Download Distinct Updip Limits to Geodetic Locking and Microseismicity at the

18
Distinct Updip Limits to Geodetic
Locking and Microseismicity at the
Northern Costa Rica Seismogenic Zone
E  T M T
Susan Y. Schwartz and Heather R. DeShon
Abstract
Results from the Costa Rica Seismogenic Zone Experiment (CRSEIZE), a joint
seismic and geodetic experiment designed to elucidate seismogenic processes
at the southern end of the Middle America Trench, indicate distinct patterns of
geodetic strain accumulation and interplate microseismicity in northern Costa
Rica. Geodetic inversions for interseismic slip on the plate interface reveal
locking equal to ~75% of the plate velocity between 8 and 15 km depth below
sea level, while shallow interplate microearthquakes begin deeper, between 15
and18 km below sea level, in a region that appears to be freely slipping. Interplate microseismicity terminates at ~30 km depth along the updip edge of a
second patch of partial geodetic locking that represents ~30 – 40% of the plate
velocity. The strike-parallel alternation of geodetically locked regions with no
microseismicity and freely slipping regions with abundant microseismicity
suggests that downdip increases in temperature and pressure may generate
two distinct transitions in mechanical behavior along the plate interface. We
interpret the updip limit of the shallow geodetically locked patch as the frictional stability transition from stable sliding to stick-slip behavior. The shallow transition correlates with models of the 100°C isotherm. The updip limit
of interplate seismicity and the freely slipping zone most likely correspond to
a change in mechanical properties along the plate interface; increased porefluid pressure resulting from basalt dehydration reactions and/or decreased
permeability in the upper plate may lead to fault weakening. Modeled temperatures at the deeper transition to the onset of seismicity are estimated at
200º – 250°C, in the range where low-grade metamorphic reactions potentially
release 0.3 – 3 wt % water and experimental data reveal significant decreases
in permeability for a variety of rock types. Our results suggest that thermally
mediated diagenetic processes, low-grade metamorphic reactions, or changes
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Distinct Updip Limits to Geodetic Locking and Microseismicity
577
in effective stress occur across several depths to produce multiple transitions
in the mechanical behavior of the seismogenic zone and explain the distinct
geodetic and seismic observations in northern Costa Rica.
Introduction
Subduction is a fundamental geological process generating and modifying
continental crust and associated with severe natural hazards (earthquakes,
volcanoes, tsunamis). Subduction zones release ~90% of the Earth’s seismic
energy [Pacheco et al., 1993] and generate almost all of the world’s great earthquakes. Most of this seismic moment reflects mechanical coupling between the
underthrusting and overriding plates along a shallow (<50 km depth) portion
of the dipping plate interface termed the seismogenic zone. The earthquake
cycle consists of accumulation of elastic strain in the seismogenic zone during
the interseismic period and rapid release during an earthquake. Factors affecting coupling and the strain accumulation/release process are important for
assessing and understanding seismic and tsunami hazard and the earthquake
process. Key factors include the efficiency of strain accumulation (completely
locked versus partially or completely unlocked), its spatial pattern (updip and
downdip limits and along-strike variability), and the relationship between
strain accumulation and seismicity.
Previous studies of convergent margin seismogenic zones have found that
regions accumulating strain during the interseismic cycle and releasing seismic
moment in large earthquakes collocate [e.g., Hyndman et al., 1997; Oleskevich
et al., 1999; Zweck et al., 2002; Igarashi et al., 2003]. It is commonly believed that
the shallow transition from aseismic to seismic behavior along the subduction
thrust can be defined either by the updip limit of large earthquake rupture, the
geodetically defined locked zone, or the onset of microseismicity [e.g., Newman et al., 2002; Obana et al., 2003]. As suggested by Scholz [2002], the shallow
transition from aseismic to seismic behavior may be dominantly temperature
controlled. For several subduction zone thrusts, the updip limit of geodetic
locking or seismicity has been correlated with models of the 100º – 150°C isotherm [e.g., Hyndman and Wang, 1993; Hyndman et al., 1995, 1997; Harris and
Wang, 2002; Currie et al., 2002], a temperature range that corresponds to several
phase transformations from weak hydrous to stronger dehydrated minerals
[Hyndman et al., 1997; Peacock and Hyndman, 1999; Moore and Saffer, 2001; Moore
et al., this volume].
In this paper we report distinct updip limits of elastic strain accumulation
or geodetic locking and microseismicity in the vicinity of the Nicoya Peninsula,
Costa Rica. We suggest that the updip limit of strain accumulation reflects the
aseismic to seismic transition and the updip extent of potential rupture during large underthrusting earthquakes; the transition is most likely thermally
controlled. A second transition to a freely slipping plate interface and the on-
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Seismic and Geodetic Studies
set of small-magnitude earthquakes suggests that additional diagenetic, metamorphic, or fluid-dominated processes occur, generating a second important
transition in mechanical behavior.
Tectonic Setting
The Nicoya Peninsula directly overlies the Costa Rica seismogenic zone,
making this section of the Middle America Trench (MAT) particularly well
suited for land based investigations of seismogenic zone coupling (fig. 18.1).
