Earthquakes and Seismology

Introduction

An earthquake is a naturally produced shock event which causes the Earth to vibrate.

Of interest are:

· their effect

· why they occur

· where they occur

· scientific study of the Earth's interior and dynamics:

the structure of the Earth,
composition of the Earth
kinematics and dynamic of plate tectonics.

How many Earthquakes?

Earthquakes are very common, many thousands per day all over the world: e.g. map for 18-25^th Oct 2008:

Most are weak (low energy) and undetected. Here are those with Mag > 2.5 (see later for definition) for that week:

Update time = Thu Sep 25 11:47:16 UTC 2008

	MAG	UTC DATE-TIME y/m/d h:m:s	LAT deg	LON deg	DEPTH km	Region
MAP	2.5	2008/09/25 11:27:50	51.881	-176.894	60.0	ANDREANOF ISLANDS, ALEUTIAN IS., ALASKA
MAP	3.0	2008/09/25 10:53:19	61.306	-147.312	14.9	SOUTHERN ALASKA
MAP	2.7	2008/09/25 09:58:22	53.024	-168.050	73.3	FOX ISLANDS, ALEUTIAN ISLANDS, ALASKA
MAP	2.6	2008/09/25 09:55:20	36.646	-121.269	7.2	CENTRAL CALIFORNIA
MAP	3.1	2008/09/25 09:05:51	62.827	-150.917	100.0	CENTRAL ALASKA
MAP	2.6	2008/09/25 07:46:28	53.413	-165.928	29.9	FOX ISLANDS, ALEUTIAN ISLANDS, ALASKA
MAP	4.6	2008/09/25 06:54:31	38.260	-30.073	10.0	AZORES ISLANDS, PORTUGAL
MAP	2.7	2008/09/25 06:33:38	62.835	-149.484	61.6	CENTRAL ALASKA
MAP	4.4	2008/09/25 04:48:54	-16.121	-173.561	111.4	TONGA
MAP	5.7	2008/09/25 01:47:13	30.927	83.449	10.0	WESTERN XIZANG
MAP	2.6	2008/09/25 01:07:05	63.221	-144.849	17.3	CENTRAL ALASKA
MAP	2.5	2008/09/25 00:41:49	44.632	-110.677	1.9	YELLOWSTONE NATIONAL PARK, WYOMING

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	MAG	UTC DATE-TIME y/m/d h:m:s	LAT deg	LON deg	DEPTH km	Region
MAP	4.8	2008/09/24 22:03:49	-3.254	-102.749	10.0	CENTRAL EAST PACIFIC RISE
MAP	2.9	2008/09/24 21:39:50	40.458	-124.732	20.7	OFFSHORE NORTHERN CALIFORNIA
MAP	2.8	2008/09/24 18:29:11	60.168	-153.883	100.0	SOUTHERN ALASKA
MAP	3.2	2008/09/24 18:00:09	51.556	178.627	28.8	RAT ISLANDS, ALEUTIAN ISLANDS, ALASKA
MAP	5.2	2008/09/24 17:59:33	36.213	71.172	96.9	HINDU KUSH REGION, AFGHANISTAN
MAP	2.6	2008/09/24 17:35:31	35.993	-117.954	8.0	CENTRAL CALIFORNIA
MAP	5.4	2008/09/24 17:12:16	-22.686	-12.771	10.0	SOUTHERN MID-ATLANTIC RIDGE
MAP	3.3	2008/09/24 16:01:49	63.403	-150.074	5.8	CENTRAL ALASKA
MAP	3.1	2008/09/24 15:20:53	54.271	-163.313	36.5	UNIMAK ISLAND REGION, ALASKA
MAP	2.5	2008/09/24 15:20:02	35.440	-92.260	0.1	ARKANSAS
MAP	2.8	2008/09/24 14:54:44	55.626	-160.180	10.2	ALASKA PENINSULA
MAP	2.5	2008/09/24 14:10:36	19.399	-155.281	1.6	ISLAND OF HAWAII, HAWAII
MAP	4.2	2008/09/24 12:19:52	63.411	-150.068	7.4	CENTRAL ALASKA
MAP	4.7	2008/09/24 12:04:54	36.561	70.944	183.4	HINDU KUSH REGION, AFGHANISTAN
MAP	2.6	2008/09/24 09:44:13	59.933	-153.143	97.7	SOUTHERN ALASKA
MAP	5.1	2008/09/24 06:00:51	-10.223	161.381	92.4	SOLOMON ISLANDS
MAP	2.7	2008/09/24 03:53:34	64.495	-147.945	3.9	CENTRAL ALASKA
MAP	2.6	2008/09/24 03:50:41	53.764	-165.010	49.2	FOX ISLANDS, ALEUTIAN ISLANDS, ALASKA
MAP	2.7	2008/09/24 02:52:44	36.761	-120.868	7.2	CENTRAL CALIFORNIA
MAP	2.9	2008/09/24 02:37:47	33.369	-116.388	6.9	SOUTHERN CALIFORNIA
MAP	6.4	2008/09/24 02:33:05	17.635	-105.520	10.0	OFF THE COAST OF COLIMA, MEXICO
MAP	4.7	2008/09/24 01:37:05	-14.634	167.749	103.0	VANUATU

