External Grant Award Number 01HQAG0009 WESTERN GREAT BASIN SEISMIC NETWORK OPERATIONS. December 1, 2000 to November 30, 2001

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1 External Grant Award Number 01HQAG0009 WESTERN GREAT BASIN SEISMIC NETWORK OPERATIONS December 1, 2000 to November 30, 2001 Glenn P. Biasi University of Nevada Reno Seismological Laboratory MS-174 Reno, NV Ph: (775) FAX: (775)

2 ABSTRACT External Grant Award Number 01HQAG0009 WESTERN GREAT BASIN SEISMIC NETWORK OPERATIONS Glenn P. Biasi University of Nevada Reno Seismological Laboratory MS-174 Reno, NV Ph: (775) FAX: (775) Principle efforts during the December 1, 2000 to November 30, 2001 period included continued operation of the Western Great Basin Seismic Network and the installation of twelve new strong motion recorders under the Advanced National Seismic System. Over 6600 earthquakes were located by the Nevada Seismological Laboratory during the year. The largest earthquakes, an Ml 5.3 event on August 10, 2001, and an Ml 4.9 earthquake on December 2, 2000 were both in the eastern Sierra Nevada mountains. Neither event caused significant damage. Seismicity follows recognized trends in the Walker Lane tectonic province, consistent with the approximately 13 mm/year of strike-slip motion passing through the region. The above noted large events in the eastern Sierras suggest that at least some of this 13 mm/yr is in the eastern mountain block. In the strong-motion component of the grant effort, eight new strong motion recorders were installed in the Reno-Carson corridor, and four were installed in the Las Vegas Valley. Radio telemetry was installed to most of the Reno area stations, while direct IP telemetry was used in Las Vegas. No strong-motion of consequence occurred during the reporting period, but events recorded since December 1, 2001 indicate that substantial site effects are present. In particular the one recorder explicitly sited on rock shows ground accelerations nearly as high as the highest of the other stations sited on sediments. Variability among stations and among geologic types should eventually yield an improved picture of seismic hazard conditions in the urban regions of Nevada.

3 INTRODUCTION This report details results and progress for the period from December 1, 2000 through November 30, 2001 for USGS External Grant 01HQAG0009, Western Great Basin Seismic Network Operations. The Nevada Seismological Laboratory (NSL) used grant funds to operate and maintain the Western Great Basin Seismic Network and to pursue objectives of the National Earthquake Hazard Reduction Program in the network area. The Western Great Basin Seismic Network covers from the eastern Sierra Nevada Mountains in California to central Nevada, with regional coverage focused on the Walker Lane Tectonic Belt. Sierran micro-plate motion relative to central Nevada, amounting for approximately 20 percent of the total Pacific-North American relative motion, is the leading source of earthquake hazard in eastern California and western Nevada. Interaction between this strike-slip regime and ongoing Basin and Range extension also gives the western Great Basin region unique characteristics for tectonic and seismic hazard research. Specific tasks for the reporting period included: 1. Seismic Network Operations 2. Real-time Integration of Networks 3. Access to and Distribution of Earthquake Information 4. ANSS Strong Motion Installation SEISMIC NETWORK OPERATIONS Calendar year 2001 was the first year during which the WGBSN operated entirely under the Antelope Real-Time software system. Through December 2000 the CUSP system was retained for backup and comparison purposes. Combining analog and threecomponent digital stations in a single processing system approximately doubled the number of located earthquakes compared to the operation of digital and analog networks separately. The increase in detected earthquakes significantly increased the workload on network analysts compared to previous years. A total of 6613 earthquakes were located in the combined seismic network between 12/1/2000 and 11/30/2001 (Figure 1). The tectonic complexity of the Walker Lane Belt is evident by the concentration of Ml>3.0 earthquakes between 37 and 38.5 degrees north, where a major right step is suggested from seismic and tectonic evidence. One earthquake large enough to be widely felt in the Las Vegas Valley occurred in the past year, an Ml 3.5 event on February 4, Areas of continuing activity include Scottys Junction (37.4N, 117.0W) and Little Skull Mountain (36.7N, 116.5W) in southern Nevada, south and east of the Long Valley region (37.5N, 118.5W), north of Pyramid Lake (40.5N, 119.5W), and in the eastern Sierra Nevada Mountains (39.5N, 120.6W).

