Preliminary Analysis of Sedimentary Deposits from the June 23, 2001 Peru Tsunami

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A Cooperative Study by the USGS, la Dirección de Hidrografía y Navegación de la Marina de Guerra del Perú, Instituto Geofísico del Perú, Instituto Geologico, Minero y Metalurgico, University of California, Santa Cruz, University of Southern California, and University of San Agustin.

September 4 - September 18, 2001

Two photographs together, one shows ruined buildings with lots of sand and the other shows 10 men standing together smiling.

Top photo, ruined buildings in Peru's La Punta District. Bottom photo, members of the Peru Tsunami Sediment Survey (from left to right): Robert Peters, University of California, Santa Cruz; Percy Colque Riega, University of San Agustin, Peru; Matt Swensson, USC; Luis Bernales Anticona, DHN; Roberto Anima, USGS; Dave Rubin, USGS; Daniel Olcese, DHN; Juan Carlos Gomez, IGP; Bruce Jaffe, USGS; Guy Gelfenbaum, USGS.

On June 23, 2001 a deadly tsunami hit the southern coast of Peru, triggered by a massive earthquake of moment magnitude 8.4, the largest earthquake recorded worldwide in 35 years. The tsunami was observed in many coastal areas of the Pacific including Peru, Chilé, Hawaii, and Japan. Hardest hit was the region around Camaná, Peru, where more than 20 people died as a result of the tsunami and thousands of homes, hotels, and restaurants were destroyed. Shortly after the tsunami, a multi-national team of scientists visited Peru to document the effects of the tsunami. This group is referred to as the first International Tsunami Survey Team (1st ITST). In early September, a second group of scientists from the United States and Peru cooperated in a study of sediment deposited by the tsunami. The group included researchers from the United States Geological Survey USGS), la Dirección de Hidrografía y Navegación de la Marina de Guerra del Perú (DHN), Instituto Geofísico del Perú (IGP), Instituto Geologico, Minero y Metalurgico (INGEMMET), University of California, Santa Cruz (UCSC), University of Southern California (USC) and University of San Agustin (UNSA). This report contains photographs from the September trip, as well as some prelimary results and conclusions.

Preliminary modeling of the tsunami was done by Eric Geist of the USGS. The modeling results predict that the largest run-ups are centered around Camaná, even though the epicenter for the earthquake was approximately 100 km to the NW.  This was because the largest slip values, which are directly linked to the initial tsunami amplitude, were centered beneath Camaná. Model results, including a tsunami animation, are available.

Visit the U.S. National Oceanic and Atmospheric Administration Pacific Marine Environmental Laboratory (NOAA/PMEL) for information on the Peru tsunami event of June 23, 2001. Previous tsunami sediment surveys include a survey of sediments deposited by the 1998 Papua/New Guinea Tsunami.

Cartoon shows how sediment is deposited from a tsunami.

Deposition of sediment by a tsunami is governed by two basic processes. In the first process, deposition occurs through gradients in transport (more sediment goes into an area than comes out). Deposition also occurs by sediment falling out of suspension. USGS scientists use data from the Peruvian tsunami sedimentary deposits to determine which process is dominant. This aids in the development of sediment transport models that may be used to determine the wave height and flow velocity of the tsunami.

Participants in the Peru Tsunami Sediment Survey

USGS: Bruce Jaffe, Guy Gelfenbaum, Roberto Anima, David Rubin
UCSC: Robert Peters
USC: Matt Swensson
DHN: Daniel Olcese, Luis Bernales Anticona
IGP: Juan Carlos Gomez, Leonidas Ocala, Porfirio Huaco, Oswaldo Velasqez Cahuas, Hector Alamain
INGEMMET: Antonio Guzman Martinez, German Valenzuela
USA: Percy Colque Riega

The Importance of Studying Tsunami Sediments

Tsunamis often leave a layer of sediments that may be preserved in the geologic record. The geologic record may then be used to help assess tsunami hazard. In some cases, this record may be the only evidence that a region may be at risk from a tsunami. Where more than one tsunami deposit is preserved in a stratigraphic sequence, the record may help determine how often a tsunami is likely to occur. Tsunami sediments may also record important information about the wave that deposited them. By examining the thickness and grain size distribution of tsunami deposits, scientists may be able to deduce the wave height and flow velocity of the wave. Wave height and flow velocity are among the most important properties of a tsunami that determine how destructive the wave is. By studying sediments from recent tsunamis, we may be better able identify and interpret tsunami sediments in the geologic record.

Map showing location of transects conducted along a coast after a tsunami, with runup measurements plotted on a graph.

