EPS 50 Lab 5 The Hayward Fault at UC Berkeley

Transcription

1 EPS 50 Lab 5 The Hayward Fault at UC Berkeley Introduction: On this field trip we will see some of the most dramatic expressions of the Hayward Fault in the Berkeley area. Most of the subtle geomorphic expressions of the fault have been removed by development and by the natural processes of landslides and erosion. Some clear expressions of the fault remain, and these are important for mapping the main trace through the intervening sections. The field trip stops are shown in Fig. 1. Answers: Please answer the questions on the separate answer sheet handed out during lab. Only the answer sheet needs to be returned. Explanations should be concise; most questions can be answered with a few sentences or less. All answers should be your own, but we encourage you to discuss and check your answers with 2-3 other students. This is a terrific way to learn! Labs are graded out of 100 points. Figure 1: Field trip stops 1

2 Stop 1: Founders Rock During a meeting of the trustees of the College of California, Frederick Billings stood atop Founders Rock, gazed off into the sunset and announced, We will call it Berkeley. It was April 16, 1860, and the college, which was to be an agriculture and mining school, had just been built in a rural area north of Oakland. Trustees and many others met at Founders Rock again in 1968 to inaugurate the university. The site was chosen for the College because it provided an important resource: a year round water supply from Strawberry Creek even during the dry season. The fog that regularly enters the bay from the cold current off the coast supplies much of the water in Strawberry Creek. The fog hangs on the ridges, where the water condenses on the trees and shrubs, drips to the ground, and eventually percolates to Strawberry Creek. Without the Berkeley Hills the fog would rapidly dissipate, and the creek bed would dry up. Without the Hayward Fault, there would be no Berkeley Hills. There would be no Strawberry Canyon, and no year-round water supply. 1) What type of fault is the Hayward Fault? Sketch the fault motion. (5 pts) 2) What can explain the presence of Hills on the east side of the fault? (5 pts) Just as the hills are a geomorphic expression of the fault, there are other smaller landforms along the Hayward Fault that help us to recognize its exact location. Up the hill from Founders Rock (at the corner of Hearst Ave. and Gayley Road) is Foothill Housing. The initial plan was to connect the buildings north and south of Cyclotron Road with a second story walkway. That plan was scuttled during the construction due to the fear that the walkway would collapse in the case of an earthquake. The fault exits the Foothill Housing complex at the gate across from Highland road. Walk south along Gayley Road, from Founders Rock toward the stadium. Just north of the Greek theater on the left there is a relatively flat area. This area has been used to construct university buildings. It is the former bed of a beheaded stream channel of the Strawberry Creek. On Fig. 2, you can clearly see the bends in the contour lines just left of the Greek Theater indicating this beheaded valley, with no continuation farther east into the hills. One of the easiest ways to map a strike-slip fault is by lining up deflected drainages along its length. Many drainages are offset by the Hayward Fault, and some of these provide primary evidence of the fault s location. 2

3 Figure 2: Landforms and culture in the area of the Hayward fault zone. Major fault related landforms are: A-A = strawberry creek channel offset; B= abandoned Mining Circle channel; C= abandoned Hearst Avenue channel. Stop 2: Maxwell Field parking lot Just north of the stadium fence you can see the small walled enclosure containing the top of the Berkeley Seismological Laboratory Hayward Fault Network borehole station (CMSB). This seismometer at the bottom of a 100 m (300 ft, about the length of the football field) hole is part of a system for studying the Hayward Fault. 3) Why do seismologists install seismometers at depth? Contrast with surface records? (5 pts) North of the seismometer you can see a shallow ditch in the hillside. This was dug several years ago to study the history of the Hayward fault using the sediments. Stop 3: Memorial stadium 3

