Vulnerable areas determination based on seismic response at Chile center region

. Chile is one of most seismic countries in the world. In the past 100 years, more than 8 major seismic events occurred in Chile, with the coastal area of central Chile being one of the most affected Since 2007, nearly 2,306 earthquakes were recorded in this area with magnitudes varying between 1.9 and 6.9 in the Ritcher and Mercalli scales. The earthquakes ’ impact on the engineering structures depends on several factors, such as: the depth of the wave, wave type, geomorphological aspects of the rock, and soil properties. Chile central region of is a very important economics area, but at the same time, very vulnerable. The cities existing on this area, are installed in coastal plains, limited by Coastal Mountains and Pacific Ocean. Most of the seismic activity episodes recorded by Nacional Seismological center (CSN) is on the ocean. The present research aims to identify the regions vulnerable to significant seismic activities based on the average shear wave velocity on the upper 30m (Vs30) and using Geographic Information Systems. The Vs30 values were measured by the United Stated Geological Service (USGS). This analysis is based on the topography, geological units classified by Chilean norms NCh433, NCh2369, and law decree 61/ 2011. The analysis indicated that last epicenter city, Cobquecura, is the low vulnerable area, considering high Vs30 values and low number of habitants. Cities as Concepción, Talcachuano, San Pedro de la Paz, Coronel y Hualpén are more vulnerable because low Vs30 values, and high population. This factor can be associated to river mouths and floodplains presents in this area.


Introdution
Chile is one of most seismic countries in the world, registering in 2017 a total of 8,094 earthquakes. The number of registered earthquakes increased by 26% from 2016 to 2017, in which there were 6,437 tremors (for the entire territory), according to a report by the National Seismological Center (Centro Sismológico Nacional -CSN) [1]. In the past 100 years, more than ten events of magnitude 8 or higher occurred in Chile [2]. An 8.8 Mw earthquake with epicenter at Cobquecura in 2010 caused infrastructure damage mainly in coastal zone at central Chile [3] near the earthquake epicenter. In this area, the historical data recorded from 2006 to 2019 indicate increase in the number of daily small earthquakes (short period and magnitude less than 6.9 Mw) in the last thirteen years. After the 2010 event, the number of daily small earthquakes increased approximately 600 times in comparison to previous years. Ground motions amplified by surficial materials can exacerbate the situation, often making the difference between minor and major damage [4] The seismic impact to the surface can be measured by the secondary wave velocity of propagation on the first 30 meters of surface. These values are related with capacity of wave propagation, expressed by material. The Vs30 is a parameter used to determine the ground susceptibility to loss of strength during an earthquake.
This research analyzed the Vs30 values of eleven cities, based on USGS data which were acquired by tectonic measurements. Mean, Maximum, Minimum and Majority Vs30 were calculated. The Vs30 values were mapped with number of seismic events, slopes, eroding zones, and population. Finally, all parameters were evaluated to determine municipalities more vulnerable to damage during or after a seismic event.
Chile lacks the inclusion of disaster risk management tools in its public policies at the community level [5]. Only the municipality of Talcahuano (one of the cities evaluated in this work) is recognized internationally for considering natural disasters problems [6,7].

Seismic wave propagation
The release of tectonic energy occurs from the movement of tectonic plates, which generate seismic waves causing ground tremble. The waves radiate in all directions from the hypocenter.
The waves are classified into two types: primary wave (P-wave) and secondary wave (S-wave). P motion travels fastest in materials, so the P-wave is the first-arriving energy on a seismogram, and generally with smaller amplitude and higher frequency than the S and surface waves, respectively. S-waves travel slower than P-waves in a solid and, therefore, arrive after the P-wave. Usually the S wave has greater amplitude than the P and feels stronger than P-waves [8].
The wave propagation intensity is felt depending on the rock geomorphological aspects. The Vs30 can be an indicator as a "proxy" to investigate seismic deformation. The Vs30 is average shear wave in the 30 m thick surface layer of the soil and/or rock. It is considered a trace of seismic behavior of foundation units floors before the seismic waves action generated by earthquake [9]. Chilean standards for building designs NCh433, NCh2609 and Decree 61/2011 have taken the VS30 as the design parameter for the seismic design of civil structures [10].