Along the Pacific coast of Costa Rica, oceanic Cocos plate formed at the East
Figure 18.1
Tectonic setting of the Costa Rica subduction zone. The Cocos plate oceanic
crust formed at the Cocos-Nazca Spreading Center at 22.7–19.4 Ma and at
19.5–14.5 Ma (CNS-1 and CNS-2 from Meschede et al. [1998] and Barckhausen et al. [1998, 2001]) subducts along this margin at a rate that varies between
~9 cm/yr at the Cocos Ridge and 8.3 cm/yr beneath the Nicoya Peninsula
[DeMets, 2001]. East Pacific Rise (EPR) derived lithosphere formed at ~24 Ma
subducts beneath the Nicoya Peninsula. Smooth seafloor offshore the Nicoya
Peninsula abruptly transitions to seamount dominated seafloor at the trace of
the Fisher seamount chain. Bathymetry is from von Huene et al. [2000].
Distinct Updip Limits to Geodetic Locking and Microseismicity
579
Pacific Rise (EPR) and the Cocos-Nazca Spreading Center (CNS) subducts
along the MAT at 8.3 – 9 cm/yr [DeMets, 2001]. The morphology, age, and
formation history of the Cocos plate varies along strike [von Huene et al.,
1995, 2000; Barckhausen et al., 2001], and the western Costa Rican margin can
be divided into three morphologic and bathymetric sections: the southern
Osa, the seamount-dominated central, and the smooth northern Nicoya segments (fig. 18.1) [von Huene et al., 1995]. In northern Costa Rica the incoming
plate exhibits variable thermal gradients across the EPR/CNS plate suture
[Langseth and Silver, 1996; Fisher et al., 2003] and is overlain by a thin sediment layer comprised of ~180 m of smectite-rich hemipelagic clays overlying
~220 m of carbonate-rich pelagic sediments [e.g., Kimura et al., 1997; Spinelli
and Underwood, 2004]. Complete sediment subduction occurs along the trench
[Kimura et al., 1997], and recent evidence suggests that the Costa Rica margin
may be actively eroding along the subduction thrust [Ranero and von Huene,
2000; Vannucchi et al., 2001, 2003].
Variability in lithospheric age, thermal history, and morphology of the
subducting Cocos plate reflect a complex tectonic history. Oceanic plate history is divided into three stages: (1) oceanic lithosphere formation along the
East Pacific Rise; (2) oceanic lithosphere formation along the Cocos-Nazca
Spreading Center (CNS-1) following a reorientation of the EPR-CNS triple
junction; and (3) lithosphere formation along the Cocos-Nazca Spreading Center (CNS-2) following a ridge jump along the CNS (fig. 18.1) [Meschede et al.,
1998; Barckhausen et al., 1998, 2001]. The oldest crust, formed along the EPR at
~24 Ma, subducts offshore the northern Nicoya Peninsula and is anomalously
cold for lithosphere of its age owing to strong hydrothermal circulation within
the oceanic crust [Langseth and Silver, 1996; Fisher et al., 2003]. Movement of
fluids mines heat out of the crust by pulling seawater into and expelling it out
through exposed basement highs, and recent bathymetric surveys show that
EPR-derived oceanic crust is characterized by numerous small seamounts,
exposed basement highs, and sediment-covered basement highs [see Hutnak
et al., this volume].
Segmentation of the Cocos plate along the southern MAT appears to
strongly influence both the seismic behavior and the characteristics of arc
magmas [Carr and Stoiber, 1990; Protti et al., 1995a; von Huene et al., 1995]. The
volcanic arc in Costa Rica is offset westward at the transition from CNS-2 to
CNS-1 oceanic crust (fig. 18.1) and shifts eastward again at the Costa RicaNicaragua border. Northern Costa Rica and Nicaragua volcanoes exhibit
higher components of slab and subducted sediment in their isotopic compositions (e.g., Ba/La, U/Th, and 10 Be) than volcanoes in central Costa Rica [see
Patino et al., 2000 for a review]. This may reflect a larger amount of hemipelagic
and carbonate-rich sediments reaching melt generation depths in the north or
the dilution of oceanic sediments by upper continental crustal material derived through subduction erosion in central Costa Rica [Morris et al., 2002].
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Seismic and Geodetic Studies
Subduction erosion has been proposed along much of the Costa Rican
margin [Meschede et al., 1998; Ranero and von Huene, 2000; Vannucchi et al., 1998,
2001, 2003] with two mechanisms postulated: (1) thinning of the upper plate
as seamounts subduct and drag material beneath the shallow part of the fore
arc; and (2) basal erosion whereby the fore arc is thinned and upper plate material is transferred to the lower plate. While evidence of fore-arc deformation
caused by seamount subduction is common as furrows and scarps along the
continental fore arc (fig. 18.1), evidence of deeper basal erosion, which may be
unrelated to subducted topography, is more difficult to document. Ranero and
von Huene [2000] argued for basal erosion along central Costa Rica based on
an interpretation of reflection seismic images of large, fault bounded lenses of
upper plate material being carried down the subduction zone. The process of
subduction erosion may have important implications for the frictional properties of the plate interface because, rather than subducted sediments, the plate
interface may be lined with fault gouge consisting of variably compacted,
consolidated oceanic sediments and upper plate material. Additionally, largescale normal faulting along the outer rise (fig. 18.1) may later develop into
topographic basins within the subduction channel and carry gouge to magmagenerating depths [von Huene et al., 2004].
