Preliminary Earthquake Reportuwiseismic.com/Downloads/20190201_SMN_Abbr_Report_ilias...2019/02/01 · Preliminary Earthquake Report 20180821 21:31 UTC Mt 6.9 (Main Event) 20180822

Preliminary Earthquake Report20180821 21:31 UTC Mt 6.9 (Main Event)20180822 13:27 UTC Mt 6.0 (Aftershock)

STRONG MOTION PARAMETERS FOR PORT OF SPAIN FROM LOCAL STRONG MOTION NETWORK &

INTENSITY REPORT FROM CITIZEN SEIMOLOGIST COMMUNITY REPORT AND

EARTHQUAKE FEEDBACK FORMS

Ilias Papadopoulos, Celeste Sobion & Cory George | Tel: 868-662-4659; Fax: 868-663-9293

Email: [email protected] | Web: www.uwiseismic.com

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The results presented are preliminary and may be subject to revision if more detailed analysis is

carried out.

The uncorrected data of the records in ASCII are available upon request.

DISCLAIMER

This report was compiled by the Engineering Seismologist of the University of the West Indies Dr.

Ilias Papadopoulos, the Research Assistant Ms Celeste Sobion and Earthquake Engineer Mr. Cory

George. The accuracy and creditability of the results do not necessary reflect those of the Centre.

For the compilation of this report the ART 3 software provided by Guralp Ltd has been used. For the

Figure 2 the codes of David Boore (Boore, 2018) for the calculation of the GMRotI50 have been used.

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Contents Introduction ............................................................................................................................................. 7

Station Locations .................................................................................................................................... 8

Executive Summary of Report ................................................................................................................ 9

Port of Spain Strong Motion Network ................................................................................................ 9

20180821 2131 UTC Main Event (Mt=6.9) .................................................................................... 9

20180822 1327 UTC Aftershock Event (Mt=6.0) ......................................................................... 13

Modified Mercalli Intensity Reports ..................................................................................................... 16

Summary ........................................................................................................................................... 16

20180821 2131 UTC Main Event (Mt=6.9) .................................................................................. 16

Demographics ............................................................................................................................... 17

20180822 1327 UTC Aftershock Event (Mt=6.0) ......................................................................... 20

Conclusions ........................................................................................................................................... 22

References ............................................................................................................................................. 24

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Introduction

This document serves as a preliminary report for the magnitude Mt=6.9 earthquake that occurred on

Tuesday 21st August 2018 at 21:31 UTC (5:31 pm local time), and the biggest aftershock which

occurred on Wednesday 22nd

August 2018 at 13:27 UTC. The epicentre for the main event was west

of Trinidad and Tobago at 10.578 N 62.728 W, while the focal depth was approximately 130 km for

the main shock and 100 km for the aftershock (source:SRC). Given the magnitude of the earthquakes

and their focal depth, the shaking was felt not only in Venezuela and Trinidad & Tobago, but

throughout the Eastern Caribbean as far north as Dominica and as far south as Suriname, for the main

event.

Six sensors from the Port of Spain Strong Motion Network (POS SMN) recorded the main event and

the data processing is presented in this report. Four of the sensors: Port of Spain General Hospital

(PSGH), Kent House (PSKH), Queen’s Royal College (PSQC) and Mucurapo Girls’ R.C. School

(PSMG), recorded the entire waveform. However, two of the sensors, Woodbrook Youth Facility

(PSWY) and Woodbrook Secondary School (PSWS) cut off during the earthquake due to power

failure in the area and did not record the full waveform and hence the data from these sensors is

considered incomplete. For the aftershock, five sensors recorded the full waveform. Station PSGH

was out of power since the whole area of General Hospital was cut off for inspection and assessment

of damages from the main event.

We also present an Intensity Report in Modified Mercalli Intensity Scale (MMI) for the main event of

21st August 2018. The report includes the intensity reports as recorded by the SRC’s Citizen

Seismology Project and the ‘Did you feel it?’ questionnaire module on SRC’s website with regards to

the main event of 21st August 2018.

For the main event, the maximum instrumentally recorded Peak Ground Acceleration value was 0.27g

in Maraval at the Kent House Station (PSKH) with the second largest value recorded in St James at

the Mucurapo Girls’ R.C. School (PSMG). The highest intensity values registered in the country was

MMI=VIII, as reported by citizens via the “Did You Feel It?” module on SRC’s website.

