A strong-motion database from the Peru–Chile subduction zone

2002

This paper shows that attenuation formula for peak ground acceleration (PGA) for Chile subduction zone, derived from a homogeneous database for thrust interplate and inslab of intermediate depth earthquakes recorded on ‘hard rock’ and ‘rock and hard soil’, give systematically higher values than universal formulas proposed for subduction zones. Also PGA Chilean values are higher than values for Mexico and Cascadia subduction zone values. Criterion of homogeneous database is defined in order to obtain PGA attenuation formulas with high correlation coefficients. Comparison of MMI attenuation formulas for Chile, Mexico and Cascadia subductions is also made. The main conclusion is not possible to obtain universal attenuation formula for PGA and MMI for subduction zones and attenuation formulas can be quite different for each American subduction zone. Formulas look to depend of the age of the converging tectonic plate, convergence velocity, stress drop, among other factors. PGA and MMI va...

Natural Hazards, 2014

The San Ramón Fault is an active west-vergent thrust fault system located along the eastern border of the city of Santiago, at the foot of the main Andes Cordillera. This is a kilometric crustal-scale structure recently recognized that represents a potential source for geological hazards. In this work, we provide new seismological evidences and strong ground-motion modeling from hypothetic kinematic rupture scenarios, to improve seismic hazard assessment in the Metropolitan area of Central Chile. Firstly, we focused on the study of crustal seismicity that we relate to brittle deformation associated with different seismogenic fringes in the main Andes in front of Santiago. We used a classical hypocentral location technique with an improved 1D crustal velocity model, to relocate crustal seismicity recorded between 2000 and 2011 by the National Seismological Service, University of Chile. This analysis includes waveform modeling of seismic events from local broadband stations deployed in the main Andean range, such as San José de Maipo, El Yeso, Las Melosas and Farellones. We selected events located near the stations, whose hypocenters were localized under the recording sites, with angles of incidence at the receiver\5°and S-P travel times\2 s. Our results evidence that seismic activity clustered around 10 km depth under San José de Maipo and Farellones stations. Because of their identical waveforms, such events are interpreted like repeating earthquakes or multiplets and therefore providing first evidence for seismic tectonic activity consistent with the crustal-scale structural model proposed for the San Ramón Fault system in the area (Armijo et al. in Tectonics 29(2):TC2007, 2010). We also analyzed the ground-motion variability generated by an M w 6.9 earthquake rupture scenario by using a kinematic fractal k -2 composite source model. The main goal was to model broadband strong ground motion in the near-fault region and to analyze the variability of ground-motion parameters computed at various receivers. Several kinematic rupture scenarios were computed by changing physical source parameters. The study focused on statistical analysis of horizontal peak ground acceleration (PGAH) and ground velocity (PGVH). We compared the numerically predicted ground-motion parameters with empirical ground-motion predictive relationships from Kanno et al. (Bull Seismol Soc Am 96:879-897, 2006). In general, the synthetic PGAH and PGVH are in good agreement with the ones empirically predicted at various source distances. However, the mean PGAH at intermediate and large distances attenuates faster than the empirical mean curve. The largest mean values for both, PGAH and PGVH, were observed near the SW corner within the area of the fault plane projected to the surface, which coincides rather well with published hanging-wall effects suggesting that ground motions are amplified there.

Bulletin of Earthquake Engineering, 2016

The Chilean subduction zone is one of the most active in the world. Six events of magnitude greater than M w ¼ 7:5 have occurred in the last 10 years, including the 2010

