Local site effect characterization and determination of the shallow high impedance layerin downtown area of Managua

Abstract:

Managua city is the capital of Nicaragua and has the most dense population of the country. It is settled in the Pacific region of the country where the largest seismic hazard exists. It is agreed by the research community, regarding site response assessment, that dynamic properties of the soils are one of the key factors, that increases the damage an earthquake can cause in a given site. In Managua city, dynamic properties of the soils has not yet been characterized to detail.

In this study, fundamental periods and an approximate thickness from the shallow material to the high impedance layer were estimated. Fundamental period was estimated based on microtremor measurements in 26 sites in the old downtown are of Managua city and the Nakamura method, whereas the thickness was estimated based on the modeling of the empirical transfer function of microtremors.

Results led to two conclusions. First, the fundamental resonance occurs in the period range of 0.18 – 0.22 s in the old downtown area of Managua city. A second mode of vibration seems to ocurrs in the range of ?? to ?? s. An increase of the fundamental period from North to South was observed.

Second, two main high impedance interfaces seem to be detected by the H/V ratio, the first high impedance interface show its larger amplitude around 0.22 s and seems to occur to a depth of 13 m where as the second impedance interfaces show its larger amplitude in ?? second and extends up to ?? s. This results are very important as in the city most of the constructions are one and two story houses and medium rise buildings.

1. INTRODUCTION

Nicaragua is an earthquake prone country. This occurs as consequence of the continuous subduction process of the Coco's plate beneath the Caribbean plate. Additional to this seismic source there is the existence of inland active shallow faulting. This faulting have historically generated earthquakes with devastating consequences for different cities in the country.

Managua is the capital city of Nicaragua, the population as of 2012 were of 1028808 million people which represents a sixth part of the total population of the country (INIDE 2012). The city is located in the Pacific region of Nicaragua. In this region there are two main tectonic features. One is the volcanic chain and the other one is the Nicaraguan depression (Frez and Gamez 2008; Figure 1). Both geological features cross the country with an approximate direction of N45W. The top layers of soil in the Nicaraguan depression, are constituted mainly by volcanic soils. and shallow faulting with strike-slip type faults is common in the depression (INETER 2004, McBirney and Williams 1965).

Historically is known that Managua city have been struck by earthquakes in 1931, 1968 and 1972 (Leeds 1974). The last one caused many collapses of buildings and dwellings in the downtown area of the city in that epoch (Brown 1974). Moreover, paleo- seismological studies have been carried out in the eastern part of the city, specifically in the eastern edge of the zone known as the airport Graben, researchers found that this faulting zone have generated earthquakes in the past, being the most recent between the period of A.D. 1650-1810 (Cowan et al. 2002).

One of the factors that affect the most the damaging potential of seismic waves is the local site conditions. This fact, have been learned from several examples around the world being one of the most infamous the 1985 Michoacan earthquake in Mexico, where the epicenter of the earthquake was located more than 300 km away of Mexico city and yet this earthquake caused devastation in buildings and dwellings. Such local response caused by the local site conditions is known as site effect (Kagawa 1996).

A research aimed to investigate site effect potential in the shallow soil deposits in Managua city was carried out. This was done as part of the cooperation program between the Institute of Geology and Geophysics (IGG) which belongs to the National Autonomous University of Nicaragua (UNAN-Managua) and the International Institute of Seismology and Earthquake Engineering (IISEE) which belongs to the Building Research Institute (BRI).

In this document preliminary results which consists in determining site effect in the old downtown area of Managua city are presented.

2. SEISMIC ACTIVITY

Potential damaging earthquakes in Managua city are generated due to three main seismic sources. These sources can be divided into three functional types:

1. Interplate earthquakes along the subduction zone. Seismic activity is generated due to the interaction of the plates along the subduction zone. According to the earthquake catalog compiled for the seismic hazard assessment of Nicaragua, this is the most seismically active area (Resis II 2008). This seismicity occurs along the Pacific coast of Nicaragua (Figure 3). A major earthquake with Mw=7.6, occurred in 1992 in this zone, this earthquake generated a Tsunami which affected Masachapa town and some other nearby towns (Resis II 2008).

2. Intraslab earthquakes in the subducting Cocos plate.

Earthquakes in this zone are also related to the seismicity generated as a consequence of the subduction of the Cocos plate beneath the Caribbean plate. However it also includes seismicity from the Forearc zone of Nicaragua (Güendel and Protti 1998). In this zone a fault system with NE-SW strike predominates, located along the Pacific coast (Funk et al. 2009; Segura et al. 1996). The biggest historical earthquake in this zone occurred in 1956 and it had a magnitude of Mw=7.3 (Resis II 2008).

