Estimation of a Source Model and Strong Motion Simulation
for Tacna City, South Peru

Paper:

Estimation of a Source Model and Strong Motion Simulation
for Tacna City, South Peru
Nelson Pulido∗1 , Shoichi Nakai∗2 , Hiroaki Yamanaka∗3 , Diana Calderon∗4 ,Zenon Aguilar∗4 , and Toru Sekiguchi∗2∗1 National Research Institute for Earth Science and Disaster Prevention

3-1 Tennodai, Tsukuba, Ibaraki 305-0006, Japan
E-mail: nelson@bosai.go.jp∗2 Chiba University, Chiba, Japan∗3 Tokyo Institute of Technology, Kanagawa, Japan∗4 Universidad Nacional de Ingenierı́a, Perú, Lima, Perú
[Received August 18, 2014; accepted September 8, 2014]

We estimate several scenarios for source models of
megathrust earthquakes likely to occur on the NazcaSouth American plates interface in southern Peru.
To do so, we use a methodology for estimating theslip distribution of megathrust earthquakes based on
an interseismic coupling (ISC) distribution model insubduction margins and on information about historical earthquakes. The slip model obtained fromgeodetic data represents large-scale features of asperities within the megathrust that are appropriate forsimulating long-period waves and tsunami modelling.
To simulate broadband frequency strong ground motions, we add small scale heterogeneities to the geodetic slip by using spatially correlated random noise distributions. Using these slip models and assuming several hypocenter locations, we calculate a set of strong
ground motions for southern Peru and incorporate site
effects obtained from microtremors array surveys in
Tacna, the southernmost city in Peru.
Keywords: strong motion, source process, 1868 earthquake, seismic hazard, South Peru

1. Introduction
To improve seismic hazard assessment of rare
megaearthquakes such as the Sumatra 2004, Chile 2010,
and Japan 2011 earthquakes, it may be useful to incorporate information on crustal deformation at plate margins. The development of global positioning systems(GPS) and satellite radar interferometry (InSAR) havemade it possible to measure strain buildup associatedwith plate coupling in many earthquake-prone regions
worldwide [1]. Recent studies suggest that subducting
plates are either locked or creeping aseismically and thata patchwork of locked or coupled regions during the interseismic period may be related to asperities in earthquakes [2–4]. We apply a methodology for characterizing
source models of megathrust earthquakes based on the interseismic coupling (ISC) spatial distribution of subducJournal of Disaster Research Vol.9 No.6, 2014

Fig. 1. Slip deficit in southern Peru and northern Chile
obtained from geodetic measurements and historical earthquakes along the Nazca subduction zone in Peru [1]. Black
arrows are velocity vectors for the Nazca Plate at the margin convergence with the South American Plate [1]. The
dashed black line encloses the source area of our scenario
earthquakes in southern Peru. The continuous white line encloses the source area of the 2001 Arequipa earthquake, Peru
(Mw 8.4) [1]. The continuous black line encloses the sourcearea of the 2014 Iquique earthquake, Chile (Mw 8.2) [8]. The
white dashed line encloses the source area of the 1877 earthquake in northern Chile. The location of the HMBCX strong
motion station in Chile is shown by a triangle.

tion zones and information on historical earthquakes [5]to southern Peru, which is a very active seismic region
characterized by the fast subduction of the Nazca plate beneath the South American plate (Fig. 1). Seismic activity
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Pulido, N. et al.

Fig. 2. Broadband source models constructed by adding short wavelength slip distributions obtained from a Von
Karman power spectral density (PSD) function with random phases to the slip model inferred from geodetic data forsouthern Peru. The panel corresponding to slip No.1 shows the 9 starting points of rupture used for our scenarios.

in Peru is divided by the Nazca ridge (15◦ S) into two regions, north or south of the ridge. The ridge is coincident
with a region of low coupling where the Nazca plate isbelieved to subduct aseismically [1, 2] and therefore possibly act as a geomorphological barrier to seismic rupture
propagation. Southern Peru has been repeatedly affectedby large earthquakes such as the great 1868 earthquake,
which had a moment magnitude of ∼8.8 [6]. In 2001, theArequipa earthquake, which measured Mw 8.4, released alarge fraction of slip accumulated in the region since the1868 event (Fig. 1). We use this information combinedwith an ISC model of central and southern Peru to construct possible scenarios for the next megathrust earthquake that would likely affect Tacna, the southernmost
city in Peru.

