TECTONICS BLOG
Rev.
08-22-2024
G.C. Herman
Structural, tectonic and other aspects of
the 2024 earthquake swarm in
Hunterdon, Morris, and Somerset Counties, New Jersey
|
Click on an image for more detail |
Figure 1. Neotectonic map (top) and cross sections (bottom) through the Ramapo seismic zone that's rooted on the Cortland igneous
intrusive complex that crops along the New York Hudson River. The 2024 Tewksbury earthquake swarm sits on the east side of the Paulins Kill arch and the west side of the Watchung trough
in an area having a historic gap in seismicity. See
Herman (2022) for further details about this interpretation that's based on the Sykes and others (2006) earthquake catalog.
Figure 2. A focused swarm of light to very
minor earthquakes occurred during April 2024 within the southeast
Highlands of central New Jersey. An initial 4.8 Mwr earthquake with an
epicenter near Oldwick, NJ was been succeeded by about 200 aftershocks
occurring within a 60-square-mile area of Hunterdon, Morris, and
Somerset Counties over 4 months. This image was captured from a USGS web
site that reports earthquakes within a 30-day time window. Therefore, if
the swarm exceeds a 30-day time span, the earliest-collected records
become omitted. Earthquake locational parameters and Mwr values are
listed in the left window pane.
Table 1. A partial list of the earthquake parameters obtained from the USGS earthquake website. The elapsed time between events was calculated
to compute the swarm running time beginning with the 4.8 Mwr earthquake.
Click here to access the
first 202 records.
Figure 3. Geographic plot of the
2024 Tewksbury swarm epicenters in Google Earth Pro. The events are time stamped and linked to a short video of
the earthquake sequence placed into a regional, neotectonic setting.
Figure 4. The 2024 central New Jersey earthquake swarm in GE Pro. Epicenters are colored and sized by magnitude range. The Epicenters
are circumscribed by an ellipse with a major:minor axes = 1.28. The epicenters straddle the boundary faults separating the New
Jersey Highlands from the Newark Basin. However, a three-dimensional analysis shows that the foci occur solely in Proterozoic
basement rocks as demonstrated below. Note the location of cross section C-C' from Drake and others (1996).
Figure 5. Epicenters seem to indicate movement associated with the Newark Basin border faults but their focal points average
about 6.0 km depth solely within upper- to mid-crustal Proterozoic basement. Focal points were constructed by dropping vertical lines from
epicenters to reported depths. Cross section C-C' from Drake and others (1996).
Figure 6. MS Excel charts of the running time (minutes) versus magnitude (Mwr - top) and depths (middle) for the April-August 2024 earthquake swarm
in central N.J. This event includes 194 records over 72 days. Earthquakes of Mwr 2.0 and greater are highlighted with orange dots.
Lines connecting each event were added to each chart along with light-gray shaded polygons that envelope the respective value
ranges. Bottom chart plots the number of earthquakes occurring in consecutive, 12-hour windows. Most of the earthquakes range between Mwr 0.5 to 1.5 and become much less frequent after 24 days. The most intense and frequent events
occur during the first day, then slowly wane in both magnitude and frequency. Aftershock appear to be focused mainly between 5 and 12 km depths.
Figure 10. Comparative fault-ellipse model surfaces after
limiting ratios for the fault-ellipse axes (1.20) and rupture
length to displacement (0.015). Ellipses A to C show relative
fault sizes for Mwr 2.0 to 4.0 earthquakes. Ellipse A is
slightly larger than a football pitch. Ellipse D is shown using
a different scale for the Mwr 4.8 that was computed to be about
three adjoining football pitches in area.
Figure 11. A. Comparative fault ellipses for the 4.8 and 3.7 Mwr
earthquakes modeled as flat, dipping, and oriented positions, B.
ellipses placed into the model at earthquake focal points, and
C. Model faults when viewed along fault strike relative to the
other foci. The colored spheres used to visualize foci exceed
the size of the computed faults when using f = 6.03. Exaggerated
faults sizes result from using f = 8.0 but provide a visual
perspective of fault orientation relative to the foci swarm.
Figure 12. Screen-captured images from a SketchUp Pro 2022 3D CAD
model of the 2024 New Jersey earthquake swarm including Map (top)
and cross sections (side views) of the earthquake swarm. The main
events (4.8 and 3.7 Mwr) are modeled with elliptical fault surfaces
scaled using f values of 6.03 and 8.0. Dimensions are listed in
table 2 of figure 8. 3D earthquake foci are approximate locations
with locational errors varying less than the width of each symbol.