Protti et al. [1995a] correlated seismic behavior with subducting seafloor
characteristics. The largest underthrusting earthquakes, with magnitude
>7.5, have historically only occurred along the Nicoya segment where relatively smooth seafloor subducts. Intermediate magnitude interplate events,
with magnitudes up to 7.4, occur in the southern Osa segment, where thickened oceanic crust of the Cocos Ridge (fig. 18.1) subducts. The central segment, where seamount dominated seafloor subducts, generates thrust events
with magnitudes of only 7.0 and is the location of a general rate increase in
background seismicity. More detailed examination of seismicity and its relationship to seafloor characteristics in the vicinity of the Nicoya Peninsula,
made possible by CRSEIZE, revealed a shallowing of the updip limit of interplate earthquakes across the EPR/CNS-1 crustal origin boundary [Newman et al., 2002]. Since EPR-derived crust is cooler than CNS-derived crust
[Langseth and Silver, 1996; Fisher et al., 2003], Newman et al. [2002] interpreted
their observations as evidence for thermal control of the updip limit of the
seismogenic zone. More recently, geodetic inversions for interseismic slip on
the plate interface in the Nicoya region identified a locked patch seaward of
the shallowest microearthquakes [Norabuena et al., 2004]. As explored further
in this paper, the geodetic inversion results suggest that the updip limit of
the seismogenic zone is defined by the shallow boundary of the geodetically
locked plate interface and that the updip limit of interplate seismicity may
correspond to second along-dip transition in the mechanical behavior of the
plate interface.
Distinct Updip Limits to Geodetic Locking and Microseismicity
581
Seismic and Geodetic Observations
and Results
Seismic Observations and Earthquake Locations
The Costa Rica Seismogenic Zone Experiment consisted of GPS observations
and two combined land and ocean bottom seismometer (OBS) seismic deployments, one northwest of the Osa Peninsula in southern Costa Rica and the
other in and around the Nicoya Peninsula in northern Costa Rica (fig. 18.2).
Figure 18.2
The Costa Rica Seismogenic Zone Experiment (CRSEIZE) consisted of
43 GPS stations (diamonds) and two passive on/offshore seismic experiments
(squares and triangles represent broadband and short-period sensors respectively) that recorded earthquakes (circles) along the Middle America Trench.
The Osa Peninsula experiment primarily recorded aftershocks of the Mw 6.9,
8/20/99 Quepos earthquake (Harvard CMT mechanism). The northern
Nicoya Peninsula experiment recorded interplate events and intraplate events
within the oceanic crust (e.g., the Mw 6.4, 7/21/00 outer rise event and its
numerous aftershocks, shown with Harvard CMT mechanism) and within
the overlying continental crust.
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Seismic and Geodetic Studies
Experiment details, analytical techniques for earthquake data processing, location, and interpretations for the seismic component of CRSEIZE are described
by Newman et al. [2002]; DeShon et al. [2003]; DeShon [2004]; and DeShon et al.
[2006]. Details describing GPS data quality, processing, modeling, and interpretation can be found in the work of Norabuena et al. [2004].
The Nicoya seismic network recorded more than 8000 regional and local
earthquakes along the Middle America subduction zone over its 18-month deployment (fig. 18.2). Shallow seismicity beneath the Nicoya Peninsula, both
crustal and along the plate interface, represents ~20% of these earthquakes.
Initial earthquake locations were determined using the global IASP91 velocity
model (seismicity shown on fig. 18.2), and ~600 well-recorded Nicoya events
were relocated using a simultaneous inversion of P- and S-wave arrival times to
solve for earthquake location and three-dimensional velocity structure [DeShon
et al. 2006]. These events have a location precision (one standard error) better
than 1 km in both horizontal and vertical components. Interplate events were
identified in this high-quality data set based on position relative to the plate
interface as defined by the seismic velocity data, with all events located within
5 km of the plate interface assumed to be interplate earthquakes (fig. 18.3).
Two aspects of the seismicity can be discerned from figure 18.3: (1) seismicity begins at shallower depth where CNS lithosphere is subducted, and (2) the
dip of the EPR slab is steeper than the CNS slab [DeShon et al. 2006]. The revised
slab geometries for both EPR- and CNS-derived lithosphere agree well with,
but extend, existing seismic refraction information [Ye et al., 1996; Christeson
et al., 1999; Sallarès et al., 1999, 2001].
Geodetic Data and Modeling
The horizontal component of GPS site velocities in northern Costa Rica relative
to the Caribbean plate display a distinct north to northwest rotation of vectors
relative to the plate convergence direction (fig. 18.4). This presumably reflects
the influence of one or more processes in addition to strain accumulation on
the plate boundary such as postseismic response to major past earthquakes
and trench-parallel motion of a fore-arc block due to oblique convergence. As
described by Norabuena et al. [2004], these are accounted for prior to estimation
of locking on the plate interface. Following Savage [1983], interseismic strain
accumulation on the plate interface is modeled with elastic dislocation theory
as a superposition of continuous slip at the plate rate and “back slip” on the
locked portions. Values of back slip, derived from inversions, therefore represent the part of the total plate rate presently locked and accumulating strain at
particular positions along the plate boundary. Figure 18.4 shows the inferred
back-slip distribution on the Nicoya portion of the plate-boundary fault for
the best-fitting solution, and the corresponding observed and calculated velocity vectors. The best-fit model includes the effects of 8 mm/yr of northwest block motion and postseismic response of the 1992 Nicaragua earthquake
Distinct Updip Limits to Geodetic Locking and Microseismicity
Figure 18.3
583
Nicoya Peninsula experiment seismicity relocated through a three-dimensional
velocity model (all circles) shown in map and cross-sectional view. Larger
black and gray circles represent EPR and CNS events respectively, identified
as occurring on the thrust interface. Shaded histograms show the number of
events in 2.5-km bins with EPR and CNS interplate event histograms overlaid.
