in NewMexico
Earthquake
activity
from1849through1961
b y A l l a n RS a n l o r d , G e o p h y s i c i s t , N e w M e x i c oM
l nisntiint ug t&eToef c h n o l o g y , S o c o n o , N M
Information on location and strength of
in New Mexico prior to 1962has
earthquakes
beenreportedprimarily on the basisof subjective reactionsand noninstrumentalobservations during a shock, and on the degreeof
damageto structures.Given a number of intensity observations,both the point of maximum intensityand the area of perceptibility
The areaof perceptibilityis
canbe established.
particularlyrelatedto the earthquakemagnitude (Richter,1958;Slemmons,1965;Wiegel,
1970).
A major weaknessin determiningstrength
and location from intensity observationsis
that the methoddependson populationdensity. In sparselysettledareassuchas much of
New Mexico, moderateshocksmay go unreported-or reported at low-intensityvalues
that fail to indicate the true strength of the
earthquake.Even in areasof relativelyhigh
populationdensity,as along the Rio Grande
valley,the point of maximumintensityor area
of perceptibilitymay be undefinedbecauseof
too few observations.
Reliabilityof earlyearthquakereportsmust
also be considered.In New Mexico, someof
the strongearthquakeintensitiesprior to 1900
are basedon reportsfrom local residentstens
of years after the shocks occurred (Bagg,
1904). The use of newspaperaccounts of
earthquakesto estimateintensityhas proved
fairly reliable. In at least two instances,
however, effects of earthquakesin the Rio
in
Crandevalleyat Socorrowereexaggerated
Albuquerqueand El Paso newspapers(Sanford, 1963;Ashcroft,1974).
Despitethe imperfect nature of the noninstrumentaldata, they are valued because
theycovera periodroughlyeighttimesgreater
than the instrumental data. Summarized
below are noninstrumentaldata that indicate
the nature of seismicactivity in New Mexico
prior to 1962.
The earliestreport of earthquakesin New
Mexico is the descriptionof an earthquake
swarmin the Rio Granderift at Socorroby a
U.S. Army surgeon.The swarmcomprised22
felt shocks, commencingon Decemberll,
1849and lastinguntil February8, 1850(Hammond, 1966).No shock in this swarm was
reportedfelt at distancesgreaterthan 25 km
from Socorro.Similarsequences
of shocksoccurring away from population centersalong
in the state
the Rio Grandevalleyor elsewhere
could easilyhavegone unreportedbeforethe
adventof instrumentalstudies.
From 1849through 1961,Northrop (1961
and 1976)citesevidence,primarily from old
newspaperfiles, for over 600 felt earthquakes
in New Mexico. About 95 percentof these
February1980
New Mexico Geology
shocksoccurredalonga 150-kmsectionof the
Rio Grande rift from Albuquerque to Socorro: the majority in the 75-kmsegmentfrom
Belen to Socorro. The concentration of
reported activity in this area cannot be attributedto populationdensity,for the population from Belen to Albuquerquehas always
exceeded the population from Belen to
Socorro.
Northrop'sdatain generalareinfluencedby
the distribution of population in the state.
Population density is higher in the
section of the Rio
Albuquerque-to-Socorro
Grandevalley than in most other sectionsof
the state. To reduce bias arising from the
distribution of population, only shockswith
maximumreportedintensities(modifiedMercalli)of V or greaterareplottedin fig. I .
F;
s
16:
theymay be relatedto injectionof magmainto
thecrust.
By far the strongestand longestearthquake
swarmat Socorro,
swarmwas the 1906-1907
apparently comparable to the Matsushiro
swarm believedby someto have beencaused
by magmatic intrusion at shallow depth
(Stuart and Johnson, 1975). Although the
evidenceis not absolutely conclusive,the
for thi 1906-1907
distributionof isoseismals
swarm suggestshypocenters beneath the
SocorroMountain horst block, a structural
featurein the central part of the rift. Other
swarmsalso appearto have originatedin the
central part of the rift, rather than the
margins. Recentbasalt flows from north of
Albuquerqueto southof Socorroaregenerally
confined to the central part of the rift. This
observation,in conjunctionwith the location
of earthquake swarms, may indicate that
magma is continuingto be injectedinto the
centralportion of the rift.
