DISCLAIMER -- THIS IS NOT AN EARTHQUAKE PREDICTION OR WARNING! The commentary provided with these map(s) is for INFORMATIONAL USE ONLY, and SHOULD NOT be construed as an earthquake prediction, warning, or advisory. Responsibility for such warnings rests with the Office of Emergency Services of the State of California. PLEASE REMEMBER -- THIS IS PRELIMINARY DATA Releasing these summaries on a timely basis requires that the data, analysis, and interpretations presented are PRELIMINARY. Of necessity they can only reflect the views of the seismologists who prepared them, and DO NOT carry the endorsement of the U.S.G.S. Thus while every effort is made to ensure that the information is accurate, nothing contained in this report is to be construed as and earthquake prediction, warning, advisory, or official policy statement of any kind, of the U.S. Geological Survey, or the U.S. Government. FOR QUESTIONS CONCERNING THIS REPORT Send e-mail to andy@pangea.stanford.edu Seismicity Report for Northern California, the Nation, and the World for the week of October 22 - 28, 1992 Data and text prepared by Steve Walter, Barry Hirshorn, and Allan Lindh U.S. Geological Survey 345 Middlefield Rd. MS-977, Menlo Park, CA 94025 Graphics by Quentin Lindh San Francisco Bay Area Following last week's flurry of small earthquakes on the creeping segment of the San Andreas, seismicity returned to the very low levels that have characterized activity in the Bay Area for much of the year. During the 7-day period ending at midnight on Wednesday, October 28, 1992 the U.S. Geological Survey office in Menlo Park recorded 16 earthquakes of magnitude one (M1) and greater within the San Francisco Bay area shown in Figure 1. None were as large as M2.0. This week's total compares to 33 earthquakes during the previous 7-day period, four of which were as large as M2. The activity along the creeping segment noted in last week's report continued for another day or so into the current reporting period, producing another four M>1 events, the largest a M1.7 event early last Thursday morning (#1 in Fig.1). Otherwise the only notable earthquake during the week was a M1.9 that occurred about 8 miles southwest of Santa Cruz, near the offshore trace of the San Gregorio fault (#2/1). Northern California Seismicity was down throughout most of northern California during the past week with the exception of the Parkfield area, discussed below, where several interesting clusters of activity continued to keep this segment of the San Andreas in the news. Cape Mendocino was again the most active area in northern California though only one of the week's earthquakes here was as large as M3, a M3.1 last Sunday evening (#1/2). Another M3.1 occurred the previous day beneath the foothills of the Sierra Nevada (#3/2). The epicenter for this event was about 15 miles north-northeast of Chico. It was followed six hours later by a M2.3 beneath the Sacramento Valley, about 16 miles west of Chico (#4/2). Activity was also down in the eastern Sierra Nevada with only a couple of M2 earthquakes in the Markleeville area (#2/2) and a pair of M2 events in the northern Owens valley, just southeast of Long Valley. Parkfield The A-level alert that was in effect at the end of last week's reporting period expired late last Thursday night without the predicted M6 Parkfield earthquake occurring. Earthquakes that did occur last Wednesday and Thursday were too small and too shallow to affect the alert status which was reduced to level D (~1% probability of the M6 within the next 72 hours). The area remained quiet until late Sunday