Figure 1. Map showing the surroundings of the study area (left). The right map shows the geomorphologic units nearby the urban area of Managua city (relief data from Farr 2007). The pink polygon in the right plot represents the study area whereas the white lines represent the current urban area of Managua city.

Figure 2. Left figure shows the geological map of Nicaragua and geological faults around Managua city. (INETER 2004). As can be seen from the figure the surface materials in the city corresponds to volcanic materials.

3. The third seismic source are the inland earthquakes in and nearby Managua city. Historic experience indicates that inland earthquakes are often highly devastating. These inland earthquakes are mainly generated due to shallow faulting systems (LaFemina et al. 2002). These faults release only a small amount of the energy generated through daily seismic activity. As consequence they accumulate high amounts of stresses and are then able to generate earthquakes of up to Mw =6.3 as experienced in 1972 (Brown et al. 1974).

Figure 3. Earthquakes epicenters (filled dots) in the period 1990-2007(INETER 2007) and geological faults (lines) around the study area.

3. STUDY AREA

The study area is the downtown area of Managua city during the 1972 Managua earthquake. This area is located in the Central-Northern part of the city between the coordinates 0577500 E – 1344250 N and 0581000 E- 1341000 N , and it has a smooth slope towards the North.

Figure 4. The map shows the study area enclosed in a gray rectangle and the location of the 26 points where microtremors data were recorded.

Managua city is settled within the Nicaraguan depression, geologists have defined the lithology of Managua soils and nearby Managua city (Figure 2 left and Figure 3). Some historical earthquakes have also occurred in this area (Resis II 2008)

In a regional scale, Managua as all of the Pacific region cities is located within a major geological feature in the shape of a fringe along the Pacific coast of Nicaragua named as the Nicaraguan depression. This is a tectonic structure that lays in the Pacific region of the country with an approximate direction of N45W and an approximate width of 75 km.

Two normal faults limits the depression thus is classified as a Graben (INETER 2004). In a local scale, Managua city is settled in a smaller Graben named as Graben of Managua.

Geomorphologically, the Urban area of Managua is surrounded by many volcanic edifices. It is limited in the Northwest by Chiltepe volcanic complex, in the Southwest by the Mateare scarp, in the Southeast by Masaya volcanic complex and in the Northeast the Tipitapa plain (Hradecky et al. 2000). To the South lays an scarp known as Las Nubes scarp, thus in general, the city have a slope towards the North which becomes smoother from the southern border of the city (Figure 1).

Lithologically, Managua city lays over volcanic soils mostly pirocalstic which comprises scorias, ashes, tuffs, pumice, lahars, ignimbrites, colluvials, paleosoils and meteorized material. All these materials lays discordantly over ignimbrites known as Grupo Las Sierras (Hradecky et al. 2000).

4. METHOD

The main goal of this research was to determine whether Managua city soils present local site effects or not based on the fundamental period of vibration of the soils as well as in the relative amplifications. The method used in this study was the HVSR or Nakamura technique as was popularized most by this author (Nakamura 1989). The main assumption of this method is that we can use the vertical component of a record as if this component does not have amplification in it, this is, as if it was a component of a statio n located in a rock site.

Microtremors has been acknowledged as being able to characterized the fundamental vibration period of a site with reliability by the scientific community whereas in the case of the relative amplification this method is not totally accurate and still some debate is going on (Bonnefoy-Claudet et al. 2006). The use of microtremors for site effect assessment was initiated in Japan (Nogoshi and Igarashi 1971) and most popularized by Nakamura (1989).

The capability of microtremors for determining fundamental periods of the soils accurately, comes from the fact that a high contrast boundary between soil and bedrock is clearly detected in the frequency spectrum with a peak, this has been studied numerically and analytically by some researchers (Uebayashi et al. 2012; Bonnefoy-Claudet et al. 2006).

Eventhough it has been pointed out the weak accuracy of the method to determine relative amplifications, in this study, it was decided to show the amplitudes of the HVSR, so we can have a rough idea of what kind of amplification values we could expect in the soils of Managua.

In order to evaluate the fundamental period of vibration and relative amplitudes the analysis of 26 microtremor records based on the H/V ratio was performed. The duration of the records was fixed to approximately 30 min and data acquisition was carried out at selected sites in the urban area of the city.

5. DATA ACQUISITION

The data acquisition had as target to determine short period and long period microtremors. Thus we recorded 30 min long records at 26 points distributed in the city (Figure 4).

