M4.7 earthquake 105 km WNW of Santa Cruz, Chile

Thursday, March 28, 1985, 04:43:28 UTC · 34.304°S, 72.443°W

A magnitude 4.7 earthquake struck 105 km WNW of Santa Cruz, Chile on Thursday, March 28, 1985, 04:43:28 UTC, at a depth of 33.0 km (shallow). It is the strongest earthquake within 300 km since an M4.7 on March 25, 1985.

Magnitude: 4.7; Light · mb
Depth: 33 km; Shallow
Felt reports: 0; Did You Feel It?
Est. shaking: —; No ShakeMap
PAGER alert: None; Not issued
Tsunami: No flag; USGS feed

Outlined circle: this earthquakeDashed circle: 50 km aftershock search area

In context

How big was it, and why here?

Energy released

169 tons of TNT

That is about 1/89 of the Hiroshima bomb. A magnitude 5.0 releases about 3 times more energy.

Tectonic setting

65 km

from the nearest mapped plate boundary, between the Nazca and South American plates. Earthquakes cluster along boundaries like this one, where plates collide, pull apart or slide past each other.

Depth: shallow

33 km

Shallow earthquakes (under 70 km) produce the strongest surface shaking for their size, so they are the ones most likely to cause damage near the epicenter.

Did You Feel It?

Where it was felt

No “Did You Feel It?” reports have been published for this event yet. If you felt it, you can add your report on the USGS website.

I felt it — report to USGS ↗

USGS PAGER estimate

Who was exposed to shaking

USGS publishes PAGER impact estimates for significant earthquakes, usually within an hour. No PAGER estimate is available for this event.

After the main event

Aftershocks within 50 km

USGS lists 8 M2.5+ earthquakes within 50 km in the following 30 days; the largest was M4.6 on April 18, 1985. Most nearby activity right after a large earthquake is aftershocks, which usually become less frequent over days and weeks.

Time after the mainshock on a logarithmic scale (first hours stretched), magnitude on the vertical axis. M2.5+ events from USGS.

Regional history

Past M5+ earthquakes within 300 km

USGS lists 218 M5.0+ earthquakes within 300 km before this one (catalog from 1900; older records are less complete). The strongest:

History · NOAA

Deadliest earthquakes in this area

NOAA’s significant-earthquake database lists 39 damaging, deadly or tsunami-generating earthquakes within 300 km of this epicenter since 1700, which together caused about 35,163 recorded deaths. The most severe:

1939M8.3
Chillan
25 Jan 1939 · 217 km away · intensity X
30,000 deaths$920 million damagetsunami
1906M8.2
South Central
17 Aug 1906 · 151 km away · intensity XI
4,000 deaths$100 million damagetsunami
1965M7.3
Central
28 Mar 1965 · 241 km away
400 deaths$125 million damage
1928M7.6
Talca, Chillan, Santiago, San Fernando
1 Dec 1928 · 87 km away · intensity IX
279 deathsSevere damage ($5–24M)tsunami
1985M8.0
Central Coast, San Antonio, Valparaiso
3 Mar 1985 · 147 km away · intensity VIII
180 deaths$1.5 billion damagetsunami
1971M7.8
Central
9 Jul 1971 · 231 km away · intensity IX
83 deaths$236 million damagetsunami

NOAA NCEI Global Significant Earthquake Database. Historical figures are estimates; older records are incomplete.

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