Hydrothermal microearthquake swarms beneath active vents at Middle Valley, northern Juan de Fuca Ridge

Abstract

Over 3000 local and regional earthquakes were recorded by a compact network of eight ocean bottom seismographs (500 m instrument spacing) from August 1996 to January 1997 in Middle Valley, a sediment-covered rift valley on the northern Juan de Fuca Ridge. Thirteen swarms of small-magnitude microearthquakes (−1.2 < Mw < 0.2) were detected beneath Dead Dog vent field, a major hydrothermal area in Middle Valley with exit fluid temperatures near 270°C. High precision relative positions for 304 events within swarms were determined using waveform cross-correlation techniques. The events were relocated into small, spatially distinct clusters. The intensity of the swarms is correlated with high heat flow with the largest swarm positioned 1.3 km beneath the Dead Dog vents. Smaller clusters of earthquakes are located up to hundreds of meters outside the vent field. The results suggest that the observed seismicity in the Dead Dog region is triggered by thermal strain (contraction) in the hydrothermal reaction zone as fluids extract heat from hot basement rock. Microearthquake swarms appear to be concentrated in regions where faulting has promoted seawater penetration through the sediment layer, cooling the crust, and yielding larger strain rates than those produced by seafloor spreading.

Full text

Hydrothermal microearthquake swarms beneath active vents at
Middle Valley, northern Juan de Fuca Ridge
Charles E. Golden,
1
Spahr C. Webb,
2
and Robert A. Sohn
3
Received 27 July 2000; revised 27 April 2002; accepted 22 August 2002; published 18 January 2003.
[1] Over 3000 local and regional earthquakes were recorded by a compact network of
eight ocean bottom seismographs (500 m instrument spacing) from August 1996 to
January 1997 in Middle Valley, a sediment-covered rift valley on the northern Juan de
Fuca Ridge. Thirteen swarms of small-magnitude microearthquakes (1.2 < M
w
< 0.2)
were detected beneath Dead Dog vent field, a major hydrothermal area in Middle Valley
with exit fluid temperatures near 270C. High precision relative positions for 304 events
within swarms were determined using waveform cross-correlation techniques. The
events were relocated into small, spatially distinct clusters. The intensity of the swarms
is correlated with high heat flow with the largest swarm positioned 1.3 km beneath the
Dead Dog vents. Smaller clusters of earthquakes are located up to hundreds of meters
outside the vent field. The results suggest that the observed seismicity in the Dead Dog
region is triggered by thermal strain (contraction) in the hydrothermal reaction zone as
fluids extract heat from hot basement rock. Microearthquake swarms appear to be
concentrated in regions where faulting has promoted seawater penetration through the
sediment layer, cooling the crust, and yielding larger strain rates than those produced by
seafloor spreading.
INDEX TERMS: 3025 Marine Geology and Geophysics: Marine seismics (0935);
3015 Marine Geology and Geophysics: Heat flow (benthic) and hydrothermal processes; 3035 Marine
Geology and Geophysics: Midocean ridge processes; 5104 Physical Properties of Rocks: Fracture and flow;
7220 Seismology: Oceanic crust; K
EYWORDS: Hydrothermal, microearthquake, swarm, middle, valley,
seismology
Citation: Golden, C. E., S. C. Webb, and R. A. Sohn, Hydrothermal microearthquake swarms beneath active vents at Middle Valley,
northern Juan de Fuca Ridge, J. Geophys. Res., 108(B1), 2027, doi:10.1029/2001JB000226, 2003.
1. Introduction
[2] Hydrothermal processes at ridge crests have been
studied with a broad range of techniques because they play
a controlling role in the thermal structure of the ridge, create
a fascinating and unique biology, and have formed many of
the important commercial ore deposits now on land. Hydro-
thermal circulation can penetrate several kilometers into
oceanic crust, but our understanding of these systems is
largely based on samples and specimens obtained from the
seafloor. As a result, hydrothermal flow patterns within the
crust are poorly understood.
[
3] Thermal gradients within newly formed oceanic crust
are among the largest naturally occurring on Earth, and
reflect the close physical proximity between 0C seawater
and 1200C basaltic magma. Simple calculations suggest
that thermal strain generated by conv ective heat flow within
these systems is several orders of magnitude greater than
tectonic strain [Sohn et al., 1999]. The brittle deformation
can occur when the rock is cooled below the rigidus
(550C), and when stresses exceed the strength of the
host rock [Lister, 1974, 1983]. In this view, the hydro-
thermal reaction zone is a seismogenic zone for micro-
earthquakes associated with contraction from thermal strain.
[
4] A close association between fluid flow and micro-
seismicity is commonly observed in subaerial geothermal
systems. Seismicity at the Krafla [Arnott and Foulger, 1994]
and Hengill-Grensdalur areas [Julian et al., 1997; Foulger,
1988a] in Iceland is triggered by thermal processes as fluids
cool hot rock emplaced by volcanic activity [Foulger and
Long, 1984; Foulger, 1988b; Arnott and Foulger, 1994].
Seismograph networks also record high levels of artificial
seismicity associated with the injection of cold water into
hot rock at the Geysers geothermal field, California [Stark,
1992]. Correlations between microseismicity and fluid injec-
tion times at the Geysers field indicate that the majority of
earthquakes near the injection sites are induced by thermo-
elastic stresses (A. Mossop and P. Segall, Induced seismicity
at the Geysers geothermal field, correlations and interpreta-
tions, manuscript in preparation). In these land-based stud-
ies, microearthquake hypocenters cluster in areas where
faults form fluid pathways in the crust, providing a unique
look at the plumbing networks of the geothermal systems.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. B1, 2027, doi:10.1029/2001JB000226, 2003
1
Scripps Institution of Oceanography, University of California, San
Diego, La Jolla, California, USA.
2
Lamont Doherty Earth Observatory, Columbia University, Palisades,
New York, USA.
3
Woods Hole Oceanographic Institution, Woods Hole, Massachusetts,
USA.
Copyright 2003 by the American Geophysical Union.
0148-0227/03/2001JB000226$09.00
EPM 2 - 1