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	MAG	UTC DATE-TIME y/m/d h:m:s	LAT deg	LON deg	DEPTH km	Region
MAP	4.9	2008/09/23 22:46:23	16.860	-99.829	45.7	GUERRERO, MEXICO
MAP	3.9	2008/09/23 22:08:27	61.580	-146.456	29.5	SOUTHERN ALASKA
MAP	2.7	2008/09/23 21:35:49	66.362	-150.336	3.2	NORTHERN ALASKA
MAP	3.9	2008/09/23 19:14:46	36.362	71.355	89.4	HINDU KUSH REGION, AFGHANISTAN
MAP	2.6	2008/09/23 19:09:49	36.617	-121.227	5.8	CENTRAL CALIFORNIA
MAP	3.0	2008/09/23 17:49:42	20.141	-155.440	31.9	HAWAII REGION, HAWAII
MAP	2.5	2008/09/23 16:34:52	53.211	-166.851	48.0	FOX ISLANDS, ALEUTIAN ISLANDS, ALASKA
MAP	2.7	2008/09/23 15:57:09	35.415	-118.921	18.0	BAKERSFIELD URBAN AREA, CALIFORNIA
MAP	4.7	2008/09/23 15:20:09	-24.298	-175.427	35.0	SOUTH OF TONGA
MAP	3.0	2008/09/23 14:43:41	37.187	-114.543	2.2	NEVADA
MAP	3.6	2008/09/23 14:24:37	34.073	73.617	35.0	PAKISTAN
MAP	4.6	2008/09/23 13:26:51	38.477	142.885	63.8	NEAR THE EAST COAST OF HONSHU, JAPAN
MAP	4.5	2008/09/23 03:48:24	15.821	-92.964	130.2	CHIAPAS, MEXICO
MAP	2.6	2008/09/23 01:18:36	19.186	-155.477	6.7	ISLAND OF HAWAII, HAWAII
MAP	2.7	2008/09/23 00:01:13	54.582	-163.447	9.2	UNIMAK ISLAND REGION, ALASKA

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	MAG	UTC DATE-TIME y/m/d h:m:s	LAT deg	LON deg	DEPTH km	Region
MAP	4.8	2008/09/22 23:34:01	54.221	-35.187	10.0	REYKJANES RIDGE
MAP	2.6	2008/09/22 23:05:43	18.492	-65.112	48.9	VIRGIN ISLANDS REGION
MAP	2.8	2008/09/22 21:21:40	64.492	-147.883	7.6	CENTRAL ALASKA
MAP	2.9	2008/09/22 20:32:25	64.506	-147.908	8.9	CENTRAL ALASKA
MAP	5.3	2008/09/22 20:20:59	-23.445	-179.574	540.1	SOUTH OF THE FIJI ISLANDS
MAP	2.6	2008/09/22 20:11:13	51.711	-178.566	10.2	ANDREANOF ISLANDS, ALEUTIAN IS., ALASKA
MAP	2.6	2008/09/22 18:09:11	36.221	-120.266	10.4	CENTRAL CALIFORNIA
MAP	4.9	2008/09/22 17:53:29	-9.765	160.214	35.2	SOLOMON ISLANDS
MAP	4.5	2008/09/22 15:44:11	8.399	-103.557	10.0	NORTHERN EAST PACIFIC RISE
MAP	3.0	2008/09/22 15:21:49	56.615	-154.649	13.5	KODIAK ISLAND REGION, ALASKA
MAP	2.7	2008/09/22 13:55:21	37.378	-104.898	5.0	COLORADO
MAP	2.6	2008/09/22 13:50:31	52.973	-167.702	46.0	FOX ISLANDS, ALEUTIAN ISLANDS, ALASKA
MAP	5.4	2008/09/22 13:30:38	15.563	96.112	35.0	NEAR THE SOUTH COAST OF MYANMAR
MAP	4.7	2008/09/22 13:10:46	-24.250	-175.906	35.0	SOUTH OF TONGA
MAP	2.8	2008/09/22 10:27:18	51.144	-179.505	24.8	ANDREANOF ISLANDS, ALEUTIAN IS., ALASKA
MAP	2.5	2008/09/22 08:55:28	57.535	-155.542	78.8	ALASKA PENINSULA
MAP	5.2	2008/09/22 08:17:24	-5.999	151.005	34.1	NEW BRITAIN REGION, PAPUA NEW GUINEA
MAP	5.7	2008/09/22 07:31:59	41.603	140.457	147.3	HOKKAIDO, JAPAN REGION
MAP	4.6	2008/09/22 05:55:58	16.886	-99.787	42.5	GUERRERO, MEXICO
MAP	3.1	2008/09/22 05:05:26	19.152	-66.777	24.7	PUERTO RICO REGION
MAP	2.6	2008/09/22 03:04:28	57.108	-155.502	31.0	ALASKA PENINSULA
MAP	3.4	2008/09/22 02:46:19	17.774	-68.911	88.8	DOMINICAN REPUBLIC REGION
MAP	2.8	2008/09/22 01:10:58	51.913	179.607	3.6	RAT ISLANDS, ALEUTIAN ISLANDS, ALASKA

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	MAG	UTC DATE-TIME y/m/d h:m:s	LAT deg	LON deg	DEPTH km	Region
MAP	2.5	2008/09/21 23:33:12	18.857	-67.428	11.9	PUERTO RICO REGION
MAP	4.6	2008/09/21 19:19:21	-15.632	-174.823	271.3	TONGA
MAP	5.2	2008/09/21 18:50:18	-31.363	-179.996	340.1	KERMADEC ISLANDS REGION
MAP	2.7	2008/09/21 18:21:48	63.524	-146.879	3.1	CENTRAL ALASKA
MAP	3.3	2008/09/21 17:34:57	18.196	-68.075	107.7	MONA PASSAGE, DOMINICAN REPUBLIC
MAP	4.6	2008/09/21 16:50:55	8.851	-102.857	10.0	NORTHERN EAST PACIFIC RISE
MAP	4.4	2008/09/21 16:23:21	-17.730	-178.619	562.2	FIJI REGION
MAP	3.3	2008/09/21 12:52:33	17.922	-68.142	107.6	DOMINICAN REPUBLIC REGION
MAP	4.7	2008/09/21 12:48:02	-3.061	138.837	40.4	PAPUA, INDONESIA
MAP	4.5	2008/09/21 12:38:52	-23.103	179.427	519.2	SOUTH OF THE FIJI ISLANDS
MAP	3.2	2008/09/21 08:45:34	61.579	-149.972	35.4	SOUTHERN ALASKA
MAP	4.4	2008/09/21 07:45:23	-20.294	-178.448	557.0	FIJI REGION
MAP	4.4	2008/09/21 07:11:07	-22.185	-179.485	583.1	SOUTH OF THE FIJI ISLANDS
MAP	4.9	2008/09/21 07:05:47	-3.976	141.437	102.4	NEW GUINEA, PAPUA NEW GUINEA
MAP	5.2	2008/09/21 06:36:25	-16.132	-173.869	76.3	TONGA
MAP	2.7	2008/09/21 05:59:23	56.876	-155.937	6.4	ALASKA PENINSULA
MAP	3.6	2008/09/21 02:47:46	19.650	-65.993	117.2	PUERTO RICO REGION