Location of tsunami deposit transects along the southern Peruvian coast near Camaná, approximately 700 km (435 miles) southeast of Lima, Peru. Transects were made at six sites: (1) Playa la Chira, (2) Playa Jahuay, (3) La Quinta, (4) Amecosupe, (5) La Bajada, and (6) Pampa Grande. Runup measurements, plotted on the graph above the map, include measurements from this survey and the 1st International Tsunami Survey Team.

Survey and Methods

USGS and Peruvian scientists measured tsunami sediment thickness, run-up, inundation distance, and topographic profiles for six shore-normal transects along 50 km of coastline in the vicinity of Camaná, Peru. More than 120 samples were collected for laboratory analyses for grain size distribution, microfossils, mineralogy, and chemistry. Sedimentary characteristics of the tsunami deposits and underlying material were logged and photo-documented. Box cores and sediment peels were taken at several sites to preserve the stratigraphy of the sediments. Erosion, flow-direction indicators, water marks on buildings, and damage to structures were also documented. Local residents were interviewed for their observations of the tsunami and local conditions before and after the tsunami. In addition to participating in the field survey, our Peruvian colleagues provided maps, supplied photographs taken before and shortly after the tsunami, and were invaluable for their knowledge of the area.

One photo has a person standing on a beach that's been heavily eroded, one photo has scattered cobbles over a beach.

Top photo: Some locations in Peru exhibited significant erosion as a result of the tsunami, such as this stretch of coast near Amecosupe. Bottom photo: The tsunami was capable of transporting large material considerable distances inland, such as these cobbles in a field at La Quinta. Large mud clasts ripped up from the agricultural fields are also present at the surface.

One photo shows mud and sand deposited in a damaged building and one photo shows a sandy beach.

Top photo: Tsunami-deposited sand with mud cap on floor of damaged building at Las Cuevas. Note water mark on wall. Bottom photo: La Bajada Transect. Tsunami crossed road in foreground of this photograph.

 

One photo shows a group of people standing together listening while one person speaks and gestures.

Top photo: Discussing our project with the local residents and interviewing eye-witnesses was an important part of our work. Bottom photo: It is necessary to document sedimentary deposits as soon after the tsunami as possible, for the need to rebuild communities and reestablish agricultural areas will often destroy valuable data. Many inundated farmlands had already been plowed before the survey team arrived and others were plowed during or soon after the survey collected data.

One photo shows a man examining sand in a hole, one photo shows two men photographing sand in a hole.

Top photo: USGS researcher Guy Gelfenbaum taking sediment samples for grain-size analysis, microfossils, mineralogy, and chemistry. Bottom photo: Two international researchers photographing tsunami deposits. 

A man pours a white liquid onto a cross-section of sand, the liquid will harden and stick to the sand to preserve the features.

Making a sediment peel of a tsunami deposit.

 

Damage to Structures

Four photographs show damage to buildings following a tsunami.

Top left photo: View of tsunami damage from highway to Camaná. Top right photo: Building destroyed by tsunami at La Punta. Note extensive erosion of sand around foundation. Bottom left photo: Watermarks on building at La Punta. Bottom right photo: Damaged structure at La Quinta.

Tsunami Runup

Map showing location of transects conducted along a coast after a tsunami, with runup measurements plotted on a graph.

Location of tsunami deposit transects along the southern Peruvian coast near Camaná, approximately 700 km (435 miles) southeast of Lima, Peru. Transects were made at six sites: (1) Playa la Chira, (2) Playa Jahuay, (3) La Quinta, (4) Amecosupe, (5) La Bajada, and (6) Pampa Grande. Runup measurements, plotted on the graph above the map, include measurements from this survey and the 1st International Tsunami Survey Team.

Two photographs show the extents to which debris was carried by tsunami waves.

Top photo: The limit of tsunami inundation was marked by lines of woody debris and often followed lows in topography, as seen on the beach at Pampa Grande. Bottom photo: The inundation line at Playa la Chira is marked by a line of debris and a color change on the slope above the beach. The highest run-up measured by the survey (8.2 m) was along this stretch of beach.

 

Sedimentary Deposits

Sedimentary deposits from the tsunami were found in most places where significant inundation occurred. Where the deposits were overlying a known preexisting surface that was texturally distinct, such as farm soils (La Quinta, Playa Jahuay), identification was fairly simple. Where the underlying material was beach sand that was similar both texturally and visually, identification was more difficult. Identification of tsunami deposits was based on several criteria, including differences in grain-size and color. In tsunami deposits, grain size generally fines upwards and rip-up clasts (pieces of material from the underlying sediment entrained by the tsunami) may be present. The base of the deposit erodes underlying structures and a heavy mineral layer may be present at the base. Underlying sands were often trampled while tsunami sands were relatively undisturbed. Many of the deposits had multiple layers. Many of the elements found in tsunami deposits from Papua New Guinea, such as rip-up clasts, multiple layers, and fining upwards sequences, were also found in Peru.