The biggest aftershock from the main event occurred the next day, 22nd

August 2018 at 09:27 am. No

additional damages have been reported to SRC from this event.

All processing of the strong-motion data is presented in the Appendix in Figures and Tables.

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Station Locations

The map below shows the location of the seven stations of the Port-of-Spain Strong Motion Network:

PSKH – Kent House

PSMG – Mucurapo Girls’ R.C. School

PSQC – Queen’s Royal College

PSGH – Port-of-Spain General Hospital

PSWY – Woodbrook Youth Facility

PSWS – Woodbrook Secondary School

PSNL – Port-of-Spain National Library

Figure 1: Aerial picture of Port of Spain. Purple circles indicate the location of the strong-motion stations

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Executive Summary of Report

Port of Spain Strong Motion Network

20180821 2131 UTC Main Event (Mt=6.9)

Six out of seven sensors in Port of Spain were operational during the main earthquake event.

Two of them had a cut off while the earthquake was still propagating, due to a power failure in the

area of Woodbrook. As a result, part of the waveform was recorded in these two stations. In Tables 1

to 6 information for the stations with regards to their location, installation and response is presented.

Table 1. PSGH information for the main earthquake of 21st August 2018.

Port of Spain General Hospital (PSGH)

Longitude W 61.50807° Soil Type Dense Soil

Latitude N 10.66094° Vs₃₀ (m/s) 420

Elevation (m) 15.5 NEHRP Soil Class C

Building Structure Wood Ground Resonance Period (s)1 0.45

Installation Surface / Concrete Sensor GURALP 5TDE

Telemetry Internet / Wired Digitizer GURALP DM24

PGA (g)2 0.186 Mean Period (s) 0.304

Table 2. PSKH information for the main earthquake of 21st August 2018.

Kent House (PSKH)


Latitude N 10.67958° Vs₃₀ (m/s) 440


Building Structure Free Field Ground Resonance Period (s) 0.31



Mean PGA (m/s²) 0.271 Mean Period (s) 0.328

Table 3. PSQC information for the main earthquake of 21st August 2018.

Queen’s Royal College (PSQC)

Longitude 61.51902° W Soil Type Stiff Soil

Latitude 10.66814° N Vs₃₀ (m/s) 350

Elevation (m) 14.7 NEHRP Soil Class D

Building Structure Concrete Ground Resonance Period (s) 0.77

Installation Vault (-2.5m) Sensor GURALP 5TDE

Telemetry Wireless Digitizer GURALP DM24


1 Resonance Period of the ground has been calculated during the Trinidad & Tobago Microzonation Project.

2 PGA(g) refers to the geometric mean of the two horizontal components, low-pass filtered @0.05 Hz.

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Table 4. PSMG information for the main earthquake of 21st August 2018.

Mucurapo Girls’ R.C. School (PSMG)

Longitude 61.5341° W Soil Type Dense Soil

Latitude 10.6722° N Vs₃₀ (m/s) 370



Installation Surface Sensor GURALP 5TDE


Mean PGA (g) 0.233 Mean Period 0.421

Table 5. PSWY information for the main earthquake of 21st August 2018.

Woodbrook Secondary School (PSWS)


Latitude 10.6614° N Vs₃₀ (m/s) 360





Mean PGA (g) 0.157 Mean Period (s) 0.391

Table 6. PSWS information for the main earthquake of 21st August 2018.



Latitude 10.6614° N Vs₃₀ (m/s) 360






The area of Maraval registered the highest value of maximum acceleration (PGA), reaching

0.270g. Table 7 shows the summary of recorded peak ground acceleration (PGA) for all stations in

Port of Spain, along with other site information related to their response.

Comparing the values of PGA from Table 7, in Maraval the recorded PGA is more than 40%

the value in Woodbrook Secondary, while the areas of Mucurapo and Woodbrook south have equal

values (with the reminder that Woodbrook sensors did not record the whole waveform due to power

failure).