Most reportedearthquakesprior to the beginningof instrumentalstudieswerealongthe
Rio Grande valley, primarily betweenAIbuquerqueand Socorro(fig. l). The concentration of seismicactivity in this belt cannot be
attributedto populationdensityinasmuchas
somereportedshocks,particularlyat Socorro,
wereof sufficientstrengthto be felt overareas
up to 245,000km' (Sanford,Olsen,and Jaksha,1979).Shocksof thisstrengthareunlikely
anywherein the state,
to havegoneundetected
of populationdistribution.
regardless
This manuscriptis an excerptfrom Circular
activityin New Mexico(1849
l7l, Earthquake
through1977),soonto be released.
References
A
j
Ashcroft, 8., 19'74,The July 1906 earthquakes in
Socorro: New Mexico Historical Review, v. 49, p.
325-330
Bagg, R. M., 1904, Earthquakes in Socorro, New
;
Mexico: American Geology, v. 34, p. 102-lO4
Hammond, J. F., 1966, A surgeon's report on Socorro, New Mexico, 1852: Stagecoach Press,
r&
Santa Fe, NM
l**
Neumann, F., 1940, United States earthquakes,
).
1 9 3 8 :U . S . D e p a r t m e n t o f C o m m e r c e , C o a s t a n d
Geodetic Survey, Serial 629
N o r t h r o p , S . A . , 1 9 6 1 ,E a r t h q u a k e so f c e n t r a l N e w
O r d e ro f m o g nt iu d e
Mexico: New Mexico Geological Society, Guide,,iiin,5-5.99 33 locotions
, . l5l-152
b o o k l 2 t h f i e l d c o n f e r e n c ep
l2locotions
Ae-esg
1976, New Mexico's earthquake history,
iz-t.sg
4 tocotions
1849-1975:New Mexico Geological Society, Spec'
2locolions
o8.Oor)
Pub. 6, p. 77-87
REPoRTED
FICURE l-LocerroNs oF EARTHeUAKES
R i c h t e r , C . F . , 1 9 5 8 ,E l e m e n t a r ys e i s m o l o g y :W . J .
npr-r rN New Mrxtco pRIoR ro 1962. Eplcrlren
Freeman. San Francisco, CA
IS PLACEDAT THEPOINTOF
FOREACHEARTHQUAKE
S a n f o r d , A . R . , t 9 6 3 , S e i s m i ca c t i v i t y n e a r S o c o r r o :
A
W
H
E
R
E
N T E N S I TE
YX C E P T
M A X I M U MR E P O R T EID
New Mexico Ceological Society, Cuidebook l4th
LARCE NUMBEROF SHOCKSARE REPORTEDFROM THE
fieldconferencep
, . 146-154
SAME POPULATIONCENTER
Sanford, A. R., Olsen, K. H., and Jaksha, L. H.,
1979, Seismicityof the Rio Crande rift, in The Rio
Characteristically, the strong Socorro
Crande rift, tectonics and magmatism: American
G e o p h y s i c aU
l n i o n , S p e c .P u b . , p . 1 4 5 - 1 6 8
shocks (as well as many other known earthquakes in the rift from Albuquerque to
S l e m m o n s ,D . N . , J o n e s , A . 8 . , a n d G i m l e t t , J . L ,
1 9 6 5 ,C a t a l o g o f N e v a d a e a r t h q u a k e s ,1 8 5 2 - 1 9 6 0 :
Socorro) are associated with earthquake
Bulletin of the Seismological Society of America,
swarms. As noted for many years, swarms are
v . 5 5 ,p . 5 3 7 - 5 8 3
observed in the vicinity of active volcanoes
S t u a r t , W . D . , a n d J o h n s o n , M . J . S . , 1 9 7 5 ,I n t r u volcanic
and in regions that have experienced
sive origin of the Matsushiro earthquake swarm:
activity in geologically recent times (Richter,
C e o l o g y ,v . 3 , p . 6 3 - 6 7
1958). The possible significance of the earth- W i e g e l , R . L . , e d . , 1 9 7 0 , E a r t h q u a k e e n g i n e e r i n g :
quake swarms in the Rio Grande rift is that
I
Prentice Hall, Inc., Englewood Cliffs, NJ
r&