Earthquake Science Center Seminars

This presentation will summarize observations by a post-earthquake reconnaissance mission to south western Puerto Rico after the sequence of earthquakes that initiated at the end of 2019 with a main event occurring on January 7th, 2020 at 8km S of Indios at 4:24am local time (8:24 UTC) with a magnitude Mw 6.4, followed by thousands of aftershocks. The ground shaking from the earthquakes damaged over 10,000 structures, including the complete collapse of at least 80 structures, most of which were single-family residential units. In addition, several bridges, roads and other infrastructure were also damaged or destroyed and two thirds of the island lost power as a result of the Mw 6.4 earthquake. The presentation will summarize some of the characteristics of strong motion recordings and damage to various types of construction.

The Greater Caucasus Mountains, located between the Black and Caspian Seas at the northern margin of the Arabia-Eurasia collision zone, presently accommodate most (70%) of the orogen-perpendicular shortening within this sector of the collision zone. Patterns in exhumation, geodetic shortening rate, and subcrustal seismicity have led to the hypothesis that the modern range records an along-strike transition from an attached subducted slab in the east to a detached slab in the west.

To test this hypothesis, we combine 1:100k structural mapping along two orogen-perpendicular transects with stratigraphic and structural analyses to constrain the magnitude of shortening accommodated within the Greater Caucasus. Using these data, we construct balanced cross sections that accommodated ~200km of shortening across the orogen. In addition, we determine the modern geologic shortening rate across the active frontal thrust (~1.1 +/-0.5 mm/yr), providing a point of comparison with both geodetic rates across the full orogen at the same longitude (3-4 mm/yr) and longer term shortening rate estimates based on low-tempertature thermochronology (6-8 mm/yr).

In combination, these results are compatible with the slab detachment model of orogen evolution, and are not as consistent with other competing hypotheses. Though more work is needed, these data suggest that the Greater Caucasus may capture the surface expression of ongoing processes associated with the transition from subduction to continental collision.

This year's Distinguished Lecture will present the emerging concept of Functional Recovery as a basis for

earthquake-resistant design. Designing buildings and infrastructure for limited downtime -- or an

acceptably quick functional recovery -- is not new, but it is receiving new attention through state and

federal legislation, and showing new feasibility through research and technology. Most intriguing is the

recognition that designing for functional recovery is a necessary tool for achieving community-wide

earthquake resilience. And if progress is to be measured at the community level, functional recovery will

also be a matter of public policy. The lecture will look at the roles EERI members can play in shaping this

thinking into design practice with four sets of questions: definitional, technical, policy, and

implementation.

The largest magnitude earthquakes we have observed occur along subduction zone thrust faults. However, the subseafloor location of these faults makes them incredible difficult to study. One useful method for studying these zones is scientific ocean drilling, which has helped to characterize the structure, composition, physical properties, role of fluids, temperature, and stress state in subduction zones. We discuss some of the challenges that faced the recent IODP Expedition 358's quest to drill the deep megathrust fault at the Nankai Trough and lessons learned in the process.

Previous subduction zone drilling has focused on the relatively shallow part of the subduction zone, ~ 1 km below the seafloor. IODP Expedition 358 was the first attempt to drill into a subduction zone fault at seismogenic depths. The expedition occurred over 167 days and attempted to extend existing IODP Hole C0002P by over 2 km to reach a depth of ~ 5200 mbsf. Multiple operational challenges prevented this goal from being accomplished. However, borehole did reach 3262.5 mbsf making it the deepest scientific ocean drilling hole to date. The lessons learned from Expedition 358 could be useful for planning future deep drilling projects.

The last decade has witnessed a renaissance in the breadth of applications of seismology, which extended into non-traditional areas with the aim of studying environmental processes. A unique kind of seismic sources, identified by the coupling of different Earth systems into the solid Earth, provides a new way for monitoring the global environment and for exploring the Earth's interior.

In this talk, I will present novel findings on the study of mass-wasting events and atmospheric-driven ocean storms through the analysis and modeling of seismic signals. In the first part of my talk, I will show how we can infer dynamics and main characteristics of remote landslides by using seismic data analysis and numerical modeling. In the second part of my talk, I will show how we can model seismic signals generated by ocean storms by numerical simulations, and how the time-varying behavior and strength of atmospheric and oceanic events can be estimated from decades of ambient seismic noise records.

The mechanisms generating intraslab intermediate-depth earthquakes (~70-200 km depth) remains uncertain because the high temperatures at pressures at those depths ought to prohibit brittle failure. Proposed mechanisms for their nucleation include dehydration-driven processes such as embrittlement and dehydration-induced stressing, and the generation of thermal shear runaways. These processes have hypothetically different observable seismological signatures, such a scaling of radiated efficiency with moment, and reduced aftershock activity in the case of thermal shear runaways. However, variability in the observed character of intermediate-depth earthquakes worldwide makes it difficult to support any particular mechanism. My research analyzes intermediate-depth earthquakes in two tectonic settings in Japan - the younger, warmer Philippine Sea Plate and the older, colder Pacific Plate. We find no significant scaling of radiated efficiency in either slab, indicating that pure thermal shear, as proposed to explain intermediate-depth earthquakes in the Bucaramanga Nest, Colombia, cannot well explain the occurrence of the subduction zone events. We also find a variation in intermediate-depth aftershock activity between the two slabs. Our findings indicate that aftershock activity may correspond to high Vp-Vs ratio and varies along-strike and with distance to the trench. Taken together these results suggest that the generation mechanisms of intermediate-depth earthquakes form a spectrum, where the relative importance of certain mechanisms is modulated by regional fluctuations in slab character.

A Discussion:

1) What Happened After the 1989 Loma Prieta Earthquake.

2) Why Early Earthquake Warning Systems are So Late Coming to California?

Please come prepared for discussion having read Bakun et al. (1994), Early Warning System for Aftershocks, available online here

Due to the large amount of expected discussion, this seminar will not be webcast in real time. However, we plan to upload a video to the ESC seminar archives afterwards.

A Discussion:

1) What Happened After the 1989 Loma Prieta Earthquake.

2) Why Early Earthquake Warning Systems are So Late Coming to California?

Please come prepared for discussion having read Bakun et al. (1994), Early Warning System for Aftershocks, available online here

Due to the large amount of expected discussion, this seminar will not be webcast in real time. However, we plan to upload a video to the ESC seminar archives afterwards.

Each large continental earthquake is a unique opportunity to get a privileged insight in earthquake mechanics. One way to go is to document in detail surface ruptures by combining remote sensing studies and field observations. As resolution increases, quantification of the deformation on- and off-fault enlarges our understanding of earthquake rupture processes. In addition, although the number of large continental earthquakes well documented remains small, in case of strike-slip events it becomes now possible to point to some generic pattern in surface rupture that can inform us on how continental strike-slip faults might be structured.