Hi All. There was an M=2.1 quake 9Km (6 miles) north of Boulder Creek today (12/19/2005) that caught me attention. This might be because I was looking for some data about the San Andreas fault in this area.

There are some quakes that have been occurring in this area over the past couple of years that seem to be increasing in numbers although the increase isn’t consistent. They are very close to the northern end of the aftershock zone of the 1989 Loma Prieta quake (28Km, 17 miles north of the Loma Prieta quake). There is a “first motion mechanism” for this quake and it shows the quake to be a thrust quake with some lateral movement, which was the same kind seen with the Loma Prieta quake.

The aftershock zone for the Loma Prieta earthquake is about 31 miles long. The aftershocks define a plane dipping about 70 degrees to the southwest. That means the San Andreas fault plane in this region is not vertical, but dips at a steep angle. For this reason the epicenter does not plot on the fault; it occurred at a depth of 11.5 miles, and is thus located to the southwest of the surface expression of the fault. There is also a slight chance the quake didn’t occur on the San Andreas fault. The portion of the Pacific Plate including Santa Cruz and Watsonville moved 6.5 feet north and 4.6 feet up relative to the portion of the North American Plate.

The catalog below covers an area of 4Km (2.5 miles) but the bunch of quakes is just a little tighter then that. I say bunch because I don’t know if this would be a cluster of quakes, or a swarm of quakes. I have a couple of ideas as to why the quakes are occurring, but neither can be proven one way or the other. Anyway time will tell, one way or the other. Take Care…Don in creepy town