[5] Midocean ridge crest ocean bottom se ismometer
(OBS) deployments near high-temperature vent fields also
detect high levels of seismic activity [e.g., Riedesel et al.,
1982; McClain et al., 1993; Sohn et al., 1995, 1999 ;
Wilcock et al., 1999]. However, only in the last few years
have marine seismograph surveys featured networks dense
enough and deployments long enough to obtain accurate
hypocenter locations and image subsurface structures com-
parable to subaerial geothermal studies [e.g., Sohn et al.,
1999; Wilcock et al., 1999]. A network of nine seismo-
graphs on the East Pacific Rise (EPR) at 950
0
N monitored a
hydrothermal cracking event consisting of a 3-hour swarm
of 162 events. The swarm, located as a thin vertical column
over the margin of the axial magma chamber, triggered a
7C increase in vent exit fluid temperatures [Fornari et al.,
1998; Sohn et al., 1999]. On the Endeavour segment of the
Juan de Fuca Ridge, 15 seismometers detected thousands of
local microearthquakes at 2–4 km depth beneath the Main,
High Rise, Salty Dog, and Mothra vent fields [Wilcock et
al., 1999].
[
6] To map region s of h ydrothe rmal cracking us ing
seismicity, we deployed a small array of eight OBSs near
Dead Dog vent field, a major area of active venting in
Middle Valley, Juan de Fuca Ridge. To locate and analyze
small microearthquakes with shallow hypocenters, the OBS
array featured interelement spacing of a few hundred meters
to ensure that seismic phases from each event would be
recorded at every instru ment. The array recorded 1220 local
microearthquakes within 5 km of the vents over a 145-day
period. Hypocenter locations with standard errors on the
order of a few hundred meters were obtained for 480 of
these events using a grid search algorithm, with travel times
computed throughout a 3D model of the area [ Golden,
2000]. Waveform cross-correlation and relative relocation
techniques were used to exploit waveshape similarity and
constrain the relative hypocenter locations to within a few
tens of meters. The microearthqu akes are located beneath
the Dead Dog vents, and are correlated with a broad area of
anomalously high seafloor heat flow. The results from this
study link the observed seismicity with hydrothermal pro-
cesses, and we use this information to develop a model of
the deep hydrothermal ci rculation and the rmal structure
beneath Dead Dog vent field.
2. Geological Setting
[7] Middle Valley is a sediment-covered rift graben on the
northernmost portion of the Juan de Fuca Ridge (Figure 1,
inset), a rise crest with a full spreading rate of 6 cm/yr
situated approximately 300 km off the northwest coast of the
United States. Middle Valley was the primary spreading
center in the region until 10 ka, when the rate of tectonic
extension began to decrease, and magmatic activity ceased.
Spreading activity on this part of the northern Juan de Fuca
Ridge is presently accommodated at the adjacent West
Valley [Davis and Villinger, 1992]. Normal faults mark the
eastern and western boundaries of Middle Valley (Figure 1),
and a prominent west-facing normal fault is located near the
valley center, separating the central rift graben from a
shallow basement bench to the east [Davis and Villinger,
1992]. This axial valley morphology is characteristic of
intermediate and slow spreading centers [e.g., Macdonald
and Luyendy k, 1977; Karsten et al., 1986; Kong et al.,
1988].
[
8] Middle Valley is bounded to the north by the Sovanco
fracture and the Nootka fault zones (Figure 1, inset),
marking a tectonically complex, seismically active triple
junction between the Pacific, Explorer, and Juan de Fuca
plates. The split between the Explorer and Juan de Fuca
plates is perpetuated as young, thin oceanic lithosphere is
subducted at different rates along the Cascadia margin only
a few hundred kilomet ers east of the ridge system [Riddi-
hough, 1977]. Faulting in this unstable region yields high
levels of seismic activity. Land-based seismograph stations
operated by the Geological Survey of Canada (GSC) and
marine OBS surveys of the area have typically recorded
moderate-magnitude earthquakes (M > 3 for GSC stations,
and 1 < M < 3 for OBSs) along the Sovanco and Nootka
fault zones [Wahlstrom and Rogers, 1991; Hyndman and
Rogers, 1981]. The seismicity distributions depict both the
Sovanco and Nootka boundaries as diffuse fault zones
rather than single transform faults.
[
9] Heat and fluid flux at Middle Valley are controlled by
a thick layer (up to 2 km) of turbidite sediment that blankets
the volcanic basement [Davis and Villinger, 1992]. The
turbidite layer is characterized by low permeability and
thermal conductivity, and it effectively insulates the hot
crustal rock from the water column. Hydrothermal recharge
may be focused into areas where permeable basement is
exposed at the seafloor, or regions where faults provide
permeable pathways through the sedimen t [ Davis and
Fisher, 1994; Stein and Fisher, 2001].
[
10] Active hydrothermal discharge in M iddle Valley
occurs at Bent Hill and Dead Dog, two distinct sites located
along the shallow basement bench to the east of the central
rift graben. In this region, the crust beneath the turbidites is
composed of an interlayered sequence of basaltic sills and
sediment, unlike the entirely extrusive layer of a standard
ridge crest structure [Davis and Villinger, 1992]. Discharge
at Bent Hill has created a massive sulfide mound that is 120
m thick, indicati ng that fluid temperatures once reached an
excess of 400C. Current exit fluid temperatures at this site
are roughly 265 C[Davis and Fisher, 1994]. Dead Dog
vent field is located 4 km west of Bent Hill, and is
delineated by a 400  800 m
2
region of high side scan
acoustic backscatter near a west-facing normal fault [Davis
and Villinger, 1992]. There are no extensive sulfide deposits
associated with Dead Dog, and it appears that the vents have
been continuously discharging moderate temperature fluids
(270C) since their formation [Davis and Fisher, 1994].
[
11] Results from Ocean Drilling Program (ODP) drilling
at Middle Valley (Legs 139 and 169) suggest a prominent
basement edifice beneath Dead Dog perpetuates venting at
the site. The structure is a small seamount that was buried
by 250 m of sediment during the Pleistocene [Shipboard
Scientific Party, 1992b]. Langseth and Becker [1994] and
Stakes and Franklin [1994] have postulated that the vol-
canic center beneath Dead Dog emplaced the basalt sills in
the eastern portion of the valley. Long-term hydrothermal
convection in the r egion is most likely driven by heat
trapped in the basaltic basement beneath the sediment
blanket.
[
12] The Dead Dog area generates an elongate positive
heat flow anomaly with a north-south orientation [Davis and
EPM 2 - 2 GOLDEN ET AL.: MICROEARTHQUAKE SWARMS