The modeled fault surfaces at f = 8.0 are exaggerated in size
because actual fault areas are barely visible at this scale. The
bottom sections shows a rotated, fault-parallel view.
Figure 13. The elliptical nature of bedrock fractures in rock
outcrops in Hunterdon County, New Jersey. These surfaces occur in
transitional-tensional arrays resembling normal dip-slip shear zones
and terminate with curvilinear, elliptical forms (Herman, 2009).
Note the Brunton compass for scale in the bottom photo on the left.
Figure 14. Neotectonic map of the central to north Atlantic continental margin
centered on the Philadelphia region.. A GPS-based vertical-derivative
map from
Herman (2022) shows areas that are slowly rising (yellow) and
sinking (blue). GPS-based horizontal drift of the North American Plate
is summarized by the white vectors. Rings of varying radii are drawn around the Chesapeake impact crater.
Figure 15. The neotectonic setting of the New York Recess now includes the 2024 central NJ earthquake swarm that fills a historical gap in
regional seismicity. This swarm appears very similar to other earthquake clusters that probably have similar faults and slip
directions as this most recent one. These very minor to mild earthquakes signal the wrinkling of our crust from compression
accompanying the slow, northwestward drift of the North American tectonic plate.
Figure 16. The orientation of the faults in the 2024 central NJ
earthquake cluster parallel some of the latest fault trends from
Newark stage crustal rifting (Herman, 1997).
Figure 17. Ground-fixed GPS stations surrounding the 2024 central NJ
earthquake swarm all showed an upward deflection of the ground
immediately following the event as part of the perpetual ground
oscillations through time (Herman, 2015). |
A focused swarm of light to very minor earthquakes began in the spring of 2024 and continue to happen during the fall season at a very low frequency in a part of central New Jersey
having spotty, historical seismicity (figs. 1 and 2; table 1). As of August 13, 2024 there have been 202 minor to very
light earthquakes occurring within a 60 sq. mi., elliptical area of central New Jersey covering parts of Hunterdon, Morris, and Somerset Counties (figs. 2 to 5).
The location (longitude, latitude), depth (m), magnitude (Mwr), and time of occurrence of each earthquake are posted on the United States Geological Survey
(USGS) earthquake web page for a period of 30 days after an event. The first 202 records
were copied, compiled and are listed in a
linked table. The USGS
records event times to the second, but they were rounded to the nearest minute for this work.
The initial 4.8 magnitude (Mwr) earthquake on April 5th
with an epicenter near Tewksbury, NJ was followed by over 200 aftershocks over
the following 3 months. The 4.8 Mwr is the second largest earthquake ever
recorded in New Jersey, falling behind a 5.3 magnitude earthquake in 1793 event
located to the northeast in Morris County. That one predated seismic
instrumentation and its magnitude was based on human reports of ground shaking
and surface disturbances.
These earthquakes have been very enlightening
and energizing for me, a career structural geologist who has spent decades in the region mapping
fractured bedrock at and near Earth's surface and characterizing how brittle rock breaks and plasticized rocks flow in the Appalachian
region. In the forty years of practicing geology centered on New Jersey, I had
never experienced an earthquake, here or anywhere else. But on that morning, I
was gardening outside the kitchen window, listening to music with ear buds
in when suddenly there was a audible roar that swept through the region from north
to the south that startled both me and the birds. It didn't sound like a transportation event along route 12 so I wondered what it could have been. It
sounded more like a locomotive derailing. Seconds later my wife announced
from just inside the patio door that an
earthquake had just occurred. As I was outside I didn't' feel the ground move but I heard the
transmitted atmospheric energy!
This article
summarizes geological aspects of this earthquake swarm and places it into spatial, temporal, and structural perspectives relative to the regional
neotectonic (current tectonic) setting. It also details the methods used
to characterize fault-plane dimensions and displacements for the two largest events
using the listed moment energies and regional magnitudes (Mwr) by applying a
derivation of Aki's (1972) moment-magnitude formulae within certain limits.
