Local Seismicity recorded at MOD

Sparse information is available about the rate and the characteristics of the seismicity in the far northeastern corner of California and vicinity (Figure 23.1). The region exhibits the lowest level of seismicity of anywhere in northern California and vicinity (Uhrhammer, 1991). The largest historical earthquakes in the region are a pair of M_L 5.1 events, which occurred on May 29 and June 3, 1968 (local time), near Adel, OR as part of a sequence of earthquakes which began on May 26, 1968 and continued daily through June 11, 1968 (Niazi et al., 1969; USCGS, 1969). There is geologic evidence for Quaternary surface displacements along the $\sim$ 80 km long Surprise Valley fault in the region (Jennings, 1975) and the fault is considered capable of generating destructive earthquakes.

Microseismicity studies of the region have been hindered to date by the dearth of permanent short-period and broadband seismic stations in the region where the nearest short-period seismic stations are $\sim$ 100+ km away and the nearest broadband station is the USNSN station WVOR in Wildhorse Valley, Oregon, 150 km to the eastnortheast. The opening of the new BDSN station MOD, in Modoc County, on October 18, 1999, provides an unprecedented opportunity to study the microseismicity in the far northeastern corner of California and vicinity. MOD, as shown in Figure 23.1, is located $\sim$ 11 km south of the Oregon border and $\sim$ 25 km west of the Nevada border in a hard rock mining drift. The low background noise PSD and the sensitivity of the broadband sensors at MOD combine to allow detection and analysis of local earthquakes as small as M_L $\sim$ 0.6 at distances of up to 35 km.

Historical Seismicity

The CNSS composite catalog lists 103 historical earthquakes that have occurred within a 100 km radius of MOD and their locations are plotted in Figure 23.1. The CNSS composite catalog for northeastern California and vicinity is comprised of contributions from the NC (BSL and USGS), NEIC, NN (UNR) and UW catalogs. However, in the northeast corner of California and vicinity, the CNSS composite catalog considers the NEIC authoritative and the resulting list of historical earthquakes is neither uniform in its coverage nor in its magnitude completeness. The earliest event in the composite catalog occurred in April 1951 and only five events $M_{L} \ge$ 3.2 are listed prior to 1968. That no events are listed prior to 1951 is attributed to a combination of the low seismicity, the sparse seismic station coverage and the lack of sufficiently sensitive seismographs in the region. The installation of the sensitive vertical component Benioff seismograph (magnification of 30k at 1 Hz) at Mineral (MIN) (205 km SSE of MOD) in February 1948 (Romney and Meeker, 1949) was crucial for detecting and analyzing these early events. Only eight events are listed within 35 km of MOD. In the 35-65 km distance range, 35 events are listed and of these 25 are in southern Oregon (22 of which are part of the 1968 Adel, OR earthquake sequence), 7 are in northeastern California, and 3 are in northwestern Nevada. In the 65-100 km distance range, 60 events are listed and of these 6 are in southern Oregon, 8 are in northwestern Nevada, and the rest are in northeastern California. Of the 18 $m_{b} \ge$ 4 (assigned by NEIC) events in the list, only one was not a part of the 1968 Adel, OR sequence.

**Figure 23.1:** Map of MOD (solid triangle) and vicinity. The solid lines are the state boundaries, the fine dashed circles with radii of 35, 65, and 100 km are given as a distance reference, and North is straight up on the map. The large straight dashed line segments are the CNSS authoritative region boundaries and the authorities are: UW - upper left; NC - lower left; NN - lower right; and NEIC - central and upper right. The large curved dashed line in the lower left is the boundary between the Pacific Ranges and the Basin and Range physiographic provinces. The medium dashed lines represent the faults in the region and only the Surprise Valley Fault (SE of MOD) is labeled. The small symbols represent the historical seismicity differentiated by decade: square - 1950's; octagon - 1960's; diamond - 1970's; plus - 1980's; and hourglass - 1990's. The asterisk in the far NW corner of Nevada is the location of the event shown in Figure 23.2.
$\begin{figure} \begin{center} \epsfig{file=bob00_5_1.eps, width=13cm}\end{center}\end{figure}$

The Adel, OR earthquake sequence was investigated by Schaff (1976) who analyzed 169 events in the sequence that occurred between May 1968 and July 1968. Schaff found a focal mechanism for the mainshock which indicates left-lateral oblique-slip motion on a plane that dips 80 $^\circ$ E and strikes N4 $^\circ$ E and that the vertical component on this plane is reverse fault motion. This disagrees with the local Basin-and-Range geomorphology and it favors models which invoke subduction tectonics in the Northern California-South Central Oregon Region. Note that the number of events analyzed by Schaff is 7.5+ times the number of events that the CNSS composite catalog lists for the sequence. This is an example of a known deficiency in the CNSS composite catalog (as indicated by caveats on the CNSS web page) that can be attributed to variations in location thresholds, magnitude thresholds, and methodologies between the NEIC (assigned as the authority because the region lies between the regional networks) and the regional networks (BSL, USGS, NN and UW) which have superior station coverage.