Villinger, 1992]. Basement topography beneath Dead Dog
focuses hydrothermal fluid flow through the locally thinner
sediment cover [Davis and Fisher, 1994], and seafloor
gravity data suggest the sediments in this high heat flow
area have been highly lithified by hydrothermal processes
[Ballu et al., 1998]. Direct hydrothermal fluid discharge at
the vent field is regulated by an indurated sediment cap
roughly 30 m below the seafloor, which appears to result in a
secondary level of hydrothermal circulation in the upper 30
m of sediment [Stein et al., 1998].
[
13] Although the Middle Valley hydrothermal fields are
well characterized in comparison with most deep-sea sys-
tems, several fundamental questions have not been
addressed. In particular, the pattern of fluid flow in the
Figure 1. Bathymetric contour map (100 m interval) of Middle Valley and regional map of the northern
Juan de Fuca Ridge system (inset). The study area is depicted on the inset map. Major fault zones as well as
Dead Dog vent field are outlined and labeled on the bathymetric map. OBS locations are denoted by white
triangles, and the seismically active region beneath the Dead Dog vents is bounded by the dashed box.
GOLDEN ET AL.: MICROEARTHQUAKE SWARMS EPM 2 - 3

basement is essentially unconstrained, as is the depth of the
hydrothermal reaction zone. In addition, recharge rates
estimated at the eastern boundary fault zone near Site 855
(25 m
3
/yr) are too small to match vent field fluid fluxes
[Davis and Fisher, 1994], and the bulk sediment perme-
abilities appear to be too small (10
17
m
2
) to account for the
difference [Stein and Fisher, 2001]. Thus the major sea-
water source regions feeding the Dead Dog circulation
system are not completely understood, and the subsurface
fluid pathways defining the lateral and vertical extent of
the hydrot hermal reaction zone in the basement remain
unknown.
3. Experiment
[14] The OBSs were free-fall deployed from the R/V
Wecoma in August 1996. Each instrument was equipped
with a three-component, 1 Hz natural period geophone
(Mark Products L-4), a hydrophone, and 6 Gbytes of disk
storage space. The vertical and hydrophone channels were
each sampled at 128 Hz, whereas the horizontal channels
were not sampled to conserve disk space and maximize the
duration of the experiment. Each OBS recorded continuous
data on the vertical and hydrophone channels for 145 days,
until January 1997. Timing was maintained within each
OBS using a temperature compensate d crystal (Seascan
Co.) with timing corrected using the drift measured over
the deployment.
[
15] The OBS seafloor locations were precisely navigated
by means of a shipboard acoustic survey of the instrument
transponders. The joint inversion method of Creager and
Dorman [1982] was used to calculate station locations from
acoustic ranges and GPS ship positions. OBS location
standard errors are less than 15 m.
[
16] Seismographs were positioned around two perma-
nent ODP boreholes, Sites 858 G and 857 D, prior to ODP
Leg 169. Scientific goals of the drilling leg included a
revisit to the Middle Valley drill sites, and a further study of
hydrothermal circulation in the area. The OBS array was
deployed to monitor background seismicity associated with
Dead Dog vent field and record any microearthquakes
induced by man-made perturbations to the hydrothermal
system during ODP Leg 169 borehole operations.
[
17] The experiment featured a compact configuration of
eight instruments on the seafloor, with approximately 500 m
distance between sensors chosen to best investigate micro-
earthquakes associated with the ODP operations at Sites 858
G and 857 D during Leg 169. The reduced interelement
spacing was crucial for locating small-magnitude micro-
earthquakes in the shallow crust because it allowed wave-
forms associated with small seismic moments to be recorded
at all of the stations. Although no microearthquakes were
detected that were clearly associated with ODP operations
during Leg 169, the OBS array did record high levels of
natural seismicity beneath the Dead Dog vents, providing an
opportunity to investigate the hydrothermal system.
4. Data
[18] During the recording period, the OBS array detected
3646 local and regional events, the majority of which exhibit
clear P and S arrivals on all instruments (Figure 2). P phases
are generally distinguished in the data set by impulsive
arrivals with a frequency near 32 Hz. By comparison, S
phases are near 8 Hz, have amplitudes up to five times the
size of P phases, and usually arrive several seconds behind
the P wave depending on the range of the microearthquake
to the OBS and the seismic velocity structure. Shear to
compressional wave conversions (SP) from sharp bounda-
ries in the seismic velocity structure are also present in the
data. In Middle Valley, the mode conversions are created at
the basement/sediment interface. Compressional to shear
wave conversions (PS ) were also occasionally observed.
In addition, water wave phases (P waves reflected off the sea
surface) are found in the OBS records, but they arrive within
the high-amplitude, low-frequency shear arrivals and are
difficult to distinguish.
[
19] Initially, P and S arrival times were picked by hand
for each local microearthquake and each station. Most events
exhibited 12 –16 pickable arrivals out of a possible total of
16. Earthquake hypocenters and errors were computed using
the 3D grid search algorithm of Sohn et al. [1998]. Travel
times were estimated by tracing synthetic rays through
compressional and shear velocity models of the Middle
Valley area spanned by the dashed box in Figure 1. Travel
times were defined at nodes every 100 m in the x direction,
200 m in the y direction, and every 100 m in the z direction.
The picks for each event were weighted by their relative
uncertainties and then used to find the grid point with the
minimum weighted RMS residual. Subsequently, a finer grid
was constructed around this minimum point with nodes
spaced every 50 m in the x and y directions, and spaced
every 25 m in the z direction. Travel times within the finer
grid were linearly interpolated, and the point with the
minimum RMS residual was chosen as the event hypocenter.
[
20] The compressional and shear velocity models were
constructed by extending 1D basement profiles beneath an
overlying sediment la yer of variable thickness and constant
velocity. Sed iment thickness variations were based on
values provided by Davis and Villinger [1992], except near
Sites 857 D and 85 8 G, where higher-resolution ODP
drilling results were implemented [Shipboard Scientific
Party, 1992a, 1992b]. The sediment layer P velocity was
defined as 1970 m/s. This value is based on correlations
between ODP well logs and multichannel seismic reflection
data [Rohr and Groschel-Becker, 1994]. The 1D compres-
sional velocity profile for the underlying basement was
based on the Endeavour segment model of Cudrak and
Clowes [1993]. The profile was modified in the upper 600
m of basement to fit results provided by a small seismic
refraction data set from this region of Middle Valley
[Golden, 2000]. The sediment shear velocity was con-
strained at 420 m/s by delay times between SP conversions
and S arrivals in this data set. Shear velocities in the upper
600 m of basement were based on values of 800 m/s for the
shallow crust [Christeson et al., 1997] and velocities in the
deeper basement were derived from t he c ompressional
velocity profile assuming V
P
/V
S
= 1.85.
5. Microearthquake Results
[21] Out of 3646 local and regional earthquakes recorded
during the 5-month experiment, roughly 70% occurred in
swarms with seismicity rates of at least 2.5 events/hr.
EPM 2 - 4 GOLDEN ET AL.: MICROEARTHQUAKE SWARMS

