Timothy Masterlark

Numerical Simulation of the 2011 Tohoku Tsunami Based on a New Transient FEM Co-seismic Source: Comparison to Far- and Near-Field Observations

In this work, we simulate the 2011 M9 Tohoku-Oki tsunami using new coseismic tsunami sources based on inverting onshore and offshore geodetic data, using 3D Finite Element Models (FEM). Such FEMs simulate elastic dislocations along the plate boundary interface separating the stiff subducting Pacific Plate from the relatively weak forearc and volcanic arc of the overriding Eurasian plate. Due in part to the simulated weak forearc materials, such sources produce significant shallow slip (several tens of meters) along the updip portion of the rupture near the trench. To assess the accuracy of the new approach, we compare observations and numerical simulations of the tsunamis far- and near-field coastal impact for: (i) one of the standard seismic inversion sources (UCSB; Shaoet al.2011); and (ii) the new FEM sources. Specifically, results of numerical simulations for both sources, performed using the fully nonlinear and dispersive Boussinesq wave model FUNWAVE-TVD, are compared to DART buoy, GPS tide gauge, and inundation/runup measurements. We use a series of nested model grids with varying resolution (down to 250 m nearshore) and size, and assess effects on model results of the latter and of model physics (such as when including dispersion or not). We also assess the effects of triggering the tsunami sources in the propagation model: (i) either at once as a hot start, or with the spatiotemporal sequence derived from seismic inversion; and (ii) as a specified surface elevation or as a more realistic time and space-varying bottom boundary condition (in the latter case, we compute the initial tsunami generation up to 300 s using the non-hydrostatic model NHWAVE). Although additional refinements are expected in the near future, results based on the current FEM sources better explain long wave near-field observations at DART and GPS buoys near Japan, and measured tsunami inundation, while they simulate observations at distant DART buoys as well or better than the UCSB source. None of the sources, however, are able to explain the largest runup and inundation measured between 39.5° and 40.25°N, which could be due to insufficient model resolution in this region (Sanriku/Ria) of complex bathymetry/topography, and/or to additional tsunami generation mechanisms not represented in the coseismic sources (e.g., splay faults, submarine mass failure). This will be the object of future work.

Rheologic and structural controls on the deformation of Okmok volcano, Alaska: FEMs, InSAR, and ambient noise tomography

Received 23 January 2009; revised 5 August 2009; accepted 13 October 2009; published 27 February 2010. [1] Interferometric synthetic aperture radar (InSAR) data indicate that the caldera of Okmok volcano, Alaska, subsided more than a meter during its eruption in 1997. The large deformation suggests a relatively shallow magma reservoir beneath Okmok. Seismic tomography using ambient ocean noise reveals two low-velocity zones (LVZs). The shallow LVZ corresponds to a region of weak, fluid-saturated materials within the caldera and extends from the caldera surface to a depth of 2 km. The deep LVZ clearly indicates the presence of the magma reservoir beneath Okmok that is significantly deeper (>4 km depth) compared to previous geodetic-based estimates (3 km depth). The deep LVZ associated with the magma reservoir suggests magma remains in a molten state between eruptions. We construct finite element models (FEMs) to simulate deformation caused by mass extraction from a magma reservoir that is surrounded by a viscoelastic rind of country rock embedded in an elastic domain that is partitioned to account for the weak caldera materials observed with tomography. This configuration allows us to reduce the estimated magma reservoir depressurization to within lithostatic constraints, while simultaneously maintaining the magnitude of deformation required to predict the InSAR data. More precisely, the InSAR data are best predicted by an FEM simulating a rind viscosity of 7.5 � 10 16 Pa s and a mass flux of � 4.2 � 10 9 kg/d from the magma reservoir. The shallow weak layer within the caldera provides a coeruption stress regime and neutral buoyancy horizon that support lateral magma propagation from the central magma reservoir to extrusion near the rim of the caldera.