Comparing the elastic response spectra of this earthquake with the design spectrum engineers

are using in Trinidad to construct seismic resilient buildings, we can conclude that the mean Spectral

Acceleration (SA) did not exceed the design values (as it can be seen in Appendix figures 19, 31, 43,

55, 67 and 79), but several other observations are proving to be significant. In Figure 2 the design

spectrum for Port of Spain is compared with each stations’ 50th percentile Angle-Independent Rotated

Geometric Mean (GMRoti50) value, @5% damping, of Spectral Acceleration for periods of

oscillation between 0.0 and 1.5s.

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Table 7. Peak Ground Acceleration (PGA) for the main earthquake of 21st August 2018.


20180821 2131 UTC Mt=6.9

Station PGA (g) Soil Class Vs30 (m/s) T03

(s) Sediments Thickness4 (m)

PSGH 0.19 C 420 0.45 70

PSKH 0.27 C 440 0.31 50

PSMG 0.23 C 370 0.71 90

PSQC 0.19 D 350 0.77 90

PSWS5 0.16 C 360 1.11 160

PSWY6 0.21 C 370 1.25 160

PSNL Not operational C 370 0.77 90

Figure 2. Comparison of design spectrum for Port of Spain, soil Class C, @5% damping, 2 &

10% probability of exceedance of MCE in 50 years, with the GMRotI50 Spectral Acceleration @5%

damping from 4 stations in Port of Spain for the main event.

The calculation of the design response spectrum has been done according to the International

Building Code (IBC 2006) and American Society of Civil Engineers (ASCE-7-05) recommendations,

3 T0 refers to the Ground Resonance Period, as determined by the Trinidad & Tobago Microzonation Project.

4 Based on simulation.

5 This sensor experienced power cut-off during recording. 6 This sensor experienced power cut-off during recording.

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and represents the 10% probability of exceedance for the Maximum Considered Earthquake (MCE) in

50 years, which is the usual design for residential constructions (Importance Factor 1.0). Also we take

into account the soil class effects for most of stations location (soil Class C), for 5% damping. The

values of SS and S1 are taken from the last Probabilistic Seismic Hazard Assessment (PSHA)

conducted by The UWI SRC for return period of 2475 years (Bozzoni et al., 2011), which can be

found at SRC’s website (http://uwiseismic.com/seishaz.aspx).

There are peaks in spectral acceleration in some areas of Port of Spain that are not captured

from the spectrum design. These peaks are related to the local site effects from the basin geometry

and geology, and have been already identified and documented during the Trinidad & Tobago

Microzonation Project (TTMP7) for Port of Spain. For the Maraval area (PSKH) the peak for this

earthquake exceeds the design spectrum at certain period of oscillation. This results in the building

response surpassing the target design response which increases the probability of damage/collapse in

buildings whose resonance period coincides with the peak period (~0.54s8 for Maraval). This effect

needs to be investigated further by SRC to specify with better accuracy and resolution the site effects

in the area. The increased spectral acceleration value observed in Maraval can be attributed to the

shape of the basin in the area as, being narrow, results in trapping, reflection and amplification of the

seismic energy on the sides of the basin. The latter is also verified by the longer duration of the

wavetrain in this location.

From the same figure, the Mucurapo area exhibits a peak in spectral acceleration at ~ 0.6s,

which again coincides with the resonance frequency of the sediments package in the area, as

identified successfully from the Port of Spain Microzonation.

The Queen’s Park Savannah west area (PSQC) exhibits also a site effect at ~ 0.85s, which is the

resonance period of the soil package above bedrock (assumed to be the Chancellor Schist in the area).

The Port of Spain General Hospital area (PSGH) appears to not be affected severely from the

local site conditions. This is due to the proximity to bedrock (Laventille Meta-Limestone), the thin

soil package (~ 70m) and two-dimensional effects caused at the edge of the basin.

In Figure 2 we have also plotted the Design Spectrum calculated for 2% probability of

exceedance for the Maximum Considered Earthquake (MCE) in 50 years, value used usually for

significant constructions (such as public buildings, hospitals, schools). The recorded values of spectral

acceleration do not reach at any period the design spectrum, hence no structural damage is expected

for these buildings in the areas monitored by the Port of Spain Strong Motion Network.

