Toward a Unified Near-Field Intensity Map of the 2015 M_w 7.8 Gorkha, Nepal, Earthquake

Abstract

We develop a unified near-field shaking intensity map for the 25 April 2015 Mw 7.8 Gorkha, Nepal, earthquake by synthesizing intensities derived from macroseismic effects that were determined by independent groups using a variety of approaches. Independent assessments by different groups are generally consistent, with minor differences that are likely due in large part to differences in spatial sampling. Throughout most of the near-field region, European Macroseismic Scale (EMS-98) intensities were generally close to 7 EMS. In the Kathmandu Valley, intensities were somewhat higher (6.5–7.5) along the periphery of the valley and in the adjacent foothills than in the central valley, where they were ≈6. The results are consistent with instrumental intensity values estimated from available data using a published relationship between peak ground acceleration (PGA) and intensity. Using this relationship to convert intensities to PGA, we estimate strong-motion PGA de-amplification factors of ≈0.7 in the central Kathmandu Valley, with amplification of ≈1.6 in adjacent foothills. The results support the conclusion that the Kathmandu Valley experienced a pervasively nonlinear response during the Gorkha main shock.

Himalayan earthquakes: a review of historical seismicity and early 21st century slip potential

“…6 inset). Weak eastwards directivity in the intensity distribution was noted in the Gorkha earthquake (Adhikari et al 2017). The distribution of observed intensity locations is insufficient to evaluate its propagation direction, but the above reasoning would suggest it propagated from east to west.…”

“…Outside the valley, EMS intensity 8 was observed only on ridges (Fig. 26e), and along the northern edge of the rupture near the interseismic locking line (Adhikari et al 2017).…”

“…In the 2015 M w = 7.8 Nepal earthquake, abundant observations of unexpectedly low shaking intensities above the 150 × 60 km rupture surface were recorded. Accelerations in the 2015 earthquake were of long duration (1-3 min) and with significant fling (incremental translation of 1-7 m causing people to have difficulty standing) but averaged less than EMS (European macroseismic scale) (Grünthal 1998) intensity 7 above the rupture zone (Mencin et al 2016;Adhikari et al 2017;Bilham et al 2017). Intensity 7 corresponds to 0.18g-0.34g.…”

“…More than 3800 accounts of the earthquake were evaluated on the EMS scale by Martin et al (2015), mostly outside Nepal (Fig. 23), and these were supplemented by 291 891 observations of building damage in Nepal (Adhikari et al 2017;McNamara et al 2017). These investigators report an average intensity for the Kathmandu Valley of EMS 6.4 ± 0.9, slightly less than the average shaking intensity above the entire rupture of EMS 6.6 determined by several investigations.…”

Overview of the 2015 Gorkha, Nepal, Earthquake and the Earthquake Spectra Special Issue

“…One such initial study is documented in this issue. Adhikari et al (2017) developed a unified near-field shaking intensity map for the M w 7.8 earthquake by synthesizing intensities derived from macroseismic effects that were determined by independent groups using a variety of approaches. The authors estimate strong-motion peak ground accelerations and describe the nonlinear response of the Kathmandu basin.…”