Thirteen distinct swarms were observed at random intervals
beneath the Dead Dog field (over 1000 microearthquakes),
with sizes ranging from just over 10 events in 4 hours
(swarm 12), to over 300 events in 2 days (swarm 4). Three
microearthquake swarms were detected outside of the Dead
Dog area (Figure 4). These swarms are much larger than
those associated with the vent field , with several hundreds
of events and time durations of over a week.
[
22] Hypocenter loca tions for 480 microearthquake s
beneath the Dead Dog field were obtained via grid search
localization, with a mean RMS travel time residual of 0.013
s and a standard error of 0.023 s (Figure 5a). The majority
of the hypocenters are concentrated at 1– 2.5 km depth,
directly beneath the vents (Figure 5b). Hypocenters are also
scattered to the north, east, and west of the vent field and
deepen to 3 km away from the vent field. During the OBS
survey, no microearthquakes were observed near the south-
ern end of the OBS array. A number of epicenters estimated
to be >3 km north of the vent field could not be reliably
located due to thei r large distance from the instruments.
Approximate locations based on relative P and S arrival
times for these swarms (numbers 6, 7, 10, and 13) are
depicted in Figure 5a.
[
23] Gridded travel time residuals about each hypocentral
point were used to compute a 3D error surface representing
the 1s limit for each event [Wilcock and Toomey, 1991;
Sohn et al., 1998]. Error bars in the x, y, and z directions
were computed from these error surfaces. The error bars
represent hypocentral misfit caused by discrepancies in the
seismic velocit y mode l and inaccuracies in picking the
event arrival times. Errors in hypocentral depth tend to
increase as microearthquakes are located further from the
seismometer array (Figure 5b), which is a common feature
of local seismic networks.
[
24] Seismic moments were estimated from long period
levels of body wave (S and P) amplitude spectra at each
Figure 2. Examples of vertical component waveforms for a correlated microearthquake pair recorded
by the OBS array. Direct compressional waves (P), direct shear waves (S), and converted S to P arrivals
generated at the sediment/basement interface (SP) are typical for this data set. The waveforms vary from
station to station, but the waveforms for different events at any one station are nearly identical. This
feature makes relative hypocenter relocations possible with these data. The cross-correlation coefficients
for the compressional (r
P
) and shear (r
S
) arrivals are shown.
GOLDEN ET AL.: MICROEARTHQUAKE SWARMS EPM 2 - 5

station [Brune, 1970; Hanks and Thatcher, 1972; Hilde-
brand et al., 1997; Sohn et al., 1998]. The seismic moment
for each event is an average of the moments obtained for all
eight instruments. Local magnitudes were computed from
the seismic moments using the relation: log
10
M
0
=16+
5M
w
[Lee and Stewart, 1981]. Magnitudes for the Dead Dog
microearthquakes range from 1.2to0.2.Focalplane
solutions were not determined because the directional
coverage in this experiment was relatively sparse.
[
25] Epicenter locat ions for the three swarms located
outside of the Dead Dog area are not well constrained since
they are well outside the OBS network. Instead, approx-
imate locations were estimated from relative first arrivals at
each instrument and delay times between P and S phases.
The swarms occur along the major fault zones in Middle
Valley (Figure 1), such as the Nootka Fault which intersects
the rift from the east roughly 12 km north of Dead Dog, the
western boundary fault, and the central fault which marks
the eastern edge of the primary rift graben. The large central
fault swarm occurred roughly 8 km to the north of the OBS
array. No earthquakes were generated along this fault south
of Dead Dog vent field. P and S arrivals were clipped on the
OBS records for a majority of the earthquakes associated
with these swarms. However, a lower magnitude bound of
M
w
> 1 was estimated using the long-period spectral levels
of the smallest-amplitude events and ranges to the instru-
ments. These local earthquakes exhibit much larger seismic
moments and a more episodic chronology than the events
under Dead Dog. Large tectonic events are common for the
northern terminus of the Juan de Fuca Ridge [e.g., Wahl-
strom and Rogers, 1991; Hyndman and Rogers, 1981].
6. Relative Relocations
[26] Microearthquakes from each swarm located beneath
the vent field form distinct groupings (Figure 5a), and
waveforms for most events within any given swarm closely
resemble each other (Figure 2). This suggests that the
seismicity within a swarm is generated by a single-source
mechanism at a common location, and that the implementa-
tion of relative relocation methods may yield more accurate
relative hypocenter locations. We applied the cross-correla-
tion/relative relocation technique of Shearer [1997] to each
Dead Dog swarm separately. Waveform cross-correlation
was utilized to quantify waveform similarity and yield
differential travel times at each instrument for each corre-
lated event pair. Compressional and shear wave arrivals were
cross-correlated independently for each microearthquake
pair, using 0.4 and 0.6 s windows around each arrival,
respectively. A minimum average correlation coefficient of
0.7 was required to define a ‘‘correlated’’ pair (‘‘doublet’’),
and the corresponding peaks of the correlation functions for
each OBS defined differential compressional and shear
travel times. The differential times were then used to
calculate new, relative hypocenter locations for each doublet
using a grid search routine. A small grid of relative compres-
sional and shear travel times was constructed about the
centroid of the original, absolute hypocenter locations for
each swarm. The grid spanned a 2  2  1.5 km
3
area in the
x, y, and z directions, with lateral spacing of 100 m and
vertical spacing of 50 m. Travel times between grid points
were linearly interpolated. Once relative hypocenter loca-
tions were obtained for each correlated pair, the entire group
of differential locations was inverted to compute a final set of
relative relocations. Standard (1s) errors for each relocated
event were estimated using bootstrap methods.
[
27] The relative location method successfully relocated
304 out of 480 total microearthquake hypocenters beneath
Dead Dog, corresponding to eight out of the nine originally
located swarms (Figures 5c and 5d). Relative errors are
greatly reduced from those associated with the original
locations, and are typically on the order of tens of meters
with the exception of swarms 8 and 9. Swarms 8 and 9 are
contemporaneous with seismicity on the Nootka fault zone,
which degrades the data qual ity of those swarms (c.f.,
Figures 3 and 4). Consequently, none of the five located
swarm 8 events correlated sufficiently, and the original,
absolute hypocenter locations are depicted in Figures 5c and
5d. Waveform cross-correlation using swarm 9 events did
yield correlation coefficients above 0.7, but the relative
errors are larger than average, just over 100 m in some
instances.
[
28] The relocated hypocenters form discrete pockets and
columns, with the two largest clusters (swarms 4 and 11)
located directly beneath the Dead Dog vents (Figures 5c and
5d). Smaller clusters surround the vent field to the north,
east, and west. Relocated swarms deepen as they extend
northward from the vent field, yielding a sloped seismo-
genic zone depicted in Figure 5d. Inaccuracies in our 3D
structural model will lead to systematic errors in the depth
of the swarms and hence in the slope of this ramp because
the events are outside of the small array, but the relative
locations between events should be stable. The primary
control on locations is derived from the time difference
between the P and S wave arrivals at each instrument.
[
29] We applied a perturbation test to each relocated
cluster independently to determine if its shape is required
by the data or a reflection of an error surface in the
inversion. For each swarm, differential compressional and
shear travel times were randomly perturbed for each corre-
lated event pair. Maximum perturbation amplitudes were
constrained to be less than one standard deviation of the
relative travel time differences for all event pairs in a given
swarm. Th e relocation method was then applied to the
randomly modified relative travel times.
[
30] Artifacts from the relocation method (e.g., network
geometry, grid spacing) are accentuated in the perturbed
results. The perturbation test indicated that microearthquake
clusters aligned with the OBS array (which is linear in the
north-south direction) tend to be smeared in the east-west
direction. This bias widens the shapes of swarms 2, 4, and
11 in Figure 5c. In addition, groups of relocated hypocenters
positioned well outside of the array can be stretched in the
vertical direction. This effect produces artificially vertical
configurations for swarms 1, 2, and 3 in Figures 5c and 5d.
However, the vertical extent of microearthquake clusters
located closest to the instruments (i.e., 4, 5, and 11) is
required by the data.
7. Source of the Vent Field Seismicity
[31] The results from our local earthqua ke survey lead us
to believe that hydrothermal processes generated the seis-
micity observed beneath Dead Dog vent field. This infer-
EPM 2 - 6 GOLDEN ET AL.: MICROEARTHQUAKE SWARMS

