The PNSN strong-motion instrument program is soliciting cooperation from regional utilities and industries for assistance in siting, telemetry, instrumentation and cooperative operations of strong-motion instrumentation within our network area. We are developing procedures for rapid communication of strong-motion parameters of engineering interest to strong-motion program participants and to other interested parties.

PNSN Response to moderate-sized earthquakes

The magnitude 5.4 Duvall earthquake on May 2, 1996 (local time) was the second magnitude 5+ earthquake within the Puget basin in the past 18 months, and the fourth damaging earthquake in the PNSN area during the past 3 years. While the PNSN responds to several dozen felt earthquakes every year, damaging earthquakes occur less frequently, but pose special problems from an operational perspective. Recent seismicity (Scotts Mills OR, March 1993; Klamath Falls OR, Sept. 1993; Robinson Point, Jan. 1995; Duvall, May 1996) has tested our procedures, and allowed us to examine what happens at the PNSN during an emergency situation.

A PNSN seismologist is always on duty, and our standard procedure is to respond to pages from our automatic earthquake detection process (for any earthquake within our network of magnitude 2.9 or larger), or from Washington or Oregon emergency management agencies, or the UW police. Our automatic detection process generates PRELIMINARY faxes and electronic mail to emergency managers and other high-priority information users for earthquakes that it judges M>=2.9. We follow-up with phone calls and faxes and e-mail with the FINAL solution. In addition to ordinary phone lines, we now have a radio link to the Washington State Dept. of Emergency Services, and an independent direct phone link to the City of Seattle EOC.

Following the Duvall earthquake, automatically generated e-mail and pager alerts were sent out within 62 seconds (the first preliminary magnitude estimate was 4.3), and the seismologist on duty called the Washington Dept. of Emergency Management (DEM) within 130 seconds. Within 12 minutes, staff (and the press!!) began to arrive at the Lab, and within 16 minutes we were able to provide DEM with a good location estimate and a 5+ estimate of the magnitude. Our audio library was updated 18 minutes after the earthquake and our Web-site was updated with a special section on the Duvall sequence within a few hours. Because of the intense interest in our Web information, we now have a shell script to initiate a Web-page as soon as a final location is available. Frequent automated updates of information are posted on the Web-page each time the analyst finalizes locations and magnitudes.

Minor operational problems following the Duvall event included congestion in the Lab caused by the press, failure of automatic data-retrieval programs to fetch broad-band data, and a Web server overload caused by intense Web activity. We have taken measures to address these problems.

Following a sizable felt earthquake, we must rapidly provide information to a wide variety of users while continuing data acquisition, analysis, and interpretation. These tasks are tightly interconnected, as we attempt to rapidly calculate and communicate the best estimates of location, depth and magnitude, while simultaneously tracking aftershocks, planning field operations, and interpreting the tectonic significance of the event while staying in close communication with emergency managers.

Seismicity

Figure 2 shows earthquakes of magnitude 2.0 or larger located in Washington and Oregon during this reporting period. The PNSN processed 5,041 events between Oct. 1, 1995 and Sept. 30, 1996. Of these, 4,129 were earthquakes or blasts within the network (1,297 of which were too small to locate). Within our network area, 2,288 earthquakes were located west of 120.5 degrees west longitude (including 623 near Mount St. Helens, which has not erupted since 1986), and 140 east of 120.5 degrees west longitude. The remaining events were blasts within the network, regional earthquakes (266) or teleseisms (646).

• Figure 2. Earthquakes magnitude 2.0 or larger between Oct. 1, 1995 and Sept. 30, 1996. Locations of a few cities are shown as white-filled diamonds. Earthquakes are indicated by red circles or green squares; red circles represent earthquakes at depths shallower than 30 km, and green squares represent earthquakes at 30 km or deeper.

During this reporting period there were 32 earthquakes reported felt west of the Cascades in Washington, ranging in magnitude from 2.2 to 5.3, and 1 (magnitude 3.1) felt east of the Cascades.

In Oregon, a total of 4 earthquakes were reported felt. Of these, one was in the Klamath Falls area, where a pair of damaging earthquakes in September of 1993 (Sept. 21, 03:29 and 05:45 UTC; magnitudes 5.9 and 6.0 respectively) were followed by a vigorous aftershock sequence which has decreased over time. Another was in the Woodburn, Oregon area (where the magnitude 5.6 "Scotts Mills" earthquake of March 25, 1993 occurred). Other felt earthquakes occurred near Mt. Hood and near Coos Bay.

The focal depth of 7 km determined for this earthquake is consistent with other seismicity in the area; there is an eastward shallowing of seismicity from Puget Sound to the Cascades. The PNSN catalog contains 390 earthquakes for the first 100 days of the Duvall sequence, where 96 aftershocks are Mc => 2.0, and 9 aftershocks are 3.0 <= Mc <= 3.4. Bradford and Waters [1934] reported on a 1932 earthquake that occurred in approximately the same location as the Duvall mainshock (Bradford, D.B and A.C. Waters, The Tolt River Earthquake and its Bearing on the Structure of the Cascade Range, BSSA, V. 24, p. 51-62.)