catalog=ANSS
start_time=2003/01/01,00:00:00
end_time=2005/12/19,23:42:53
minimum_magnitude=0.1
maximum_magnitude=10
event_type=E
delta=0 km to 04 km from (37.213,-122.115)
Date Time Lat Lon Depth Mag Magt Nst Gap Clo RMS SRC Event ID
----------------------------------------------------------------------------------------------
2003/01/10 19:11:35.04 37.2075 -122.1395 9.05 1.14 Md 12 100 3 0.12 NC 21263229
2003/01/10 22:48:00.01 37.1962 -122.1125 9.85 1.15 Md 9 184 6 0.09 NC 21263252
2003/01/11 22:01:01.24 37.2088 -122.1157 11.04 1.31 Md 17 66 5 0.08 NC 21263356
2003/01/12 00:09:05.73 37.2112 -122.1143 10.88 1.55 Md 30 65 5 0.06 NC 21263364
2003/01/14 21:15:09.48 37.2145 -122.1177 10.90 1.16 Md 16 75 5 0.02 NC 21263575
2003/01/24 03:49:37.24 37.2108 -122.1222 11.06 1.00 Mx 8 171 4 0.05 NC 21264828
2003/01/25 07:39:17.32 37.2180 -122.1482 9.24 1.24 Md 28 74 2 0.13 NC 21264953
2003/02/08 08:32:09.18 37.2083 -122.1040 8.51 1.08 Mx 8 186 6 0.09 NC 21267017
2003/02/17 02:52:29.98 37.2118 -122.1122 11.57 1.31 Md 20 64 8 0.09 NC 21268082
2003/02/18 12:09:59.38 37.2132 -122.1147 11.50 1.81 Md 52 65 5 0.09 NC 21268166
2003/02/18 14:53:39.42 37.2182 -122.1230 12.18 0.98 Md 12 89 4 0.04 NC 21268178
2003/02/23 04:52:54.15 37.2120 -122.1168 10.81 1.13 Mx 8 159 5 0.02 NC 21268700
2003/02/23 08:42:28.34 37.2058 -122.1150 9.90 1.06 Md 16 124 5 0.06 NC 21268724
2003/02/27 20:15:35.43 37.2137 -122.1160 10.44 1.26 Md 11 155 5 0.08 NC 21269254
2003/03/06 21:51:44.63 37.2127 -122.1100 10.30 1.67 Md 19 63 5 0.07 NC 21269982
2003/06/02 09:13:38.29 37.2063 -122.1168 10.69 1.59 Md 18 67 5 0.10 NC 21284956
2003/06/08 02:36:11.24 37.2135 -122.1175 11.77 1.91 Md 44 66 5 0.10 NC 21286532
2003/07/18 12:47:17.65 37.2025 -122.1212 10.26 1.13 Md 17 69 5 0.06 NC 21293007
2003/07/22 19:56:45.17 37.2147 -122.1160 12.49 2.31 Md 55 66 5 0.09 NC 21294098
2003/07/23 01:40:15.52 37.2060 -122.1213 9.76 1.39 Mx 11 95 5 0.09 NC 21294169
2003/07/24 20:53:19.38 37.2072 -122.1178 10.80 1.26 Md 13 93 5 0.05 NC 21294498
2003/09/12 10:22:49.19 37.1947 -122.1120 9.63 1.12 Md 11 185 6 0.06 NC 30226259
2003/10/26 18:36:15.03 37.2245 -122.1362 8.76 2.22 Md 41 61 3 0.14 NC 21315387
2003/12/24 03:27:17.60 37.2112 -122.1168 11.27 1.22 Md 10 153 5 0.12 NC 40149513
2004/03/21 16:28:31.38 37.2312 -122.1115 12.57 1.13 Md 12 125 6 0.05 NC 21352595
2004/03/21 22:18:21.06 37.2475 -122.1122 13.68 1.03 Md 18 68 4 0.09 NC 21352716
2004/04/07 15:56:09.47 37.1997 -122.1100 8.98 1.25 Md 6 182 6 0.01 NC 21356448
2004/07/04 21:43:24.58 37.2202 -122.1233 8.51 0.81 Md 5 144 4 0.02 NC 40158562
2004/11/03 16:28:51.23 37.2447 -122.1025 6.34 0.91 Md 10 132 5 0.05 NC 21416928
2005/04/21 02:50:14.54 37.2115 -122.1145 8.58 1.11 Md 9 95 5 0.11 NC 21451042
2005/04/24 05:15:05.99 37.2098 -122.1110 12.47 1.80 Md 52 64 5 0.11 NC 21451817
2005/04/24 06:51:20.44 37.2112 -122.1147 11.05 1.06 Md 16 66 5 0.12 NC 21451827
2005/04/24 08:09:25.24 37.2123 -122.1113 11.99 1.88 Md 63 63 5 0.11 NC 21451841
2005/04/24 08:12:18.63 37.2060 -122.1168 8.91 1.18 Md 15 66 5 0.06 NC 21451842
2005/04/24 14:34:48.63 37.2100 -122.1112 10.33 1.31 Md 32 64 5 0.10 NC 21451889
2005/04/25 19:39:38.59 37.2062 -122.1087 13.02 1.24 Md 16 63 9 0.07 NC 21452179
2005/04/27 03:29:09.92 37.2113 -122.1108 12.00 2.21 Md 57 64 5 0.12 NC 21452471
2005/04/27 03:39:07.84 37.2107 -122.1103 11.80 1.69 Md 51 63 5 0.13 NC 21452473
2005/04/27 03:53:06.52 37.2035 -122.1093 9.87 1.11 Md 9 117 6 0.08 NC 21452475
2005/05/04 14:25:55.51 37.2102 -122.1117 11.26 1.67 Md 24 64 5 0.11 NC 21454049
2005/05/11 03:52:36.15 37.2137 -122.1148 11.69 1.65 Md 25 65 5 0.11 NC 21455853
2005/05/25 17:28:20.90 37.2015 -122.1145 10.46 1.35 Md 14 66 5 0.09 NC 51160308
2005/08/16 23:49:22.00 37.2275 -122.1403 9.29 1.03 Md 22 55 3 0.16 NC 21477614
2005/09/09 22:57:37.06 37.2093 -122.1158 13.15 1.15 Md 14 88 8 0.06 NC 21482469
2005/09/18 07:04:38.14 37.2060 -122.1253 14.12 2.46 Md 132 71 4 0.18 NC 30229463
2005/09/18 07:04:50.45 37.2045 -122.1177 7.13 0.92 Md 44 68 5 0.14 NC 30229462
2005/09/18 07:05:10.42 37.1960 -122.1298 13.68 2.32 Md 126 74 4 0.23 NC 21483966
2005/09/18 07:10:10.39 37.2103 -122.1232 13.23 2.53 Md 116 69 4 0.16 NC 21483967
2005/09/18 07:16:01.56 37.2170 -122.1180 12.35 2.22 Md 68 66 5 0.11 NC 21483969
2005/09/18 07:22:01.29 37.2158 -122.1175 12.39 1.97 Md 58 66 5 0.12 NC 21483970
2005/09/18 07:29:15.40 37.2157 -122.1147 12.25 1.84 Md 50 65 5 0.12 NC 21483972
2005/09/18 08:05:52.76 37.2103 -122.1148 11.87 1.32 Md 28 65 8 0.07 NC 21483977
2005/09/18 11:56:27.41 37.2105 -122.1135 12.65 1.21 Md 25 65 8 0.06 NC 21484010
2005/10/03 11:39:30.09 37.2115 -122.1108 12.24 1.14 Md 10 104 8 0.07 NC 21487250
2005/10/09 09:20:31.07 37.2082 -122.1187 10.51 1.12 Md 18 68 5 0.11 NC 21488595
2005/10/09 14:39:36.19 37.2097 -122.1195 12.29 1.28 Md 16 79 8 0.07 NC 21488619
2005/10/25 20:17:00.81 37.2013 -122.1142 9.92 1.18 Md 11 102 5 0.11 NC 21491605
2005/10/26 02:37:28.77 37.2387 -122.1448 6.02 1.22 Md 24 72 5 0.09 NC 21491691
2005/12/10 07:10:27.37 37.2418 -122.1418 1.79 1.09 Md 13 77 4 0.10 NC 21498959
2005/12/19 01:54:27.39 37.2102 -122.1085 6.55 1.35 Md 9 71 6 0.04 NC 40181974
2005/12/19 22:26:17.22 37.1862 -122.1057 8.03 1.07 Md 5 189 7 0.02 NC 40181994
2005/12/19 22:26:38.55 37.2127 -122.1152 11.95 2.10 Md 37 65 5 0.13 NC 40181995

http://www.johnmartin.com/earthquakes/eqpapers/00000079.htm

• Loma Preita