ence is based on the spatial and temporal patterns of the
observed seismicity, the correlation of seismicity with heat
flow measurements, b-value estimates, and the geology of
Middle Valley. Here we review this evidence, and use our
results to deduce fluid flow patterns in the basement beneath
the Dead Dog vents.
[
32] The spatial and temporal characteristics of the micro-
earthquakes in this study are similar to events recorded near
the Bio9/P and Tube Worm Pillar/Y hydrothermal fields at
950
0
N on the EPR [Sohn et al., 1999]. Seismicity beneath
both the Middle Valley and EPR sites is characterized by
brief and fairly intense swarms of small-magnitude (gen-
erally M
w
< 0), densely clustered, vertically aligned, repeat-
able events. The simplest way to explain this combination
of parameters is with seismic triggering from contraction of
cooled rock in regions of high thermal stress in the hydro-
thermal reaction zone. The other conceivable source mech-
anisms, magmatism and tectonism, are much more difficult
to reconcile with the observations.
[
33] Middle Valley is essentially an extensional graben,
and the sediments and crust are cut by a variety of normal
faults [Rohr and Schmidt, 1994]. The normal faults may
play some role in the microearthquakes observed, but the
vertical columns and small pockets of microearthquakes
beneath Dea d Dog vent field do not define fault-like
structures. We are unable to determine focal mechanisms
for the events given the limited array and emergent charac-
ter of many of the arrivals.
[
34] The locations of the seis mic swarms beneath Dead
Dog are correlated with results from seafloor heat flow
studies of Middle Valley (Figure 6a). Thermal gradient and
thermal conductivity have been measured at 550 sites
spaced a few hundred meters apart using probes equipped
with multiple-thermistor arrays [Davis and Villinger, 1992].
Dead Dog vent field is denoted by a positive heat flow peak
of 24 W/m
2
, and the associated region of high heat flow
(>1.0 W/m
2
) extends roughly 1 km to the south and 4 km to
the north of the vents creating a linear feature on the contour
map. The seismicity beneath Dead Dog is concentrated
within this heat flow anomaly, and the epicenters follow
its north-south orientation. The vigor of seismicity is also
roughly correlated with the heat flow values. The two
Figure 3. Timeline of seismic activity recorded at Dead Dog vent field during the experiment. Swarms
are outlined and numbered. Swarms 6, 7, 10, and 13 are located too far north of the OBS array to obtain
accurate hypocentral estimates.
GOLDEN ET AL.: MICROEARTHQUAKE SWARMS EPM 2 - 7

largest swarms are located beneath the heat flow peak, and
the smaller swarms scatter northward within regions of
reduced seafloor heat flow anomalies.
[
35] The frequency-magnitude distribution of earthquakes
beneath Dead Dog vent field produces a local b-value of
1.49, which reflects the nearly constant seismic moment
release of the observed events. The b-values are commonly
used as a basis for distinguishing tectonic and volcanic
events, with values less than one being characteristic of
tectonism, and values greater than 1.3 being characteristic of
Figure 4. Seismicity timeline of earthquakes generated outside of the Dead Dog area (outside of the
dashed box in Figure 1). The OBS array detected large swarms triggered by tectonic faulting in Middle
Valley. Earthquakes are labeled according to the fault with which they are associated (Figure 1). The
number of events per swarm is much larger for these tectonic swarms than for the seismic activity
beneath Dead Dog (Figure 5).
Figure 5. (opposite) (a) Map view of epicenter locations near Dead Dog vent field computed using a 3D grid search
algorithm. Colored circles reflect event locations for each distinct swarm, while clear circles denote epicentral estimates for
microearthquakes not associated with any swarm. Circle sizes depict subtle differences in the nearly identical moment
magnitudes (1.2 < M
w
< 0.2). Events from swarms 6, 7, 10, and 13 could not be located accurately due to their large
proximity from the station array, and their approximate locations are shown north of the vent field. No microearthquakes
were observed further than 1 km south of Dead Dog during the experiment. (b) Ridge parallel (south-north) cross section
showing grid search hypocentral estimates with 1s error limits. Symbol sizes reflect event magnitudes as in Figure 5a. The
major seismic layers incorporated into velocity models used for event localization are shown. (c) Relative relocations.
Ridge normal (west-east) cross section of relocated hypocenters with 1s error bars. Dashed boxes depict the absolute 1s
error limit for each cluster. Swarm 8 events did not correlate due to noise within the frequency band of the earthquakes, and
the original grid search locations are shown for that group. (d) Ridge parallel cross section of relocated hypocenters are as
per Figure 5c. The swarms delineate a cracking front that ramps downward north of the vent field.
EPM 2 - 8 GOLDEN ET AL.: MICROEARTHQUAKE SWARMS

