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.
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.
Coseismic: Joint Seismic and Geodetic Analysis of the 2009 Padang, Sumatra Intraslab Earthquake
The Mw 7.6 Padang earthquake occurred on 30 September 2009 offshore of central Sumatra (Figure 1). Seismicity in the Sumatra region is driven by the oblique subduction of the Indian and Australian plates beneath the Burma forearc block and Sunda plate at the Sunda trench. The Sunda megathrust has been extremely active ever since the 2004 Mw 9.2 Sumatra-Andaman earthquake, with additional megathrust earthquakes in 2005, 2007, and 2010. The last remaining section of the Sunda megathrust without a modern great earthquake is the Siberut segment, which lies offshore of Padang. Thus, it is especially important to understand the fault mechanism of the 2009 Padang earthquake in order to assess how it affects the stress levels on the Siberut segment of the megathrust.
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.
Postseismic: Postseismic variations in seismic moment and recurrence interval of repeating earthquakesProject Page
In laboratory experiments, longer stationary contact time leads to larger seismic moment during repeated ruptures. However, not all observations in natural fault systems agree with the prediction. We analyze a subset of 34 M 0.4 ~ 2.1 repeating earthquake sequences (RES) from 1987-2009 at Parkfield to examine the variation of their recurrence properties in space and time.
Interseismic and Creep: A case study of stress proxies on the Hayward fault
Currently, there are a number of seismicity analysis techniques that have been used to infer the state of stress within the lithosphere. While many of these "stress proxies" are based on theoretically plausible foundations and could potentially be incorporated in earthquake forecasting, rigorous comparisons between stress inversion results and other data that also reflect stress conditions at depth (e.g., fault creep rates) have not been performed. The Hayward fault presents an ideal locale in which to test the accuracy of these stress proxies in that we can qualitatively infer the stress along the fault by assuming that fault creep rates are inversely related to stress and thus that stress is high along asperities locked since the 1868 earthquake and low along sections that are accommodating a substantial amount of their slip budget through creep.
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).
Interseismic and Creep: Creep and Strain Accumulation on the Central San Andreas Fault
We are using campaign and continuous GPS data from the BAVU dataset, as well as stacked ERS1 & ERS2 interferograms to improve our knowledge of the regional deformation field and creep rates on the Central San Andreas fault system. Some of the current questions we hope to answer include: How much of these faults is creeping and how much is locked? What kinds of fault geometries (dip, locking depth) are required by our datasets? and What kind of fault zone rheologies are required?
Interseismic and Creep: Strain accumulation along the Hayward Fault
The Hayward fault is a major strand of the San Andreas fault system accommodating nearly 25% of the deformation across the plate boundary. The fault exhibits a diversity of slip behaviors including large coseismic rupture, frequent micro-seismicity, and aseismic creep. Surface creep is observed along the entire length of the fault in the range of 3-9 mm/yr and the rates appear to be consistent over the past several decades. The geologic slip rate on the fault is estimated at ~9 mm/yr. The difference between the geologic and contemporary creep rates suggests that a slip deficit exists and that the accumulated strain will presumably be released in a future earthquake.
The most significant earthquake to occur on the Hayward fault in historic times is a M6.8 event in 1868. The event nucleated on the southern portion of the fault and ruptured as far north as Berkeley. No earthquake greater than M4.5 has been observed on the Hayward fault since seismic instrumentation was installed in the early twentieth century. However, paleoseismic data suggest that several large earthquakes have produced surface rupture on both the northern and southern portions of the Hayward fault.
Interseismic and Creep: Detecting transient deformation along the central San Andreas Fault using InSAR
We use campaign and continuous GPS measurements as well as InSAR to measure the deformation from slow slip events on the San Andreas Fault. A series of slip events have been observed (1992, 1996, 1998) using strain- and creep-meters. Each event has been close to M5; as large as the largest seismic events in these areas. We have made two interferograms spanning the 1998 slow slip event which show significant shallow slip along the central San Andreas. These interferograms allow for the possibility that slip extended further south than inferred from the creep- and strain-meter records.
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.
Aseismic Slip and Fault Interaction from Repeating Earthquakes in the Loma Prieta Aftershock Zone
Along creeping sections of the San Andreas and other faults, small asperities in the fault zone load and fail in characteristic repeating earthquake sequences. By calculating their slip based on moment magnitude, they can be used as sub-surface creepmeters. We are using these virtual creepmeters to examine and compare slip rates on the creeping section of the San Andreas Fault (SAF) and on the associated nearby faults.
