From: andy@pangea.Stanford.EDU (Andy Michael USGS Guest)
Subject: N. CA, USA, and World Quake Report 8/20-26/92
Organization: Stanford Univ. Earth Sciences
Date: Fri, 28 Aug 1992 01:25:41 GMT

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
   August 20 - 26, 1992

Data and text prepared by
  Steve Walter, Barry Hirshorn, Nan Macgregor Scott, and Allan Lindh
U.S. Geological Survey
345 Middlefield Rd.  MS-977, Menlo Park, CA  94025
Graphics by Quentin Lindh

San Francisco Bay Area   
   
    For the 7-day period ending at midnight on Wednesday, August 26, 1992
the U.S. Geological Survey office in Menlo Park recorded 22 earthquakes of
magnitude one (M1) and greater within the San Francisco Bay area shown in
Figure 1.  Only one of these was as large as M2.0.  The weekly total
compares with 17 earthquakes recorded during the previous 7-day period,
five of which were as large as M2.0. 
   The San Andreas fault experienced a slight increase in activity with
small events along the creeping segment, the Loma Prieta segment, the
Sargent fault, and even one earthquake on the Zayante fault (see Fig. 1). 
A couple of M1 earthquakes also occurred offshore of the Santa Cruz area
including a M1.3 event in northern Monterey Bay (#1/1) and a M1.3 off Ano
Nuevo (#5/1).  The only other earthquake of note in the Peninsula area
was a M1.0 earthquake that occurred beneath the Sunset District of San
Francisco late Monday evening (#4/1). 
   Activity in the East Bay consisted of a M1.3 event about 1 mile east
of Hayward (#2/1), a M1.5 about 9 miles south-southeast of Concord (#6/1),
and a pair of events in the central Diablo Range, about 23 miles southeast
of Livermore (#3/1).  The central Calaveras fault was very quiet with a
single M1.2 11 miles east of San Jose.

Northern California
    
    Seismicity remained low along the San Andreas fault in both northern
and central California.  The only area that experienced any activity to
speak of was the Cape Mendocino area which is still producing aftershocks
to the M7.1 earthquake of late April.  During the past week four of the
aftershocks were as large as M2, including one M3.0 event that was the
largest earthquake of the week in northern California (#1/2).  Two M2
earthquakes occurred after midnight on Monday in the vicinity of Lake
Pilsbury, about 23 miles east of Willits (#8/2).
    If the San Andreas was quieter than usual, the region east and
northeast of the Sierra Nevada was noticeably more active.   Several M2
earthquakes occurred in the Modoc Plateau area of northeastern California
including two events about 5 miles southeast of Susanville (#2/2) and one
event about 27 miles southeast of Alturas (#3/2).  One M2.1 event occurred
in the Cascade Range about 32 miles northwest of Lassen Peak (#4/2), the
same location as a series of "triggered" earthquakes that followed the
M7.5 Landers earthquake in late June.  The eastern margin of the Sierra
Nevada was also active with a M2.0 eight miles northeast of Markleeville
(#6/2), a M2.2 seven miles west-southwest of Bridgeport (#5/2), and a M2.1
along the western shore of Mono Lake (#7/2).  
    Finally, a M2.4 occurred beneath the western foothills of the Sierra
Nevada, about 25 miles east of Merced (#9/2).  
         
Long Valley Caldera
        
    The only activity within the caldera consisted of a half dozen M1
earthquakes at the western end of the south moat area, about two miles
south and southeast of Mammoth Lakes.  South of the caldera, the only
earthquake of note was a M2.2 event just east of Mt. Crocker (#1/3).      
   
USA Seismicity
    
    The National Earthquake Information Service recorded a M5.5 earthquake
along the central Blanco fracture zone offshore of southern Oregon (#1/4). 
The Blanco fracture zone separates the Gorda and Juan de Fuca plates, with
numerous large earthquakes being produced as these ocean plates move past
each other.
    A M4.1 earthquake occurred last Friday in South Carolina (#2/4).  It
was felt in the Charleston-Summerville area, at Columbia, and at Augusta,
GA.  Charleston was the site of one of the largest historic earthquakes in
the eastern United States, a Mb6.7 earthquake that occurred in 1886,
killing upwards of 100 people and producing damage estimated at $25
million (1978 dollars). 
    Aftershocks to the M7.5 Landers earthquake continue to be felt, though
they are declining in both size and number.  A M3.7 aftershock occurred   
in the Barstow area (#3/4) and a M3.9 event topped the list of aftershocks
in the Landers area (#4/4)/.   
     
The Planet Earth

    Aftershocks continue from last week's devastating M7.5 earthquake in
Kyrgyzstan, the largest a M5.2 event last Sunday (#1/5).  The largest
earthquake worldwide during the week was a M6.2 event beneath southwestern
Hokkaido, Japan (#6/5).  Other earthquakes of interest include a M5.1 in
the Mauritius-Reunion region of the central Indian Ocean (#2/5), a pair of
M4.8 events near of coast of Guerrero, Mexico and in southern Peru
(#4,#5/5), and a M5.2 in the Chile-Argentina border region (#2/5).   
 