Earthquake records and parameters
The first earthquake in the 2024 central NJ swarm was
followed seven hours later by another notable, 3.7 Mwr earthquake that was the
18th in the sequence including seven others in the Mwr 2.0 to 2.7 range (table 1
and fig. 6). These two events were further analyzed by the U.S. Geological Survey (USGS) using elastodynamic
principles to resolve the amount of energy released and the probable orientations
of the associated fault planes and slip directions (fig. 7). The two most
probable, nodal-plane solutions (NP1) are oblique faults with both reverse and
strike-slip components that strike just East of North, dip from 36o to 45o eastward at
mid-crustal depths of 6 to 7 km within Proterozoic basement
rocks (figs. 7 and 10 to 13) The dynamic nature of the other aftershocks are
uncertain as they lack nodal-plane solutions.
The USGS data were sequentially copied into a MS Excel (2019) worksheet
as a table of earthquake parameters (table 1). Two other worksheets in the workbook were
made for graphing the temporal (fig. 6) and spatial (fig. 8) and aspects of
this swarm and for calculating magnitude-to-fault areas,
displacements, elliptical dimensions (figs. 9 to 12), and their seismic moments (energy
released; fig. 9). Figure 6 has three graphs that summarize the first 194 earthquake
magnitudes and depths versus minutes elapsed after the first event, and
frequency of occurrence with respect to running, 12-hour intervals.
Earthquakes in the eastern United States are infrequent but not unexpected. Since 1950,
forty other earthquakes of magnitude 3.0 and larger have occurred within 250 km of
this swarm, with the April 5, 2024 earthquake being the
largest recorded with instrumentation. Within 500 km of the April 5th earthquake, 13 earthquakes M 4.5 and
larger have been recorded since 1950, the largest being the August 23, 2011, M
5.8 Virginia earthquake which caused substantial damage and was felt
throughout the eastern U.S.
The April 5, 2024, earthquake occurred in a region
where faults have been previously identified and are periodically reactivated
(fig. 2).
This swarm falls within a geographic area outlined by a map ellipse having an aspect
ratio of the major- and minor axes of ~ 1.2 (figs. 3 to 5).
Regional fault scaling and surface modeling of earthquakes based on moment energies and magnitudes
The Richter scale is the most popular earthquake scale
because it was the first one formulated in the 1930s using seismographic instrumentation,
and henceforth a reference basis for assessing societal impacts in southern California from catastrophic
earthquakes leading to extreme ground shaking and the loss of human life and
infrastructure. The 1.0 to 10.0 magnitude Richter scale was based on measuring
seismic-wave arrival times and amplitude differences on seismographs rather than the
subsequent, more rigorous principles implemented through elastodynamic earthquake theory that
determines earthquake aspects based on rock mechanics, but whose application
still requires the use of regional factors for characterizing various tectonic
settings having varying seismic intensities. This
dependency is exemplified below for the mid-Atlantic region of the North American
tectonic plate that has relatively low-level seismicity occurring in a passive continental
margin setting (fig. 3;
Herman, 2015). New Jersey straddles the junction between the central and
northern Appalachian region where seismicity is minor and spotty, but mainly
focused near deep-rooted igneous plutons that preferentially resist slow
horizontal drift of the plate (fig. 1).
Aki established elastodynamic earthquake theory
in 1972 with a moment-magnitude
mathematical formulae relating the quantity of energy released at the moment of
brittle failure to the physical properties of the ruptured rock mass that
generates body and surface waves. It was the first
elastodynamic treatment of the amount of energy released from displacing earth
material along a planar fault surface that incorporated a factor taking into
account the rock's resistance to shear stress (the shear modulus;
μ) and geometric aspects of fault rupture and slip. In this seminal geophysical treatment of
earthquake phenomenon he proposed that the seismic moment (Mo in Newton-meters)
is the energy released when rupture occurs on a fault of
specific surface area (A) and slip distance (D).
His equation relates these factors to the magnitude of the earthquake in the
following manner:
equation 1.
Magnitude (Mw) = 2/3log10 Mo
– f
,
where
Mo is the seismic moment, or energy release (N-m), and
f
is
the moment-magnitude factor that can vary in a regional sense for very-large to
very-small earthquakes.
Mo is a product of the rock's shear modulus
(μ;
N/m2), the
rupture area (A; m2) of the fault plane, and the total
slip or displacement on the fault D (m). As specified by Aki (1972):
equation 2.
Mo =
(μ*A*D).