Transient stress-coupling between the 1992 Landers and 1999 Hector Mine, California, earthquakes

A three-dimensional finite-element model (FEM) of the Mojave block region in southern California is constructed to investigate transient stress-coupling between the 1992 Landers and 1999 Hector Mine earthquakes. The FEM simulates a poroelastic upper-crust layer coupled to a viscoelastic lower-crust layer, which is decoupled from the upper mantle. FEM predictions of the transient mechanical be- havior of the crust are constrained by global positioning system (GPS) data, inter- ferometric synthetic aperture radar (InSAR) images, fluid-pressure data from water wells, and the dislocation source of the 1999 Hector Mine earthquake. Two time- dependent parameters, hydraulic diffusivity of the upper crust and viscosity of the lower crust, are calibrated to 10 2 m 2 •sec 1 and 5 10 18 Pasec respectively. The hydraulic diffusivity is relatively insensitive to heterogeneous fault-zone permeabil- ity specifications and fluid-flow boundary conditions along the elastic free-surface at the top of the problem domain. The calibrated FEM is used to predict the evolution of Coulomb stress during the interval separating the 1992 Landers and 1999 Hector Mine earthquakes. The predicted change in Coulomb stress near the hypocenter of the Hector Mine earthquake increases from 0.02 to 0.05 MPa during the 7-yr interval separating the two events. This increase is primarily attributed to the recovery of decreased excess fluid pressure from the 1992 Landers coseismic (undrained) strain field. Coulomb stress predictions are insensitive to small variations of fault-plane dip and hypocentral depth estimations of the Hector Mine rupture.

Homogeneous vs heterogeneous subduction zone models: Coseismic and postseismic deformation

A finite-element model (FEM) incorporating geologic properties characteristic of a subduction zone is compared with FEMs approximating homogeneous elastic half-spaces (HEHS)s to investigate the effect of heterogeneity on coseismic and postseismic deformation predictions for the 1995 Colima-Jalisco M_w =8.0 earthquake. The FEMs are used to compute a coefficient matrix relating displacements at observation points due to unit dislocations of contact-node pairs on the fault surface. The Greens function responses are used to solve the inverse problem of estimating dislocation distributions from coseismic GPS displacements. Predictions from the FEM with heterogeneous material properties, loaded with either of the HEHS dislocation distributions, significantly overestimate coseismic displacements. Postseismic deformation predictions are also sensitive to the coseismic dislocation distribution, which drives poroelastic and viscoelastic relaxation. FEM-generated Greens functions, which allow for spatial variations in material properties, are thus preferable to those that assume a simple HEHS because the latter leads to dislocation distributions unsuitable for predicting the postseismic response.

Active deformation across the Sumatran forearc over the December 2004 Mw9.2 rupture

A 220-km-long, single-channel seismic reflection profile crosses the northern Sumatra margin and presumed rupture zone of the December 2004 M w 9.2 tsunamigenic earthquake and images active deformation across the forearc. At the largest wavelength (tens of kilometers), the forearc surface is defined by a steep, 55-km-wide outer slope, a 110-km-wide upper slope forming a broad depression between two forearc highs, and a 25-km-wide steep inner slope between the landward high and forearc basin. Superimposed on these prism-wide variations are anticlinal ridges spaced ∼13 km apart; the inner and outer slopes are characterized by landward and seaward fold vergence, respectively. Between anticlines, growth strata deposited in slope basins are folded at ∼2–3 km wavelengths. These small folds deform the seafloor and increase in amplitude with depth, verging toward anticlinal hinges. We suggest that long-wavelength variations are consistent with variations in strength across the forearc. The ∼13 km anticline spacing implies deformation of a slope apron that deforms independently of a stronger wedge interior. Growth strata geometries indicate ongoing deformation within individual basins. Our model for prism architecture suggests that the wedge interior advances during great earthquakes like the 2004 M w 9.2 event, peeling up shallower and less competent trench fill, deforming the toe and the upper slope of the forearc, and producing seabottom uplift responsible for the tsunami.

Poroelastic coupling between the 1992 Landers and Big Bear Earthquakes

A three-dimensional finite element model was constructed to investigate the significance of poroelastic coupling between the 1992 Landers and Big Bear earthquakes in southern California. The homogeneous poroelastic model predicted a maximum increase in left lateral slip potential (change in shear stress less the change in effective fault normal stress scaled by a coefficient of friction) along the southwest part of the Big Bear fault, consistent with the epicentral location. In contrast, slip potential calculated for a weak fault zone in a state of isotropic stress for drained conditions, indicated a maximum increase along the northeast part of the Big Bear fault.

