Earthquake Science Center Seminars

NOTE: Technical difficulties with Audio. Problem resolved and audio begins at 7:18. We apologize for the inconvenience.

Everybody knows―or thinks they know―Charles Darwin, the father of evolution and the man who altered the way we view our place in the world. But what most people do not know is that Darwin was on board the HMS Beagle as a geologist―on a mission to examine the land, not flora and fauna. Or about Darwin's seminal role in demonstrating and exploring the ups and downs of the Earth's crust, focusing on what we now call tectonics. This is the story told in Rob Wesson's book, Darwin's First Theory, and that he will share with us.

Retracing Darwin's footsteps in South America and beyond, Rob trekked across the Andes, cruised waters charted by the Beagle, hunted for fossils in Uruguay and Argentina, and explored sites of long vanished glaciers in Scotland and Wales. As he followed Darwin's path―literally and intellectually―he experienced the land as Darwin did, engaged with his observations, and tackled the same questions Darwin had about our ever-changing Earth.

Upon his return from his five-year journey aboard the Beagle, after examining the effects of earthquakes, tsunamis, volcanic eruptions, and more, Darwin conceived his theory of subsidence and uplift―his first theory. These concepts and attitudes―the vastness of time; the enormous cumulative impact of almost imperceptibly slow change; change as a constant feature of the environment―underlie Darwin's subsequent discoveries in evolution. And this peculiar way of thinking remains vitally important today as we enter the human-dominated Anthropocene age.

The New York Times Book Review wrote that Rob's book "dares, thank goodness, to work some of the rare Darwinian territory that is actually underexplored." Nature described it as "a wonderful evocation of Darwin's great theory of subsidence and uplift, the substrate of his later, explosive discovery of evolution."

The Mendocino Triple Junction (MTJ) is a rapidly deforming plate boundary zone at the intersection of the San Andreas fault, the Cascadia Subduction Zone (CSZ), and the Mendocino Fault Zone. Earthquake cycle deformation in this region results in large present-day strain rates and seismicity rates. In this project, we analyze GPS time series in the region and find slight time dependence in the interseismic velocity field. We find that in one area, the GPS velocity field changes in spatially coherent patterns following large offshore earthquakes and we find that these changes cannot be fully explained by postseismic deformation, slow slip events, or hydrological loading. Instead, we believe that these observations present a rare example of dynamically triggered changes in interface coupling, and we speculate on the possible mechanical causes of these changes. We evaluate the relationship between coupling changes and tremor. These observations, together with similar examples elsewhere in the world, suggest that faults outside of the ETS zone may have time-dependent coupling during the interseismic period, a phenomenon that we need to understand and account for in future hazard models.

The phenomenon of remote dynamic triggering has been well-established since the

1992 Landers, California, earthquake triggered earthquakes from California to Yellowstone.

Since then, there have been many other studies that have clearly shown a direct link between

remote mainshocks and small magnitude local seismicity. With the development of matched

filter and other detection algorithms, there is an increasing number of instances documenting

remote triggering. However, in many studies the triggering criteria is based on examining the

number of earthquakes in small time windows surrounding the arrival of the teleseismic waves

and rarely is the null hypothesis, that the rate change is merely a random occurrence, fully tested.

In this study using 33 years (1985-2017) of data, we investigate if 500 mainshocks (489 global,

11 regional) statistically trigger remote aftershocks. We compare these results to results

determined using the beta statistic and evaluating rate changes assuming a Poisson distribution.

While the main goal of this work is to explore the statistics used as criteria for determining

dynamic triggering and what is meant by statistical significance, we also identify previously

undocumented cases of dynamic triggering, such as triggering in Yellowstone following the

1985 M8.0 Mexico City earthquake.

I present new seismic images of the key structural elements of the upper mantle beneath North America. These images are significantly different from all previous studies. Unlike earlier work, we successfully image the cratonic lithosphere-asthenosphere boundary (LAB), and the 250-km-deep Lehmann discontinuity. We identify a pronounced, if enigmatic, low velocity layer (LVL) above the mantle transition zone (410-discontinuity). These results have important implications for the evolution of the upper mantle.

Reducing losses from megathrust earthquakes and tsunamis demands accurate estimate of future rupture scenarios. Given recent advances in geodetic data, interseismic locking models along the megathrust now become useful to qualitatively evaluate future earthquake potential. However, an individual earthquake's true rupture potential is challenging, as it depends on more than just a static image of prior locking. Here, we test the determinism of interseismic locking models using spontaneous rupture simulations and the well-refined and understood processes associated with the 2012 Mw 7.6 Nicoya earthquake. To do so, we estimate initial megathrust stress from locking, then initiate spontaneous ruptures, testing different nucleation points. For ruptures initiating at the hypocentre of the 2012 Nicoya earthquake, we find scenarios that approximate the coseismic slip distribution and final earthquake moment magnitude derived from seismic and geodetic observations. However, we find that only ~40% of nucleations develop into large earthquakes of Mw>7.2 based on present interseismic locking models. Of these events, those nucleated from deeper depths have a tendency for larger-amplitude shallow slip, suggesting increased tsunami potential. Furthermore, irrespective of the input locking models we do not observe rupture scenarios of earthquakes with intermediate magnitudes between 6 and 7, a result consistent with observations outside of the aftershock period in Nicoya. The results of hypocentre-dependent earthquake magnitudes and tsunamigenic potential not only pose challenges in estimating rupture extents from locking models, but also underscore the significance of quantitatively evaluating seismic and tsunami hazard in subduction zones.