The onset of interplate events occurs at deeper depth for EPR events (18 km)
compared with CNS events (15 km). The plate boundaries are from refraction
information (lines on map) from Christensen et al. [1999] and Sallarès et al.
[1999, 2001] for EPR (solid lines on cross sections) and from Ye et al. [1996]
for CNS (dashed lines).
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Seismic and Geodetic Studies
Figure 18.4
Best-fit model for locked slip on the plate boundary, from Norabuena et al.
[2004]. Blue vectors are calculated velocities at the GPS sites, white are
observed. Note elliptical patch of high locked slip (maximum, 6 cm/yr)
beneath the coastal area, elongated parallel to the trench, and a second patch
farther downdip (maximum, ~3 cm/yr). The black arrow is the plate convergence direction calculated using the Cocos-Caribbean angular velocity
of DeMets [2001].
[Norabuena et al., 2004]. The slip distribution shows two patches of locking,
with the maximum back slip (6 cm/yr or ~70% of the plate velocity) centered
at 14 ± 2 km depth, and a deeper patch (~3 cm/yr or ~35 % of the plate velocity) centered at 40 ± 5 km depth. A region of lower back slip (<2 cm/yr) centered at 25 km depth separates the two patches exhibiting highest back slip.
Distinct Updip Limits to Geodetic Locking and Microseismicity
585
Checkerboard resolution tests indicate that the patches could either represent
the gradational distribution illustrated, or perhaps two smaller, fully locked
patches separated by a freely slipping region. Along-strike variations in back
slip cannot currently be resolved beyond the Nicoya peninsula, related to station distribution. The occurrence of two patches of locked slip near the updip
and downdip limits of the seismogenic zone is present in all of the low-misfit
models for Nicoya, regardless of parameters used to calculate postseismic response to the 1992 Nicaragua earthquake and sliver motion [Norabuena et al.,
2004]. Inversions that constrain all locking to occur at depths below 15 km
yield significantly degraded fits to the data and were deemed unacceptable.
Therefore shallow geodetic locking between ~8 and 15 km depth is a robust
result that is not sensitive to the details of the inversion parameterization.
Controls on Frictional/Mechanical
Transitions
Updip Limit of the Seismogenic Zone
The updip limit of seismogenic zones has frequently been identified by the
shallowest occurrence of microearthquakes [e.g., Marone and Scholz, 1988; Pacheco et al., 1993; Newman et al., 2002; Obana et al., 2003] and favorably compared to other geophysical indicators of the onset of seismogenesis, such as the
shallowest extent of large earthquake rupture and geodetically locked regions
[e.g., Hyndman and Wang, 1993; Hyndman et al., 1997; Oleskevich et al., 1999; Currie et al., 2002]. The implicit assumption has been that any of these observations
or combinations therein potentially defines the shallow transition from stable
sliding to stick-slip behavior. Figure 18.5 superimposes the geodetic back-slip
model and interplate seismicity pattern for the Nicoya Peninsula. It clearly
shows that the microseismic and geodetic definitions of the updip limit do not
agree in this region over the time span sampled by the data.
While the geodetic data indicate a locked patch between ~30 and 65 km
inland from the trench, microseismicity does not begin until 70 – 75 km from
the trench. In northern Costa Rica the updip limit of the seismogenic zone corresponds to the shallowest region able to accumulate significant strain and is
distinct from the updip limit of microseismicity.
Within error limits, the updip extent of the geodetically locked patch also
corresponds to the updip limit of rupture in the last large earthquake in this
region. Figure 18.6 plots the aftershock area of the 1950 Nicoya earthquake
(Ms 7.7) determined by Güendel [1986] on the pattern of locked slip for the
Nicoya Peninsula region. The epicentral location of this event was recomputed
by Avants et al. [2001] relative to the well-located 1990 Gulf of Nicoya (Mw 7.0)
event [Protti et al., 1995b], and the entire aftershock pattern has been shifted to
the position of the relocated epicenter. Although details of the coseismic slip
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Seismic and Geodetic Studies
distribution for the 1950 event are not known, preventing a comparison between coseismic slip and interseismic strain accumulation, the updip limit of
the 1950 aftershock area appears to correspond reasonably well with the updip
edge of the locked patch, providing confirmation that both of these adequately
define the updip limit of the seismogenic zone offshore the Nicoya Peninsula,
Costa Rica.
Hyndman and Wang [1993] and Oleskevich et al. [1999] suggested that the
updip limit of the seismogenic zone corresponds to the 100º – 150°C isotherm.
Using the thermal model of Spinelli and Saffer [2004], we compared the updip
locking limit in the Nicoya region to the location of their 100°C isotherm (fig.
