Lecture 12: Water, Earthquakes & Water Supply in the Bay Area: Combining Seismicity with Mapping

Transcription

1 Lecture 12: Water, Earthquakes & Water Supply in the Bay Area: Combining Seismicity with Mapping Dynamics & geochemistry of the Siesta Valley paleo-lake environment History of major earthquakes related to the Hayward fault system Water supply issues related to potential fault motions Putting our field mapping into a broader context of regional seismicity What can seismic data tell us about the faults we are mapping? Are the faults seismically active or inactive ancient features? Pre-folding, syn-folding or post-folding? Paleo-tectonic or neo-tectonic processes? What are the magnitudes, depths, focal mechanisms and are are the latter consistent with deformation we map? 1

2 2

3 Bio-genic Pyrite in Siesta Valley Limestone Recent oxidation Geochemistry: Pyrite was precipitated from aqueous H 2 S by sulfate reducing bacteria during limestone deposition within an anoxic part of the lake Only when the pyrite was exposed to the atmosphere 8 Ma later, did it make sulfuric acid that released ferrous iron. The calcite neutralized the acid so the iron traveled only a short distance. Eh in volts by meters What is the geochemistry of the water in the paleo- Siesta Valley Lake Environment such that pyrite formed? ph papers or ph meters 3

4 Atmosphere Surface Waters Solid Earth Ground Waters ph (acidity) Sulfur speciation depends on Eh and ph 4

5 Euxinic marine (anoxic) is due to sulfate reduction To H2S and Pyrite Eh-pH controls is also exerted by common rocks Magnetite-Hematite equilibria SO4 2- Hematite Magnetite 5

6 Common rock mineral assemblages can buffer Eh and ph Biotite (annite)-hematite-orthoclase equilibria: 2KFe 3 AlSi 3 O 10 (OH) 2 +H 2 O = 3Fe 2 O 3 +2KAlSi 3 O8+ 6H + +6e - G r o = 25.7Kcals Eh= pH HM MT Biotite Ground waters Hematite+ Orthoclase Bedrocks control the Eh-pH of Ground waters by buffering along common Fe-mineral equil. Start with traditional use of seismicity: Earthquake Hazards 6

7 Claremont Tunnel San Lorenzo San Lorenzo Tunnel 7

8 Claremont Water Tunnel Quiz 8

9 Slope Stability 9

10 Show San Francisco link- hold over a faults and read name 10

11 Our field area: The Land between big fault systems Is that all there is to it? 11

12 EPS 101/271 Field Area What can we learn from these earthquake epicenters that t do not fll fall along the Hayward Fault zone? Are the faults we map active? Are the focal mechanisms consistent with the apparent fault offsets? Key questions to consider: Since we rarely can see fault planes, can we use other means to infer the dips of the fault planes? Is the Orinda/Claremont contact an active fault? What is the stress field that causes faulting? What is the stress field that causes folding? Are faulting and folding possibly related? 12

13 How can we access earthquake data? Latitude Longitude Depth Magnitude UTM Easting UTM Northing (km) (meters) (meters) earthquakes between 1984 and 2004 in field area Box: Lat, Long Lat Long 13

14 Convert Latitude/Longitude data into UTM coordinates NAD 27 Datum Latitude Longitude Depth Magnitude UTM Easting UTM Northing (km) (meters) (meters) GeoMapper base map Berkeley Hills seismicity as DRG file 14

15 Earthquakes in Our Mapping Area East of the Hayward Fault Magnitude Depth (km) Where were these earthquakes? 15

16 How close are the epicenters to our faults? 16

17 Chance to integrate sets of data from other fields with Be careful our digital in interpreting mapping in cause GeoMapper and effect Besides earthquake epicenters. Base maps could ldbe for an endless variety of things: fossil localities archeological sites etc. (Brimhall and Dreger, 2005) 17

18 Dip Direction is perpendicular to strike What is the predominant strike of these focal planes? (Brimhall and Dreger 2005) Rake is rotation of the hanging wall block from horizontal (Brimhall and Dreger, 2004) 18

19 Issues to resolve: Are the faults we are mapping inactive, small, relatively insignificant features? Or do they persist well down into the continental crust? If they are active, what is the stress field that makes them? Maximum compressive stress direction What is the maximum compressive stress direction for the Siesta Vll Valley syncline? Is the maximum comprehensive stress direction possibly the same for the faults and the folds? What process could be causing the stress field? Mapping Intervolcanic Limestone Marker beds to define Faults North-trending valley beds Bald Peak Basalt? Basal Siesta Valley limestone 19

20 Demonstrate GeoMapper tools for the day Please be very careful in: (1) placing charging plug back onto tablet (2) putting spare battery back on charge in the table (3) locking your locker (4) Where is battery # 13? 20