Shear wave velocity and Geology Based Seismic Microzonation of Port-au-Prince, Haiti

WELCOME TO OUR PRESENTATION ON
“SHEAR WAVE VELOCITY- AND GEOLOGY-BASED
SEISMIC MICROZONATION OF PORT-AU-PRINCE, HAITI”
AUTHORS: BRADY R. COX, JEFF BACHHUBER, ELLEN RATHJE, CLINTON M. WOOD,
RANON DULBERG, ALBERT KOTTKE, RUSSELL A. GREEN AND SCOTT M. OLSON
PRESENTED BY:
JOHANA SHARMIN (0417042206)
MD. NAIMUL ISLAM MOHIM (1017042221)

HAITI EARTHQUAKE, 2010
 DATE AND ORIGIN TIME: 12th
January, 2010
at 16:53:10 EST
 MAGNITUDE: 7.0
 EPICENTER: near the town of Leogane,
approximately 25 kilometers (16 mi) west
of Port-au-Prince.
 DEPTH: 13km
 MAX. INTENSITY: IX (Violent)
 PEAK ACCELERATION: 0.5g
 AFTERSHOCKS: 52 nos of magnitude 4.5
 CASUALTIES: 100,000 to 300,000 deaths
 MAJOR DAMAGES: Presidential Palace,
the National Assembly building, the Port-au-
Prince Cathedral, the main jail and
communication systems.

REASONS
 Poor building practices, which were unregulated and not in accord with
modern seismic building codes.
 The rupture process of the main Enriquillo–Plantain Garden fault zone may
have involved slip on multiple blind thrust faults with only minor, deep,
lateral slip along or near the fault zone.

OBJECTIVES OF THIS PAPER
 Seismic site classification microzonation based on 35 shear wave
velocity (Vs) profiles collected throughout the city and a new geologic
map of the region.
 Vs profiles were obtained using MASW (multichannel analysis of surface
waves) method
 Geologic map was developed from a combination of field mapping
and geomorphic interpretation of a digital elevation model (DEM).

TERMINOLOGY
 Shear Modulus: In materials science, shear modulus or modulus of rigidity,
denoted by G is defined as the ratio of shear stress to the shear strain.
 Shear Wave: Shear wave is transverse wave that occurs in an elastic
medium when it is subjected to periodic shear. Shear is the change of
shape, without change of volume, of a layer of the substance, produced
by a pair of equal forces acting in opposite directions along the two faces
of the layer. If the medium is elastic, the layer will resume its original shape
after shear, adjacent layers will undergo shear, and the shifting will
be propagated as a wave.
 Shear Wave Velocity: The velocity of a shear wave is equal to the square
root of the ratio of shear modulus (G), a constant of the medium,
to density (ρ) of the medium, Vs = √G/ρ.
 Vs30: Shear-wave velocity (Vs) has long been known to be an essential
parameter for evaluating the dynamic properties of soils. The average
shear-wave velocity in the top 30 m, based on travel time from the surface
to a depth of 30 m, is known as Vs30.

TERMINOLOGY
 Seismic Site Classification: Vs30 is used in the NEHRP Provisions (BSSC, 1994)
and the 1997 Uniform Building Code to separate sites into different classes
for engineering design, with the expectation that sites in the same class will
respond similarly to a given earthquake. The 2000 International Building
Code (IBC) permits a similar approach for site classification, the average
shear wave velocity of the upper 100 ft. These site classes are as follows:
 Class A – hard rock – VS30 > 1500 m/s (UBC) or VS100’ > 5,000 f/s (IBC)
 Class B – rock – 760 < VS30 ≤ 1500 m/s (UBC) or 2,500 < VS100’ ≤ 5,000 f/s (IBC)
 Class C – very dense soil and soft rock – 360 < VS30 ≤ 760 m/s (UBC) or 1,200 <
VS100’ ≤ 2,500 f/s (IBC)
 Class D – stiff soil – 180 < VS30 ≤ 360 m/s (UBC) or 600 < VS100’ ≤ 1,200 f/s (IBC)
 Class E – soft soil – VS30 < 180 m/s (UBC) or VS100’ < 600 f/s (IBC)
 Class F – soils requiring site-specific evaluation

