Foreshocks to Major Earthquakes In Nevada. Craig M. depolo Nevada Bureau of Mines and Geology

Transcription

1 Foreshocks to Major Earthquakes In Nevada Craig M. depolo Nevada Bureau of Mines and Geology

2 Review foreshocks, particularly the 2008 Mogul earthquake, foreshock statistics and implications, multiple earthquakes and implications, tectonic implications, and a few conclusions.

3 Nevada Earthquakes M 6 21 events eliminate aftershocks and those w/o records leaves 17 events all but one are pre-instrumental

4 Major Historical Nevada Earthquakes Examined for Foreshocks Date Magnitude Slip Foreshock Foreshock Foreshock Timeframe Reference (GMT) Type within 120 Sequence Magnitude (Days) Days /29 6.0? Yes small 0.02 depolo et al. (2003); Toppozada + (2000) /27 6.4? Yes moderate? 8 depolo et al. (2003) /3 6.5? No depolo et al. (2003) /21 6.1? Yes Yes moderate 3 depolo and Garside (2006) /18 6? Yes? small week before depolo and Garside (2006) /24 6.4? Yes Yes M 6; small 65; 0.9 depolo and Garside (2006) /3 6.1 N? Yes Yes M Slemmons et al. (1965) /3 7.3 N Yes Yes M Slemmons et al. (1965) / SS Yes moderate 0.03 Gianella & Callaghan (1934) /25 6? Yes moderate 2 Neumann (1935) / N Yes Yes M 4.1; M ; 1 Callaghan & Gianella (1935) /29 6? Yes Yes small; M ; 1.5 depolo et al. (2008) /6 6.2 SS Yes M Bolt and Miller (1975) / SS Yes Yes M 6.2; mod. 49; ~1 Slemmons et al. (1965) / NO Yes small 1.2 Slemmons et al. (1965) / N Yes M Slemmons et al. (1965) / N Yes Yes M 3.7; M ; 5.4 Smith et al. (2011) N normal dip slip SS strike slip NO normal-oblique slip

5 (Slemmons et al., 1965; depolo et al., 2008) Reno Fire Department went into an alert status 36 hours before the M 6

6 50 days of foreshocks Earthquakes recorded by the Nevada Seismological Laboratory thin vertical bar one eq. on day thick bar events M 3

7 (Anderson, NSL, 2008)

8

9 2008 Mogul Earthquake Sequence Peavine Mtn. station Day -12 Day -11 Seismic Records are from Ken Smith Nevada Seismological Laboratory

10

11 Day -10 Day -9

12

13 Mogul Earthquakes Days -8 thru 0

14 Day -1 Day 0 M w 5 Mogul Eq.

15

16 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 ed 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,

17 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.

18 17 Nevada Events with Records for Pre-Event Time Periods of 1 to 365 Days Time Percentage Earthquakes Earthquakes Earthquakes Period with with Foreshocks with Possible without Foreshocks Foreshocks Foreshocks 365 days 88 94% days 88 94% days 82 88% days 76% days 71% day 47% Carson Valley eq. no foreshocks for 1 year before in daily newspaper; just hot spring changes Major earthquakes can occur without warning!

19 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.

20 Nevada s Multiple M 6 Earthquakes Year First Event Elapsed Time Second Event Location 1869 M hrs M 6.2 Virginia City 1914 M 6 65 days M 6.4 Reno 1915 M hrs M 7.3 Pleasant Valley 1954 July M hrs M 6.1 Rainbow Mountain 1954 July M days M 6.8 Stillwater 1954 Dec. M min, 20 s M 6.9 Fairview Pk.-Dixie V.

21 Oct Helena, Montana Eq. Remarkable foreshock sequence recorded (62 events), Multiple M 6 earthquakes (M 5.9 [12 th ]; M 6.25 [19 th ]; M 6.0 [31 st ], Two recovery workers killed 12½ days after the 1 st 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.

22 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.

23 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)

24 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.

25 Remember events still will occur without warning.

26 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.