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	MAG	UTC DATE-TIME y/m/d h:m:s	LAT deg	LON deg	DEPTH km	Region
MAP	4.9	2008/09/20 22:17:10	35.561	140.121	55.1	NEAR THE EAST COAST OF HONSHU, JAPAN
MAP	4.5	2008/09/20 21:12:16	8.291	-82.174	35.5	PANAMA-COSTA RICA BORDER REGION
MAP	3.1	2008/09/20 20:42:14	56.920	-155.780	5.7	ALASKA PENINSULA
MAP	4.7	2008/09/20 19:27:23	-7.080	126.256	444.8	KEPULAUAN BARAT DAYA, INDONESIA
MAP	3.5	2008/09/20 17:20:05	49.991	-178.389	33.2	SOUTH OF THE ALEUTIAN ISLANDS
MAP	4.9	2008/09/20 16:30:10	15.349	-91.691	204.0	GUATEMALA
MAP	5.4	2008/09/20 14:43:05	-16.126	-73.761	41.5	NEAR THE COAST OF SOUTHERN PERU
MAP	2.8	2008/09/20 14:18:47	51.974	175.000	22.4	RAT ISLANDS, ALEUTIAN ISLANDS, ALASKA
MAP	2.6	2008/09/20 12:05:21	19.147	-67.046	13.1	PUERTO RICO REGION
MAP	2.7	2008/09/20 09:50:38	19.037	-66.793	13.2	PUERTO RICO REGION
MAP	2.9	2008/09/20 08:23:56	51.100	179.365	15.8	RAT ISLANDS, ALEUTIAN ISLANDS, ALASKA
MAP	4.8	2008/09/20 07:58:17	14.083	50.244	10.0	NEAR THE COAST OF YEMEN
MAP	5.2	2008/09/20 05:16:11	63.588	-129.029	10.0	NORTHWEST TERRITORIES, CANADA
MAP	5.2	2008/09/20 04:52:27	14.459	-92.211	60.7	OFFSHORE GUATEMALA
MAP	2.8	2008/09/20 04:16:35	52.112	-168.203	32.9	FOX ISLANDS, ALEUTIAN ISLANDS, ALASKA
MAP	4.6	2008/09/20 04:00:24	-29.309	-70.506	67.6	COQUIMBO, CHILE
MAP	4.3	2008/09/20 03:59:43	19.327	-155.106	9.7	ISLAND OF HAWAII, HAWAII
MAP	5.3	2008/09/20 02:54:38	38.420	73.423	146.1	TAJIKISTAN
MAP	4.4	2008/09/20 02:26:00	13.383	-90.476	88.0	OFFSHORE GUATEMALA
MAP	2.6	2008/09/20 02:03:36	53.785	-165.248	56.0	FOX ISLANDS, ALEUTIAN ISLANDS, ALASKA
MAP	4.2	2008/09/20 00:04:37	36.524	70.958	189.0	HINDU KUSH REGION, AFGHANISTAN

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	MAG	UTC DATE-TIME y/m/d h:m:s	LAT deg	LON deg	DEPTH km	Region
MAP	2.6	2008/09/19 23:22:24	62.980	-149.291	73.3	CENTRAL ALASKA
MAP	2.8	2008/09/19 22:49:45	53.719	-164.062	25.7	UNIMAK ISLAND REGION, ALASKA
MAP	5.5	2008/09/19 22:49:03	-11.167	164.491	29.2	SANTA CRUZ ISLANDS REGION
MAP	3.0	2008/09/19 22:01:38	50.217	178.402	14.9	RAT ISLANDS, ALEUTIAN ISLANDS, ALASKA
MAP	3.1	2008/09/19 21:48:08	53.730	-164.092	4.4	UNIMAK ISLAND REGION, ALASKA
MAP	3.1	2008/09/19 21:38:28	32.922	-115.878	7.0	SOUTHERN CALIFORNIA
MAP	4.9	2008/09/19 21:17:35	-7.098	-13.025	10.0	ASCENSION ISLAND REGION
MAP	4.6	2008/09/19 20:32:28	23.787	142.921	15.0	VOLCANO ISLANDS, JAPAN REGION
MAP	4.5	2008/09/19 19:27:52	26.152	128.786	17.7	RYUKYU ISLANDS, JAPAN
MAP	2.8	2008/09/19 18:46:35	52.096	-175.439	14.3	ANDREANOF ISLANDS, ALEUTIAN IS., ALASKA
MAP	5.1	2008/09/19 18:34:24	-23.990	-175.907	55.9	TONGA REGION
MAP	3.8	2008/09/19 18:08:55	18.470	-66.035	119.9	PUERTO RICO REGION
MAP	4.8	2008/09/19 17:29:54	-24.529	-175.856	53.2	SOUTH OF TONGA
MAP	3.4	2008/09/19 17:23:36	58.785	-140.335	36.2	OFF THE COAST OF SOUTHEASTERN ALASKA
MAP	3.3	2008/09/19 17:23:35	58.783	-140.311	15.8	OFF THE COAST OF SOUTHEASTERN ALASKA
MAP	2.7	2008/09/19 15:02:03	32.114	-115.816	6.0	BAJA CALIFORNIA, MEXICO
MAP	2.6	2008/09/19 13:35:46	54.366	-164.074	19.6	UNIMAK ISLAND REGION, ALASKA
MAP	2.7	2008/09/19 13:35:37	53.769	-164.081	25.6	UNIMAK ISLAND REGION, ALASKA
MAP	2.5	2008/09/19 13:32:09	53.677	-163.995	32.2	UNIMAK ISLAND REGION, ALASKA
MAP	4.7	2008/09/19 13:04:52	5.042	94.243	35.0	NORTHERN SUMATRA, INDONESIA
MAP	4.6	2008/09/19 09:47:31	-13.827	-172.582	61.3	SAMOA ISLANDS
MAP	2.6	2008/09/19 09:09:39	54.813	-161.720	43.5	ALASKA PENINSULA
MAP	2.7	2008/09/19 09:09:34	54.451	-161.510	7.9	ALASKA PENINSULA
MAP	2.8	2008/09/19 07:57:36	63.292	-151.492	11.5	CENTRAL ALASKA
MAP	2.9	2008/09/19 07:57:36	63.296	-151.595	5.0	CENTRAL ALASKA
MAP	2.8	2008/09/19 07:49:15	37.538	-118.865	8.5	CENTRAL CALIFORNIA
MAP	3.1	2008/09/19 06:22:09	33.963	-117.789	15.5	GREATER LOS ANGELES AREA, CALIFORNIA
MAP	4.4	2008/09/19 03:24:42	-17.558	-178.584	584.2	FIJI REGION
MAP	2.6	2008/09/19 01:29:11	50.075	-178.422	28.4	ANDREANOF ISLANDS, ALEUTIAN IS., ALASKA
MAP	2.6	2008/09/19 01:25:16	37.417	-113.129	1.2	UTAH
MAP	2.7	2008/09/19 01:18:26	49.971	-178.461	26.7	SOUTH OF THE ALEUTIAN ISLANDS
MAP	2.8	2008/09/19 00:44:26	51.710	-179.012	3.2	ANDREANOF ISLANDS, ALEUTIAN IS., ALASKA
MAP	3.0	2008/09/19 00:44:25	51.619	-179.036	4.3	ANDREANOF ISLANDS, ALEUTIAN IS., ALASKA

