iPlex Lunch - fall-2015

Sept. 30, 2015
noon - 12:50 p.m.
Geology 1810

Presented By:

• Paul Davis - UCLA

Earthquakes occur in crust or mantle that is below a critical temperature for the tectonic strain-rate such that stress builds up to the breaking point before it can relax due to creep. The limiting temperature depends on pres- sure, which is taken into account by finding a critical homologous temper- ature THc = T/TM above which earthquakes cease (where T, TM are tem- perature and average melting temperature of constituent minerals). The G layer of ocean plates is an uppermost mantle layer of thickness 50 km that is thought to be composed of harzburgite and depleted peridotite from which basalt has been removed to form ocean crust. Thus it has a higher melting temperature than the peridotite of the surrounding mantle, or the lower halves of plates. We find that THc for ocean plates is ~0.55. For California earth- quakes, it is also close to 0.55. Thicknesses of seismicity in deep subduction zones, determined from 2D polynomial fits to a relocated earthquake catalog, are ~50km, which suggests that the earthquake channel is confined to the G layer. We construct thermal models to find homologous temperatures that take into account the variation of thermal parameters as a function of temperature, pressure and mineral content based on laboratory and equation of state es- timates for pyrolite and harzburgite, and latent heat from phase changes. We find that seismicity thicknesses in subducted slabs are also, on average, confined to TH < 0.55 \pm 0.05. The cutoff for deep earthquakes is not sharp. However they appear unlikely to occur if homologous temperature is high TH > 0.55. Exceptions to the rule are anomalously deep earthquakes such as those beneath the Hawaiian hotspot where THc >0.66. These can be explained if volcanic strain-rates are 2 to 3 orders of magnitude higher than the strain-rates associated with tectonic earthquakes.

Oct. 7, 2015
noon - 12:50 p.m.
Geology 1810

Presented By:

• Rob Clayton - Caltech

Seminar Description coming soon.

Oct. 14, 2015
noon - 12:50 p.m.
Geology 1810

Presented By:

• An Yin - UCLA

Relative stress magnitudes of plate margins and interiors as constrained by strike-slip fault spacing

Rigid plate motion on Earth implies that the plate interiors are mechanically strong and/or have lower stress than that along plate margins. Although the strength contrast between plate interiors and margins has been quantified, their relative stress magnitude has never been investigated. Here we use a mechanical model that relates active strike-slip fault spacing to fault strength to show that the average strength of faults, which places a bound on the magnitude of stress in the crust, is about a factor of two weaker in the continental interior of Asia than that along the San Andreas transform plate boundary in California. This result provides a critical insight into the dynamic state of plate tectonics on Earth: a higher stress regime along plate boundaries and a lower stress regime within plate interiors enable classical plate-like rigid-body motion on a continental scale. As fault strength is a key parameter in modeling continental deformation and the mechanical behavior of earthquakes, our approach also provides a new approach of estimating the absolute frictional strength of active strike-slip faults on Earth.

Oct. 21, 2015
noon - 12:50 p.m.
Geology 1810

Presented By:

• Victor Tsai - Caltech

Arctic Mechanics: Studying the Interactions of Ice and Water

Oct. 28, 2015
noon - 12:50 p.m.
Geology 1810

Presented By:

• Chao An - UCLA

Seminar Description coming soon.

Nov. 4, 2015
noon - 12:50 p.m.
Geology 1810

Presented By:

• Monica Kohler - Caltech

Pacific-North America Lithospheric Structure and 2011 Tohoku Tsunami Observations from the Offshore Southern California ALBACORE project.

Nov. 18, 2015
noon - 12:50 p.m.
Geology 1707

Presented By:

• Peter Bird - UCLA

Blending data and dynamics into equilibrium for the Community Stress Model

With 3-D tensor models of stress in the lithosphere in southern California, SCEC could: (a) determine the shear stress on active faults to constrain the physics of slip; (b) predict Coulomb stress changes for operational earthquake forecasting; and (c) test the realism of long-term earthquake sequence simulators. One basis for models is data: stress directions (from focal mechanisms and boreholes), and stress intensity (only from boreholes). But earthquakes rarely occur deeper than 15~20 km, and boreholes rarely deeper than ~6 km. Therefore, data must be supplemented by dynamic models using: laboratory flow laws, a geotherm model, a Moho model, relative plate motions, and locations of active faults. One dynamic model uses code Shells, which solves for 2-D equilibrium of vertically-integrated stresses using 2-D velocity models and 3-D structure. While this model predicts full stress tensors, they are discontinuous and noisy. A newer approach is to model the stress anomaly field as the sum of topographic and tectonic stress anomaly fields. In program FlatMaxwell, the topographic stress is defined as the convolution of topography (and deep density anomalies) with analytic solutions for an elastic half-space. The tectonic stress is modeled by sums of derivatives of a Maxwell vector potential field. The whole stress field is then best-fit (by weighted least squares) to both data and the dynamic model. In practice, FlatMaxwell models are limited in spatial resolution to no more than 6 wavelengths along each side of the model domain. Thus they are quite smooth, and cannot represent stress discontinuities at the Moho predicted by the Shells model. Results to date show a low-amplitude stress anomaly, with peak shear stress of 120 MPa and peak vertically-integrated shear stress of 2.9×1012 N/m. Channeling of deviatoric stress along the strong Peninsular Ranges and Great Valley is seen. In southern California, deviatoric stress and long-term strain-rate are negatively correlated because regions of low heat-flow act as stress guides while deforming very little. In contrast, active faults lie in areas with higher heat-flow, and their low strength keeps deviatoric stresses locally modest. Opportunities for future CSM advances include: [1] collecting more data; [2] tuning the Shells dynamic model; [3] using a different dynamic modeling code; and/or [4] applying a similar Maxwell equilibrium filter to models of the interseismic stress rate.

Dec. 2, 2015
noon - 12:50 p.m.
Geology 1707

Presented By:

• David D. Jackson - UCLA EPSS

What geology and geodesy reveal about future earthquakes

Instrumental recordings of large California earthquakes cover just over a century, and accurate data for moderate quakes are available only since the mid 1930s. Thus deformation rates inferred from geological and geodetic data are needed to estimate the rates of potentially damaging earthquakes. But these data provide indirect information, and important assumptions are needed to infer earthquake rates from surface deformation. Large earthquake rates inferred from geology generally exceed those observed instrumentally, leaving open the question of whether recent rates are anomalously low (and perhaps due to increase), or instead that measurement uncertainties or incorrect assumptions explain the differences. How can we resolve this question?