4 Figure 1. (next page) Nevada seismicity from December 1, 2000 through November 30, Circles indicate earthquakes of Ml = 3.0 to 5.3, scaled by magnitude. Yucca Mountain is in the southwest corner of the Nevada Test Site in southern Nevada.

5 The density of small earthquakes in southern Nevada reflects the presence of the seismic network there monitoring seismicity for the Department of Energy in support of the Yucca Mountain Site Characterization. Approximately 1800 were located within 50 kilometers of Yucca Mountain in the reporting period. There were 32 three-component, high dynamic range recorders during the year DOE investments in telemetry and infrastructure for their project needs have collateral benefits for monitoring in much of the state. The largest earthquake in the Western Great Basin Seismic Network in the past year was an Ml 5.3 event on August 10, 2001 at 20:19:26, 15 km northwest of Portola, California (Figure 2). The network was recently extended into this area because telemetry infrastructure was available and the area has produced several moderate earthquakes in recent years. The mainshock was felt as far south as Carson City, but no damage was reported in the epicentral area, probably because of the depth of the event, 17.8 km. By contrast, a shallower M3.7 earthquake 21 months earlier caused some breakage in local stores. Most of the located aftershocks occurred at similar depths to the mainshock. Two portable recorders were deployed to record three-component waveforms on scale and to confirm the depths with clear S-minus-P times. The August 10, 2001 event had a strikeslip mechanism consistent with trends in previous seismicity. The exact contribution of this NW-trending seismic lineation to overall Sierran block motion is not known, but it appears to be significant. The second largest earthquake in the reporting period was also in the Sierras and widely felt in western Nevada. It had a local magnitude of 4.9, and occurred in the eastern Sierras, just north of the I-80 corridor, northwest of Lake Tahoe, and about 12 km from the nearest WGBSN station. REAL-TIME INTEGRATION OF NETWORKS Extensive integration of networks was maintained by exchange of real-time waveform data throughout the reporting period. Real-time waveform exchanges included networks in southern California, University of California San Diego, University of California Berkeley, and the USGS NSN, and with the IRIS Data Management Center. Stations imported from Caltech, UCSD, and the Berkeley Digital Seismic Network were used to stabilize hypocentral solutions for regional distance events outside the WGBSN. Capabilities were extended during the reporting period by USGS addition under the ANSS task of a new Ultra-10 computer dedicated to file and waveform sharing, and the purchase of a disk array to improve data archive reliability and access. In the arena of network cooperation, discussions were initiated with the California Integrated Seismic Network to clarify ANSS boundaries and reporting relationships. Historic, physical, and physiographic realities put areas of eastern California within the WGBSN, but ANSS has been organized on state boundaries, necessitating procedural adjustments and closer network cooperation.

6 Figure 2. Seismic activity in the eastern Sierras in and near the WGB Seismic Network. Scaled circles mark earthquakes with Ml>=3.0. The Portola area mainshock (39.8N, W) had a local magnitude of 5.3. Triangles indicate WGBSN stations.

7 ACCESS TO AND DISTRIBUTION OF EARTHQUAKE INFORMATION NSL distributed seismic data during the reporting period by several methods. The primary public interface for earthquake information is the NSL web page, This site presently averages 4500 requests/day, spanning a wide spectrum of public interests. The earthquake catalog from 1852 to the present was made available with mapping capabilities in early Felt reports are obtained for events greater than M3 in western Nevada. The site was linked this year to the USGS "ShakeMap" pages ( and to an Intermountain-West region added there by USGS Pasadena staff. Selected data are being archived presently at two locations, the Northern California Data Center and at the IRIS Data Management Center. These data feeds are maintained by Antelope system transfers directly from the real-time acquisition system. Strong-motion data gathered in the Portola earthquake of August 10, 2001 is the first contribution to a publicly available archive of strong-motion data. Intended in the future to serve the most common type of data requests, records are accessible by links from the NSL home page. ANSS STRONG MOTION INSTALLATION A June 2001 to the Western Great Basin Seismic Network grant provided funds for the installation and operation of twelve new Kinemetrics K2 strong motion seismographs. These instruments were provided under the urban strong motion component of the USGS FY 2001 Advanced National Seismic System initiative. Siting and site installation planning was started early in the reporting period, so that construction could begin in the early summer. Of the twelve instruments, eight were installed in the Reno-Carson urban corridor and four were installed in Las Vegas. All were installed and operating in at least triggered mode by the end of the federal fiscal year. Telemetry design was intended to avoid continuing line charges where possible. We installed BreezeCom IP 2.4 Ghz IP or 900 MHz Freewave radio telemetry for the Reno area stations. Engineering and permitting is in progress to extend the radio network into the Carson Valley, approximately 30 km south of Reno. Two of the Las Vegas area instruments are presently being received directly over the Internet, and a third is expected to come on line presently. Internet access approval was withdrawn at the fourth Las Vegas station, requiring us to explore other telemetry options. Table 1 gives the location of the K2 recorders installed in the reporting period, and also the locations of the Kinemetrics Etna recorders installed with other funds in recent years. Telemetered data are recorded directly into an Antelope Real Time System in parallel with other network stations. The Antelope interface also provides remote command, control, and event retrieval functions.