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	MAG	UTC DATE-TIME y/m/d h:m:s	LAT deg	LON deg	DEPTH km	Region
MAP	2.5	2008/09/18 22:53:16	52.341	-169.777	138.4	FOX ISLANDS, ALEUTIAN ISLANDS, ALASKA
MAP	2.5	2008/09/18 22:53:16	52.341	-169.777	138.4	FOX ISLANDS, ALEUTIAN ISLANDS, ALASKA
MAP	5.3	2008/09/18 20:32:45	10.917	91.783	12.9	ANDAMAN ISLANDS, INDIA REGION
MAP	4.7	2008/09/18 19:43:53	59.502	-152.793	89.9	SOUTHERN ALASKA
MAP	4.7	2008/09/18 19:43:53	59.502	-152.793	89.9	SOUTHERN ALASKA
MAP	2.9	2008/09/18 19:31:59	66.929	-157.471	6.2	NORTHERN ALASKA
MAP	2.9	2008/09/18 19:31:59	66.929	-157.471	6.2	NORTHERN ALASKA
MAP	5.0	2008/09/18 18:46:45	-4.712	153.271	83.7	NEW IRELAND REGION, PAPUA NEW GUINEA
MAP	3.3	2008/09/18 17:32:09	35.188	-119.430	19.2	CENTRAL CALIFORNIA
MAP	2.7	2008/09/18 15:51:44	18.876	-64.402	18.9	VIRGIN ISLANDS REGION
MAP	3.1	2008/09/18 14:56:25	60.581	-137.101	7.3	SOUTHERN YUKON TERRITORY, CANADA
MAP	4.6	2008/09/18 13:29:29	34.960	136.806	323.6	WESTERN HONSHU, JAPAN
MAP	5.9	2008/09/18 11:58:51	51.969	158.346	67.4	NEAR THE EAST COAST OF KAMCHATKA, RUSSIA

Large Earthquakes

Large energy earthquakes are rare but cause wide scale social disruption.

May 2008 Eastern Sichuan, China >87,000 fatalities

Oct 2005 Pakistan >86,000 fatalities

Dec 2004 Sumatra-Andaman >227,000 fatalities

July 1976  Tangshan, China  >650,000 fatalities
May 1970  Lima, Peru    >66,000 fatalities

Oct 1948 Ashgabat, USSR >110,000 fatalities
May 1927  Nan-Shan, China  >200,000 fatalities
Sep 1923  Tokyo, Japan   >142,000  fatalities

Dec 1920 Haiyuan, China >200,000 fatalities

Dec 1908 Messina, Italy >72,000 fatalities

1556 China ~830,000 fatalities

1138 Syria ~230,000 fatalities

856 Iran ~200,000 fatalities

Largest Recorded Earthquake

Chile 1960 May 22 19:11:14 UTC, Magnitude 9.5

More than 2,000 killed, 3,000 injured, 2,000,000 homeless, and $550 million damage in southern Chile;

Tsunami caused 61 deaths, $75 million damage in Hawaii;

138 deaths and $50 million damage in Japan;

32 dead and missing in the Phillippines; and

$500,000 damage to the west coast of the United States.

Why are there Earthquakes?

Earthquakes result from sudden displacement of crust, usually along faults.

Often come in a series of quakes - "minor" aftershocks common.

The earth rings like a bell after a large earthquake occurs.

Very long wavelength surface waves circle the globe and interfere constructively to set up harmonic oscillations.

The shape of an oscillation is determined by the wavelength of the waves, and the frequency of the oscillation by the internal velocity structure.

Analysed by spherical harmonics….

l=3 m=0 l=3, m= +/-1

l=3, m=+/- 2 l=3, m=+/-3

Shocks give rise to landslips, turbidity currents,…..

….and tsunamis:

e.g. Sumatra-Andaman Earthquake 26^th Dec2004

Magnitude 9.0

Large rupture zone:

Rupture lasted 8 mins, and travels at 2.8km/sec.

Produced massive tsunami:

Surface waves seen in seismograms as they travel around the world:

In this Lecture will discuss:

• earthquake cause and mechanism
• earthquake locations and distribution
• earthquake size: magnitude and intensity
• seismic waves and the interior of the earth
• seismotectonics
• earthquake prediction and control

Measurement of seismic waves

Earthquake waves recorded on a SEISMOMETER -> SEISMOGRAM.

Seismometers originally consisted of a rigid frame attached to the Earth, a chart recorder attached to frame, a heavy mass suspended from frame by a pivot and a pen attached to mass.

Modern ones were based on solenoids and give digitised data, etc.

When the frame is shaken by an earthquake, the mass and pen remain stationary. 

The scale of the displacement is recorded on chart.

The mass is stationary because of inertia.

The pivot restricts recording to just one plane. 

Three seismometers are needed to measure earthquakes. 

One for vertical movements and two for horizontal movements.

