Strong ground motion

In seismology, strong ground motion is the strong earthquake shaking that occurs close to (less than about 50 km from) a causative fault. The strength of the shaking involved in strong ground motion usually overwhelms a seismometer, forcing the use of accelerographs (or strong ground motion accelerometers) for recording. The science of strong ground motion also deals with the variations of fault rupture, both in total displacement, energy released, and rupture velocity.

ShakeMap for the 2001 Nisqually earthquake

As seismic instruments (and accelerometers in particular) become more common, it becomes necessary to correlate expected damage with instrument-readings. The old Modified Mercalli intensity scale (MM), a relic of the pre-instrument days, remains useful in the sense that each intensity-level provides an observable difference in seismic damage.

After many years of trying every possible manipulation of accelerometer-time histories, it turns out that the extremely simple peak ground velocity (PGV) provides the best correlation with damage.[1][2] PGV merely expresses the peak of the first integration of the acceleration record. Accepted formulae now link PGV with MM Intensity. Note that the effect of soft soils gets built into the process, since one can expect that these foundation conditions will amplify the PGV significantly.

"ShakeMaps" are produced by the United States Geological Survey, provide almost-real-time information about significant earthquake events, and can assist disaster-relief teams and other agencies.[3]

Correlation with the Mercalli scale

The United States Geological Survey created the Instrumental Intensity scale, which maps peak ground velocity on an intensity scale comparable to the felt Mercalli scale. Seismologists all across the world use these values to construct ShakeMaps.

Instrumental
Intensity		 Velocity
(cm/s)		 Perceived shaking Potential damage
I < 0.0215 Not felt None
II–III 0.135 – 1.41 Weak None
IV 1.41 – 4.65 Light None
V 4.65 – 9.64 Moderate Very light
VI 9.64 – 20 Strong Light
VII 20 – 41.4 Very strong Moderate
VIII 41.4 – 85.8 Severe Moderate to heavy
IX 85.8 – 178 Violent Heavy
X+ > 178 Extreme Very heavy

Notable earthquakes
PGV
(max recorded)		 Mag Depth Fatalities Earthquake
318 cm/s[4]7.733 km2,4151999 Jiji earthquake
183 cm/s[5]6.718.2 km571994 Northridge earthquake
170 cm/s[4]6.917.6 km6,4341995 Great Hanshin earthquake
152 cm/s[4]6.610 km112007 Chūetsu offshore earthquake
147 cm/s[4]7.31.09 km31992 Landers earthquake
145 cm/s[4]6.613 km682004 Chūetsu earthquake
138 cm/s[4]7.210.5 km356 injured1992 Cape Mendocino earthquakes
117.41 cm/s9.1[6]29 km19,7472011 Tohoku earthquake and tsunami
108 cm/s[7]7.88.2 km8,857April 2015 Nepal earthquake
38 cm/s[8]5.515.5 km02008 Chino Hills earthquake
20 cm/s (est)[9]6.410 km115-1201933 Long Beach earthquake

See also

References
  1. "INSTRUMENTAL SEISMIC INTENSITY MAPS". Archived from the original on 2000-09-03.
  2. Wu, Yih-Min; Hsiao, Nai-Chi; Teng, Ta-Liang (July 2004). "Relationships between Strong Ground Motion Peak Values and Seismic Loss during the 1999 Chi-Chi, Taiwan Earthquake". Natural Hazards. 32 (3): 357–373. CiteSeerX 10.1.1.470.4890. doi:10.1023/B:NHAZ.0000035550.36929.d0. S2CID 53479793.
  3. "ShakeMaps". Archived from the original on 2014-03-30. Retrieved 2014-04-01.
  4. "How Fast Can the Ground Really Move?" (PDF). INSTITUTE FOR DEFENSE ANALYSES. Archived (PDF) from the original on June 1, 2022. Retrieved 2021-11-03.
  5. "ShakeMap Scientific Background". Archived from the original on 2011-06-23. Retrieved 2021-11-03.
  6. "M9.1 - Tohoku, Japan". United States Geological Survey. Retrieved 2021-11-03.
  7. Takai, Nobuo; Shigefuji, Michiko; Rajaure, Sudhir; Bijukchhen, Subeg; Ichiyanagi, Masayoshi; Dhital, Megh Raj; Sasatani, Tsutomu (26 January 2016). "Strong ground motion in the Kathmandu Valley during the 2015 Gorkha, Nepal, earthquake. Earth Planets Space 68, 10 (2016)". Earth, Planets and Space. Takai, N., Shigefuji, M., Rajaure, S. et al. 68 (1): 10. doi:10.1186/s40623-016-0383-7. hdl:2115/60622. S2CID 41484836. Retrieved 2021-11-03.
  8. "M5.5 - Greater Los Angeles area, California". USGS. Retrieved 2021-11-06.
  9. Hough, S. E.; Graves, R. W. (22 June 2020). "Nature:The 1933 Long Beach Earthquake (California, USA) Ground Motions and Rupture Scenario". Scientific Reports. 10 (1): 10017. doi:10.1038/s41598-020-66299-w. PMC 7308333. PMID 32572047. S2CID 219959257.

Sources

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