1. INTRODUCTION
Lima, the capital of Peru is a coastal city in western South America with a population of about 10 million [1]. The city lies in the Pacific Ring of Fire, which is considered as one of the most seismically active zones in the world. The most destructive earthquakes in Peruvian history are a consequence of the interaction of two tectonic plates, the Nazca plate that subducts at a velocity of roughly 70 mm/year beneath the South American Plate, according to GPS measurements ( [2], [3]). This interaction has determined the geology and geomorphology of this region. Most parts of Metropolitan Lima lie in alluvial deposits that were originated by the Chillon, Rimac and Lurin rivers, commonly known as Lima conglomerate. This type of material is a heterogeneous mixture of cobbles and gravels in a silty-sandy matrix with intercalations of silty-clayey or sandy lenses. Rocky outcrops of either intrusive, volcanic, or sedimentary nature can also be observed in the outskirts of the city [4].
Several earthquakes have occurred along the Peruvian margin, of which the largest took place in 1746 with an estimated moment magnitude (Mw) between 8.5 and 9.0 [5]. Other historical seismic events are those that occurred in 1966, 1970 and 1974, which drastically damaged Lima and the Andean regions. However, very few records are available for these events due to scarce strong motion instrumentation at that time. To overcome this situation, continuous seismic implementation in the city of Lima has been performed in the very recent years. In that sense, only five strong motion stations in Lima recorded the 2007 Mw 7.9 Pisco earthquake, whereas 55 stations recorded the 2021 Mw 6.0 Mala earthquake, in a similar situation to the 2019 Mw 8.0 Lagunas event.
The current number of seismic instrumentations in Lima city provides an adequate opportunity to improve soil characterization. The objective of this study is to determine the site dominant frequency (
2. BACKGROUND
Soil amplification is an important factor in determining the level of damage caused by earthquakes. Usually, the time-averaged shear-wave velocity in the first 30 m (VS30) is the parameter used for explaining site effects conditions. Despite the inherent limitations, it has been widely used as a basis for soil characterization in building codes worldwide, such as the ASCE/SEI 7-16 [6], the EUROCODE-8 [7], the E030-2018 [8]. To overcome this matter, in recent decades,
The
In the last decade, the Japan-Peru Center for Earthquake Engineering Research and Disaster Mitigation (CISMID) has been developing a microzoning map for the Lima city which considered geotechnical and dynamic soil information obtained from various types of tests, such as soil pits, boreholes, multichannel analysis of surface waves (MASW) and single-point and microtremor array measurements. Despite the limitations in terms of the maximum exploration depth and the number of tests available, it is the most comprehensive study available, and it is continuously being updated with new information. According to this microzoning study, Lima city has been divided into five zones (Fig. 1a) [17]. Most areas of Lima lie in Zone I which are characterized by dense to very dense gravels or sands, hard silts or clays and rocky formations, with dominant periods shorter than 0.30 s (i.e.,
3. METHODOLOGY
3.1 NETWORK AND DATABASE
Currently, there are three strong motion networks in Lima city, which are operated by the Geophysical Institute of Peru (IGP), by the Japan-Peru Center for Earthquake Engineering Research and Disaster Mitigation (henceforth REDACIS) and by the Graduate School of the Faculty of Civil Engineering in agreement with the Peruvian Society of Engineers (UPG-CIP). For this study, we have only considered the latter two. The REDACIS network comprises seismic stations installed in schools, hospitals, municipalities, and universities throughout Metropolitan Lima. The instruments have been renewed from analog to digital equipment since 2001. In 2011, and within the framework of the Science and Technology Research Partnership for Sustainable Development (SATREPS) project [18] CV-374A2 Tokyo Sokushin sensors were installed. Since 2017, 130-SMA REFTEK sensors were also deployed and currently represent the largest number of the operating equipment. On the other hand, the UPG-CIP network, which started its implementation in 2013, operates sensors installed nationwide, mainly in universities and local offices of the Peruvian Society of Engineers. The implemented equipment is either 130- SMA or 130-SMHR REFTEK sensors.
It is important to mention that both networks, in agreement with the National Training Service for the Construction Industry (SENCICO), also operate one Basalt Kinemetrics and two Sigma-TS4G-ACC Gaiacode sensors. Finally, the sampling frequency of all the three-component accelerometers is set to 200 Hz.
We compiled all available events recorded on REDACIS and UPG-CIP networks from 2011 to 2021. Fig. 1 shows the geographic distribution of the strong motion stations and the epicenters of earthquakes used in this study, whereas coordinates of the stations are given in TABLE I. We identified a total of 51 stations, from which we processed and analyzed 2075 records from 292 earthquakes in total. We used records having a moment magnitude (Mw) greater than 3.5. The number of events for each station varies from 1 to 134. Fig. 2 shows the magnitude-depth, magnitude-epicentral distance (Depi), and peak ground acceleration (PGA)-magnitude distribution of the database, in addition to the histogram of the number of records at the stations. Most of the observed data are records of small-to-moderate earthquake magnitudes (3.5≤Mw≤8.0) at regional distances (3 km ≤ Depi ≤ 1000 km). The largest magnitude considered in this group is the 2019 Mw 8.0 Lagunas earthquake, a far-field event with a Depi value of about 727 km. However, the maximum PGA (230 gal) was recorded for the 2021 Mw 6.0 Mala earthquake, with a focal depth of 32 km and located 85 km from the center Lima city. It is important to mention that, due to its historical importance, records from the 2007 Mw 7.9 Pisco earthquake are also included. Finally, a summary of the data used in this study is given in TABLE II.