At the time, Aki (1972) derived an f value of 16.0 for earthquakes exceeding magnitude 7.0
based on his work in the active tectonic setting of Japan. Another notable
advancement in the magnitude-scaling of earthquakes was made by
Hanks and Kanamori (1979) who derived an
f
value
of 10.77 for earthquakes in the 5.0 to 7.0 range, with the latter being the
observed upper limit of many events across the globe. More recent work by
Edwards and others (2010)
for the Switzerland region derived
an
f
value
of 6.03 for
use in the automatic computation and derivation of earthquake elastodynamic parameters for
earthquakes in the 1.0 to 6.0 range
with similar
magnitude ranges as those reported for this swarm. The two lower,
regional
value of
f are
used in this study to investigate probable fault
aspects for this region by using fixed ratios of fault-strike versus slip distance, axial ratios,
and earthquake magnitudes (fig. 9).
Equations 3 to 8 below are derivations of Aki's (1972) moment-magnitude
equation that were used here to compute fault areas, dimensions, and slip
when using
a 30 gigapascal (GPa) shear modulus, a D:RLeMax
ratio of 0.015 and a RLeMax:RLeMin ratio of 1.20.
Equation 9 is the MS Excel formulae equivalent to equation 8. The shear modulus is a common standard, and Nicol
and others (1996) have shown that fault aspect ratios for the major and minor
axes of faults range from 3.5 down to 0.5, with
1.0 being circular. Walsh and Watterson (1989) found that rupture length
versus fault-displacement ratios typically
ranges between 0.010 and 0.020. These two findings helped limit the parameters
when modeling elliptical fault planes using a CAD system. Elliptical
fault planes are used rather than rectangular ones primarily because that what
is observed in outcrop (fig. 13). Fault-tip terminations in fractured rock are generally
curvilinear except where fractures and faults intersect pre-existing
planar structures. We simply don't know how these earthquakes relate to
preexisting, blind structures so elliptical fault planes are modeled with the major axis
corresponding to a fault strike and rupture length.
Therefore, because:
equation 3. A = Mo / μ * D
,
and
equation 4. Ae =
(RLeMax * RLeMin *3.14159),
different values for Ae were first calculated using the reported USGS
moment (Mo) values for the 4.8 and 3.7 Mwr events by repeatedly trying different RLeMax
values to reach solutions where RLeMax:RLeMin
= 1.20. In order to facilitate this
process, I constructed a MS Excel Worksheet
that contains two different calculation
methods to compute fault-rupture areas and fault displacements. Trial-and-error calculations
used the moment energies for the Mwr 4.6 and 3.8 events to derive fault area
and slip parameters by constraining the displacement and
ellipticity ratios. The results are listed in figure 9.
Method B then used a derivation of Aki's (1972) formulae to test different f
values to see what value closely matches the fault parameters gained from using
the listed seismic moments. Method B solves for fault area (A) using the following
derivations:
equation
5. Mw +
f
= 2/3(log10μ
+
log10A + log10D) or
equation
6. Mw +
f =
2/3log10μ + 2/3log10 A + 2/3log10D, therefore
equation 7.
log10 A
= 3/2((Mw +
f)
– 2/3log10μ - 2/3log10D), and finally
equation 8.
A = 103/2((Mw +
f)
– 2/3log10μ
- 2/3log10D).
equation 9.
A = POWER(10,1.5*((Mw +
f)- 0.667*LOG10(μ)-0.667*LOG10(D)))
Method B calculations show that the reported USGS earthquake parameters use f
~ 6.03
like Edward's and
others (2010) because the fault aspects are consistent with those derived using
the listed moment energies (fig. 9). Thus, f = 6.03 was used with equation
9 to calculate
the various fault lengths, areas,
and slip distances to assess their relative
sizes accompanying one-integer increases in magnitude from Mwr 2.0 to 6.0. (fig.
9). When using this approach, the length of a fault associated with the 4.8 Mwr
event computes to a maximum rupture length of about 245 meters and a slip of
3.68 meters. The 3.7 Mwr equates to a fault that's about one third in
size at about 70 meters long with about 1 meter of slip (fig. 9).
Figures 5, 11, 12, 15 and 17 show how and where the earthquake foci and nodal fault planes were added to a pre-existing SketchUp Pro 2022 (SUP)
CAD model of the New York Recess using
geographic coordinates. The epicenters were digitized as points in the SUP model and
vertical drop lines were then constructed from each epicenter downward to its
focal depth, represented using different colored spheres of graduated size (fig.
