Foreshocks to
Major Earthquakes
In Nevada
Craig M. dePolo
Nevada Bureau of Mines and Geology
Review foreshocks, particularly the 2008 Mogul earthquake,
foreshock statistics and implications,
multiple earthquakes and implications,
tectonic implications, and a few conclusions.
Nevada
Earthquakes
≥M 6
21 events
eliminate aftershocks
and those w/o records
leaves 17 events
all but one are
pre-instrumental
Major Historical Nevada Earthquakes Examined for Foreshocks
Date
(GMT)
Magnitude
Slip
Type
Foreshock
within 120
Days
1868 5/29
1869 12/27
1887 6/3
1910 11/21
1914 2/18
1914 4/24
1915 10/3
1915 10/3
1932 12/20
1933 6/25
1934 1/30
1948 12/29
1954 7/6
1954 8/24
1954 12/16
1954 12/16
2008 2/21
6.0
6.4
6.5
6.1
6
6.4
6.1
7.3
7.1
6
6.1
6
6.2
6.8
7.1
6.9
6.0
?
?
?
?
?
?
N?
N
SS
?
N
?
SS
SS
NO
N
N
Yes
Yes
No
Yes
Yes?
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
N – normal dip slip
SS – strike slip
NO – normal-oblique slip
Foreshock
Sequence
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Foreshock
Magnitude
Timeframe
(Days)
small
moderate?
0.02
8
Reference
dePolo et al. (2003); Toppozada + (2000)
dePolo et al. (2003)
dePolo et al. (2003)
moderate
3
dePolo and Garside (2006)
small
week before dePolo and Garside (2006)
M 6; small
65; 0.9
dePolo and Garside (2006)
M5
0.1
Slemmons et al. (1965)
M 6.1
0.2
Slemmons et al. (1965)
moderate
0.03
Gianella & Callaghan (1934)
moderate
2
Neumann (1935)
M 4.1; M 5.6 261; 1
Callaghan & Gianella (1935)
small; M 4.3 84; 1.5
dePolo et al. (2008)
M3
0.04
Bolt and Miller (1975)
M 6.2; mod. 49; ~1
Slemmons et al. (1965)
small
1.2
Slemmons et al. (1965)
M 5.9
107
Slemmons et al. (1965)
M 3.7; M 1.4 358; 5.4
Smith et al. (2011)
(Slemmons et al., 1965; dePolo et al., 2008)
Reno Fire Department went into an alert status 36 hours before the M 6
50 days of foreshocks
Earthquakes recorded by the Nevada Seismological Laboratory
thin vertical bar ≥one eq. on day
thick bar events ≥M 3
(Anderson, NSL, 2008)
2008 Mogul Earthquake Sequence – Peavine Mtn. station
Day -12
Day -11
Seismic Records are from Ken Smith – Nevada Seismological Laboratory
Day -10
Day -9
Mogul Earthquakes
Days -8 thru 0
Day -1
Day 0 – Mw 5 Mogul Eq.
Response to Mogul Foreshocks
• Day -10 M 3s; Washoe Co. emergency
responders alerted; no timeframe – review
emergency response plans & check their houses,
1000 Living w/Eqs (mailboxes, library, firehouses);
Public – purchased eq insurance,
• Day -8 Emailed Sierra Pacific Power – advised
putting an ad in the paper on shutting off gas;
Ken Smith got press conference together,
• Day -7 Gave talk at Governor’s office to State
PIOs on the swarm & earthquakes,
Response to foreshocks cont.
• Day -6 to -3 Continued press releases from
NSL, local GPS stations installed,
• Day -2 M 4s – immediate foreshock seq.;
Public – est. $200k+ value mitigation occurred,
• Day 0 Emergency response to affected area
within minutes.
17 Nevada Events with Records for
Pre-Event Time Periods of 1 to 365 Days
Time
Period
Percentage
with
Foreshocks
365 days
120 days
30 days
5 days
3 days
1 day
88 – 94%
88 – 94%
82 – 88%
76%
71%
47%
Earthquakes
with Foreshocks
Earthquakes
with Possible
Foreshocks
Earthquakes
without
Foreshocks
15
1
1
15
1
1
14
1
2
13
0
4
12
0
5
8
0
9
1887 Carson Valley eq. – no foreshocks for 1 year before in daily newspaper;
just hot spring changes
Major earthquakes can occur without warning!
Implications
• 5-day elevated probability – 47% (8 events w/
≥mod. foreshock),
• 8 events w/foreshock sequences,
• Higher chance 24-hr foreshock than San Andreas
f. (47% NV vs. 35% SAF),
• Mod. events and eq. sequences near
communities create windows of opportunity for
preparedness and mitigation.
Nevada’s Multiple M ≥6 Earthquakes
Year
First Event
1869
1914
1915
1954 July
1954 July
1954 Dec.
M 6.4
M6
M 6.1
M 6.2
M 6.2
M 7.1
Elapsed Time
8 hrs
65 days
5 hrs
11 hrs
49 days
4 min, 20 s
Second Event
Location
M 6.2
M 6.4
M 7.3
M 6.1
M 6.8
M 6.9
Virginia City
Reno
Pleasant Valley
Rainbow Mountain
Stillwater
Fairview Pk.-Dixie V.
Oct. 1935 Helena, Montana Eq.
• Remarkable foreshock sequence recorded
(62 events),
• Multiple M 6 earthquakes (M 5.9 [12th]; M
6.25 [19th]; M 6.0 [31st],
• Two recovery workers killed 12½ days after
the 1st M 6.25 by shaking from the second M
6. They were bringing a factory chimney down
from the top when the second quake struck.
Implications of Multiple Eqs.
• The public, emergency responders, and
recovery workers need to be mindful that a
strong aftershock or second earthquake could
follow a major event and use the appropriate
caution that this dictates.
• Short-term earthquake warning systems may
give a life-saving warning to emergency
responders.
Why all the foreshocks?
• Inefficient (evolving) seismotectonic pattern –
highly pre-conditioned crust (poorly oriented for
contemporary strain).
– Many foreshocks & aftershocks
– Multiple major events
– Complex Quaternary fault patterns
• Triggered earthquakes – (NV highly susceptible
to triggered events)
Summary Recommendations
• Eq. sequences & M 4 events near communities
should be treated as windows of opportunity for
seismic preparedness and mitigation.
• 5-day warnings of elevated probabilities for M 4
and greater events is a useful message for major
earthquakes.
• Running probabilities may be useful in objectively
characterizing eq. sequence behavior.
• Remember events still will occur
without warning.
Eq. Forecasting Realities
(What people have to understand)
• Low probabilities 1 to 10%,
• Many eq. forecasts without event,
• Events without warning.
But:
• Unacceptable consequences when unprepared
and an earthquake strikes – makes the above
realities worth dealing with.