Impoundment of the Zipingpu reservoir and triggering of the 2008 Mw 7.9 Wenchuan earthquake, China

Abstract Impoundment of the Zipingpu reservoir (ZR), China, began in September 2005 and was followed 2.7 years later by the 2008 Mw 7.9 Wenchuan earthquake (WE) rupturing the Longmen Shan Fault (LSF), with its epicenter ~12 km away from the ZR. Based on the poroelastic theory, we employ three‐dimensional finite element models to simulate the evolution of stress and pore pressure due to reservoir impoundment, and its effect on the Coulomb failure stress on the LSF. The results indicate that the reservoir impoundment formed a pore pressure front that slowly propagated through the crust with fluid diffusion. The reservoir loading induced either moderate or no increase of the Coulomb failure stress at the hypocenter prior to the WE. The Coulomb failure stress, however, grew ~9.3–69.1 kPa in the depth range of 1–8 km on the LSF, which may have advanced tectonic loading of the fault system by ~60–450 years. Due to uncertainties of fault geometry and hypocenter location of the WE, it is inconclusive whether impoundment of the ZR directly triggered the WE. However, a small event at the hypocenter could have triggered large rupture elsewhere on fault, where the asperities were weakened by the ZR. The microseismicity around the ZR also showed an expanding pattern from the ZR since its impoundment, likely associated with diffusion of a positive pore pressure pulse. These results suggest a poroelastic triggering effect (even if indirectly) of the WE due to the impoundment of the ZR.

Solid modeling techniques to build 3D finite element models of volcanic systems: An example from the Rabaul Caldera system, Papua New Guinea

Simulating the deformation of active volcanoes is challenging due to inherent mechanical complexities associated with heterogeneous distributions of rheologic properties and irregular geometries associated with the topography and bathymetry. From geologic and tomographic studies we know that geologic bodies naturally have complex 3D shapes. Finite element models (FEMs) are capable of simulating the pressurization of magma intrusions into mechanical domains with arbitrary geometric and geologic complexity. We construct FEMs comprising pressurization (due to magma intrusion) within an assemblage of 3D parts having common mechanical properties for Rabaul Caldera, Papua New Guinea. We use information of material properties distributed on discrete points mainly deduced from topography, geology, seismicity, and tomography of Rabaul Caldera to first create contours of each part and successively to generate each 3D part shape by lofting the volume through the contours. The implementation of Abaqus CAE with Python scripts allows for automated execution of hundreds of commands necessary for the construction of the parts having substantial geometric complexity. The lofted solids are then assembled to form the composite model of Rabaul Caldera, having a geometrically complex loading configuration and distribution of rheologic properties. Comparison between predicted and observed deformation led us to identify multiple deformation sources (0.74MPa change in pressure in the magma chamber and 0.17m slip along the ring fault) responsible for the displacements measured at Matupit Island between August 1992 and August 1993.

Interpretation of Rayleigh-wave ellipticity observed with multicomponent passive seismic interferometry at Hekla Volcano, Iceland

The 2010 eruption of Eyjafjallajokull has drawn increased attention to Icelands Eastern Volcanic Zone (EVZ) due to the threat it poses to the heavily used air-traffic corridors of the northern Atlantic Ocean. Within the EVZ, Hekla is historically one of the most active volcanoes and has exhibited a decadal eruption pattern for the past 40 years. Hekla most recently erupted in 2000 and is thus ripe for another decadal eruption. Because Hekla is generally aseismic, except for a brief time period (hours) leading up to an eruption, monitoring has previously depended on precursory deformation signals (Linde et al., 1993). As a result, seismic tomography of the internal structure of the volcano using phase arrivals of local earthquakes is not possible. Motivated by Heklas practically aseismic behavior in inter-eruptive periods, we installed a temporary network of four broadband seismometers around the volcanic edifice in late August 2010 with the intention of investigating the applicability of passive seismic...

Interferometric synthetic aperture radar studies of Alaska volcanoes

Interferometric synthetic aperture radar (InSAR) imaging is a recently developed geodetic technique capable of measuring ground-surface deformation with centimeter to subcentimeter vertical precision and spatial resolution of tens-of-meter over a relatively large region (/spl sim/10/sup 4/ km/sup 2/). The spatial distribution of surface deformation data, derived from InSAR images, enables the construction of detailed mechanical models to enhance the study of magmatic and tectonic processes associated with volcanoes. This paper summarizes our recent InSAR studies of several Alaska volcanoes, which include Okmok, Akutan, Kiska, Augustine, Westdahl, and Peulik volcanoes.