Current Conference:
The Geological Fingerprints of Slow Earthquakes

Santa Catalina Island, California, USA | 27 June–1 July 2021

Little Harbor, on the west coast of Santa Catalina Island, looking north. The cliffs on the left side of the image are in bluechist facies metasedimentary rocks. Hills in the center and right of the image are underlain by higher grade metamorphic rocks and serpentinized peridotite. Photo by John Paul Platt.

Conveners

James Kirkpatrick, McGill University, Dept. of Earth and Planetary Sciences, Montréal, Québec, Canada, james.kirkpatrick@mcgill.ca

Melodie French, Rice University, Dept. of Earth, Environmental and Planetary Sciences, Houston, Texas, USA, mefrench@rice.edu

John Platt, University of Southern California, Dept of Earth Sciences, Los Angeles, California, USA, jplatt@usc.edu

Christie Rowe, McGill University, Dept. of Earth and Planetary Sciences, Montréal, Québec, Canada, christie.rowe@mcgill.ca

David Schmidt, University of Washington, Dept. of Earth and Space Sciences, Seattle, Washington, USA, dasc@uw.edu

Cosponsors

The Geological Society of America

National Science Foundation

NSF-GeoPRISMS

Southern California Earthquake Center

Description and Objectives

The discovery of slow earthquakes 20 years ago revolutionized the understanding of how plate motions are accommodated at major tectonic boundaries. Slow earthquakes are a family of events that include slow slip events (SSE), tectonic tremor, and low frequency earthquakes (LFE). In some systems, SSEs occur together with tectonic tremor. When these events occur periodically, they are known as episodic tremor and slip, or “ETS.” Compared to regular earthquakes, the slip across a fault during a slow earthquake occurs slowly, but significantly faster than plate-rate creep. Slow earthquakes are observed predominantly near the plate interface of subduction zones and on transform plate boundary faults. They are often associated with “transitional” regions at the edges of seismogenic zones but occur both updip and downdip, so encompass a wide range of pressure and temperature conditions. Understanding slow earthquakes is critical to developing better constraints on regional seismic hazards and may also provide information on the physical conditions and fault-loading rates at depth.

Seismological, geophysical, and geodetic tools have been applied extensively to study the range of slow earthquake types, where they occur, their relations to each other, and the characteristics that distinguish them from regular earthquakes and creep. However, there are numerous outstanding issues regarding the basic processes that control slow earthquake characteristics. For example, what deformation processes and mechanisms are critical to their occurrence? What controls slip rates? Do all faults that host slow earthquakes share common physical characteristics? Are the multiple potential mechanisms for tremor and slow slip common among the different tectonic settings where these phenomena are observed, including subduction zones and continental transform faults?

The geological structures that formed during slow earthquakes and are preserved in exhumed systems can provide critical insights into the sources of slow earthquakes and how they interact with the regular earthquake cycle. Although different hypotheses are proposed on a regular basis in the nascent field of slow earthquake geology, there is no “smoking gun” evidence of slow earthquakes in the rock record. Reconciling the geophysical insights with geological observations is therefore an ongoing challenge, but it is increasingly clear that only field geological observations on exhumed structures can differentiate between models for slow earthquake occurrence, as geodesy and seismology cannot resolve the relevant length scales. For this conference, we solicit contributions that use geological observations, lab measurements, or numerical models to aid in understanding the physics of slow earthquakes. We encourage researchers studying analog systems from any tectonic setting or metamorphic grade relevant to modern tremor and slow earthquakes in order to build a wide range of geological perspectives. Contributions that address outstanding questions regarding deformation mechanisms, limits on rates of deformation, and environmental conditions are encouraged. New multidisciplinary approaches are needed to define the physical controls on slow earthquakes and to develop new insights into disparate datasets. For this conference, we aim to stimulate contributions from geological-focused, particularly field-based, investigators and to engage geophysicists with a range of backgrounds to define key unknowns and debate possible models.

Sheeted vein complex in blueschist facies metasediment, Little Harbor. The veins represent repeated hydraulic fracturing events reflecting fluid pressure transiently greater than lithostatic. The veins are tightly folded, indicating on-going ductile deformation during or after hydraulic fracture events. Photo by John Paul Platt.

Preliminary Outline of Thematic Sessions

1. Cutting-edge observations of slow earthquakes;

2. Current understanding of the mechanics of slow earthquakes;

3. Geological perspectives on slow earthquakes;

4. Processes and physical properties of rocks that might be relevant to slow earthquake physics;

5. Key unknowns in slow earthquake physics; and

6. The hypotheses for slow earthquake mechanisms.

Poster sessions will be held every evening between talks and dinner, and will continue after dinner. Group discussions and breakout sessions will accompany all of the thematic sessions to promote cross-disciplinary interactions. One of the main goals of the meeting is to provide a forum for geologists and geophysicists to discuss how the existing information from the rock record should be integrated with the geophysical insights, as well as what new geological observations are needed to develop our understanding of the physics of slow slip further.