Chilean building code NCh 433/ NCh 2639
Chilean Seismic Standards NCh 433 (Seismic Design of Buildings) and NCh 2369 (Seismic Design of Industrial Structures and Facilities) were developed to reduce vulnerability of structures at seismic events. The first was created in 1996 and modified in 2008. The second was created in 2003 and is considered a more complete version than the NCh 433 because it incorporates the seismic design of industrial facilities. The NCh 433 is based on seismic events that occurred particularly in the years between 1960 and 1985 [11,12].
These seismic regulations provide guidelines for civil construction, through the characterization by seismic parameters. The Chilean territory is classified into three seismic zones, being study area is in Zone 3 (Boundary of Biobio and Ñuble regions, near to ocean). These zones represent characterized areas by geology, soils and geotechnical parameters.
The codes shall be applied in conjunction with the specific design standards for each construction material to achieve structures that: a) are able to resist seismic movements of moderate intensity without significant damage; b) limit damage to non-structural elements during medium intensity earthquakes; c) avoid collapse during earthquakes of exceptionally severe intensity. In addition to the type of construction, material aspects, soil type, topography and geomorphology are considered, which will dictate how the foundations of these structures will be impacted. That is, the seismic wave has different perceptions and impacts depending on the medium in which they propagate.

Study area
The coast of the Biobio and Ñuble regions are one of the most important areas of Chile, due to their economic activities, location of ports and forestry industries. These regions are in the macro central region of Chile, in the "Gran Concepcion" area. The "Gran Concepcion area is the second most populated region of Chile [13]. The studied areas included the municipalities of Cobquecura, Tomé , Penco, Talcahuano, Hualpé n, Concepción, San Pedro de la Paz, Coronel, Lota, Arauco and Lebu ( Figure  1). All studied areas are located on the coastal zone, where various disasters happened throughout history, mostly related to seismic activity.

Antecedents
The municipalities located on the coastal edge arose without urban planning, affecting natural systems [14], occupying the coastal plain, product of erosion of the Mountain range of the Coast. Therefore, part of urban settlements is in slope areas. According to Figure 2, the coastal edge is occupied by the cities mentioned above, which have great erosive potential, where most of the area of these cities are classified based on geomorphology aspects as erosion potential [15]: very severe, severe and moderate. For instance, most of the Cobquecura area, the edge of Tomé , Talcahuano and Hualpé n stand out for "very severe" erosive potentials (black color in map). Most of the area within the studied cities are classified as "severe" erosion potential (red color).

Fig. 2.
Roding on the Biobí o coast; source: own elaboration, based on Biobí o regional government data.

Historical seismic situation
In addition to the erosive potential, the studied municipalities have as a common factor to be located at a seismically active area. In 2010 there was a historical earthquake of 8.8 Mw, which affected more than 13 million people, which represents approximately 80% of the country's population [16]. The municipalities located in the vicinity of the epicenter, presented greater seismic impact than other regions, generating secondary events such as landslides and tsunamis [17]. Figure 3 shows the earthquakes with the greatest destructive capacities throughout history, note that they are located near to studied municipalities. Within the history, the most recent earthquakes occurred in the years 2010 and 2011 due to the damages generated, the Ministry of Housing and Urban Development published Decree of Law No. 61, in 2011. This Decree considers the use of the Vs30 [18]. As of 2006, Chile begins to record earthquakes of short duration and magnitudes since 2.0. Historical data will record 2,306 daily earthquakes, for this area in the last 12 years, measured on the Ritcher and Mercalli scales (taking as a reference a study area).  Figure 4 shows that the most active municipalities are Concepción and Cobquecura with 25% and 22% of the measured seismic events, respectively, in relation to the other municipalities. This data considers the magnitudes between 2.0 and 6.9 registered in the period (Ricther and Mercalli Scales). It should be noted that the location of the seismic points was taken as a reference, some municipalities shown are not part of the studied area but belong to the region.
After the 2010 earthquake, it can be observed through Figure 5 that this coast was activated, registering for 2010 about 667 seisms accumulated, considering the study area as reference.

Geomorphology
The geological units of Chile are mainly composed of igneous, sedimentary, metamorphic and unconsolidated deposits of various origins, covering ages from the Ordovician to the Holocene. The islands of Chile are composed of volcanic rocks with specific sectors covered by unconsolidated deposits that form units of smallthickness soil. In general terms, the geology of continental Chile has been, and is, conditioned by location in a convergent margin of tectonic plates [19].The coastal boarder of the cities is located is structured in fault tectonics; the relief presents classic units of Central Chile. From West to East they follow sedimentary platforms and coastal plains. Coastal mountain range composed of granite and intensely weathered Paleozoic slates; the central depression in whose sine cones of Piedmont mainly fluviovolcanic and Andean, solid and volcanic mountain range are constituted [20].
The slopes of sun rocks ("solanas"), rocky, steep and heavily trafficked, have a high potential for erodibility. Particularly, they are threatened by the erosion of old terraces covered with trumao ("volcanics chilean soil, high porosity -Andosol"), raincoats that, under conditions of agricultural activities, increase this process on moderate slopes. [21]. Most of the urban centers located on coast are settled on coastal plain caused by sedimentation of the Cordillera de la Costa (Coastal Mountains).

