Earthquake Science Center Seminars

Recent results from an EGS project in Finland suggest a possibly successful physics-based approach in controlling stimulation-induced seismicity in geothermal projects. We analyzed the temporal evolution of seismicity and the growth of maximum observed moment magnitudes for a range of past and present stimulation projects. Our results show that the majority of the stimulation campaigns investigated reveal a clear linear relation between injected fluid volume, hydraulic energy and cumulative seismic moments. For most projects studied, the observations are in good agreement with existing physical models that predict a relation between injected fluid volume and maximum seismic moment of induced events. This suggest that in cases seismicity results from a stable, pressure-controlled rupture process at least for an extended injection period. Overall evolution of seismicity is independent of tectonic stress regime and is most likely governed by reservoir specific parameters, such as the preexisting structural inventory. However, EGS stimulation campaigns have also shown that in addition to total fluid volume injected also the rates of injection and fluid pressure increase affect seismic moment release. This aspect is not included in current models. To investigate the effect of injection rate on slip characteristics, strain partitioning and energy budget, we performed laboratory fluid injection experiments on reservoir sandstone samples in a triaxial deformation apparatus equipped with a 16-channel acoustic emission (AE) recording system. We injected fluid in sawcut samples containing a critically stressed fault at different pressurization rates. In general, fluid-induced fault deformation is dominantly aseismic. We find slow stick-slip events are induced at high fluid pressurization rate while steady fault creep occurs in response to low fluid pressurization rate. The released total seismic moment is found to be related to total injected volume, independent of fault slip behavior. Seismic moment release rate of AE is related to measured fault slip velocity. Total potential energy change and fracture energy release rate are defined by fault stiffness and largely independent of injection rate. Breakdown power density scales with slip rate and is significantly higher for fast injection and pressurization rates. The relation between moment release and injected volume is affected by fault slip behavior, characterized by a linear relation for slip at constant rate and fault creep while a cubic relation is evident for unstable and dynamic slip. Our experimental results allow separating a stable pressure-controlled injection phase with low rate of energy dissipation from a run-away phase, where breakdown power is high and cumulative moment release with injected volume is nonlinear.

Ecuador, located in South America, is roughly the size of Nevada, but is very active geologically. There are 84 Quaternary volcanoes, and three large fault systems that represent important sources of seismic hazard. These fault systems are the Nazca subduction zone, which sourced the 2016 Mw7.8 Pedernales earthquake and is the most studied fault system, the Subandean backarc fold and thrust belt, and the Interandean Valley fault system. The latter two have had well-documented historical and instrumental seismicity, but remain poorly understood and will be the focus of my talk. In the Interandean Valley fault system (analogous to the Sumatran fault system) we are currently carrying out several projects mapping intermontane basins to help constrain its Neogene history and with InSAR to constrain active fault processes. In the Subandean backarc fold and thrust belt we have carried out preliminary mapping in its northern section to constrain the structural geometry there. Our goal is to provide accurate and detailed source faults that can be used to improve the seismic hazard assessment for major cities and infrastructure in the country.

Indonesian seismicity provides important insights into the tectonics and hazards of a region that is characterized by a remarkable diversity in faulting, including subduction, extension, thrusting, and strike-slip faulting. We present a synthesis of Indonesian seismotectonics by documenting the distributions of hypocenters ( M 4.6) and focal mechanisms ( M 5.0) over ~20 years, quantifying seismicity rates, and comparing observed seismicity trends with proposed tectonic models. Of the 20,622 events M 4.6 observed in the study region, ~77% of seismicity are shallow ( 70 km depth) and of magnitudes < M 5.0 (68%). 61 events M 7.0 occurred, five of which exceeded M 8.0, including the 2004 Mw 9.1 Sumatra-Andaman earthquake. Regionally, about ~320 M 5.0 earthquakes occur per year, and rates decrease exponentially between 50-300 km with significantly elevated seismicity in the Mantle Transition Zone (MTZ). Intermediate and deep events ( 70 km depth) trace the Wadati-Benioff zones of several subducting slabs exhibiting a geometry consistent with recent tomography models. Seismicity extends to a maximum depth of 678 km. Oblique convergence, lithospheric age, ambient mantle temperatures and viscous resistance at the 410, 520, and 660 km phase boundaries likely contribute to the non-uniform depth distribution of intermediate and deep earthquakes. Shallow seismicity provides insight into how complex oblique convergence is accommodated near the surface, with primary sources including megathrusting, crustal faulting, and shallow intraslab faulting. All sources of shallow seismicity constitute significant seismic hazards.