5). Elliptical fault planes were
next added for the
Mwr 4.8 and 3.7 earthquakes using
regional scaling variables of f = 8.0 and 6.03 to see how the three-dimensional (3D) distribution of the
surrounding aftershocks correlate geospatially with different sizes of faults
(figs. 11 to 12).
The f = 8.0 ellipses exaggerate
the respective sizes of the faults
which is necessary for visualization because faults sized at f = 6.3 are smaller than the 3D spheres used
to virtually symbolize earthquake foci at regional scales (~200 to 600 meter
diameters; figs. 11
and 12).
A part of regional cross section
C-C' from the New Jersey state geological map (Drake and others, 1996)
runs through the northern reaches of the swarm and was added to the SUP model to see how the
earthquakes
spatially relate to interpreted basement
faults in the New Jersey Highlands (figs. 5 and 12). The 3D distribution of the earthquake
foci agree very well with the orientation and spacing of major basement faults
that mostly dip southeastward at moderate angles and project upward towards the Long
Valley area in the southeastern NJ Highlands. Although the epicenters straddle a border fault
separating the Phanerozoic-Highlands from the Mesozoic Newark
Basin, the 3D perspectives show that the foci are situated solely within
Proterozoic basement rocks at depth.
The 2024 Central NJ earthquake swarm with respect to
New York Recess neotectonics
The prior structural geological interpretation of the New York Recess based on a
detailed 3D earthquake catalog was constructed as part of a regional
neotectonic investigations (fig. 3 and Herman, 2015;
Herman, 2022). The interpretation included a SketchUp Pro 2022
computer-aided drafting (CAD) model of the 3D seismicity occurring in the
New York and Philadelphia region as reported by Sykes and others (2006)
and the US Geological Survey National Earthquake Information Center (NEIC). The model used 3D foci symbolized with varying
colors and sizes of spheres that were placed into their respective positions
using the same technique described above. This project has a regional, E-W
striking cross section depicting the Cortland and Beemerville plutons, the depth
limit of seismicity, and a structural framework that includes at least three,
overlapping tectonic stages (fig. 3). The Late Ordovician plutons
were emplaced at the end of the Taconic orogeny before subsequent Alleghanian stages of Paleozoic
contraction from plate convergence and orogenesis. The model shows that the
Cortland and Beemerville igneous plutons are resistant plugs where seismicity
is now focused due to their preferential resistance to plate drift, similar to what is seen with the
White Mountains of New Hampshire and the Adirondack
Mountains of New York (Herman, 2015). Figure 14 show the string of seismic zones that spot the Atlantic
margin including the Adirondack, White Mt. (New Hampshire), Ramapo (New York), and Lancaster
(Pennsylvania) seismic zones. The rising
Poughkeepsie and Paulin's Kill arches together with the Watchung and Allentown
troughs are neotectonic wrinkles in the Atlantic margin zones of seismicity
are focused around deep-seated igneous plutons intruded on rising continental
platform to the north and a sinking one to the south. Part of the grain in the vertical derivative map can be
attributed to the architecture of the bolide impact and crater formation in the
Chesapeake Bay Maryland at 35.5 Ma (Poag, 1999; Herman, 2015; Mathur and others,
2015).
Discussion
Cowie and Scholz (1992) provide a synthesis of results
and discussion of the scaling relationship between maximum fault displacement (DMAX)
and the maximum rupture length (L) of faults. Although it is generally assumed
that this relationship is a power function of the form DMAX = cLn
where c is a constant and n ranges between 0.5 and 2, a more straightforward
approach uses a simple linear relationship between these two variables with
g =
DMAX / L ranging between 1.0 and 0.01. The near-minimum value of 0.015 was
used in this study. No doubt that this
represents only an approximation, and the actual fault size and geometry is
nearly impossible to derive without higher-resolution data or direct evidence
from drill core. But this seemed like a
good starting place that provides a sense of relative fault sizes with respect
to minor to low-levels of seismicity. An unexpected result from these
spreadsheet computations are some simple rule-of-thumb mathematical relationships
concerning the scaling of fault sizes relative to the corresponding magnitude
classifications. Namely, as summarized in figure 8, for each integer increase in magnitude
there is about a three-fold (3.16 x) increase in both fault displacement and
strike length, a 10-fold (10.01 x) increase in fault rupture area, and a 30-fold
(31.6) increase in energy released. I had previously heard of the latter
relationship but the not the former two. As you probably noticed, the 3.16
factor is very close to Pi (3.14) which makes me wonder if this is just
coincidental, or stems from the lack of precision in the calculations. That
remains to be determined.