8 Table 1: Nevada Seismological Laboratory Strong Motion Stations Name Location GPS Coordinates Recorder Latitude Longitude Type SN Reno RF05 Reno Fire Station # K RF07 Reno Fire Station # Etna 1565 RF08 Reno Fire Station # K RF10 Reno Fire Station # Etna 1568 RF11 Reno Fire Station # Etna 1567 NOAA National Weather Station K PICO Picolo Elementary School K UNRX University Extension K PAML 301 Roberts, Reno, NV K Sparks SF02 Sparks Fire Station # Etna 1569 Carson City CF01 Carson City Fire Station # Etna CF02 Carson City Fire Station # K CF03 Carson City Fire Station # K Las Vegas LV04 Las Vegas Fire Station # Etna 2093 LV09 Las Vegas Fire Station # Etna 2095 LVSP Las Vegas Springs K CHYN Cheyenne College K UNLV UNLV K DOE DOE Building, Las Vegas K No true strong motion occurred near the installed K2 recorders during the reporting period. On February 5, 2002, however, an Ml 3.1 earthquake occurred southeast of Reno which was well recorded by the Reno area stations. Figures 3a and 3b show vertical and east component accelerograms for Reno area stations, plotted in order of epicentral distance. Peak horizontal accelerations were between 0.18 and 0.55 cm/sec**2, consistent with felt reports. Shear-waves are clearly isolated in the east-west component recordings and clearly recorded. A variety of geologic site conditions are sampled in the stations shown. Station PICO is on a very large alluvial fan surface. UNRX is on the eastern flood plain of the Truckee River. Sites RF05 and RF08 are on well consolidated fine-grained Pliocene terrace deposits. Station NOAA is nearly 20% farther from the epicenter, but recorded nearly as high accelerations. It is sited on Miocene volcanic rock on the north end of the Truckee Meadows. Figure 4 shows peak accelerations in narrow band-pass windows as a function of frequency after adjusting for the epicentral distance of station PICO. It confirms the qualitative impression from Figure 3b that the NOAA site has higher accelerations at most frequencies than the other stations. Amplitudes from

9 a nearby M2 event (not shown) follow a similar pattern. From Figures 3b and 4 it appears that the one site installed on rock is not likely to be a suitable "rock" reference site. Obviously a larger sampling will be required to truly characterize site response, but Figures 3 and 4 show that good data will be forthcoming. SUMMARY Over 6600 earthquakes were located in the course of Western Great Basin Seismic Network operations between December 1, 2000 and November 30, The largest earthquake within the network, (Ml 5.3) occurred west of Portola, California on August 10, The second largest located event was also in the Sierran block, 12 km west of the nearest WGB station. Twelve new strong-motion recorders were installed and are presently operating in the Reno-Carson corridor and in the Las Vegas Valley. Preliminary recordings by the Reno area stations show variations in distance-equilibrated ground motion of over a factor of two, and that the station physically on rock is not a good candidate for a reference site in ground motion amplification and site effect studies. BIBLIOGRAPHY Grant-supported data was not used in refereed papers by the Nevada Seismological Laboratory published during this grant period. Figure 3 (a) Vertical component accelerograms from Reno K2 stations. Waveforms are plotted at the same scale to illustrate absolute amplitude differences. (b) East-West component accelerograms. Figure 4. Narrow-band filtered peak amplitudes for the accelerograms in Figure 3b.

10 Figure 3a Figure 3b

11 :036 Narrow Band Peak Accelerations, Reno ANSS Stations NOAA E PICO E UNRX E RF05 E RF08 E 10 1 Acceleration (cm/sec**2) Band Center Frequency (Hz) Figure 4