18.5). This isotherm estimate corresponds approximately to the updip limit
of the geodetically determined locked patch and the updip rupture extent of
the last large earthquake (fig. 18.6). The thermal model of Spinelli and Saffer
[2004] accounts for abrupt difference in seafloor heat flow values obtained on
EPR- versus CNS-derived crust and includes 1 – 2 km of hydrothermal cooling of the EPR crust [Fisher et al., 2003]. An offset in their 100°C isotherm,
coincident with the change in oceanic crustal origin is subtly mimicked in
a shallowing of the updip edge of the geodetically locked patch across the
EPR-CNS boundary (figs. 18.5 and 18.6). Temperatures at the plate boundary
derived from heat flux measurements and the depth to the bottom simulating
reflector (BSR) (I. Grevemeyer, GEOMAR, Germany unpublished data, 2003)
agree with the Spinelli and Saffer [2004] values where well-resolved (figs. 18.5
and 18.6).
Temperature-influenced mechanisms that have been proposed to govern
the transition from stable sliding to stick-slip behavior include a suite of diagenetic and metamorphic reactions that occur near 100º – 150°C and lead to a
change in the frictional behavior of the plate interface or a decrease in fluid
production and fluid pressure [Moore and Saffer, 2000]. Vrolijk [1990] suggested
the clay-mineral transition of smectite, common in oceanic sediments, to illite controls the updip location of the aseismic to seismic transition. However,
recent work on the frictional characteristics of illite under increasing normal
stress, and sliding velocity has shown it to exhibit only velocity-strengthening
or stable sliding behavior [Saffer and Marone, 2003]. Saffer and Marone [2003]
suggested that processes such as cementation, consolidation, and localization
of slip in clay minerals may exert a significant influence on the frictional state
of the subduction thrust. Moore et al. [this volume] suggest that chemical byproducts of the smectite-illite transition, such as calcium, can facilitate carbonate cementation, which in turn may affect the frictional behavior of the thrust
interface. Spinelli and Saffer [2004] investigated the likelihood that opal and
clay dehydration reactions are responsible for the aseismic to seismic transition
documented by Newman et al. [2002] at the Nicoya Peninsula. They found an
offset in the location at which sediment dewatering is completed (between 90º
and 160°C) that mimics the offset of shallow seismicity across the EPR/CNS
boundary but lies ~15 – 20 km seaward of it. Our recent observations suggest
that the shallow extent of geodetic locking, not microseismicity, may define the
Distinct Updip Limits to Geodetic Locking and Microseismicity
Figure 18.5
587
Comparison of well-located earthquakes from CRSEIZE (yellow circles are
plate interface events), distribution of locked slip from inversion of the GPS
data [Norabuena et al., 2004], 100º–250°C isotherm estimates from Spinelli
and Saffer [2004] calculated assuming hydrothermal cooling of the upper 1 km
of the EPR crust (solid red lines), 120°C isotherm estimate from I. Grevemeyer (unpublished data, 2003) (orange dashed line; filled squares indicated
well-resolved data), 300°C isotherm estimate from Harris and Wang [2002],
and continental Moho/oceanic slab intersection from DeShon [2004]. Earthquakes reach an updip limit ~70 km from the trench where temperatures reach
200º–250°C. The updip extent of locked slip (~6 cm/yr) occurs ~35 km from
the trench approximately coincident with the 100º–150°C isotherms that shallows across the East Pacific Rise to Cocos-Nazca Spreading Center generated
crustal boundary (dashed line).
aseismic to seismic transition for the Nicoya Peninsula over the entire seismic
cycle. The onset of geodetic locking agrees with the location of pore-pressure
dissipation from Spinelli and Saffer [2004], but potential along-strike variations
in locking remain unresolved.
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Seismic and Geodetic Studies
Figure 18.6
Comparison of the distribution of locked slip from inversion of GPS data
and the rupture area of the Ms = 7.7 1950 Nicoya earthquake, modified from
Norabuena et al. [2004]. The mainshock epicenter (yellow box) was relocated
(yellow star) by Avants et al. [2001] relative to the well-located Mw = 7.0 1990
event (black star) and the aftershock area was shifted accordingly. Estimates of
the 100° and 120°C isotherms are from Spinelli and Saffer [2004] and I. Grevemeyer (unpublished data, 2003) respectively and coincide with the updip limit
of the rupture area and the locked patch.
Most of the progress made on understanding controls on the aseismic to
seismic transition has come from the study of accretionary convergent margins where the plate interface is dominated by subducted trench sediments.
In this environment, continual burial and heating transforms trench sediments in a series of progressive diagenetic/low-grade metamorphic reactions
leading to the onset of seismic behavior. The association between temperature
estimates of 100º – 150°C and the onset of seismogenesis has been documented
for erosional margins as well (e.g., Costa Rica (this study) and northeast Japan
Distinct Updip Limits to Geodetic Locking and Microseismicity
589
[Oleskevich et al., 1999]); however, in this environment, material eroded from
the underside of the upper plate subducts and may line the plate interface
[Ranero and von Huene, 2000; von Huene et al., 2004]. This material most likely
has not undergone the same progressive transformations experienced by accretionary margin sediments. The correlation of frictional stability and thermal structure at both accretionary and erosional margins suggests that fundamental processes not tied to specific mineralogies, such as smectite, operate
at both types of margins to govern the transition from aseismic to seismic
behavior.
