Constraining the Source of the M w 8.1 Chiapas, Mexico Earthquake of 8 September 2017 Using Teleseismic and Tsunami Observations
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The September 2017 Chiapas (Mexico) normal-faulting intraplate earthquake (M w 8.1) occurred within the Tehuantepec seismic gap offshore Mexico. We constrained the finite-fault slip model of this great earthquake using teleseismic and tsunami observations. First, teleseismic body-wave inversions were conducted for both steep (NP-1) and low-angle (NP-2) nodal planes for rupture velocities (Vr) of 1.5–4.0 km/s. Teleseismic inversion guided us to NP-1 as the actual fault plane, but was not conclusive about the best Vr. Tsunami simulations also confirmed that NP-1 is favored over NP-2 and guided the Vr = 2.5 km/s as the best source model. Our model has a maximum and average slips of 13.1 and 3.7 m, respectively, over a 130 km × 80 km fault plane. Coulomb stress transfer analysis revealed that the probability for the occurrence of a future large thrust interplate earthquake at offshore of the Tehuantepec seismic gap had been increased following the 2017 Chiapas normal-faulting intraplate earthquake.
KeywordsPacific ocean tsunami 2017 Chiapas earthquake tsunami modeling teleseismic body-wave inversion Coulomb stress transfer
From the regional tectonic point of view, the Chiapas earthquake occurred within the North American Plate at ~ 100 km from the Middle America Trench where the Cocos Plate is subducting beneath the North American Plate (Fig. 1b). As shown in Fig. 1b, the epicentral area is located within a seismic gap zone along offshore Mexico, which is called the Tehuantepec gap (e.g., Singh et al. 1981). Based on the USGS catalog, 38 M > 7 earthquakes were recorded in this subduction zone including 15 tsunamigenic events (Fig. 1b) (Hatori 1995). The latest notable tsunamis in this region were generated following the M w 8.0 thrust earthquake on 9 October 1995 (Ortiz et al. 1998; Synolakis and Okal 2005), M 7.6 earthquake on 21 September 1985 (Ortiz et al. 2000; Okal and Synolakis 2004) (M 7.5 as reported by Hatori 1995) and M 7.6 thrust earthquake on 29 November 1978 (Singh et al. 1980) (Fig. 1b).
The recent 2017 Chiapas event is important because it is the first significant tsunami along the Mexican coast in the past 22 years (since 1995). A small tsunami was reported in this region following the 20 March 2012 M w 7.4 earthquake (M.T. Ramirez-Herrera; written communications). In addition, it is a tsunami event generated by a steep normal-faulting earthquake which is not frequent. The other recent tsunamis generated by normal-faulting earthquakes occurred offshore Solomon Islands on 18 July 2015 (M w 7.0) (Heidarzadeh et al. 2016b), offshore Kuril Islands in 2007 (Rabinovich et al. 2008; Fujii and Satake 2008) and offshore Fukushima (Japan) on 21 November 2017 (Gusman et al. 2017). The purposes of this study are: 1) to constrain the finite-fault slip model of the 2017 Chiapas earthquake using teleseismic and tsunami observations, and 2) to investigate changes in the Coulomb stress for the Tehuantepec gap region. The source model obtained in this study helps understand future earthquake and tsunami hazards offshore Mexico and adds to the existing knowledge on tsunami genesis of normal-faulting earthquakes.
2 Data and Methods
The data employed here were 18 tsunami (Fig. 1a) and 76 teleseismic body-wave records (Fig. 1c). Among 18 tsunami records, 4 were deep-ocean assessment and reporting of tsunamis (DART) records, downloaded from the US National Oceanic and Atmospheric Administration website (http://www.ndbc.noaa.gov/dart.shtml), and 14 were tide gauge records provided by the Intergovernmental Oceanographic Commission (http://www.ioc-sealevelmonitoring.org/) and the Mexican Servicio Mareográfico Nacional (http://www.mareografico.unam.mx/portal/). All tsunami observations had a sampling interval of 1 min. The tidal signals were estimated by a polynomial-fitting approach and were then subtracted from the original tsunami observation to produce tsunami waveforms. The 76 dataset of teleseismic records include 64 P and 12 SH waves. These data belonged to distances 30°–100° from the epicenter (Fig. 1) and were retrieved from the Data Management Center of the Incorporated Research Institutions for Seismology (IRIS; https://www.iris.edu/hq/). All teleseismic data were filtered in the frequency band range of 0.004–1.0 Hz and were deconvolved into the ground displacements. The duration of the waveforms used in the teleseismic inversion was 90 s from the calculated P or SH wave arrival times. The velocity structures used in this study were based on CRUST 1.0 (Laske et al. 2013) and ak135 (Kennett et al. 1995).