     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 AUG 20  709 57.54 40 30.32 124 17.04  5.06 22 1 .12  .3  .4 EUR   2.2
92 AUG 20 1651 39.88 38 47.47 122 44.78  2.24 19   .16  .4  .9 GEY   2.3
92 AUG 20 1817  0.63 37 28.76 118 48.66  9.97 40 1 .10  .3  .9 MOR   2.4
92 AUG 20 2350 52.80 40 21.76 120 35.26  7.90 22 1 .14 1.8 4.7 MOD   2.6
92 AUG 21  638  0.02 41  8.99 120 16.97 11.57 19   .14 3.1 1.3 MOD   2.8

92 AUG 21 2126 53.58 38 47.83 122 48.31  5.87 24   .12  .3  .6 GEY   2.0
92 AUG 21 2143 28.68 40 56.67 121 39.69  4.99  9 1 .11  .611.0 SHA-- 2.1
92 AUG 22  454  7.64 38 12.12 119 22.85  2.31 12   .15  .8 7.1 WAK   2.4
92 AUG 23   44 45.51 38 48.54 119 45.23 12.88 12 3 .10  .9 7.2 WAK   2.2
92 AUG 23  155  6.80 40 21.93 120 34.87  9.15 31 2 .13  .9 1.8 MOD   2.8

92 AUG 23 1535 48.57 38 47.99 122 47.19  0.83 29   .13  .2  .4 GEY   2.1
92 AUG 23 1638 11.00 37 26.07 121 28.26  2.33 25 2 .20  .5 5.3 HAM   2.0
92 AUG 24  152 43.04 38  2.55 119  9.68 13.94 24   .10  .8  .9 MOL   2.3
92 AUG 24  302 25.24 35 59.80 118 22.35  5.00  7   .02  .5 1.8 WWF   2.2
92 AUG 24  425 43.17 38 47.16 122 45.13  1.90 33 2 .15  .3  .7 GEY   2.5

92 AUG 24  836 36.42 40 18.86 124 28.00  7.25 18 1 .09  .8  .4 MEN   2.4
92 AUG 24  858  7.15 39 28.54 122 55.83 14.35 49 5 .17  .2  .5 BAR   2.8
92 AUG 24 1036 27.62 39 28.64 122 55.91 13.48 28 1 .12  .3  .6 BAR   2.1
92 AUG 24 1809 33.59 38 49.76 122 49.25  2.56 23   .12  .3 1.0 GEY   2.5
92 AUG 25   41 19.60 40 20.66 124 32.80 26.59 12   .17 3.3 1.8 MEN   2.1

92 AUG 25  101 56.82 38 49.26 122 49.36  1.40 18   .06  .2  .6 GEY   2.0
92 AUG 25 1219 33.73 37 19.86 120  2.13 12.45 10 2 .16  .9 2.0 KAI   2.4
92 AUG 26 1058  8.56 40 13.93 124  9.42 11.41 20   .14  .4  .4 MEN   3.0

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
----------------------------------------------------------------------------
AUG 20
122249.3* 42.376N  73.836E  19D 4.8 4.3 1.2  20 KYRGYZSTAN
133711.6* 25.772S  68.512W 104D 5.2     0.7  44 CHILE-ARGENTINA BORDER REGION.
        Felt from Taltal to Copiapo, Chile.
AUG 21
010217.4  43.926N 128.362W  10G 5.5 5.0 1.1  98 OFF COAST OF OREGON
095451.4* 18.276S  65.128E  10G 5.1     0.7  18 MAURITIUS-REUNION REGION
163155.0  33.047N  80.124W  10G 4.1     0.7  12 SOUTH CAROLINA. Felt in the
        Charleston-Summerville area and at Columbia. Also felt at Augusta,
        Georgia.
AUG 22
015258.9  34.110N 116.934W   5G         0.7  10 SOUTHERN CALIFORNIA. MD 3.3
073542.9? 15.06 S  70.15 W 172D 4.8     0.7  31 SOUTHERN PERU
131259.7* 51.119N 178.846E  33N 4.2     1.3  13 RAT ISLANDS, ALEUTIAN ISLANDS
132153.0* 51.265N 178.884E  33N 4.3     1.3  21 RAT ISLANDS, ALEUTIAN ISLANDS
224200.6  43.596N  17.571E  10G         0.9  27 NORTHWESTERN BALKAN REGION. ML
                                                3.9 (VIE), 3.6 (ROM).
AUG 23
025510.3  34.218N 116.716W   5G         0.5  10 SOUTHERN CALIFORNIA. MD 3.2
051128.8* 16.020N  98.023W  19D 4.8     1.0  41 NEAR COAST OF GUERRERO, MEXICO
053023.2  34.169N 116.746W   5G         0.7  12 SOUTHERN CALIFORNIA. MD 3.2
064043.1  35.017N 116.988W   5G         0.6  17 CENTRAL CALIFORNIA. MD 3.7
090436.5? 42.30 N  74.32 E  33N 5.2     0.7  20 KYRGYZSTAN
174930.8  34.086N 116.775W   5G         0.9  11 SOUTHERN CALIFORNIA. MD 3.2
AUG 24
030438.8* 24.368N 123.432E  33N 5.1     0.9  31 SOUTHWESTERN RYUKYU ISLANDS
065939.3* 42.015N 140.748E 114? 6.2     0.7  91 HOKKAIDO, JAPAN  REGION.
        Mo=3.2*10**18 Nm (PPT). Felt (IV JMA) at Obihiro; (III JMA) at Nemuro
        and Kushiro. 
124301.8  34.930N 116.752W   5G         0.7  16 SOUTHERN CALIFORNIA. MD 3.4
135145.0  34.267N 116.797W   5G         0.7  17 SOUTHERN CALIFORNIA. ML 3.9
152121.6* 20.197S 169.490E  33N 5.3     0.7  23 VANUATU ISLANDS
182141.5  34.262N 116.777W   5G         0.8  11 SOUTHERN CALIFORNIA. MD 3.4
Newsgroups: sci.geo.geology