Dissipation of pore-fluid pressure and its concomitant increase in effective normal stress is the common factor that has been identified as important in governing the aseismic to seismic transition at both types of margins
[e.g., Saffer and Bekins, 2002; Saffer, 2003; Spinelli and Saffer, 2004]. Von Huene
et al. [2004] proposed a generalized model of subduction erosion appropriate for margins with small frontal prisms such as Costa Rica. This model
requires extensive hydrofracturing within the upper plate and decreased effective normal stress across the décollement due to sediment and oceanic
crust dewatering. Mechanically weakened continental plate causes a vertical
upward shift in the décollement, allowing inclusion of upper plate material
into the subduction channel. In this model, the cessation of hydrofracturing
and dissipation of pore pressures with depth terminates subduction erosion
and results in an increase in effective normal stress and the beginning of
seismogenesis.
Prestack depth-migrated profiles offshore the Osa Peninsula in central
Costa Rica, where slab dip is significantly shallower than the Nicoya Peninsula region, image the plate boundary as high-amplitude landward dipping
reflections [Ranero and von Huene, 2000]. High reflectivity is often associated
with fluids since their presence causes a large change in impedance contrast
easily detectable with seismic reflection methods. Plate-boundary reflectivity
decreases beneath the continental shelf break, some 30 – 35 km from the trench
where heat flux and depth to the BSR indicate a temperature of ~150°C [Pecher
et al., 2001; I. Grevemeyer, GEOMAR, Germany, unpublished data, 2003]. Assuming that the high-amplitude plate boundary reflections are caused by
abundant free water, their termination at ~8 km depth most likely represents
the downdip extent of significant water release along the plate boundary.
The location of an Mw 6.4 underthrusting earthquake northwest of the Osa
Peninsula, the shallowest earthquake to occur on the plate boundary in the
south central Costa Rica region, when projected onto this reflection line, occurs
where the plate-boundary reflectivity significantly decreases. This suggests
that unlike the Nicoya Peninsula, the updip limit of midmagnitude seismicity
in central Costa Rica coincides with the aseismic to seismic transition, but like
the Nicoya region, this transition occurs where plate boundary temperatures
attain 100º – 150°C.
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Transition from Locked with No Microseismicity to
Creeping with Microseismicity
Aseismic slip along particular segments of strike-slip faults, such as the San
Andreas and the North Anatolian systems, has been recognized for some
time [e.g., Steinbrugge et al., 1960; Ambraseys, 1970; Aytun, 1980; Lienkaemper
et al., 1991; Lienkaemper and Galehouse, 1998]. Creeping segments within these
fault systems were first identified by offsets of cultural features and an abundance of microseismicity in the absence of sizeable earthquakes. More recently,
creeping strike-slip fault segments have been documented and interseismic
slip rates quantified through GPS geodesy. Inversion of geodetic data for slip
at depth on the Hayward fault in California [Bürgmann et al., 2000; Simpson
et al., 2001; Malservisi et al., 2003] reveals that the fault zone consists of a patchwork of frictional properties with behaviors ranging from completely locked
patches (no observable slip) with little microseismicity, to high-slip rate zones
(unlocked and moving at the surface velocity) with abundant earthquakes.
Malservisi et al. [2003] report the largest concentrations of microearthquakes at
transition zones between locked and more freely slipping regions on the Hayward fault in California. They interpreted this to indicate accelerated strain
accumulation at the transition zones. Increased strain at locked to creeping
transition zones would result in deformation within the surrounding volume
and might explain the abundant upper plate as well as interplate seismicity we
observe near the strong gradient in locked slip (fig. 18.7).
Because much of the plate interface lies below the seafloor at subduction
zones, less is known about patterns of locked and slipping segments in this
environment. Our work in northern Costa Rica and recent work at convergent margins in northeast Japan, Nankai subduction zone, and Alaska have
revealed distinct regions that are geodetically locked and have little to no microseismicity during the interseismic cycle and become the loci of major moment release during large earthquakes. Other regions remain unlocked with
abundant microseismicity during the interseismic cycle and are sites of little to
no moment release during major earthquakes. In northeastern Japan, Igarashi
et al. [2003] found that frequently repeating small earthquakes, and adjacent
freely slipping regions colocated with low-slip regions of coseismic moment
release associated with two large plate-boundary earthquakes in 1968 (Mw 8.2)
and 1994 (Mw 7.7). Their distribution of freely slipping regions obtained from
repeating earthquakes also matched the pattern of freely slipping regions estimated from GPS. In the region of the great 1964 (Mw 9.2) Alaskan earthquake,
Zweck et al. [2002] also found a correspondence between regions of interseismic strain accumulation or strong geodetic coupling and patches of high coseismic slip during the 1964 earthquake. However, only the Nicoya Peninsula
possesses downdip alternation between geodetically locked, microseismically
quiescent regions and freely slipping, microseismically active regions during
an interseismic period.
Distinct Updip Limits to Geodetic Locking and Microseismicity
Figure 18.7
591
Schematic of northern Costa Rica subduction zone. Color shading represents
P-wave velocities from DeShon [2004]. The aseismic to seismic transition is
defined by the shallowest region presently locked (shaded) and accumulating strain [Norabuena et al., 2004] and is coincident with the 100º–120°C
isotherm estimates. A second transition in mechanical behavior characterized
by the onset of thrust earthquakes and geodetic unlocking occurs between 200º
and 250°C (modeled by Spinelli and Saffer [2004]), where the upper plate velocity increases velocity. A second deeper locked patch (shaded) exists below the
continental Moho (Mc); however geodetic resolution at this depth is poor.