Note: The superscripts R and U denote stations belonging to the REDACIS and UPG-CIP networks, respectively. Stations operated in agreement with SENCICO are represented by subscript S
* Corresponding zone according to the current microzoning map.
Network | |||
---|---|---|---|
REDACIS | UPG-CIP | ||
No. of events | 219 | 132 | |
No. of records | 1695 | 380 | |
No. of recording stations | 42 | 9 | |
Date recorded | January 2011 - November 2021 | November 2016 - November 2021 | |
Sensors | 130-SMA REFTEK, ETNA Kinemetrics, CV-374A2 Tokyo Sokushin, Basalt Kinemetrics* , Sigma-TS4G-ACC Gaiacode* | 130-SMA REFTEK, 130-SMHR REFTEK, Basalt Kinemetrics*, Sigma-TS4G-ACC Gaiacode* | |
Sampling frequency (Hz) | 200 | 200 | |
Recording institution | Japan-Peru Center of Earthquake Engineering Research and Disaster Mitigation | Graduate School of the Faculty of Civil Engineering - Peruvian Society of Engineers | |
Magnitude range (Mw) | 3.5 - 8.0 | 3.5 - 8.0 | |
Epicentral distance range, Respect to Lima City (km) | 3 - 1093 | 4-1087 | |
Depth range (km) | 11-175 | 12 - 141 |
* in agreement with SENCICO
3.2 IDENTIFICATION OF THE SITE DOMINANT FREQUENCY
We followed the same methodology introduced in Hassani & Atkinson [19]. In brief, to calculate H/V, first we computed the pseudo-response spectral acceleration (PSA, 5% damped) of each acceleration component using Newmark’s direct integration method over 501 structural frequencies in a bandwidth ranging from 0.2 Hz to 20 Hz. Thus, we calculated the mean of the two horizontal components (in log10 units). Then, H/V is obtained by dividing the geometric mean of the horizontal spectra by the amplitude spectrum of the vertical component, as follows.
where
in which
In equation 3,
To evaluate the sensitivity of
4. RESULTS
Despite the different values of dominant frequencies and their respective amplitudes, H/V response spectral ratios can be grouped based on their overall shape. For the subset that considered all available events (
For the ease of visualization, results of the spatial distribution of
Fig. 4 shows the northern part of Metropolitan Lima where, for example, LIM020 station shows a clear peak at 5.6 Hz, a value that agrees with reports that indicate that the area nearby is formed by eolian sands overlying alluvial gravel deposits very close to an andesitic volcanic formation ( [20], [21]). On the other hand, stations such as LIM022, LIM023, CAL012 and CAL006 show lower dominant frequencies, in the vicinity of 2 Hz, that correspond to thicker eolian formations or clayey deposits (LIM022, LIM023) and the effect of softer materials close to the coast (CAL006) or swampy soils nearby (CAL012).
Two dominant frequencies were identified in most of the stations located in the south-western area of Callao province (Fig. 5) where it is known that soil deposits consist of thick layers of soft clay and Vs30 values of about 300 m/s [17]. Hence, the observed two peaks represent both the influence of surficial soft soils (higher
Central and south-eastern areas of Lima city are shown in Fig. 6. Considerable variation of the values of
A comparison between the calculated values of
Fig. 7 shows the H/V response spectral ratios for CAL001 station grouped according to the criteria described in Table III. While H/V ratios have peaks with reasonably similar
In most cases, variability of
Note: The superscripts R and U denote stations belonging to the REDACIS and UPG-CIP networks, respectively. Stations operated in agreement with SENCICO are represented by subscript S. The values in parentheses are a second
CONCLUSIONS
Site dominant frequencies (
Two-peaked spectral shapes, with one of the dominant frequencies in the vicinity of 1 Hz, were observed for the coastal areas of Callao province, where the underlying soil consist of clayey/sandy deposits and regions with an important impedance contrast in the deeper part of the substructure. These values increment towards the center of the city coinciding with surficial gravel deposits (Lima conglomerate). The
Record selection criteria were adopted including minimum values of epicentral distance, moment magnitude and PGA. The average H/V response spectral ratios do not change significantly. However, amplitudes vary, changing the order of importance of the low and high dominant frequencies for some stations. This is mainly observed in far-field records (Depi>100km) and Mw>5.0 events.
Finally, results obtained in this study can be used for future updates of the microzoning map of Metropolitan Lima and for the estimation of reliable shear-wave velocity profiles by means of joint inversion procedures. Further analyses will consider the comparison of the estimated H/V response spectral ratio with those obtained from ambient vibration records and the compilation of information from the IGP seismic network.