For completion purposes, the theoretical elastic displacement response spectrum is also compared to

the displacement spectra produced by the strong motion instruments. This spectrum is mainly used

when considering displacement–based seismic design and assessments of long period structures as

opposed to the force-based design approach. A conservative method used in seismic design codes

which is regarded as the most simple and straight forward solution (Bommer & Elnashai, 1999) is

shown below. The results are presented in Figure 3 in a semi-logarithmic scale.

Spectral Displacement (SD):𝑆𝐷(𝑡) = 𝑆𝑎(𝑇) [𝑇

2𝜋]2

7 TTMP is a 10-years collaboration between the UWI SRC and the Ministry of Planning & Development, The

Republic of Trinidad & Tobago. 8 This corresponds approximately to a 16m concrete moment-resisting frame building

http://uwiseismic.com/seishaz.aspx

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The tallest building located in Port of Spain does not exceed 130 meters in height, therefore, the

period range of interest lies between 0 and 4s.The results are presented in Figure 3 in semi -

logarithmic scale.

The plot comparisons indicate that the displacement spectra developed from the records of the strong

motion instruments, match or exceed the developed displacement curve at periods less than 1 second.

This corresponds to low-rise and medium-rise buildings. For periods greater than 1s, the displacement

values are much less than the developed displacement spectrum. To determine the impact this

earthquake would have on long period structures, an in depth investigative structural assessment

would be required taking into consideration soil properties, source mechanism and frequency content

of the earthquake.

Figure 3. Elastic displacement response spectrum compared with the spectral displacement

spectra of the four stations of Figure 2.

20180822 1327 UTC Aftershock Event (Mt=6.0)

On 2018/08/22 five of the strong-motion network sensors were operational and recorded the

full waveform of the biggest aftershock, registered at 09:31 am local time with magnitude Mt=6.0. In

Tables 8 to 12 information for the stations with regards to their location, installation and response is

presented.

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Table 8. PSKH information for the aftershock earthquake of 22nd

August 2018.

Kent House (PSKH)


Latitude N 10.67958° Vs₃₀ (m/s) 440


Building Structure Free Field Ground Resonance Period (s) 0.31




Table 9. PSQC information for the aftershock earthquake of 22nd

August 2018.

Queen’s Royal College (PSQC)

Longitude 61.51902° W Soil Type Stiff Soil

Latitude 10.66814° N Vs₃₀ (m/s) 350

Elevation (m) 14.7 NEHRP Soil Class D


Installation Vault (-2.5m) Sensor GURALP 5TDE



Table 10. PSMG information for the aftershock earthquake of 22nd

August 2018.

Mucurapo Girls’ R.C. School (PSMG)


Latitude 10.6722° N Vs₃₀ (m/s) 370





Mean PGA (g) 0.077 Mean Period 0.224

Table 11. PSWY information for the aftershock earthquake of 22nd

August 2018.

Woodbrook Youth Facility (PSWY)


Latitude 10.6614° N Vs₃₀ (m/s) 370






Table 12. PSWS information for the aftershock earthquake of 22nd

August 2018.



Latitude 10.6614° N Vs₃₀ (m/s) 360






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As it can be seen from Table 13, Maraval area registered a PGA more than 40% higher than in

Woodbrook and Mucurapo area, same as with the main event the previous day. The rest of the

stations recorded the same value of PGA (within 10% margin). The aftershock has significant shorter

duration compared with the main event, and although it was widely felt on the island it did not cause

any additional damage to structures and properties.

Table 13. Peak Ground Acceleration (PGA) for the aftershock earthquake event of 22nd

August 2018.


20180822 1331 UTC Mt=6.0

Station PGA (g) Soil Class Vs30 (m/s) f0 (Hz) Sediments Thickness (m)

PSGH Not operational C 420 2.2 70

PSKH 0.11 C 440 3.2 50

PSMG 0.07 C 370 1.4 90

PSQC 0.06 D 350 1.3 90

PSWS 0.06 C 360 0.9 160

PSWY 0.07 C 370 0.8 160

PSNL Not operational C 370 1.3 90

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Modified Mercalli Intensity Reports

Summary

20180821 2131 UTC Main Event (Mt=6.9)

The analysis of the felt reports collected from The UWI SRC via its Citizen Seismologist Project

(CSP) and “Did You Feel It?” (DYFI) module is presented on a map in Figure 4. The duration of the

survey was 3 days for the CSP, while SRC received reports via the DYFI module until the next day

morning, when the biggest aftershock occurred. Further details of the survey and processing of the

CSP data can be found in the Appendix, Section 2.