Examining the mechanical behavior and evolution of the southern San Andreas fault system through determination of late Quaternary slip rates and distinct element simulations
To understand the processes and mechanisms driving fault growth and crustal deformation we combine (1) geologic field mapping of offset landforms, (2) Quaternary geochronology to obtain precise estimates of fault slip rates, and (3) distinct element models (DEM) to simulate the behavior and interactions of faults within the southern San Andreas fault system. The southern San Andreas fault zone of the SAFS is an ideal structure to investigate and model processes of crustal deformation because a high rate of strain is localized on a relatively simple set of faults, the Mission Creek and Banning faults. Furthermore, it is the most poorly understood section of the southern SAFS with respect to slip rate and timing of past large-magnitude earthquakes; therefore, its seismic hazard is difficult to quantify. Nonetheless, the seismic hazard that it presents is likely to be high since this section of the fault system is the only major section that has not ruptured in historic time. The last earthquake to rupture occurred over 300 years ago c. 1690. Accordingly, the long lapse time since the last surface rupture implies that this section of the SAFS is in the late phase of its earthquake cycle, and that strain accumulated over the past 300 years is likely to be relieved in a large-magnitude earthquake.
The nature of deformation at Lake Pillsbury, CA
In May of 2000 the Pillsbury Lake region, a seismically quiescent region bounded by the Ma’acama and Bartlett Springs faults, experienced a burst of seismic activity lasting for six months and culminating in an M4.2 event. A similar pattern of earthquake activity occurred in April 2007 with mainshock of M4.8. Previous studies have interpreted this anomalous activity as evidence of magma transport in the crust caused by the northward migration of the Mendocino Triple Junction. However, a magma intrusion does not fully explain the regional geophysical activity and the spatial seismicity patterns associated with the two swarms. The question to be addressed in this study is whether this theory is correct, namely, is the seismicity near Pillsbury Lake due to crustal dike propagation?
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.
Fault structure and kinematics from characteristically repeating earthquakesProject Page
We first relocated seismicity at the juncture of these two major faults to determine the subsurface seismic structure. Next we identified characteristically repeating earthquakes (CREs) using waveform cross-correlation techniques to investigate how slip at depth is being partitioned in this complex juncture zone. CREs are earthquakes with extremely similar waveforms that we believe are caused by repeated slip on the same fault zone patch. Essentially, our method treats each CRE sequence as a "creepmeter" at depth and derives a slip rate estimate at the location of the earthquake.
Barnes Ice Cap: an InSAR study of post-glacial rebound
The objective of this study is to determine the elastic properties of the lithosphere in Baffin Island Canada by observing and modeling the lithospheric response to the rapid melting of the Barnes ice cap. We anticipate a substantial vertical deformation signal is present due to the glaciodynamics associated with Barnes ice cap. In order to observe this signal we plan to assemble multiple interferograms from the regional SAR acquisitions dating from 1991. The magnitude, spatial extent, temporal evolution, and thinning rate estimates can be modeled to constrain the regional elastic properties of the lithosphere surrounding Barnes ice cap. Additionally, a study of this nature would help to inform the debate on the roles of and interaction between the crust and upper mantle in supporting topographic loads.
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.
Coseismic DeformationCoseismic and postseismic deformation data and models of the August 17, 1999 Izmit earthquake and the November 12, 1999 Duzce earthquake
Surface deformation of the 2001 Bhuj earthquake from satellite imagery and InSAR data
Postseismic DeformationProbing the Rheology of Tibet using Postseismic Motion from Large Earthquakes
Earthquake triggering from the 1992 Landers and 1999 Hector Mine earthquakes
Powerlaw relaxation following the Landers and Hector Mine earthquakes
Time-dependent depth distribution of aftershocks
Postseismic deformation following the 2002 Denali earthquake
Interseismic Deformation and CreepCreep on the Central San Andreas Fault
Surface creep along the Longitudinal Valley Fault, Taiwan from InSAR measurments
Indian Plate Motion, Deformation, and Plate Boundary Interactions
Unrest at large Quaternary silicic calderas, Long Valley Caldera, southern CA
Paleoseismic Analysis of the South Flank of Kilauea Volcano
Quantifying frictional heating on the San Gabriel Fault in southern CA using fission-track thermochronometry