The April 28, 1969 earthquake
M5.5near Borrego Springs, CA
A magnitude 5.5 earthquake 3.7 miles deep, centred in San Diego County. The 213th strongest in California’s record since 1900, and the strongest within 50 miles since a magnitude 6.6 on April 8, 1968. 7 aftershocks of magnitude 2.5 or more followed within 30 days.
The earthquake
- Magnitude
- 5.5
- local magnitude (ml)
- Depth
- 3.7 mi
- 6.0 km below the surface
- Local time
- Monday, April 28, 1969
- 16:20 PDT; 23:20 UTC
- In California’s record
- 213th
- strongest of 97,065 since 1900
USGS ANSS Comprehensive Earthquake Catalog, event ci3327613 (“1km ENE of Borrego Springs, CA”). The USGS event page carries the ShakeMap, felt reports and the official summary.
Aftershocks
Earthquakes of magnitude 2.5 or more within 20 miles of the epicentre in the 30 days after.
7
Aftershocks in 30 days
4.5
Largest aftershock, May 19, 1969
2
On the first day
0
In the 7 days before
- The search radius is the surface rupture length expected for an earthquake of this magnitude (Wells and Coppersmith, 1994), at least 20 and at most 600 miles, so a great earthquake's sequence is counted along its whole fault.
- Before the modern seismic networks of the 1970s many small aftershocks went unrecorded, so early sequences read short.
Counties and cities within 50 miles
Distance from the epicentre to each county boundary (zero when inside) and to each city centre.
Counties
- San Diego CountyCA · inside
- Imperial CountyCA · 19.6 mi
Cities, largest first
- EscondidoCA · 42.1 mi
- TemeculaCA · 47.3 mi
- El CajonCA · 47.0 mi
- San MarcosCA · 47.6 mi
- IndioCA · 33.4 mi
- HemetCA · 49.2 mi
- SanteeCA · 45.7 mi
- Palm DesertCA · 33.1 mi
- Cathedral CityCA · 40.3 mi
- PowayCA · 42.6 mi
- CoachellaCA · 32.3 mi
- Palm SpringsCA · 39.0 mi
Earthquakes around the epicentre
2,349 earthquakes of magnitude 3 or more within 50 miles of this epicentre since 1973.
Earlier earthquakes of magnitude 5 or more within 50 miles, strongest first
- April 21, 1918M6.7
- October 21, 1942M6.6
- April 8, 1968M6.6
- March 19, 1954M6.4
- March 25, 1937M6.0
- August 15, 1945M5.8
- September 29, 1916M5.7
- October 21, 1942M5.6
- March 19, 1954M5.5
- July 29, 1950M5.4
USGS ANSS Comprehensive Earthquake Catalog. Local time is the time zone of the county nearest the epicentre. Magnitudes are as the catalogue reports them.