The UWI SRC also received the processed results from FUNVISIS for Venezuela and IPGP for the

French territories in the Eastern Caribbean, which are presented altogether in Figure 5.

The strongest intensity from the felt reports was magnitude VIII on the Modified Mercalli Scale (8 in

total). Five of the reports came from western Trinidad. The different locations are as follows:

Cedros,

St James,

Belmont,

Queen’s Park Savannah,

Wrightson Road, Port of Spain,

Diego Martin,

Barataria,

Sangre Grande.

The reports from these areas indicated that there was significant damage with some of the felt reports

indicating some or all of the following occurring:

Pictures moved, Items rattled, toppled over or fell off shelves or cupboards, Liquids splashed

or spilled, Standing vehicles rocked, Moving vehicles rocked, Car alarms were triggered,

Large Cracks in Walls,

Mirrors or windows shattered,

Masonry fell from block or brick wall(s),

Free standing walls swayed.

The average intensity experienced in Trinidad was V. This corresponds to pictures moving, items

rattling, toppled over or fell off shelves or cupboards, liquids splashing or spilling, standing vehicles

rocking, moving vehicles rocking and the triggering of car alarms.

In Port-of-Spain (including downtown and uptown POS, Woodbrook, St James and Belmont)

however, on the north-western side of the island, there were four reports of VIII intensities, six reports

of VII intensity and three intensity VI reported. The average intensity in Port-of-Spain, VII was

higher than the country average. This was expected as the epicentre for the earthquake was off the

west coast of Trinidad and as such, areas such as Carenage, Diego Martin and Port-of-Spain would

have experienced the most intense shaking. There was also significant damage in the south of

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Trinidad with visible surface deformation (lateral liquefaction) in Los Iros and an intensity VIII report

in Cedros.

On the map, there were two reports in Tobago, intensity VII close to Canaan (western side of the

island) and intensity V in Mount St. George.

Demographics

Table 14. Demographics from “Did you feel it?” module (http://uwiseismic.com/EarthquakeFeedback.aspx)

Reports # Demographics #

Submitted 424 Urban Not available

Complete and Usable 173 Rural Not available

Unreliable 0 Female 273

Not usable 251 Male 121

Table 15. Demographics from Citizen Seismologist Community Project (communication via email to

registered users)

Reports # Demographics #

Submitted 114 Urban Not available

Complete and Usable 68 Rural Not available

Unreliable 1 Female 77

Not usable 45 Male 37

Comments:

Submitted: Total submitted online reports from persons located in Trinidad and Tobago

Complete: Reports with all fields filled.

Usable: Reports with exact location to be placed on map

Unreliable: Reports that are considered to have not been filled with good will and faith.

Not usable: Reports that were submitted but the address could not be found or a general location was given eg Arima.

Female: Total submitted reports filled by females.

Male: Total submitted reports filled by males.

http://uwiseismic.com/EarthquakeFeedback.aspx

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Figure 4. Nationwide Modified Mercalli Intensity Reports, as registered by the SRC’s CSP and the DYFI modules.

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Figure 5. Modified Mercalli Intensity Scale, Eastern Caribbean and South America

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20180822 1327 UTC Aftershock Event (Mt=6.0)

The largest aftershock from this sequence happened 16 hours after the main event, on 22/08/2018

13:31 UTC (09:31 local time). Given the close proximity with the main event, The UWI SRC decided

to not stress more the CSP participants by issuing another report, instead citizens on their own

reported at the DYFI module on The UWI SRC’s website. It was reported widely felt at the west coast

of Trinidad, and we received one report from Tobago.

A total of 148 reports has been received from The UWI SRC, of which we were able to place on map

81 (55%). The strongest intensity from the felt reports was magnitude VII on the Modified Mercalli

Scale (2 in total), in the areas of San Juan and Arima. On average this event is registered for Trinidad

& Tobago as IV (76%).

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Figure 6. Nationwide Modified Mercalli Intensity Reports, as registered by the SRC’s DYFI module.