Often have more seismometers with different masses to record shocks with different frequencies - seismic vibrations have spectrum of frequencies.

o Typically record waves with short-periods - 12s,

o and  long-periods  - 20s.

Earthquake causes and mechanisms

Major earthquakes occur at plate boundaries – reasonable to infer that the cause of earthquakes is the relative motion of sections of the lithosphere.

San Andreas pacific-american

Mount San Jacinto, California

Indian into Eurasian plate

Explanation of earthquakes is elastic rebound theory, put forward by H. F. Reid in 1906 after the San Francisco earthquake.

An earthquake derives from a fault, that separates two blocks which are attempting to move relative to each other.

Friction or cementing will resist the movement. 

Under these conditions crustal stresses do not move blocks, but a state of strain builds up in the region of the fault.

If crustal stresses increase (because of movement elsewhere on the fault, etc.) they will reach a level where they overcome frictional restraint, and the two blocks will slip relative to each other leading to an earthquake.

This process can occur time after time, so called stick-slip movement.

Time between movements can be years. 

The longer the time, the larger the stress and strain, the larger the earthquake.

Types of fault

There are three main types of fault movement, each give distinctive EQ patterns (see below):

normal fault

thrust fault

strike-slip fault

oblique-slip fault

strike-slip in S.California

Reverse faults

Normal faults

Earthquake location and distribution

Focus - The source - a small part of a fault (approx. a few kms³) - can be regarded as a point .
Epicentre - The point on the surface, vertically above the focus.
Focal Depth  - The distance from the epicenter to the focus.

Shock waves radiate out from focus in all directions. 

They are first felt at the surface at the epicentre.

Location of epicentre

If we have seismographs from three stations relatively close to the epicentre, we can deduce the position of the epicentre by the time intervals between the P and the S waves at each station - given knowledge of the P and S wave velocities (from standard seismic models).

If local P and S near-surface velocities are V_P and V_S and the station is X km from epicentre, then

 Time for P wave arrival  (t_p) =  X / V_P
 Time for S wave arrival  (t_s) =  X / V_S

Therefore interval between P and S arrival

Δt = t_S-t_P  =  X / V_S  -  X / V_P

Thus re-arranging to find X gives

X  =  V_S.V_P. Δt / (V_P - V_S)

Units - V_P,V_S in kms^-1; Δt in s; X in km

To locate the epicentre we need values of X from three stations -> Triangulation. 

The epicentre is at the intersection of three circles of radii X₁, X₂ and X₃ based at stations 1, 2 and 3.

World-wide have there are several thousand stations.  

These are often arranged as SEISMIC ARRAYS in a well defined geometrical distribution. This can be either for local use such as the Evergreen Basin seismic imaging study (below left) or a broader array such as the Eastern Turkey Seismic Array (below right).

Necessarily most seismic stations and arrays are land based, so there is a significantly uneven coverage of the seismic structure of the Earth. Recent attempts to address this problem include the MERMAID project that uses passively drifting autonomous hydrophones recording hundreds of distant earthquakes.

Arrays can give both near-surface data and also information about the deeper structure of Earth.

Depth of focus (h)

· In order to establish the depth of focus, we need to determine the epicentral angle – Δ

· This is done from travel time curve, as t_s-t_p is a simple function of Δ:

· Then, a plot of t_s-t_p v. t_p for several stations give t₀, the time the earthquake occurred.

· Then, if V is the P-wave velocity, distance of detector (D) to focus is:

D = (t_p-t₀)V

· Now can obtain h, since:

· X = R . Δ . (2π/360) and so:

h ~ √(D² – X²)

Global Distribution

· All earthquakes occur at depths < 700 km and are confined to the rigid lithosphere.

· Three classifications

Shallow focus    0-70 km
Intermediate focus  70-300 km
Deep focus   300-700 km

· With large Earthquakes (magnitude > 7 ):

o 75% Shallow,  

o 90%  Intermediate,

o ~100% Deep occur around the margins of the Pacific

· Those not in circum-Pacific belt occur in Alpine-Himalayan belt and along plate boundaries.

· Deep Earthquakes largely from subducted slabs:

· Also high concentration of shallow Earthquakes (mag. 4-6) along ocean ridge system.

· Intra-plate Earthquakes are rarer (but do occur, e.g. Lisbon 1755).

· Shallow Earthquakes much more common than Intermediate and Deep.

· Most surface damage is caused by Shallow Earthquakes.

Earthquake size: magnitude and intensity

Earthquake magnitude is an “absolute” measure of size, and is related to the energy released.  

Magnitude is determined from the seismic wave amplitude.

Earthquake intensity refers to amount of damage caused – more subjective.

Magnitude

Magnitude, given by general equation:-  

M  =  log₁₀ A/T + a

M = magnitude
log - base 10
A = max. amplitude of wave in µm (= 10^-6m)
T = period of wave, (P or S) in secs. (time taken for one wavelength to pass a fixed point)
a= factor correcting for epicentral distance, focal depth and type of wave.  These can be read from published tables. However, since A needs to be obtained from observation the "absolute" measure of Earthquake magnitude is still a subject of debate.  Error in magnitude should be < ± 0.3.

Recall, the epicentral distance from the point of measurement to the epicentre is measured by the angle subtended by these 2 points at the centre of the  Earth - the epicentre angle (Δ) :

Magnitude scale is logarithmic, therefore M = 7.0 is 100 times larger than M = 5.0

There are many definitions of magnitude, depending on what is being measured:

· Richter was amongst first to develop a scale for earthquakes in 1935 based on P-wave amplitudes. 

An earthquake's Richter magnitude was originally defined to be the amplitude of shaking on a Wood-Anderson seismometer of an earthquake 100km away. Since there are very few working Wood-Anderson seismometers around these days, scientists approximate the magnitude using calibration scales based on the distance from the source and the amplitude of seismic waves.

· The Richter local magnitude is given by:

M_R = log₁₀(A/T) + g(Δ,h) + a

• where M is the magnitude
• A is the maximum wave amplitude
• T is the wave period
• g is a function that corrects for the decrease in wave amplitude(A)
• Δ is the angular distance from seismometer to epicenter
• h is the focal depth of the earthquake
• and (a) is an empirical constant

· BODY-WAVE magnitude - At large distances from epicentre use body-wave amplitude (usually P-wave), because body-wave attenuation is less than surface-waves.  