GOLDEN ET AL.: MICROEARTHQUAKE SWARMS EPM 2 - 9

EPM 2 - 10 GOLDEN ET AL.: MICROEARTHQUAKE SWARMS

volcanism and large thermal strain [Warren and Latham ,
1970; Wiemer and McNutt, 1997].
8. Model of Middle Valley Hydrothermal Flow
[36] If the seismicity beneath Dead Dog is predominantly
triggered by thermal strain in the reaction zone, then the
microearthquakes and their hypocentral distributions delin-
eate the depth extent of hydrothermal circulation beneath
the vent field (Figures 6b and 6c). We therefore surmise that
the reaction zone beneath Dead Dog is oriented as a ramp
that shallows to the south toward the high-temperature vents
from a maximum depth of about 2 km to the north. The
absence of seismicity more than 1 km south of Dead Dog is
difficult to interpret given the relatively short period of
observation, but the events are definitely biased to the north
of the vents. The majority of the fluid discharged at Dead
Dog vent field may be drawn into the system locally, via the
faults to the north of the vent field.
[
37] The maximum depth of seismicity, and hence our
inferred hydrothermal reaction zone, is 1.5 km below the
surface of the basement. The downward velocity (u)ofa
hydrothermal cracking front into basement can be estimated
using the water penetration theory of Lister [1974]. Using
parameters appropriate for Dead Dog we obtain
u ¼
Q
rc
r
T
1
 T
w
ðÞ
 1cm=yr ð1Þ
where Q is the heat flow generated in the basement rock (1
W/m
2
)[Stein and Fisher, 2001], r is crustal density (2900
kg/m
3
), c
r
is the specific heat capacity of the crust (1000 J/
kg K), T
1
is the initial hot rock temperature (1500K), and
T
w
is the hot fluid temperature (573K). Stein and Fisher
[2001] use seafloor heat flow data to estimate Q by
summing all conductive and advective heat flow outputs
within a 260 km
2
area surrounding Dead Dog vent field.
[
38] If we assume that the cracking surface has migrated
downward with a constant velocity of 1 cm/yr over the life
span of hydrothermal circulation in this portion of Middle
Valle y, the total time to penetrate 1.5 km of basement
(inferred from swarm depths) is 150,000 years. This calcu-
lation provides a maximum age limit (because the heat flow
in this area was probably higher than 1 W/m
2
, when Middle
Valley was actively spreading during the Pleistocene (A.
Fisher, personal communication), but is consistent with the
models of Lang seth and Becker [1994] and Davis and
Fisher [1994]. These models suggest that Dead Dog is a
very old hydrothermal system (i.e., hundreds of ka) that was
created as magmatic activity ceased and rapid sedimentation
immediately insulated the volcanic center, setting up mod-
erate vent field temperatures (270C) that have remained
relatively constant.
[
39] The cracking front velocity estimate of 1 cm/yr is
about two orders of magnitude slower than theoretical
values for unsedimented hydrothermal s ystems [Lister,
1982]. The result, however, is consistent with observations
of hydrothermal seismicity at the adjacent Endeavor seg-
ment of the Juan de Fuca Ridge. Although there are lava
flows in the North Endeavour Valley [Karsten et al., 1990]
that are more recent than the turbidite sediments that blanket
Middle Valley [Davis and Villinger, 1992], the depth of
seismicity beneath the North Endeavour is 1–2 km deeper
[Wilcock et al., 1999] than seismicity beneath Dead Dog.
The depth difference emphasizes the insulating effect of the
turbidites in Middle Valley, a feature that is absent at the
Endeavour vent field areas. The thick sediment layer
surrounding Dead Dog inhibits seawater recharge into the
hydrothermal system, limiting the rate of cracking front
propagation and ultimately shoaling t he maximum depth of
any microearthquake activity.
9. Conclusions
[40] Microearthquake activity from August 1996 to Jan-
uary 1997 beneath Dead Dog vent field in the Middle Valley
of the Juan de Fuca Ridge is dominated by 13 seismic
swarms. Hypocentral estimates for 480 of these events were
obtained with a grid search method, and 304 of these
hypocenters were relocated using waveform cross-correla-
tion. Relocated events map into discrete clusters, with an
apparent deepening of events moving away from the vent
field to the north. The character of the seismicity is most
compatible with a thermal mechanism, which we interpret
as thermal strain resulting from the cooling of hot rock by
hydrothermal fluids. The microearthquakes may therefore
delineate the hydrothermal reaction zone during our experi-
ment, placing it about 1 –2 km directly under the vents. This
suggests that the reaction zone is propagating downward at
approximately 1 cm/yr, which is in agreement with calcu-
lations based on the extraction of heat by hydrothermal
processes. This propagation rate is extremely slow and
likely reflects the effect of the low-permeability turbidite
sediment cover on heat flow within the Dead Dog system.
[
41] This study illustrates the ability of local seafloor
seismic networks to record small-magnitude microearth-
quakes associated with hydrothermal vent fields. It further
Figure 6. (opposite) (a) Seafloo r heat flow in Middle Valley with relocated microearthquake epicenters superimpo sed
(contour map from Davis and Villinger [1992]). Note that the two largest seismic swarms are associated with the thermal
anomaly peak at Dead Dog vent field, and smaller swarms follow the elevated heat flow pattern as it extends northward. (b)
Top view of microearthquake swarms relative to Dead Dog (denoted by brown chimneys and blue hydrothermal fluid) and
ODP boreholes (yellow stars). (c) 3D cartoon of microearthquake swarms and the hydrothermal processes that trigger them.
The top surface is the flat seafloor. The gridded surface represents the sediment/basement interface (i.e., top of the sediment/
sill complex). This boundary is derived from the sediment thickness map in Davis and Villinger [1992] and drilling depths
recorded by ODP Leg 139 [Shipboard Scientific Party, 1992a, 1992b]. Local fault structure promotes water penetration to
the north of the vent field through the low-permeability sediments. Recharged seawater circulates down through the
permeable basement until it contacts hot rock associated with the buried volcanic center. Heat is extracted as the basal rock
is cooled, triggering rock fracture and swarms of small-magnitude earthquakes. Hydrothermal fluid rises (red arrows) and is
trapped beneath the sediments in a reservoir [Stein and Fisher, 2001], recirculating until it is eventually discharged through
the seafloor at Dead Dog.
GOLDEN ET AL.: MICROEARTHQUAKE SWARMS EPM 2 - 11