This along-dip transition occurs where temperature estimates are between
200º and 250°C (fig. 18.5), and the plate interface lies between 15 and 19 km
depth or 0.5 – 0.6 GPa. This transition most likely reflects a weakening of the
plate interface that allows it to slip freely through both creep and small earthquakes. Because this depth range is clearly in the seismogenic zone based
both on historic Mw > 7 seismicity and the occurrence of microseismicity, the
subduction megathrust is in the frictional unstable regime [e.g., Scholtz, 1998].
We suggest that small increases in pore-fluid pressure decrease the effective
normal stress and shear strength of the fault without driving the subduction
megathrust back into the frictional stability regime such as exists at the trench.
In this way, strain buildup along small-scale heterogeneities, perhaps at localized regions of increased effective normal stress due to changing pore pressure
or material properties, serve as initiation regions for microseismicity while the
fault predominantly creeps aseismically.
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Seismic and Geodetic Studies
An increase in pore pressure can be achieved through either or both an
increase in fluid production or a reduction in permeability. Fluids in the depth
range of interest (18 – 22 km) are most likely generated through low-grade metamorphic reactions occurring in subducted oceanic crust and/or marine sediments. Chemically bound water in oceanic crust and marine sediments will
be released at different temperature-pressure conditions as hydrous silicates
become progressively unstable. Most metamorphic devolatilization of siliceous limestones and clay-rich sediments occurs at temperatures greater than
attained within the seismogenic zone [Kerrick and Connolly, 2001]. However,
an average marine sediment bulk composition with an initial 7.3 wt % water
(taken from the database of Plank and Langmuir [1998]) begins devolatilization
at temperatures and pressures corresponding to the noted transition in mechanical behavior, releasing up to 0.5 wt % water [Kerrick and Connolly, 2001].
Although unaltered oceanic basalt typically contains <0.5 wt % water
[Dixon et al., 1988], hydrothermal alteration of basalt at mid-ocean ridges can
substantially increase the amount of chemically bound water in the oceanic
crust and lead to a highly variable water content. Estimates of the average
water content of oceanic crust range from 1 – 2 wt % [Peacock, 1990] to 6 wt %
[Anderson et al., 1976]. Depending on the degree of hydration of the oceanic
crust, dehydration will occur in association with different metamorphic facies
reactions. Assuming a large degree of hydration (5 – 6 wt % water) and metamorphic phase equilibrium computed by Peacock [1993], P-T paths followed by
subducted crust beneath the Nicoya Peninsula progress through zeolite (ZE)
to lawsonite-chlorite (LC) to pumpellyite-actinolite (PA) facies in the depth
range of 13 – 25 km releasing ~0.3 wt % water across the LC-PA boundary at 16
and 22 km for hotter CNS or colder EPR crust respectively (fig. 18.8). Oceanic
crust containing over 8 wt % water (the maximum amount of water accommodated in zeolite facies rocks) would begin dehydration across the zeolite
to lawsonite-chlorite boundary releasing 3.1 wt % water in the depth range
between 14 and 17 km (fig. 18.8). The near coincidence of the onset of these
basalt dehydration reactions with the updip edge of microseismicity (fig. 18.7)
makes a compelling case for weakening of the plate interface due to increased
pore-fluid pressure.
However, this mechanism requires hydrous oceanic crust to be subducted
and thus additional information on the level of hydration is critical. Ranero et
al. [2003] imaged pervasive bending-related normal faults in the oceanic lithosphere offshore the Nicoya Peninsula and postulated that these faults promote
hydration of the surrounding crust and mantle. The faults imaged in their seismic data extend at least to 20 km into the plate and provide a mechanism for
infiltration of water into the crust and mantle. Husen et al. [2003] suggested the
existence of a shallow (<40 km) hydrous layer in the lithosphere of the downgoing Cocos plate in northern Costa Rica based on seismic tomography and
petrologic modeling. Although they acknowledged that the anomaly might
be due to low velocity material above the plate interface in the subduction
Distinct Updip Limits to Geodetic Locking and Microseismicity
Figure 18.8
593
Pressure-temperature diagram of metamorphic facies modified from Peacock
[1993] with low-temperature geotherms appropriate for EPR and CNS crust
from Spinelli and Saffer [2004] and depth range of 300°C isotherm from
Harris and Wang [2002]. T1 and T2 represent the depth ranges of the two
mechanical transitions discussed in this paper. ZE, Zeolite (~8.5 wt % H2O);
LC, Lawsonite-Chlorite (~5.4 wt % H2O); PP, Prenite-Pumpellyite (~5.9 wt
% H2O), PA, Prenhite-Actinolite (~5.1 wt % H2O); LB, Lawsonite-Blueschist
(5.9 wt % H2O); GS, Greenschist (~3.4 wt % H2O) where maximum H2O
content is indicated for a basaltic bulk composition.
channel, they favored an interpretation requiring strong hydration of the oceanic crust. Fluids released by metamorphic reactions in both the oceanic basalt and marine sediments can most efficiently increase pore-fluid pressure in
the presence of reduced permeability. Recent permeability measurements on
rocks from the Shimanto complex, an ancient exhumed subduction zone fault
in Japan, show a decrease in permeability at temperatures above ~250°C for
all rock types sampled [Kato et al., 2004]. Moore and Saffer [2001] list a variety of
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Seismic and Geodetic Studies
diagenetic/metamorphic processes such as quartz cementation and pressure
solution that reduce permeability that initiate at temperatures as low as 150°C
but do not operate at full efficiency until 200º – 250°C.