M_B  = log₁₀ (A/T) + 0.01Δ + 5.9

where T ~ 12 secs.

· The surface wave magnitude is given by

M_S    = log₁₀ (A/T) + 1.66 log₁₀ Δ  + 3.3

• where T is ~17-23 s
• Δ is ~20-160 degrees
• and h (depth of focus) is less than 50 km

For extremely deep earthquakes the magnitude must be corrected to compensate for the increased distance due to depth.

· The seismic moment is given by

M_O = G.A.m

• where G is the rigidity
• A is the surface area of the fault
• and m is the average fault displacement

and the moment magnitude is given by

M_M = 0.666log₁₀M_O - 6

· The moment magnitude can be related to the surface wave magnitude:

log₁₀M_O = 1.5M_S + 16.1

Largest Earthquakes in the World 1900-2003

	Location	Date	Mag Coordinates
1.	Chile	1960 05 22	9.5	38.26 S	72.15 W
2.	Prince William Sound, Alaska	1964 03 28	9.2	61.02 N	147.65 W
3.	Andreanof Islands, Aleutian Islands	1957 03 09	9.1	51.57 N	175.34 W
4.	Kamchatka	1952 11 04	9.0	52.75 N	159.50 E
5.	Off the Coast of Ecuador	1906 01 31	8.8	1.0 N	81.5 W
6.	Rat Islands, Aleutian Islands	1965 02 04	8.7	51.23 N	178.52 E
7.	India-China Border	1950 08 15	8.6	28.5 N	96.5 E
8.	Kamchatka	1923 02 03	8.5	54.0 N	161.0 E
9.	Banda Sea, Indonesia	1938 02 01	8.5	5.25 S	130.5 E
10.	Kuril Islands	1963 10 13	8.5	44.9 N	149.6 E

Largest Earthquakes in the World 1900-present (spot the difference)

	Location	Date UTC	Magnitude	Lat.	Long.	Reference
1.	Chile	1960 05 22	9.5	-38.24	-73.05	Kanamori, 1977
2.	Prince William Sound, Alaska	1964 03 28	9.2	61.02	-147.65	Kanamori, 1977
3.	Off the West Coast of Northern Sumatra	2004 12 26	9.1	3.30	95.78	Park et al., 2005
4.	Kamchatka	1952 11 04	9.0	52.76	160.06	Kanamori, 1977
5.	Off the Coast of Ecuador	1906 01 31	8.8	1.0	-81.5	Kanamori, 1977
6.	Rat Islands, Alaska	1965 02 04	8.7	51.21	178.50	Kanamori, 1977
7.	Northern Sumatra, Indonesia	2005 03 28	8.6	2.08	97.01	PDE
8.	Assam - Tibet	1950 08 15	8.6	28.5	96.5	Kanamori, 1977
9.	Andreanof Islands, Alaska	1957 03 09	8.6	51.56	-175.39	Johnson et al., 1994
10.	Southern Sumatra, Indonesia	2007 09 12	8.5	-4.438	101.367	PDE
11.	Banda Sea, Indonesia	1938 02 01	8.5	-5.05	131.62	Okal and Reymond, 2003
12.	Kamchatka	1923 02 03	8.5	54.0	161.0	Kanamori, 1988
13.	Chile-Argentina Border	1922 11 11	8.5	-28.55	-70.50	Kanamori, 1977
14.	Kuril Islands	1963 10 13	8.5	44.9	149.6	Kanamori, 1977

Energy and Magnitude

· The energy released by an earthquake (E in Joules) is related to magnitude.

· An empirical relation between energy and magnitude is given by:  

Log₁₀ E  = 1.5M_S + 4.8

· An increase in magnitude of 1.0 unit in M_S gives ~30 x increase in energy.

· The estimated annual energy loss via earthquakes is ~10¹⁸ J.  

· Most of this comes from the few quakes with M_S ~ 7 or 8.

· The energy of an earthquake is turned largely into heat via friction of vibrating rock particles, etc.. 

· The total contribution of an earthquake to heat loss is small.  

· The total Earth heat flow is ~ 10²¹ J per y.  The total U.S.A. energy consumption is ~10¹⁹ Jy^-1!

Intensity

· The intensity of an earthquake is based on its effects at the surface.

·   For a given location intensity is reported as Roman Numerals according to the modified Mercalli Scale:

Intensity	Description
I	Not Felt.
II	Felt by persons at rest on upper floors.
III	Felt indoors; hanging objects swing; vibration like passing of light trucks.
IV	Vibration like passing of heavy trucks; standing cars rock; windows, dishes, doors rattle; walls, frames creak.
V	Felt outdoors; sleepers wake; liquids spill; small objects move; doors swing; shutters and pictures move.
VI	Felt by all; frightening; people walk unsteadily; windows, objects broken; objects knocked off shelves, pictures off walls; furniture moved or overturned; weak plaster cracked; small bells ring; trees and bushes shaken.
VII	Difficult to stand; furniture broken; damage to weak materials; cracking of masonry; fall of plaster, loose bricks, and tile; waves on ponds; water muddy; small slides along sand or gravel banks; large bells ring.
VIII	Steering cars affected; damage to, and partial collapse, of masonry; fall of chimneys, towers; frame houses moved on foundations if not bolted down; changes in flow of springs and wells.
IX	General panic; frame structures shifted off foundations if not bolted down; frames cracked; serious damage even to partially reinforced masonry; underground pipes broken; reservoirs damaged; cracks in ground.
X	Most masonry and frame structures destroyed with their foundations; serious damage to dams and dikes; large landslides; rails bent slightly.
XI	Rails bent greatly; underground pipelines out of service.
XII	Damage nearly total; large rock masses shifted; objects thrown into the air.