suggests the utility of marine seismological methods for
constraining the nature of fluid convection in oceanic crust.
[42] Acknowledgments. We thank Jacques Lemire and Tom Deaton
for engineering and maintenance of the OBSs, Peter Shearer for assistance
with the relative microearthquake relocations, Russ Johnson, Mark McDo-
nald, Valerie Ballu, Wayne Crawford, and the officers and crew of the R/V
Wecoma for help with the instruments at sea, and Jo Griffith for illustration
support. This research was supported by the National Science Foundation
(OCE 95-21282).
References
Arnott, S. K., and G. R. Foulger, The Krafla spreading segment, Iceland, 1,
Three-dimensional crustal structure and the spatial and temporal distribu-
tion of local earthquakes, J. Geophys. Res., 99, 23,801 – 23,825, 1994.
Ballu, V. S., J. H. Hildebrand, and S. C. Webb, Seafloor gravity evidence
for hydrothermal alteration of the sediments in Middle Valley, Juan de
Fuca Ridge, Mar. Geol., 150, 99–111, 1998.
Brune, J. N., Tectonic stress and the spectra of seismic shear waves from
earthquakes, J. Geophys. Res., 75, 4997 – 5009, 1970.
Christeson, G. L., P. R. Shaw, and J. D. Garmany, Shear and compressional
wave structure of the East Pacific Rise, 9 –10N, J. Geophys. Res., 102,
7821 – 7835, 1997.
Creager, K. C., and L. M. Dorman, Location of instruments on the seafloor
by joint adjustment of instrument and ship positions, J. Geophys. Res.,
87, 8379 – 8387, 1982.
Cudrak, C. F., and R. M. Clowes, Crustal structure of Endeavour ridge
segment, Juan de Fuca Ridge, from a detailed seismic refraction survey,
J. Geophys. Res., 98, 6329 – 6349, 1993.
Davis, E. E., and A. T. Fisher, On the nature and consequences of hydro-
thermal circulation in the Middle Valley sedimented rift: Inferences from
geophysical and geochemical observations, Leg 139, Proc. Ocean Drill.
Program Sci. Results, 139, 695 – 717, 1994.
Davis, E. E., and H. Villinger, Tectonic and thermal structure of the Middle
Valley sedimented rift, Northern Juan de Fuca Ridge, Proc. Ocean Drill.
Program Initial Rep., 139, 9 – 41, 1992.
Fornari, D. J., T. Shank, K. L. Von Damm, T. K. P. Gregg, M. Lilley,
G. Levai, A. Bray, R. M. Haymon, M. R. Perfit, and R. Lutz, Time-series
temperature measurements at high-temperature hydrothermal vents: East
Pacific Rise 949
0
Nto951
0
N: Monitoring dike intrusion and crustal
cracking events, Earth Planet. Sci. Lett., 160, 419 – 431, 1998.
Foulger, G. R., Hengill triple junction, SW Iceland, 1, Tectonic structure
and the spatial and temporal distribution of local earthquakes, J. Geophys.
Res., 93, 13,493 – 13,506, 1988a.
Foulger, G. R., Hengill triple junction, SW Iceland, 2, Anomalous earth-
quake focal mechanisms and implications for process within the geother-
mal reservoir and at accretionary plate boundaries, J. Geophys. Res., 93,
13,507 – 13,523, 1988b.
Foulger, G. R., and R. E. Long, Anomalous focal mechanism solutions:
Evidence for tensile crack formation on an accreting plate boundary,
Nature, 310, 43 – 45, 1984.
Golden, C. E., Investigation of oceanic spreading center hydrothermal pro-
cesses using ocean bottom seismometers, Ph.D. thesis, Univ. of Calif.,
San Diego, La Jolla, 2000.
Hanks, T. C., and W. Thatcher, A graphical representation of seismic source
parameters, J. Geophys. Res., 77 , 4393–4405, 1972.
Hildebrand, J. A., M. A. McDonald, and S. C. Webb, Microearthquakes at
intermediate spreading-rate ridges: The Cleft segment Megaplume site on
the Juan de Fuca Ridge, Bull. Seismol. Soc. Am., 87, 684 – 691, 1997.
Hyndman, R. D., and G. C. Rogers, Seismicity surveys with ocean bottom
seismographs off western Canada, J. Geophys. Res., 86, 3867 – 3880, 1981.
Julian, B. R., A. D. Miller, and G. R. Foulger, Non-double-couple earth-
quake mechanisms at the Hengill-Grensdalur volcanic complex, south-
west Iceland, Geophys. Res. Lett., 24, 743 – 746, 1997.
Karsten, J., S. R. Hammond, E. E. Davis, and R. G. Currie, Detailed
geomorphology of the Endeavour segment of the Juan de Fuca Ridge,
Geol. Soc. Am. Bull., 97, 213 – 221, 1986.
Karsten, J. L., J. R. Delaney, J. M. Rhodes, and R. A. Liias, Spatial and
temporal evolution of magmatic systems beneath the Endeavour segment,
Juan de Fuca Ridge: Tectonic and petrologic constraints, J. Geophys.
Res., 95, 19,235 – 19,256, 1990.
Kong, L. S. L., R. S. Detrick, P. J. Fox, L. F. Mayer, and W. B. F. Ryan, The
morphology and tectonics of the MARK area from Sea Beam and Sea
MARC I observations (Mid-Atlantic Ridge 23N), Mar. Geophys. Res.,
10, 59 – 90, 1988.
Langseth, M. G., and K. Becker, Structure of igneous basement at sites 857
and 858 based on Leg 139 downhole logging, Proc. Ocean Drill. Pro-
gram Sci. Results, 139, 573 – 583, 1994.
Lee, W. H. K., and S. W. Stewart, Principles and Applications of Micro-
earthquake Networks, Academic, San Diego, Calif., 1981.