A Mcseis MT Neo instrument was used as datalogger (OYO corp.) with a sampling frequency of 100 sps and three long period seismometers (Tokyo Sokushin) with a bandwith of 0.2Hz to 100Hz (Figure 6). The time window for analysis was of 87 s (Figure 5 right). The length was defined by keeping only, the windows with less cultural noise, this was done by using and sta/lta algorithm. Moreover, In order to select the less noisy part we checked the deviation of the average HVSR with respect to +/- one standard deviation.

Figure 5. Right figure shows the microtremor record with a part highlighted indicating the portion to analyze and the left figure shows the highlighted part of the right figure in a zoom in.

Figure 6. Frequency response of the long period sensors.

Figure 7. Flow chart of the steps followed in the analysis of the microtemors records.

5.1 Data processing

The data acquired was first converted from binary to ascii. As mentioned the analysis was based on the HVSR technique, this procedure was applied to each record to determine the FFT and the H/V ratio. The analysis was performed based on the flowchart shown in Figure 7.

The procedure could be summarized as follows: in the time domain, we tapered the record by applying a sinusiodal function increasing from 0 to the maximum amplitude in the first 10 percent of the time window selected and then decreasing from the maximum amplitude to 0 in the last 10 percent of the selected window. If the time window was not a power of two, zero padding was applied until this was a power of 2.

Then we applied the Fast Fourier Transform and smoothed the record in the frequency domain by applying the Parzen window. Then we obtained the H over V spectral ratio by taking the square root of the sum of the squared horizontal components spectra and divided by the vertical component spectrum.

Result was taken as good or bad based on the deviation of +/- one standard deviation from the average HVSR curve.

6. RESULTS AND ANALYSIS

A total of 31 ambient vibrations records were collected in old downtown area of Managua city.

Figure 8. H/V ratio curves grouped in three types according to its frequency content. The information shown in the microzonation maps was obtained from the H/V ratio curves. The curves were grouped according to its characteristics in three families (see Figure 8), this is, according to the frequency content of each record. Moreover zonations are proposed based on the periods and on the relative amplitudes of HVSR.

The zonation maps were plotted taking into account the spatial distribution of points and based on the range shown in the legend we tried to plot the "best" involving contours to our criteria.

The proposed zonations, based on the Fundamental periods of HVSR and relative amplitudes of HVSR are detailed below:

6.1 Fundamental periods of HVSR

The proposed zonation is based on fundamental periods of HVSR, this periods were divided in three categories 0.10 – 0.18 s; 0.18 – 0.22 s; 0.22 – 0.37 s (Figure 9).

These ranges were chosen by taking bins of equal value and according to the spat ial distribution of the fundamental periods of HVSR. This criterion was chosen in order to avoid a too complex zonation map.

Zone 1: The soils in this zone have fundamental periods between 0.10 s to 0.18 s. Taking into account the fundamental periods observed in this zone and the Lithology information collected by ENACAL (2004) we could infer that in this part of the city the shallow soils have a smaller thickness compared to the other parts of the area. In this zone, it is important to avoid the coincidence of the natural period of vibration of buildings with 1 to 2 stories height with that of the soil.

Zone 2: This zone have fundamental periods between 0.18 s – 0.22 s. Taking into account the fundamental periods observed in this zone and the Lithology data collected by ENACAL (2004) we could infer that in this part of the city the thickness of the shallow soils increase outwards the diameter of the Tiscapa Lagoon. In this zone, it is important to avoid the coincidence of the natural period of vibration of buildings with 2 to 3 stories height with that of the soil.

Zone 3: This zone have fundamental periods between 0.22 s – 0.37 s. Taking into account the fundamental periods observed in this zone and the lithology data collected by ENACAL (2004) we could infer that in this part of the city the thickness of the soils of fine grain it is a bit larger than the other 2 zones. It is important to avoid the coincidence of the natural period of vibration of buildings with 2 to 4 stories height with that of the soil.

Figure 9. Zonation based on the fundamental periods.

6.2 Amplitudes of HVSR

The relative amplitudes of HVSR observed in the old urban area of Managua city range from a minimum of 1.7 times to a maximum of 5.2 times (Figure 10). Predominating the values in the range of 3.9 to 5.3. In this study relative amplification is shown for reference purposes as the HVSR cannot determine accurately the soil amplification. The zonation presented here, can be explained as follows:

Zone 1: This zone presents HVSR amplitudes in the range of 1.7 to 3.0 times. This amplitude values can be explained as the change in the thickness of the lithology towards the Tiscapa Lagoon, since the soil layers with big impedance contrast reduce its thickness, thus the amplitudes reduces.

Figure 10. Zonation based on the relative amplification.