For years, there have been lines of evidence that inform the weak vs. strong fault debate. Strong faults, or fault segments, are mainly characterized by a granular load-bearing structure, LBS, with a frictional strength controlled by the Byerlee's rule of friction. With increasing strain, localization along a principal slip zone favors the passage from rate strengthening to rate weakening behavior. Along other faults, or fault segments, fluid-assisted reaction softening promotes the development of interconnected phyllosilicate networks, IFN, that produce a significant reduction in friction and a marked rate-strengthening behaviour. LBS are prone to develop relatively high permeabilities (k > 10-16 m2), in particular at low effective normal stresses and within the damage zone, whereas IFN tend to reduce permeability significantly (k < 10-19 m2).

Fluid pressure stimulations conducted on laboratory faults show that: 1) within the LBS fluid pressure steps easily diffuse within the fault and promote acceleration of slip that spontaneously evolves into dynamic failure; 2) in the IFN fluid pressure steps are associated with periodic accelerations and decelerations, that seem to be modulated by fluid pressure diffusion within the shear zones. In addition, for high levels of pressurization, slip velocity remains slow (i.e. v~200 µm/s), never approaching dynamic slip rates.

Our integrated dataset of structural and mechanical data from natural and experimental structures indicate that fault slip behaviour and instability are strongly controlled by the coupling between structural, frictional and permeability properties of faults.

Topography affects earthquake ground motions by amplifying shaking at topographic highs and reducing shaking at topographic lows. Amplification is typically greatest at a characteristic frequency related to the topography's shape and varies based on the wavefield's polarization relative to a feature's principal axis of elongation. While topographic effects have been invoked to help explain patterns of increased damage during some major earthquakes, they are not generally considered in modeling contexts. This is largely because topographic effects are highly variable from one event to the next, and they often only manifest at higher frequencies. As a result, questions remain over how topography may contribute to the maximum predicted ground motion at a given location, and how dependent the effects are on source parameters. We simulate M7.0 earthquakes on the Seattle Fault in Washington State using a spectral element method code (SPECFEM3D) to help constrain the sensitivity of topographic effects to kinematic earthquake rupture parameters. The simulations use a 3D velocity model that includes local sedimentary basin structure and random near-surface velocity heterogeneities. A 30m mesh spacing near the surface of the model is used to specify the topography and allows for accurate ground motion prediction up to 3Hz. We isolate the effects of topography by comparing simulations with and without a topographic surface, using the ratios of peak ground velocity and spectral acceleration to identify systematic amplification. By running multiple earthquake scenarios on the same sections of fault, we evaluate the sensitivity of topographic effects to hypocenter location and slip distribution, as well as demonstrate the relative importance of topographic amplification on high frequency ground shaking.

The San Francisco Bay area's Hayward fault produces large earthquakes, with the last occurring in 1868. We use dynamic rupture modeling to numerically simulate large earthquakes on the Hayward fault and its connected companions to the north and south, the Rodgers Creek and Calaveras faults. We incorporate information about this fault system's 3D geology and 3D geometry, and inferences about its interseismic creep-rate pattern and rock-friction behavior, into a finite-element computer code to perform 3D dynamic earthquake rupture simulations. We find that crucial factors controlling rupture behavior in our models are the earthquake nucleation locations, the fault geometry, and the data that reveal where the fault system is creeping or locked. Our findings suggest that large Rodgers Creek-Hayward-Calaveras-Northern Calaveras (RC-H-C-NC) fault-system earthquakes may result from dynamic rupture that starts in a locked part of the fault system, but is then stopped by creeping parts, leading to high-magnitude-6 earthquakes; or, from dynamic rupture that starts in a locked part of the fault system, then cascades through some of the creeping parts, leading to magnitude-7 earthquakes.

Harris, R. A., Barall, M., Lockner, D. A., Moore, D. E., Ponce, D. A., Graymer, R. W., Funning, G., Morrow, C. A., Kyriakopoulos, C., and Eberhart-Phillips, D. (2021). A geology and geodesy based model of dynamic earthquake rupture on the Rodgers Creek-Hayward-Calaveras fault system, California. Journal of Geophysical Research: Solid Earth, 126, e2020JB020577. https://doi.org/10.1029/2020JB020577

Oceanic transform faults (OTFs) are a significant component of the global plate boundary system. How and why fault slip on OTFs is accommodated is still not fully understood. Here, we conducted a global survey of seismic segmentation on OTFs by relocating M5 earthquakes on OTFs. We show that creeping segments are ubiquitous, and they contribute significantly to the earthquake segmentation and low seismic coupling ratio on OTFs. We also found five types of along-strike distribution of creeping segments, depending on geological structures and fault zone heterogeneity. The mechanism of fault creep on OTFs is still unclear but the richness of fluid in the environment seems to be very important.