Lister, C. R. B., On the penetration of water into hot rock, Geophys. J. R.
Astron. Soc., 39, 465 – 509, 1974.
Lister, C. R. B., ‘‘Active’’ and ‘‘passive’’ hydrothermal systems in the ocea-
nic crust: Predicted physical conditions, in The Dynamic Environment of
the Ocean Floor, pp. 441 – 470, D. C. Heath, Lexington, Mass., 1982.
Lister, C. R. B., The basic physics of water penetration into hot rock, in
Hydrothermal Processes at Seafloor Spreading Centers, pp. 141– 168,
Plenum, New York, 1983.
Macdonald, K. C., and B. P. Luyendyk, Deep tow studies of the structure of
the mid-Atlantic Ridge crest near lat 37N, Geol. Soc. Am. Bull., 88,
621 – 636, 1977.
McClain, J. S., M. L. Begnaud, M. A. Wright, J. Fondrk, and G. K. Von
Damm, Seismicity and tremor in a submarine hydrothermal field: The
northern Juan de Fuca Ridge, Geophys. Res. Lett., 20, 1883 – 1886, 1993.
Riddihough, R. P., A model for recent plate interactions off Canada’s west
coast, Can. J. Earth Sci., 14, 384 – 396, 1977.
Riedesel, M., J. A. Orcutt, K. C. Macdonald, and J. S. McClain, Micro-
earthquakes in the black smoker hydrothermal vent field, East Pacific
Rise at 21N, J. Geophys. Res., 87, 10,613 – 10,623, 1982.
Rohr, K. M. M., and H. Groschel-Becker, Correlation of well logs, physical
properties, and surface seismic reflection data, Middle Valley, Juan de Fuca
Ridge, Proc. Ocean Drill. Program Sci. Results, 139, 585 – 596, 1994.
Rohr, K. M. M., and U. Schmidt, Seismic structure of Middle Valley near
sites 855 – 858, Leg 139, Juan de Fuca Ridge, Proc. Ocean Drill. Pro-
gram Sci. Results, 139, 3 – 17, 1994.
Shearer, P. M., Improving local earthquake locations using the L1 norm and
waveform cross-correlation: Application to the Whittier Narrows, Cali-
fornia aftershock sequence, J. Geophys. Res., 102, 8269 – 8283, 1997.
Shipboard Scientific Party, Site 857, Proc. Ocean Drill. Program Initial
Rep., 139, 283 – 429, 1992a.
Shipboard Scientific Party, Site 858, Proc. Ocean Drill. Program Initial
Rep., 139, 431 – 569, 1992b.
Sohn, R. A., J. A. Hildebrand, S. C. Webb, and C. G. Fox, Hydrothermal
microseismicity at the Megaplume site on the southern Juan de Fuca
Ridge, Bull. Seismol. Soc. Am., 85, 775 – 786, 1995.
Sohn, R. A., J. A. Hildebrand, and S. C. Webb, Postrifting seismicity and a
model for the 1993 diking event on the CoAxial segment, Juan de Fuca
Ridge, J. Geophys. Res., 103, 9867 – 9877, 1998.
Sohn, R. A., J. A. Hildebrand, and S. C. Webb, A microearthquake survey
of the high-temperature vent fields on the volcanically active East Pacific
Rise (950
0
N), J. Geophys. Res., 104, 25,367 – 25,377, 1999.
Stakes, D. S., and J. M. Franklin, Petrology of igneous rocks at Middle
Valley, Juan de Fuca Ridge, Proc. Ocean Drill. Program Sci. Results,
139, 79 – 102, 1994.
Stark, M. A., Microearthquakes—A tool to track injected water in the Gey-
sers reservoir, Geotherm. Resour. Counc. Spec. Rep., 17, 111 – 120, 1992.
Stein, J. S., and A. T. Fisher, Multiple scales of hydrothermal circulation in
Middle Valley, northern Juan de Fuca Ridge: Physical constraints and
geologic models, J. Geophys. Res., 106, 8563 – 8580, 2001.
Stein, J. S., A. T. Fisher, M. Langseth, W. Jin, G. Iturrino, and E. Davis,
Fine-scale heat flow, shallow heat sources, a nd decoupled circulation
systems at two sea-floor hydrothermal sites, Middle Valley, northern Juan
de Fuca Ridge, Geology, 26, 1115 – 1118, 1998.
Wahlstrom, R., and G. C. Rogers, Relocation of earthquakes west of Van-
couver Island, British Columbia, 1965 – 1983, Can. J. Earth Sci., 29,
953 – 961, 1991.
Warren, N. W., and G. V. Latham, An experimental study of thermally
induced microfracturing and its relation to volcanic seismicity, J. Geo-
phys. Res., 75, 4455 – 4464, 1970.
Wiemer, S., and S. R. McNutt, Variations in the frequency-magnitude dis-
tribution with depth in two volcanic areas: Mount St. Helens, Washing-
ton, and Mt. Spurr, Alaska, Geophys. Res. Lett., 24, 189 – 192, 1997.
Wilcock, W. S. D., and D. R. Toomey, Estimating hypocentral uncertainties
for marine microearthquake surveys: A comparison of the generalized
inverse and grid search methods, Mar. Geophys. Res., 13, 161– 171, 1991.
Wilcock, W. S. D., S. D. Archer, J. Almendros, and G. M. Purdy, Micro-
earthquakes on the Endeavour segment of the Juan de Fuca Ridge, poster
presented at the RIDGE Juan de Fuca Symposium, RIDGE, Seattle,
Washington, D.C., Nov. 7 – 9, 1999.

C. E. Golden, Scripps Institution of Oceanography, University of
California, San Diego, La Jolla, CA 92093-0205, USA. (charles.golden@
cox.net)
R. A. Sohn, Woods Hole Oceanographic Institution, Woods Hole, MA
02543, USA. (rsohn@whoi.edu)
S. C. Webb, Lamont Doherty Earth Observatory, Columbia University,
Palisades, NY 10964, USA. (scw@ldeo.columbia.edu)
EPM 2 - 12 GOLDEN ET AL.: MICROEARTHQUAKE SWARMS