Thomas H. Heaton

Research

Although smaller earthquakes are far more numerous, large earthquakes (M > 7.5) account for most of the slip in plate tectonics. That is, the number of earthquakes generally decreases by a factor of ten for each unit increase in magnitude, but the energy of an individual earthquake increases by a factor of 32. If we assume that M 8.0 is the largest earthquake magnitude that an earthquake can have in California, then there is three times as much radiated energy in the M 7 to 8 earthquakes as there is in all other earthquakes smaller than M 7. Therefore, we see that although large earthquakes are infrequent, they are the major actors in plate tectonics; in this sense, large earthquakes are inevitable.

What will happen when one of these large magnitude earthquakes hits one of our cities? Recent engineering studies have concluded that since many structures weathered the onslaught of the 1994 M 6.7 Northridge earthquake and the 1995 M 6.9 Kobe earthquake, that our building standards are adequate to handle the coming earthquakes. However, can we expect to survive a M 7.9 earthquake with 32 times as much energy?

There seems to be an inconsistency between earth scientists and earthquake engineers about the significance of large magnitude earthquakes. Much of our work is aimed at a more complete understanding of the nature of ground shaking close to large earthquakes. That is, ground motions from large earthquakes are simulated by propagating waves through 3-dimensional earth structure models. The models produce realistic estimates of the large displacements (several meters in several seconds) that occur in great earthquakes. While accelerations that are associated with these large displacements may not be large enough to cause failure of strong, shear-wall structures, they may cause severe deformations in flexible buildings that rely heavily on ductility for their performance in large earthquakes. This work is closely coordinated with Prof. John F. Hall.

We (Jing Yang) are investigating the potential performance of steel moment-resisting-frame buildings in large subduction zone earthquakes. We have simulated the deformations and damage that would have occurred to such buildings in the M 8.3 Tokachi-Oki earthquake (2003). Although there we no such buildings present on the island of Hokkaido during this earthquake, there were 275 strong motion records which we are using as the basis of our study. In addition, we are using this data as the basis of an empirical Green’s function study of the potential effects of a giant (M>9) subduction earthquake on high-rise buildings in the cities of Seattle, Portland, and Vanvouver.

We (Anna Olsen) are also studying the performance of steel moment-resisting-frame buildings and base-isolated buildings in simulations of large crustal earthquakes in California. These include simulations of the 1906 San Francisco earthquake (collaboration with Brad Aagaard) and simulations of several plausible earthquakes in the Los Angeles Basin.

Much of the deformation of the Earth's crust occurs as earthquake rupture. Therefore, it is of critical importance to understand the fundamental dynamics of earthquake rupture to understand the stress state of the crust. A short description of the problem can be found at Live Science. We are particularly interested in understanding the origins of spatially heterogeneous slip in earthquakes. There is compelling evidence that slip in earthquakes and stress in the Earth’s crust are spatially heterogeneous, and perhaps fractal. We have been pursuing two different approaches to understand the dynamic properties of this system.

The first approach is a long-standing collaboration with Dr. Brad Aagaard (USGS, Menlo Park) and it consists of constructing 3-dimensionional finite-element models of the Earth's crust, which are controlled by dynamic friction on fault planes. The models include the effects of gravity so that crustal stresses are consistent with the topography of the Earth's surface and density variations in the crust. The models allow us to follow the partitioning of elastic and gravitational potential energy into radiated seismic waves, fracture energy, and frictional heating on faults. Using estimates or bounds on wave energy, facture energy, and heat energy, it is possible to put bounds on crustal deviatoric stress.

Despite steady progress in simulating dynamic earthquake ruptures, there are limitations of this approach to understanding the dynamic properties of the crust. In particular, recent experiments in dynamic friction suggest that there are rapid transitions between high static friction (>200 MPa at 10 km depth) and very low dynamic friction (<5 MPa). These strong transitions in friction point to very localized slip pulses that propagate unsteadily along faults. Unfortunately, simulation of dynamic rupture with these friction laws requires enormous spatial grids with very fine time resolution. We (Jing Liu-Zeng) have constructed fractal models of slip that are compatible with observations of slip vs. rupture length scaling and also with earthquake frequency vs. magnitude statistics.