Geophysical Journal International, 2015

, a magnitude M w 8.1 interplate thrust earthquake ruptured a densely instrumented region of Iquique seismic gap in northern Chile. The abundant data sets near and around the rupture zone provide a unique opportunity to study the detailed source process of this megathrust earthquake. We retrieved the spatial and temporal distributions of slip during the main shock and one strong aftershock through a joint inversion of teleseismic records, GPS offsets and strong motion data. The main shock rupture initiated at a focal depth of about 25 km and propagated around the hypocentre. The peak slip amplitude in the model is ∼6.5 m, located in the southeast of the hypocentre. The major slip patch is located around the hypocentre, spanning ∼150 km along dip and ∼160 km along strike. The associated static stress drop is ∼3 MPa. Most of the seismic moment was released within 150 s. The total seismic moment of our preferred model is 1.72 × 10 21 N m, equivalent to M w 8.1. For the strong aftershock on 2014 April 3, the slip mainly occurred in a relatively compact area, and the major slip area surrounded the hypocentre with the peak amplitude of ∼2.5 m. There is a secondary slip patch located downdip from the hypocentre with the peak slip of ∼2.1 m. The total seismic moment is about 3.9 × 10 20 N m, equivalent to M w 7.7. Between the rupture areas of the main shock and the 2007 November 14 M w 7.7 Antofagasta, Chile earthquake, there is an earthquake vacant zone with a total length of about 150 km. Historically, if there is no big earthquake or obvious aseismic creep occurring in this area, it has a great potential of generating strong earthquakes with magnitude larger than M w 7.0 in the future.

The great subduction earthquakes that occurred recently in Peru, Chile, and Japan have provided unprecedented information about the ground motions generated by such earthquakes. The 23 June 2001 M 8.4 Peru earthquake was recorded at eight strong-motion stations; the 27 February 2010 M 8.8 Maule, Chile, earthquake was recorded at over 10 strong-motion stations; and the 11 March 2011 M 9.0 Tohoku, Japan, earthquake was recorded at more than a thousand stations and produced the most extensive dataset of recordings for any earthquake. For the first time, data are available to guide the generation of ground-motion simulations from great subduction earthquakes. Broadband ground-motion simulations can enhance the usefulness of the recordings of these earthquakes by providing a means of interpolating and extrapolating the recorded data. Once they have been validated, broadband ground-motion simulations can be used for forward predictions of the ground motions of great subduction events in regions such as Cascadia, in which there are no strong-motion recordings of large subduction earthquakes. In this study, we test our ability to use a hybrid method to simulate broadband strong-motion recordings of megathrust earthquakes by demonstrating that our simulations reproduce the amplitudes of the recorded ground motions without systematic bias. We use simulations to study the distribution of various intensity measures of ground motion caused by these earthquakes and to validate our ground-motion simulation method by comparing the simulated ground motions with recorded ground motions as well as with empirical ground-motion prediction models.

2016

Northwestern Peru is a highly seismically active region situated above the downgoing Nazca plate within the South America subduction zone. Historically, a total of 28 earthquakes of moment magnitude (M) 7.0 and greater have occurred in the region, including the 1619 M 7.7 to 8 Trujillo earthquake. Several active crustal faults have been identified in the region but likely many more have yet to be discovered. Probabilistic seismic hazard analysis (PSHA) was performed to assess the hazard in the region as exemplified by the city of Cajamarca. The basic inputs into the PSHA include the seismic source model, ground motion prediction models, and a description of the geologic and geotechnical conditions beneath the site. The seismic source model includes 21 Quaternaryage (active) crustal faults, the South America subduction zone, including both the megathrust and intraslab (WadatiBenioff) zone, and an areal crustal source zone. As part of reconnaissance field investigations, several previ...