night when a cluster of 8 earthquakes occurred within a 2-hour period. The cluster included two M3 earthquakes, the largest a M3.9 at 11:27p PST (#5/2 and Fig. 3). This event boosted the alert level back to a B-level, corresponding to about a 10% probability that the predicted earthquake would follow in the next 3 days. The California Office of Emergency Services was immediately notified of the change in alert status; however a level-B status does not require that personnel in the affected counties be placed on alert. The B-level alert expired Wednesday night at the end of the 72-hour window but declined only to a C-level because of a pair of M2 events on Wednesday that fell in the "Alert Box". The C-level status will expire just after 3pm on Saturday, barring further significant activity. Incidentally, this is the 33rd C-level status since 1985. The cluster of events Sunday night was located 2-3 km northwest of the M4.7 earthquake and, at a depth of about 11 km, was somewhat deeper than the activity that prompted the A-level alert (see Fig. 3). Both clusters of activity occurred near the bottom of the "stuck patch" of the San Andreas. This patch is nicely defined in the cross-section in Figure 3 by the lack of earthquakes in the depth range between 6 and 10 km southeast of the 1966 mainshock. This cross-section shows the recent activity in relationship to the previous 20+ years of earthquakes on this segment. The sudden increase in activity both above and below this "stuck patch" explains as clearly as anything, the heightened concern that this part of the San Andreas might be preparing to rupture. Long Valley Caldera A small swarm of earthquakes occurred Tuesday morning in the south moat area, just west of the Fish Hatchery (#2/4). The swarm included four M2 earthquakes, the largest a M2.6 event mid-morning Wednesday. Focal depths for all well-located events were between 5 and 6 km. The area south of the caldera was quiet with only a few scattered earthquakes. The largest were a pair of M2.1 events, the first west of the southern end of the Hilton Creek fault (#1/4), the second east of the Round Valley fault about 9 miles west-northwest of Bishop(#3/4). USA Seismicity According to data provided by the National Earthquake Information Service there were several earthquakes in the lower 48 states during the past week, other than the California events discussed in this report. The first of these was a M3.0 event Saturday evening in western Nevada, approximately 30 miles north of Beatty (#2/5). NEIS also reported two small events in central Washington (#3/5). The first of these was a M3.5 on Sunday northwest of Yakima that was felt in Selah, WA. The next day a M2.5 occurred in the Cascade Range south of Mount Rainier. In southern California aftershocks to the M7.5 Landers earthquake continued to decline. During the past week there were no events as large as M4 and only a handful as large as M3. The Planet Earth For the third week in a row people were killed, injured, and made homeless by earthquakes in various parts of the world. A relatively minor M4.3 earthquake in Cairo, Egypt killed 4 people and injured at least 50 (#2/6). The earthquake was an aftershock to a Mb 5.9 event earlier in the month that killed hundreds of people, injured thousands, and left many structures badly weakened and susceptible to further failure. A moderate M5.3 earthquake that was felt throughout much of Morocco killed at least two people in the city of Rissani (#3/6). Last Friday a Ms6.6 in the eastern Caucasus killed one person, injured 10, and damaged several homes in the