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PENROSE REPORT
slow earthquake sources in places with well-constrained thermal Penrose attendees are developing a set of papers to address these
structures should be undertaken to connect the deformation condi- next steps and will invite community participation.
tions with slow earthquake characteristics.
ACKNOWLEDGMENTS
2. How can geological observations test the slow earthquake The 2022 Penrose Conference met on Pimu (Catalina Island),
hypotheses? ancestral home of the Gabrielino/Tongva Nation, the Juaneño Band
Multiple hypotheses have been proposed that can explain how of Mission Indians Acjachemen Nation, and the Payómkawichum
slip velocities during slow earthquakes are limited, thereby differ- (Luiseño) nations. This conference was made possible thanks to
entiating them from regular earthquakes. Many of the hypotheses financial support from the Geological Society of America, the
are developed from the rate and state friction framework and are National Science Foundation GeoPRISMS program (2025105), and
supported by laboratory rock friction experiments. Others call for the Southern California Earthquake Center (SCEC-19133). We
coupled frictional-viscous deformation. The relevance to natural thank GSA meeting manager Becky Sundeen for organizational and
systems is largely untested. Attendees examined the hypotheses administrative support during the challenging planning process and
and asked what geological structures might serve as records that the staff at the Wrigley Center for their huge efforts.
the hypothesized mechanisms were active in ancient slow earth-
quakes, what geological features would lead the hypothesis to be
rejected, and whether there are new geological observations that
should be collected to test the hypotheses. For most hypotheses
there are both deformation structures that are predicted to be con-
sistent with the hypothesis and characteristics of deformed rocks
that would reject the hypothesis. Some, such as a specific velocity-
neutral condition and a dislocation creep-based mechanism might
be indistinguishable from perceived “steady-state” structures.
Work is needed from both the observational and geological com-
munities to reconcile the relevant scales of deformation process
and structures so that future field and microscale observations Photo credit: Daniel Ortega-Arroyo.
can contribute positively toward understanding slow slip.
PARTICIPANTS
CONSENSUS Alexis Ault, Nick Beeler, Maia Cohen, Cailey Condit,
Ultimately, the breakout discussions concluded that no signature Meghomita Das, Sara De Caroli, Eric Dunham, Åke Fagereng,
has been identified in the rock record that uniquely identifies slow Melodie French, Joan Gomberg, Djordje Grujic, Shuoshuo Han,
slip phenomena as observed with contemporary geophysical sen- Kayleigh Harvey, Jessica Hawthorne, Greg Hirth, Will Hoover,
sors but that does not radiate seismic waves. Overcoming this bar- Charis Horn, Matt Ikari, Jamie Kirkpatrick, Alissa Kotowski,
rier may depend on a combination and linkage of deformation- Peter Lindquist, Xi Lu, Olivia Marcelli, Shea McLafferty, Tim
related features, rather than a “smoking gun” (in contrast to Melbourne, Frencesca Meneghini, Margo Odlum, Kristina
frictional heat anomalies that record seismic slip). Defining the Okamoto, Daniel Ortega-Arroyo, Zhigang Peng, Sarah Penniston-
mechanisms accommodating slow slip is important for informing Dorland, Sandra Piazolo, John Platt, Jacqueline Reber, Emily
the physics of slow slip, but will likely require definition of differ- Roland, Christie Rowe, Heather Savage, David Schmidt, Susan
ent geological signatures in different settings. Future cross-disci- Schwartz, Caroline Seyler, Chris Thom, Kohtaro Ujiie, Laura
plinary studies are needed to reconcile the observations of active Wallace, Jessica Warren, Randy Williams, Stuart Williams,
slow slip with the rock record. This type of work will benefit from Baoning Wu.
a clarification of terminology so that aspects of the deformation
associated with slow slip can be compared across fields. The

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