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.
James Kirkpatrick, McGill University, Dept. of Earth and Planetary
Sciences, Montréal, Québec, Canada, email@example.com
Melodie French, Rice University, Dept. of Earth, Environmental
and Planetary Sciences, Houston, Texas, USA, firstname.lastname@example.org
John Platt, University of Southern California, Dept of Earth
Sciences, Los Angeles, California, USA, email@example.com
Christie Rowe, McGill University, Dept. of Earth and Planetary
Sciences, Montréal, Québec, Canada, firstname.lastname@example.org
David Schmidt, University of Washington, Dept. of Earth and
Space Sciences, Seattle, Washington, USA, email@example.com
- The Geological Society of America
- National Science Foundation
- 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
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.
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
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.