city of Barisakho, Georgia (#5/6). A fourth potentially deadly earthquake occurred just after noon on Wednesday in central Burma, about 150 miles north of Rangoon. Word of injuries or damage had not been received by the time this was written. Several large earthquake sequences occurred in the southwest Pacific. The Kermadec Islands, north of New Zealand, experienced three M6 events during the week, the largest a M6.6 last Thursday (#1/6). The New Britain region of Papua New Guinea also was rocked by numerous M5 earthquakes during the week, the largest a Ms6.7 last Friday (#4/6). Table 1. Central California Seismicity (M>2.0) --ORIGIN TIME (UT)-- -LAT N-- --LON W-- DEPTH N N RMS ERH ERZ DUR YR MON DA HRMN SEC DEG MIN DEG MIN KM RD S SEC KM KM REMKS MAG 92 OCT 22 717 5.88 38 47.26 122 46.28 1.34 36 .17 .3 .8 GEY 2.6 92 OCT 22 1015 40.87 40 30.20 124 16.84 4.60 11 .09 .5 .8 EUR 2.9 92 OCT 22 1233 16.93 37 14.16 118 34.35 14.96 16 .07 .7 1.0 KAI 2.1 92 OCT 22 1256 13.59 38 43.68 119 40.10 0.63 7 .06 2.9 3.7 WAK * 4.5 92 OCT 22 1257 57.06 38 47.58 122 46.93 0.49 14 .07 .2 .7 GEY 2.1 92 OCT 22 1327 28.53 38 45.67 119 37.89 6.96 19 .12 1.2 2.2 WAK 2.5 92 OCT 22 1551 51.99 38 43.86 118 45.54 0.54 27 .17 1.8 5.9 WAK- 2.8 92 OCT 23 510 48.84 40 20.97 124 34.37 23.90 12 1 .12 1.2 .8 MEN 2.1 92 OCT 23 920 14.95 37 28.12 118 47.38 12.05 30 .11 .4 .8 MOR 2.1 92 OCT 23 1226 19.75 36 16.76 120 20.03 9.48 19 .13 .4 .3 COA 2.0 92 OCT 24 216 5.14 40 28.01 124 39.31 5.00 13 .41 5.4 4.6 MEN 2.9 92 OCT 24 224 23.37 40 21.33 124 22.68 16.07 21 1 .12 .6 .3 MEN 2.0 92 OCT 24 1824 34.02 39 58.32 121 42.58 18.53 32 .20 .5 1.2 ALM 3.1 92 OCT 25 38 44.76 39 44.03 122 6.65 20.04 13 .15 .6 1.6 SAC 2.3 92 OCT 26 319 39.20 40 18.42 124 32.67 19.80 10 .08 1.9 .4 MEN * 3.1 92 OCT 26 517 3.69 40 18.99 124 24.90 15.34 7 .10 1.6 .8 MEN 2.5 92 OCT 26 655 14.53 35 56.73 120 29.62 10.24 60 1 .13 .2 .3 MID 3.3 92 OCT 26 705 57.54 35 56.63 120 29.64 10.34 32 .12 .3 .5 MID 2.2 92 OCT 26 727 40.51 35 56.79 120 29.19 10.24 67 .18 .3 .4 MID 3.9 92 OCT 26 1235 6.69 38 49.17 122 48.11 4.60 30 .11 .2 .5 GEY 2.4 92 OCT 26 2307 51.24 40 28.36 124 45.34 27.10 15 .12 1.8 2.1 MEN * 2.9 92 OCT 27 442 56.99 40 18.98 124 25.59 9.62 13 .17 1.4 .7 MEN 2.4 92 OCT 27 1114 37.20 37 38.15 118 52.24 5.65 28 .12 .4 .5 SMO 2.2 92 OCT 27 1202 37.21 37 38.08 118 52.27 5.87 31 .13 .3 .4 SMO 2.4 92 OCT 27 1252 6.95 37 25.19 118 33.04 7.35 21 .08 .3 .8 RVL 2.1 92 OCT 27 1456 39.86 36 22.01 120 40.86 10.83 21 .20 .5 1.1 CRV 2.1 92 OCT 27 2020 44.69 35 46.92 118 23.70 10.16 13 .06 .4 1.1 WWF 2.0 92 OCT 28 818 5.71 37 38.16 118 52.22 5.16 20 .12 .4 .5 SMO 2.2 92 OCT 28 1659 50.31 37 38.07 118 52.25 5.67 24 .13 .4 .5 SMO 2.6 92 OCT 28 1741 30.72 35 56.39 120 29.49 9.95 39 .14 .3 .5 MID 2.3 Notes: Origin time in the list is in GMT, in the text and on maps it is in local time. N RD: is the number of readings used to locate the event. N S: is the number of S waves in N RD. RMS SEC: is the root mean squared residual misfit for the location is seconds, the lower the better, over 0.3 to 0.5 seconds is getting bad, but this is machine, not hand timed, data. ERH: is the estimated horizontal error in kilometers. ERZ: is the estimated vertical error in kilometers. N FM: is the number of readings used to compute the magnitude. REMKS: obtuse region codes that denote the velocity model used to locate the event. DUR MAG: is the magnitude as determined from the duration of the seismograms, not the amplitude. Sort of like going to echo canyon and measuring how loud your yell is by counting echos. FIG: denotes the figure/event number in the maps posted separately. TABLE 2. Data from National Earthquake Information Center (NEIC) UTC TIME LAT LONG DEP GS MAGS SD STA REGION AND COMMENTS HRMNSEC MB Msz USED ------------------------------------------------------------------------------- OCT 22 074028.9* 59.597N 140.658W 10G 0.8 24 SOUTHEASTERN ALASKA. ML 3.1 081235.2 60.700N 151.777W 92 3.1 0.8 75 KENAI PENINSULA, ALASKA 083927.2 33.631N 117.474W 5G 0.9 15 SOUTHERN CALIFORNIA. MD 3.5 090425.0* 29.959S 177.328W 33N 5.9 6.6 1.4 113 KERMADEC ISLANDS, NEW ZEALAND. 