Figure 16 from Herman (1997) is augmented with bold N-S striking lines that
emphasize the strikes of the latest faults mapped in the Newark rift basin.
It's highly likely that these late-Mesozoic faults are now being reactivated
with reverse slip in the modern, compressional stress field. Figure 17 is added
at the end to show that ground-fixed GPS stations in the area surrounding the
earthquake swarm all registered a positive vertical deflection at the time of
the event that be only coincidental, but nevertheless congruent with this event
having a reverse slip component that locally compounds crustal thickness through
reverse faulting.
The occurrence of this earthquake swarm in central New
Jersey in an area historically lacking such focused activity has given us a glimpse into
how perpetual tectonic processes
operate in our crust to slowly impart topographic relief and influence the manner
in which rivers flow and sediment is produced and accumulates. Figure 2
shows that the course of the Delaware River partly flows along the boundary
between relatively more- and less slowly subsiding areas, and whose headwaters zig zag along the
crest of the Paulin's Kill arch. Conversely, the Jurassic Watchung
syncline is nestled comfortably into a neotectonic trough where the
landscape that surrounds us evolves at such a slow pace that we
ordinarily attribute it only to past events and fail to notice how our current landscape is
constantly evolving with distributed zones of local tectonic adjustment. The location of this earthquake swarm on the eastern limb
of the Paulin's Kill arch is just one of many temporal swarms that have
occurred in different areas of the same region that altogether cause the differential rise and fall of land leading to the
rebounding Canadian shield in response to glacial unloading (figs. 14 and 15).
A short movie was produced that captures the temporal sequence of 194 events that are
mapped and symbolized using varying magnitudes, sizes, and colors and placed into a
3D tectonic perspective (fig. 4). The SUP CAD model of seismicity is available
upon request.
I hope you enjoyed learning about some New York and New Jersey
earthquakes. If you have any questions, thoughts, or find some mistakes please email me
at gcherman56@yahoo.com.
References
Aki, H, 1972, Scaling law of
earthquake source-time function: Geophysical Journal of the Royal Astronomical
Society, vol. 31, p. 3 - 25. DOI 10.1111/j.1365-246X.1972.tb02356.x.
Cowie, P. A., and Scholz, C. H., 1992,
Displacement-length scaling relationships for faults: data synthesis and
discussion: Journal of Structural Geology vol. 14, no. 10, p. 1149 to 1156.
Drake, A. A., Jr., Volkert, R. A., Monteverde, D. H.,
Herman, G. C., Houghton, H. F., Parker, R. A., and Dalton, R. F., 1996, Bedrock
geological map of northern New Jersey: U.S. Geological Survey Miscellaneous
Investigation Series Map I-2540-A, scale 1:100,000, 2 sheets.
Edwards, B., Allman, B., Fäh, D., and Clinton, J.,
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Herman, G. C.,pan> 2009,
Steeply-dipping extension fractures in the Newark
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MB PDF), Journal of Structural Geology, V. 31, p. 996-1011.
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and borehole geophysics of fractured-bedrock aquifers, in HeHerman,
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the Newark basin: N.J. Geological Survey Bulletin 77, Chapter F., p. F1-F45.
Herman, G. C., 2015,
Neotectonics
of the New York Recess, in Herman,
G. C., and Macaoay
Ferguson, S., Neotectonics
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Geological Association of New Jersey, Lafayette College, Easton, Pa., p. 80-151
Herman, G. C. and Curran, John, 2010,
Borehole geophysics and hydrogeology studies in the
Newark basin, New Jersey, in Herman,
G. C., and Serfes, M. E., eds., Contributions to the geology and hydrogeology of
the Newark basin: N.J. Geological Survey Bulletin 77, Appendixes 1-4, 245 p.
Nicol, A., Watterson, J., Walsh, J. J., and Childs, C., 1997, The shapes, major
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Poag, C.W., 1999,
Chesapeake Invader: Discovering America’s giant meteorite crater: Princeton
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Earthquakes in Greater New York-Philadelphia Area: Catalog 1677 to 2005 and
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City-Philadelphia Area: 1677-2004.
Walsh, J. J., and Watterson, J., 1989, Displacement gradients on fault surfaces:
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