MOD Instrumentation

The BDSN seismic station on the Modoc Plateau (MOD in Figure 23.1) is sited in a hard rock mining drift on the western flank of the Warner Mtns. The vault is constructed from a steel Sea-Land shipping bin and reinforced concrete and there is $\sim$ 5 m of overburden to help maintain thermal stability. The seismic sensors consist of a three component set of Streckeisen STS-1 broadband seismometers and a triaxial Kinemetrics FBA-ES-T strong motion accelerometer. Auxiliary sensors include a thermistor to measure the seismic pier temperature and a microbarograph to measure local atmospheric pressure fluctuations. There is also a co-sited geodetic GPS receiver that is part of the BARD network. A Quanterra Q935 24-bit integer resolution datalogger records the signals and they are telemetered to BSL in near real-time via a NSN satellite link.

**Figure:** MOD three-component broadband recording of a local M_L 3.0 earthquake that occurred on April 30, 2000, (distance 33km; azimuth N75 $^\circ$ E) in the far NW corner of Nevada (asterisk in 23.1. All components are plotted to the same scale.
$\begin{figure} \begin{center} \epsfig{file=bob00_5_2.eps, width=7cm}\end{center}\end{figure}$

Local Micro-earthquakes

The high-frequency ( $\ge$ 1 Hz) background noise PSD observed at MOD is sufficiently low that the broadband sensors can detect and locate local microearthquakes ( $\Delta \le$ 35 km) down to M_L $\sim$ 0.6. The energetic high-frequency spectral response of these local microearthquakes requires that the acausal FIR anti-aliasing filter effects (Scherbaum, 1996) be removed in order to unambiguously determine the onset time and propagation azimuth of the seismic phase arrivals. This is particularly important since the local microearthquakes are typically not recorded with usable signal levels at stations other than MOD. An example of a local earthquake recording is shown in Figure 23.2. This M_L 3.0 earthquake occurred on April 30, 2000 at 0353 UT and it was located $\sim$ 33 km N85 $^\circ$ E of MOD in the far northwestern corner of Nevada (star symbol in Figure 23.1). Spectral analysis of this event shows that the signal-to-noise ratio is $\sim$ 60 dB at $\sim$ 4 Hz and that the seismic signal is above the background noise at frequencies up to 26 Hz. This event was not present in any of the web-accessible seismicity lists generated by the regional networks or by the NEIC.

Analysis of Local Earthquakes

It should be noted that the procedures that are used cooperatively by the USGS and the BSL for the routine detection and analysis of earthquakes which occur in Northern California and vicinity failed to detect the local earthquakes that have been recorded by MOD, including the M_L 3.0 event shown in Figure 23.2 and six other local earthquakes that occurred between April 26 and May 6, 2000. Thus we can not rely on the routine procedures to analyze the local earthquakes which occur near MOD and we will have to set up a special procedure to insure that these earthquakes are included in the BSL routine analysis. At the present time, we are extracting the data associated with HHZ triggers in the MOD log file and visually looking at the waveforms to identify the local events. We find that the HHZ channel is triggering on numerous regional and teleseismic signals as well as on local earthquakes and local cultural noise. The BSL started recently receiving a continuous 40 Hz telemetry feed from the USNSN broadband station WVOR, located in southern Oregon 150 km ENE of MOD, which should greatly aid in the detection and analysis of the local earthquakes in the region.

References

Jennings, C. W., Fault Map of California with locations of volcanoes, thermal springs, and thermal wells, CDMG, California Geologic Data Map Series, Map 1, scale 1:750,000, 1975.

Niazi, M., M. Somerville, and L. Dengler, Bulletin of the Seismographic Stations, University of California Press, 38, No.1, pp. 161, 1969.

Romney, C. F. amd J. E. Meeker, Bulletin of the Seismographic Stations, University of California Press, 18, No.1, pp. 1-89, 1949.

Schaff, S. C., The Adel, Oregon earthquake sequence: evidence for present day subduction tectonics in northwestern Nevada, EOS, Transactions, American Geophysical Union, 57, 958, 1976.

Scherbaum, F., Of Poles and Zeros: Fundamentals of Digital Seismology, Modern Approaches in Geophysics, Volume 15, G. Nolet, Managing Ed., Kluwer Academic Press, Dordrecht, The Netherlands, pp. 256 + xi, 1996.

Uhrhammer, R. A., Northern California seismicity, The Geology of North America, Decade Map Volume I, Chapter 7, 99-106, 1991.

USCGS, Abstracts of earthquake reports for the United States, U.S. Coast and Geodetic Survey, April-June 1968, MSA-138, 1969.