The 2003 Kikuchi and Kanamori’s teleseismic body-wave inversion program (http://www.eri.u-tokyo.ac.jp/etal./KIKUCHI/) was applied for estimating the finite-fault slip model. Both nodal planes (NP) (steep and low-angle faults with dip angles of 77° and 13°; called hereafter as NP-1 and NP-2, respectively) from the GCMT focal solution were examined to investigate which nodal plane better explained the waveform data. We used subfaults with length and width of 10 km (along strike and dip) over the total extent of 100–130 km for teleseismic inversion by allowing maximum rupture duration of 9.5 s for each subfault. Six rise-time triangles were used and each triangle had a duration of 3 s overlapped by 1.5 s with the neighboring triangles. The rupture velocity (Vr) was varied from 1.5 to 4.0 km/s with 0.5 km/s intervals; therefore, 12 slip distributions were estimated using teleseismic inversions: 6 for NP-1 and the other 6 for NP-2. The reason for producing 12 slip distributions was to investigate which nodal plane (i.e., NP-1 or NP-2) and which Vr better reproduced the teleseismic and tsunami observations. As reported by various authors (e.g., Lay et al. 2014; Gusman et al. 2015; Zhang et al. 2017; Heidarzadeh et al. 2016a, 2017a), the results of teleseismic inversions are not unique due to the uncertainties associated with Vr and therefore they need to be constrained by other types of observations such as tsunami observations.
The numerical package of Satake (1995) was employed for simulations of tsunami propagation on the bathymetry data provided by GEBCO-2014 (The General Bathymetric Chart of the Oceans) digital atlas having a resolution of 30 arc-sec (Weatherall et al. 2015). The model includes bottom friction and the Coriolis forces and solves the shallow-water equations over a spherical domain. This numerical model was successfully applied for the modeling of a number of large tsunamis including the 2011 Tohoku (Japan) tsunami (Satake et al. 2013; Satake 2014). A time step of 1.0 s was used for linear simulations. Tsunami simulations were initiated using coseimsic seafloor deformations obtained from the Okada’s (1985) analytical solution for coseimsic dislocation. The quality of fit between observations and simulations was measured by using the normalized root mean square (NRMS) misfit equation of Heidarzadeh et al. (2016a, b).
3 Finite-Fault Slip Model
The best finite-fault slip model, based on teleseismic body-wave inversions and forward tsunami simulations, belongs to the NP-1 (i.e., steep fault plane) with Vr = 2.5 km/s. The dimension of the fault is 130 km in length × 80 km in width, with maximum and average slip amounts of 13.1 and 3.7 m (Fig. 2b). The main rupture unilaterally propagates toward northwest and the large slip patch (slip = 7–13 m) is located at the depth range of 30–50 km. The duration of the earthquake rupture was 56.5 s and the seismic moment was estimated to be 1.91 × 1021 Nm giving M w = 8.1. Our source model obtained from joint tsunami and teleseismic data is consistent with that of Gusman et al. (2018) and Adriano et al. (2018), which are based on tsunami inversion.
4 Stress Transfer from the 2017 Chiapas Earthquake
The maximum tide gauge and DART zero-to-crest tsunami amplitudes, among the observed data, were 133.8 cm (Salina Cruz) and 8.8 cm (DART 43413) for the 2017 Chiapas tsunami, respectively.
To resolve the actual fault plane between the steep (NP-1) and low-angle (NP-2) nodal planes, teleseismic inversions were performed for both nodal planes employing rupture velocities (Vr) of 1.5–4.0 km/s. While teleseismic inversions favored NP-1 over NP-2, it was not possible to select the best Vr.
Tsunami simulations revealed that NP-1 is a better fault plane than NP-2 in terms of agreement between tsunami observations and simulations and conclusively guided the selection of Vr = 2.5 km/s as the best source model. We report a source model with dimensions of 130 km (strike-wise) × 80 km (dip-wise), maximum and average slips of 13.1 and 3.7 m, respectively, belonging to the steep fault plane (NP-1). Duration of the earthquake, seismic moment and M w were 56.5 s, 1.91 × 1021 Nm and 8.1, respectively.
Coulomb stress transfer analysis revealed that the shallower part of the Tehuantepec gap (i.e., near the trench axis) received positive stress, while negative stress was transferred to the deeper part (i.e., along the shoreline). The probabilities for the occurrence of future large thrust interplate earthquakes in the region may have been increased following the 2017 Chiapas M w 8.1 intraplate earthquake.
Tsunami DART data used here came from the US National Oceanic and Atmospheric Administration (NOAA) website (http://www.ndbc.noaa.gov/dart.shtml). Tide gauge data were downloaded from Intergovernmental Oceanographic Commission website (http://www.iocsealevelmonitoring.org/). We obtained teleseismic data for inversions from the Incorporated Research Institutions for Seismology (http://www.iris.edu/wilber3/find_event). We used The GMT software by Wessel and Smith (1998) in this study. MH is grateful to the Brunel University London for the funding provided through the Brunel Research Initiative and Enterprise Fund 2017/18 (BUL BRIEF). The authors declare that they have no competing interests regarding the work presented in this article. We are grateful to two anonymous reviewers.
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