090433.5& 60.600N 150.800W 40 55 KENAI PENINSULA, ALASKA. ML 2.5 162107.5 34.267N 116.405W 5G 0.7 12 SOUTHERN CALIFORNIA. MD 3.2 173858.3 29.472N 31.394E 10G 4.3 1.1 10 EGYPT. MD 4.2 (HLW). Four people killed and at least 50 injured in the Cairo area. 175119.0 33.942N 116.292W 5G 0.8 11 SOUTHERN CALIFORNIA. ML 3.0 182554.5 4.212S 126.160E 295 5.4 1.1 69 BANDA SEA 230830.6 29.749S 177.247W 33N 5.5 6.4 1.3 103 KERMADEC ISLANDS, NEW ZEALAND. OCT 23 041123.8% 35.539N 140.311E 64? 0.8 11 NEAR EAST COAST OF HONSHU, JAPAN 052639.3* 25.688N 109.862W 10G 4.1 0.9 24 GULF OF CALIFORNIA 091109.1 31.319N 4.351W 28D 5.1 5.3 1.0 75 MOROCCO. At least two people killed at Rissani. Felt throughout much of Morocco from Fes in the northeast to Marrakech in the southwest. 092802.9* 6.841N 77.001W 10G 5.0 5.0 1.0 57 NEAR WEST COAST OF COLOMBIA 130441.5 5.170S 152.498E 33N 5.5 6.7 0.9 62 NEW BRITAIN REGION, P.N.G. 133130.5? 5.18 S 152.68 E 33N 4.9 0.8 10 NEW BRITAIN REGION, P.N.G. 141502.9? 5.46 S 152.69 E 33N 4.8 1.8 10 NEW BRITAIN REGION, P.N.G. 152913.2 5.094S 152.659E 33N 5.6 5.7 1.0 39 NEW BRITAIN REGION, P.N.G. 164544.8* 5.034S 152.564E 33N 5.1 1.2 21 NEW BRITAIN REGION, P.N.G. 172837.0* 5.340S 152.804E 33N 5.0 1.0 17 NEW BRITAIN REGION, P.N.G. 191643.0 5.243S 152.574E 31D 5.4 5.6 1.0 38 NEW BRITAIN REGION, P.N.G. 212435.9* 9.451S 122.710E 33N 5.9 5.7 1.5 41 SAVU SEA 231947.2 42.515N 45.148E 33N 6.1 6.6 0.9 118 EASTERN CAUCASUS. At least one person killed, 10 injured and several houses damaged in the Barisakho, Georgia area. Landslides reported in the epicentral area. OCT 24 082304.6? 29.24 S 177.36 W 33N 5.8 6.2 1.0 57 KERMADEC ISLANDS, NEW ZEALAND. 092636.9* 5.376S 152.647E 33N 5.0 0.9 14 NEW BRITAIN REGION, P.N.G. 134651.9* 29.680S 177.262W 33N 4.9 0.8 14 KERMADEC ISLANDS, NEW ZEALAND 140258.9 58.377N 151.348W 33N 1.1 15 KODIAK ISLAND REGION. ML 3.8 142223.4 34.399N 116.464W 5G 0.6 8 SOUTHERN CALIFORNIA. MD 3.0 150018.7 51.286N 179.043E 33N 4.7 1.0 42 RAT ISLANDS, ALEUTIAN ISLANDS. 185948.4 5.700S 145.554E 33N 5.7 4.4 0.6 31 EASTERN NEW GUINEA REG., P.N.G. 221607.0 11.950N 141.984E 33N 5.4 4.8 0.9 66 WESTERN CAROLINE ISLANDS 230034.1 29.584S 177.414W 40D 5.4 5.2 0.7 43 KERMADEC ISLANDS, NEW ZEALAND OCT 25 002919.4 29.532S 177.563W 28D 5.6 5.8 1.0 63 KERMADEC ISLANDS, NEW ZEALAND 012300.2 33.884N 118.682W 10G 0.7 24 SOUTHERN CALIFORNIA. MD 3.3 030735.7 37.367N 116.671W 5G 0.6 14 SOUTHERN NEVADA. ML 3.0 (GS). 105901.6 59.046N 151.081W 78? 0.6 12 KENAI PENINSULA, ALASKA 111642.5? 2.50 N 97.21 W 10G 4.5 4.4 1.0 17 WEST OF GALAPAGOS ISLANDS 150003.9* 6.045S 103.872E 33N 5.2 1.0 21 SOUTHWEST OF SUMATERA, INDONESIA OCT 26 042915.5? 7.17 N 76.23 W 10G 5.0 4.4 1.1 41 NORTHERN COLOMBIA 065514.5& 35.946N 120.498W 11 13 CENTRAL CALIFORNIA. MD 3.4 072740.4& 35.946N 120.495W 11 19 CENTRAL CALIFORNIA. MD 3.9 075635.1& 46.840N 120.711W 0 101 WASHINGTON. MD 3.5 Felt at Selah. OCT 27 061027.1& 46.600N 121.768W 9 13 WASHINGTON. MD 2.5 OCT 28 070209 Q 19.0 N 96.4 E 33N 5.7 0.7 58 MYANMAR OCT 29 072420 Q 6.7 S 153.9 E 33N 5.9 0.9 33 NEW BRITAIN REGION, P.N.G.