TERMINOLOGY
 Surface Wave: Surface waves, as the name suggests, travel at or near the
surface of the ground. They are characterized by low velocity, low
frequency, and relatively high amplitude. They are often referred to as
“ground roll” and are a source of noise in shallow seismic reflection.
 Surface Wave Techniques: Surface waves are dispersive; different
frequencies travel at different velocities in a manner similar to light. As it
happens, surface wave phase velocities – the velocities of different
frequencies – are a useful proxy for shear wave velocity, and because of
their high amplitude, are relatively easy to measure. They are easily
generated by both active (hammer and plate, weight drop) and passive
(microtremors) sources.
 Active and passive surface wave techniques are relatively new in situ
seismic methods for determining shear wave velocity (Vs) profiles. These
techniques include the spectral analysis of surface waves (SASW) and multi-
channel analysis surface wave (MASW) methods.

TERMINOLOGY
 Difference Between SASW and MASW
methods:
 The SASW method is optimized for conducting
Vs depth soundings. A dynamic source is used
to generate surface waves of different
wavelengths (or frequencies) which are
monitored by two or more receivers at known
offsets. A dynamic signal analyzer is typically
used to calculate the phase and coherence of
the cross spectrum of the time history data
collected at a pair of receivers.
 The MASW field layout is similar to that of the
seismic refraction technique. Twenty four, or
more, geophones are laid out in a linear array
with 1 to 2m spacing and connected to a multi-
channel seismograph as shown below. This
technique is ideally suited to 2D Vs imaging,
with data collected in a roll-along manner
similar to that of the seismic reflection
technique.

TERMINOLOGY
 Seismic Microzonation: Seismic microzonation is defined as the process of
subdividing a potential seismic or earthquake prone area into zones with respect
to some geological and geophysical characteristics of the sites such as ground
shaking, liquefaction susceptibility, landslide and rock fall hazard, earthquake-
related flooding, so that seismic hazards at different locations within the area
can correctly be identified. Microzonation provides the basis for site-specific risk
analysis, which can assist in the mitigation of earthquake damage. In most
general terms, seismic microzonation is the process of estimating the response of
soil layers under earthquake excitations and thus the variation of earthquake
characteristics on the ground surface.
 Digital Elevation Model (DEM): A digital elevation model (DEM) is a digital model
or 3D representation of a terrain's surface — commonly for a planet (including
Earth), moon, or asteroid — created from terrain elevation data.

GEOLOGY OF PORT-AU-PRINCE, HAITI
 The Port-au-Prince region is a physiographically diverse area that has
undergone a complex geologic history of tectonism, erosion, and
sedimentation within an area of regional deformation associated with the
active Enriquillo-Plantain Garden fault zone (EPGFZ) and other associated
secondary faults.
 The EPGFZ is located approximately 5 to 7 km south of Port-au-Prince. This
section of the fault did not rupture during the 12 January 2010 earthquake (the
epicenter was approximately 30 km to the west), but has geomorphic
expression consistent with Holocene-active faulting.
 Immediately following the 12 January 2010 Haiti earthquake, the Geotechnical
Extreme Events Reconnaissance (GEER) organization deployed a team to
document the geotechnical and geological aspects of the earthquake.
 The GEER team was able to locate only one small-scale (1:250,000) published
geologic map for the entire country of Haiti. This map indicates that nearly all of
greater Port-au-Prince is founded on three broad geologic units : (1) Quaternary
deposits (Qa), (2) Pliocene fan deposits (P), and (3) Miocene deposits (Ms).

GEOLOGY OF PORT-AU-PRINCE
 The map is regional, it lacks
detail, particularly with respect
to Quaternary geology. As a
result, the unit boundaries did
not match our field
observations of geology and
damage patterns.
 The GEER team anticipated
that younger, softer
Quaternary deposits actually
underlay the downtown
district and that these deposits
amplified the ground shaking
in the area.
Figure: Previously published regional geologic
map including Port-au-Prince, Haiti

GEOLOGY OF PORT-AU-PRINCE
 The authors returned to Haiti in late April 2010 to perform additional in-
depth field studies of geotechnical-driven damage patterns and soil
liquefaction from the earthquake. This research was included detailed
geologic mapping of Port-au-Prince and shear wave velocity profiling
throughout the city as two primary goals.
 The new map is based on field reconnaissance mapping and geomorphic
interpretation of a 10-m (spatial resolution) DEM.

