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PENROSE REPORT
Geological Fingerprints of Slow Earthquakes

Santa Catalina Island, California, USA
1–5 April 2022

CONVENERS coupling between metamorphic and structural processes, the
James Kirkpatrick, McGill University, Dept. of Earth and importance of mechanical heterogeneity during deformation, and
Planetary Sciences, Montréal, Québec, Canada, james.kirkpatrick@ the rates at which veining/fracture and distributed deformation
mcgill.ca occur. In the Catalina Amphibolite Unit, blocks of mafic and
John Platt, University of Southern California, Dept. of Earth ultramafic rocks in a metasomatized matrix attested to fluid-rock
Sciences, Los Angeles, California, USA interaction and the accompanying progressive changes in rock
David Schmidt, University of Washington, Dept. of Earth and rheology leading to mechanical heterogeneity.
Space Sciences, Seattle, Washington, USA The keynote presentations emphasized that the well-resolved
Melodie French, Rice University, Dept. of Earth, Environmental source parameters of slow earthquakes (e.g., ~10 m/s slip rate,
–7
and Planetary Sciences, Houston, Texas, USA km/day rupture propagation rate, and ~10 kPa stress drop for
Christie Rowe, McGill University, Dept. of Earth and Planetary SSEs) are distinct from regular earthquakes. Furthermore, to first
Sciences, Montréal, Québec, Canada order the geodetic and seismic records of slow earthquakes appear
to be similar, regardless of tectonic setting or depth of the slip
INTRODUCTION event. A broadly held view was that because the SSEs accommo-
This Penrose Conference assembled a multidisciplinary group of date substantially more of the plate motion budget and exhibit
scientists to debate how geological evidence can contribute toward much larger seismic moments than LFEs, SSEs are the dominant
understanding why slow earthquakes occur and to explain their member of the slow earthquake family and should be the focus of
characteristics. Slow earthquakes are a family of fault and shear future efforts to understand the physics of slow slip. However,
zone slip events that include slow slip events (SSEs), tectonic some participants argued that the LFE components of SSE were
tremor, and low frequency earthquakes (LFEs). In some systems the best-resolved geophysically and should be instrumental in
these different events occur together, and they are known as epi- driving more specific comparisons to geologic features. The
sodic tremor and slip, or “ETS.” Compared to earthquakes, the slip breakout discussions therefore explored the significance of exist-
across a fault during a slow earthquake occurs slowly, but signifi- ing geological observations and the future research needs from
cantly faster than plate-rate creep. Slow earthquakes are widely geological work in two themes:
observed where geophysical networks are robust, contribute signifi-
cantly to the overall slip budget in portions of some plate boundary 1. Is there any process, condition, or structure common to slow
faults, and may elucidate stress transfer between portions of plate earthquakes that could explain their characteristics?
boundaries with different modes of fault slip. Understanding slow The consistent geodetic and seismological characteristics of slow
earthquakes is therefore critical to better constrain regional seismic slip suggest there may be a common process or set of conditions at
hazards and may also constrain the physical conditions and fault- the sources of SSEs and possibly another for tectonic tremor and
loading rates at depth. LFEs. Attendees debated if and how grain-scale deformation mech-
The conference was held on 1–5 April 2022 at the University of anisms, deformation structure geometry, and deformation condi-
Southern California Wrigley Institute for Environmental Studies tions (e.g., effective pressure, temperature, etc.) could be different
on Pimu (Santa Catalina Island), California, USA, where the across the range of slow earthquake occurrences but combine to
exceptional exposures and structural complexity of the Catalina yield similar slip phenomena. A combination of deformation mecha-
Schist provided inspiration for the discussions. Forty-seven attend- nisms involving frictional sliding plus some viscous-type mecha-
ees, including ~45% early-career scientists, traveled from eight nism likely promotes slow earthquakes. Further work is needed to
different countries, including Canada, Germany, Japan, Italy, the determine how the two mechanisms interact and are preserved in
Netherlands, New Zealand, and the UK. The meeting launched the rock record, recognizing that both the interactions and the pres-
with a series of keynote talks presenting cutting-edge perspectives ervation may vary between different SSE settings. Attendees agreed
on slow earthquakes from seismological, geodetic, experimental, that geological field observations from slow earthquake source
modeling, and geological communities. The subsequent breakout depths indicate the deformation associated with these events likely
discussion sessions and poster sessions were devoted to research affects a volume rather than a planar or quasi-planar surface, which
presentations from the participants, framing relevant hypotheses is also allowed by the geophysical constraints. Heterogeneity is
that explain slow earthquakes and debating how geological evi- ubiquitous, but further work is needed to determine what structures
dence can be leveraged to test those hypotheses. or rheological contrasts are relevant to slow slip, and how slow
A day-long field trip informed much of the meeting discussions, slip can propagate over long distances despite the heterogeneity.
with participants taking in exposures of the Catalina Schist that Although slow slip phenomena occur over a broad range of meta-
were metamorphosed and deformed under conditions similar to morphic conditions, attendees noted that low effective stresses pro-
where some slow earthquakes occur today. Exposures included a mote slow slip by driving slip toward frictionally neutral stability,
mélange and a sheeted vein complex in the Catalina Blueschist though low effective stresses alone cannot explain why slow slip
Unit, which focused attention on pore pressure constraints, occurs rather than regular earthquakes. More detailed analyses of

38 GSA TODAY | October 2022