The estimated location of the 250°C isotherm along the plate interface [Spinelli and Saffer, 2004] and the onset of microseismicity occur where the upper
plate material transitions to higher P-wave velocities (fig. 18.7). This increase
in velocity may reflect a decrease in permeability achieved through fracture
cementation or a fundamental change in rock type [i.e., DeShon et al., 2006].
A combination of fluid production from low-grade metamorphic reactions in
basaltic crust and marine sediments coupled with a decrease in permeability around 250°C has potentially led to elevated pore pressure and sufficient
weakening of the thrust interface to permit failure during small-magnitude
earthquakes. In contrast, the region just updip is so strongly coupled that rupture only occurs during large-magnitude earthquakes.
Transition from Creeping with Microseismicity to
Locked with No Microseismicity
All acceptable geodetic models of locked slip on the plate interface produce
a deeper locked patch downdip of most of the microseismicity (fig. 18.5), although it is less well constrained in magnitude and downdip extent than the
shallow locked patch. The few microearthquakes that occur in this region are
somewhat diffuse and have been interpreted as intraplate events within the
overriding plate and subducted lithosphere (fig. 18.3) [DeShon et al., 2006]. The
better-resolved updip edge of this deeper locked patch at ~35 km, corresponds
to the depth of the continental Moho/oceanic slab interface (figs. 18.5 and 18.6)
[Sallarès et al., 2001; DeShon et al. 2006]. Temperature estimates at this depth have
been calculated to be ~300 – 350°C (with some degree of shear heating) [Harris
and Wang, 2002], below those associated with stable sliding in mantle material.
The significant change in upper plate composition from continental to mantle
material may result in significant strengthening of the fault and account for the
transition back to geodetically locked. The downdip extent of the seismogenic
zone would coincide with the downdip edge of deep strain accumulation; unfortunately, our present model does not have the resolution to determine this.
Conclusion
The Nicoya segment of the Middle America seismogenic zone possesses two
shallow transitions in mechanical behavior during the interseismic period. We
interpret the updip aseismic to seismic frictional transition as the shallowest
region presently accumulating strain (geodetically locked); this occurs between 30 and 40 km from the trench. Although we believe the frictional properties of the plate interface below this transition to be velocity weakening, no
Distinct Updip Limits to Geodetic Locking and Microseismicity
595
earthquakes have occurred on this portion of the plate interface during the
time span of our experiment. We interpret the absence of microseismicity to
reflect high strength (high shear stress) along this section of the fault. The onset of abundant interplate microseismicity, coincident with a decrease in strain
accumulation (fault creep), occurs between 70 and 75 km from the trench. We
interpret this as a weakening of the fault, most likely resulting from increased
pore pressure. Metamorphic reactions in hydrated subducted oceanic crust
coupled with low permeability are most likely responsible for generating elevated pore pressures. This deeper transition occurs where temperature estimates are between 200º and 250°C, in the range of the lawsonite-chlorite to
pumpellyite-actinolite facies boundary where up to 0.3 wt % water may be
released for highly hydrated crust. For a variety of rock types a significant
decrease in permeability has been demonstrated at ~250°C, allowing a small
influx of fluid to generate significant pore pressures.
There is already great interest in the state of hydration of subducted lithosphere due to the likely role of slab dehydration at depth in producing arc
lavas and generating intermediate depth earthquakes in downgoing slabs.
Slab dehydration at shallower depths and the resulting serpentinization of the
fore-arc mantle wedge has also been identified as a possible mechanism for
the termination of interplate earthquake rupture [e.g., Peacock and Hyndman,
1999]. Our observation in northern Costa Rica that the onset of interplate microseismicity is coincident with the downdip edge of a geodetically locked
patch and estimates of the ~250ºC isotherm (fig. 18.5) has now implicated slab
dehydration in the initiation of interplate earthquakes and thus increases the
importance of determining the state of hydration of subducting lithosphere.
Although we know of no other seismogenic zones that possess downdip
transitions in mechanical behavior like those described here, they may exist but
be beyond the present resolution of most seismic and geodetic observations.
While we can generate a uniform slip solution in Nicoya with a large region of
partial coupling (~40% of the plate rate), the data density and proximity to the
trench allow finer resolution and indicate that the seismogenic plate interface
is better represented by relatively small fully locked (or nearly so) patches,
surrounded by regions that are essentially freely slipping with localized microseisms. This suggests that some subduction thrusts, previously inferred to
be partially coupled based on spatially limited geodetic data, might be better
represented by alternating locked and slipping regions. Spatially dense, for
many subduction zones, seafloor geodetic observations will be required to resolve this issue.
Acknowledgments
Some of the ideas contained in this paper were developed while participating
in a graduate seminar on the Seismogenic Zone at UC Santa Cruz. We thank
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Seismic and Geodetic Studies
the students in that seminar and especially co-instructor Casey Moore for their
stimulating discussions. We also thank Tim Dixon, Nick Beeler, and Demian
Saffer for their insightful comments that vastly improved this manuscript. This
work was supported by NSF grant EAR-0229876 to S.Y.S.
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