2. Source Models for Central and SouthernPeru
2.1. ISC Model and Scenario Earthquakes
An ISC model in Peru was obtained by inverting velocities from GPS campaigns at 87 sites surveyed from1993 to 2003 [1]. The model for ISC assumes that con926

vergence between the Nazca and South American plates isdominantly accommodated along the megathrust, including 4mm/year of back-arc shortening absorbed in the subAndean fold and thrust belt regions [1]. ISC results indicate the existence of two strongly coupled regions – thefirst near the source of the Arequipa earthquake (white
contour line in Fig. 1) and the second near Tacna. Fig. 1
shows the slip deficit rate, obtained by multiplying the
ISC distribution by the plate convergence rate proposedin [7]. In southern Peru, we assume that the interseismic process has stood since the 1868 earthquake. Removing the slip released by the Arequipa earthquake in 2001
(Mw 8.4) results in an earthquake scenario with a moment
magnitude of 8.5. We place a southern limit for our scenario fault model at the bend of the Nazca plate, near theborder region between Peru and Chile, the source area enclosed by the black dashed line in Fig. 1. A large subduction earthquake occurred in the northern Chile regionin April 2014 (Mw 8.2) [8], and this probably released alarge fraction of the slip accumulating in north Chile sincethe 1877 earthquake (Fig. 1), providing an additional constraint for a possible fault rupture further south of thebend.

Journal of Disaster Research Vol.9 No.6, 2014

Estimation of a Source Model and Strong Motion Simulation
for Tacna City, South Peru

Table 1. Parameters for strong motion simulation.

3. Strong Motion Simulation Method and Parameters

Fig. 3. a) Slip, b) Stress drop, c) Rise time distributions
across the fault plane for the slip scenario in southern Peru
(slip No.4 in Fig. 2). The 9 starting points of rupture areshown by white stars.

2.2. Slip Scenario Earthquakes
The fault dimensions of our source models for southern Peru are 280 km (along strike) by 200 km (along dip)
(Fig. 1). This model features a smooth distribution of asperities partly due to the large grid spacing used to estimate the ISC model (20 km) (Fig. 1). This model isappropriate for simulating long-period seismic waves andfor tsunami modelling. For simulating a strong broadband ground motion, we introduce small-scale complexities into the source slip to simulate high-frequency groundmotion, for which we construct a “broadband” source
model in which large-scale features of the slip are obtained from the ISC model and short wavelength slip
distribution is obtained from spatially correlated random
noise [5]. To calculate a set of broadband slip scenarios
based on the ISC to be used for strong motion simulation,
we calculated a set of 12 short wavelength slip models andadded them to the long wavelength slip. The 12 broadband slip models used for strong motion simulation are
shown in Fig. 2. Slip distributions have been obtained fora subfault size of 10 km.
Journal of Disaster Research Vol.9 No.6, 2014

To perform strong motion simulation, we used a hybrid
methodology [5, 9] that combines the deterministic simulation of ground motion at low frequencies with semistochastic simulation at high frequencies and assume a flatlayered 1D velocity structure. Broadband simulation forthe present study is performed in the 0.05 Hz–30 Hz frequency range. The methodology has been extensively
tested and validated in previous studies [9–14]. To calculate strong motion simulation for the Tacna region, weused the 12 broadband slip source models obtained as detailed in the previous section and assumed 9 different locations for starting points of the rupture (white stars in
Fig. 3a), yielding 108 fault rupture scenarios. Hypocenter locations were determined based on [15], indicatingthat earthquakes ruptures tend to nucleate close to large
slip regions. The main parameters for simulation are
slip, stress drop, and rise time distribution across the fault
plane and rupture velocity (Fig. 3 and Table 1).

4. Velocity Models and Strong Motion Simulations in Tacna
In Tacna, we calculated the EW, NS, and UD components of strong ground motions at the 8 points shownin Fig. 5 for a seismic bedrock condition of Vs =2790 m/s. Crustal velocity models for these calculations (Table 2) were obtained by combining results from
P and S wave seismic tomography from a regional deployment of 18 seismic stations [16] and P-wave tomography and receiver function analysis from a linear array
deployment of 50 broadband seismic stations in southern
Peru [17]. We calculated accelerograms at the surface byperforming convolution between seismic bedrock waveforms with soil transfer functions at the 8 points in Fig. 5.
Deep soil velocity models up to 1 km deep were obtained927

Pulido, N. et al.

Fig. 4. a) Velocity models from seismic bedrock (Vs = 2790 m/s) up to the surface at 8 sites in Tacna obtained
from array microtremor measurement [18]. See site locations in Fig. 5. b) Transfer functions for velocities in a).

Fig. 5. a) Average PGA distributions in Tacna for all scenarios in southern Peru. b) Same for PGVs. Values
incorporate shallow site effects.

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Estimation of a Source Model and Strong Motion Simulation
for Tacna City, South Peru

Fig. 6. a) Simulated accelerogram in Tacna (UNJBG station) for a scenario earthquake in southern Peru
(Slip No.4, hypocenter 1 in Fig. 3) (black line). The red line is the observed accelerogram of the April 1,
2014, Iquique earthquake, North Chile (Mw 8.2) at the HMBCX strong motion station in Chile. The blue lineis the observed accelerogram of the Iquique earthquake in Tacna (UNJBG station). b) Velocity waveformsof records in a). Acceleration and velocity response spectra for all waveforms are shown in c) and d) using
the same color code.