4 The stadium was built in 1923 as a tribute to UC Berkeley alumni who died in World War I; 250,000 cubic yards of fill had to be brought in for its construction! It straddles the Hayward Fault. Renovations carried out in to improve seismic safety included the placement of surface rupture blocks in each end zone, where the fault line passes beneath the stadium. These blocks are designed to allow the two halves of the stadium to move independently, up to 6 feet. The stadium sits on a shutter ridge that blocks the stream and deflects the flow of Strawberry Creek to the north. In fact, an 1100-foot offset of Strawberry Creek was buried in a culvert beneath the site. The creek emerges from a culvert behind the Women s Faculty Club near the intersection of Stadium Rim Road and Gayley Road. The channel that is now Hearst Avenue (C in Fig. 2) is likely an even older channel of Strawberry Creek, as was the Mining Circle Channel. 4) Today the Strawberry Creek shows an offset of about 300 m by the Hayward Fault (Figure 2). Analyses estimate the age of the channel at ~ 30,000 years. Calculate the average movement of the fault in mm/yr. (5 pts) 5) Compare your result with the total Pacific-North American Plate Boundary motion (~50 mm/yr). How do you explain the difference? (5 pts) Prior to renovations, the stadium structure had been offset by slow creep of the Hayward fault. More than 14 inches (355 mm) of offset had accumulated across the stadium since it was built. Some evidence of this displacement is still visible at the southern exterior of the stadium. 6) Discuss the difference between a stick-slip fault and a creeping fault. (5 pts) Figure 3: Velocities for of four benchmark in UC Berkeley's Memorial Stadium, shown relative to station STAC. The Hayward fault cuts down the middle of the stadium. These 4

5 observations show that the Hayward fault creeps at a rate of about 4.2 mm/yr (D Alessio et al., 2002). Fig. 3 shows four benchmarks in UC Berkeley s Memorial Stadium that are used for geodesic studies (GPS). The current creep rate along this portion of the fault is about 4.2 mm/yr. 7) Compare your result for the long-term slip rate of the fault (Question 4) with the present-day slip rate derived from GPS. Why are they different? What does the present-day slip rate indicate compared to the long-term slip rate? Is the fault releasing all the stress by creep? (10 pts) 8) What is the slip deficit (in cm) today considering that the last earthquake occurred in 1868? (5 pts) 9) If an earthquake occurs on the Hayward Fault today, would the stadium be a safe place to be? Are the recent renovations a good solution to the problems posed by the fault? Comment in terms of earthquake hazard and contrast with earthquake risk. (10 pts) Stop 4: Hamilton Creek Walk down Prospect St. to the first intersection on the left, then turn onto Hillside and walk to the bridge over Hamilton Creek. To the left of the bridge, there is an iron gate leading to a public footpath the goes uphill and alongside the creek. About 30 yards from the gate the creek turns sharply to the right. Walk down into the creek bed and follow it (a property wall will be on your right) another 30 yards or so until the creek turns sharply to the left and uphill. You have just walked through an offset stream channel. The portion between the two sharp bends lies directly on the Hayward Fault. You can observe the offset of the creek bed due to a topographic high (a smaller shutter ridge, the property within the wall) that blocked it from flowing straight downhill to the bay. 10) Sketch and describe the site. (5 pts) 11) The offset at Hamilton Creek is approximately 50 meters. Using both the presentday slip rate and the long-term slip rate estimate the time it took for the fault to create this offset. (10 pts) Stop 5: Dwight Way fault scarp Once on Dwight Way, walk left up the hill to the corner near the top of the street. As you walk up the steep slope, you are walking on the Hayward Fault scarp. As you get to the flatter area at 5

6 the top, turn around and stand facing the bay. Look at the curbs on either side of the street. Note the distinct displacement of the curb towards the right (at the level of the wooden electricity pole if you are standing on the left side of the street, facing the bay). The curb has recently been repaired on both sides of the street, but the offset is still visible. 12) Calculate the recurrence time of magnitude 6.8 earthquakes on the Hayward Fault considering the long-term slip rate and an average slip during the events of ~ 1.5 m. (5 pts) The following table shows the results of a trench study (Lienkaemper et al., 2007) on the southern Hayward Fault, in Fremont. Event Mean age (yr) Time interval (yr) ) Calculate the time intervals in year between each earthquake. Plot the recurrence time intervals versus the event date. (5 pts) 14) Calculate the average recurrence time of Magnitude earthquakes on the Hayward fault using the table. (5 pts) 15) Compare your results obtained in Question 13 and Question 15. What can be said about the earthquake hazards on the East Bay today? From your results when should the next earthquake occur? (Don t forget to talk about the uncertainties of the forecast). (5 pts) 6

7 Name: GSI: EPS 50 Lab 1 The Hayward Fault at UC Berkeley Answer sheet 1) 2) 3) 4) 7

8 5) 6) 7) 8) 9) 8

9 10) 11) 12) 9

10 13) Event Mean age (yr) Time interval (yr) Number of earthquake Time (year) 10

11 14) 15) 11