In addition we (Deborah Smith) have constructed a 3-dimensional fractal model of tensor stress that we use to simulate catalogs of earthquake locations and focal mechanisms. This model predicts that traditional inversions of focal mechanism catalogs for average stress orientation may provide results that are seriously biased towards the orientation of the stress rate function. It also predicts that the strength of the crust depends on the length scale over which failure occurs. We (Ahmed Elbanna) are currently investigating the statistical relationship between fractal stress and fractal slip.
Earthquake Warning Systems

We (Georgia Cua and Masumi Yamada) are helping to develop new tools to mitigate earthquake disasters by providing comprehensive information as quickly as possible during and immediately after significant earthquakes. We are developing computer algorithms that will analyze earthquake waves while the earthquake is still rupturing. This will allow a short-term warning (seconds to tens of seconds) to be broadcast to regions that are about to be shaken by seismic waves that are propagating towards them. Such warning may allow short-term mitigation actions to lessen the impact of shaking. We have named our system the Virtual Seismologist (VS method) since it is based on the type of robust analysis that a human would perform if they had the time. We use envelopes of acceleration, velocity, and displacement as the basic data input to a Bayesian framework that also incorporates other types of information (e.g., topology of the seismic network, recent seismic activity). We are currently testing this algorithm on data recorded by the California Integrated Seismic Network. We are also working on methodologies that will provide real-time estimates of rupture geometry and fault slip. Knowledge of the present value of slip can be used to probabilistically predict the eventual size of an earthquake even before it is finished rupturing. There's a nice story on earthquake warning systems in IEEE's Spectrum.

We are investigating the vibrations of buildings that are excited by a wide number of sources, including wind, machinery, and earthquakes of all sizes. We have installed an advanced seismic station that continuously records the 9-story Millikan Library on the CIT campus, a building which has been the source of many interesting mysteries. For example, when the building's fundamental modes (north-south, east-west, and torsion) are excited by a 1-hp eccentric shaker operated on the building's roof, harmonic seismic waves are observed at the building's eigen-frequencies throughout the Pasadena area; they can even be detected on seismometers just north of the US-Mexican border, which is about 250 km away (see Javier Favela’s dissertation).

Another interesting mystery of Millikan Library is the fact that the natural frequencies of the fundamental modes (north-south, east-west, and torsion) all increase by several percent just following significant rain storms. These increases in frequency slowly decrease over a period of several days. We have been using advanced time-frequency representations (the Wigner-Ville distribution) to investigate how these natural frequencies change during shaking to both damaged and undamaged buildings (see Casey Bradford’s dissertation).

Nineke Oerlemans, 1999, MS in Geophysics from Utrecht Univ. (co-advised with H. Paulssen), Sorting Source Parameters to Produce Coherent Record Sections. pdf

Brad Aagaard, Ph.D. 2000, CE (co-advised with John Hall); Finite-element simulations of earthquakes. pdf

Javier Favela, Ph.D. 2004, Geophysics, Energy radiation from a multi-story building. pdf

John Clinton, Ph.D. 2004, CE Modern digital seismology - instrumentation, and small amplitude studues in the engineering world. pdf

Georgia Cua, Ph.D. 2004, Creating the Virtual Seismologist: developments in ground motion characterization and seismic early warning pdf

Deborah Smith, PhD 2006, Geophysics A new paradigm for interpreting stress inversions from focal mechanisms; how 3D stress heterogeneity biases the inversions toward the stress rate pdf

Casey Bradford, Ph.D. 2006, CE, Time-frequency analysis of systems with changing dynamic properties pdf

Masumi Yamada, Ph.D. 2007, CE, Early warning for earthquakes with large rupture dimension. pdf

Jing Yang, CE (4thyear; research topic; ground motions and simulated high-rise response in great and giant subduction earthquakes)

Anna Olsen, CE (3rd year; research topic; Characterizing the non-linear response of high-rise buildings to anticipated ground motions in the metropolitan Los Angeles region)

Ahmed Elbanna, CE (2nd year; research topic; Statistical physics of rupture mechanics)

1995 Meritorious Service award from the U.S. Dept. of Interior

2007 Fellow of the American Geophysical Union

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