2016

A megathrust subduction earthquake (M w 7.8) struck the coast of Ecuador on 16 April 2016 at 23:58 UTC. This earthquake is one of the best-recorded megathrust events to date. Besides the mainshock, two large aftershocks have been recorded on 18 May 2016 at 7:57 (M w 6.7) and 16:46 (M w 6.9). These data make a significant contribution for understanding the attenuation of ground motions in Ecuador. Peak ground accelerations and spectral accelerations are compared with four ground-motion prediction equations (GMPEs) developed for interface earthquakes, the global Abrahamson et al. (2016) model, the Japanese equations by Zhao, Zhang, et al. (2006) and Ghofrani and Atkinson (2014), and one Chilean equation (Montalva et al., 2017). The four tested GMPEs are providing rather close predictions for the mainshock at distances up to 200 km. However, our results show that high-frequency attenuation is greater for back-arc sites, thus Zhao, Zhang, et al. (2006) and Montalva et al. (2017), who are not taking into account this difference, are not considered further. Residual analyses show that Ghofrani and Atkinson (2014) and Abrahamson et al. (2016) are well predicting the attenuation of ground motions for the mainshock. Comparisons of aftershock observations with the predictions from Abrahamson et al. (2016) indicate that the GMPE provide reasonable fit to the attenuation rates observed. The event terms of the M w 6.7 and 6.9 events are positive but within the expected scatter from worldwide similar earthquakes. The intraevent standard deviations are higher than the intraevent variability of the model, which is partly related to the poorly constrained V S30 proxies. The Pedernales earthquake produced a large sequence of aftershocks, with at least nine events with magnitude higher or equal to 6.0. Important cities are located at short distances (20-30 km), and magnitudes down to 6.0 must be included in seismic-hazard studies. The next step will be to constitute a strong-motion interface database and test the GMPEs with more quantitative methods. Electronic Supplement: Figures of V S30 values based on topography versus rupture distance and difference between reference V S30 and V S30 based on topography versus distance, residuals, event terms, and intraevent standard deviations.

Earthquake Spectra

The correlation between spectral accelerations is key in the construction of conditional mean spectra, the computation of vector-valued seismic hazard, and the assessment of seismic risk of spatially distributed systems, among other applications. Spectral correlations are highly dependent on the earthquake database used, and thus, region-specific correlation models have been developed mainly for earthquakes in western United States, Europe, Middle East, and Japan. Correlation models based on global data sets for crustal and subduction zones have also become available, but there is no consensus about their applicability on a specific region. This study proposes a new correlation model for 5% damped spectral accelerations and peak ground velocity in the Chilean subduction zone. The correlations obtained were generally higher than those observed from shallow crustal earthquakes and subduction zones such as Japan and Taiwan. The study provides two illustrative applications of the correl...

Geophysical Research Letters, 2001

Seven Mw • 6 events occurred between July 1997 and January 1998 along the shallow dipping subduction zone of Central Chile. We used body waveform modeling and master event relocation to study them. During July 1997, typical shallow interplate thrust events located close from each other followed a southward migration path and were located close from each other. On 15 October, the largest shock of the series (Mw=7.1) occured at 68 km depth. From the directivity, we found that its rupture plane was almost vertical with a downward rupture. It was a down-dip compressional mechanism which is rare in Chile. Then, several thrust events occurred above it, along the plate interface. The rupture zones of the July 1997 events followed a cascade pattern with strong stress interaction. The slab push event of 15 October does not seem to have been triggered by static stress transfer from the July swarm.

Bulletin of the Seismological Society of America, 2016

The generation of accelerograms using stochastic methods has been a very useful methodology for solving the problem of the lack of appropriate strongmotion records for seismic design. Here, we propose the generation of synthetic strong motion for subduction earthquakes that present well-developed P waves and energetic arrivals of S waves associated with the main asperities of the source of these events. The first few seconds of these accelerograms are dominated by P waves; however, the strong motion is a mixture of S and P waves arriving at the same time. The traditional method considers only S waves. We propose to improve the stochastic generation of accelerograms taking into account a stratified velocity model, incident and azimuthal angles, free surface factors, and energy partition to incorporate the P and SV waves in the simulation. Finally, the simulated accelerograms are compared with the observed data recorded on rock by the Integrated Plate boundary Observatory Chile (IPOC) network during the 2007 Tocopilla and 2014 Iquique earthquakes. The use of P, SV, and SH waves in the stochastic simulation allowed us to generate three-component synthetic records. The early seconds are clearly associated with P waves, and the three components reproduce the shape and the amplitude in time and spectral domains for the observed and simulated records. Online Material: Figures showing fit between observed and simulated waveforms, maximum amplitude of acceleration response spectra, peak ground velocities, and peak ground accelerations. BSSA Early Edition / 1

2008

We present the results of a study aimed at choosing the more suitable strong-motion models for seismic hazard analysis in the Central America (CA) Region. After a careful revision of the state of the art, different models developed for subduction and volcanic crustal zones, in tectonic environment similar to those of CA, were selected. These models were calibrated with accelerograms recorded in Costa Rica, Nicaragua and El Salvador. The peak ground acceleration PGA and Spectral Acceleration SA (T) derived from the records were compared with the ones predicted by the models in similar conditions of magnitude, distance and soil. The type of magnitude (M s , M b , M w), distance (R hyp , R rup , etc) and ground motion parameter (maximum horizontal component, medium, etc) was taken into account in the comparison with the real data. As results of the analysis, the models which present a best fit with the local data were identified. These models have been applied for carrying out seismic hazard analysis in the region, in the frame of the RESIS II project financed for the Norway Cooperation Agency (NORAD). .