Seismic hazard in the Los Angeles area is controlled in large part by the near-surface shear-wave velocity. Maps of Vs30 have traditionally filled this role, but it is increasingly recognized that we need estimate that extend down to 2-3 km or deeper to properly explain the level or lateral variations of strong ground motions. In this talk we present shear wave velocity models determined using dense seismic arrays. Specifically, we present results for the coastal area, the central LA Basin and the northern LA Basin. We will also present a plan to conduct a survey across the entire LA Basin region using a dense array.

Recent current events have shined a harsh light on the nature and consequences of continuing systemic inequalities in society, inspiring the scientific community to take an overdue look in the mirror. While some progress has been made in recent decades, improving the gender diversity in geosciences, very little progress has been made improving racial diversity in a field that has historically been the purview of white, male scientists. As steps are now taken to improve diversity of the geoscience community, it is important to note that systemic socioeconomic biases potentially impact not only our community, but also geoscience itself. In this talk I focus on earthquake observations contributed by human observers, which provide an invaluable source of information to investigate both historical and modern earthquakes. Commonly, the observers whose eyewitness accounts are available to scientists come from a self-selected minority of those who experience a given earthquake. As such these may not be representative of the overall population that experienced shaking from the event. Eyewitness accounts can contribute to modern science only if they are recorded and archived in an accessible repository. I discuss the extent to which geopolitics and socioeconomic disparities can limit the number of earthquake observers whose observations can contribute to science. I first revisit a late-19th century earthquake in the central U.S. in 1882 that provides an illustrative example of an event that has been poorly characterized due to a reliance on English-language archival materials. For modern earthquakes, I consider data collected for recent earthquakes in California and India via the online "Did You Feel It?" system. In California, online data-collection systems appear to be increasingly effective in gathering eyewitness accounts from a broad range of socioeconomic groups. In India, however, a recent study by Hough and Martin (SRL, 2021) shows that responses to the "Did You Feel It?" system reveal a strong bias towards responses from urban areas as opposed to rural settlements, as well a strong bias with literacy rate. The dissimilarity of these results from modern earthquakes in the U.S. and India provides a caution that, in some parts of the world, contributed felt reports can still potentially provide a grossly unrepresentative view of earthquake effects, especially if online data collection systems are not designed to be inclusive. This limitation can in turn potentially shape our understanding of an earthquake's impact and the characterization of seismic hazard. A lesser degree of bias is found in DYFI data for California, suggesting there is room for improved engagement with underserved communities.

When an earthquake occurs, slip models of the event may be produced by inverting geodetic data for slip on a finite fault. This process generally requires coseismic Green's functions, which must be calculated in advance. The vast majority of such studies use Green's functions that ignore any 3D structure that is present in the region. The planet's largest earthquakes, magnitude 8-9 subduction zone ruptures, occur underneath regions with the largest topographic gradients ranging from deep ocean trenches up to onshore mountain ranges. These regions also include wide variations in material properties. Inland earthquakes can also occur in regions with significant heterogeneity, such as mountain ranges or sedimentary basins, which have the potential to affect the deformation field.

Here, we investigate the effects of 3D structure on forward models of coseismic deformation and on earthquake static slip inversions. Using a newly-developed software package, SPECFEM-X, we show that the presence of topography alters the shape of predicted surface deformation patterns for the 2010 Maule and 2015 Gorkha earthquakes. We then compute coseismic Green's functions for these earthquakes in domains with and without topography, and perform Bayesian inversions using geodetic data. In both cases, we find that the use of Green's functions with topography yields a different distribution of slip. We then turn our attention to 3D elastic structure, and use SPECFEM-X to examine the impact of sedimentary basins on forward models of coseismic deformation. Our analysis of the 1984 Morgan Hill earthquake, which occurred near the Evergreen Basin, and of a hypothetical earthquake on the Sanchiao Fault near the Taipei Basin show that sedimentary basins generally affect the magnitude rather than the shape of surface deformation patterns. These findings suggest that the effects of 3D elastic structure may be qualitatively different from those of topography, and that 3D structure should be accounted for when estimating static slip in regions with significant heterogeneity.