Preliminary Earthquake Report

Transcription

1 Preliminary Earthquake Report :31 UTC Mt 6.9 (Main Event) :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: ; Fax: Web:

2 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. 2

3 Contents Introduction... 7 Station Locations... 8 Executive Summary of Report... 9 Port of Spain Strong Motion Network UTC Main Event (M t =6.9) UTC Aftershock Event (M t =6.0) Modified Mercalli Intensity Reports Summary UTC Main Event (M t =6.9) Demographics UTC Aftershock Event (M t =6.0) Conclusions References

4 Introduction This document serves as a preliminary report for the magnitude M t =6.9 earthquake that occurred on Tuesday 21 st August 2018 at 21:31 UTC (5:31 pm local time), and the biggest aftershock which occurred on Wednesday 22 nd August 2018 at 13:27 UTC. The epicentre for the main event was west of Trinidad and Tobago at N 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 21 st August 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 21 st August 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, 22 nd 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. 7

5 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 8

6 Executive Summary of Report Port of Spain Strong Motion Network UTC Main Event (M t =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 21 st August Port of Spain General Hospital (PSGH) Longitude W Soil Type Dense Soil Latitude N Vs₃₀ (m/s) 420 Elevation (m) 15.5 NEHRP Soil Class C Building Structure Wood Ground Resonance Period (s) Installation Surface / Concrete Sensor GURALP 5TDE Telemetry Internet / Wired Digitizer GURALP DM24 PGA (g) Mean Period (s) Table 2. PSKH information for the main earthquake of 21 st August Kent House (PSKH) Longitude W Soil Type Dense Soil Latitude N Vs₃₀ (m/s) 440 Elevation (m) 25.4 NEHRP Soil Class C Building Structure Free Field Ground Resonance Period (s) 0.31 Installation Surface / Concrete Sensor GURALP 5TDE Telemetry Internet / Wired Digitizer GURALP DM24 Mean PGA (m/s²) Mean Period (s) Table 3. PSQC information for the main earthquake of 21 st August Queen s Royal College (PSQC) Longitude W Soil Type Stiff Soil Latitude 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 Mean PGA (m/s²) Mean Period (s) 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 Hz. 9

7 Table 4. PSMG information for the main earthquake of 21 st August Mucurapo Girls R.C. School (PSMG) Longitude W Soil Type Dense Soil Latitude N Vs₃₀ (m/s) 370 Elevation (m) 8.1 NEHRP Soil Class C Building Structure Concrete Ground Resonance Period (s) 0.71 Installation Surface Sensor GURALP 5TDE Telemetry Wireless Digitizer GURALP DM24 Mean PGA (g) Mean Period Table 5. PSWY information for the main earthquake of 21 st August Woodbrook Secondary School (PSWS) Longitude W Soil Type Dense Soil Latitude N Vs₃₀ (m/s) 360 Elevation (m) 5.5 NEHRP Soil Class C Building Structure Concrete Ground Resonance Period (s) 1.11 Installation Surface Sensor GURALP 5TDE Telemetry Wireless Digitizer GURALP DM24 Mean PGA (g) Mean Period (s) Table 6. PSWS information for the main earthquake of 21 st August Woodbrook Secondary School (PSWS) Longitude W Soil Type Dense Soil Latitude N Vs₃₀ (m/s) 360 Elevation (m) 5.5 NEHRP Soil Class C Building Structure Concrete Ground Resonance Period (s) 1.11 Installation Surface Sensor GURALP 5TDE Telemetry Wireless Digitizer GURALP DM24 Mean PGA (g) Mean Period (s) 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 50 th percentile Angle-Independent Rotated Geometric Mean (GMRoti50) damping, of Spectral Acceleration for periods of oscillation between 0.0 and 1.5s. 10

8 Table 7. Peak Ground Acceleration (PGA) for the main earthquake of 21 st August Port of Spain Strong Motion Network UTC M t =6.9 Station PGA (g) Soil Class Vs 30 (m/s) T 3 0 (s) Sediments Thickness 4 (m) PSGH 0.19 C PSKH 0.27 C PSMG 0.23 C PSQC 0.19 D PSWS C PSWY C PSNL Not operational C Figure 2. Comparison of design spectrum for Port of Spain, soil Class damping, 2 & 10% probability of exceedance of MCE in 50 years, with the GMRotI50 Spectral 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 T 0 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. 11