· Factors that tend to increase intensity:

o magnitude of Earthquake

o distance to focus

o substrate

o

· 1989 Loma Prieta event:

Seismotectonics

Earthquake Focal Mechanisms

o In any given direction P waves radiate from focus as compression or dilatation. The pattern of initial motion (first motion) characterizes the nature of the fault motion that produced the earthquake (normal, reverse, strike-slip)

o Thus of a strike slip fault:

Can define the fault plane from the plane normal to fault (the auxiliary plane)

o The first shock reveals a good deal about the nature of the fault. Depending on whether a particular area is compressed or extended by movement along the fault, the first motion p-wave will be upwards (positive) or downwards (negative), respectively.

o The pattern of first motions obtained from a number of seismic stations can be summarized on a stereogram, depicting quadrants of compression and dilation.

o On a stereonet there are two planes, drawn along great circles, separating the compressional (positive) p-waves from the extensional (negative) p-waves.

o These planes are known as nodal planes.

o From this we can infer nature of faulting – focal mechanism solutions (black- compression, white- dilatation).

o Note: the observed pattern of arrivals does not uniquely determine the orientation of the fault.

o From P waves alone cannot tell fault plane from auxiliary plane:

o If the source is a single- couple one, can tell from S-waves distribution:

o But if movement on the fault is complex (“double couple”), then the solution is still ambiguous:

o To determine the correct plane, the locations of aftershocks and/or surface geology and regional tectonics may be necessary.

Mixed Faults

It is very unusual to find a pure strike-slip, normal or thrust fault in nature. Most faults consist mainly of one motion with a small component of another. Pure strike-slip faults have first motion diagrams with nodal planes that intersect in the center of the stereonet (as pictured above). Strike-slip faults that have some component of thrust or normal faulting intersect off-center.

This is either a left-lateral or right-lateral strike slip fault with a component of thrust faulting. The nodal planes for pure normal faults and pure thrust faults intersect along the primitive circle (as pictured previously). Strike-slip faults can have components of either normal or thrust faulting, but normal and thrust faults can only have components of strike-slip faulting. Here's the first motion patterns of an oblique normal fault and an oblique thrust fault with a small component of strike slip motion.

Oblique thrust

The first motion pattern for thrust faulting with a small component of left lateral strike-slip motion would look the same (except the compression and extension circles would be reversed).

Oblique normal

Earthquakes and Plate Tectonics

Different earthquakes are characteristic of different tectonic setting revealed by the study of focal mechanisms.

Divergent boundaries - shallow focus earthquakes, normal and strike slip faults along mid-ocean ridge:

Convergent boundary - earthquakes at all depth and all magnitudes. Extentional or compressional depending on position:

o Thrust faults in upper part of subducting slab,

o normal faults at depth

Continental collision zones show only shallow and intermediate depth EQ.

Earthquake prediction

Social impact of earthquakes in regions of high population are obvious.

If we could predict when an earthquake will occur, it could reduce effect of quake on death toll, etc..

U.S.A., Japan and China have large programmes.

Tokyo and Peking are "at risk" cities.

Current results in prediction are poor - still need more data on earthquake processes.

Prediction at several levels

(1) Most large earthquakes will occur at plate boundaries.

If no one lived in these regions, earthquakes would not be socially very important. But since people will not move, how and when will these large, plate boundary quakes occur?

(2) Earthquakes often occur in cycles.

There will almost certainly be a large earthquake in San Francisco again.

But when - within 200 years - 100% prob.; 50 years - 50% prob.; 5 years ?

(3) To be socially useful, we need to be precise as to time and place.

To make progress need to understand:

Earthquake precursors

Geodetic Measurements

Prior to an earthquake, does the Earth bulge due to build up of strain, etc.?

These could be detected by geodetic measurements - study of land shape, tilt and position -> periodic survey :

The average strain rate ε₁₂ (right-lateral shear across a vertical plane striking N W) in the San Francisco Bay area within each of the 32 polygonal regions, in nanostrain/yr .

(http://seismo.berkeley.edu/annual_report/ar97_98/node31.html)

and GPS monitoring (but need high precision):

Velocities of GPS sites in the Coast Ranges and western Great Valley, relative to Point Reyes NCMN on the Point Reyes peninsula ( N, W) with 95% confidence regions. Virtually all velocities are parallel to the San Andreas fault system (sinuous NW-trending lines, principally comprised from west to east of the San Andreas, Ma'acama, and Bartlett Springs faults). The NOAM and SNGV arrows show predicted NUVEL-1A North America and Sierra Nevada-Great Valley motions (Don Argus, personal communication, 1998). (http://seismo.berkeley.edu/annual_report/ar97_98/node31.html)

Studies in Japan, USSR and elsewhere, show that there are changes of height, etc. in regions which subsequently suffer from earthquakes. Also water levels in wells change:

Studies show upwelling and downwelling before an earthquake. However, the time scale can be in 10's of years so it is not easy to predict when it will happen.

Tide-gauges can also be useful. Large retreats of sea can occur before large submarine earthquake. In 1872 in Japan the sea went out over 200 m, 20 mins. before the earthquake.

Tide gauge data:

Continuous observation of land level - Tiltmeters (c.f. surveys) e.g. water filled tube 30-40 m. in length.

Find that increase in rate of ground tilting occurs during immediate pre-earthquake period.

Tilting produced by change in volume of rocks during deformation.

As rock shears, cracks open up - dilation - which increases volume.

Big earthquakes occurs when cracks join up to give a large fracture.

Now Synthetic Aperture Radars (SAR) can be used to see land deformation after Earthquake (and bulging before):

7.1 magnitude Hector Mine earthquake of October 16, 1999 40 km

northwest of Barstow, California (edge of Mojave Desert) from ERS2 satellite.

Seismic Activity

In seismically active regions, the distribution of seismic activity can give clues as to where large earthquakes may occur.

Along San Andreas there are areas with lots of small earthquakes (microearthquakes M_s approx. 1 to 3).

These are continuously releasing the strain in rocks and it is unlikely that a large EQ will happen in these sections.

Some areas are locked and show no microseismic behaviour. Expect large build-up of strain in these Locked Sections.

This will be where the big earthquakes occur –

e.g. Loma Prieta Earthquake of 17^th Oct 1989:

Earthquake filled the seismic gap:

Major damage where the shockwaves where in soft mud – high amplitude:

Still a gap in the San Fransico Area – when will it happen?

Changes in seismic wave velocity may give the answer.

When rocks deform there are changes in V_p and V_s.

Find V_p/V_s during stress build up.

Normally V_p/V_s ~ 1.7, but this drops prior to an earthquake, then shows rapid increase to original value of even greater prior to an earthquake.