Both P - wave first motions and three-component broadband seismograms reveal that the mainshock had a thrust mechanism with an east-west compression axis. This is unusual since focal mechanisms from most Puget Sound earthquakes indicate regional north-south compression. A least-squares moment tensor inversion using PNSN broadband seismograms yields a strike = 350 degrees, dip = 53 degrees (to the east), rake = 83 degrees, and M_W = 5.0.

Collecting felt reports over the Internet, the PNSN rapidly estimated ground motion intensity at 845 sites in the region. An intensity contour map was generated two weeks after the mainshock. The map uses the Community Decimal Intensity (CDI) scale developed by Lori Dengler at Humboldt State University. The CDI scale is calibrated to the U. S. Geological Survey Modified Mercalli intensity scale but differs in that it uses a rigorous scheme for averaging responses over a community area. An advantage of the CDI scale is that it allows automation of the intensity estimate.

Public Education and Outreach

Staff from the PNSN provide an educational outreach program to better inform the public, policy makers, and emergency managers about seismicity and natural hazards. In addition to information sheets, lab tours, workshops, and media interviews, we have an audio library with several tapes, including a frequently updated message on current seismic activity. Similar information, and much more, is available via Internet on the World-Wide-Web (WWW):

Publications

Ludwin, R.S., A.I. Qamar, S.D. Malone, R.S. Crosson, S. Moran, G,C. Thomas, and W.P. Steele (in preparation), Earthquake Hypocenters in Washington and Oregon, 1990-1994, Washington State Dept. of Natural Resources Information Circular

Ma, L., R.S. Crosson, and R.S. Ludwin, (1996), Focal Mechanisms of western Washington earthquakes and their relationship to regional tectonic stress, in: Volume 1, USGS Professional Paper 1560 "Assessing Earthquake Hazards and Reducing Risk in the Pacific Northwest", p. 257-284.

Malone, S.D., 1995, The Internet- a quick recap of where and how to get seismological information, Seismological Research Letters V66, N6, 89-91.

Malone, S.D., 1996, The International Internet- Global Connectivity, Seismological Research Letters V67, N1, 28-29.

Malone, S.D., 1996, Spyders on the Internet, Seismological Research Letters V67, N5, 58-63.

Malone, S.D., 1996, "Near" Realtime Seismology", Seismological Research Letters V67, N6, 52-54.

Stanley, W. D., S. Y. Johnson, A. Qamar, C. S. Weaver, and J. M. Williams, 1996, Tectonics and seismicity of the southern Washington Cascades range, Bull. Seism. Soc. Am., V. 86, p. 1-18.

Thomas, G.C., R.S. Crosson, D.L. Carver, and T.S. Yelin, 1996, The 25 March 1993 Scotts Mills, Oregon, Earthquake and aftershock sequence: spatial distribution, focal mechanisms, and the Mount Angel Fault, BSSA, V. 86, N. 4, p. 925-935.

Univ. of Wash. Dept. of Earth and Space Sciences, 1996, Quarterly Network Reports; 95-D, 96-A,96-B, and 96-C; Seismicity of Washington and Oregon

Crosson, R.S., 1996, Seismic root structure of the Cascade Volcanic Arc of Washington: evidence for deep crustal magmatism, EOS, V. 77, No. 46, p. F514.

Crosson, R.S., S.R. Dewberry, N.P. Symons, and G.C. Thomas, 1996, Crustal earthquake hazards in Cascadia: characteristics of moderate and small earthquakes west of the Cascade Range, The Geological Society of America, 92nd annual meeting, 1996 Abstracts with Programs. p. 58.

Ludwin, R.S. and A.I. Qamar, 1996, A new database catalog of historic earthquakes in the Cascadia Region, The Geological Society of America, 92nd annual meeting, 1996 Abstracts with Programs. p. 86.

Malone, S.D., 1996, Volcanic earthquake hazards in the Pacific Northwest, The Geological Society of America, 92nd annual meeting, 1996 Abstracts with Programs. p. 87.

Qamar, A.I., K. Meagher, and T. Yelin, 1996, Implications of the 1993 Klamath Falls earthquakes on earthquake potential in the Pacific Northwest, The Geological Society of America, 92nd annual meeting, 1996 Abstracts with Programs. p. 103.

Thomas, G.C., R.S. Crosson, B. Cohee, T. Qamar, and P. Lombard, 1996, The May 2, 1996, Duvall, Washington earthquake and aftershock sequence, EOS, V. 77, No. 46, p. F523.

Symmons, N.P. and R.S. Crosson, 1996, P-wave Tomography in the Puget Sound Region, Washington: preliminary results, EOS, V. 77, N. 46, p. f466

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