Hydrochemical response of groundwater following the 2020 Monte Cristo
Range Earthquake sequence within Mineral and Esmeralda counties, NV
Abstract
The 2020 Monte Cristo Earthquake sequence in western Nevada began with a
M6.5 shock on 5/15/20, and was the largest to occur in Nevada since
1954. The event exhibited left-lateral slip along an eastward extension
of the Candelaria fault and extensive distributed surface faulting in
the epicentral area. Groundwater monitoring and strain analysis were
conducted to evaluate hydrochemical effects on the regional groundwater
systems following the initial event. Physio-chemical monitoring,
(started on 5/16 and still ongoing) includes measurements of temperature
(temp), pH, specific conductance (SpC), flow rate, alkalinity and
collection of samples for major ions and trace element analysis. Since
sites had not been monitored prior to the initial shock, measurements
were evaluated against a year of post-event data to gauge response to
seismicity. Four sites were monitored: a well from Columbus Marsh (CM)
located 5 km from the epicenter; an artesian thermal well from Fish Lake
Valley (FL); a well at Willow Ranch (WR) tapping cool water above the FL
waters; and a spring along Mina Dump Road (MD) located 15 km north of
the Candelaria fault on the Benton Springs Fault. GPS and InSAR
measurements were used to create a model of the slip from which we
estimated coseismic strain at each sampling location. All but one sample
site, MD, experienced positive dilation and CM experienced the greatest
amount of strain (15-17 microstrains). Hydrologic and chemical changes
were observed following the initial shock, varying between sites. CM had
significantly lower SpC values in the week following the event, as well
as changes in major ion composition. Other sites showed minor changes;
MD showed fluctuations in pH values and FL experienced a slight drop in
temp. These waters showed minimal changes in major ions and trace
elemental composition. Clear responses were observed throughout three
>M5 aftershocks (6/30/20, 11/13/20, and 12/1/20),
especially in SpC and alkalinity. A remarkable change in elemental
concentration (an increase in Ca, K, SO4, Fe, and
decrease in Na, Cl, Li, and Ba) was observed in CM. WR experienced a
transient increase in temp measured two weeks prior to the 11/13/20
earthquake. Strain analyses of the smaller (>M5) events are
planned to further evaluate observed responses and to clarify factors
affecting groundwater response.