslab Earthquake Activity Following the 2011 Tohoku-Oki Earthquake

Understanding the mechanics of the subduction of cold oceanic lithosphere is crucial to the theory of plate tectonics and subduction zone seismogenesis. The locked zone between the subducting and overriding plates host the planet’s largest earthquakes, including all of the recorded events of moment magnitude 9.0 or larger. These thrust events have been extensively studied, however a significant portion of subduction zone events are intraplate events, which occur within, rather than along the boundary of, the subducting slab. These intraplate earthquakes are frequent and often damaging events that pose a large seismic hazard to large populations along convergent boundaries across the globe.

Fault interaction and predictability of repeating earthquake sequences at Parkfield

What determines the timing of earthquake recurrences and their regularity is of fundamental importance in understanding the earthquake cycle and has important implications for earthquake probability and risk estimates. This question cannot be answered without statistically sufficient observations of recurrence properties in natural earthquake populations.

Seismicity Changes and Aseismic Slip on the Sunda Megathrust Preceding the Mw 8.4 2007 Earthquake

The September 12, 2007 Sumatra Mw 8.4 earthquake initiated 750 km south of the 2005 epicenter. Twelve hours later a deeper Mw 7.9 aftershock ruptured further to the north. Their occurrence, close in time and space to the 2004 Mw 9.2 Sumatra-Andaman earthquake, 2005 Mw 8.7 Nias earthquake, and 2000 Mw 8.0 Enggano earthquake, suggest the possibility of these being triggered events. Coulomb failure stress models have shown that the 2000 earthquake had a larger impact at the 2007 hypocenter than the 2004 and 2005 earthquakes, and could have contributed to its southern location. We investigate seismicity changes and GPS-measured velocities, in the 2007 rupture region, for alternative triggering evidence.

Fault Structure, Geometry and Frictional Properties (top)

Tremor-tide correlations and near-lithostatic pore pressures on the deep San Andreas fault

New observations of tidal triggering of non-volcanic tremor near Parkfield, CA present a uniqueopportunity to better understand the nature of tremor and the conditions under which it occurs. Here we perform a full tidal analysis to determine the stress orientations and magnitudes that favor tremor generation on the lower-crustal San Andreas fault. Our results show that extremely small shear stress perturbations primarily influence tremor activity levels while much larger normal stress fluctuations and stressing rates have little to no influence. These findings are indicative of near-lithostatic pore pressures in the deep San Andreas fault zone and suggest that low effective normal stresses explain the response of non-volcanic tremor to tidal forcing.

Non-tectonic Deformation (top)

Predictability of hydraulic head changes and characterization of aquifer system properties from InSAR-derived ground deformation

We use InSAR time-series analysis of ERS, Envisat, and ALOS data to resolve 1992-2011 vertical ground deformation associated with hydrological processes in the Santa Clara Valley, California. We extract temporally variable deformation patterns embedded in the multi-decadal time series without a-priori constraints using T-mode Principal Component Analysis (TPCA). The longer-term deformation, which corresponds to poroelastic rebound of the aquifer system following recovery of hydraulic heads after the 1960s low stand, occurs mostly prior to 2000, leading to uplift one order of magnitude smaller than its preceding subsidence. Two patterns of seasonal deformation also exist, both sharply partitioned by the Silver Creek Fault (SCF), the fault being a barrier to across fluid flow. We show that by combining this seasonal deformation with hydraulic head data we can characterize basin-wide aquifer system properties and that after callibration, we can accurately estimate water level changes from the observed deformation without well measurements.

Inactive Research

Earthquake Cycle Deformation (top)

Plate Tectonics (top)

Volcano Deformation (top)

Fault Structure, Geometry, and Frictional Properties (top)

Non-Tectonic Deformation (top)

Active Research

Inactive Research