Preliminary Agenda

This five-day meeting will be held at the University of Southern California Wrigley Institute for Environmental Studies, Santa Catalina Island, California, USA. The meeting format will be a balance of invited talks, breakout discussions, pop-up talks, and poster presentations, with a day-long field trip on day three. All nights will be spent at the Wrigley Institute. Participants will be expected to observe the GSA Code of Ethics & Professional Conduct throughout the meeting.

The meeting will begin with a ferry ride to Santa Catalina Island, followed by an icebreaker on the evening of day one (27 June). Day two will focus on plenary talks presenting cuttingedge observations of slow earthquake phenomena, as well as mechanical insights from geophysical and numerical modeling studies. Geological characteristics of potential slow earthquake sources will be introduced. Days four and five will involve a combination of poster presentations, talks from participants, pop-up talks, panel discussions, and breakout groups to define hypotheses regarding the geological mechanisms of slow earthquake slip and explore interdisciplinary collaborations to further our understanding of slow earthquake phenomena. A return ferry will depart for the mainland targeting arrival around 5 p.m. for evening flight departures from Los Angeles International Airport.

Day three will be an all-day field trip for all participants, taking in several exposures within about a 30–40-minute drive from the Wrigley Institute. Santa Catalina Island has extensive exposures of a metamorphic complex of Cretaceous age that is generally considered to be part of the Franciscan Complex. Protolith rock-types include graywacke, pillow basalt, chert, and serpentinite, which have been variously metamorphosed under blueschist, high-pressure greenschist, amphibolite, and eclogite-facies conditions at depths of 35–50 km and temperatures of 300–700 °C in the Mesozoic subduction zone along the western North American margin. This range of depths and metamorphic temperatures encompasses the likely range of conditions found in the source areas for slow earthquakes on active margins such as Nankai or Cascadia, and beneath the Parkfield segment of the San Andreas fault. Deformational features include block-in-matrix mélanges with sedimentary or ultramafic matrixes, as well as extensive tracts of coherent but strongly deformed rock showing polyphase folding, ductile shear zones, and a variety of deformational fabrics. On Santa Catalina Island, the complex displays a variety of structural features that have been suggested as possibly associated with slow slip, including blocks of effectively rigid rock in a viscous matrix, sheeted vein complexes, and shear zones showing evidence for solution-redeposition creep associated with microfolding and dilational cracking.

Folded glaucophane-lawsonite schist, Little Harbor area, Santa Catalina Island. Fold hinge lines are exposed on the right side of the image, beneath the compass: hinge lines are sub-parallel to the dominant stretching direction in the rock, represented by a strong crystallographic preferred orientation of glaucophane prisms. This type of intense ductile deformation is characteristic of large volumes of coherent metamorphic rock in the Catalina Schist Terrane. Photo by John Paul Platt.

Attendees and Estimated Costs

Thanks to the generous support of the sponsoring agencies, the majority of costs for the meeting are covered. The anticipated registration fee will be US$100. The registration fee will cover four nights of lodging, meals, transportation to/from Santa Catalina Island, transportation for field trips, and facility usage. Participants will be expected to pay for travel expenses from their home to Southern California. However, we have funds to support participant travel, which will be prioritized toward participants from underrepresented groups as well as early career and student participants. All participants will be expected to make their own travel arrangements to arrive at Long Beach, California, USA, in time for a scheduled ferry to the Wrigley Institute on Santa Catalina Island.

Applications and Registration

Application period opens: 1 Sept. 2020

Application deadline: 8 Jan. 2021

Registration deadline: 5 Mar. 2021

GSA and the meeting conveners are committed to fostering diversity, equity, inclusive excellence, and belonging in the geoscience community. For this meeting, we welcome and encourage applications from all gender identities, Black, Indigenous, Latinx, and People of Color, people with disabilities, LGBTQIA+ individuals, and other groups that are currently underrepresented within the earth-science community. The conference will be limited to 64 participants, and each participant will have to commit to attending for the full duration of the conference, because transport to/from Catalina Island is provided at the beginning and end of the meeting but otherwise is limited. To apply, please submit your application through the form on the meeting website: https://sites.google.com/view/penrose2021/home.

As part of the application, we ask that you prepare a brief statement of your interests and relevance of your recent work to the conference themes (max. 300 words) as well as a tentative title for a proposed poster presentation (required) and short pop-up talk (optional). After the registration deadline, participants will be asked to write a short review that summarizes the processes and/ or mechanisms they believe are important to slow slip and tremor and the supporting evidence for these mechanisms. Applicants will be notified in early February 2021.

COVID-19 contingencies: The meeting conveners are closely monitoring the ongoing pandemic and are considering how this might impact the conference. It is hoped that a vaccine will be widely available prior to the conference, which will allow for group gatherings. Additional requirements or changes may be imposed to help mitigate the risks.