Three photos showing cross-sections of sand and labeled with various features to show the structure of tsunami deposits.

Top photo: Layered tsunami deposit overlying trample structures at Amecosupe. Middle photo: Tsunami deposit at La Bajada overlying trampled beach sand. Bottom photo: Tsunami deposit overlying mud layer at Pampa Grande.

Two photos showing cross-sections of sand and labeled with various features to show the structure of tsunami deposits.

Top photo: Tsunami deposit at La Quinta overlying agricultural soil. A thin layer of mud divides the deposit into two distinct layers. A mud cap also covers the surface of the deposit. Note mud rip-up clasts above soil in bottom layer and mud rip-up beneath mud cap in top layer. Normal grading is clearly visible in the lower layer. Bottom photo: Tsunami deposit at Amecosupe. Coarse material at base was eroded from a roadbed. Note eroded trample structures in underlying sand, thin mud layer near top of deposit, and current ripples from return flow at surface.

Two photos showing cross-sections of sand and labeled with various features to show the structure of tsunami deposits.

Top photo: Tsunami deposit (gray sand) overlying river sand (reddish sand) at Playa Jahuay. Tsunami deposit has 2 layers while river sand has multiple layers. Layering in both tsunami and river deposits may be defined by a heavy mineral lag. Bottom photo: Tsunami deposit overlying beach sand at Playa la Chira. Tsunami sand is lighter gray color. Underlying beach sand is darker and contains feathers.

 

Graphs show sand elevations at various locations inland from a beach.

Top graph: The deposits from the tsunami exhibited a wide variety of forms throughout the study area. Thickness varied both with distance inland and with site. The thickest deposits measured were at Amecosupe. While sedimentary structures were usually absent, many deposits contained 2-4 internal layers. Each layer is believed to represent deposition from a single wave within the tsunami wave train. Eyewitness accounts report that the tsunami consisted of 3-4 waves, with the second and third waves being the largest. The greatest run-up measured (from mid-swash, uncorrected for tides) was 8.2 meters at Playa la Chira. The greatest inundation distance measured (from mid-swash, uncorrected for tides) was 760 meters at La Quinta. The 1st ITST measured inundation distances of up to 1 km at some locations. Bottom graph: Topographic profiles from four sites surveyed by the team.

Results

  • Sediment deposits were found at all sites.
  • Sediment deposit thickness is variable, effected by distance from the shoreline, local topography, and change in slope.
  • Only a few sites had easily identifiable deposits.
  • Some deposits were normally graded, some were inversely graded, and some had no visible grading.
  • Flow indicators suggest significant onshore flow and weaker, but significant offshore flow.

The deposits from the tsunami exhibited a wide variety of forms throughout the study area. Thickness varied both with distance inland and with site. The thickest deposits measured were at Amecosupe. While sedimentary structures were usually absent, many deposits contained 2-4 internal layers. Each layer is believed to represent deposition from a single wave within the tsunami wave train. Eye-witness accounts report that the tsunami consisted of 3-4 waves, with the second and third waves being the largest. The greatest run-up measured (from mid-swash, uncorrected for tides) was 8.2 meters at Playa la Chira. The greatest inundation distance measured (from mid-swash, uncorrected for tides) was 760 meters at La Quinta. The 1st ITST measured inundation distances of up to 1 km at some locations.

Acknowledgments

Funding for the participation of the USGS in the 2001 Peru Tsunami Sediment Survey came from the USGS Coastal and Marine Geology Program, Dr. John Haines, Program Coordinator. We would like to thank Admiral Hector Soldi Soldi, Director de Hidrografia y Navagacion, Peruvian Navy, for inviting the USGS to participate in this survey, for DHN's assistance with logistics, and for the support of DHN scientists who participated in this survey. We thank Rómulo Mucho, Chairman of the Board of Directors of the INGEMMET, and Dr. Ronald Woodward, President IGP for the support of the IGP and INGEMMET scientists who participated in the survey. We thank Miguel Ypez, Foreign Service National, US Embassy, Lima for his help and Jean Weaver, USGS/GD International Programs Latin America leader for assisting in arranging for participation of the USGS personnel. We send our condolences for those lost in the tsunami. We thank the people of Camaná for their warm welcome and support, and wish them a rapid recovery in these difficult times.