2002

The Civil Engineering Department of the University of Chile, together with international institutions deployed strong motion stations in the northern Chile seismic gap. These networks recorded the June 23, 2001 Mw=8.4 earthquake that occurred in Southern Peru, which is the strongest event in the last 25 years. This earthquake exhibited, at stiff soil sites, in northern Chile relatively large maximum velocities and accelerations although the recording stations are located more than 400 km away from the epicentral region and 200 km from the southern edge of the rupture. Damage to infrastructure was observed in adobe housing and low strength structures. Typical accelerations are in the order of 0.30 g, considerably larger than those expected from typical attenuation formulas if epicentral distances are considered.

Geophysical Research Letters, 1995

The Bolivia earthquake was remarkable for being felt in North America, as far as 8680 km from the epicenter. This paper examines the characteristics of ground motions in North America from the perspective of strong motion seismology. Data on ground motions is mostly from highgain stations, but we also have two strong motion accelerograms. The largest accelerations and highest frequencies are associated with the P and PcP phases, which also correlate with the felt reports in several instances. Felt locations are associated with a combination of favorable factors. Most are in a distance range where P-wave amplitudes tend to be relatively high, as observed in the empirical correction factors for body wave magnitude, and in regions with relatively high values of Q in the lithosphere. In addition, site effects, structural amplification, or both, seem to contribute to the selective detection of the earthquake in the regions where some people noticed it. Motions were far below design spectra for earthquake resistant design, and far below levels likely to be associated with damage.

Journal of Seismology, 2006

The Departments of Civil Engineering and Geophysics of the University of Chile, together with international institutions, deployed strong-motion stations in the northern Chile seismic gap. These instruments recorded the June 23, 2001 M w = 8.4 earthquake that occurred in Southern Peru. This earthquake exhibited at stiff deep soil sites in northern Chile, relatively large maximum accelerations although the recording stations are located more than 400 km away from the epicentral region and 200 km from the southern edge of the rupture. Typical accelerations at these distances are in the order of 0.30 g, considerably larger than those expected from recently presented attenuation formulae. Frequency and Wavelet Decomposition of the signals are presented from which the evolution of the amplitude, as a function of selected frequency bands, is analyzed. Typical Central Frequency varies from 3 to 4.8 Hz for horizontal records and 4.5 to 9.5 Hz for vertical records. Ninety five percent of the record energy is concentrated below 11 Hz.

Pure and Applied Geophysics, 1999

The 12 November 1996 M w 7.7 Peru subduction zone earthquake occurred off the coast of southern Peru, near the intersection of the South American trench and the highest topographical point of the subducting Nazca Ridge. We model the broadband teleseismic P-waveforms from stations in the Global Seismic Network to constrain the source characteristics of this subduction zone earthquake. We have analyzed the vertical component P-waves for this earthquake to constrain the depth, source complexity, seismic moment and rupture characteristics. The seismic moment determined from the nondiffracted P-waves is 3-5 ×10 20 N•m, corresponding to a moment magnitude M w of 7.6-7.7. The source time function for the 1996 Peru event has three pulses of seismic moment release with a total duration of approximately 45-50 seconds. The largest moment release occurs at approximately 35-40 seconds and is located 90 km southeast of the rupture initiation. Approximately 70% of the seismic moment was released in the third pulse. We find that the 1996 event reruptured part of the rupture area of the previous event in 1942. The location of the 1996 earthquake corresponds to a region along the Peru coast with the highest uplift rates of marine terraces. This suggests that the uplift may be due to repeated earthquakes such as the 1996 and 1942 events.