Directionality and polarization of response spectral ordinates in the 2023 Kahramanmaras, Türkiye earthquake doublet

Abstract

Until recently, the orientation of maximum horizontal spectral response was generally believed to not have a predominant orientation at rupture distances greater than 5 km. However, a recent study found that the orientation of maximum spectral response for strike-slip earthquakes in the NGA-West2 database tends to occur close to the epicentral transverse orientation, that is, an orientation perpendicular to a line connecting the epicenter to the station. This article investigates directionality in the 6 February 2023 Türkiye doublet earthquakes ( Mw 7.8 and 7.5) with strike-slip faulting. The orientation of the maximum response of 5%-damped linear elastic oscillators was studied. The spatial distribution of the level of polarization, which in this article refers to the amount of directionality, and intensities at specific orientations were also studied. The maximum spectral response was found to occur systematically close to the epicentral transverse orientation, consistent with previous observations for other strike-slip earthquakes. For the Mw 7.8 event where the location of maximum slip was relatively far from the epicenter, it was found that the orientation of maximum spectral response is, on average, closer to the maximum slip transverse orientation (i.e. perpendicular to a line connecting the station to the surface projection of the point of maximum slip) when compared to the epicentral transverse orientation over most period ranges. This suggests that the maximum slip transverse orientation may be a better estimator for determining the orientation of maximum spectral response in large-magnitude strike-slip earthquakes, although further study using more events is warranted. Polarized motions were observed over large geographical areas, and the orientation of maximum spectral response was found to be close to the epicentral or maximum slip transverse for Joyner–Boore distances up to the farthest studied (400 km). These findings further support the case for the development of orientation-dependent ground motion models for strike-slip earthquakes.

Directionality of FIV3 ground-motion intensities during the 6 February 2023 Kahramanmaraş, Türkiye earthquake doublet

“…In other words, the predictability of the direction of maximum FIV3 intensity increases if the surface projection of the maximum slip point is considered as the seismic source instead of the epicenter. These results are consistent with those found by Girmay et al (2023) who also explored the use of the surface projection of the point of maximum slip for estimating the direction of maximum 5%-damped response spectral ordinates during the 2023 Tu¨rkiye earthquake doublet.…”

“…Fifth, the ratio between the intensity at the radial component and RotD100 decreases monotonically as the period increases, ranging from 0.78 at 0.1 s to 0.6 at 10 s. Noticeably, all these observations are valid for both earthquakes. For periods smaller than about 2 s, the directionality of FIV 3 intensities is larger than that reported by Girmay et al (2023) for response spectral ordinates for the same doublet considered herein. Finally, some of these ratios such as the intensity at the transverse orientation and RotD50 could be used to generate correction factors for DHM2020 GMM to yield accurate ground-motion IMs at the transverse orientation.…”

“…RotD50 FIV 3 intensities). These results, together with those presented by Girmay et al (2023), indicate that it is possible in the future to develop orientation-specific GMMs that provide improved estimates of ground-motion intensities as a result of strike-slip earthquakes that are of primary concern to seismic regions, such as Tu¨rkiye or California. In particular, the development of such GMMs for FIV 3 would be an important contribution as this ground-motion IM outperforms many other IMs in terms of efficiency, sufficiency, predictability, and scaling factor robustness for estimating seismic structural collapse within the PBEE framework.…”

“…A selection of ground-motion records was made following the criteria used by Girmay et al (2023), which are as follows: (1) using only stations that recorded both horizontal components with reported azimuths of both components and (2) using only records with a peak ground velocity (PGV) of at least 1 cm/s in one of the recorded horizontal components, aimed at obtaining a strong signal-to-noise ratio over a wide range of periods. Records satisfying these two criteria were used only up to the maximum usable oscillator period (Boore, 2004), defined as 1 over 1.25 times the low-pass corner frequency applied (Abrahamson and Silva, 1997).…”

Analysis of Near-Fault Ground Motions in the February 2023 Kahramanmaras, Türkiye, Earthquake Sequence

“…The mainshock rupture propagated north-east transferring to the East Anatolian fault and triggering a sequence of aftershocks. The hypocenter of the Mw 7.8 event was estimated at 37.043 。 E and 37.288 。 N with a focal depth of 8.6 km; while the hypocenter of the Mw7.5 event was at 37.239 。 E and 38.089 。 N with a depth of 7.0 km (e.g., Goldberg et al, 2023;Özkan et al, 2023;Girmay et al, 2024;Lashgari et al, 2023;Pujol et al, 2024. Both events occurred in the Eastern Anatolian Fracture Zone (EAFZ), which is located in the southern and northern branches of the fracture zone, respectively.…”