Observations and Modeling of the August 27, 2012 Earthquake and Tsunami affecting El Salvador and Nicaragua

Abstract

On 27 August 2012 (04:37 UTC, 26 August 10:37 p.m. local time) a magnitude M _w = 7.3 earthquake occurred off the coast of El Salvador and generated surprisingly large local tsunami. Following the event, local and international tsunami teams surveyed the tsunami effects in El Salvador and northern Nicaragua. The tsunami reached a maximum height of ~6 m with inundation of up to 340 m inland along a 25 km section of coastline in eastern El Salvador. Less severe inundation was reported in northern Nicaragua. In the far-field, the tsunami was recorded by a DART buoy and tide gauges in several locations of the eastern Pacific Ocean but did not cause any damage. The field measurements and recordings are compared to numerical modeling results using initial conditions of tsunami generation based on finite-fault earthquake and tsunami inversions and a uniform slip model.

Appendix

For Source 2, we use a scaling approach similar to that used in Newman et al. (2011) for the 2010 earthquake and tsunami in the Mentawai Islands offshore Sumatra, Indonesia. Following this, an appropriate scaling parameter for the displacement along the fault plane can be determined from the following relationship:

$$ D/D_{0} = \left( {V_{{{\text{S}} - {\text{ref}}}} /V_{\text{S}} } \right)^{2} , $$

(1)

where D/D ₀ is the ratio of the scaled to original slip, V _S-ref is a reference shear wave velocity for the region where the earthquake occurred, and V _S is the actual shear wave velocity deduced from teleseismic observations of the event and is calculated from the earthquake rupture velocity (V _R) by assuming that V _S = 1.25 V _R. In general, since V _R can be highly variable, individual V _S values are calculated for each subfault in the finite fault solution, However, inspection of the finite fault model from this event reveals a consistent rupture velocity (V _R) of approximately 2.25 km/s, yielding a shear wave velocity (V _S) 2.81 km/s. From the Crust 2.0 model (Bassin et al. 2000) used in the finite fault analysis, the V _S-ref along the sections of greatest slip is assumed to be 3.9 km/s. Using these values in Eq. 1 leads to an average scale factor of 1.92. We note that this factor is much less than what was deduced from the 2010 Mentawai event where rupture velocities in the main slip zones were often around 1.5 km/s. In using this approach, we emphasize that this is only a first-order approximation, as pointed out in Newman et al. (2011). This is primarily due to the fact that seismic excitation at teleseismic distances includes some upper-plate signal that cannot be fully deconvolved leading to an overamplification of the result. Secondly, because the solution is entirely based on the square of the determination of rupture velocity, any inaccuracies in the solution of V _R can have a very large impact on the final scale factor.