EXPLANATION OF THE NEW MAP
 Lmst = Miocene limestone bedrock forms
the steep mountain front along the south
margin of our study area. Little of the
greater Port-au-Prince population center is
built on the Lmst deposits.
 Mpb = North of the Lmst unit, at the base of
the range front, is a band of Mio-Pliocene
“fanglomerate/breccias” comprised of
dense angular limestone blocks, variably
cemented in a finer-grained sandy and
calcareous matrix. Extensive development
has occurred on the Mpb unit breccias,
including bidonvilles that cover many ridge
crests and slopes.

 Pf = A large Pliocene fan complex extends
northward from the Mio-Pliocene breccia
at the range front and forms low hills and
ridges through the eastern part of Port-au-
Prince and adjacent suburbs. The Pf unit is
comprised of crudely bedded siltstone,
sandstone, claystone, conglomerate, and
localized limestone with some zones of
dense unlithified sand or very stiff clay.
 Ppf = A large, flat surface was eroded into
the central part of the Pliocene fan
surface (Pe´tion-Ville area) by an ancestral
stream, and this is a beveled strath
(erosional) surface rather than a
depositional fan deposit. The planated
surface is designated Ppf on the geologic
map, indicating that it is an assumed
Pleistocene surface.

 A sequence of Pleistocene fans (Qpf) and
Pleistocene-Holocene stream terraces (Qphf)
surround the Pliocene fan, extend along the
margins of stream and river valleys, and
propagate outward into the valley and toward
the shoreline. These geologic units consist of
sediment derived by erosion of the older Pf and
Mpb units.
 Qham = The central district of Port-au-Prince
occupies a narrow, Holocene coastal alluvial
plain bounded to the east by the Qpf fans or Pf
hills, and extending to the shoreline. Alluvial
deposits underlying this coastal plain were
deposited in a marine margin setting, and
include interbedded terrestrial alluvium and
estuarine muds with a high organic content.
Much of the central district of Port-au-Prince is
located on this unit.

 Af = Artificially filled ground extends west
of the original shoreline location in the
central district of the city. Liquefaction
and lateral spreading were
documented in some Af deposits during
the earthquake, particularly those
closest to the current coastline.
 Qhac = Holocene fluvial alluvium has
been differentiated principally along the
Grey River and its major tributaries in the
study area. It typically occurs under river
channels and active alluvial floodplains
and low areas.

 Qht1 = Some liquefaction occurred in
Qhac unit, but was primarily confined
to the most recent stream
deposits/terraces along the distal
reach of the Grey River near the
coast, where the Holocene stream
deposits are dominated by sand, silt,
and fine gravel.
 The early Holocene to Pleistocene
terrace (Qht2) and fan deposits
(Qphf) are quite dense/stiff, elevated
above the ground water table, or
gently sloping without free faces, and
therefore resistant to liquefaction and
lateral spreading.

SHEAR WAVE VELOCITY PROFILES IN PORT-AU-PRINCE
 The authors conducted MASW testing at 36 sites in and around Port-au-
Prince, Haiti from 19–25 April 2010 as a means to rapidly and non-intrusively
obtain Vs profiles across the city.
 Potential test locations initially were laid out on a 1.5-km grid pattern to
provide adequate coverage of the city.
 All of the MASW surveys performed for this study were completed using a
linear array of 24 receivers (4.5-Hz geophones) with a constant receiver
spacing of approximately 1 m (total array length of 23 m).
 A 7.3-kg sledgehammer was used as the dynamic source and signals from
at least five impacts were averaged at each source location. The hammer
was struck directly on the ground surface when testing on stiff surficial
material, while a steel plate was struck when testing on soft materials.

SHEAR WAVE VELOCITY PROFILES IN PORT-AU-PRINCE
Table: Seismic site classifications for surface
wave test locations (TL)
Figure: Locations and names of sites in Port-au-
Prince, Haiti where (MASW) testing was
performed to obtain shear wave velocity profiles

RELATIONSHIPS BETWEEN Vs30 VALUES AND SURFICIAL GEOLOGY
 With the exception of the Qhac and Qht1 units, which occupy narrow strips
of land adjacent to the Grey River that runs along the eastern and northern
sides of Port-au-Prince, all significant units on the new geologic map
contain at least one Vs profile. The number of Vs profiles range from one,
for the Ppf, Qht2, and Lmst units, to eight, for the Pf and Qpf units.
Figure: Locations and names of sites in Port-au-Prince, Haiti where (MASW) testing was
performed to obtain shear wave velocity profiles (shown relative to the new geologic map)
This site class is
based on Vs
profiles