Table 2. Velocity model of southern Peru.

from microtremor array measurement at TAC and UNJB
sites [18]. Shallow soil velocity models up to 100 m wereobtained from array measurement by CISMID at six additional sites (A01 to A06) (Figs. 4 and 5). Our simulated
average PGA and PGV values at these sites for all scenarios are 160 ∼ 280 cm/s2 and 20 ∼ 24 cm/s. These values
are approximately 3 times smaller than simulated valuesin Lima for an earthquake scenario with a moment magnitude of 8.9 [5], mainly due to the greater distance that
Tacna is from the source area than Lima and the smaller
Journal of Disaster Research Vol.9 No.6, 2014

magnitude of the southern Peru scenario compared to thescenario in Central Peru [5]. Fig. 6 (black line) shows
the simulated EW accelerogram in Tacna at the UNJBG
strong motion station location (Fig. 5) for the scenario
earthquake in southern Peru (Slip No.4, Hypocenter 1 inFig. 2). The red line is the observed accelerogram of theApril 1, 2014, Iquique earthquake in north Chile (Mw 8.2),
at the HMBCX strong motion station (Fig. 1). The blueline is the observed accelerogram of the Iquique earthquake in Tacna (UNJBG station). Note that the amplitudeof simulated strong motion of our scenario for southern
Peru is comparable to observed values for the Iquique
earthquake at HMBCX station for similar slip centroid
distances of 130 km vs 100 km and magnitudes Mw 8.5
vs Mw 8.2 (Fig. 6).
Although our selection of the number of broadband
slips and rupture models is arbitrary (108), we consider
that these scenarios generate a wide range of ground motion for the megaearthquake likely to occur in southern
Peru as discussed in Section 2.1. We cannot rule out thepossibility that more critical scenarios in ground shaking
for Tacna could be obtained from a 1868 type earthquakeor other types of tectonic sources such as active faults, butthat is beyond the scope of this study.

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5. Conclusions
We applied a methodology for estimating slip scenarios for megathrust earthquakes combining models of ISC
distribution in subduction margins obtained from geodetic
data, information on historical earthquakes, and spatiallycorrelated random noise distribution representing small
scale heterogeneities of slip in the southern Peru region.
Our results for southern Peru suggest an earthquake scenario with a moment magnitude of Mw = 8.5, a source
rupture area of approximately 280 km (along strike) and
200 km (along dip). Our strong motion simulation resultsfor Tacna show values of PGA and PGV of approximately
240 cm/s2 and 22 cm/s, which are much smaller thanstrong motion simulation values for Lima for a scenario
earthquake in central Peru (Mw 8.9) [5]. This difference
can be explained because of the smaller magnitudes of the
scenarios for Tacna as compared to scenarios for Lima, as
well as the larger fault-distance for Tacna as compared toLima. Amplitudes of the simulated strong motion of ourscenario for southern Peru (Mw 8.5) are comparable to values observed for the 2014 Iquique earthquake (Mw 8.2) fora similar source distance.
Acknowledgements
This study was supported by the SATREPS project Enhancementof Earthquake and Tsunami Mitigation Technology in Peru. We
thank Fernando Lazares, CISMID, Peru, and Chile University(IPOC network) for kindly providing strong motion records for
the Iquique earthquake in the UNJBG station in Tacna and the
HMBCX station in north Chile.

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Name:
Nelson E. Pulido H.

Affiliation:
Senior Researcher, National Research Institute for Earth Science and Disaster Prevention(NIED)[3] N. Moreno, M. Rosenau, and O. Oncken, “2010 Maule earthquake slip correlates with pre-seismic locking of Andean subduction zone,” Nature, 467, pp. 198-204, 2010.
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Sekiguchi, S. Nakai, and F. Yamazaki, “Scenario source models andstrong ground motion for future mega-earthquakes: Application to
Lima, Central Peru,” Bull. Seism. Soc. Am., 2014 (in press).
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pp. 551-576, 1990.
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Lauria et al., “The Nazca-South America Euler vector and its rateof change,” J. South Am. Earth Sci., Vol.16, No.2, pp. 125-131,
doi:10.1016/S0895-9811(1003)00028-00022, 2003.
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930

Address:
3-1, Tennodai, Tsukuba, Ibaraki 305-0006, Japan

Brief Career:
2012- Senior Researcher, Earthquake and Volcano Research Unit, National
Research Institute for Earth Science and Disaster Prevention (NIED),Japan

Selected Publications:• N. Pulido, Y. Yagi, H. Kumagai, and N. Nishimura, “Rupture process
and coseismic deformations of the February 2010 Maule earthquake,
Chile,” Earth, Planets and Space, Vol.63, No.8, pp. 955-959,
doi:10.5047/eps.2011.04.008, 2011.
• N. Pulido and L. Dalguer, “Estimation of the high-frequency radiation of
the 2000 Tottori (Japan) earthquake based on a dynamic model of fault
rupture: Application to the strong ground motion simulation,” Bull. Seism.
Soc. Am., Vol.99, No.4, pp. 2305-2322, doi: 10.1785/012008016, 2009.

Academic Societies & Scientific Organizations:• American Geophysical Union (AGU)• Seismological Society of America (SSA)• Seismological Society of Japan (SSJ)• Japan Association for Earthquake Engineering (JAEE)

Journal of Disaster Research Vol.9 No.6, 2014