Why?

As stress builds up, cracks are formed.

Air-filled cracks show seismic waves (P affected more than S, since effective bulk modulus reduced).

When cracks are nearly all joined up, water can flow in.

Water replaces air (or vacuum) and increases bulk modulus (water more incompressible than air), but not shear modulus, therefore V_p/V_s increases.

Water in cracks weakens rock so it now fails, and an earthquake occurs (stress corrosion, lubrication, etc.).

This theory is supported by experiments on rocks. It could be a way of predicting earthquakes.

But ....

The "Dilatancy Cycle" takes longer in areas where rocks are strong - i.e. in areas where earthquakes are likely to be of high energy.

The time interval between V_p/V_s drop and the occurrence of earthquake increases with increasing magnitude of quake.

The larger the quake, the longer the warning time - good!

The less precise the estimate of occurrence of quake - bad!

If M_s > 6, V_p/V_s anomaly occurs ~ 3 years before quake!

Other Techniques

Magnetic and electrical effects:

Low frequency magnetic signals recorded for 31 days around the 1989

Due to enhanced water connectivity and so electrical conduction.

Also have strain on quartz in rock that may set up piezoelectric field.

Friction might give heating and enhanced IR emission:

An infrared image of the region surrounding Gujarat, India, on January 21, 2001. Yellow-orange areas trace thermal anomalies that appeared days before the Jan. 26th quake. The boxed star denotes the quake's epicenter.

Crushed rocks give off IR because of friction.

Noble gas escape due to cracking:

Location and radon variations of 6 springs in the vicinity of

a M=2.4 earthquake a few km west of the Abant Lake end of 1990.

Shown are changes relative to 1988.

animals go mad, etc.

Earthquake general sequence of events:

Stage I	Elastic strain builds up along a fault due to plate movement. All parameters are at their normal state. No uplift, radon increase, etc.
Stage II	Cracks begin to develop in crustal rocks in the pre-quake area. The build-up begins to be visible as an uplift of the area. The cracked rocks do not propagate P-waves as easily and their velocity slows in the area. Radon gas can escape through the newly formed cracks, and electrical resitivity decreases. The newly forming cracks and increasing stress may also result in a tiny increase in local seismicity.
Stage III	Groundwater from surrounding areas can now flow into the new cracks. Because the cracks are now filled again, the P-wave velocity can increase back to normal. The ground's uplifting also ceases and radon gas emission decreases. Electrical resistivity is still decreasing.
Stage IV	THE EARTHQUAKE

Man-made earthquakes

Man produces earthquakes in several ways.

The study of these "man-made" earthquakes sheds light on how it may be possible to control natural earthquakes and so prevent major, damaging shallow earthquakes.

Nuclear Explosions

An explosion naturally produces a shock

1 megaton ~ M_b ~ 6.5

Also find that the explosion gives rise to a series of after-shocks. Usually small M_b < 4, but sometimes M_b > 4.

Cause: shock of explosion unsticks locked up faults, and so allows the release of stored strain energy in the form of after-shocks.

Conclude: Could release strain in a large fault by explosion.

Problem: Unpredictable; not respectable (Superman stuff!)

Study of shock effects of explosions was not motivated by earthquake control, but for nuclear test ban studies, etc..

How to identify a nuclear explosion?

(1) First motion studies - an explosion only generates compressive first waves, no dilational first arrivals. Fault plane solution:

(2) Usual M_b, M_s relations do not hold for nuclear explosions:

M_b = 0.56 M_s = 2.9 for natural earthquakes but not for explosions.

M_b is larger than predicted; also energy not transformed as much into surface waves, therefore M_s < M_b.

Also the rock in which the explosion occurs affects M_b, M_s. Alluvium adsorbs more energy than granite, therefore M_b is less in Alluvium explosion than M_b in granite.

~ 90% of nuclear tests can be distinguished from natural earthquakes; larger explosions easier to distinguish than those ~<M_b = 4.

Reservoirs

It is now widely known that filling a dammed reservoir initiates a series of earthquakes, even in previously aseismic areas.

Make a lake, cause a quake!

Largest occurred in December 1967: after filling the local reservoir, an earthquake occurred with M_s = 6.4 in Koyna, India, killing 180 people.

This occurred in area of the Indian PreCambrian shield, with no previous seismic history.

Also note:

(1) Epicentres of shocks are always close to reservoir.

(2) Seismic activity maximum at early stages of filling. Diminishes with time.

(3) Most earthquakes are <M_s = 4.

Causes:

(1) Increased load of water may change stress loads, therefore triggering an earthquake?

Unlikely because the stress due to loading of even 1 or 2 km of water is small in comparison with crustal stresses.

(2) Water in the reservoir increases the pore-water pressure in local rocks?

More likely - water enters cracks and lubricates faults to allow strain release (c.f. Dilatancy Cycle of natural earthquakes).

Pumping Studies

Similar earthquake generation found in areas where chemical waste pumped into ground.

Earthquakes started in a previously aseismic area.

Find that, often, increased pumping pressure increased earthquake activity.

Conclude:-

(1) Increase in pore-pressure reduces shear strength of rock, therefore allows release of stored up strain energy.

(2) Water acts as lubricant (stress corrosion) and hence allows slip.

Earthquake Control

Aim: To release strain in locked fault zone in a controlled manner. Hence no more damage!

Approach: Use controlled changes in pore pressure to allow limited slip:

Drill holes A -> D along fault.

-----------A---------------------B----------------------C-------------D------------

Pump out H₂O at A and C to lock fault still further - dry fault zone.

Pump in H₂O at B to allow limited slip (locking at A and C will prevent catastrophic failure along whole fault)!

Now repeat, but with centre on C, etc.

Has worked in small scale experiments.

Problems:

(1) don't know what pressure is needed to allow slip;

(2) can't be certain that movement will be stopped at dry wells;

(3) don't know what M_s of earthquake will be when it is triggered,

(4) relation of energy and magnitude means that to release same amount of energy as an M=6 earthquake, need to generate ~ 1000 M=4 shocks.

Conclusion:

San Francisco 1906

Earthquake seismology is a major field of continuing active research – for obvious reasons:

(i) it is difficult

(ii) it is important!

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