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Conclusions

The earthquake of 21st August 2018 is the strongest registered in Trinidad for the past several

decades. While the epicentral distance is over 100 km away and at a significant focal depth, it was

felt and reported strong from the majority of Trinidadians. Due to its long duration (the significant

duration in Port of Spain is on average more than 50 seconds) it caused fear to citizens. Since it

happened early in the evening (5:31 pm local time) most of the citizens were awake. The majority of

reports mention an acoustic effect coming from inside the building. Anyone with animals inside or

close to the house reported that the animals reacted to the shaking motion.

For the Port of Spain area, the strong-motion network operated by The UWI SRC recorded the

seismic motion in six locations. The strong-motion analysis shows that the recorded values of

acceleration did not exceed the design values for the area, but there are some considerations for the

future. While the PGA value is considerable smaller than the design, in some areas there are certain

periods of oscillation that the Spectral Acceleration recorded is close or even above the design value.

These periods coincide in all areas with the resonance period of the sediment package, as it has been

identified and documented by the Trinidad & Tobago Microzonation Project. This serves as proof of

the importance and significance of the TTMP, and the ability to reveal important information

otherwise not available from the low-resolution PSHA and not represented in the Design Spectrum of

any building code (regional or international).

The aftershock that occurred the next morning (2018/08/22 @13:27UTC, 09:27 local time),

while it had much shorter duration, alerted the citizens as they were already influenced from the major

event. No cumulative damage has been reported to The UWI SRC from the biggest aftershock (or

any other aftershock from this sequence). Given the very short time interval between the two big

events, citizens were still reporting at the DYFI module the main shock observations of the previous

night. Given the close proximity of the two events, we still collected 148 responses via the DYFI it

module.

Due to the shape of Trinidad’s west coast, the epicentral distance is almost the same for all

major urban centres at the west coast of the island (120 km for Port of Spain, 130 km for San

Fernando, and 110 km for Point Fortin). This attributes to the uniform reporting of damages from the

entire west coast of the island. Independent reports to The UWI SRC mention significant building

damage to several places on the island, which were reported directly to the Regional Corporation

Disaster Management Units (not presented in this report). Most of buildings affected severely by the

shaking were built without engineering supervision or design. In Port of Spain and Diego Martin at

the north of the island, several recently built mid-rise buildings suffered minor damages on the walls,

but no structural damage on beams, columns or joints has been reported to The UWI SRC.

Since the last earthquake report on 26th June 2016 (Mt=5.5) we have an increase in received

reports by 35.6%. This can be explained by the much stronger shaking of the current event, and

hopefully due to SRC’s outreach activities. While the usage of reports on 26th June 2018 was 36.5%,

the reports used in the current report represent 44.8% of total reports. While this is a significant

increase in used reports, the percentage is still small and we need to keep up on our efforts to sensitize

the citizens to properly report their observations, in order to maximize the usage of information

received.

Significant ground deformation has been reported in the areas of Los Iros and Moruga in

Siparia and Princes Town regions, respectively. While the research is still ongoing, the initial results

indicate lateral liquefaction spreading due to the saturation of soil, the long duration of the earthquake,

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and the high level of shaking. Water discharge and mud diapirism appeared in both cases,

accompanied by oil content in the extracted fluid. In the Los Iros case a slump followed the lateral

spreading in an area that was prior identified as prone to landslide. The UWI SRC is investigating the

case with geophysical and geotechnical exercises, supported by GPS and remote sensing techniques.

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References

Boore, D.M., 2018, TSPP---A collection of FORTRAN programs for processing and

manipulating time series, U.S. Geological Survey Open-File Report 2008-1111 (Revised 27 February

2018).

Bommer,J.J. & A.S. Elnashai, 1999, Displacement spectra for seismic design. Journal of

Earthquake Engineering 3(1), 1-32.

Bozzoni, F., M. Corigliano, C. G. Lai, W. Salazar, L. Scandella, E. Zuccolo, J. Latchman, L. Lynch,

and R. Robertson, 2011, Probabilistic seismic hazard assessment at the eastern Caribbean Islands,

Bull. Seismol. Soc. Am. 101, no. 5, 2499 – 2521

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Preliminary Earthquake Reportuwiseismic.com/Downloads/20190201_SMN_Abbr_Report_ilias...2019/02/01 · Preliminary Earthquake Report 20180821 21:31 UTC Mt 6.9 (Main Event) 20180822