Seismic response
The surface seismic response of a soil deposit and the seismic solicitation that develops on a structure located on a deposit are mainly dependent on: the stiffness at low deformations of the upper extracts of the foundation grounds, the fundamental period of the deposits of the soil, the level of damping developed by the different constituent soils of the land, and of the seismic disturbance (seismic activity) [22].
For the study area, the Vs30 raster was download from United States Seismological Service (USGS) web page, in a regular grid of 90-second arc cells, WGS 84 projection system The data relates Vs30 (m / s) and slope (m / m) for each Vs30 measurement point for data in active tectonic areas [23].
It was calculated the statistics per zone (Mean, Maximun, Majority and Minimun) for each city, using software Arcgis 10.1 version. After that, the soil/lithology composition was classified based on Vs30 values, proposed by Decree 61/2011.
Each information contained in the pixels was analyzed at the municipal level. This information gives: a) The capacity of wave propagation at zonal coast, represented by Vs30 map values; b) the soil composition based on statistical analysis per cities.

Results
The results indicate that, in general, the steepest slopes of the studied area have higher Vs30 values and lower Vs30 values may be encountered in the flat areas, as shown in Figure  6. According to the decree No. 61/2011encountered in both NCh433 and NCh2609 building codes, the soil profile is classified based on the soil shear wave velocity (Table 1). A first experiment overview relating the soil shear wave velocity and slope inclination is shown in Figure 6. Figure 6 indicates that for the Chilean topography, the Slope's inclination and Vs30 are directly related. Regions with Vs30 values above approximately 600m/s overlaps the regions with steep slopes, in rocky outcrops. In the other hand, in flat areas near river mouths and floodplains, the measured Vs30 values were below 180m/s. River mouths and floodplain areas generally comprises saturated loose to poorly compacted sands and soft to firm fine-grained soils. During a seismic event, these soils are susceptible to liquefaction and strength softening, respectively [24].   Table 2 present the Vs30 values mean, maximum, majority (most frequencies) and minimum values measured at different municipalities. The ground of each municipality was classified based on the values presented in Table 1.  Vulnerability exists when there is a possibility of physical damage, material and exposed elements [25]. Therefore, to determine the most critical areas, the number of habitants affected by the possible disaster should be analyzed collectively with historical data, measured Vs30, and geotechnical and geological aspects. According to Table 3  The priority areas are Concepcion, Talcahuano, Hualpen, San Pedro de la Paz and Coronel, based on ground condition (Classified due Vs30 values), seismic historical data and number of habitants.
Although Cobquecura is the municipality with the highest amount of daily seismic activity and very severe erosive potential, the generally high Vs30 and low number of habitants suggest that this municipality has a low vulnerability to seismic damage.
The Cobquecura, Tomé, Talcahuano and Hualpen has severe erosive potential and 22% of the daily earthquakes that affect Concepcion also reach Hualpé n, Talcahuano and San Pedro de la Paz due to the proximity between the three municipalities. These municipalities should consider disaster risk management in their public policies, since they meet the minimum requirements for disasters occurrences, may be a consequence of seismic activity, or they may cause other secondary disasters such as landslides, tsunamis and floods.

Conclusions
The Vs30 is the average shear wave velocity of the material encountered below ground level. This parameter combined with historical earthquake data and the number of habitants in a region may be considered to determine the vulnerable zones to seismic damage. Evaluating these parameters in a small scale allows the determination of priority zones and may help on the municipality's decision-making process.
The most vulnerable municipalities are Concepción, Talcahuano, San Pedro de la Paz, Coronel and Hualpen, considering the majority and minimum Vs30 values and number of habitants.
Zones with low Vs30 values are more susceptible to loss of strength during an earthquake. The soil loss of strength, also called soil liquefaction for cohesionless soils, may result in severe settlement of structures, landslides, and other hazards. The minimum Vs30 values designate which areas had a concerning seismic response. This factor may occur due to proximity of this areas to water bodies as well as bordering and mouth of the rivers and floodplains. A next step of this research would be to understand the dynamics of the earthquake phenomenon, the analysis of precipitation pattern, and in situ experiments for the determination of the soil and rock strength parameters, to demonstrate more precisely the influence of the earthquake phenomenon on the landslides occurring on the coastal zones of Chile.