9 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 S S and S 1 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 ( 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 (TTMP 7 ) 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 (~0.54s 8 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):S D (t) = S a (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 12

10 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 UTC Aftershock Event (M t =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 M t =6.0. In Tables 8 to 12 information for the stations with regards to their location, installation and response is presented. 13

11 Table 8. PSKH information for the aftershock earthquake of 22 nd August Kent House (PSKH) Longitude W Soil Type Dense Soil Latitude N Vs₃₀ (m/s) 440 Elevation (m) 25.4 NEHRP Soil Class C Building Structure Free Field Ground Resonance Period (s) 0.31 Installation Surface / Concrete Sensor GURALP 5TDE Telemetry Internet / Wired Digitizer GURALP DM24 Mean PGA (m/s²) Mean Period (s) Table 9. PSQC information for the aftershock earthquake of 22 nd August Queen s Royal College (PSQC) Longitude W Soil Type Stiff Soil Latitude 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 Mean PGA (m/s²) Mean Period (s) Table 10. PSMG information for the aftershock earthquake of 22 nd August Mucurapo Girls R.C. School (PSMG) Longitude W Soil Type Dense Soil Latitude N Vs₃₀ (m/s) 370 Elevation (m) 8.1 NEHRP Soil Class C Building Structure Concrete Ground Resonance Period (s) 0.71 Installation Surface Sensor GURALP 5TDE Telemetry Wireless Digitizer GURALP DM24 Mean PGA (g) Mean Period Table 11. PSWY information for the aftershock earthquake of 22 nd August Woodbrook Youth Facility (PSWY) Longitude W Soil Type Dense Soil Latitude N Vs₃₀ (m/s) 370 Elevation (m) 2.1 NEHRP Soil Class C Building Structure Concrete Ground Resonance Period (s) 1.25 Installation Surface Sensor GURALP 5TDE Telemetry Internet / Wired Digitizer GURALP DM24 Mean PGA (g) Mean Period (s) Table 12. PSWS information for the aftershock earthquake of 22 nd August Woodbrook Secondary School (PSWS) Longitude W Soil Type Dense Soil Latitude N Vs₃₀ (m/s) 360 Elevation (m) 5.5 NEHRP Soil Class C Building Structure Concrete Ground Resonance Period (s) 1.11 Installation Surface Sensor GURALP 5TDE Telemetry Wireless Digitizer GURALP DM24 Mean PGA (g) Mean Period (s)

12 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 22 nd August Port of Spain Strong Motion Network UTC M t =6.0 Station PGA (g) Soil Class Vs 30 (m/s) f 0 (Hz) Sediments Thickness (m) PSGH Not operational C PSKH 0.11 C PSMG 0.07 C PSQC 0.06 D PSWS 0.06 C PSWY 0.07 C PSNL Not operational C

13 Modified Mercalli Intensity Reports Summary UTC Main Event (M t =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 16

14 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 ( 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 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. 17

15 Figure 4. Nationwide Modified Mercalli Intensity Reports, as registered by the SRC s CSP and the DYFI modules. 18

16 Figure 5. Modified Mercalli Intensity Scale, Eastern Caribbean and South America 19

17 UTC Aftershock Event (M t =6.0) The largest aftershock from this sequence happened 16 hours after the main event, on 22/08/ :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%). 20

18 Figure 6. Nationwide Modified Mercalli Intensity Reports, as registered by the SRC s DYFI module. 21

19 Conclusions The earthquake of 21 st 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 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 26 th June 2016 (M t =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 26 th 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, 22

20 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. 23

21 References Boore, D.M., 2018, TSPP---A collection of FORTRAN programs for processing and manipulating time series, U.S. Geological Survey Open-File Report (Revised 27 February 2018). Bommer,J.J. & A.S. Elnashai, 1999, Displacement spectra for seismic design. Journal of Earthquake Engineering 3(1), 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,