Zone 2: This zone presents HVSR amplitudes in the range of 3.0 to 3.95 times. This amplitude values can be explained similarly as the last zone, this is, the high impedane layers increase their thickness outwards the diameter of the Tiscapa lagoon, thus the relative amplitudes increases its values.

Zone 3: This zone presents HVSR amplitudes in the range of 3.95 to 5.17 times. This amplitude values follow the same behavior as Zone 2. This is, as the high impedance contrast layers increases in thickness so does the amplitudes values.

6.3 Soils thickness

Soil model was obtained here based on Arai and Tokimatsu (2005). According to these authors the soil empirical transfer function can be modeled by means of fitting the theoretical phase velocity of Rayleigh waves.

By following the above mentioned procedure we were able to model a soil structure which show two main impedance contrasts, one around 0.25 s and the other one from 0.6 to 2 s. These result indicates that although the fundamental period of vibration of Managua soils is of 0.25 s, there is another vibration period which influences the dynamic behavior of the soils and is in the range of 0.6 to 2 s (Figure 11).

Figure 11. Soil modeling using H/V empirical transfer function.

The soil model we obtained by modeling is shown in the table below:

Table 1. Soil structure obtained from H/V empirical transfer function.

The Nicaraguan national building code (RNC 07) classifies the soils in four groups according to shear waves velocities as follows:

TYPE I: Outcrop with Vs > 750 m/s,

TYPE II: Stiff soil with 360 < Vs <=750 m/s,

TYPE III: Moderately soft soil, with 180 <= Vs <= 360 m/s

TYPE IV: Very soft soil, with Vs < 180 m/s.

From the previous results, it appears that the engineering basement in Managua city is located to an approximate depth range of 13 to 37 m.

7. CONCLUSIONS

Based on the analysis carried out in this study we can summarize the most important findings in the following conclusions:

Two types of zonations are proposed in this study, one is based on fundamental periods of HVSR and the other based on amplitudes of HVSR. The fundamental period zonation was proposed by dividing the vertical to horizontal spectra in three categories 0.10 to 0.18 s; 0.18 – 0.22 s; 0.22 – 0.37 s.

Regarding relative amplifications two zones were identified, the first zone have amplifications that ranges between 1.7 to 3.0 times; the second zone show values in the range between 3.0 to 4.0 times; and the third zone show values from 4.0 to 5.2 times. Managua soils show moderate soft soil in the shallow layes (depth < 10 m) and then the rigidity increases in such a way that can be considered as stiff soil according to the Nicaraguan building code soil classification. Despite the fact that the fundamental periods changes as we moved towards the southern part of the study area, it is clear that from the fundamental periods of HVSR that the soils of the old downtown area of Managua city shows a similar lithological composition.. The difference of fundamental periods observed in the city can be interpreted as a consequence of the thickness of the shallow material rather than as a change in the material.

As exposed by the fundamental period results, Managua shallow soil response coincide s with the vibration mode of one, two and three stories houses. Thus it is important for the authorities to take into account the information found here to better design the dwellings and buildings in the future.

Finally, two main high impedance interfaces seem to be detected by the H/V ratio, the first high impedance show its larger amplitude around 0.22 s and seems to occurs to a depth of 13 m whereas the second impedance interface show its larger amplitude in ?? seconds and seems to occur to a depth of ?? m. This results are very important as in the city most of the constructions are one story houses and medium rise buildings. Moreover to date, the highest building in Nicaragua, formerly known as the BAMER building have 16 stories which is in the range of the second mode detected here. Thus is very important to assess in further studies whether this building is affected by this second mode or not.

The results seems to be in agreement with the geology and geotechnical information of the study area. From the results, it appears that two interfaces of high impedance was detected in the spectra of the records. Based on these results it appears that the engineering basement in the old downtown area of Managua is located to an approximate depth interval of ?? to ?? m.

8. ACKNOWLEDGEMENTS

The first author would like to thank to his former students Julio César Muñoz, Yoel Morales and Stanly Pérez for their help in field measurements.

The instruments used for data acquisition were donated by the Japan International Cooperation Agency (JICA) and facilitated by the Instituto de Geologia y Geofisica (IGG) from National Autonomous University of Nicaragua (UNAN).

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Autor:

Edwin Nadir Castrillo

National Autonomous University of Nicaragua (UNAN-Managua).

International Institute of Seismology and Earthquake Engineering (IISEE), Building Research Institute (BRI), Japan.

Lund University, Department of Geology, Sweden.

Eto Kiminobub

Tokyo Soil Research, inc.

Toshiaki Yokoic

International Institute of Seismology and Earthquake Engineering (IISEE), Building Research Institute (BRI), Japan.

Peter Ulriksend

Lund University, Department of Geology, Sweden.