 Based on limited exposures and geologic
map interpretation, the Pleistocene and
Holocene geologic deposits are probably
on the order of tens-of meters thick, and
Mio-Pliocene deposits are probably on
the order of many tens-of-meters to
several hundred meters thick.
 The individual Vs profiles, median VS
profile with one standard deviation, and
coefficients of variation (COV) as a
function of depth for geologic units Af,
Qham, Qphf, Qpf, Pf, and Mpb are
presented in Figure. In general, COV
values for the geologic units are greatest
near the surface and decrease with
depth. This is believed to be a result of
variable surficial weathering (intensity
and thickness) across the units.

 The Vs30 values calculated from the median Vs profile in each geologic unit are
presented in Table. The number of Vs profiles used to develop the median Vs
profile for each geologic unit and the NEHRP site classifications are also provided.
 It is interesting to compare the median Vs profiles for each geologic unit with the
same seismic site classification. The NEHRP Site Class D median profiles for
geologic units Af, Qham, and Qht2 are compared in next Figure.

RELATIONSHIPS BETWEEN VS30 VALUES AND SURFICIAL GEOLOGY
 The median Vs profiles for the Af and
Qham units are remarkably similar.
 The Vs profile for the Qht2 unit is also very
similar until an abrupt impedance
contrast (rock) is encountered at a
depth of 15 m.
 While the profiles from all three of these
units yield Site Class D classifications, it is
obvious that the strong impedance
contrast in the Qht2 unit would result in a
very different site response.
 Since bedrock was not encountered in
the top 30 m in the Qham and Af units, it
is likely that these units amplified
frequencies less than 2.5 Hz (periods
longer than 0.4 sec), while the Qht2 unit
likely amplified frequencies around 5 Hz
(periods around 0.2 sec).

 The NEHRP Site Class C median Vs profiles
for geologic units Qpht, Ppf, Pf, Mpb,
and Qpf are similarly compared in this
Figure.
 The median Vs profiles for all of these
units are very similar and, on average, no
major differences in site response would
be expected based on the profile
shapes.
 The median Vs30 values within these units
only range from 480 to 564 m/s, placing
them well within the bounds of NEHRP
Site Class C. As a result, it is likely that
most sites located within these geologic
units classify as NEHRP Site Class C.

 The median Vs profiles for all geologic
units are compared in this Figure. It is
obvious that the Lmst unit (NEHRP Site
Class B) is significantly stiffer than the
other units.
 At this scale, it is still possible to distinguish
between the NEHRP C and D profiles, but
the differences are not as obvious.
 Despite classifying as NEHRP Site Class D,
this profile appears to be a hybrid
between the NEHRP Site Class D and Site
Class C profiles.

SEISMIC SITE CLASSIFICATION MAP FOR PORT-AU-PRINCE
 Only a small portion of the mapped area
classifies as NEHRP Site Class B.
 Much of the greater Port-au-Prince area
classifies as Site Class C or D, based strictly
on Vs30.
 However, it should be stressed that some
areas within the mapped Holocene alluvial
deposits and coastal artificial fills may
classify as Site Class E or F, depending on
local subsurface conditions (e.g., presence
of soft clay or liquefiable soil) that cannot
be determined solely with Vs profiling.
 As liquefaction and lateral spreading were
documented following the earthquake in
some artificial fill (Af) deposits along the
coast and in the youngest Holocene
alluvium (Qht1) along the Grey River these
areas have conservatively been assigned a
Site Class F classification.

CONCLUSIONS
 A seismic site classification microzonation map for the greater Port-au-Prince area
has been presented herein. It is based on Vs profiles obtained using the MASW
method and surficial geology defined while developing a new geologic map of the
city.
 Much of Port-au-Prince is founded on soil deposits that classify as either NEHRP Site
Class C or D, based on Vs30.
 This microzonation map is a preliminary effort that can be refined as additional Vs
measurements, further geologic mapping, and other information such as
fundamental site period are gathered throughout the city. This initial microzonation
map is a critical step toward facilitating code based on site classification and
seismic design throughout Port-au-Prince as the rebuilding process begins.

Shear wave velocity and Geology Based Seismic Microzonation of Port-au-Prince, Haiti

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